Boundless Bio has finally given on its hopes on finding redemption for two solid tumor drugs that had both come up short in the clinic by packaging them up as a combination treatment.
The Californian company had previously opted against advancing one of these drugs, an RNR inhibitor called BBI-825, as a monotherapy back in December 2024 after seeing a lack of dose-proportional exposure.
Boundless persevered with the other drug, a CHK1 inhibitor BBI-355, as a monotherapy for oncogene-amplified solid tumors. But the clinical data showed signs of hematological toxicity that would have narrowed the therapeutic window too much to make the therapy viable.
The company had also tested BBI-355 in combination with the EGFR inhibitor erlotinib and the FGFR inhibitor futibatinib, which Taiho Oncology sells as Lytgobi. However, the combinations were not well tolerated at the doses the biotech believed were needed for robust, sustained antitumor activity.
Boundless’ decision in May 2025 to end work on BBI-355 as a monotherapy involved the company laying off a third of its staff. But the biotech still hoped it could salvage its work on BBI-355 and BBI-825 by combining them as a treatment for oncogene-amplified cancers. Boundless began studying the combo in a phase 1/2 study last year, pointing to “recent preclinical data” the company said provided a “strong mechanistic rationale” for exploring the treatment.
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But even that last glimmer of hope for the drugs has now been extinguished. After a “strategic portfolio review,” the company said in a release Tuesday morning that it has “elected to cease enrollment” of the trial.
“This decision reflects market considerations, clinical data and the company’s prioritization of programs with the greatest potential to deliver meaningful clinical impact and long-term value,” the biotech said.
Instead, Boundless is going all-in on its preclinical Kinesin oral degrader program BBI-940, which the FDA has now granted permission to enter human trials. The company hopes to launch a phase 1 study in the first half of this year.
“The acceptance of the BBI-940 IND marks an important milestone for our first-in-class Kinesin oral degrader program, enabling us to advance this differentiated anti-cancer approach into clinical development,” Boundless CEO Zachary Hornby said in Jan. 20 release.
“In parallel, our portfolio prioritization and disciplined capital allocation sharpen our focus on BBI-940, maximizing our potential to deliver high-impact therapies for patients with high unmet need cancers,” Hornby added.
Scrapping the remaining work on BBI-355 and BBI-825 means Boundless should be able to keep the cash flowing through to the second half of 2028, enough time for a readout from the BBI-940 trial. The company ended September 2025 with $117.6 million in the bank.
