Summit Therapeutics (Nasdaq: SMMT) has agreed to sell ridinilazole, a narrow-spectrum antibiotic that completed Phase III development for Clostridioides difficile infection, to Toronto-based Biossil, Inc. for USD 500,000 upfront plus up to USD 104.5 million in regulatory and commercial milestones and tiered royalties on net sales. The deal is a clean exit: Summit retains no development rights and assumes no future costs.
The economics reflect the asset's history. Ridinilazole failed to meet the superiority primary endpoint in Summit's Ri-CoDIFy Phase III trial, results announced in December 2021 and subsequently published in Clinical Infectious Diseases. The study showed a numerically higher sustained clinical response rate versus vancomycin and demonstrated microbiome preservation, but the primary endpoint was not met within the protocol's time boundaries, leaving the asset requiring at least one additional Phase III trial before any regulatory submission becomes viable.
The upfront payment represents less than 1% of the transaction's headline value, an unusually back-loaded structure for a late-stage clinical asset. The economics indicate that the vast majority of value is contingent on Biossil successfully advancing and commercializing ridinilazole.
Summit's rationale is straightforward. The company raised USD 500 million in a private placement in October 2025 and has since proposed and withdrawn a further USD 500 million public offering as it funds multiple global Phase III trials for ivonescimab (SMT112), its PD-1/VEGF bispecific antibody licensed from Akeso Inc. in a deal worth up to USD 5 billion. The contrast between the two transactions — USD 500 million paid for ivonescimab versus USD 500,000 received for ridinilazole — captures the complete reorientation of Summit's capital allocation toward oncology. Maintaining ridinilazole's IP, regulatory standing, and FDA correspondence carried ongoing costs with no foreseeable return under Summit's strategy.