Sutro's antibody-drug conjugate impresses in hard-to-treat ovarian cancer

Sutro Biopharma’s lead antibody-drug conjugate beat back ovarian cancer in a small phase 1 study, shrinking tumors in 13% of patients who had tried a median of six other treatments.

The study tested various doses of the treatment, STRO-002, an ADC targeting folate receptor alpha (FolRα), in 39 patients who had received multiple kinds of cancer treatment, including platinum-based chemotherapy, Genentech’s Avastin, PARP inhibitors and checkpoint inhibitors. Patients could join the study regardless of whether their tumors expressed FolRα.

In the the 31 evaluable patients, there were four confirmed responses—three patients saw their tumors shrink while the fourth patient’s tumors were cleared completely, according to a statement.

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Six other patients had their tumor shrink too, but their responses need to be confirmed with repeat measurements. If confirmed, these patients would bring the drug’s response rate to 32%.

The showing was “impressive,” Jefferies analyst Roger Song wrote in a note to clients on Friday, adding that the final, confirmed response rate could tick upwards of 23% and “put ‘002 ahead of close comps w/caveats of cross trial comparison.”

The patients spent a median of 19 weeks on treatment with 58% of patients achieving disease control at 16 weeks, meaning their cancer did not worsen or get better.

This “durable clinical benefit is extremely meaningful” for this patient group, some of whom had tried as many as 11 other treatments and “who do not expect activity from current standard of care,” Song wrote.

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Most of the side effects, 86%, were mild or moderate, with the most common severe side effect being neutropenia, or low levels of white blood cells called neutrophils. About 15% of patients suffered severe joint pain (arthralgia) and 7% had neuropathy, but these effects were resolved with standard treatment, including reducing or delaying doses of the treatment.

Sutro has picked two dose levels, 4.3 mg/kg and 5.2 mg/kg to carry into a dose expansion study. It plans to test the ADC in patients with ovarian cancer who’ve tried fewer lines of treatment than in the current trial, as well as in a group of patients whose tumors express FolRα.

"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, CEO of Sutro Biopharma, in the statement. “Additional data from the dose-expansion will inform regulatory discussions, accelerate registration strategy, and identify the broadest population that may benefit from STRO-002.”