Merck & Co's investigational clostridium difficile treatment has been hit with a setback in the US after regulators extended the review time for the drug by three months.

Bezlotoxumab is designed to neutralise C. difficile toxin B, which can damage the gut wall and cause inflammation, leading to diarrhoea.

The firm is hoping that the drug, given in conjunction with standard of care antibiotics used to fight the bug, could offer the first therapy that prevents infection recurrence, which remains a key challenge for healthcare workers.

The US Food and Drug Administration is undertaking a priority review of bezlotoxumab and was initially to make a decision on by July 23, but has now requested new data and analyses from the MODIFY I and MODIFY II clinical trials, which form the basis of the submission.

These pivotal Phase III clinical studies showed the rate of infection recurrence through week 12 to be significantly lower in patients given bezlotoxumab (17.4 percent and 15.7 percent) or bezlotoxumab and actoxumab (15.9 percent and 14.9 percent) than those taking a placebo (27.6 percent) and (25.7 percent), respectively.

According to the US Centers for Disease Control and Prevention, C. difficile is estimated to have caused almost half a million infections in the US alone in 2011, with 29,000 deaths, often within 30 days of initial diagnosis.

Around one-in-four patients experience a recurrence after the initial episode, and more than 40 percent of these have further recurrence, highlighting the need for new options able to break the infection cycle.