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FDA breakthrough status withdrawn for C. difficile drug candidate

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Synthetic Biologics announced it has withdrawn the breakthrough therapy designation for its Clostridium difficile drug candidate SYN-004 (ribaxamase) after an FDA review of safety data from a phase 2b clinical trial.

The FDA determined that criteria for breakthrough status were no longer met because of an imbalance in fatal adverse events in the study that could not be fully evaluated due to its limited safety database, and its method of statistical treatment of patients who failed to complete the trial for any reason, according to a press release.

The company plans to address these safety concerns in a phase 3 trial, which it has discussed with the FDA. Based on these discussions, the company expects the trial will use separate co-primary endpoints to evaluate the efficacy and safety of the drug for the treatment of patients with C. difficile infection who are also receiving treatment with a representative selection of IV beta-lactam antibiotics. The primary efficacy endpoint of the trial will be the reduction of CDI incidence in a cohort that receives ribaxamase compared to placebo, and the primary safety endpoint will be to assess mortality risk.

“We are grateful to have received guidance and continued support from the FDA as part of ribaxamase's development and remain encouraged that a robust, controlled, and well-designed clinical trial may provide sufficient efficacy and safety data to support a pathway towards marketing approval for ribaxamase,” Steven A. Shallcross, Interim CEO and chief financial officer of Synthetic Biologics said in the press release. “Following recent collaborative discussions with the FDA, we have gained clarity on several significant elements of the proposed phase 3 clinical program, which we believe provides a path forward to develop ribaxamase, a product which, if approved, may address the serious and unmet health impacts associated with antibiotic-mediated CDI.”

The company plans to share additional information about the trial design after its End-of-Phase 2 meeting with the FDA is completed during the second half of 2018, and it expects the phase 3 trial to begin during the second half of 2019.

Disclosures: Shallcross is employed by Synthetic Biologics.