Background:  The G516T and C1459T single nucleotide polymorphisms (SNP) in cytochrome P450 (CYP) 2B6 affect hepatic expression of this key enzyme in non-nucleoside reverst transcriptase inhibitor (NNRTI) metabolism. Furthermore, the G516T SNP is associated with altered pharmacokinetics, and toxicity of NNRTI. CYP3A enzymes have a more minor role for NNRTI, but are important for metabolism of the protease inhibitors (PI) and also exhibit polymorphic expression. Since CYP2B6 and CYP3A4 are expressed in lymphocytes, we have performed a proof-of-concept study to assess the effect of genetic polymorphism on enzyme expression within HIV-replication competent cells.

Methods:  Whole blood was obtained from 20 healthy volunteers by venopuncture and genomic DNA and lymphocytes prepared. All SNP were genotyped by real-time polymerase chain reaction (PCR) allelic discrimination with specific primers and probes. Quantitative real-time PCR was used to examine the effect of CYP2B6 (G516T, C1459T), CYP3A4 (*1B), and CYP3A5 (*3) SNP on lymphocyte expression of the enzymes.

Results:  The allele frequencies for CYP2B6 516T, CYP2B6 1459T, CYP3A4*1B, and CYP3A5*3 were 17.5%, 12.5%, 0%, and 5%, respectively. The median (range) expression was 2.8 (2.3 to 22.9) for CYP2B6 and 2.3 (2.3 to 11.7) for CYP3A4. No correlation was observed between 2B6 and 3A4 (r² = 0.01; p = 0.67). A significant association was observed between G516T and lymphocyte CYP2B6 expression (10.1±2.4 in G homozygotes vs 2.3±0.03 in heterozygotes; p = 0.003, 95%CI 3.4, 12.2; see the figure). No differences in CYP2B6 and CYP3A4 expression were observed for the other SNP. 

Conclusions:  These data indicate that the previously reported association between CYP2B6 SNP and expression is not limited to hepatic tissue. Metabolism of NNRTI at their site of action may be an additional factor impacting on response..