Pharmacology 4th semester

1. PHRMACODYNAMICS
SATYAJIT GHOSH
B. PHARM 4TH
SEMESTER

2.  Introduction:
- Pharmacodynamics is the study of the biochemical, cellular, and physiological effects of drugs and their mechanisms of action. The effects of most
drugs result from their interaction with macromolecular components of the organism. The term drug receptor or drug target denotes the cellular
macromolecule or macromolecular complex with which the drug interacts to elicit a cellular or systemic response.
- Drugs commonly alter the rate or magnitude of an intrinsic cellular or physiological response rather than create new responses. Drug receptors are
often located on the surface of cells but may also be located in specific intracellular compartments, such as the nucleus, or in the extracellular
compartment, as in the case of drugs that target coagulation factors and inflammatory mediators.
- Many drugs also interact with acceptors (e.g., serum albumin), which are entities that do not directly cause any change in biochemical or physiological
response but can alter the pharmacokinetics of a drug’s actions.
 Physiological Receptors:
- Many drug receptors are proteins that normally serve as receptors for endogenous regulatory ligands.
- These drug targets are termed physiological receptors. Drugs that bind to physiological receptors and mimic the regulatory effects of the endogenous
signalling compounds are termed agonists. If the drug binds to the same recognition site as the endogenous agonist, the drug is said to be a primary
agonist. Allosteric (or allotopic) agonists bind to a different region on the receptor, referred to as an allosteric or allotopic site.
- Drugs that block or reduce the action of an agonist are termed antagonists. Antagonism generally results from competition with an agonist for the same
or overlapping site on the receptor (a syntopic interaction), but can also occur by interacting with other sites on the receptor (allosteric antagonism),
by combining with the agonist (chemical antagonism), or by functional antagonism by indirectly inhibiting the cellular or physiological effects of the
agonist.

3. - Agents that are only partially as effective as agonists are termed partial agonists. Many receptors exhibit some constitutive activity in the absence of a
regulatory ligand; drugs that stabilize such receptors in an inactive conformation are termed inverse agonists. In the presence of a full agonist, partial
and inverse agonists will behave as competitive antagonists.
 Specificity of Drug Responses:
- The strength of the reversible interaction between a drug and its receptor, as measured by the dissociation constant, is defined as the affinity of one for
the other. Both the affinity of a drug for its receptor and its intrinsic activity are determined by its chemical structure.
- The chemical structure of a drug also contributes to the drug’s specificity. A drug that interacts with a single type of receptor that is expressed on only
a limited number of differentiated cells will exhibit high specificity.
- Conversely, a drug acting on a receptor expressed ubiquitously throughout the body will exhibit widespread effects. Many clinically important drugs
exhibit a broad (low) specificity because they interact with multiple receptors in different tissues.
- Such broad specificity might not only enhance the clinical utility of a drug but also contribute to a spectrum of adverse side effects because of off-target
interactions. One example of a drug that interacts with multiple receptors is Amiodarone, an agent used to treat cardiac arrhythmias. Amiodarone also
has a number of serious toxicities, some of which are caused by the drug’s structural similarity to thyroid hormone and, as a result, its capacity to
interact with nuclear thyroid receptors.
- Some drugs are administered as racemic mixtures of stereoisomers. The stereoisomers can exhibit different pharmacodynamic as well as
pharmacokinetic properties. For example, the antiarrhythmic drug Sotalol is prescribed as a racemic mixture; the d- and l-enantiomers are equipotent
as K+
channel blockers, but the l-enantiomer is a much more potent β adrenergic antagonist.
- A drug may have multiple mechanisms of action that depend on receptor specificity, the tissue-specific expression of the receptor(s), drug access to
target tissues, different drug concentrations in different tissues, pharmacogenetics, and interactions with other drugs.

4. - Chronic administration of a drug may cause a down regulation of receptors or desensitization of response that can require dose adjustments to maintain
adequate therapy. Chronic administration of nitro vasodilators to treat angina results in the rapid development of complete tolerance, a process known
as tachyphylaxis.
- Some drug effects do not occur by means of macromolecular receptors. For instance, aluminium and magnesium hydroxides [Al (OH)3 and Mg (OH)2]
reduce gastric acid chemically, neutralizing H+
with OH+
and raising gastric pH.
- Mannitol acts osmotically to cause changes in the distribution of water to promote diuresis, catharsis, expansion of circulating volume in the vascular
compartment, or reduction of cerebral edema.
- Anti-infective drugs such as antibiotics, antivirals, and antiparasitic achieve specificity by targeting receptors or cell processes that are critical for the
growth or survival of the infective agent but are nonessential or lacking in the host organism.
- Resistance to antibiotics, antivirals, and other drugs can occur through a variety of mechanisms, including mutation of the target receptor, increased
expression of enzymes that degrade or increase efflux of the drug from the infective agent, and development of alternative biochemical pathways that
circumvent the drug’s effects on the infective agent.
 Quantitative Aspects of Drug Interactions with Receptors:
- Receptor occupancy theory assumes that a drug’s response emanates from a receptor occupied by the drug, a concept that has its basis in the law of
mass action. The dose-response curve depicts the observed effect of a drug as a function of its concentration in the receptor compartment. Following
figure shows a typical dose-response curve:

5. L+R ⇌ LR ⇌ LR*
K-1
K-2
K+2
K+1
 Affinity, Efficacy, and Potency:
- In general, the drug-receptor interaction is characterized by (1) binding of drug to receptor and (2) generation of a response in a biological system, as
illustrated in following equation:
where the drug or ligand is denoted as L and the inactive receptor as R. The first reaction, the reversible formation of the ligand-receptor complex LR,
is governed by the chemical property of affinity.
- LR*
is produced in proportion to [LR] and leads to a response. This simple relationship illustrates the reliance of the affinity of the ligand (L) with
receptor (R) on both the forward or association rate k+1 and the reverse or dissociation rate k–1.
- At any given time, the concentration of ligand-receptor complex [LR] is equal to the rate of formation of the bimolecular complex LR (k+1[L][R]),
minus the rate of dissociation of LR into L and R (k–1[LR]).
- At equilibrium (i.e., when δ[LR]/δt = 0), k+1[L][R] = k–1[LR]. The equilibrium dissociation constant KD is then described by ratio of the off and on
rate constants, k–1/k+1.Thus, at equilibrium:
K =
[L][R]
[LR]
=
K
K
- The affinity constant or equilibrium association constant KA is the reciprocal of the equilibrium dissociation constant (i.e., KA = 1/KD); thus, a high-
affinity drug has a low KD and will bind a greater number of a particular receptor at a low concentration than a low-affinity drug.
- As a practical matter, the affinity of a drug is influenced most often by changes in its off rate (k–1) rather than its on rate (k+1). Above equation
permits us to describe the fractional occupancy (f) of receptors by agonist L as a function of [R] and [LR]:
f =
[Ligand − receptor complex]
[Total receptor]
=
[LR]
[R] + [LR]

6. f can also be expressed in terms of KA (or KD) and [L]:
f =
K [L]
1 + K [L]
=
[L]
1
K + [L]
=
[L]
[K ] + [L]
This equation shows that when the concentration of drug equals the KD (or 1/KA), f = 0.5, that is, the drug will occupy 50% of the receptors. This
equation describes only receptor occupancy, not the eventual response that may be amplified by the cell.
Because of downstream amplification, many signalling systems can reach a full biological response with only a fraction of receptors occupied.
- Basically, when two drugs produce equivalent responses, the drug whose dose-response curve lies to the left of the other (i.e., the concentration
producing a half-maximal effect [EC50] is smaller) is said to be the more potent.
- Efficacy reflects the capacity of a drug to activate a receptor and generate a cellular response. Thus, a drug with high efficacy may be a full agonist,
eliciting, at some concentration, a full response. A drug with a lower efficacy at the same receptor may not elicit a full response at any dose. A drug
with a low intrinsic efficacy will be a partial agonist. A drug that binds to a receptor and exhibits zero efficacy is an antagonist.
 Principle of drug action:
- Drugs can produce effect by following ways:
Principle Note
Stimulation - It refers to selective enhancement of the level of activity of specialized cells. e.g. adrenaline stimulates heart,
pilocarpine stimulates salivary glands.
- Excessive stimulation is often followed by depression of that function. e.g. high close of picrotoxin, a central nervous
system (CNS) stimulant, produces convulsions followed by coma and respiratory depression.
Depression - It means selective diminution of activity of specialized cells, e.g. barbiturates depress CNS, quinidine depresses heart,
omeprazole depresses gastric acid secretion.
- Certain drugs stimulate one type of cells but depress the other, e.g. acetylcholine stimulates intestinal smooth muscle
but depresses SA node in heart.

