A drug-to-metabolite ratio-based quantitative analytical approach for evaluation of combination therapy through impact assessment of brigatinib on the pharmacokinetics of tazemetostat and its primary metabolite.

Coadministration of multiple drugs may significantly alter each other's therapeutic profiles by altering their in vivo drug metabolism and pharmacokinetics. Quantitative profiling of metabolites along with the parent drug may be critical for establishing the potential of combination therapy. In this study, a drug-to-metabolite ratio-based quantitative liquid chromatography-tandem mass spectrometry analytical method has been developed and validated for the simultaneous quantification of tazemetostat (TAZ) and its N-deethylated metabolite in rat plasma. The N-deethylated metabolite was generated in-house through controlled photoacidic degradation of TAZ, followed by structural confirmation using liquid chromatography-tandem mass spectrometry and NMR spectroscopy. The method exhibited excellent linearity, precision, and accuracy over a 1-1000 ng/mL range and was successfully applied to investigate the impact of repeated dosing of brigatinib (BRT) on the pharmacokinetics of TAZ and its N-deethylated metabolite in rats. The study design involved oral coadministration of BRT and TAZ on different dosing regimens. Pharmacokinetic results demonstrated a significant reduction in TAZ exposure following repeated BRT treatment, as reflected by lower AUC0-24, AUC0-∞, and Cmax values. In contrast, systemic exposure of N-deethylated TAZ increased notably on day 7. The overall drug-to-metabolite ratio decreased by approximately 51%, with a geometric mean ratio of 0.49 (95% CI, 0.30-0.79; P = .024). These results indicate that repeated BRT administration substantially alters the pharmacokinetic behavior of TAZ, possibly through modulation of metabolic or transporter pathways. The analytical method and drug-to-metabolite ratio-based approach could provide a reliable framework for assessing potential interactions during preclinical pharmacokinetic evaluations of such combinations. SIGNIFICANT STATEMENT: In this study, a drug-to-metabolite ratio-based quantitative liquid chromatography-tandem mass spectrometry method has been established for simultaneous quantification of tazemetostat and its N-deethylated metabolite in rat plasma. This bioanalytical method combined with a drug-to-metabolite ratio-based framework may provide a reliable approach for assessing potential drug-drug interactions during preclinical pharmacokinetic evaluations.