Drug classes and mechanisms: antibiotics (MOA), antihypertensives, psychotropics

Intuition Beginner

Every drug works by interacting with specific body molecules — usually proteins such as receptors, enzymes, ion channels, and transporters. Antibiotics exploit differences between bacteria and humans: penicillin blocks cell-wall synthesis (which humans lack), tetracyclines jam the bacterial ribosome (which differs from ours). Antihypertensives lower blood pressure by different routes — ACE inhibitors block angiotensin, beta-blockers slow the heart, diuretics remove salt and water. Psychotropics alter brain chemistry — antidepressants boost serotonin, antipsychotics block dopamine, anxiolytics enhance GABA. Knowing mechanisms lets physicians pick the right drug, predict side effects, and avoid interactions. It also explains addiction (reward-circuit activation; see 35.05.03) and antibiotic resistance (selection; see 35.02.02). Ehrlich called such compounds "magic bullets."

Visual Beginner

Drug class	Target family	Example drug	What it does
Antibiotic (cell wall)	Bacterial enzyme (PBP)	Penicillin	Blocks wall cross-linking
Antibiotic (ribosome)	30S/50S subunit	Tetracycline	Stops protein synthesis
ACE inhibitor	Enzyme (ACE)	Lisinopril	Blocks angiotensin II
Beta-blocker	Receptor (beta-1)	Metoprolol	Slows heart rate
SSRI	Transporter (SERT)	Fluoxetine	Raises synaptic serotonin
Antipsychotic	Receptor (D2)	Haloperidol	Blocks dopamine

The table and diagram share one organising idea: a drug is classified by the molecule it binds. Two drugs in the same class share a target and therefore share side effects; two drugs with different targets can treat the same disease through different routes.

Worked example Beginner

Worked example: selective toxicity — why penicillin hurts bacteria but not you

Penicillin binds penicillin-binding proteins, the enzymes that cross-link peptidoglycan in the bacterial cell wall. When cross-linking fails, osmotic pressure ruptures the bacterium. Human cells have no cell wall and no peptidoglycan, so penicillin's target simply does not exist in the patient. This is selective toxicity — damage the pathogen, spare the host — and it is the property that makes an antibiotic an antibiotic rather than a poison. Tetracyclines hit the 30S ribosomal subunit, which differs enough from the human 80S ribosome that binding is selective; fluoroquinolones hit bacterial DNA gyrase, an enzyme humans replace with topoisomerase II.

Worked example: reading a drug name as its class

The suffix of a generic name often encodes the mechanism. Drugs ending in -pril (ramipril, lisinopril) are ACE inhibitors; -sartan (losartan, valsartan) are angiotensin receptor blockers; -olol (metoprolol, atenolol) are beta-blockers; -dipine (amlodipine, nifedipine) are calcium channel blockers; -oxetine (fluoxetine) and -pramine block monoamine transporters. Reading the suffix tells you the class, the target, and most of the side-effect profile before you read any further.

Check your understanding Beginner

Formal definition Intermediate+

A drug class is a set of compounds that share a molecular target and therefore a mechanism of action. Pharmacology organises its pharmacopeia by target family — receptors, ion channels, enzymes, and transporters — and within each family by the pathway the target controls.

The four target families

Receptors are proteins that transduce an extracellular signal into an intracellular response. The G-protein coupled receptors (GPCRs) are the largest family and the target of roughly a third of all marketed drugs; they include the beta-adrenergic receptors (blocked by -olol drugs), the angiotensin II type-1 receptor (blocked by -sartan drugs), and the opioid and dopamine receptors (see 17.07.* — signalling). Ligand-gated ion channels such as the nicotinic and GABA-A receptors couple binding directly to ion flux (see 17.09.* — action potential). Catalytic receptors (insulin, HER2) and intracellular nuclear receptors (glucocorticoid, estrogen, thyroid) complete the family.

