FORCES INVOLVED IN BINDING OF DRUGS TO RECEPTORS.• The driving force for the drug-receptor interaction can be considered as a low energy state of the drug-receptor complex,• Where kon is the rate constant for formation of the drug-receptor complex, which depends on the concentration of the drug and the receptor• koff is the rate constant for breakdown of the complex, which depends on the concentration of the drug-receptor complex as well as other forces.• The biological activity of drug is related to its affinity for the receptor, i.e., the stability of the drug-receptor complex.• This stability is commonly measured by how difficult is for the complex to dissociate, which is measured by its kd, the dissociation constant for the drug-receptor complex at equilibrium.
INTERACTIONS INVOLVED IN THE DRUG-RECEPTOR COMPLEX• Covalent bonding• Ionic interactions• Ion-dipole and dipole-dipole interactions,• Hydrogen bonding• Charge transfer interactions• Hydrophobic interactions, and• Van der waals interactions
Development of Drug-receptor theory• a. Langley(1878): Intercounter of atropine with pilocarpine in salivary excretion.• b. Langley(1906):Intercounter tubocurarine with nicotine in skeletal muscle – “receptive substance”• c. Ehrlich(1908): “lock and key (receptor)”• d. Clark(1926-33): Acetylcholine on heart contraction.• e. Dale, Ahlquist, Gaddum, Schild, Sutherland, et al.
• Receptor theory was propounded by Alfred Joseph Clark, a theory of drug action based on occupation of receptors by specific drugs and the cellular function can be altered by interaction of the receptors with the drugs.• The interaction between the drug (D) and receptor (R) is governed by the Law of action; the rate at which new DR complexes are formed is proportional to the concentration of D.• This equation is derived from Langmuir absorption isotherm, the interaction of drug (D) with receptor (R) on forward or association rate constant (k1) and the reverse or dissociation (k2).• It has been accepted that occupation of the receptor is essential but itself not sufficient to elicit a response; the agonist must be able to induce conformational change in the receptor.
THEORIES OF DRUG RECEPTOR INTERACTIONS1. OCCUPATION THEORY:2. RATE THEORY3. THE INDUCED-FIT THEORY OF ENZYME-SUBSTRATE INTERACTION4. MACROMOLECULAR PERTURBAION THEORY5. ACTIVATION-AGGREGATION THEORY6. TWO STATE MODEL OF RECEPTOR ACTIVATIONOther theories The receptor cooperativity model The mobile receptor Model
Occupation theory (1926) Drugs act on independent binding sites and activate them, resulting in a biological response that is proportional to the amount of drug-receptor complex formed. The response ceases when this complex dissociates. Intensity of pharmacological effect is directly proportional to number of receptors occupied D + R ↔ DR ⇒ RESPONSE Response is proportional to the fraction of occupied receptors Maximal response occurs when all the receptors are occupie d Does not rationalize how two drugs can occupy the same receptor and act differently
Rate theory (1961)• The response is proportional to the rate of drug-Receptor complex formation.• Activation of receptors is proportional to the total number of encounters of a drug with its receptor per unit time.• According to this view, the duration of Receptor occupation determines whether a molecule is agonist, partial agonist of antagonist.• Does not rationalize why different types of compounds exhibit the characteristics they do.
THE INDUCED-FIT THEORY: (1958)• States that the morphology of the binding site is not necessarily complementary with even the preferred conformation of the ligand.• According to this theory, binding produces a mutual plastic molding of both the ligandand the receptor as a dynamic process.• The conformational change produced by the mutually induced fit in the receptor macromolecule is then translated into the biological effect, eliminating the rigid and obsolete “ key and lock” concept of earlier times• Agonist induces conformational change – response• Antagonist does not induce conformational change – no response• Partial agonist induces partial conformational change - partial response
Macromolecular perturbation theory:• Suggests that when a drug-receptor interaction occurs, one of two general types of Macromolecular perturbation is possible:• a specific conformational perturbationleads to a biological response (agonist),• whereas a non specific conformational perturbation leads to no biologic response (Antagonist
Ariensresponse is proportional to the fraction of occupied receptors and the intrinsic activity Stephenson response is a FUNCTION of occupancy maximum response can be produced WITHOUT 100% occupation, i.e. tissues have spare receptors Receptors are said to be sparespare for a given pharmacological response when the maximal response can be elicited by an agonist at a concentration that does not result in occupancy of the full complement of available receptorsSpare receptors More receptors available than needed to elicit maximum response allow maximal response without total receptor occupancy – increase sensitivity of the system Agonist has to bind only a portion of receptors for full effect
Activation-Aggregation Theory Monad, Wyman, Changeux (1965) Karlin (1967)is an extension of the Macromolecular perturbation theorySuggests that a drug receptor (in the absence of a drug) still exists in an equilibrium between an activated state (Bioactive) and an inactivated state (Bio-inactive); agonists bind to the activated state and antagonist to the inactivated state
Activation-Aggregation TheoryReceptor is always in a state of dynamicequilibrium between activated form (Ro)and inactive form (To).
