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Ribaxamase prevented C. difficile infections by protecting microbiome

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Comment by Daniel Ouellette, MD, FCCP

Dr. Daniel Oullette

Aggressive treatment of septic patients with antibiotics has become the cornerstone of modern sepsis management. Like all such treatments, adverse effects confound clinical outcomes. Intensive care units have experienced epidemics of C. difficile colitis related to antibiotic use. The oral agent ribaxamase shows promise in this regard. This beta-lactamase breaks down surplus antibiotics in the gut and may offer needed adjunctive therapy to our sepsis regimens. Further study will be needed to confirm positive effects on clinical endpoints.



– An investigational beta-lactamase reduced Clostridium difficile infections by 71% in patients receiving extended antibiotic therapy for respiratory infections but not by killing the opportunistic bacteria.

Rather, ribaxamase prevented C. difficile infections (CDI) by breaking down excess therapeutic antibiotics in the gut before they could injure an otherwise healthy microbiome, John Kokai-Kun, PhD, said at the European Society of Clinical Microbiology and Infectious Diseases annual congress.

Dr. John Kokai-Kun Michele G Sullivan

Dr. John Kokai-Kun

“Up to 50% of an antibiotic dose is excreted into the small intestine, where it starts to disrupt the bowel microbiome and predisposes you to pick up C. difficile,” said Dr. Kokai-Kun, vice president of nonclinical affairs at Synthetic Biologics, Rockville, Md. “Ribaxamase is designed to block this cascade. If we protect the microbiome, any C. difficile that finds its way in would not find a gut conducive to the germination of vegetative cells.”

Ribaxamase is an oral enzyme that breaks the lactam ring in penicillins and cephalosporins. It’s formulated to release at a pH of 5.5 or higher, an environment that begins to develop in the upper small intestine near the bile duct – the same place that excess antibiotics are excreted.

“The drug is intended to be administered during, and for a short time after, intravenous administration of specific beta-lactam–containing antibiotics,” Dr. Kokai-Kun said. Ribaxamase doesn’t work on carbapenem-type antibiotics, he noted, and Synthetic Biologics is working on an effective enzyme for those as well.

In early human studies, ribaxamase was well tolerated and didn’t interfere with the pharmacokinetics of therapeutic antibiotics (Antimicrob Agents Chemother. 2017 Mar;61[3]:e02197-16). It’s also effective in patients who are taking a proton pump inhibitor, he said.

Dr. Kokai-Kun reported the results of a phase IIb study of 412 patients who received IV ceftriaxone for lower respiratory infections. They were assigned 1:1 to either 150 mg ribaxamase daily or placebo throughout the IV treatment and for 3 days after.

The primary endpoint was prevention of C. difficile infection. The secondary endpoint was prevention of non–C. difficile antibiotic-associated diarrhea. An exploratory endpoint examined the drug’s ability to protect the microbiome. Patients were monitored for 6 weeks after treatment stopped.

The cohort was a mean 70 years old. One-third of patients also received a macrolide during their hospitalization, and one-third were taking proton pump inhibitors. The respiratory infection cure rate was about 99% in both groups at both 72 hours and 4 weeks.

Eight patients in the placebo group (3.8%) and two in the active group (less than 1%) developed C. difficile infection. That translated to a statistically significant 71% risk reduction, with a P value of .027, Dr. Kokai-Kun said. Ribaxamase did not hit its secondary endpoint of preventing all-cause diarrhea or antibiotic-associated diarrhea that was not caused by C. difficile infection.

Although not a primary finding, ribaxamase also inhibited colonization by vancomycin-resistant enterococci, which occurred in about 70 (40%) patients in the placebo group and 40 (20%) in the ribaxamase group at both 72 hours and 4 weeks.

All patients contributed stool samples at baseline and after treatment for microbiome analysis. That portion of the study is still ongoing, Dr. Kokai-Kun said.

Synthetic Biologics sponsored the study and is developing ribaxamase. Dr. Kokai-Kun is the company’s vice president of nonclinical affairs.

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