Mechanisms of Drug Action Mechanisms of Drug Action

Dr. Robert G. Lamb Direct Effect: antacid (base) neutralizes Professor excess acid in stomach Pharmacology & Toxicology Indirect Effect: drug interacts with cell receptor and initiates a sequence of cellular events.

Procaine’s Mechanism of Action Drug Action Sites

Drug Action (mechanism) Drugs influence normal cellular processes. Drug Effect (therapeutic effect) Drugs do not produce new cell functions.

Procaine ⇒ Action Site ⇒ Mechanism ⇒ Effect

Local Sodium Blocks Reduced Anesthetic Channels Nerve Cell Pain Nerve Cells Conductance

Extracellular Sites of Drug Action Cellular Sites of Drug Action

Stomach: neutralize acid with base (antacids) Antibiotics inhibit bacterial but not host functions. Blood: bind metals (chelation) like lead with EDTA Penicillin inhibits cell wall formation. GI Tract: bind drugs (adsorption) with Cholestyramine. Tetracycline inhibits protein synthesis. GI Tract: increase water by osmotic effects (laxatives)

Kidney: increase water elimination (diuretics) Erythromycin inhibits protein synthesis.

1 Signal Transduction I

Cellular Sites of Drug Action Lipid-soluble drug cross membrane and Drug interacts with receptor. Outside Hormones, Steroids, Vitamins and Neurotransmitters Membrane Receptor may be Inside enzyme(activated) or alter cell functions by interacting with cellular receptors. cell gene regulator.

Receptor Gene regulator enters nucleus and increases Four specific examples of receptor-mediated protein synthesis transmembrane signaling processes. Nucleus Results in increased enzyme activity.

Signal Transduction II Signal Transduction III

Ca Na Drug binds to extracellular Drugs bind to sodium and domain of transmembrane calcium channels allowing the protein and activates influx of sodium and calcium. intracellular proteins such as Tyrosine Kinase (TK). Increase in cellular sodium and TK calcium alters cell functions. Activated TK alters enzyme Y Y ~ P activity as a result of protein phosphorylation. Ca Na

Signal Transduction IV Agonist cAMP second messenger pathway

Rec Gs AC

Drugs bind to receptor linked to ATP cAMP 5'-AMP effector enzymes (AC, GC, PLC) AC by a G protein. PDE R PLC R • cAMP G GC 2 Activated effector enzyme R2C2 generates second messengers 2C* (cAMP, cGMP, IP3 and DG) ATP ADP X Y that alter cell functions. S S~P

Pi P'ase

Response

2 Agonist Calcium/Phosphoinositide pathway

PIP Examples of Cell Receptors R G PLC 2 DAG Cellular Enzymes (altered activity) IP 3 PK-C

ER ATP ADP Transmembrane Signaling Processes Ca 2+ CaM S S-P

Pi Cellular Macromolecules (DNA, RNA, etc.)

E CaM-E*

Response

Drug Receptor Complex Drug-Dependent Dose Response

D + R DR RECEPTOR THEORY Affinity Efficacy MASS ACTION LAW Possible k1 k R + D DR 3 effect H-bond site k2 25% Anionic group Possible Cavity Cavity electron Flat region donor group 50% CH3 CH3 -OH + + 75% N CH2 CH2 O C VDW CH 100% CH3 3 5Å ACETYLCHOLINE

Terminology of Antagonists Michaelis-Menten Dose Response Curves 100 A AGONIST

Emax 50 Competitive: competes with agonist for receptor, Ic effect is reduced by increasing dose of agonist. EFFECT KD = EC % OF MAXIMAL 50 0 DOSE Non-Competitive: reduces number of active receptors, Inc effect is not reduced by increasing dose 100 100 A AGONIST of agonist. A AGONIST E +IC max Reversible Antagonist: I and R have weak chemical bonds. 50 50 EFFECT EFFECT K +INC K OF MAXIMAL % D Irreversible Antagonist: I and R have strong chemical bonds. OF MAXIMAL % D 0 0 DOSE DOSE Competitive Inhibitor Non-competitive Inhibitor

3 Log Dose Response Curves C AGONIST 100 B AGONIST

KD / MAX Emax EFFECT 1/EFFECT 50 -1/KD 1 / MAX EFFECT EFFECT

KD 1/DOSE % OF MAXIMAL 0 LOG DOSE Competitive Inhibitor Non-competitive Inhibitor

+I +I C NC 100 B 100 B AGONIST AGONIST AGONIST AGONIST Emax 1/E 1/E 50 +IC 50 +INC 1/EMAX EFFECT 1/EMAX EFFECT -1/KD -1/KD KD KD 1/DOSE 1/DOSE % OF MAXIMAL % OF MAXIMAL 0 0 Lineweaver-Burke Dose Response Curves LOG DOSE LOG DOSE

Summary of Antagonist Effects Occupancy Theory of Drug Action

Competitive: same Emax (efficacy) K1 K3 D (Drug) + R (Receptor) ↔ DR → Response higher KD (lower affinity and potency) K2 2. Reversible drug-receptor interaction

Non-Competitive: same KD (affinity and potency) 3. Response is proportional to number of receptors occupied.

lower Emax (efficacy) 4. Maximum response when all receptors are occupied.

4. Agonist (high Kl, K2 and K3)

5. Antagonist (high Kl, little or no K2 and K3).

Effect of Spare Receptors on Dose Response -I +45 I +90 I Modifications of Occupancy Theory

Drug concentration that produces 50% of maximal response A B C +94 I (EC50) is not equal to KD (saturation of 50% of receptors).

EC50 is not equal to KD when tissues have spare receptors. D Agonist effectAgonist Heart tissue has spare receptors (90%) which means that only 10 +97 I of 100 receptors have to be occupied to obtain maximal response. E Under these conditions EC50 is equal to 5 (50% of 10 receptors) and

KD is equal to 50 (50% of 100 receptors). EC50 (A) EC50 (B) EC50 (C) EC50 (D,E) = KD

Agonist concentration (C) (log scale)

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