Objectives: Folate receptor alpha (FolRa) is a cell-surface glycoprotein, highly expressed in ovarian and endometrial adenocarcinoma, and is thus a promising target for cancer therapy using antibody drug conjugates (ADCs). Most ADCs currently in development are generated by random attachment of the cytotoxic payload to naturally occurring amino acids within the antibody and result in a heterogeneous mixture. These mixtures are comprised of many different forms that are likely to vary in stability and activity, and therefore may be suboptimal therapeutic agents.

We have employed an E. coli cell-free expression system (Xpress CF+™) and a site-specific conjugation technology, to rapidly optimize the lead ADC by selection of the best antibody, drug-linker, conjugation site and drug-antibody ratio (DAR) to generate STRO-002, a novel, FolRa-targeting ADC.We have conducted preclinical studies to evaluate the stability of STRO-002 and characterize the pharmacological properties of the cytotoxic metabolite SC209. In vitro cytotoxicity assays and in vivo efficacy studies were conducted to evaluate the activity of STRO-002 in multiple ovarian cancer cell lines and xenografts. IND enabling toxicology studies were conducted to determine the safety profiles for STRO-002 and its metabolite SC209 in cynomolgous monkeys and rats, respectively.

Results:Based on optimization studies, the anti-FolRa human IgG1 antibody (SP8166) conjugated to a proprietary cleavable drug-linker (SC239) was selected for the lead ADC STRO-002. SC239 contains a tubulin-targeting 3-aminophenyl hemiasterlin warhead, SC209, which has potent cytotoxic activity and is a weak substrate for efflux pumps. Cytotoxic activity of SP8166 conjugated to SC239 was optimal in ADCs with DAR of ~4 with SC239 incorporated at two positions on each heavy chain, which were selected based on stability and cytotoxic activity.

The drug-linkage in STRO-002 is highly stable and the released warhead, SC209, is a very weak substrate for cellular drug-resistance efflux pumps and is cleared rapidly from plasma. STRO-002 has potent cytotoxic activity (0.1-3 nM) on multipleFolRa-positive ovarian and endometrial cancer cell lines in vitro and anti-tumor activity in ovarian and endometrial xenograft models. STRO-002 exhibits dose-dependent tumor growth inhibition in Igrov-1 tumor xenografts at a single dose and complete regression is achieved in Igrov-1 and OVCAR-3 tumors with a single dose at 10 and 5 mg/kg, respectively. In addition, administration of STRO-002 in combination with carboplatin confers added benefit in efficacy in Igrov-1 tumors. Toxicology studies show favorable safety profiles for STRO-002 and SC209. The main toxicity finding in monkeys dosed up to 9 mg/kg consists of reversible hematopoietic/lymphoid tissue toxicity, which is considered antigen-independent and is consistent with the anti-proliferative effects of SC209 observed in single-dose toxicology studies in rats. No evidence of ocular toxicity due to SC209 were observed in either species.

Conclusions: STRO-002 is an ADC with minimal drug moiety release in circulation and the potential for an improved safety and activity profile, and a reduced risk of tumor drug resistance. Our data supports the advancement of STRO-002 to the clinic as a potential treatment of FolRa expressing malignancies such as ovarian and endometrial cancer.