7. Irritation - This connotes a non-selective, often noxious effect and is particularly applied to less specialized cells (epithelium,
connective tissue).
- Strong irritation results in inflammation. corrosion. necrosis and morphological damage. This may result in diminution
or loss of function.
Replacement - This refers to the use of natural metabolites. hormones or their congeners in deficiency states. e.g. levodopa in
parkinsonism. insulin in diabetes mellitus, iron in anaemia.
Cytotoxic
action
- Selective cytotoxic action on invading parasites or cancer cells, attenuating them without significantly affecting the
host cells is utilised for cure/palliation of infections and neoplasms, e.g. penicillin, chloroquine. zidovudine,
cyclophosphamide, etc.
 Mechanism of drug action:
- Drug show their actions on different receptors by following mechanisms:
 Receptors That Affect Concentrations of Endogenous Ligands:
- A large number of drugs act by altering the synthesis, storage, release, transport, or metabolism of endogenous ligands such as neurotransmitters,
hormones, and other intercellular mediators. For example, some of the drugs acting on adrenergic neurotransmission include α-methyltyrosine
(inhibits synthesis of NE), cocaine (blocks NE reuptake), amphetamine (promotes NE release), and selegiline (inhibits NE breakdown by MAO).
- There are similar examples for other neurotransmitter systems, including ACh, DA, and 5HT. Drugs that affect the synthesis and degradation of
circulating mediators such as vasoactive peptides (e.g., ACE inhibitors) and lipid-derived autocoids (e.g., cyclooxygenase inhibitors) are also widely
used in the treatment of hypertension, inflammation, myocardial ischemia, and heart failure.
 Drug Receptors Associated with Extracellular Processes:
- Many widely used drugs target enzymes and molecules that control extracellular processes such as thrombosis, inflammation, and immune responses.
For instance, the coagulation system is highly regulated and has a number of drug targets that control the formation and degradation of clots, including

8. several coagulation factors (thrombin and factor Xa), antithrombin, and glycoproteins on the surface of platelets that control platelet activation and
aggregation.
 Receptors Utilized by Anti-infective Agents:
- Anti-infective agents such as antibacterials, antivirals, antifungals, and antiparasitic agents target receptors that are microbial proteins. These proteins
are key enzymes in biochemical pathways that are required by the infectious agent but are not critical for the host.
- A novel approach to preventing infections such as that of the mosquito-borne malaria parasite is to genetically engineer the vector organism to be
resistant to infection by the parasite using techniques such as the CRISPR-Cas9 system.
 Receptors That Regulate the Ionic Milieu:
- A relatively small number of drugs act by affecting the ionic milieu of blood, urine, and the GI tract. The receptors for these drugs are ion pump sand
transporters, many of which are expressed only in specialized cells of the kidney and GI tract.
- Most of the diuretics (e.g., furosemide, chlorothiazide, amiloride) act by directly affecting ion pumps and transporters in epithelial cells of the nephron
that increase the movement of Na+
into the urine or by altering the expression of ion pumps in these cells (e.g., aldosterone).
- Another therapeutically important target is the H+
/K+
-ATPase (proton pump) of gastric parietal cells. Irreversible inhibition of this proton pump by
drugs such as esomeprazole reduces gastric acid secretion by 80%–95%.
 Drug action-effect sequence:
 Drug action:
- It is the initial combination of the drug with its receptor resulting in a conformational change in the receptor (in case of agonists), or prevention of
conformational change through exclusion of the agonist (in case of antagonists).

9.  Drug effect:
- It is the ultimate change in biological function brought about as a consequence of drug action, through a series of intermediate steps (transducer).
Receptors serve two essential functions, viz recognition of the specific ligand molecule and transduction of the signal into a response.
- Accordingly, the receptor molecule has a ligand binding domain and an effector domain which undergoes a functional conformational change. The
perturbation in the receptor molecule is variously translated into the response.
 Targets of drugs actions:
- There are four major types of bio macromolecular targets of drug action:
1. Enzyme
2. Ion channels
3. Transporters
4. Receptors
 Enzymes:
- Enzymes are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may
act are called substrates, and the enzyme converts the substrates into different molecules known as products.
- Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life. Metabolic pathways depend upon
enzymes to catalyse individual steps. Enzymes are known to catalyse more than 5,000 biochemical reaction types. Other biocatalysts are catalytic
RNA molecules, called ribozymes.
- Enzymes increase the reaction rate by lowering its activation energy. Some enzymes can make their conversion of substrate to product occur many
millions of times faster.

10. - Drug can either increase or decrease the rate of enzymatically mediated reactions. Enzyme stimulation may occur due to some natural metabolites
only, e.g., pyridoxine acts as a cofactor and increases decarboxylase activity.
- Several enzymes are stimulated through receptors and second messengers, e.g. adrenaline stimulates hepatic glycogen phosphorylase through p
receptors and cyclic AMP.
- Stimulation of an enzyme increases its affinity for the substrate so that rate constant (kM) of the reaction is lowered. Apparent increase in enzyme
activity can also occur by enzyme induction, i.e. synthesis of more enzyme protein.
Enzyme inhibition:
Some chemicals (heavy metal salts strong acids and alkalis, formaldehyde, phenol, etc.) denature proteins and inhibit all enzymes non-selectively.
Such inhibition is either competitive or non-competitive.
A- Competitive inhibitors:
- The drug being structurally similar competes with the normal substrate for the catalytic binding site of the enzyme so that the product is not
formed or a non-functional product is formed, and a new equilibrium is achieved in the presence of the drug.
- Such inhibitors increase the kM but the Vmax remains unchanged i.e. higher concentration of the substrate is required to achieve ½ maximal
reaction velocity, but if substrate concentration is sufficiently increased, it can displace the inhibitor and the same maximal reaction velocity
can be attained.
Enzyme Endogenous substrate Competitive inhibitor
Cholinesterase Acetylcholine Physostigmine, Neostigmine
Monoamine-oxidase A (MAO-A) Catecholamines Moclobemide
Dopa decarboxylase Levodopa Carbidopa, Benserazide
Xanthine oxidase Hypoxanthine Allopurinol
Angiotensin converting enzyme (ACE) Angiotensin-1 Captopril
5α -Reductase Testosterone Finasteride
Aromatase Testosterone, Androstenedione Letrozole, Anastrozole
Bacterial folate synthase Para-aminobenzoic acid (PABA) Sulfadiazine

11. - A nonequilibrium type of enzyme inhibition can also occur with drugs which react with the same catalytic site of the enzyme but either form
strong covalent bonds or have such high affinity for the enzyme that the normal substrate is not able to displace the inhibitor, e.g.
a- Organophosphates react covalently with the esteratic site of the enzyme cholinesterase.
b- Methotrexate has 50,000 times higher affinity for dihydrofolate reductase than the normal substrate DHFA. In these situations, kM is
increased and Vmax is reduced.
B- Non-competitive inhibitors:
- The inhibitor reacts with an adjacent site and not with the catalytic site, but alters the enzyme in such a way that it loses sits catalytic
property. Thus, kM is unchanged but Vmax is reduced. Examples:
Non-competitive inhibitor Enzyme
Acetazolamide Carbonic anhydrase
Aspirin, indomethacin Cyclooxygenase
Disulfiram Aldehyde dehydrogenase
Omeprazole H+ K+ ATPase
Digoxin Na+ K+ ATPase
Theophylline Phosphodiesterase
Propylthiouracil Peroxidase in thyroid
Lovastatin HMG-CoA reductase
Sildenafil Phosphodiesterase-5
 Ion channel:
- Ion channels are pore-forming membrane proteins that allows the ions to pass through the channel pore. To establish the electrochemical gradients
required to maintain a membrane potential, all cells express ion transporters for Na+
, K+
, Ca2+
, and Cl–
.
- For example, the Na+
, K+
-ATPase expends cellular ATP to pump Na+
out of the cell and K+
into the cell. The electrochemical gradients thus established
are used by excitable tissues such as nerve and muscle to generate and transmit electrical impulses, by non-excitable cells to trigger biochemical and
secretory events, and by all cells to support a variety of secondary symport and antiport processes.
- They can also be classified as voltage-activated, ligand-activated, store-activated, stretch-activated, and temperature activated channels.

12. - Certain drugs modulate opening and closing of the channels, e.g.:
i. Quinidine blocks myocardial Na+
channels.
ii. Dofetilide and amiodarone block myocardial delayed rectifier K+
channel.
iii. Nifedipine blocks L-type of voltage sensitive Ca2+
channel.
iv. Nicorandil opens ATP-sensitive K+
channels.
v. Sulfonylurea hypoglycaemics inhibit pancreatic ATP-sensitive K+
channels.
vi. Amiloride inhibits renal epithelial Na+
channels.
vii. Phenytoin modulates (prolongs the inactivated state of) voltage sensitive neuronal Na+
channel.
viii.Ethosuximide inhibits T-type of Ca2+
channels in thalamic neurones.
 Transporters:
- Several substrates are translocated across membranes by binding to specific transporters (carriers) which either facilitate diffusion in the direction of
the concentration gradient or pump the metabolite/ion against the concentration gradient using metabolic energy.
- Many drugs produce their action by directly interacting with the solute carrier (SLC) class of transporter proteins to inhibit the ongoing physiological
transport of the metabolite/ ion. Examples are:
i. Desipramine and cocaine block neuronal reuptake or noradrenaline by interacting with norepinephrine transporter (NET).
ii. Fluoxetine inhibit neuronal reuptake or 5-HT by interacting with serotonin transporter (SERT).
iii. Amphetamines selectively block dopamine reuptake in brain neurons by dopamine transporter (DAT).
iv. Reserpine blocks the vesicular reuptake of noradrenaline and 5-HT by the vesicular monoamine transporter (YMAT-2).
v. Hemicholinium blocks choline uptake into cholinergic neurones and depletes acetylcholine.
vi. The anticonvulsant tiagabine acts by inhibiting reuptake of GABA into brain neurone by GABA transporter GAT-I.

