Preclinical Evaluation of STI-8811, a Novel Antibody Drug Conjugate Targeting BCMA for the Treatment of Multiple Myeloma

Abstract Treatment for patients with Multiple Myeloma (MM) has experienced rapid development and improvement in recent years, however, patients continue to experience relapse and MM remains largely incurable. B cell maturation antigen (BCMA) has been widely recognized as a promising target for treatment of MM due to its exclusive expression in B cell linage cells and its critical role in the growth and survival of malignant plasma cells. Here, we introduce STI-8811, a BCMA-targeting antibody drug conjugate linked to an auristatin-derived duostatin payload via an enzymatically cleavable peptide linker, using our proprietary C-lock technology. STI-8811 exhibits target specific binding activity and rapid internalization, leading to G2/M cell cycle arrest, caspase 3/7 activation and apoptosis in BCMA-expressing tumor cells in vitro. Soluble BCMA (sBCMA) is shed by MM cells into the blood and increases with disease progression, competing for ADC binding and reducing its efficacy. We report enhanced cytotoxic activity in the presence of high levels of sBCMA compared to a belantamab mafodotin biosimilar (J6M0-mcMMAF). STI-8811 demonstrated greater in vivo activity than J6M0-mcMMAF in solid and disseminated multiple myeloma models, including tumor models with low BCMA expression and/or in large solid tumors representing soft tissue plasmacytomas. In Cynomolgus monkeys, STI-8811 was well tolerated, with toxicities consistent with other BCMA targeting ADCs with auristatin payloads in clinical studies. STI-8811 has the potential to outperform current clinical candidates with lower toxicity and higher activity under conditions found in patients with advanced disease.