STRO-002, an investigational antibody-drug conjugate, continues to show promising safety and anti-tumor activity in women with advanced ovarian cancer, according to preliminary data from a Phase 1 clinical trial.

The findings were presented by Wendel Naumann, MD, principal investigator of the trial, in a recent virtual presentation at the American Association for Cancer Research (AACR) Virtual Annual Meeting.

Sutro Biopharma’s STRO-002 contains an antibody that specifically binds to folate receptor alpha (FRα), a protein found in high levels in most types of ovarian cancer. This prompts the release of a toxic compound to kill tumor cells, while sparing healthy cells.

The safety, tolerability, and preliminary efficacy of STRO-002 are being evaluated in an open-label Phase 1 trial (NCT03748186) in heavily pre-treated women with recurrent platinum-resistant or refractory ovarian cancer (including fallopian and primary peritoneal cancer) and endometrial cancer.

Currently recruiting patients in the U.S., the trial intends to enroll about 120 participants.

The study is divided into two parts: an initial dose-escalation part, in which women will receive increasing doses of STRO-002 to determine the therapy’s optimal dose; and a dose expansion part, in which participants will be treated with the recommended dose. This second phase is planned to start in the second half of this year.

While the dose-escalation part of the trial will include only patients with ovarian, fallopian, or primary peritoneal cancer, the dose expansion phase will include an additional group of women with endometrial cancer.

Earlier data from this study showed that STRO-002 was generally safe and well-tolerated. One woman receiving the experimental treatment at a dose of 2.9 mg/kg had achieved a partial response (partial cancer eradication), and two others a state of confirmed stable disease.

As of April 20, a total of 30 women with advanced ovarian cancer had enrolled in the study, and received intravenous infusions (directly into the bloodstream) of STRO-002 in 21-day cycles, at doses ranging from 0.5 mg/kg to 6.4 mg/kg.

Updated findings have shown that 15 ovarian cancer patients (75%) receiving STRO-002 at a dose of 2.9 mg/kg or higher responded to treatment. Respondents included one woman achieving a partial response, and 14 attaining disease stabilization.

“This level of tumor control is typically very difficult to achieve in these patients who have been heavily pre-treated, with a median of five prior lines of other therapies, and who have such advanced disease,” Bill Newell, Sutro’s CEO, said in a press release.

Findings also revealed that 13 women had a reduction greater than 50% or normalization of the levels of CA-125, a cancer biomarker. Apart from one patient who was not yet eligible for treatment response assessments, the remaining 12 within this subgroup achieved either partial response or stable disease.

Treatment was safe and well-tolerated, with 89% of the adverse events (side effects) reported so far being mild (grade 1) or moderate (grade 2) in severity.

Severe (grade 3) adverse events included neutropenia (low white blood cell counts), diarrhea, joint pain, peripheral neuropathy (nerve damage), and myalgia (muscle pain). Only one life-threatening (grade 4) adverse event (neutropenia) was reported, and all cases of neutropenia were resolved within one week.

“The preliminary evidence of anti-tumor activity we observed is encouraging, particularly in this heavily pre-treated patient population,” said Naumann, who is also a gynecologic oncologist at Levine Cancer Insitute. “With limited therapeutic options for these patients, we are excited to continue to advance this clinical program to further investigate its therapeutic potential.”

“It is extremely encouraging that we see this preliminary evidence of anti-tumor activity at this stage of development,” said Arturo Molina, MD, chief medical officer of Sutro.

“As we advance STRO-002 in the clinic, we plan to share additional data on the efficacy and safety of STRO-002 by the end of 2020 and we look forward to the potential to bring a new treatment option to ovarian cancer patients,” Molina added.

The maximum tolerated dose of STRO-002 has not been reached. The company is planning to continue exploring the effects of the highest doses of the medication (5.2 to 6.0 mg/kg) while seeking to determine the best dose to be used in the study’s second part.