Ion channels are gated pores that set membrane excitability. Voltage-gated calcium channels in vascular smooth muscle are blocked by the dihydropyridines (-dipine drugs), while local anaesthetics block voltage-gated sodium channels.

Enzymes are catalytic targets: angiotensin-converting enzyme (ACE) generates angiotensin II and is blocked by -pril drugs; HMG-CoA reductase drives cholesterol synthesis and is blocked by the statins (see 35.03.02); cyclooxygenase (COX) generates prostaglandins and is blocked by aspirin and ibuprofen; bacterial DNA gyrase is blocked by fluoroquinolones; dihydrofolate reductase is blocked by trimethoprim and methotrexate.

Transporters move small molecules across membranes. The serotonin transporter (SERT) is blocked by SSRIs, the dopamine and norepinephrine transporters by stimulants and bupropion (see 29.02.03), and the gastric H+/K+ ATPase by proton pump inhibitors. P-glycoprotein, an efflux transporter at the gut and blood-brain barrier, determines which substrates reach the brain at all (see 35.07.02).

Antibiotics by mechanism

Antibiotics sort by the bacterial structure they disable (see 35.02.02 — bacterial pathogenesis and resistance). Cell-wall inhibitors — the penicillins and cephalosporins (beta-lactams that acylate penicillin-binding proteins), vancomycin (which binds D-Ala-D-Ala directly), and bacitracin — exploit a structure humans entirely lack. Protein-synthesis inhibitors target ribosomal subunits that differ from the human 80S ribosome: tetracyclines and aminoglycosides hit the 30S subunit (aminoglycosides cause mistranslation and are bactericidal), while macrolides, lincosamides, chloramphenicol, and the oxazolidinone linezolid hit the 50S subunit. DNA inhibitors include the fluoroquinolones (DNA gyrase and topoisomerase IV) and metronidazole (DNA damage in anaerobes). Folate antagonists block the two-step bacterial folate pathway — sulfonamides inhibit dihydropteroate synthase (the first synthetic antibiotics; see 34.03.*), trimethoprim inhibits dihydrofolate reductase — and the combination cotrimoxazole is synergistic (see 35.02.02 — resistance mechanisms). Isoniazid inhibits mycolic-acid synthesis in mycobacteria; rifampin inhibits bacterial RNA polymerase.

Antihypertensives by mechanism

Antihypertensives intervene in the regulatory loops that set blood pressure (see 35.03.02 — cardiovascular; 35.01.02 — homeostasis; 18.08.* — renal). The renin-angiotensin-aldosterone system (RAAS) branch is blocked at three points: ACE inhibitors (-pril) prevent angiotensin II formation (and raise bradykinin, causing cough and angioedema); angiotensin receptor blockers (-sartan) block the AT1 receptor directly; the direct renin inhibitor aliskiren blocks the first step. The sympathetic branch is blocked by beta-blockers (-olol), which reduce cardiac output and renin release, and by alpha-blockers. Vascular smooth muscle is relaxed by the dihydropyridine calcium channel blockers (-dipine), while the non-dihydropyridines verapamil and diltiazem slow the heart. Volume is reduced by diuretics — thiazides, the loop diuretic furosemide, and potassium-sparing agents — which lower pressure by shedding salt and water. Direct vasodilators (hydralazine, nitroprusside) are reserved for refractory or emergency hypertension.

