Preclinical characterization of STRO-227: A PTK7-targeting dual-payload ADC with topoisomerase 1 and tubulin inhibitors

PTK7 is a catalytically inactive, cell surface protein tyrosine kinase and a marker of tumor-initiating cancer stem cells. It plays a key role in tumor biology by regulating pathways involved in cell migration, invasion, and metastasis, and is upregulated in multiple solid tumors, including ovarian, triple-negative breast, non-small cell lung, esophageal, and head and neck cancers. Its combination of tumor-restricted expression and functional relevance in cancer stem cells makes PTK7 an attractive target for antibody-drug conjugates. We present the preclinical characterization of STRO-227, an investigational PTK7-targeting antibody-drug conjugate (ADC) generated using Sutro's XpressCF+® cell-free protein synthesis platform, which enables the site-selective incorporation of non-natural amino acids. This platform allows precise conjugation of two distinct payloads: exatecan, a topoisomerase I inhibitor, and monomethyl auristatin E (MMAE), a tubulin inhibitor. Each of these potent payloads is attached via a hydrophilic beta-glucuronidase-cleavable linker. The final ADC possesses a defined drug-to-antibody ratio (DAR) of 8 exatecan to 2 MMAE payloads. In preclinical studies, STRO-227 demonstrated favorable pharmacokinetics and in vivo stability. In multiple xenograft and patient-derived xenograft (PDX) models, STRO-227 exhibited superior anti-tumor activity compared to the corresponding single-payload ADCs. In addition, STRO-227 demonstrated a favorable safety and pharmacokinetic profile in non-human primates, with a highest non-severely toxic dose (HNSTD) comparable to established MMAE- and topoisomerase I-based single-payload ADC benchmarks.