Rib-X Pharmaceuticals, Inc. today announced that it has closed the first tranche of a $67.5 million Series 2 preferred stock financing. A second tranche is anticipated to close around the end of 2012. The round was led by new investor Vatera Healthcare Partners and included existing shareholders Warburg Pincus, ABS Ventures and Vox Equity Partners. The Company plans to use the proceeds to initiate the Phase 3 clinical program for delafloxacin for the treatment of acute bacterial skin and skin structure infections (ABSSSI). Delafloxacin is being developed as a first-line therapy for use initially in hospitals prior to the availability of a specific diagnosis.

"We are entering an exciting phase as a company, as we continue to advance development of an urgently needed new antibiotic treatment option to patients," said Mark Leuchtenberger, Chief Executive Officer of Rib-X. "The continued support from our existing investors as well as the participation of new investor Vatera Healthcare Partners validates the potential of delafloxacin and our progress to date. We are very pleased to have the long term financial and strategic commitment that Vatera brings to their portfolio companies."

Mr. Leuchtenberger continued, "With this financing, we plan to initiate the Phase 3 program for delafloxacin, which performed successfully in Phase 2b against all endpoints, including the new objective endpoints issued by the FDA. Delafloxacin has consistently demonstrated its broad utility as a well-tolerated, broad spectrum antibiotic that effectively targets resistant pathogens. The recently granted QIDP designation from the FDA means delafloxacin will receive priority review and is eligible for fast-track status. Furthermore, if approved, this designation gives delafloxacin an additional five years of market exclusivity in the U.S."

"Vatera is pleased to support Rib-X and the development of delafloxacin, as there is a serious and growing need for new antibiotics that can address life-threatening drug resistant infections," commented Dr. Thomas Koestler, Executive Director of Vatera Holdings LLC. "We believe its broad spectrum activity and IV to oral potential give delafloxacin a strong market position and we look forward to working with the Rib-X management team to advance this important new antibiotic towards the market."

About Delafloxacin:

Delafloxacin is being developed for use as an effective and convenient first-line antibiotic initially in hospitals prior to the availability of a specific diagnosis. Delafloxacin has the potential to offer broad spectrum coverage as a monotherapy, including for methicillin-resistant Staphylococcus aureus (MRSA), with both intravenous (IV) and oral formulations. With the exception of Zyvox® (linezolid), all other currently approved treatments for MRSA offer only IV delivery. In addition to strong Gram-positive potency, delafloxacin has shown excellent in vitro activity against susceptible Gram-negative bacteria. Rib-X recently presented data at ICAAC from a successful Phase 2b study in which delafloxacin met or exceeded primary and secondary efficacy endpoints evaluated in comparison to Zyvox, with and without aztreonam, and vancomycin, with and without aztreonam, including endpoints based on the new draft guidance from the US Food and Drug Administration (FDA) for ABSSSI.

Delafloxacin has been through four Phase 2 trials where it has shown promising results for the treatment of lung infections, including pneumonia and bronchitis, and skin infections. Rib-X is developing both IV and oral formulations of delafloxacin to enable patients who begin IV treatment in the hospital setting to transition to oral dosing for home-based care, offering the potential to increase patient convenience, lower the overall cost of treatment and reduce the length of hospital stays. These attributes, combined with delafloxacin's safety profile and reduced probability of resistance, demonstrate the potential of delafloxacin to become a new standard of care for first-line treatment of serious infections and thereby reduce the need to switch to second-line, narrow spectrum antibiotics.

About Vatera Healthcare Partners

Vatera Healthcare Partners LLC is a venture capital firm established by Michael Jaharis, founder of Kos Pharmaceuticals, Inc., a specialty pharmaceutical company sold to Abbott Laboratories in 2006 for $4.2 billion, and of Key Pharmaceuticals, Inc., a specialty pharmaceutical company merged with Schering-Plough in 1986 for $836 million. Vatera focuses on investing in biopharmaceutical firms and products with the goal of building and growing companies by leveraging the team's collective experience and expertise in the pharmaceutical industry and strong network of relationships within industry and academia.

About Rib-X:

Rib-X Pharmaceuticals, Inc. is a biopharmaceutical company developing new antibiotics to provide superior coverage, safety and convenience for the treatment of serious and life-threatening infections. The Company's proprietary drug discovery platform provides an atomic-level, three-dimensional understanding of interactions between drug candidates and their bacterial targets and enables design of antibiotics with enhanced characteristics. Rib-X has two antibiotic candidates in clinical development. Delafloxacin is an enhanced spectrum IV/oral antibiotic intended for use as first-line monotherapy primarily in hospitals and recently completed a Phase 2b clinical trial for the treatment of acute bacterial skin and skin structure infections. Radezolid is a next-generation IV/oral oxazolidinone designed to be a potent antibiotic with a safety profile permitting long-term treatment of resistant infections. The Company's pipeline also includes its preclinical RX-04 program, partnered with Sanofi, S.A., and other discovery stage anti-infective programs. For more information, please visit www.rib-x.com.