Psychotropics by mechanism

Psychotropic drugs act on the neurotransmitter systems of the brain (see 29.10.03 — biological treatments; 29.02.03 — neurotransmitters). Antidepressants turn on the monoamine hypothesis: SSRIs (fluoxetine/Prozac, sertraline/Zoloft) block serotonin reuptake; SNRIs (venlafaxine, duloxetine) block serotonin and norepinephrine reuptake; the tricyclics and MAOIs are older but mechanistically related; bupropion acts on dopamine and norepinephrine (see 29.09.02 — mood disorders). Mood stabilisers for bipolar disorder include lithium — whose precise mechanism remains unsettled (inositol depletion, GSK-3 inhibition) — valproate, and lamotrigine (see 29.09.02). Antipsychotics split into typical D2 blockers (haloperidol, chlorpromazine) and atypicals that combine 5-HT2A and D2 antagonism (risperidone, olanzapine, quetiapine, clozapine); clozapine remains the most effective but is reserved for treatment-resistant cases because of agranulocytosis risk (see 29.09.04 — psychotic disorders). Anxiolytics include the benzodiazepines (positive allosteric modulators of the GABA-A receptor) and buspirone (a 5-HT1A partial agonist). Stimulants for ADHD — methylphenidate and the amphetamines — raise synaptic dopamine and norepinephrine (see 29.09.04; 29.10.03).

Analgesics and cancer chemotherapy

Analgesics close the survey. Opioids (morphine, fentanyl, oxycodone) are mu-opioid receptor agonists — powerfully analgesic but addictive, the pharmacology behind the opioid epidemic (see 35.05.03). NSAIDs (aspirin, ibuprofen, celecoxib) inhibit cyclooxygenase and cut prostaglandin production; acetaminophen/paracetamol acts centrally by an unsettled mechanism and carries hepatotoxicity risk (see 29.03.03 — pain). Cancer chemotherapy (see 35.03.03 — cancer biology) extends the same logic. Cytotoxic classes attack proliferation: alkylating agents (cisplatin, cyclophosphamide), antimetabolites (methotrexate, 5-fluorouracil), microtubule inhibitors (vincristine, paclitaxel), and topoisomerase inhibitors (doxorubicin, etoposide). Targeted agents hit tumour-specific drivers — imatinib/Gleevec inhibits the BCR-ABL fusion kinase of chronic myeloid leukaemia; trastuzumab/Herceptin blocks HER2 in breast cancer. Immunotherapy completes the framework: the checkpoint inhibitors pembrolizumab (anti-PD-1) and ipilimumab (anti-CTLA-4) release the brakes on T-cell attack (see 35.03.03 — cancer immunotherapy).

Key result: the dose-response relation and the meaning of selective toxicity Intermediate+

Pharmacodynamics answers a single quantitative question: how much effect does a given concentration produce? The answer is the Hill-Langmuir equation, which generalises the equilibrium binding of a drug to its receptor into a relation between concentration and effect (see 37.* — probability and dose-response curves):

$$E=\frac{E_{\max }\cdot [D{]}^n}{E{C}_{50}^n+[D{]}^n}$$

Here $E$ is the observed effect, $E_{\max }$ the maximal effect the drug can produce, $[D]$ the drug concentration, $E{C}_{50}$ the concentration producing half-maximal effect, and $n$ the Hill coefficient. The equation has three load-bearing consequences.

Potency versus efficacy. $E{C}_{50}$ measures potency — how little drug is needed — while $E_{\max }$ measures efficacy — how much effect is achievable. The distinction is nontrivial clinically: a partial agonist may be highly potent (low $E{C}_{50}$) yet achieve less maximal effect than a less potent full agonist. This is exactly why buprenorphine (a high-affinity partial agonist at the mu-opioid receptor) can both relieve pain and block the effect of a co-administered full agonist such as morphine — it occupies the receptor without fully activating it (see 35.05.03).

Agonism and antagonism. A full agonist has high intrinsic activity and reaches $E_{\max }$; a partial agonist plateaus below it; an antagonist has zero intrinsic activity but still binds, displacing agonists and shifting their curve rightward. The Schild relation quantifies competitive antagonism through the dose-ratio $DR$ — the factor by which agonist concentration must rise to restore a given effect:

$$DR-1=\frac{[B]}{K_B}$$

where $[B]$ is the antagonist concentration and $K_B$ its equilibrium dissociation constant. A parallel rightward shift with unchanged $E_{\max }$ is the signature of competitive antagonism; a depressed $E_{\max }$ signals non-competitive (irreversible or allosteric) blockade.