THE TWO-STATE (MULTISTATE) RECEPTOR MODEL• Was developed on the basis of the kinetics of competitive and allostericinhibition as well as through interpretation of the results of direct binding experiments.• It postulates that a receptor, regardless of the presence or absence of a ligand,exists in two distinct states: the R(relaxed, active or on) and T(Tense, inactive or off) states, which are in equilibrium with each other.Molecular level conceptual model of Receptor• These models emphasize the fact that many receptors are not just simple macromolecules, which interact with a drug in “hand in glove” fashion.• On the contrary, some receptors are extremely dynamic, existing as a family of low-energy conformers existing in equilibrium with each other.• Other receptors have complex multi-unit structures, being composed of more than one protein; facilitatoryand inhibitory interactions exist between these subunits and may alter the drug-receptor interaction.• Some receptors are not only dynamic in terms of their shape, but also mobile, drifting in the membrane like an iceberg in the ocean.
Two-state (Multi-state) Receptor Model• R and R* are in equilibrium (equilibrium constant L), which defines the basal activity of the receptor.• Full agonists bind only to R*• Partial agonists bind preferentially to R*• Full inverse agonists bind only to R• Partial inverse agonists bind preferentially to R• Antagonists have equal affinities for both R and R* (no effect on• basal activity)• In the multi-state model there is more than one R state to account for variable agonist and inverse agonist behavior for the same
• An agonist (Drug, D) has a high affinity for the R state and will shift the equilibrium to the right• An antagonist (Inhibitor, I) will prefer the T state and will stabilize the TI complex.• Partial agonists have about equal affinity for both forms of the receptor.• In contrast to the classical occupation theory the agonist in the two-state model does not activate the receptor but shifts the equilibrium toward the Rform.
Terminologies regarding drug receptor interaction Affinity Efficacy Potency Ligand
Affinity: measure of propensity of a drug to bind receptor; the attractiveness of drug and receptorEfficacy: Potential maximum therapeutic response that a drug can produce.Potency: Amount of drug needed to produce an effect.Ligand: Molecules that binds to a receptor
Classification of Ligands a. agonist b. partial agonistc. antagonist pharmacological vs. physiological vs. che mical pharmacological antagonists - competitive surmountable - noncompetitive
Drug ↔ Receptor interaction - Primary way for drug to produce an action Targets of drug action non-specific receptors neurotransmitters hormones enzymes transport systems • ion channels • active transporters, e.g. uptake blockers
DESENSITIZATION OF RECEPTORS - Receptor structure change - Receptor inactivation (protein inhibitors, modifications) - Down regulation of receptor endocytosis or degradatio
Receptor “agonist” Any drug that binds to a receptor and stimulates the functional activities e.g.: adrenaline (epinephrine) Receptor Effect Epinephrine Cell
AgonistDrugs that cause a responseDrugs that interact with and activate receptors;They possess both affinity and efficacyTypesFull agonists An agonist with maximal efficacy (response) has affinity plus intrinsic activityPartial agonists An agonist with less then maximal efficacy has affinity and less intrinsic activity
Agonists differing in potency and maximum efficacy
PARTIAL AGONISTS - EFFICACY Even though drugs may occupy the same # of receptors, the magnitude of their effects may differ. 1.0 Full Agonist% Maximal Effect 0.8 Partial agonist 0.6 Partial agonist 0.4 0.2 0.0 0.01 0.10 1.00 10.00 100.00 1000.00 [D] (concentration units)
Receptor antagonist Any drug which can influence a receptor and produce no response e.g.: propranolol (a beta blocker) propranolol epinephrine Competitive Antagonist: both the drug and its antagonist compete for the same site of the receptor Non-competitive Antagonist: the drug and its antagonist do not compete for the same site
Antagonist Interact with the receptor Have affinity but NO efficacy Block the action of other drugs Effect only observed in presence of agonist
Types of AntagonistsCompetitive Noncompetitive- Decrease(Surmountable) apparent maximum efficacydecrease apparent Potency
Competitive Antagonistcompetes with agonist for receptor with increasing agonistsurmountable concentrationdisplaces agonist dose response curve to the right (dextral shift)Only affinity, no efficacy
Noncompetitive Antagonistdrug binds to receptor and stays boundirreversible – does not let go of receptorproduces slight dextral shift in the agonist DR curve in the low concentration rangebut, as more and more receptors are bound (and essentially destroyed),the agonist drug becomes incapable of eliciting a maximal effect