13. vii. Furosemide inhibits the Na+
K+
2CI-
cotransporter in the ascending limb of loop of Henle.
viii.Hydrochlorothiazide inhibits the Na-Cl symporter in the early distal tubule.
ix. Probenecid inhibits active transport of organic acids (uric acid, penicillin) in renal tubules by interacting with organic anion transporter (OAT).
 Receptors:
- The largest number of drugs do not bind directly to the effectors, viz. enzymes, channels, transporters, structural proteins, template biomolecules, etc,
but act through specific regulatory macromolecules which control the above listed effectors. These regulatory macromolecules or the sites on them
which associate with and interact with the drug are called ' receptors'.
- Receptor: It is defined as a macromolecule or binding sile located on the surface or inside the effector cell that serves to recognize the signal
molecule/drug and initiate the response to it, but itself has no other function.

14.  Structural and Functional Families of Physiological Receptors (classification of receptors):
- Receptors for physiological regulatory molecules can be assigned to functional families that share common molecular structures and biochemical
mechanisms. There are six functional families of physiological receptors:

15.  G Protein–Coupled Receptors (GPCR):
 Molecular structure:
- GPCRs consist of a single polypeptide chain, usually of 350–400 amino acid residues. Their characteristic structure comprises seven transmembraneα-
helices, with an extracellular N-terminal domain of varying length, and an intracellular C-terminal domain.
- GPCRs are divided into three main classes – A, B and C. They share the same seven transmembrane helix (heptahelical) structure, but differ in other
respects, principally in the length of the extracellular N-terminus and the location of the agonist binding domain.
- Class A is by far the largest, comprising most monoamine, neuro peptide and chemokine receptors. Class B includes receptors for some other peptides,
such as calcitonin and glucagon. Class C is the smallest, its main members being the metabotropic glutamate and GABA receptors, and theCa2+
-sensing
receptors.
 G proteins and their roles:
- G proteins comprise a family of membrane-resident proteins whose function is to respond to GPCR activation and pass on the message inside the cell
to the effector systems that generate a cellular response.

16. - GPCRs couple to a family of heterotrimeric GTP-binding regulatory proteins termed G proteins. G proteins are signal transducers that convey the
information from the agonist-bound receptor to one or more effector proteins.
- The G protein heterotrimer consists of a guanine nucleotide-binding α subunit, which confers specific recognition to both receptors and effectors, and
an associated dimer of β and γ subunits that helps confer membrane localization of the G protein heterotrimer by prenylation of the γ subunit.
- Guanine nucleotides bind to the α subunit, which has enzymic (GTPase) activity, catalysing the conversion of GTP to GDP. The β and γ subunits
remain together as a βγ complex. The ‘γ’ subunit is anchored to the membrane through a fatty acid chain, coupled to the G protein through a reaction
known as prenylation.
- In the ‘resting’ state, the G protein exits as an αβγ trimer, which may or may not be pre coupled to the receptor, with GDP occupying the site on the
αsubunit. When a GPCR is activated by an agonist this induces small changes in residues around the ligand-binding pocket that translate to larger
rearrangements of the intracellular regions of the receptor that open a cavity on the intracellular side of the receptor into which the G protein can bind,
resulting in a high-affinity interaction of αβγ and the receptor.
- This agonist-induced interaction of αβγ with the receptor causing the bound GDP to dissociate and to be replaced with GTP, which in turn causes
dissociation of the G protein trimer, releasing α–GTP from the βγ subunits; these are the ‘active’ forms of the G protein, which diffuse in the
membrane and can associate with various enzymes and ion channels, causing activation of the target.

17. - The α subunit had a signalling function & the βγ complexes control effectors in much the same way as the α subunits. Association of α or βγ subunits
with target enzymes or channels can cause either activation or inhibition, depending on which G protein is involved.
- G protein activation results in amplification, because a single agonist–receptor complex can activate several G protein molecules in turn, and each of
these can remain associated with their effector enzyme for long enough to produce many molecules of product.
- The product is often a ‘second messenger’, and further amplification occurs before the final cellular response is produced. Signalling is terminated
when the hydrolysis of GTP to GDP occurs through the inherent GTPase activity of the αsubunit. The resulting α–GDP then dissociates from the
effector, and reunites with βγ, completing the cycle.

18. - Four main classes of G protein (Gs, Gi, Go and Gq) are of pharmacological importance. These differ primarily in the αsubunit they contain. G proteins
show selectivity with respect to both receptor and the effector with which they couple, having specific recognition domains in their structure
complementary to specific G protein-binding domains in the receptor and effector molecules.
 Targets for G proteins:
- The main targets for G proteins, through which GPCRs control different aspects of cell function, are:
a- Adenylyl cyclase, the enzyme responsible for cAMP formation;
b- Phospholipase C, the enzyme responsible for inositol phosphate and diacylglycerol (DAG) formation;
c- Ion channels, particularly calcium and potassium channels;
d- Rho A/Rho kinase, a system that regulates the activity of many signalling pathways controlling cell growth, proliferation and motility, smooth
muscle contraction, etc.;
e- Mitogen-activated protein kinase (MAP kinase), a system that controls many cell functions, including cell division and is also a target of several
kinase-linked receptors
α-GTP
GαS
↑AC
↑cAMP
↑Ca2+
↑Na+
GαI/O
↓AC
↓cAMP
↓Ca2+
Na+
↑K+
GαQ
↑Phospholipase C
↑DAG, IP3
↑Ca2+
Gα12/13
Cl-/K+/Na+/H+ exchanger

19.  Second-Messenger Systems:
A- Cyclic AMP:
- cAMP is synthesized by the enzyme adenylyl cyclase (AC) from ATP; stimulation is mediated by the Gαs subunit, inhibition by the Gαi subunit.
It is produced continuously and inactivated by hydrolysis to 5’ AMP by the action of phosphodiesterase (PDEs).
ATP
( )
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ cAMP
- There are nine membrane-bound isoforms of AC and one soluble isoform found in mammals. cAMP generated by ACs has three major targets in
most cells:
a- the cAMP-dependent protein kinase A (PKA);
b- cAMP-regulated GEFs termed EPACs
c- via PKA phosphorylation, a transcription factor termed CREB.
- In cells with specialized functions, cAMP can have additional targets, such as CNG and HCN, and cyclic nucleotide-regulated PDEs.
 PKA
- The PKA holoenzyme consists of two catalytic (C) subunits reversibly bound to a regulatory (R) subunit dimer to form a heterotetrameric
complex (R2C2).
- When AC is activated and cAMP concentrations increase, four cAMP molecules bind to the R2C2 complex, two to each R subunit, causing a
conformational change in the R subunits that lowers their affinity for the C subunits, resulting in their activation.
- The active C subunits phosphorylate serine and threonine residues on specific protein substrates. There are multiple isoforms of PKA;
molecular cloning has revealed α and β isoforms of both the regulatory subunits (RI and RII), as well as three C subunit isoforms Cα, Cβ, and
Cγ.

20. - The R subunits exhibit different subcellular localization and binding affinities for cAMP, giving rise to PKA holoenzymes with different
thresholds for activation. PKA function also is modulated by subcellular localization mediated by A-kinase anchor protein (AKAPs).
Roles of PKA:
a- Lipase (Inactive) ⎯ Lipase (Active) → ↑ se lipolysis
b- Glycogen synthase ⎯ Glycogen synthase (Active) → ↓ se glycogen synthesis
c- Increases glycogenolysis in liver: -
d- PKA phosphorylates myosin light chain kinase enzyme in smooth muscle of lungs leads to relaxation.
e- In heart it causes contraction in cardiac muscle.
f- It inhibits platelet aggregation.
 PKG:
- Stimulation of receptors that raise intracellular cGMP concentrations leads to the activation of the cGMP-dependent PKG that
phosphorylates some of the same substrates as PKA and some that are PKG-specific.
- Unlike the heterotetramer (R2C2) structure of the PKA holoenzyme, the catalytic domain and cyclic nucleotide-binding domains of PKG are
expressed as a single polypeptide, which dimerizes to form the PKG holoenzyme.