Selective toxicity and the therapeutic index. Ehrlich's magic bullet is, quantitatively, a separation between the dose-response curve for the desired effect and the dose-response curve for toxicity. The therapeutic index $TI=T{D}_{50}/E{D}_{50}$ — the ratio of the dose toxic in half the population to the dose effective in half — measures that separation. Penicillin has an enormous therapeutic index because its target is absent in humans; lithium and warfarin have narrow indices (often $TI<3$) because their targets sit inside pathways the host also needs, and their dosing therefore demands concentration monitoring (see 35.07.02). The same dose-response arithmetic that classifies a drug as potent or efficacious also classifies it as safe or dangerous, and it is the formal content of the clinician's risk-benefit calculation.

Exercises Intermediate+

Advanced results Master

Drug discovery and the development pipeline

Modern drug discovery runs from target identification through high-throughput screening (see 33.07.* — computing and bioinformatics), medicinal chemistry and rational design (see 33.04.* — the chemistry revolution), preclinical testing in animal models (see 17.* — molecular biology and model organisms), and the four clinical-trial phases — Phase I safety, Phase II efficacy and dosing, Phase III definitive randomised comparison, Phase IV post-marketing surveillance (see 35.02.04 — epidemiology and RCT design). The ADME and pharmacokinetic principles of 35.07.02 enter at every transition, and FDA or EMA approval is the regulatory gate that closes the pipeline (see 33.08.* — big science and drug regulation). Attrition is brutal: of thousands of screened hits, roughly one in ten thousand reaches the market, and most failures occur in Phase II for lack of efficacy.

Pharmacodynamics in depth: tolerance, signal transduction, and receptor theory

Beyond the Hill equation, pharmacodynamics treats the dynamic response of the receptor system. Signal transduction through second messengers — cAMP, IP3, calcium, the MAP-kinase cascade — links a single binding event to a cellular response that can be amplified, desensitised, or cross-regulated (see 17.07.*). Tolerance emerges as receptor downregulation, G-protein uncoupling, or compensatory changes in opposing pathways — the mechanism behind opioid tolerance, benzodiazepine tolerance, and the tachyphylaxis to indirectly acting sympathomimetics (see 35.05.03 — neuroadaptation). Spare receptors explain why a maximal tissue response can occur at fractional occupancy, and inverse agonists — which reduce constitutive receptor activity rather than merely blocking it — distinguished themselves once constitutively active receptors were recognised.

Rational drug design and structure-based discovery

Structure-based design rests on solving the three-dimensional structure of a drug target — historically by X-ray crystallography (see 33.06.* — protein crystallography) and increasingly by cryo-electron microscopy and by AlphaFold prediction (see 33.07.* — deep learning). Computational docking screens virtual libraries against the solved structure, molecular dynamics simulates binding, and fragment-based design assembles a lead from small chemical fragments that each occupy part of the active site. The newest modality — proteolysis-targeting chimeras (PROTACs) — abandons inhibition entirely: a bifunctional molecule recruits an E3 ubiquitin ligase to the target, marking it for proteasomal degradation, which converts "occupy and block" into "remove entirely" and opens drug action against targets long considered undruggable (see 43.* — numerical methods; 33.07.* — AI in drug discovery).

Biologics and the pharmacology of macromolecules

Biologics — monoclonal antibodies (the -mab suffix: trastuzumab, rituximab, pembrolizumab), fusion proteins (etanercept), recombinant cytokines, and cellular therapies — extend drug classification beyond small molecules. Their targets are extracellular by necessity (antibodies do not cross membranes), their pharmacokinetics follow proteolytic rather than CYP-mediated clearance, and their manufacture is inseparable from the product itself — the reason biosimilars face a more demanding regulatory standard than small-molecule generics (see 35.07.02; 35.03.03). CAR-T cells (see 35.03.03 — cancer immunotherapy; 33.06.* — genetic engineering), gene therapy (see 35.08.02 — CRISPR), and the mRNA therapeutics licensed at scale during the COVID-19 pandemic (see 35.06.03 — vaccine science) push the boundary of what counts as a drug toward living or information-encoded agents.

