5.1 Pharmacodynamics and Dose-Response

Drug-Receptor Interactions

Most drugs produce effects by binding to specific receptors — cellular macromolecules (usually proteins) that recognize endogenous ligands and translate binding into a biological response. The classic lock-and-key model treats the receptor as a rigid pocket with a fixed shape complementary to the ligand. The more dynamic induced-fit model recognizes that both receptor and ligand undergo conformational changes on binding, which explains how a single receptor can accommodate structurally related ligands and how partial agonists stabilize distinct active states.

Two quantitative properties describe receptor interactions. Affinity is the strength of binding, conventionally expressed as the dissociation constant Kd — the concentration at which 50% of receptors are occupied. Lower Kd means higher affinity. Intrinsic activity (efficacy) is the ability of the bound drug to activate the receptor and generate a response. A drug can bind tightly (high affinity) yet produce no effect (zero intrinsic activity, e.g., a pure antagonist).

Receptor Classifications

Pharmacology classifies receptors by structure and signal transduction mechanism:

Nuclear receptors act as ligand-activated transcription factors; their delayed onset explains why levothyroxine and corticosteroids take days to weeks to produce clinical effects.

Agonists and Antagonists

A full agonist produces the maximal response the tissue can give (Emax = 1). A partial agonist has intrinsic activity between 0 and 1 — it cannot reach Emax even when all receptors are occupied, and in the presence of a full agonist it behaves as a competitive antagonist, lowering the overall response. Clinical examples include pindolol at beta receptors and buprenorphine at mu opioid receptors. An inverse agonist produces an effect opposite to that of an agonist by stabilizing the inactive conformation of a receptor that exhibits constitutive activity (spontaneous signaling without ligand); examples include naloxone at mu receptors and mirtazapine at H1/5-HT2A receptors.

Antagonists are classified by mechanism:

• Competitive (surmountable) — bind reversibly to the same site as the agonist; increasing agonist concentration overcomes the block. The dose-response curve shifts rightward in parallel with no change in Emax.
• Noncompetitive (insurmountable) — bind irreversibly or to an allosteric site; the curve shows reduced Emax that cannot be overcome by more agonist.
• Physiological — two drugs produce opposing effects through different receptors (e.g., insulin vs. glucagon on blood glucose).
• Chemical — one drug binds another directly in solution (e.g., protamine neutralizing heparin, chelators binding heavy metals).

Dose-Response Relationships

The graded dose-response curve (individual subject, log dose on x-axis, effect on y-axis) is a sigmoid curve; it describes potency and efficacy for a single patient. The quantal dose-response curve (population) describes the cumulative proportion of subjects achieving a predefined response at each dose and is used to derive ED50, TD50, and LD50:

• ED50 — dose effective in 50% of the population
• TD50 — dose toxic in 50% of the population
• LD50 — dose lethal in 50% of the population

Potency vs Efficacy

A highly potent drug is not necessarily more clinically effective — efficacy is what determines whether the therapeutic goal is achievable. Potency mainly affects the dose required.

Therapeutic Index and Safety

Therapeutic index (TI) = TD50 / ED50. A larger TI indicates a wider safety margin. Drugs such as penicillin have a very large TI, whereas digoxin, lithium, warfarin, and chemotherapeutics have a narrow TI and require monitoring. The therapeutic window is the range between the minimum effective concentration and the minimum toxic concentration — the practical clinical correlate of TI used in therapeutic drug monitoring.

Spare receptors exist when a maximal response is achieved while only a fraction of receptors are occupied; this signal amplification explains why high-affinity agonists can produce full effects at low receptor occupancy, particularly at GPCRs.

Tachyphylaxis, Tolerance, and Desensitization

Tachyphylaxis is an acute, rapid loss of response after repeated dosing over minutes to hours (e.g., ephedrine, indirect sympathomimetics that deplete norepinephrine stores). Tolerance is a slower loss of response over days to weeks, often involving pharmacokinetic induction (e.g., CYP induction by carbamazepine) or homeostatic receptor downregulation (e.g., opioids). Desensitization is receptor-level refractoriness — rapid phosphorylation and internalization of GPCRs (e.g., beta-agonists in asthma) — and is often reversible within minutes after the agonist is withdrawn.