21. - Protein kinase G exists in two homologous forms, PKG-I and PKG-II. PKG-I has an acetylated N terminus, is associated with the cytoplasm,
and has two isoforms (Iα and Iβ) that arise from alternate splicing.
- PKG-II has a myristylated N-terminus, is membrane-associated, and can be localized by PKG-anchoring proteins.
- Pharmacologically important effects of elevated cGMP include modulation of platelet activation and relaxation of smooth muscle.
 PDEs:
- Cyclic nucleotide PDEs form another family of important signalling protein whose activities are regulated via the rate of gene transcription as
well as by second messengers (cyclic nucleotides or Ca2+
) and interactions with other signalling proteins such as β arrestin and PKs.
- PDEs hydrolyse the cyclic 3′,5′-phosphodiester bond in cAMP and cGMP, thereby terminating their action. The PDEs comprise a
superfamily with more than 50 different proteins.
- The substrate specificities of the different PDEs include those specific for cAMP hydrolysis and for cGMP hydrolysis and some that hydrolyse
both cyclic nucleotides.
- PDEs (mainly PDE3 forms) are drug targets for treatment of diseases such as asthma, cardiovascular diseases such as heart failure,
atherosclerotic coronary and peripheral arterial disease, and neurological disorders.
- PDE5 inhibitors (e.g., sildenafil) are used in treating chronic obstructive pulmonary disease and erectile dysfunction.
 EPACs:
- EPAC, also known as cAMP-GEF, is a novel cAMP-dependent signalling protein that plays unique roles in cAMP signalling. cAMP signalling
through EPAC can occur in isolation or in concert with PKA signalling.
- EPAC serves as a cAMP-regulated GEF for the family of small Ras GTPases, catalysing the exchange of GTP for GDP, thus activating the
small GTPase by promoting formation of the GTP-bound form.

22. - Two isoforms of EPAC are known, EPAC1 and EPAC2; they differ in their architecture and tissue expression. Both EPAC isoforms are
multidomain proteins that contain a regulatory cAMP-binding domain, a catalytic domain, and domains that determine the intracellular
localization of EPAC.
- Compared to EPAC2, EPAC1 contains an additional N-terminal low-affinity cAMP-binding domain. The expression of EPAC1 and EPAC2
are differentially regulated during development and in a variety of disease states.
- EPAC2 can promote incretin-stimulated insulin secretion from pancreatic β cells through activation of Rap1. Sulfonylureas, important oral
drugs used to treat type II diabetes mellitus, may act in part by activating EPAC2 in β cells and increasing insulin release.
B- Gq-PLC-DAG/IP3-Ca2+
Pathway:
- Calcium is an important messenger in all cells and can regulate diverse responses, including gene expression, contraction, secretion, metabolism,
and electrical activity. Ca2+
can enter the cell through Ca2+
channels in the plasma membrane or be released by hormones or growth factors from
intracellular stores.
- The basal Ca2+
level in cells is maintained by membrane Ca2+
pumps that extrude Ca2+
to the extracellular space and a sarco/endoplasmic
reticulum Ca2+
ATPase or SR Ca2+
ATPase (SERCA) in the membrane of the ER that accumulates Ca2+
into its storage site in the ER/SR.
- Hormones and growth factors release Ca2+
from its intracellular storage site, the ER, via a signalling pathway that begins with activation of PLC,
of which there are two primary forms, PLCβ and PLCγ.
- GPCRs that couple to Gq activate PLCβ by activating the Gα subunit and releasing the βγ dimer. Both the active, Gq-GTP–bound α subunit and
the βγ dimer can activate certain isoforms of PLC-β. PLC-γ isoforms are activated by tyrosine phosphorylation, including phosphorylation by
receptor and nonreceptor tyrosine kinases.

23. - The PLCs are cytosolic enzymes that translocate to the plasma membrane on receptor stimulation. When activated, they hydrolyse a minor
membrane phospholipid, phosphatidylinositol-4,5-bisphosphate (PIP2), to generate two intracellular signals, Inositol (1, 4, 5) triphosphate
(IP3) and the lipid Diacylglycerol (DAG).
Inositol phosphates and intracellular calcium:
- Inositol (1,4,5) trisphosphate (IP3) is a water-soluble mediator that is released into the cytosol and acts on a specific receptor– the IP3
receptor – which is a ligand-gated calcium channel present on the membrane of the endoplasmic reticulum.
- The main role of IP3, is to control the release of Ca2+
from intracellular stores. Because many drug and hormone effects involve intracellular
Ca2+
, this pathway is particularly important.
Diacylglycerol and protein kinase C:
- The main effect of DAG is to activate a protein kinase, protein kinase C (PKC), which catalyses the phosphorylation of several intracellular
proteins. DAG is highly lipophilic and remains within the membrane.
- It binds to a specific site on the PKC molecule, causing the enzyme to migrate from the cytosol to the cell membrane, thereby becoming
activated. There are at least 10 different mammalian PKC subtypes, which have distinct cellular distributions and phosphorylate different
proteins.
- Several are activated by DAG and raised intracellular Ca2+
, both of which are produced by activation of GPCRs. PKCs are also activated by
phorbol esters (highly irritant, tumour-promoting compounds produced by certain plants), which have been extremely useful in studying the
functions of PKC.
- One of the subtypes is activated by the lipid mediator arachidonic acid generated by the action of phospholipase A2 on membrane
phospholipids, so PKC activation can also occur with agonists that activate this enzyme.

24. - Release of Ca2+
from these intracellular stores raises Ca2+
levels in the cytoplasm and activates Ca2+
-dependent enzymes such as some of the PKCs
and Ca2+
/calmodulin-sensitive enzymes such as one of the cAMP-hydrolysing PDEs and a family of Ca2+
/calmodulin-sensitive PKs (e.g.,
phosphorylase kinase, MLCK, and CaM kinases II and IV).
- Depending on the cell’s differentiated function, the Ca2+
/calmodulin kinases and PKC may regulate the bulk of the downstream events in the
activated cells.
 Ion channels as targets for G proteins:
- Another major function of GPCRs is to control ion channel function directly by mechanisms that do not involve second messengers such as cAMP or
inositol phosphates. Direct G protein–channel interaction, through the βγ subunits of Gi and Go proteins, appears to be a general mechanism for
controlling K+
and Ca2+
channels.
- In cardiac muscle, for example, mAChRs enhance K+
permeability in this way (thus hyperpolarising the cells and inhibiting electrical activity).
Similar mechanisms operate in neurons, where many inhibitory drugs, such as opioid analgesics, reduce excitability by opening certain K+
channels –
known as G protein-activated inwardly rectifying K+
channels (GIRK) – or by inhibiting voltage-activated N and P/Q type Ca2+
channels, thus
reducing neurotransmitter release.
 The Rho/Rho kinase system:
- This signal transduction pathway is activated by certain GPCRs (and also by non-GPCR mechanisms), which couple to G proteins of the G12/13 type.
The free G protein α subunit interacts with a guanosine nucleotide exchange factor, which facilitates GDP–GTP exchange at another GTPase, Rho.
- Rho–GDP, the resting form, is inactive, but when GDP–GTP exchange occurs, Rho is activated, and in turn activates Rho kinase. Rho kinase
phosphorylates many substrate proteins and controls a wide variety of cellular functions, including smooth muscle contraction and proliferation, cell
movement and migration, angiogenesis and synaptic remodelling.

25.  Ion Channels:
- Ion channels are pore-forming membrane proteins that allows the ions to pass through the channel pore. To establish the electrochemical gradients
required to maintain a membrane potential, all cells express ion transporters for Na+
, K+
, Ca2+
, and Cl–
.
- For example, the Na+
, K+
-ATPase expends cellular ATP to pump Na+
out of the cell and K+
into the cell. The electrochemical gradients thus established
are used by excitable tissues such as nerve and muscle to generate and transmit electrical impulses, by non-excitable cells to trigger biochemical and
secretory events, and by all cells to support a variety of secondary symport and antiport processes.
- They can also be classified as voltage-activated, ligand-activated, store-activated, stretch-activated, and temperature activated channels.

26.  Voltage-Gated Channels:
- Humans express multiple isoforms of voltage-gated channels for Na+
, K+
, Ca2+
, and Cl–
ions. In nerve and muscle cells, voltage-gated Na+
channels are
responsible for the generation of robust action potentials that depolarize the membrane from its resting potential of –70 mV up to a potential of +20
mV within a few milliseconds.
- These Na+
channels are composed of three subunits, a pore-forming α subunit and two regulatory β subunits. The α subunit is a 260-kDa protein
containing four domains that form a Na+
ion–selective pore by arranging into a pseudo tetramer shape.
- The β subunits are 36-kDa proteins that span the membrane once. Each domain of the α subunit contains six membrane-spanning helices (S1–S6)
with an extracellular loop between S5 and S6, termed the pore forming or P loop; the P loop dips back into the pore and, combined with residues from
the corresponding P loops from the other domains, provides a selectivity filter for the Na+
ion.
- Four other helices surrounding the pore (one S4 helix from each of the domains) contain a set of charged amino acids that form the voltage sensor and
cause a conformational change in the pore at more positive voltages, leading to opening of the pore and depolarization of the membrane.
- The voltage-activated Na+
channels in pain neurons are targets for local anaesthetics, such as Lidocaine and Tetracaine, which block the pore, inhibit
depolarization, and thus block the sensation of pain. They are also the targets of the naturally occurring marine toxins Tetrodotoxin and Saxitoxin.