Drug regulation, ethics, and access

Regulation is the social machinery that decides which mechanisms reach patients. The thalidomide tragedy and the 1962 Kefauver-Harris Amendment made proof of efficacy, not merely safety, the condition of approval (see 18.09.* — teratology). The opioid epidemic — Purdue Pharma's marketing of OxyContin and the downstream fentanyl crisis — is the defining case of pharmacological harm driven by corporate behaviour, and it reframes drug-class control as a question of criminal justice as much as medicine (see 35.05.03 — addiction; 30.06.02 — criminal justice). Off-label prescribing (see 20.02.* — autonomy), direct-to-consumer advertising (see 36.* — media literacy), insulin pricing (see 30.04.* — class and access), and the WHO Essential Medicines List as a global-access instrument (see 31.06.* — global health) complete the ethical map: a drug class is not only a mechanism but also a political-economic object.

Personalised drug therapy and the future of the field

Pharmacogenomics makes genotype a covariate of dosing — CYP2D6 and CYP2C19 polymorphisms shape antidepressant and clopidogrel response, HLA-B*57:01 predicts abacavir hypersensitivity, DPYD deficiency predicts fluoropyrimidine toxicity (see 35.07.02; 35.08.* — genomic medicine). Companion diagnostics and therapeutic drug monitoring move prescribing from population averages toward individual estimates (see 35.08.03 — precision medicine). The forward edge of the field is heterogeneous: AlphaFold-driven structure prediction (see 33.07.*), RNA therapeutics including siRNA, antisense, and mRNA (see 33.06.*), microbiome-targeted drugs (see 35.02.02), psychedelic therapeutics — psilocybin, MDMA (see 29.10.03) — and the candidate anti-ageing pharmacology of metformin, rapamycin, and senolytics (see 31.04.* — longevity; 29.06.04 — ageing). Each extends the magic-bullet idea into territory Ehrlich did not foresee.

Connections Master

Foundation-of: pharmacology and therapeutics

This unit is the systematic complement to 35.07.01 (principles) and 35.07.02 (pharmacokinetics): it supplies the what the drug does to the body half of pharmacology, organised by target and mechanism. Every later clinical unit — cardiovascular (35.03.02), infectious disease (35.02.*), mental health (35.05.*, 29.10.03), cancer (35.03.03), and future medicine (35.08.*) — consumes this classification.

Molecular biology: the target inventory

The four target families — receptors, ion channels, enzymes, transporters — are the molecular-biological objects catalogued in 17.07.* (signalling and GPCR pharmacology), 17.09.* (action potential and voltage-gated channels), 17.04.* (enzyme biochemistry), and 17.02.* (membrane transporters). The recurrence is structural: every drug class is a chemical handle on a protein family that molecular biology names and characterises.

Cardiovascular, renal, and homeostatic physiology

Antihypertensive pharmacology is applied cardiovascular and renal physiology: the RAAS, sympathetic tone, vascular smooth-mass contraction, and salt-and-water balance are exactly the regulatory loops of 35.03.02, 18.08.*, and 35.01.02. A diuretic, a beta-blocker, and an ACE inhibitor lower pressure through three different physiological levers, and the choice between them is a choice about which loop to perturb.

Neuroscience and psychopharmacology

Psychotropic classification mirrors neurotransmitter anatomy: the monoamine systems of 29.02.03 (serotonin, dopamine, norepinephrine), the GABA-glutamate balance of 29.10.03, and the clinical syndromes of 29.09.02 (mood), 29.09.04 (psychosis, ADHD). The drug-class taxonomy and the disease taxonomy are nearly parallel — a convergence that is both the strength and the explanatory limit of present-day psychiatry.

