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Rib-X Pharmaceuticals, Inc. initiates Phase 2 Trial for RX-3341 in Complicated Skin and Skin Structure Infections
 — Studies to investigate safety and efficacy in treatment of multi-drug resistant infections

New Haven, Connecticut. July 15, 2008 — Rib-X Pharmaceuticals, Inc., a development stage company focused on the discovery and development of novel antibiotics for the treatment of antibiotic resistant infections, today announced the initiation of a Phase 2 clinical trial for an intravenous form of antibiotic compound RX-3341 in the treatment of complicated skin and skin structure infections (cSSSIs). The safety and efficacy study will be conducted at 35 sites across the United States. As a precursor to this news the Company also announced positive results of a two-part Phase 1 study with the same candidate.

"We have made significant progress in advancing this next generation broad spectrum antibiotic further toward clinical use," said Dr. Susan Froshauer, President and Chief Executive Officer of Rib-X. "We intend to rapidly move forward with the development of our intravenous dosage form to meet the need for a broad spectrum antibiotic in the hospital setting, particularly one that is active against quinolone resistant MRSA. We also hope to further progress our oral dosage form to ensure a greater diversity of use in the treatment of serious infections in a number of settings."

Phase 2 Study Design
This Phase 2 double-blind study (study RX-3341-201) will evaluate the safety and efficacy of RX-3341 at two different doses administered intravenously to hospitalized cSSSI patients every 12 hours for 5 to 14 days, as compared to tigecycline (Tygacil®). The study's primary endpoint is the assessment of RX-3341 efficacy, safety and tolerability at the two different doses compared to that of tigecycline's standard dosing regimen. A secondary endpoint for the study is the assessment of clinical efficacy of RX-3341 compared to tigecycline in patients with cSSSIs caused by methicillin resistant Staphylococcus aureus (MRSA).

Phase I Results
The two-part Phase 1 study (RX-3341-103) compared the safety, tolerability and pharmacokinetics of two intravenous formulations of RX-3341. Part 1 of the study was designed to compare the safety and pharmacokinetics of the two intravenous (IV) formulations, with the purpose of optimizing the formulation of the IV dosage form. Twelve individuals received one dose of each of the formulations in a crossover design. The two formulations were thus shown to be comparable in terms of exposure.

Part 2 of the study was designed to assess the safety, tolerability and pharmacokinetics of the optimized intravenous RX-3341 formulation. Results showed that the chosen intravenous RX-3341 formulation was well tolerated using multiple doses for 14 days.

About RX-3341
RX-3341 is a novel, broad spectrum fluoroquinolone antibiotic which has shown increased activity against Gram-positive organisms compared to other quinolones, and similar or better activity to that of ciprofloxacin against Gram-negative organisms in in vitro studies and twelve Phase 1 and two Phase 2 clinical trials of the oral dosage form.

About Rib-X Pharmaceuticals
Rib-X Pharmaceuticals, Inc. is a product-driven small-molecule drug discovery and development company focused on developing and commercializing antibiotics to treat highly-resistant bacterial infections. Based in New Haven, Connecticut, the Company has raised over $170 million and has two programs, delafloxacin and radezolid, in advanced clinical trials. In addition, a preclinical program featuring an entirely new class of broad-spectrum antibiotics with the potential to treat even highly- resistant Gram-negative infections is scheduled to begin clinical trials in 2011.
Rib-X designs and develops novel small-molecule antibiotics by leveraging the Company's Nobel Prize- winning discovery platform to perform structure-based design and optimization. While many commercially valuable classes of antibiotics bind to the bacterial ribosome, including oxazolidinones, tetracyclines and macrolides, traditional antibiotic discovery relies heavily on serendipity. Rib-X's key competitive advantage is its proprietary understanding of ribosome structure, which enables it to generate antibiotics that overcome known resistance mechanisms and have broad-spectrum activity.
Rib-X's integrated research strategy combines state-of-the-art, proprietary computational analysis, X-ray crystallography, medicinal chemistry, microbiology and biochemistry, thereby allowing Rib-X to rapidly synthesize new agents designed to avoid typical antibiotic resistance mechanisms. Rib-X's iterative intelligent engine has yielded several distinct new antibiotic classes.

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Kari Watson
MacDougall BioMedical Communications
781-235-3060
kari@macbiocom.com
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Sarah Cavanaugh
MacDougall BioMedical Communications
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