27. - Voltage-activated Na+
channels are also important targets of many drugs used to treat cardiac arrhythmias.
- Voltage-gated Ca2+
channels have a similar architecture to voltage-gated Na+
channels with a large α subunit (four domains of five membrane-spanning
helices) and three regulatory subunits (the β, δ, and γ subunits).
- Ca2+
channels can be responsible for initiating an action potential (as in the pacemaker cells of the heart) but are more commonly responsible for
modifying the shape and duration of an action potential initiated by fast voltage-gated Na+
channels.
- These channels initiate the influx of Ca2+
that stimulates the release of neurotransmitters in the central, enteric, and autonomic nervous systems and
that control heart rate and impulse conduction in cardiac tissue. Ca2+
channel antagonists such as Nifedipine, Diltiazem, and Verapamil are effective
vasodilators and are widely used to treat hypertension, angina, and certain cardiac arrhythmias.
- Voltage-gated K+
channels are the most numerous and structurally diverse members of the voltage-gated channel family and include the voltage-gated
Kv channels, the inwardly rectifying K+ channel, and the tandem or two-pore domain “leak” K+ channels.
- The inwardly rectifying channels and the two-pore channels are voltage insensitive, regulated by G proteins and H+
ions, and greatly stimulated by
general anaesthetics.
- Increasing K+
conductance through these channels drives the membrane potential more negative, thus, these channels are important in regulating
resting membrane potential and restoring the resting membrane at -70 to -90 mV following depolarization.
 Ligand-Gated Channels:
- They are commonly referred to as ionotropic receptors, are a group of transmembrane ion channel proteins which open to allow ions to pass through
membrane in response to a binding chemical messenger or a ligand.
- Major ligand-gated channels in the nervous system are those that respond to excitatory neurotransmitters such as ACh or glutamate (or agonists such
as AMPA and NMDA) and inhibitory neurotransmitters such as glycine or GABA.

28. - Activation of these channels is responsible for the majority of synaptic transmission by neurons both in the CNS and in the periphery. In addition,
there are a variety of more specialized ion channels that are activated by intracellular small molecules and are structurally distinct from conventional
ligand-gated ion channels.
 Molecular structure:
- Ligand-gated ion channels have structural features in common with other ion channels. It consists of a pentameric assembly of different subunits,
of which there are four types, termed α, β, γ and δ, each of molecular weight (Mr) 40–58 kDa. The subunits show marked sequence homology,
and each contains four membrane-spanning α-helices, inserted into the membrane.
- The pentameric structure (α-2, β, γ, δ) possesses two ligand binding sites, each lying at the interface between one of the two α subunits and its
neighbour. Both must bind ligand molecules for the receptor to be activated.
- Each subunit spans the membrane four times, so the channel comprises no fewer than 20 membrane-spanning helices surrounding a central pore.
 Transmembrane Receptors Linked to Intracellular Enzymes (Enzyme linked receptors):
 Receptor Tyrosine Kinases:
- The receptor tyrosine kinases include receptors for hormones such as insulin; growth factors such Epidermal Growth Factor (EGF), PDGF, Nerve
Growth Factor (NGF), FGF, VEGF; and ephrins. With the exception of the insulin receptor, which has α and β chains, these macromolecules consist
of single polypeptide chains with large, cysteine-rich extracellular domains, short transmembrane domains, and an intracellular region containing one
or two protein tyrosine kinase domains.
- Activation of growth factor receptors leads to cell survival, cell proliferation, and differentiation. Activation of the ephrin receptors leads to neuronal
angiogenesis, axonal migration, and guidance.

29. - The inactive state of growth factor receptors is monomeric; binding of ligand induces dimerization of the receptor and cross-phosphorylation of the
kinase domains on multiple tyrosine residues. The phosphorylation of other tyrosine residues forms docking sites for the SH2 domains contained in a
large number of signalling proteins.
- In addition to recruiting enzymes, phosphotyrosine-presenting protein scan interact with SH2 domain-containing adaptor molecules without activity
(e.g., Grb2), which in turn attract GEFs such as Sos that can activate the small GTP-binding protein Ras.
- Activation of members of the Ras family leads in turn to activation of a PK cascade termed the Ras-MAPK pathway. Activation of the MAPK pathway
is one of the major routes used by growth factor receptors to signal to the nucleus and stimulate cell growth.
 Jak-STAT Receptor Pathway:
- These receptors have no intrinsic enzymatic activity; rather, the intracellular domain binds a separate, intracellular tyrosine kinase termed a Jak. On
dimerization induced by ligand binding, Jaks phosphorylate other proteins termed Signal Transducer and Activator of Transcription (STATs), pairs of
phosphorylated STATs get dimerises translocate to the nucleus and regulate transcription.
- The entire pathway is termed the Jak-STAT pathway. There are four Jaks and six STATs in mammals that, depending on the cell type and signal,
combine differentially to activate gene transcription.

30.  Nuclear receptors:
- Nuclear receptors are a class of proteins found within the cells that are responsible for sensing steroid and thyroid hormones and other certain
molecules.
- They have the ability to direct bind to DNA and regulate expression of adjacent genes, hence these genes are classified as transcription factors.

31.  Structure of nuclear receptors:
- Most nuclear receptors have molecular masses between 50,000 and 100,000 daltons. Nuclear receptors are modular in structure and contain the
following domains:
a- (A-B) N-terminal regulatory domain:
Contains the activation function 1 (AF-1) whose action is independent of the presence of ligand. The transcriptional activation of AF-1 is normally
very weak, but it does synergize with AF-2 in the E-domain to produce a more robust upregulation of gene expression. The A-B domain is highly
variable in sequence between various nuclear receptors.

32. b- (C) DNA-binding domain (DBD):
Highly conserved domain containing two zinc fingers that binds to specific sequences of DNA called hormone response elements (HRE).
c- (D) Hinge region:
It is flexible domain that connects the DBD with the LBD. Influences intracellular trafficking and subcellular distribution with a target
peptide sequence.
d- (E) Ligand binding domain (LBD):
The structure of the LBD is referred to as an alpha helical sandwich fold in which three anti parallel alpha helices (the "sandwich filling") are
flanked by two alpha helices on one side and three on the other (the "bread").
The ligand binding cavity is within the interior of the LBD and just below three anti parallel alpha helical sandwich "filling". Along with the DBD,
the LBD contributes to the dimerization interface of the receptor and in addition, binds coactivator and corepressor proteins.
The LBD also contains the activation function 2 (AF-2) whose action is dependent on the presence of bound ligand, controlled by the
conformation of helix 12 (H12).
e- (F) C-terminal domain: Highly variable in sequence between various nuclear receptors.
 Mechanism of action:
- Small lipophilic substances such as natural hormones diffuse through the cell membrane and bind to nuclear receptors located in the cytosol (type I
NR) or nucleus (type II NR) of the cell.
- Binding causes a conformational change in the receptor which, depending on the class of receptor, triggers a cascade of downstream events that direct
the NRs to DNA transcription regulation sites which result in up or down-regulation of gene expression.
- They generally function as homo/heterodimers. In addition, two additional classes, type III which are a variant of type I, and type IV that bind DNA as
monomers have also been identified.

33. - Accordingly, nuclear receptors may be subdivided into the following four mechanistic classes:
 Type I:
- Ligand binding to type I nuclear receptors in the cytosol results in the dissociation of heat shock proteins, homo-dimerization, translocation
(i.e., active transport) from the cytoplasm into the cell nucleus, and binding to specific sequences of DNA known as hormone response
elements (HREs).
- Type I nuclear receptors bind to HREs consisting of two half-sites separated by a variable length of DNA, and the second half-site has a sequence
inverted from the first (inverted repeat).
- Type I nuclear receptors include members of subfamily 3, such as the androgen receptor, estrogen receptors, glucocorticoid receptor, and
progesterone receptor.
- The nuclear receptor/DNA complex then recruits other proteins that transcribe DNA downstream from the HRE into messenger RNA and
eventually protein, which causes a change in cell function.

34.  Type II:
- Type II receptors, in contrast to type I, are retained in the nucleus regardless of the ligand binding status and in addition bind as hetero-dimers
(usually with RXR) to DNA.
- In the absence of ligand, type II nuclear receptors are often complexed with corepressor proteins. Ligand binding to the nuclear receptor causes
dissociation of corepressor and recruitment of coactivator proteins. Additional proteins including RNA polymerase are then recruited to the
NR/DNA complex that transcribe DNA into messenger RNA.
- Type II nuclear receptors include principally subfamily 1, for example the retinoic acid receptor, retinoid X receptor and thyroid hormone receptor.