Microbiology and the evolution of resistance

Antibiotic classes are defined against bacterial structures documented in 35.02.01 and 35.02.02; the resistance mechanisms that defeat them — beta-lactamases, target modification, efflux, permeability loss — are the evolutionary response to the selection each class imposes. Antibiotic pharmacology and evolutionary microbiology are inseparable: the drug class selects for its own obsolescence (see 19.* — eco-evo-biology).

Oncology and the logic of targeted therapy

Cytotoxic-versus-targeted-versus-immunotherapy (35.03.03) is the cancer-pharmacology instance of the magic-bullet criterion developed here. Imatinib, trastuzumab, and the checkpoint inhibitors each select a tumour-specific molecule as the drug target, and the therapeutic-index arithmetic of this unit quantifies the gain in selective toxicity that results.

Chemistry, drug design, and computing

Structure-based design, docking, PROTACs, and AI-driven discovery (33.04.*, 33.06.*, 33.07.*, 43.*) supply the tools by which a new mechanism is turned into a new drug class. The link runs both ways: AlphaFold's predicted structures convert target identification into a computational operation, and the classification system of this unit is the ontology that the computational pipeline aims to populate.

Genomic medicine, ethics, and public health

Pharmacogenomics (35.08.*), drug regulation and the opioid epidemic (30.06.*, 35.05.03), and global access (31.06.*) situate drug classes inside their social and individual-genomic context. The same compound can be a cure, a commodity, or a controlled substance depending on the framework applied, and the classification of a drug class by mechanism cannot be separated from its classification by access and harm.

Historical and philosophical context Master

Ehrlich and the magic bullet

Paul Ehrlich coined Zauberkugel — the magic bullet — to name the ideal of a compound that strikes the disease-causing organism without injuring the host. His 1910 Salvarsan (arsphenamine), the first effective treatment for syphilis, was the first deliberate instantiation of the idea, developed through the systematic screening of arsenical dyes in his Frankfurt laboratory. Ehrlich framed the search as therapia sterilisans magna: a single curative dose that exterminates the parasite. The empirical reality was messier — Salvarsan was toxic and required many doses — but the principle of selective toxicity, the quantitative separation of the effective dose from the toxic dose, became the organising criterion of pharmacology. The therapeutic index of this unit's Key result is Ehrlich's magic bullet made arithmetic.

Fleming, Florey, Chain, and the antibiotic era

Alexander Fleming's 1928 observation that a Penicillium mould had killed staphylococci on a contaminated plate is among the most famous accidents in science, but the observation languished for a decade. It was Howard Florey, Ernst Chain, and their Oxford team who, in 1940-41, purified penicillin, demonstrated its curative power in infected mice, and scaled production in time for the Second World War, sharing the 1945 Nobel Prize with Fleming. The beta-lactam antibiotics that descended from penicillin defined the cell-wall-inhibitor class and, with the sulfonamides (Domagk, 1932 — the first synthetic antibacterials; see 34.03.*), inaugurated the antibiotic era that transformed surgery, childbirth, and infectious-disease prognosis within a single generation.

The receptor concept: Langley, Clark, and Ariëns

The idea that drugs act through specific receptive substances is older than the molecules themselves. J. N. Langley, working on the autonomic nervous system at Cambridge, proposed in 1905 that cells carry "receptive substances" with which curare and nicotine compete — a pharmacological argument for the existence of receptors decades before any receptor was isolated. A. J. Clark's 1933 The Mode of Action of Drugs on Cells cast the idea quantitatively: drug effect follows the same mass-action binding isotherm that governs oxygen and haemoglobin, giving the Hill-Langmuir equation its pharmacological home. E. J. Ariëns in the 1950s then split Clark's single parameter into two — affinity (binding) and intrinsic activity (the effect produced once bound) — which is the formal basis of the agonist/partial-agonist/antagonist distinction that still organises the Key result of this unit.

