Rib-X Pharmaceuticals, Inc. announced today the receipt of a $3 million milestone payment from Sanofi (EURONEXT: SAN and NYSE: SNY). The payment is the fourth received for milestones achieved following the signing in July 2011 of a worldwide research collaboration and option for license with Sanofi for novel classes of antibiotics resulting from Rib-X’s RX-04 program for the treatment of drug resistant Gram-negative and Gram-positive pathogens. The payment, which was part of the pre-specified terms of the agreement, was for the achievement of research-based milestones.

“This milestone payment from Sanofi reflects the continued progress we have made in advancing the RX-04 program toward the clinic,” said Mark Leuchtenberger, President and Chief Executive Officer at Rib-X Pharmaceuticals. “This is the fourth milestone we have reached since signing the partnership with Sanofi and we look forward to working hard toward our joint continued success.”

Under the RX-04 agreement, Sanofi has the right to license an unlimited number of product candidates targeting a discrete binding site within the ribosome. Rib-X will retain all rights pertaining to their proprietary drug discovery platform, including all other binding sites within the ribosome and all future programs, as well as to any RX-04 compound that Sanofi does not exercise its option to develop during the three-year term of the collaboration. Rib-X has received $22.0 million to date in upfront and milestone payments under the collaboration. Rib-X may receive up to a total of $86.0 million per product for the achievement of research, development and regulatory milestones, up to a total of $100.0 million per product for the achievement of commercial milestones, and tiered percentage royalties up to the low double digits on commercial sales. Rib-X also has the right under the collaboration to co-commercialize one RX-04 product of the Company’s choosing with Sanofi in the United States.

The RX-04 program is focused on using a discrete, novel binding site within the ribosome to design and develop new classes of antibiotics to treat some of the most deadly and difficult-to-treat, multi-drug resistant Gram-positive and Gram-negative infections. Using Rib-X’s proprietary drug discovery platform, the Company has developed three novel classes of antibiotics in less than three years that bind to this ribosome site.

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.