35.  Type III:
- Type III nuclear receptors (principally NR subfamily 2) are similar to type I receptors in that both classes bind to DNA as homodimers. However,
type III nuclear receptors, in contrast to type I, bind to direct repeat instead of inverted repeat HREs.
 Type IV:
- Type IV nuclear receptors bind either as monomers or dimers, but only a single DNA binding domain of the receptor binds to a single half site
HRE. Examples of type IV receptors are found in most of the NR subfamilies.
 Receptor theories:
A- Occupation theory:
- This was given by Gaddum and Clark. This theory states that the intensity of the pharmacological effect is directly proportional to the number of
receptors occupied by the drug or therapeutic agents.
- The interaction between two molecular species i.e. drug (D) and receptor (R) is governed by the laws of mass action, and the effect (E) is a direct
function of the drug-receptor (DR) complex formed:
D + R ⇌ DR → E
- The rate of interaction of drug and receptors can be given as:
𝑟 ∝ 𝐾 [𝑅] [𝐷]
Where,
K1=Association constant
- At equilibrium,
K [R] [D] = K [DR]
Where,
K2= Dissociation constant

36. B- Rate theory:
- This theory was given by Parton and Rang in 1965. This theory states that drug action is the rate at which drug receptor combination occurs. The
pharmacological activity of a drug is the rate of association and dissociation of drug with the receptor and not the number of occupied receptors.
- At equilibrium the rate of association and dissociation of drug-receptor reactions can be written as:
r =
D
1 +
DR
D
C- Induced fit theory:
- It states that when drug approaches receptor and orients its essential binding sites, then conformational changes are induced results in the
initiation of the various biological responses.
 Regulation of receptors:
- Receptors are regulated feedback regulation by their own signalling outputs. Continued stimulation of cells with agonists generally results in a state of
desensitization (also referred to as adaptation, refractoriness, or downregulation) such that the effect of continued or repeated exposure to the same
concentration of drug is diminished. This phenomenon, called tachyphylaxis.
- Desensitization can result from temporary inaccessibility of the receptor to agonist or from fewer receptors being synthesized. Phosphorylation of
GPCRs by specific GRKs plays a key role in triggering rapid desensitization. Phosphorylation of agonist-occupied GPCRs by GRKs facilitates the
binding of cytosolic proteins termed arrestins to the receptor, resulting in the uncoupling of G protein from the receptor.
- The β arrestins recruit proteins, such as PDE4, which limit cAMP signalling, and clathrin and β2 adaptin, which promote sequestration of receptor
from the membrane (internalization), thereby providing a scaffold that permits additional signalling steps.
- Conversely, supersensitivity to agonists also frequently follows chronic reduction of receptor stimulation. As an example, supersensitivity can be
noticeable following withdrawal from prolonged receptor blockade (e.g., the long-term administration of β adrenergic receptor antagonists such as

37. RESPONSE
100%
70%
30%
metoprolol) or in the case where chronic denervation of a preganglionic fibre induces an increase in neurotransmitter release per pulse and to greater
postsynaptic effect, indicating postganglionic neuronal supersensitivity.
DOSE-RESPONSE RELATIONSHIP
i. When a drug is administered systemically, the dose response relationship has two components:
a- Dose-plasma concentration relationship
b- Plasma concentration-response relationship
ii. The former is determined by pharmacokinetic consideration and the latter is considered as dose-response relationship and contribute to dose-response
curve.
iii. The dose-response curve is a curve obtained by the plotting a graph by taking dose in x-axis and response in y-axis. Generally, intensity of response
increases with increase in dose, but at higher dose the intensity becomes progressively less and the curve is rectangular hyperbola.
DOSE

38. iv. The drug-receptor interaction obeys laws of mass action, which is described by following equation:
E =
E × [D]
K + [D]
Where,
E= Observed effect
[D] = Dose of the drug
EMAX = Maximal response
KD = Dissociation constant of the drug-receptor complex
v. If the dose is plotted on a logarithmic scale, the curve becomes sigmoid and a linear relationship between log of dose and the response is seen in the
intermediate (30%-70% response).
vi. A wide range of drug dose can be easily displaced on a graph. Comparison between agonist and study of antagonists are easier.
 Drug potency and efficacy: -
i. Drug potency refers to the amount of drug needed to produce a certain response. The position of DRC on dose axis is the index of drug potency. A DRC
positioned rightward indicate slower potency.
ii. Relative potency is often more meaningful than absolute potency, and is generally defined by comparing the dose(concentration) of the two agonists at
which they elicit half maximal response (EC50). Thus, if 1O mg of morphine = 100 mg of pethidine as analgesic, morphine is 1O times more potent than
pethidine.
iii. Drug potency is clearly a factor in choosing the dose of a drug.
iv. Drug efficacy refers to the maximal response that can be elicited by the drug, e.g. morphine produces a degree of analgesia not obtainable with any dose of
aspirin – morphine is more efficacious than aspirin. The upper limit of DRC is the index of drug efficacy.

39. v. The drug potency and efficacy can vary independently:
a- Aspirin is less potent as well as less efficacious analgesic than morphine.
b- Pethidine is less potent but equally efficacious analgesic as morphine.
c- Furosemide is less potent but more efficacious diuretic than metolazone.
d- Diazepam is more potent but less efficacious CNS depressant than pentobarbitone.
vi. The slope of the DRC is also important. A steep slope indicates that a moderate increase in dose will markedly increase the response, while a flat one
implies that little increase in response will occur over a wide dose range. Hydralazine has a steep, while hydrochlorothiazide has a flat DRC of
antihypertensive effect.

40.  Therapeutic efficacy & Therapeutic index: -
i. The 'therapeutic efficacy' or 'clinical effectiveness is a composite attribute of a drug different from the foregoing pharmacological description of 'potency'
and 'efficacy'. It depends not only on the relative potency and efficacy of the drug but on many pharmacokinetic and pathophysiological variables as well.
ii. It is often expressed in terms of:
a- Degree of benefit/relief afforded by the drug in the recommended dose range i.e. graded dose-response relationship.
b- Success rate in achieving a defined therapeutic end point, i.e. quantal dose-response relationship, e.g. an antibiotic which cures 95% cases of
gonorrhea is a more efficacious anti-gonococcal drug than one which cures only 75% patients.
iii. The therapeutic index (therapeutic ratio) is a quantitative measurement of the relative safety of a drug. It is a comparison of the amount of a therapeutic
agent that causes the therapeutic effect to the amount that causes toxicity.
iv. The terms therapeutic window or safety window refer to a range of doses which optimize between efficacy and toxicity, achieving the greatest therapeutic
benefit without resulting in unacceptable side-effects or toxicity.
v. Therapeutic index refers to the ratio of the dose of drug that causes adverse effects at an incidence not compatible with the targeted indication (e.g. toxic
dose in 50% of subjects, TD50) to the dose that leads to the desired pharmacological effect (e.g. efficacious dose in 50% of subjects, ED50).
vi. In the early days of pharmaceutical toxicology, TI was frequently determined in animals as lethal dose of a drug for 50% of the population (LD50) divided
by the minimum effective dose for 50% of the population (ED50). Today, more sophisticated toxicity endpoints are used.
Therapeutic index =
LD
ED

41. DRUG INTERACTION (DRUG-BODY INTERACTION)
i. A drug interaction is a change in the action or side effects of a drug caused by concomitant administration with a food, beverage, supplement, or another
drug.
ii. A cause of a drug interaction involves one drug which alters the pharmacokinetics of another medical drug. Alternatively, drug interactions result from
competition for a single receptor or signaling pathway
iii. The interactions between a drug and the body are conveniently divided into two classes:
a- The actions of the drug on the body are termed pharmacodynamic processes. These properties determine the group in which the drug is classified, and
they play the major role in deciding whether that group is appropriate therapy for a particular symptom or disease.
b- The actions of the body on the drug are called pharmacokinetic processes. Pharmacokinetic processes govern the absorption, distribution, and
elimination of drugs and are of great practical importance in the choice and administration of a particular drug for a particular patient.
 Pharmacodynamic Principles: -
1. Most drugs must bind to a receptor to bring about an effect. However, at the cellular level, drug binding is only the first in a sequence of steps:
a- Drug (D) + receptor-effector (R) → drug-receptor-effector-complex → effect
b- D + R → drug-receptor complex → effector molecule → effect
c- D + R → D-R complex → activation of coupling molecule →effector molecule → effect
d- Inhibition of metabolism of endogenous activator → increased activator action on an effector molecule → increased effect
2. When two drugs are used together, their effects can be as follows: -

