rs12721627 — CYP3A4 *16

Missense variant reducing CYP3A4 enzyme activity by 50–74% depending on substrate, found primarily in East Asian populations at ~2% allele frequency

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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.

Drug Interactions

atorvastatin increased_toxicity literature
simvastatin dose_adjustment literature
midazolam dose_adjustment literature
carbamazepine dose_adjustment literature
irinotecan dose_adjustment literature
citalopram dose_adjustment literature

Genotype Interpretations

What each possible genotype means for this variant.

GG Normal Metabolizer Normal

Normal CYP3A4*16 — standard drug metabolism expected

You carry two copies of the reference CYP3A4 allele at this position, meaning your CYP3A4 enzyme is not impaired by the *16 variant. Standard dosing of CYP3A4-metabolized drugs is appropriate based on this genotype. This is the most common result worldwide: essentially all Europeans and Africans carry this genotype, and approximately 96% of East Asians do as well.

CG Intermediate Metabolizer Intermediate

One copy of CYP3A4*16 — moderately reduced CYP3A4 activity

In vitro studies show CYP3A4*16 reduces intrinsic clearance by 50–74% per enzyme molecule depending on the substrate. With one copy, your overall hepatic CYP3A4 activity is likely 25–40% below the reference level. This is clinically meaningful for drugs that rely heavily on CYP3A4 and have narrow therapeutic windows. Statin doses above 20 mg for atorvastatin or above 20 mg for simvastatin may produce higher plasma levels than expected, increasing myopathy risk.

CC Poor Metabolizer Poor

Two copies of CYP3A4*16 — substantially reduced CYP3A4 activity

Pharmacokinetic modeling using CYP3A4*16 kinetic parameters estimated atorvastatin Cmax 3.3-fold higher and AUC 4.2-fold higher in homozygous carriers compared to wild-type individuals. These exposures are comparable to those observed when a strong CYP3A4 inhibitor (such as clarithromycin or itraconazole) is added to statin therapy — a combination flagged in drug interaction warnings. For carbamazepine, the 74% reduction in in vitro intrinsic clearance predicts similarly elevated serum levels and a higher risk of neurotoxicity at standard doses. A full pharmacogenomics consultation before starting any high-dose CYP3A4 substrate is strongly recommended.