Pharmacodynamics and Pharmacokinetics
Start with one fact that the rest of this chapter unpacks: the body never responds to the dose written on the chart. It responds to the concentration of drug that actually reaches the target, for as long as it stays there. A 500 mg tablet swallowed by a woman who is vomiting, the same 500 mg given intravenously in septic shock, and 500 mg in a woman with acute kidney injury are three completely different pharmacological events, even though the prescription looks identical. Everything that follows is an attempt to predict, from first principles, what concentration-over-time a given dose will actually produce, and what that concentration will then do.
Pharmacology answers that with two complementary questions. Pharmacokinetics (PK) asks what the body does to the drug — how it is absorbed, distributed, metabolised and eliminated, which together determine the concentration-time profile. Pharmacodynamics (PD) asks what the drug does to the body — how that concentration, once present, produces an effect at a target. A memorable shorthand: PK is the journey, PD is the destination.
Every O&G prescription sits where those two questions cross. Oxytocin contracts the uterus only if its receptors are present, its signalling is intact (PD) and enough drug reaches the myometrium (PK). Magnesium sulphate prevents eclamptic seizures only if the concentration is therapeutic (PK) and the neuromuscular effect is achieved without toxicity (PD), which in turn depends on renal clearance. Antibiotics save a septic patient only if exposure relative to the organism is adequate (PD) and the drug actually reaches the tissue fast enough (PK). The clinical reasoning chain the Primary candidate must internalise is:
dose -> concentration at the site of action over time -> receptor or target effect -> tissue response -> benefit or toxicity
O&G then layers specific modifiers onto that chain: pregnancy physiology, the placenta, the fetus, breastfeeding, labour, haemorrhage, sepsis, renal and liver function, antiretroviral and TB therapy, anaesthesia, and the time pressure of an emergency. The aim of this chapter is not to memorise drugs but to be able to take any unfamiliar drug and reason through where, in pregnancy, the prediction will break.
We begin with pharmacodynamics, because the target is the simplest place to anchor: it answers "what is this drug trying to do?" before we worry about how it gets there.
Pharmacodynamics: Drug Effect at the Target
Drug effect begins with a target. A drug molecule binds something — a protein, a channel, a strand of DNA, an enzyme's active site — and that binding changes the target's behaviour. Most O&G drugs act at receptors, ion channels, enzymes, transporters, microbial targets, DNA/RNA machinery or immune pathways.
| Target | First-principle effect | O&G example |
|---|---|---|
| Receptor agonism | Activates a receptor and mimics a signal | Oxytocin at the oxytocin receptor |
| Receptor antagonism | Blocks endogenous ligand effect | Progesterone-receptor antagonism in selected reproductive contexts |
| Ion-channel effect | Alters electrical activity, contraction or conduction | Local anaesthetic sodium-channel block; nifedipine calcium-channel block |
| Enzyme inhibition | Blocks a biochemical reaction | NSAID inhibition of cyclo-oxygenase; aromatase inhibition |
| Transporter effect | Changes movement of molecules across membranes | Renal drug secretion interactions; monoamine transporters |
| Microbial target | Exploits microbial-human differences | Beta-lactams inhibit bacterial cell-wall synthesis |
| DNA/RNA or mitotic machinery | Alters replication or division | Chemotherapy and antiviral principles |
| Immune modulation | Suppresses or redirects inflammation | Corticosteroids, biologics in autoimmune disease |
Two binding ideas underpin all of this. Affinity is how tightly a drug binds its target; selectivity is how much it prefers that target over others. A drug with high affinity occupies its target at low concentrations; a drug with poor selectivity occupies several targets and produces "off-target" effects — for example, a beta-agonist intended to relax myometrium also stimulating maternal cardiac beta-receptors and causing tachycardia. Binding is usually reversible: the drug occupies, signals, then dissociates, which is why a steady concentration must be maintained for a steady effect.
Do not memorise a drug name alone. In an exam answer, name the target and then explain the tissue effect. "Oxytocin is a uterotonic" is weaker than: oxytocin receptor activation increases intracellular calcium in myometrium, causing contraction and compression of uterine vessels.
Agonists, Antagonists, Potency and Efficacy
Once a drug occupies a receptor it either switches it on or it does not — and that single distinction generates the whole vocabulary of agonism and antagonism. An agonist binds and activates a receptor, mimicking the natural signal. An antagonist binds without activating and, by occupying the site, blocks the endogenous ligand. A partial agonist activates but produces a smaller maximal response than a full agonist even when every receptor is occupied — it has high affinity but submaximal efficacy. An inverse agonist reduces a receptor's baseline (constitutive) activity, a less common but real idea.
Antagonism itself comes in two mechanistic flavours, and the difference is testable. A competitive (reversible) antagonist competes for the same binding site; its block can be overcome by raising agonist concentration, so on a dose-response curve it shifts the curve to the right without lowering the maximum. A non-competitive (or irreversible) antagonist either binds irreversibly or acts at a different site; the block cannot be fully overcome by more agonist, so the maximum response falls. Recognising which is which explains why some drug effects can be "out-competed" by giving more of the natural agonist and others cannot.
| Term | Meaning | Clinical trap |
|---|---|---|
| Potency | Dose or concentration needed to produce an effect | More potent means less drug is needed, not necessarily a better maximum effect |
| Efficacy | Maximum effect a drug can produce | A less potent drug can be more efficacious |
| EC50 | Concentration producing 50% of maximal effect | Describes potency in a system |
| Ceiling effect | Higher doses do not increase effect further | Toxicity may still increase despite no extra benefit |
| Therapeutic index | Toxic dose relative to effective dose | Narrow index drugs require monitoring |
A dose-response curve teaches the distinction. A left shift means greater potency. A higher plateau means greater efficacy. A steeper curve means small dose changes produce larger effect changes, which can be clinically dangerous.
There are two kinds of dose-response relationship, and exams exploit the confusion between them. A graded curve plots the size of effect in one individual against concentration (more drug, bigger response, up to a ceiling); this is where EC50, potency and efficacy live. A quantal curve plots the proportion of a population showing an all-or-nothing response (seized / did not seize; converted / did not convert) against dose. From the quantal curve come the population landmarks: the ED50 (median effective dose — the dose producing the desired effect in 50% of subjects), the TD50 (median toxic dose), and in preclinical work the LD50 (median lethal dose). These are the numbers that define how safe a drug is, which leads directly to the therapeutic index discussed later in this chapter.