P-glycoprotein (PGP) is a membrane protein which determines drug disposition in humans (e.g. digoxin). It is also expressed in various leukocyte lineages with highest expression in CD56+ natural killer cells. Recently, a polymorphism in exon 26 (C3435T) of this gene was shown to correlate with intestinal PGP expression and function in humans. Carriers homozygous for this polymorphism (TT) showed more than two-fold lower PGP expression and higher digoxin plasma concentrations compared to the CC group. However, it is not known whether this mutation in the MDR1 gene is also associated with altered PGP function in peripheral blood cells. We therefore assessed efflux of the PGP-substrate rhodamine 123 from CD56+ natural killer cells. Leukocytes were isolated from whole blood of 10 CC, 10 CT and 11 TT healthy Caucasian individuals. Using flow cytometry, rhodamine fluorescence was determined in CD56+ cells. Moreover, MDRI mRNA was quantified in leukocytes by real-time polymerase chain reaction. Subjects with CC genotype revealed a significantly lower rhodamine fluorescence (i.e. higher PGP function) compared to individuals with TT genotype (51.1 +/- 11.4% versus 67.5 +/- 9.5%, p < 0.01). Heterozygous individuals had an intermediate rhodamine fluorescence (61.4 +/- 6.3%). MDR1 mRNA normalized for cyclophilin was lowest in the TT population (1.29 +/- 1.01), intermediate in heterozygous subjects (1.60 +/- 0.76) and highest in the CC group (1.91 +/- 0.94; not significant). In summary, subjects being homozygous for C in position 3435 of the MDR1 gene have a more pronounced efflux of rhodamine from CD56+ natural killer cells and a higher MDR1 mRNA expression in leukocytes than subjects with the TT genotype. Measurement of rhodamine efflux using flow-cytometry from peripheral blood cells allows assessment of genetically determined differences in P-glycoprotein function.

Aims The C3435T polymorphism in the human MDR1 gene is associated with lower intestinal P-glycoprotein expression, reduced protein function in peripheral blood cells and higher plasma concentrations of the P-glycoprotein substrate digoxin. Using fexofenadine, a known P-glycoprotein substrate, the hypothesis was tested whether this polymorphism also affects the disposition of other drugs in humans. Methods Ten Caucasian subjects homozygous for the wild-type allele at position 3435 (CC) and 10 individuals homozygous for T at position 3435 participated in this study. A single oral dose of 180 mg fexofenadine HCl was administered. Plasma and urine concentrations of fexofenadine were measured up to 72 h using a sensitive LC/MS method. In addition, P-glycoprotein function was assessed using efflux of the P-glycoprotein substrate rhodamine 123 from CD56 + cells.Results Fexofenadine plasma concentrations varied considerably among the study population. However, fexofenadine disposition was not significantly different between the CC and TT groups (e.g. AUC(0,?) CC vs TT: 3567.1t1535.5 vs 3910.1t1894.8 ng ml x1 h, NS; 95% CI on the difference x1364.9, 2050.9).In contrast, P-glycoprotein function was significantly decreased in CD56 + cells of the TT compared with the CC group (rhodamine fluorescence CC vs TT: 45.6t7.2% vs 61.1t12.3%, P<0.05; 95% CI on the difference 5.6, 25.5).Conclusions In spite of MDR1 genotype-dependent differences in P-glycoprotein function in peripheral blood cells, there was no association of the C3435T polymorphism with the disposition of the P-glycoprotein substrate fexofenadine in this German Caucasian study population. These data indicate that other mechanisms including uptake transporter function are likely to play a role in fexofenadine disposition.

Using segmental intestinal perfusion, we provide direct evidence that intestinal P-glycoprotein mediates substantial drug elimination after intravenous administration from the systemic circulation into the gut lumen and prevents entry of luminally administered P-glycoprotein substrates into the enterocytes. These data also highlight the relative importance of direct intestinal drug secretion in comparison with drug elimination through bile.

St John's wort (SJW) is known to induce cytochrome P450 (CYP) 3A4 and P-glycoprotein through pregnane X-receptor activation. Our study evaluated the effects of long-term SJW administration on oral and intravenous pharmacokinetics of the nonmetabolized in vivo probe of P-glycoprotein, talinolol, in relation to intestinal P-glycoprotein expression. In a controlled, randomized study (N=9), the pharmacokinetics of oral (50 mg) and intravenous talinolol (30 mg) was determined before and after 12 days SJW (900 mg daily, Jarsin 300). Duodenal biopsies were taken and MDR1 genotypes assessed. SJW reduced the oral talinolol bioavailability by 25% (P=0.049) compared with water control. A 93% increase in oral clearance (P=0.177) and a 31% reduction in area under the serum concentration time curve (AUC; P=0.030) were observed. Renal and nonrenal clearance (CLNR), elimination half-life, peak serum drug concentration (Cmax), and time to reach Cmax were not significantly altered. After intravenous talinolol, SJW affected only CLNR (35% increase compared with water, P=0.006). SJW increased MDR1 messenger ribonucleic acid (mRNA) as well as P-glycoprotein levels in the duodenal mucosa. Subjects with the combined MDR1 genotype comprising 1236C>T, 2677G>T/A, and 3435C>T polymorphisms had lower intestinal MDR1 mRNA levels and displayed an attenuated inductive response to SJW as assessed by talinolol disposition. Long-term SJW decreased talinolol AUC with a corresponding increase in intestinal MDR1 expression, suggesting that SJW has a major inductive effect on intestinal P-glycoprotein. Interestingly, the magnitude of induction appeared to be affected by MDR1 genotype.

Multiple genetic polymorphisms of MRP3 exist in Caucasians. The -211C>T promoter polymorphism appears to be associated with altered hepatic MRP3 mRNA expression.