Design characteristics of mass balance studies conducted for protein kinase inhibitors

Protein kinase inhibitor (PKI) therapy increasingly relies on personalized approaches such as therapeutic drug monitoring and pharmacogenetic testing. Understanding PKI metabolism is essential for these methods to be effective, notably for determining the substance to be quantified and the enzyme for which the genotypic metabolizer status needs to be predicted. For most drugs, however, human in vivo metabolite profiles are derived from the mass balance studies conducted by drug developers, typically in small and homogeneous populations.

We studied mass balance study characteristics for PKIs, assessing number of participants, sex, and health state, as well as dosing regimen, study location, study duration, and consideration of genetic variation in metabolic enzymes and transporters. Data were primarily extracted from European Public Assessment Reports, supplemented with information from the Drugs@FDA database, ClinicalTrials.gov, and peer-reviewed mass balance study publications.

Mass balance studies were conducted for 68 out of the 69 PKIs listed as "authorized" human drugs in the European Medicines Agency database on July 7, 2025 (updated April 18, 2026; +3 PKIs). The studies included 2-12 participants (median = 6), with 90% enrolling only healthy volunteers, 88% enrolling only males, 97% administering a single dose, and four studies mentioning consideration of genetic variation in metabolic enzymes during participant inclusion.

PKI mass balance studies exhibit similar, rather homogeneous design characteristics, potentially limiting the generalizability of their insights into drug metabolism across diverse patient populations. Still, most align with recent FDA guidelines on mass balance studies, despite being issued after study conduct.