Identification and Clinical Evaluation of Potential Biomarkers for Breast Cancer Resistance Protein (<scp>BCRP</scp>/<scp><i>ABCG2</i></scp>)

Clinical inhibition and genetic variation of the Breast Cancer Resistance Protein (BCRP/ABCG2) efflux transporter can significantly influence drug exposure, highlighting the need for reliable BCRP functional biomarkers. This study aimed to identify and evaluate biomarkers predictive of BCRP function in humans. A comprehensive analysis of metabolomic genome-wide association studies (mGWAS) was conducted to discover potential BCRP biomarkers, followed by evaluation in in vitro transporter assays and a clinical drug-drug interaction (DDI) study. Across multiple mGWAS datasets, plasma concentrations of three herbicide derivatives-4-hydroxychlorothalonil (4HC), 3-bromo-5-chloro-2,6-dihydroxybenzoic acid (BCDBA), and 3,5-dichloro-2,6-dihydroxybenzoic acid (DCDBA)-were significantly elevated (P < 5E-8) in individuals carrying reduced function ABCG2 polymorphisms. These compounds were confirmed as novel BCRP substrates via transporter uptake assays and selected for clinical evaluation alongside riboflavin, a known BCRP substrate and potential BCRP biomarker. In a DDI study with 11 healthy subjects, eltrombopag, a BCRP inhibitor, increased rosuvastatin concentrations by approximately twofold (P = 0.002). No significant changes in the plasma concentrations of organic anion transporting polypeptide 1B (OATP1B) biomarkers (CP-I and CP-III) or potential BCRP biomarkers (4HC, BCDBA, DCDBA, or riboflavin) were observed. Notably, two subjects were heterozygous carriers for the ABCG2 p.Q141K variant and exhibited significantly higher baseline concentrations of 4HC (P = 0.004) and BCDBA (P = 0.0003), consistent with reduced BCRP function. These findings suggest that 4HC and BCDBA are promising biomarkers for baseline BCRP function in specific populations, such as those harboring reduced function genetic polymorphisms, but do not appear suitable for detecting acute BCRP inhibition.