Clinical Psychopharmacology · Paper A
Acetylator status and metabolism
Genotype predicts drug toxicity risk before a single dose is given, because N-acetyltransferase 2 (NAT2, a liver enzyme) comes in two inherited speeds (slow and rapid) and that speed alone changes how a patient handles several common drugs. Slow acetylators build up NAT2-metabolised drugs at standard doses, raising toxicity risk; rapid acetylators clear them faster, risking subtherapeutic levels. A patient of European ancestry, where slow acetylators predominate, e.g. someone started on standard-dose isoniazid, needs closer watching for isoniazid-induced peripheral neuropathy and liver toxicity.
NAT2 gene variants are inherited in an autosomal recessive pattern, producing slow acetylators (two low-activity alleles) and rapid acetylators (at least one fully functional allele); slow acetylators predominate in European and Middle Eastern populations, rapid acetylators in East Asian populations. Slow acetylators accumulate isoniazid, hydralazine and the MAOIMonoamine oxidase inhibitor, an older antidepressant with strict food and drug interaction rules.Read more → (monoamine oxidase inhibitor) phenelzine, raising the risk of peripheral neuropathy, drug-induced lupus, and dose-related side effects respectively. A patient on phenelzine who is a slow acetylator and reports increasing side effects on a standard dose, e.g. worsening dizziness and headache over a few weeks, needs phenelzine titrated more cautiously, since their genotype predicts higher plasma levels at the same dose.
A fresh doctor might treat "individual variation in drug response" as unpredictable. NAT2 acetylator status is the opposite: a single, well-characterised genetic polymorphism that predicts specific toxicities before a drug is even started. Hold the phrase GENOTYPE PREDICTS TOXICITY through every section below.
Acetylation is a phase II (conjugation) metabolic reaction carried out by the liver enzyme N-acetyltransferase 2 (NAT2). NAT2 activity is genetically determined and falls into two broad phenotypes: slow acetylators, who metabolise NAT2-dependent drugs more slowly, and rapid (fast) acetylators, who metabolise them more quickly. This single genetic difference changes both toxicity risk and treatment efficacy for a specific group of drugs.
A common mistake is assuming rapid acetylators are always the "safer" phenotype because they clear drugs quickly. Current genotyped cohort studies confirm isoniazid hepatotoxicity risk is actually higher in slow acetylators, superseding an older idea that rapid acetylators carried greater risk through hydrazine metabolite accumulation. Rapid acetylators instead carry the opposite risk profile: undertreatment and relapse, not toxicity, from standard dosing.
NAT2 gene variants are inherited in an autosomal recessive pattern (a trait that appears only when both inherited gene copies carry a low-activity variant). Slow acetylators, who carry two low-activity alleles, predominate in European and Middle Eastern populations, while rapid acetylators, who carry at least one fully functional allele, predominate in East Asian populations. Knowing a patient's likely acetylator phenotype from ancestry alone is a useful, though imperfect, first approximation before genetic testing.
Slow acetylators accumulate NAT2-metabolised drugs at standard doses because the enzyme clears them more slowly than expected. This raises the risk of isoniazid-induced peripheral neuropathy (nerve damage causing tingling and weakness) and hepatotoxicity (liver injury), hydralazine- and procainamide-induced drug-induced lupus (a lupus-like autoimmune reaction to certain drugs), and dapsone toxicity, for example a patient of European ancestry started on standard-dose isoniazid who needs closer monitoring for neuropathy and abnormal liver function.
Rapid acetylators clear the same drugs faster, risking subtherapeutic levels and treatment failure: a rapid acetylator started on standard-dose isoniazid for tuberculosis may not achieve adequate plasma levels, risking relapse. Isoniazid also inhibits phenytoin oxidation via a separate hepatic pathway, so a patient taking both drugs can develop phenytoin toxicity, a risk compounded further in a slow acetylator whose isoniazid levels are already higher than expected.
NAT2 acetylation should not be confused with poor metabolism, which affects oxidative metabolism of tricyclic antidepressants, codeine, and many antipsychotics through the system rather than phase II conjugation. It should also not be confused with HLA-B*15:02-associated Stevens-Johnson syndrome, an immune-mediated hypersensitivity to carbamazepine rather than a dose-dependent pharmacokinetic effect, or with pseudocholinesterase deficiency, which prolongs neuromuscular blockade from suxamethonium through a completely separate plasma esterase pathway.
Acetylator status is not established through routine clinical diagnosis in everyday practice, but two real methods exist and are worth knowing for the exam: direct NAT2 genotyping (identifying which gene variants a patient carries) and phenotyping via a caffeine metabolite ratio test (measuring how quickly a standard caffeine dose is acetylated in urine or plasma), both used mainly in research or specialist pharmacogenomic settings rather than routine tuberculosis clinics. In everyday practice, diagnosis of a problem in an individual patient is instead reactive: recognising peripheral neuropathy, deranged liver function tests, or a lupus-like syndrome (joint pain, rash, positive antinuclear antibody) in a patient on isoniazid, hydralazine, procainamide, or dapsone, and considering slow acetylator status as the mechanism.
Management relies on close clinical and biochemical monitoring, liver function tests and clinical screening for neuropathy symptoms, in patients on isoniazid, hydralazine, or dapsone, particularly those from populations where slow acetylation predominates, following BNF and standard tuberculosis-treatment prescribing guidance. Pyridoxine (vitamin B6) is co-prescribed with isoniazid specifically to reduce the risk of peripheral neuropathy, a practical first-line preventive measure regardless of known genotype. If toxicity develops, first-line management is dose reduction or drug withdrawal alongside supportive treatment of the specific complication (for example stopping hydralazine if drug-induced lupus develops, which typically resolves after discontinuation).
The high-yield pearl: slow acetylators get TOXICITY, rapid acetylators get TREATMENT FAILURE: the exam loves testing this the wrong way round. Vignettes often describe a patient of European ancestry on standard-dose isoniazid developing peripheral neuropathy and expect recognition of slow acetylator status as the underlying mechanism. Another trap describes a patient on hydralazine developing joint pain, rash, and a positive antinuclear antibody test and expects recognition of drug-induced lupus in a likely slow acetylator. A further trap contrasts NAT2 acetylation with or HLA-mediated reactions, testing whether the candidate can distinguish a dose-dependent, pharmacokinetic pharmacogenomic effect from an immune-mediated hypersensitivity reaction.
NAT2 acetylator status, inherited as autosomal recessive, determines whether a patient is a slow or rapid metaboliser of isoniazid, hydralazine, procainamide, and dapsone. Slow acetylators, more common in European and Middle Eastern populations, risk toxicity (peripheral neuropathy, hepatotoxicity, and drug-induced lupus) at standard doses, while rapid acetylators, more common in East Asian populations, risk subtherapeutic levels and treatment failure. Genotyping and caffeine-ratio phenotyping exist but are not routine; management instead relies on clinical and biochemical monitoring, pyridoxine co-prescription, and dose adjustment, guided by BNF prescribing practice.