rs12721627 — CYP3A4 *16

CYP3A4*16 — The East Asian Reduced-Metabolizer Variant

CYP3A4 is the most abundant drug-metabolizing enzyme in the human liver and intestine, handling roughly 50% of all clinically used drugs¹¹ 50% of all clinically used drugs
CYP3A4 metabolizes approximately half of all drugs on the market, including statins, immunosuppressants, calcium channel blockers, antiepileptics, and many oncology agents. The CYP3A4*16 allele (rs12721627, c.554C>G on the coding strand, p.Thr185Ser) substitutes a serine for threonine at position 185 of the enzyme's active site, impairing its catalytic machinery. This variant is found almost exclusively in East Asian populations²² almost exclusively in East Asian populations
In gnomAD and ALFA datasets, the C allele frequency in Europeans and Africans rounds to 0.000; East Asian frequency is ~1–2.2% at an allele frequency of approximately 1–2%, making it relevant for the approximately 4 billion people of East Asian descent worldwide.

The Mechanism

The Thr185Ser substitution sits within the substrate recognition site 2 (SRS-2) of CYP3A4. Functional expression in a baculovirus-insect cell system³³ Functional expression in a baculovirus-insect cell system
Wild-type and variant CYP3A4 co-expressed with human NADPH-P450 reductase in Sf21 insect cells showed that the variant protein is produced at normal levels — the problem is not quantity but catalytic efficiency. The Thr-to-Ser change reduces substrate binding affinity (increased Km) and, for some substrates, reduces the maximal turnover rate (Vmax). The result is substrate-dependent reduced intrinsic clearance⁴⁴ substrate-dependent reduced intrinsic clearance
CYP3A4.16 shows the substrate-dependent altered kinetics compared with CYP3A4.1: 50% lower clearance of midazolam (via 1'-hydroxylation), and a striking 74% lower clearance of carbamazepine. This means carriers need lower doses of affected drugs to achieve the same plasma concentration — or reach toxic levels on standard doses.

The Evidence

The foundational pharmacokinetic study of CYP3A4*16 used recombinant protein expression⁵⁵ recombinant protein expression
Maekawa K et al. Xenobiotica 2009 to measure enzyme kinetics directly. For midazolam 1'-hydroxylation, intrinsic clearance (Vmax/Km) was halved. For carbamazepine 10,11-epoxide formation — the primary oxidative pathway — intrinsic clearance dropped by 74%, representing a greater impairment than seen with the more commonly studied CYP3A4*22 splice variant.

The clinical stakes were quantified in a pharmacokinetic modeling study of Japanese patients⁶⁶ pharmacokinetic modeling study of Japanese patients
Adachi K et al. Drug Metab Pharmacokinet 2023 receiving atorvastatin. Homozygous CYP3A4*16 carriers showed virtual plasma Cmax 3.3-fold higher and AUC 4.2-fold higher than *1/*1 individuals — exposures comparable to those produced by concomitant use of strong CYP3A4 inhibitors. Among 483 Japanese patients on atorvastatin monotherapy in an adverse drug event database, over half (258) experienced statin intolerance severe enough to require discontinuation. The authors suggest CYP3A4*16 is a contributing factor.

In 177 Japanese cancer patients receiving irinotecan⁷⁷ 177 Japanese cancer patients receiving irinotecan
Sai K et al. Cancer Chemother Pharmacol 2008, male *16B carriers showed approximately 50% lower AUC ratio of APC (inactive metabolite) to parent irinotecan — evidence that the variant alters the metabolic routing of this chemotherapy agent. No significant difference in total irinotecan clearance or severe toxicity was observed, suggesting the clinical impact on irinotecan may be modest.

For antidepressants, an in vitro study of 21 CYP3A4 variants and citalopram⁸⁸ 21 CYP3A4 variants and citalopram
Wang P et al. J Psychopharmacol 2025 found CYP3A4*16 among 12 variants that significantly reduced intrinsic clearance. The substrate-dependent kinetics pattern means the magnitude of effect varies considerably from drug to drug.

Practical Actions

Because CYP3A4*16 is almost exclusive to East Asian populations, its population-level impact is concentrated in Japan, China, Korea, and neighboring regions. Heterozygotes (*1/*16) represent approximately 4% of East Asians and are expected to show intermediate impairment. Homozygotes (*16/*16), though rare (~0.05%), carry the most substantial risk of toxic drug accumulation on standard doses of CYP3A4 substrates.

The drugs most likely to cause problems are those heavily dependent on CYP3A4 for clearance with a narrow therapeutic index: high-dose statins (atorvastatin, simvastatin), carbamazepine, and some immunosuppressants. For drugs with wide therapeutic windows or multiple metabolic pathways, the effect of CYP3A4*16 is likely less clinically significant.

Interactions

CYP3A4*16 acts additively with CYP3A4*22 (rs35599367) — both reduce the same enzyme. An individual heterozygous for both would have substantially lower total CYP3A4 activity than either variant alone. Combined genotyping with CYP3A5 (rs776746) is important: CYP3A5 expressers have a compensatory CYP3A pathway; East Asians have high CYP3A5*3 frequency (~75%), meaning most CYP3A4*16 carriers also lack functional CYP3A5, amplifying the clinical impact of the CYP3A4 impairment.

Co-administration of CYP3A4 inhibitors (azole antifungals, macrolide antibiotics, grapefruit juice) on top of CYP3A4*16 can compound the drug exposure increase. Prescribers of East Asian patients on high-dose statins or narrow-therapeutic-index CYP3A4 substrates should consider this variant when unexplained intolerance arises.