The psychopharmacology revolution: chlorpromazine, lithium, and the antidepressants

The modern psychiatric pharmacopeia emerged in a single remarkable decade. In 1949 the Australian psychiatrist John Cade reported that lithium calmed manic patients — a serendipitous finding from guinea-pig experiments that became the first specific mood stabiliser. In 1952 the French surgeon Henri Laborit observed that the antihistamine chlorpromazine produced an indifferent calm in surgical patients, and Pierre Delay and Jean Delay used it at the Sainte-Anne hospital in Paris to treat psychosis, launching the antipsychotic era and making large-scale deinstitutionalisation conceivable. The antidepressants followed from another accident: iproniazid, an anti-tubercular drug, elevated mood in TB patients, and was recognised in 1957 (by Kuhn) as an MAOI antidepressant; imipramine, a failed antihistamine, was found in the same year to relieve depression and defined the tricyclic class. Each of these classes — lithium, the antipsychotics, the antidepressants — was discovered by clinical observation of an unexpected effect, and each was only later assigned a mechanism. The receptor-based mechanistic explanation of this unit is, for psychiatry, largely a post hoc rationalisation of discoveries made empirically, and the field still bears the marks of that ordering.

Black, propranolol, and rational drug design

James Black's development of propranolol — the first beta-blocker, 1962 — and of cimetidine, the first H2-blocker, marked a methodological turn. Where earlier drugs had been found by screening, Black designed antagonists to a chosen receptor from a physiological hypothesis: that blocking the beta-adrenergic receptor would reduce cardiac work in angina. The method earned him a Nobel Prize and established the model of rationale-first drug discovery that the targeted therapies of the late twentieth century — imatinib, trastuzumab, the checkpoint inhibitors — would extend. The -olol, -mab, and -tinib suffixes of the contemporary pharmacopeia all descend from Black's commitment to designing a molecule for a named target rather than screening for an effect.

Köhler, Milstein, and the biologics frontier

The 1975 hybridoma technique of Georges Köhler and César Milstein made monoclonal antibodies indefinitely producible, opening the -mab class that now dominates the list of top-selling drugs and the biologics frontier of this unit's Advanced results. The same logic — a single molecular target addressed by a single molecular species — generalised Ehrlich's magic bullet from small molecules to proteins, and from proteins, in the CAR-T and mRNA era, to cells and to information.

The philosophical content: classification by mechanism versus classification by disease

This unit classifies drugs by mechanism, but clinical practice also classifies them by indication — a "drug for high blood pressure" or "a drug for depression." The two taxonomies do not align cleanly: the same mechanism (an SSRI) treats depression, anxiety, and premature ejaculation; the same disease (hypertension) is treated by five mechanistically distinct classes. Pharmacology's commitment to the mechanistic taxonomy reflects its scientific ambition — to explain effect by cause — but the indicational taxonomy reflects the clinician's practical need, to choose among options for a given patient. The tension between the two is the everyday content of therapeutics, and it is why a mechanism-centred curriculum still ends at the bedside rather than at the laboratory bench. The magic bullet was Ehrlich's wager that mechanism and indication would converge — that the right molecule for the right target would be the right drug for the right disease. A century of pharmacology has made the convergence closer but never complete.

