BioXcel Spinout Posts Promising Results in Rare Prostate Cancer


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BioXcel Therapeutics’ foray into oncology is looking hopeful. Its subsidiary, OnkosXcel,  announced positive top-line results from a Phase II trial in patients with small cell neuroendocrine metastatic castration-resistant (SCNC) prostate cancer Wednesday morning.

BXCL701 is an innate immune activator being studied in combination with Merck’s Keytruda.

OnkosXcel spun out of BioXcel last April as a wholly owned subsidiary to focus on developing its oncology asset. The drug was acquired from Point Therapeutics years after it failed in a Phase III trial as a monotherapy for NSCLC.

Immunotherapies often fail to help cancers that appear “cold” or uninflamed to the immune system. BioXcel’s oral therapy is designed to turn cold tumors hot, lighting them up for the immune system to recognize and attack.

SCNC is a rare but growing population that emerges in approximately 20% of metastatic castration-resistant prostate cancer patients. The difficult-to-treat indication currently has no FDA-approved therapies for the nearly 11,000 patients whose cancer progressed to SCNC just last year. Experts believe numbers will only increase as checkpoint inhibitor use grows.

The trial included 28 patients on a twice-daily dose for two weeks alongside three weeks of intravenously administered Keytruda. Primary and secondary endpoints included a composite response rate, duration of response, progression-free and overall survival and biomarker evaluation.

Patients in the trial had previously failed to respond to platinum therapy. In an earlier release of interim trial data, the treatment regimen elicited a response in four out of the twelve radiographically evaluable patients. There was also evidence of a durable response rate in excess of nine months as of the last report. 

“Keytruda has a negligible response rate in this group [MS stable and TMB low population], so we are highly encouraged to see those responses,” Vincent O’Neill, Chief Research & Development Officer, OnkosXcel Therapeutics, told BioSpace.

“Bear in mind that these are elderly men who are pretty beaten up, having already had platinum chemo. We now have a duration of response in excess of nine months in a patient population that literally would have weeks or at best months to live."

O’Neill believes regulators will want evidence of individual contribution, so the next step will be a randomization of the Keytruda combo versus a noncomparative randomization of BXCL701 alone.

“We know, however, from all the preclinical work we've done, it really is the synergy between the checkpoint and 701. So we'll be having those conversations with the regulators. What type of data set would be required for us to get this drug to market?” O’Neill said.

One of the drug’s biggest differentiators from other innate immune activators is its oral delivery. TLRs and STING agents are intertumoral, injected directly into the tumor, yet a large percentage of patients don’t have an injectable lesion available. The asset also inhibits a novel target – DPP89 - for inflammasome activation.

BXCL701 has received Orphan Drug Designation from the FDA for four other indications and is being tested in a basket trial in combination with Keytruda for other types of advanced “hot” solid tumors.

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