Rib-X Pharmaceuticals' RX-04 Program Yields Compounds with In Vitro Potency Against Multi-drug Resistant P. aeruginosa

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Rib-X Pharmaceuticals, Inc. announced that results from the Company's RX-04 development program will be featured today in the Novel Antimicrobial Agents poster session at the 49th Annual Meeting of the Infectious Diseases Society of America (IDSA) being held in Boston, Mass. on October 20-23, 2011. The study, conducted by researchers at Hartford Hospital, demonstrated that three representative RX-04 compounds possessed greater in vitro potency (MIC distributions ranged from 2-8 µg/mL)

than currently utilized antimicrobials for Pseudomonas aeruginosa respiratory infections. The compounds were tested against hospital-derived respiratory isolates, including multi-drug resistant (MDR) strains. P. aeruginosa is a common cause of pneumonia in the hospital setting and is associated with high rates of morbidity and mortality.

The RX-04 program is focused on the design and development of new classes of antibiotics with broad-spectrum coverage of some of the most deadly and difficult-to-treat MDR Gram-negative and Gram-positive infections, including NDM-1 strains of E. coli and K. pneumonia, as well as P. aeruginosa, A. baumannii and MRSA. In June 2011, Rib-X signed an exclusive worldwide research collaboration agreement and option for license with Sanofi for novel classes of antibiotics resulting from the RX-04 program.

"Many currently prescribed antibiotics are losing their capacity to cure difficult Gram-negative infections due to ß-lactamases and overexpressed efflux pumps," said Erin Duffy, Ph.D., Chief Scientific Officer at Rib-X. "We are unaware of any other new antibiotics in development with potency against multi-drug resistant P. aeruginosa, yet its association with high morbidity and mortality rates in the hospital setting makes this an area of significant unmet need. We believe the distinct in vitro potency observed with the RX-04 compounds could be due to their novel binding site on the ribosome and proprietary chemical characteristics that are unique from all other known classes of broad-spectrum or Gram-negative therapies."

About the Study

Non-duplicate P. aeruginosa respiratory isolates were selected from 29 United States hospitals over the period of October 2005 to June 2010. Out of a total of 111 isolates, 62 were collected from the ICU and 49 from ward patients. MDR strains accounted for 27 of the isolates. All three novel RX-04 compounds displayed similar activity against ICU- and ward-collected P. aeruginosa respiratory isolates with MIC90 values ranging between 4-8 µg/mL. All three RX-04 compounds also demonstrated greater in vitro potency than the currently utilized antibiotics meropenem, ceftazidime, and levofloxacin, which had respective MIC90 values of 16, =128, and 32 µg/mL. The data were presented by Seth Housman, PharmD, MPA, of the Nicolau lab at the Center for Anti-Infective Research and Development at Hartford Hospital, in poster number 594 titled, "In Vitro Potency of Rib-X Novel Compounds against Respiratory Pseudomonas aeruginosa isolates".

About Rib-X Pharmaceuticals, Inc.

Rib-X Pharmaceuticals is developing potent broad-spectrum antibiotics to provide a superior combination of coverage, safety and convenience for the treatment of patients with life-threatening infections. The Company's drug discovery platform, based on Nobel Prize-winning science, enables a unique understanding of how antibiotics combat infection and has generated a robust pipeline of product candidates. www.rib-x.com