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.