Sutro Biopharma has provided a clinical update from the company's ongoing dose-escalation Phase 1 study of STRO-002, a folate receptor alpha (FolRα) targeting antibody-drug conjugate (ADC), for patients with ovarian cancer.

STRO-002-GM1 is a single-arm monotherapy dose-escalation study for patients with ovarian cancer not selected based on their FolRα-expression levels. The dose-escalation portion of the study was fully enrolled with 39 patients in August 2020. Patients were heavily pre-treated and had a median of 6 prior lines of therapy, including standard of care platinum-based regimens, bevacizumab, PARP inhibitors, and checkpoint inhibitors.

The dose-escalation study included 34 patients treated with clinically active dose levels, 2.9 mg/kg or higher, of which 31 patients had post-baseline scans and were evaluable for RECIST responses. At the data cutoff of October 30, 2020, median time on treatment was 19 weeks and 10 patients remained on treatment. Results out of 31 evaluable patients included:

• 10 patients met RECIST criteria for response. Of which, 1 patient achieved a complete response (CR) and 9 patients achieved a partial response (PR). Of the PRs, 3 were confirmed PRs (cPRs) and 6 unconfirmed PRs (uPRs)

• 23 patients (74%) achieved disease control at 12 weeks

• 18 patients (58%) achieved disease control at 16 weeks

• 4 patients (13%) were on treatment for 52 weeks. 3 patients remained on treatment beyond 64 weeks

STRO-002 continues to be well-tolerated and 86% of all treatment-emergent adverse events (AEs) were Grade 1 or 2. Of note, prophylactic corticosteroid eye drops have not been required and no ocular toxicity signals have been observed. The most common Grade 3 and 4 AEs were reversible neutropenia. Grade 3 arthralgia (15.4%) and neuropathy (7.7%) were observed and managed with standard medical treatment, including dose reductions or delays without evidence of compromised efficacy.

"We are encouraged to see meaningful clinical benefit from STRO-002 for patients with advanced platinum-resistant and refractory ovarian cancer. The women on the study are heavily pretreated and have limited treatment options as many have received experimental agents and participated in other clinical trials," said Dr. Lainie P. Martin, Leader of Gynecology/Oncology Program at Hospital of the University of Pennsylvania and an investigator on the STRO-002 study. "The deepening of responses in patients as well as disease control over time demonstrates STRO-002 to be an important potential treatment option for patients with ovarian cancer."

"We are seeing improved outcomes in disease control and RECIST responses as the data matures and will continue to follow the patients who remain on study for further deepening of responses or clinical benefit," said Dr. Arturo Molina, Chief Medical Officer of Sutro Biopharma. "The broad therapeutic index of STRO-002 should allow for long-term dosing and dose intensity. Although a maximum tolerated dose was not reached, we have identified dose levels of 4.3 and 5.2 mg/kg that we plan to randomize in the dose-expansion. We plan to dose the first patient January 2021 and will be treating less heavily pre-treated ovarian cancer patients. An expansion cohort for FolRα-selected endometrial cancer is planned for next year."

"We are rapidly moving forward with further development of STRO-002, the FolRα-targeted ADC program. Based on emerging IHC data from our dose-escalation, we have seen responses and stable disease at various FolRα-expression levels. For dose-expansion, we will be collecting required tissue samples at enrollment and using an established assay to determine if a FolRα-selection enrichment strategy is needed," said Bill Newell, Chief Executive Officer of Sutro Biopharma. "Additional data from the dose-expansion will inform regulatory discussions, accelerate registration strategy, and identify the broadest population that may benefit from STRO-002."