MDR1 genotypes do not influence the absorption of a single oral dose of 1 mg digoxin in healthy white males

Aims A noncoding single nucleotide polymorphism (SNP) in exon 26 3435C > T of the highly polymorphic MDR1 gene has been demonstrated to alter digoxin absorption after induction of the MDR1 gene product P-glycoprotein by rifampicin or after multiple oral dosing. The aim of the study was to investigate the effects of the major known MDR1 SNPs on the absorption of digoxin after a single oral dose in a large sample without drug pretreatment. Methods Fifty healthy white male subjects between the age of 18 and 40 years were enrolled. Following an overnight fast, all subjects received a single oral dose of 1 mg digoxin. Venous blood samples were taken at intervals up to 4 h post dose to obtain a pharmacokinetic profile. Results AUC(0,4 h), C (max) and t (max) , used as indices of digoxin absorption, were not significantly different in any of the genotype groups tested. In particular, there was no significant difference between homozygous carriers of the C and T allele in exon 26 3435 (AUC(0,4 h) 9.24 and 9.38 mug l(-1) h, C (max) 4.73 and 3.81 mug l(-1) , t (max) 0,83 and 01.14 h). Conclusions This lack of effect of the major MDR1 SNPs on digoxin absorption might be explained by saturation of the maximum transport capacity of intestinal Pgp at the dose used.