42. Synergism: -
When the action of one drug is facilitated or increased by the other, they are said to be synergistic. In a synergistic pair. both the drugs can have action
in the same direction or given alone one may be inactive but still enhance the action of the other when given together. Synergism can be:
Additive: -
The effect of the two drugs is in the same direction and simply adds up:
Effect of drug A + B = Effect of drug A + Effect of drug B
Side effects of the components of an additive pair may be different do not add up. Thus, the combination is better tolerated than higher dose of one
component.
Additive drug combination
Aspirin + Paracetamol As analgesic / antipyretics
Nitrous oxide + Halothane As general anesthetics
Amlodipine + Atenolol As antihypertensive
Ephedrine + Theophylline As bronchodilator
Supra additive (potentiation): -
The effect of combination is greater than the individual effects or the components:
Effect of drug A + B > Effect of drug A + Effect of drug B
This is always the case when one component given alone produces no effect, but enhances the effect of the other (potentiation). Examples are:
Supra additive drug combination
Drug pair Basis of potentiation
Acetylcholine + Physostigmine Inhibition of breakdown
Levodopa + Carbidopa / Benserazide Inhibition of peripheral metabolism
Adrenaline + Cocaine / Desipramine Inhibition of neuronal uptake
Sulfamethoxazole + Trimethoprim Sequential blocked
Tyramine + MAO inhibitors Increase releasable CA store

43. Antagonism: -
When one drug decreases or abolishes the action of another, they are said to be antagonistic:
Effect of drug A + B < Effect of drug A + Effect of drug B
Depending on the mechanism involved, antagonism may be:
a- Physical antagonism: -
Based on the physical property of the drugs, e.g. charcoal adsorbs alkaloids and can prevent their absorption – used in alkaloidal poisoning .
b- Chemical antagonism: -
The two drugs react chemically and form an inactive product, e.g.
a- KMnO4 oxidizes alkaloids- used for gastric lavage in poisoning.
b- Tannins + alkaloids- insoluble alkaloidal tannate is formed.
c- Chelating agents (BAL, Cal. Disodium edetate) complex toxic metals (As, Pb).
d- Nitrites form methemoglobin which reacts with cyanide radical.
Drugs may react when mixed in the same syringe or infusion bottle:
a- Thiopentone sod. + succinylcholine chloride
b- Penicillin-G sod. + succinylcholine chloride
c- Heparin + penicillin / tetracyclines / streptomycin / hydrocortisone
c- Physiological / functional antagonism: -
The two drugs act on different receptors or by different mechanisms, but have opposite overt effects on the same physiological function, i.e. have
pharmacological effects in opposite direction, e.g.
a- Histamine and adrenaline on bronchial muscles and on BP.
b- Hydrochlorothiazide and amiloride on urinary K+
excretion.

44. c- Glucagon and insulin on blood sugar level.
d- Receptor antagonism: -
One drug (antagonist) blocks the receptor action of the other (agonist). Receptor antagonists are selective, i.e. an anticholinergic will oppose
contraction of intestinal smooth muscle induced by cholinergic agonists, but not that induced by histamine or 5-HT (they act through a different
set of receptors). Receptor antagonism can be competitive or noncompetitive.
Competitive Non competitive
Antagonist bind with the same receptor as the agonist Binds to another sites of the receptor
Antagonists resembles chemically with the agonist Doesn’t resemble
Parallel rightward shift of agonist DRC Flattening of agonist DRC
It can be reversible on increasing the dose of agonist Maximal response is suppressed
Example: Ach – Atropine Example: Diazepam – Bicuculline
3. Both synergy and antagonism can occur during different phases of the interaction between a drug, and an organism. The different responses of a receptor
to the action of a drug have resulted in a number of classifications, such as: -
Pharmacodynamic interaction
Pharmacological
receptor
Homodynamic
Agonist
Full agonist
Partial agonist
Inverse agonist
Antagonist
Competitive
antagonist
Reversible Irreversible
Non - competitive
antagonist
Heterodynamic
Signal transduction
mechanism
Antagonic physiological
system

45.  Pharmacological receptors:
From a pharmacodynamic perspective, two drugs can be considered to be:
Heterodynamic competitors, if they act on distinct receptors & Homodynamic, if they act on the same receptor. Homodynamic can be following types:
 Agonists:
- An agonist is a chemical or drug that binds to a receptor and activates the receptor to produce a biological response. An agent which activates a
receptor to produce an effect similar to that of the physiological signal molecule is known as agonist.
- Agonist can be following types:
A- Full agonists:
- It binds to and activate a receptor with the maximum response that an agonist can elicit at the receptor. One example of a drug that can
act as a full agonist is Isoproterenol, which mimics the action of adrenaline at β adrenoreceptors. Another example is morphine, which
mimics the actions of endorphins at μ-opioid receptors throughout the central nervous system.
- However, a drug can act as a full agonist in some tissues and as a partial agonist in other tissues, depending upon the relative numbers
of receptors and differences in receptor coupling.
B- Co-agonist:
- It works with other co-agonists to produce the desired effect together. NMDA receptor activation requires the binding of both
glutamate, glycine and D-serine co-agonists. Calcium can also act as a co-agonist at the IP3 receptor.
C- Selective agonist:
- It is selective for a specific type of receptor. E.g. buspirone is a selective agonist for serotonin 5-HT-1A.

46. D- Partial agonists:
- It also binds and activate a given receptor, but have only partial efficacy at the receptor relative to a full agonist, even at maximal
receptor occupancy. Example of drugs are: - Buspirone, Aripiprazole, Buprenorphine, Or Nor-Clozapine
- Agents like buprenorphine are used to treat opiate dependence for this reason, as they produce milder effects on the opioid receptor
with lower dependence and abuse potential.
- Partial agonist does not stabilise the receptor configuration as fully as full agonist. Such drugs have low intrinsic efficacy.
E- Inverse agonist:
- It is an agent that binds to the same receptor binding-site as an agonist for that receptor and inhibits the constitutive activity of the
receptor. Inverse agonists exert the opposite pharmacological effect of a receptor agonist, not merely an absence of the agonist effect as
seen with an antagonist.
- An example is the Cannabinoid inverse agonist rimonabant.
 Antagonists:
- A receptor antagonist is a type of receptor ligand or drug that blocks or dampens a biological response by binding to and blocking a receptor
rather than activating it like an agonist.
- They are sometimes called blockers; examples include alpha blockers, beta blockers, and calcium channel blockers.
- In pharmacology, antagonists have affinity but no efficacy for their cognate receptors, and binding will disrupt the interaction and inhibit the
function of an agonist or inverse agonist at receptors.
- Antagonists mediate their effects by binding to the active site or to the allosteric site on a receptor, or they may interact at unique binding sites
not normally involved in the biological regulation of the receptor's activity.

47. - Antagonist activity may be reversible or irreversible depending on the longevity of the antagonist–receptor complex, which, in turn, depends on
the nature of antagonist–receptor binding.
Types of antagonists:
A- Competitive antagonists:
- It binds to receptors at the same binding site (active site) as the endogenous ligand or agonist, but without activating the receptor.
Agonists and antagonists "compete" for the same binding site on the receptor.
- Once bound, an antagonist will block agonist binding. Sufficient concentrations of an antagonist will displace the agonist from the
binding sites, resulting in a lower frequency of receptor activation.
- The level of activity of the receptor will be determined by the relative affinity of each molecule for the site and their relative
concentrations. High concentrations of a competitive agonist will increase the proportion of receptors that the agonist occupies, higher
concentrations of the antagonist will be required to obtain the same degree of binding site occupancy.
- Competitive antagonists are used to prevent the activity of drugs, and to reverse the effects of drugs that have already been consumed.
- Naloxone (also known as Narcan) is used to reverse opioid overdose caused by drugs such as heroin or morphine. Similarly, Ro15-4513
is an antidote to alcohol and flumazenil is an antidote to benzodiazepines.
- Competitive antagonists are sub-classified as reversible (surmountable) or irreversible (insurmountable) competitive antagonists,
depending on how they interact with their receptor protein targets.
- Reversible antagonists, which bind via noncovalent intermolecular forces, will eventually dissociate from the receptor, freeing the
receptor to be bound again.
- Irreversible antagonists bind via covalent intermolecular forces. Because there is not enough free energy to break covalent bonds in the
local environment, the bond is essentially "permanent", meaning the receptor-antagonist complex will never dissociate.

48. - The receptor will thereby remain permanently antagonized until it is ubiquitinated and thus destroyed.
B- Non-competitive antagonists:
- A non-competitive antagonist is a type of insurmountable antagonist that may act in one of two ways: by binding to an allosteric site of
the receptor, or by irreversibly binding to the active site of the receptor.
- Unlike competitive antagonists, which affect the amount of agonist necessary to achieve a maximal response but do not affect the
magnitude of that maximal response, non-competitive antagonists reduce the magnitude of the maximum response that can be attained
by any amount of agonist.
- This property earns them the name "non-competitive" because their effects cannot be negated, no matter how much agonist is present.
- An antagonist that binds to the active site of a receptor is said to be "non-competitive" if the bond between the active site and the
antagonist is irreversible or nearly so.
- The second form of "non-competitive antagonists" act at an allosteric site. These antagonists bind to a distinctly separate binding site
from the agonist, exerting their action to that receptor via the other binding site. They do not compete with agonists for binding at the
active site.
- The bound antagonists may prevent conformational changes in the receptor required for receptor activation after the agonist binds.
Cyclothiazide has been shown to act as a reversible non-competitive antagonist of mGluR1 receptor.
 Signal transduction mechanisms:
1. These are molecular processes that commence after the interaction of the drug with the receptor. For example, it is known that hypoglycemia in an
organism produces a release of catecholamines, which trigger compensation mechanisms thereby increasing blood glucose levels.
2. The release of catecholamines also triggers a series of symptoms, which allows the organism to recognize what is happening and which act as a
stimulant for preventative action (eating sugars).

