Quetiapine

Quetiapine (brand name Seroquel) is a second-generation atypical antipsychotic used for schizophrenia, bipolar disorder (mania, depression, and maintenance), and as an adjunct in major depressive disorder. It is also widely prescribed off-label as a sleep aid.

Quetiapine is heavily metabolized by CYP3A4 to its active metabolite norquetiapine and to inactive metabolites. CYP3A4 activity varies between individuals, both genetically and through drug-drug interactions with CYP3A4 inducers and inhibitors, and this variability is the main pharmacogenetic story for quetiapine.

CYP3A4 is the dominant enzyme clearing quetiapine. The CYP3A4*22 allele (rs35599367) is the most well-characterized reduced-function variant, present in roughly 5–7% of European-ancestry populations. Carriers of *22 produce less CYP3A4 enzyme and clear CYP3A4 substrates more slowly, raising plasma concentrations of those drugs.

Most CYP3A4 pharmacogenetic effects are smaller in magnitude than CYP2D6 or CYP2C19 effects, because CYP3A4 substrates are often co-metabolized by other enzymes and because non-genetic factors (drug interactions, hepatic function) typically dominate. For quetiapine, the *22 effect is meaningful but not dramatic at standard doses.

CYP3A4 reduced-function carriers (*22 carriers and similar) have moderately higher quetiapine plasma concentrations, with corresponding increases in sedation, orthostatic hypotension, and metabolic side effects. Normal-function individuals show standard exposure.

Concurrent CYP3A4 inhibitors (e.g., clarithromycin, ketoconazole, ritonavir) and inducers (e.g., carbamazepine, rifampin) can produce effects that overshadow the genetic effect, so checking for relevant drug interactions remains important regardless of phenotype.

CYP3A4 Normal Function

• Effect: Standard quetiapine exposure.
• Recommendation: Standard dosing.

CYP3A4 Intermediate / Reduced Function

• Effect: Higher quetiapine plasma concentrations than expected.
• Clinical consequence: Higher risk of sedation, orthostatic hypotension, and metabolic side effects.
• Recommendation: Start at the low end of the dose range; titrate cautiously; reassess if side effects emerge.

Indeterminate / Not Available

• Recommendation: Standard dosing; check for interacting CYP3A4 inhibitors / inducers.

Atypical antipsychotics that do not rely primarily on CYP3A4 include olanzapine (CYP1A2 / glucuronidation) and risperidone (CYP2D6). Each has its own pharmacogenetic considerations and side-effect profile, particularly metabolic risk for olanzapine and EPS for risperidone.

• DPWG — Pharmacogenetics Working Group recommendations
• PharmGKB — Quetiapine

Learn more about how genetics may affect your response to Quetiapine and related medications:

• Haloperidol (Haldol) Side Effects: When CYP2D6 Genetics Are Behind Them
• Quetiapine (Seroquel) Side Effects: When CYP3A4 Genetics Make It Harder
• Zuclopenthixol (Clopixol) Side Effects: CYP2D6 Genetics Explained