1) Pharmacodynamics describes how drugs act on the body by interacting with receptors to produce effects. Most drugs bind to receptors to initiate signaling cascades through second messengers. 2) There are four major receptor families - ligand-gated ion channels which open to conduct ions, G protein-coupled receptors which activate G proteins and second messengers, enzyme-linked receptors with enzymatic activity, and intracellular receptors which bind ligands inside cells. 3) Signal transduction amplifies small signals and protects cells from overstimulation through mechanisms like desensitization, downregulation, and upregulation of receptors over time. The magnitude of drug effects depends on its dose, concentration at receptor sites, and the properties of

1. PHARMACODYNAMICS
PRESENTED BY Dr. MAHEEN SALEEM

2. PHARMACODYNAMICS
• Pharmacodynamics describes the actions of a drug on the body.
• The influence of drug concentrations on the magnitude of the response.
• Most drugs exert their effects, both beneficial and harmful, by interacting with
receptors.

3. SIGNAL TRANSDUCTION.
• Drugs act as signals, and their receptors act as signal detectors.
• Receptors transduce their recognition of a bound agonist by initiating a series of
reactions that ultimately result in a specific intracellular response.
• “Second messenger” or effector molecules are part of the cascade of events that
translates agonist binding into a cellular response.

4. The drug–receptor complex
• Cells have many different types of receptors, each of which is specific for a
particular ligand and produces a unique response.
• The magnitude of the response is proportional to the number of drug– receptor
complexes.
• Most receptors are named for the type of agonist that interacts best with it. For
example, the receptor for histamine is called a histamine receptor.
• It is important to know that not all drugs exert their effects by interacting with a
receptor, for example; Antacids,

5. RECEPTOR STATE
• Receptors exist in at least two states, inactive (R) and active (R*), that are in
reversible equilibrium with one another.
• Binding of agonists causes the equilibrium to shift from R to R* to produce a
biologic effect.
• Antagonists occupy the receptor but do not increase the fraction of R*.
• Agonists, antagonists, and partial agonists are examples of ligands, or molecules
that bind to the activation site on the receptor.

6. MAJOR RECEPTOR FAMILIES
• 1) ligand-gated ion channels
• 2) G protein– coupled receptors
• 3) enzyme-linked receptors
• 4) intracellular receptors

7. https://slideplayer.com/slide/14109197/

8. 1. Transmembrane ligand-gated ion channels:
• The extracellular portion of ligand-gated ion channels usually contains the ligand
binding site.
• Allows ions to flow across cell membranes.
• The channel is usually closed until the receptor is activated by an agonist.
• which opens the channel briefly for a few milliseconds. Depending on the ion
conducted through these channels.

9. https://www.news-medical.net/health/Importance-of-Ion-Channels-

10. 2. Transmembrane G protein–coupled receptors:
• The extracellular domain of this receptor contains the ligand-binding area, and the
intracellular domain interacts (when activated) with a G protein or effector
molecule.
• There are many kinds of G proteins (for example, Gs, Gi, and Gq), but they all are
composed of three protein subunits.
• The α subunit binds guanosine triphosphate (GTP), and the β and γ subunits
anchor the G protein in the cell membrane.

11. CONTI…
• Binding of an agonist to the receptor increases GTP binding to the α subunit,
causing dissociation of the α-GTP complex from the βγ complex.
• These two complexes can then interact with other cellular effectors, usually an
enzyme, a protein, or an ion channel,
• The activated effectors produce second messengers that further activate other
effectors in the cell, causing a signal cascade effect.

12. https://www.nature.com/scitable/topicpage/gpcr-14047471

13. 3. Enzyme-linked receptors:
• This family of receptors consists of a protein that may form dimers.
• When activated, these receptors undergo conformational changes resulting in
increased cytosolic enzyme activity.
• The most common enzyme linked receptors (epidermal growth factor, platelet-
derived growth factor, atrial natriuretic peptide, insulin and others).

14. CONTI…
• possess tyrosine kinase activity as part of their structure.
• The activated receptor phosphorylates tyrosine residues on itself .
• In turn, the phosphorylated receptor phosphorylates other peptides or proteins
that subsequently activate other important cellular signals.

15. https://open.lib.umn.edu/pharmacology/chapter/enzyme-linked-receptors/

16. 4. Intracellular receptors:
• The fourth family of receptors differs considerably from the other three in that the
receptor is entirely intracellular, and, therefore, the ligand must diffuse into the cell
to interact with the receptor.
• In order to move across the target cell membrane, the ligand must have sufficient
lipid solubility.
• The primary targets of these ligand– receptor complexes are transcription factors
in the cell nucleus.

