Aims: An in vitro study has suggested that risperidone is a substrate of P-glycoprotein, which is coded by MDR-1 gene. Thus, we studied the effects of major polymorphisms of the MDR-1 gene on plasma drug concentrations.
Methods: Subjects were 85 schizophrenic patients receiving 3 mg twice daily of risperidone. Sample collections were conducted 12 h after the bedtime dosing. Plasma concentrations of risperidone and 9-hydroxyrisperidone were quantified using LC/MS/MS. MDR-1 genotypes (C3435T and G2677T/A) and CYP2D6 genotypes were identified using PCR-RFLP methods.
Results: There was no difference in geometric mean (95% CI) of steady-state plasma concentration of risperidone between C3435T genotypes [C/C, C/T, T/T; 2.06 (1.63, 6.47), 2.96 (3.10, 7.91), 2.28 (1.81, 8.04) ng ml(-1), P = 0.759] or G2677T/A genotypes [G/G, G/T or A, T or A/T or A; 1.62 (0.08, 6.07), 2.64 (3.25, 7.10), 2.71 (2.77, 8.72) ng ml(-1), P = 0.625] or 9-hydroxyrisperidone between C3435T genotypes [38.3 (33.7, 50.1), 34.9 (32.9, 42.0), 35.7 (31.7, 42.3) ng ml(-1), P = 0.715] or G2677T/A genotypes [40.6 (33.0, 51.8), 35.0 (33.3, 42.4), 36.1 (32.8, 47.2) ng ml(-1), P = 0.601]. Multiple regression analyses including CYP2D6 genotypes, sex, and age revealed that steady-state plasma concentration of risperidone correlated with the number of mutated alleles for CYP2D6 (standardized partial correlation coefficients (beta) = 0.540, P < 0.001) and those of 9-hydroxyrisperidone (standardized beta = 0.244, P = 0.038) and active moiety (standardized beta = 0.257, P = 0.027) correlated with age.
Conclusions: These findings suggest that the MDR-1 variants are not associated with steady-state plasma concentration of risperidone or 9-hydroxyrisperidone, but CYP2D6 genotypes and age are determinants of these concentrations.