Background: The CYP4503A5*1 genotype is associated with lower tacrolimus concentrations. Although its effect is important, it incompletely explains the variability in tacrolimus concentrations and has a relatively low minor allele frequency in whites relative to African Americans (AA).
Methods: We studied clinical and recipient genetic correlates of dose-normalized tacrolimus troughs (n=12,277) in the first 6 months posttransplant using a customized single-nucleotide polymorphism chip with 2722 variants in a large, ethnically diverse (144 AA and 551 non-AA) adult kidney transplant population through a seven-center consortium.
Results: During the 6-month study, AAs had consistently lower median (interquartile range) troughs than non-AAs, 6.2 (4.4-8.4) ng/mL vs. 8.3 (6.4-10.4) ng/mL (P<0.0001), despite 60% higher daily doses, 8 (5-10) mg vs. 5 (4-7) mg (P<0.0001). The median tacrolimus trough concentration in week 1 posttransplant was particularly low in AAs (2.1 [1.2-3.5] ng/mL) compared with non-AAs (5.0 [3.1-8.2] ng/mL) (P<0.0001), despite similar initial doses. In single-variant analysis, CYP3A5*3 (rs776746) was the top variant (P=2.4×10) associated with troughs. After adjustment for CYP3A5*3, clinical factors and race, 35 additional variants were identified (P<0.01, not significant at false discovery rate 20%). In the final multivariant, regression models beginning with these variants and clinical factors, seven variants were identified in the non-AA and seven variants in the AA group towards the first trough concentrations. Rs776746 (CYP3A5), rs2239393 (COMT) and diabetes were the only factors common in both populations.
Conclusion: We identified variants beyond CYP3A5*3, which may further explain pharmacokinetic variability of tacrolimus and demonstrated that important variants differ by race.