49. 3. If a patient is taking a drug such as insulin, which reduces glycaemia, and also be taking another drug such as beta-blockers for heart disease, then
the beta-blockers will act to block the adrenaline receptors. This will block the reaction triggered by the catecholamines and produces
hypoglycemic episode.
4. Therefore, the body will not adopt corrective mechanisms and there will be an increased risk of a serious reaction resulting from the ingestion of
both drugs at the same time.
 Antagonic physiological systems:
1. Imagine a drug A that acts on a certain organ. This effect will increase with increasing concentrations of physiological substance S in the organism.
Now imagine a drug B that acts on another organ, which increases the amount of substance S. If both drugs are taken simultaneously it is possible
that drug A could cause an adverse reaction in the organism as its effect will be indirectly increased by the action of drug B.
2. An actual example of this interaction is found in the concomitant use of digoxin and furosemide. The digoxin acts on cardiac fibres and its effect is
increased if there are low levels of potassium (K) in blood plasma. Furosemide is a diuretic that lowers arterial tension but favours the loss of K+.
This could lead to hypokalemia (low levels of potassium in the blood), which could increase the toxicity of digoxin.
 Pharmacokinetic Principles: -
Modifications in the effect of a drug are caused by differences in the absorption, transport, distribution, metabolism or excretion of one or both of the drugs
compared with the expected behavior of each drug when taken individually.
 Absorption interactions: -
A- Changes in motility: -
Some drugs, such as the prokinetic agents increase the speed with which a substance passes through the intestines. If a drug is present in the digestive
tract's absorption zone for less time its blood concentration will decrease. The opposite will occur with drugs that decrease intestinal motility.

50. B- pH:
i. Drugs can be present in either ionized or non-ionized form, depending on their pKa (pH at which the drug reaches equilibrium between its
ionized and non-ionized form).
ii. The non-ionized forms of drugs are usually easier to absorb, because they will not be repelled by the lipidic bilayer of the cell, most of them can
be absorbed by passive diffusion, unless they are too big or too polarized (like glucose or vancomycin), in which case they may have or not have
specific and non-specific transporters distributed on the entire intestine internal surface, that carries drugs inside the body.
iii. Obviously increasing the absorption of a drug will increase its bioavailability, so, changing the drug's state between ionized or not, can be
useful or not for certain drugs.
iv. Certain drugs require an acid stomach pH for absorption. Others require the basic pH of the intestines. Any modification in the pH could
change this absorption.
v. In the case of the antacids, an increase in pH can inhibit the absorption of other drugs such as zalcitabine (absorption can be decreased by
25%), tipranavir (25%) and amprenavir (up to 35%).
C- Drug solubility:
The absorption of some drugs can be drastically reduced if they are administered together with food with a high fat content. This is the case for oral
anticoagulants and avocado.
D- Formation of non-absorbable complexes:
Chelation:
The presence of di- or trivalent cations can cause the chelation of certain drugs, making them harder to absorb. This interaction frequently occurs
between drugs such as tetracycline or the fluoroquinolones and dairy products (due to the presence of Ca2+
).

51. Binding with proteins: -
Some drugs such as sucralfate binds to proteins, especially if they have a high bioavailability. For this reason, its administration is contraindicated in
enteral feeding.
E- Acting on the P-glycoprotein of the enterocytes:
P-glycoprotein 1 (permeability glycoprotein, abbreviated as P-gp) also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-
family B member 1 (ABCB1) or cluster of differentiation 243 (CD243) is an important protein of the cell membrane that pumps many foreign substances
out of cells.
It is an ATP-dependent efflux pump with broad substrate specificity. It exists in animals, fungi, and bacteria, and it likely evolved as a defense
mechanism against harmful substances.
This appears to be one of the mechanisms promoted by the consumption of grapefruit juice in increasing the bioavailability of various drugs, regardless
of its demonstrated inhibitory activity on first pass metabolism.
 Transport and distribution interactions: -
The main interaction mechanism is competition for plasma protein transport. In these cases, the drug that arrives first binds with the plasma protein,
leaving the other drug dissolved in the plasma, which modifies its concentration.
The organism has mechanisms to counteract these situations (by, for example, increasing plasma clearance), which means that they are not usually
clinically relevant.
 Metabolism interactions: -
Many drug interactions are due to alterations in drug metabolism. Further, human drug-metabolizing enzymes are typically activated through the
engagement of nuclear receptors.
One notable system involved in metabolic drug interactions is the enzyme system comprising the cytochrome P450 oxidases.

52. CYP450: -
i. Cytochrome P450 is a very large family of hemoproteins that are characterized by their enzymatic activity and their role in the metabolism of a large
number of drugs.
ii. Of the various families that are present in human beings the most interesting in this respect are the 1, 2 and 3, and the most important enzymes are
CYP 1A2, CYP 2C9, CYP 2C19, CYP 2D6, CYP 2E1 and CYP 3A4.
iii. The majority of the enzymes are also involved in the metabolism of endogenous substances, such as steroids or sex hormones, which is also important
should there be interference with these substances.
iv. As a result of these interactions the function of the enzymes can either be stimulated (enzyme induction) or inhibited (enzyme inhibition).
Enzymatic inhibition: -
If drug A is metabolized by a cytochrome P450 enzyme and drug B inhibits or decreases the enzyme's activity, then drug A will remain with high levels
in the plasma for longer as its inactivation is slower. As a result, enzymatic inhibition will cause an increase in the drug's effect. This can cause a wide
range of adverse reactions.
It is possible that this can occasionally lead to a paradoxical situation, where the enzymatic inhibition causes a decrease in the drug's effect: if the
metabolism of drug A gives rise to product A2, which actually produces the effect of the drug. If the metabolism of drug A is inhibited by drug B the
concentration of A2 that is present in the blood will decrease, as will the final effect of the drug.
Enzymatic induction: -
If drug A is metabolized by a cytochrome P450 enzyme and drug B induces or increases the enzyme's activity, then blood plasma concentrations of
drug A will quickly fall as its inactivation will take place more rapidly. As a result, enzymatic induction will cause a decrease in the drug's effect.
It is possible to find paradoxical situations where an active metabolite causes the drug's effect. In this case, the increase in active metabolite A2
produces an increase in the drug's effect.

53.  Excretion interactions: -
A- Renal excretion: -
i. Only the free fraction of a drug that is dissolved in the blood plasma can be removed through the kidney. Therefore, drugs that are tightly bound to
proteins are not available for renal excretion, as long as they are not metabolized when they may be eliminated as metabolites.
ii. Creatinine clearance is used as a measure of kidney functioning but it is only useful in cases where the drug is excreted in an unaltered form in the
urine.
iii. The excretion of drugs from the kidney's nephrons has the same properties as that of any other organic solute: passive filtration, reabsorption and
active secretion.
iv. In the latter phase, the secretion of drugs is an active process that is subject to conditions relating to the saturability of the transported molecule
and competition between substrates. Therefore, these are key sites where interactions between drugs could occur.
v. Filtration depends on a number of factors including the pH of the urine, it having been shown that the drugs that act as weak bases are increasingly
excreted as the pH of the urine becomes more acidic, and the inverse is true for weak acids. This mechanism is of great use when treating
intoxications (by making the urine more acidic or more alkali) and it is also used by some drugs and herbal products to produce their interactive
effect.
Weak acids Weak bases Weak acids Weak bases
Acetylsalicylic acid Reserpine Theophylline Amiloride
Furosemide Amphetamine Phenytoin Terbutaline
Ibuprofen Procaine Paracetamol
Levodopa Ephedrine
Acetazolamide Atropine
Sulfadiazine Diazepam
Chlorothiazide Hydralazine
Ampicillin Pindolol
Ethacrynic acid Propranolol
Phenobarbital Salbutamol
Warfarin Alprenolol

54. B- Bile excretion: -
Bile excretion is different from kidney excretion as it always involves energy expenditure in active transport across the epithelium of the bile duct
against a concentration gradient. This transport system can also be saturated if the plasma concentrations of the drug are high.
Bile excretion of drugs mainly takes place where their molecular weight is greater than 300 and they contain both polar and lipophilic groups. The
glucuronidation of the drug in the kidney also facilitates bile excretion.
Substances with similar physicochemical properties can block the receptor, which is important in assessing interactions. A drug excreted in the bile
duct can occasionally be reabsorbed by the intestines (in the enterohepatic circuit), which can also lead to interactions with other drugs.