Sutro Biopharma, Inc. provided a clinical update from the Company's ongoing, fully enrolled, dose-expansion Phase 1 study of STRO-002, a folate receptor alpha (FolRa)-targeting antibody-drug conjugate (ADC), for patients with advanced ovarian cancer. Discussion of these data will be held at a STRO-002 Virtual Event at 2 pm PT/5 pm ET today.  The dose-expansion cohort for ovarian cancer enrolled 44 patients who had experienced up to three prior lines of therapy. As of the interim data cutoff date of Nov.

8, 2021, 43 patients had been randomized into dose levels starting at 4.3 mg/kg and 5.2 mg/kg, and one patient had not yet been dosed. 81% of the patients were platinum-resistant, and 63% and 65% of the patients had been treated previously with bevacizumab and PARP inhibitors, respectively. Of the 43 patients, 33 had at least one post-baseline scan and, therefore, were evaluable for RECIST v1.1 responses.

As of the Nov. 8, 2021, the interim data cutoff date, the Best Overall Response (BOR) for evaluable patients were as follows (N=33): Seven patients had achieved partial responses (PR), which were confirmed with at least two post-baseline scans. Five patients had unconfirmed partial responses (PRu), based on having received only one post-baseline scan as of the interim data cutoff date.

Their next scheduled scan, subsequent to the interim data cutoff date, revealed the following: Four PRs were confirmed and one patient was in stable disease (SD). An ORR of 33% (11 PRs out of 33 patients) was demonstrated in all evaluable patients, unenriched for FolRa-expression levels at both dose levels. 14 total patients experienced SD, and 8 patients had progressive disease (PD).

Dose response was observed, with an ORR of 47% (8 PRs out of 17 patients), for patients who started at the 5.2 mg/kg dose level, unenriched for biomarker status. Higher FolRa expression levels using TPS are correlated with higher response rates. TPS has been identified as a potentially appropriate scoring algorithm for STRO-002 with respect to the biomarker enrichment strategy.

Based on an exploratory cut-off of TPS > 25%, a 40% ORR (10 PRs out of 25 patients) was observed. TPS = 25% demonstrates 13% ORR. Based on our data, an enrichment approach of TPS > 25% FolRa expression may enable treatment of about 70% of the advanced ovarian cancer patient population.

Safety signals from the 43 safety evaluable patients at the 5.2 mg/kg and 4.3 mg/kg starting dose levels were consistent with data from the dose-escalation cohort. No new safety signals were observed in the dose-expansion cohort, including the absence of keratopathy. 85.5% treatment-emergent adverse events (TEAEs) were Grade 1-2. Neutropenia was the leading TEAE, resulting in treatment delay or dose reduction.

The majority of the cases of neutropenia were generally asymptomatic and resolved with a one week dose delay or, in other cases, with standard medical treatment, including the use of G-CSF. There were limited observed cases of febrile neutropenia, including one Grade 5 event at the 5.2 mg/kg starting dose level and one Grade 3 event at the 4.3 mg/kg starting dose level. The trial protocol was subsequently updated to require dose reduction for Grade 4 neutropenia.

Data from the STRO-002 dose-expansion cohort are expected to provide further information to inform regulatory discussions and a global registration strategy.