Coproporphyrin‐I (CP‐I) in plasma is a sensitive and specific endogenous probe for phenotyping organic anion transporting polypeptides 1B (OATP1B, encoded by SLCO1B). A few small‐scale studies suggested that plasma CP‐I concentration is affected by OATP1B1 polymorphism, but detailed studies are lacking. In this large‐scale study, we measured plasma CP‐I concentrations in 391 subjects from the Japanese general population, and evaluated the relationship between plasma CP‐I concentrations and OATP1B1 polymorphisms to further assess the utility of plasma CP‐I concentrations as an endogenous OATP1B probe. Plasma CP‐I concentrations were 0.45 ± 0.12, 0.47 ± 0.16, 0.47 ± 0.20, 0.50 ± 0.15, 0.54 ± 0.14, and 0.74 ± 0.31 ng/mL in participants with OATP1B1*1b/*1b (n = 103), *1a/*1b (n = 122), *1a/*1a (n = 40), *1b/*15 (n = 74), *1a/*15 (n = 41), and *15/*15 (n = 11), respectively, showing an ascending rank order with significant difference (P < 0.0001). Post hoc analysis revealed significant increases in plasma CP‐I concentration in OATP1B1*1b/*15 (P = 0.036), *1a/*15 (P = 0.0005), and *15/*15 (P = 0.0003) groups compared with the OATP1B1*1b/*1b group. There was no significant difference among OATP1B genotypes in plasma concentration of 3‐carboxy‐4‐methyl‐5‐propyl‐2‐furanpropanoic acid, a uremic toxin reported to decrease OATP1B activity in vivo. These findings confirm the utility of plasma CP‐I concentrations as an endogenous biomarker for phenotyping of OATP1B activity. Plasma CP‐I concentration is potentially useful for the study of drug‐drug interactions via OATP1B or individual dose adjustment of OATP1B substrates.

Aims: Chlorzoxazone is the paradigm marker substrate for CYP2E1 phenotyping in vivo. Because at the commonly used milligram doses (250–750 mg) chlorzoxazone acts as an inhibitor of the CYP3A4/5 marker substrate midazolam, previous attempts failed to combine both drugs in a common phenotyping cocktail. Microdosing chlorzoxazone could circumvent this problem.Method: We enrolled 12 healthy volunteers in a trial investigating the dose–exposure relationship of single ascending chlorzoxazone oral doses over a 10,000‐fold range (0.05–500 mg) and assessed the effect of 0.1 and 500 mg of chlorzoxazone on oral midazolam pharmacokinetics (0.003 mg).Results: Chlorzoxazone area under the concentration–time curve was dose‐linear in the dose range between 0.05 and 5 mg. A nonlinear increase occurred with doses ≥50 mg, probably due to saturated presystemic metabolic elimination. While midazolam area under the concentration–time curve increased 2‐fold when coadministered with 500 mg of chlorzoxazone, there was no pharmacokinetic interaction between chlorzoxazone and midazolam microdoses.Conclusion: The chlorzoxazone microdose did not interact with the CYP3A marker substrate midazolam, enabling the simultaneous administration in a phenotyping cocktail. This microdose assay is now ready to be further validated and tested as a phenotyping procedure assessing the impact of induction and inhibition of CYP2E1 on chlorzoxazone microdose pharmacokinetics.