Bibliography Master

1. Katzung, B. G. (ed.) (2021). Basic and Clinical Pharmacology (15th ed.). McGraw-Hill. Chs. 38-47 (antibacterial, antifungal, antiviral agents); Chs. 11-12 (autonomic and cardiovascular pharmacology); Chs. 27-30 (psychopharmacology); drug classification by mechanism of action. ^{[source pending]}

2. Stahl, S. M. (2021). Stahl's Essential Psychopharmacology: Neuroscientific Basis and Practical Applications (5th ed.). Cambridge University Press. Antidepressants, mood stabilisers, antipsychotics, anxiolytics, and stimulants — receptor and circuit mechanisms with neuroanatomical illustrations of drug action. ^{[source pending]}

3. Brunton, L. L., Chabner, B. A. and Knollmann, B. C. (eds.) (2018). Goodman & Gilman's The Pharmacological Basis of Therapeutics (13th ed.). McGraw-Hill. General principles and pharmacodynamics; chemotherapy of microbial and neoplastic disease; cardiovascular-renal and central nervous system pharmacology. ^{[source pending]}

4. Rang, H. P., Ritter, J. M., Flower, R. J. and Henderson, G. (2024). Rang and Dale's Pharmacology (10th ed.). Elsevier. Drug targets (receptors, ion channels, enzymes, transporters); dose-response and agonism/antagonism; systematic pharmacology grouped by mechanism and drug class. ^{[source pending]}

5. Ehrlich, P. (1910). "Aus Theorie und Praxis der Chemotherapie." Zeitschrift für ärztliche Fortbildung, 7. The magic-bullet programme and the selective-toxicity criterion that organises modern drug classification.

6. Fleming, A. (1929). "On the antibacterial action of cultures of a penicillium, with special reference to their use in the isolation of B. influenzae." British Journal of Experimental Pathology, 10(3), 226-236. The founding observation of the penicillin/antibiotic era.

7. Chain, E., Florey, H. W., Gardner, A. D., Heatley, N. G., Jennings, M. A., Orr-Ewing, J. and Sanders, A. G. (1940). "Penicillin as a chemotherapeutic agent." The Lancet, 236(6104), 226-228. The Oxford purification and the animal proof of curative efficacy.

8. Langley, J. N. (1905). "On the reaction of cells and of nerve-endings to certain poisons, chiefly as regards the reaction of striated muscle to nicotine and to curare." Journal of Physiology, 33(4-5), 374-413. The pharmacological proposal of "receptive substances" anticipating receptor theory.

9. Clark, A. J. (1933). The Mode of Action of Drugs on Cells. Edward Arnold, London. The mass-action binding isotherm applied to drug effect — the origin of the Hill-Langmuir dose-response relation.

10. Ariëns, E. J. (1954). "Affinity and intrinsic activity in the theory of competitive inhibition." Archives Internationales de Pharmacodynamie et de Thérapie, 99, 32-49. The split of Clark's parameter into affinity and intrinsic activity — the agonist/partial-agonist/antagonist distinction.

11. Cade, J. F. J. (1949). "Lithium salts in the treatment of psychotic excitement." Medical Journal of Australia, 2(10), 349-352. The serendipitous founding paper of lithium as a mood stabiliser.

12. Delay, J. and Deniker, P. (1952). "Le traitement des psychoses par une méthode neurolytique dérivée de l'hibernothérapie." Comptes Rendus du Congrès des Médecins Aliénistes et Neurologistes de France et des Pays de Langue Française, 50, 497-502. The clinical introduction of chlorpromazine and the start of the antipsychotic era.

13. Black, J. W. and Stephenson, J. S. (1962). "Pharmacology of a new adrenergic beta-receptor-blocking compound (Nethalide)." The Lancet, 2(7251), 311-314. The first beta-blocker and the founding paper of rationale-first drug design.

14. Köhler, G. and Milstein, C. (1975). "Continuous cultures of fused cells secreting antibody of predefined specificity." Nature, 256(5517), 495-497. The hybridoma technique that opened the monoclonal-antibody (-mab) class of biologics.

15. Druker, B. J. et al. (2001). "Efficacy and safety of a specific inhibitor of the BCR-ABL tyrosine kinase in chronic myeloid leukemia." New England Journal of Medicine, 344(14), 1031-1037. The clinical validation of imatinib/Gleevec and of targeted therapy against a tumour-specific driver.