Summit Sells Phase III Antibiotic Ridinilazole to Biossil in Up to $105 Million Deal

Summit Therapeutics has entered an agreement to sell ridinilazole, its investigational Phase III precision antibiotic for Clostridioides difficile infection (CDI), to Toronto-based Biossil Inc. Under the agreement, Summit will receive $500,000 upfront, up to $104.5 million in regulatory and commercial milestone payments, and tiered royalties on future net sales. The transaction allows Summit to sharpen its focus on its oncology pipeline, including the development of ivonescimab, while Biossil assumes responsibility for advancing ridinilazole.

Ridinilazole Targets C. difficile While Preserving the Gut Microbiome

Ridinilazole is an oral, microbiome-sparing antibiotic developed to treat CDI, a serious bacterial infection that commonly develops after antibiotic exposure and remains a leading healthcare-associated infection. Unlike broad-spectrum antibiotics, ridinilazole selectively targets C. difficile while preserving much of the normal gut microbiome, an approach that may help reduce infection recurrence and limit antibiotic-associated gut dysbiosis.

Phase III Ri-CoDIFy Trial Showed Higher Sustained Clinical Response

Ridinilazole was evaluated in the global Phase III Ri-CoDIFy clinical program (NCT03595553). comparing ridinilazole with vancomycin in adults with Clostridioides difficile infection. The study assessed Sustained Clinical Response (SCR), defined as clinical cure of the treated infection with no recurrence through 30 days after completing therapy. Although ridinilazole produced a higher observed SCR rate than vancomycin, it did not achieve statistical superiority within the predefined analysis boundaries, preventing the study from meeting its primary endpoint.

Despite that outcome, exploratory analyses and earlier clinical studies consistently showed several clinically relevant trends favoring ridinilazole. These included preservation of the gut microbiome, lower recurrence rates, reduced toxin production, convenient dosing, and favorable tolerability compared with vancomycin. The Phase III findings were reported in 2021 and later published in Clinical Infectious Diseases.

Companies Highlight Strategic and Clinical Rationale

Summit Chairman and Co-Chief Executive Officer Robert W. Duggan said ridinilazole remains a scientifically differentiated therapy for CDI and noted that previous clinical studies generated encouraging exploratory findings supporting further investigation. He added that Biossil’s AI-native drug development capabilities and focus on serious unmet medical needs position the company to continue evaluating the antibiotic while Summit concentrates on its oncology strategy.

Biossil Chief Scientific Officer and Chair Alexander Mosa, MD, PhD, said the development of microbiome-preserving antibiotics has not kept pace with the growing clinical need. He highlighted recurrent CDI, antibiotic resistance, and gut microbiome disruption as persistent healthcare challenges and said Biossil intends to advance ridinilazole with urgency.

C. difficile Infection Continues to Pose a Major Healthcare Burden

CDI remains a major public health concern worldwide. The infection causes severe diarrhea, abdominal pain, colitis, dehydration, sepsis, and can become fatal, particularly in hospitalized and older patients. Nearly 500,000 CDI cases occur annually in the United States, contributing to more than 29,000 deaths each year. Recent U.S. surveillance data continue to show substantial disease burden across both healthcare and community settings, with most infections linked to recent antibiotic use. Recurrent CDI also imposes significant healthcare costs and remains an important unmet medical need.

Path Forward for Ridinilazole

Ridinilazole has not been approved by the U.S. Food and Drug Administration or any other regulatory authority, and its safety and efficacy remain under regulatory review as Biossil prepares to continue its clinical development. The acquisition enables Summit to concentrate on its oncology portfolio while providing Biossil with a late-stage anti-infective asset that may offer a microbiome-preserving alternative for patients with CDI.

Reference

Summit Therapeutics Inc. – Summit Therapeutics Signs Agreement to Sell Phase III Asset Ridinilazole to Biossil, Inc.