Effect of ABCC2 (MRP2) transport function on erythromycin metabolism.

The macrolide antiobiotic erythromycin undergoes extensive hepatic metabolism and is commonly used as a probe for cytochrome P450 (CYP) 3A4 activity. By means of a transporter screen, erythromycin was identified as a substrate for the transporter ABCC2 (MRP2) and its murine ortholog, Abcc2. Because these proteins are highly expressed on the biliary surface of hepatocytes, we hypothesized that impaired Abcc2 function may influence the rate of hepatobiliary excretion and thereby enhance erythromycin metabolism. Using Abcc2 knockout mice, we found that Abcc2 deficiency was associated with a significant increase in erythromycin metabolism, whereas murine Cyp3a protein expression and microsomal Cyp3a activity were not affected. Next, in a cohort of 108 human subjects, we observed that homozygosity for a common reduced-function variant in ABCC2 (rs717620) was also linked to an increase in erythromycin metabolism but was not correlated with the clearance of midazolam. These results suggest that impaired ABCC2 function can alter erythromycin metabolism, independent of changes in CYP3A4 activity.

Environmental and genetic factors affecting transport of imatinib by OATP1A2.

The bioavailability of orally administered imatinib is >90%, although the drug is monocationic under the acidic conditions in the duodenum. In vitro, we found that imatinib is transported by the intestinal uptake carrier organic anion transporting polypeptide (OATP1A2) and that this process is sensitive to pH, rosuvastatin, and genetic variants. However, in a study in patients with cancer, imatinib absorption was not associated with OATP1A2 variants and was unaffected by rosuvastatin. These findings highlight the importance of verifying in a clinical setting the drug-transporter interactions observed in in vitro tests.

Pharmacogenetics of drug transporters.

During the last decade, a greater focus has been given to impact of genetic variation in membrane transporters on the pharmacokinetics and toxicity of numerous therapeutic drugs. While the majority of transporter-related pharmacogenetic research has been in regards to classic genes encoding the outward-directed ATP-binding cassette (ABC) transporters, such as ABCB1 (P-glycoprotein), ABCC2 (MRP2), and ABCG2 (BCRP), more studies have been conducted in recent years evaluating genes encoding solute carriers (SLC) that mediate the cellular uptake of drugs, such as SLCO1B1 (OATP1B1) and SLC22A1 (OCT1). The distribution of ABC and SLC transporters in tissues key to pharmacokinetics, such as intestine (absorption), blood-brain-barrier (distribution), liver (metabolism), and kidneys (excretion), strongly suggests that genetic variation associated with changes in protein expression or function of these transporters may have a substantial impact on systemic drug exposure and toxicity. In this current article, we will review recent advances in understanding the contribution of critical ABC and SLC transporters to interindividual pharmacokinetic and dynamic variability of substrate drugs.

Pharmacogenetic pathway analysis of docetaxel elimination.

The purpose of this study was to evaluate the affinity of docetaxel for 14 transporter proteins and assess the functional significance of 17 variants in five genes involved in drug elimination. Among the transfected models investigated, OATP1B3 (SLCO1B3) was identified as the most efficient influx transporter for docetaxel. None of the observed genotypes (SLCO1B3, ABCB1, and ABCC2) was related with docetaxel clearance in 92 white patients (P > 0.17). However, the simultaneous presence of the CYP3A4*1B and CYP3A5*1A alleles was associated with a 64% increase in docetaxel clearance (P = 0.0015), independent of both sex and CYP3A activity (as determined using the erythromycin breath test). This haplotype was also associated with increased midazolam clearance in another population (P = 0.0198). An analysis of the CYP3A locus among CEPH-HapMap samples revealed that CYP3A4*1B is present exclusively among a subset of CYP3A5 expressors. Therefore, future studies should first stratify the population on the basis of CYP3A5 genotype and then compare CYP3A activity between individuals with and without the CYP3A4*1B allele.

Influence of solute carriers on the pharmacokinetics of CYP3A4 probes.

We hypothesized that the assessment of baseline CYP3A4 activity is influenced by probe-specific differences in hepatocellular uptake mechanisms. There was no significant correlation between the erythromycin breath test (ERMBT) parameters and midazolam clearance in 30 cancer patients (R(2) < 0.01), regardless of their CYP3A5 genotype status. In cellular models overexpressing 10 different solute carriers, erythromycin uptake was significantly increased by OATP1A2 (P < 0.005) and OATP1B3 (P < 0.01). Midazolam was not a substrate for any of the tested transporters. In a separate cohort of 119 patients, 6 nonsynonymous variants in the OATP1B3 gene SLCO1B3 were identified. Individuals carrying two copies of the T allele at the 334 locus had a 2.4-fold lower value for ERMBT 1/T(max) (P = 0.001), a measure reflecting more rapid hepatic uptake. These findings suggest that differential affinities for solute carriers should be considered when selecting an appropriate phenotypic probe to allow tailored dosing of pharmaceuticals that are CYP3A4 substrates.