17. CONTI…
• Binding of the ligand with its receptor generally activates the receptor via
dissociation from a variety of binding proteins.
• The activated ligand–receptor complex then translocates to the nucleus, where it
often dimerizes before binding to transcription factors that regulate gene
expression.
• The activation or inactivation of these factors causes the transcription of DNA into
RNA and translation of RNA into an array of proteins.

18. Some characteristics of signal transduction
• Signal transduction has two important features:
• 1) the ability to amplify small signals
• 2) mechanisms to protect the cell from excessive stimulation.

19. Signal amplification:
• A characteristic of G protein–linked and enzyme-linked receptors is their ability to
amplify signal intensity and duration.
• For example, a single agonist–receptor complex can interact with many G
proteins, thereby multiplying the original signal manyfold.
• activated G proteins persist for a longer duration than does the original agonist–
receptor

20. 2. Desensitization and down-regulation of receptors:
• When a receptor is exposed to repeated administration of an agonist, the receptor
becomes desensitized resulting in a diminished effect. This phenomenon, called
tachyphylaxis.
• In addition, receptors may be down-regulated such that they are internalized and
sequestered within the cell, unavailable for further agonist interaction.
• Similarly, repeated exposure of a receptor to an antagonist may result in up-
regulation of receptors.

21. CONTI…
• Up-regulation of receptors can make the cells more sensitive to agonists and/or
more resistant to the effect of the antagonist.
• These receptors may be recycled to the cell surface, restoring sensitivity, or,
alternatively, may be further processed and degraded, decreasing the total
number of receptors available.
• During this recovery phase, unresponsive receptors are said to be “refractory.”

22. DOSE-RESPONSE RELATIONSHIP
• The magnitude of the drug effect depends on the drug concentration at the
receptor site.
• which, in turn, is determined by both the dose of drug administered and by the
drug’s pharmacokinetic profile

23. Graded dose–response relations
• As the concentration of a drug increases, its pharmacologic effect also gradually
increases until all the receptors are occupied (the maximum effect).

24. POTENCY
• Potency is a measure of the amount of drug necessary to produce an effect of a
given magnitude.

25. EFFICACY
• Efficacy is the magnitude of response a drug causes when it interacts with a
receptor. Efficacy is dependent on the number of drug–receptor complexes
formed and the intrinsic activity of the drug.
• Efficacy is a more clinically useful characteristic than is drug potency

26. INTRINSIC ACTIVITY
• An agonist binds to a receptor and produces a biologic response based on the
concentration of the agonist and the fraction of activated receptors.

27. Full agonists
• If a drug binds to a receptor and produces a maximal biologic response that
mimics the response to the endogenous ligand, it is a full agonist.
Partial agonists
• Partial agonists have intrinsic activities greater than zero but less than one. Even
if all the receptors are occupied, partial agonists cannot produce the same Emax
as a full agonis.

28. Inverse agonists
• Inverse agonists, unlike full agonists, stabilize the inactive R form and cause R* to
convert to R. This decreases the number of activated receptors to below that
observed in the absence of drug.
• Thus, inverse agonists have an intrinsic activity less than zero, reverse the
activity of receptors, and exert the opposite pharmacological effect of agonists.

29. ANTAGONIST
• Antagonists bind to a receptor with high affinity but possess zero intrinsic activity.
• An antagonist has no effect in the absence of an agonist but can decrease the
effect of an agonist when present.
• Antagonism may occur either by blocking the drug’s ability to bind to the receptor
or by blocking its ability to activate the receptor.

30. 1. Competitive antagonists:
• If both the antagonist and the agonist bind to the same site on the receptor in a
reversible manner, they are said to be “competitive.”
• The competitive antagonist prevents an agonist from binding to its receptor and
maintains the receptor in its inactive state.

31. 2. Irreversible antagonists:
• Irreversible antagonists bind covalently to the active site of the receptor, thereby
reducing the number of receptors available to the agonist.

32. 3. Allosteric antagonists:
• This type of antagonist binds to a site (“allosteric site”) other than the agonist-
binding site and prevents the receptor from being activated by the agonist.

33. 4. Functional antagonism:
• An antagonist may act at a completely separate receptor, initiating effects that are
functionally opposite those of the agonist.

34. REFERENCES:
Whalen K. Feild C. & Radhakrishnan R. (2019). Lippincott illustrated reviews :
pharmacology (Seventh). Wolters Kluwer.