Extensive Biotransformation Profiling of AZD8205, an Anti-B7-H4 Antibody-Drug Conjugate, Elucidates Pathways Underlying Its Stability <i>In Vivo</i>

What happens to macromolecules <i>in vivo</i>? What drives the structure-activity relationship and <i>in vivo</i> stability for antibody-drug conjugates (ADCs)? These interrelated questions are increasingly relevant due to the re-emerging importance of ADCs as an impactful therapeutic modality and the gaps that exist in our understanding of ADC structural determinants that underlie ADC <i>in vivo</i> stability. Complex macromolecules, such as ADCs, may undergo changes <i>in vivo</i> due to their intricate structure as biotransformations may occur on the linker, the payload, and/or at the modified conjugation site. Furthermore, the dissection of ADC metabolism presents a substantial analytical challenge due to the difficulty in the identification or quantification of minor changes on a large macromolecule. We employed immunocapture-LCMS methods to evaluate <i>in vivo</i> changes in the drug-antibody ratio (DAR) profile in four different lead ADCs. This comprehensive characterization revealed that a critical structural determinant contributing to the ADC design was the linker, and competition of the thio-succinimide hydrolysis reaction over retro-Michael deconjugation can result in superb conjugation stability in vivo. These data, in conjunction with additional factors, informed the selection of AZD8205, puxitatug samrotecan, a B7-H4-directed cysteine-conjugated ADC bearing a novel topoisomerase I inhibitor payload, with durable DAR, currently being studied in the clinic for the potential treatment of solid malignancies (NCT05123482). These results highlight the relevance of studying macromolecule biotransformation and elucidating the ADC structure-<i>in vivo</i> stability relationship. The comprehensive nature of this work increases our confidence in the understanding of these processes. We hope this analytical approach can inform future development of bioconjugate drug candidates.