Pfizer's preventative vaccine against Clostridium difficile has passed a phase 2 test and will start registration studies in the next few months, in hot pursuit of a rival candidate from Sanofi.

Infections caused by C. diff are on the rise and resistance to antibiotics is a growing problem, so there is intense interest in a vaccine that could protect patients from the pathogen, which is a leading cause of hospital-acquired infections.

Pfizer's candidate—PF-06425090—is a three-dose recombinant vaccine designed to stimulate an antibody against the two main toxins (A and B) produced by C. diff, which cause the characteristic diarrhea and colitis.

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An interim analysis of the phase 2 trial showed that the vaccine was safe and stimulated a C. diff-targeted immune response, said Pfizer, giving it the confidence to press ahead with a phase 3 program in the first half of 2017. The FDA gave PF-06425090 fast-track status in 2014.

Rival Sanofi has an ongoing phase 3 trial of a C. diff toxoid vaccine—based on inactivated toxins A and B—which started in 2013 and generated positive phase 2 data a year later. Analysts at Credit Suisse have predicted that if the vaccine reaches the market in 2019 it could make sales of $525 million or more the following year, with Pfizer's candidate just a little behind at $450 million.

Pfizer's other main rival in this category is French biotech Valneva, which completed a phase 2 trial of VLA84, a recombinant fusion protein consisting of toxin A and B binding regions, last July and is now seeking a partner to help fund a phase 3 study.

Last year, Merck & Co bagged approval for Zinplava (bezlotoxumab) a B toxin-neutralizing antibody used to prevent the recurrence of C. diff infections in high-risk patients that has been tipped as a $250-million-a-year product.

An estimated 450,000 cases of C. diff are diagnosed in the U.S. annually, and antibiotic treatment has significant limitations, with recurrent infections occurring in around 20% of cases.

"Despite improved infection control measures, C. diff disease continues to rise, further augmenting an already urgent public health threat with particular negative impact on older adults," said Kathrin Jansen, Pfizer's head of vaccine R&D.

"We are very encouraged by these interim immunogenicity and safety results demonstrating robust increases in vaccine-elicited neutralizing antibodies to both toxins that we believe could provide protection against C. diff disease."