October 5, 2005

Rib-X Granted Key Antibiotic Drug Discovery Patents

Company Also Announces First Ever Gram-Positive 50S Ribosome Structure

NEW HAVEN, Conn., Oct. 5 /PRNewswire/ -- Rib-X Pharmaceuticals, Inc., a privately held biopharmaceutical company that is designing and developing next generation antibiotics, today announced the issuance of four U.S. patents (U.S. patent number 6,952,650, 6,947,845, 6,947,844, and 6,939,848). Rib-X was founded to use high resolution crystal structures of the 50S ribosomal subunit and novel, proprietary structure-based drug design approaches to efficiently design new classes of antibiotics that are active against drug- resistant bacteria. The ribosome, found in all cells, is a complex of protein and RNA which promotes protein synthesis, and its 50S subunit has long been known to be a valuable, clinically relevant antibiotic target. This growing patent estate reinforces the Company's significance to the important area of antibiotic drug development.

The new patents describe Rib-X's proprietary position on crystal structures of the 50S subunit of the ribosome and the complexes it forms with known antibiotics, which define functional sub-spaces of the ribosome ripe for drug discovery. These patents are based on the groundbreaking research of two Rib-X founders, Yale professors Peter Moore, Ph.D. and Thomas Steitz, Ph.D., (also a Howard Hughes Medical Institute investigator) as well as the work of Rib-X scientists. Rib-X is the exclusive worldwide commercial licensee of the Yale patents. To complete its access to ribosome high resolution structural information, Rib-X has also exclusively licensed the structure of the small subunit of the ribosome, the 30S, from the Medical Research Council, London, and the structure of the entire ribosome, the 70S, from the University of California.

Rib-X also announced today a new ribosome crystal structure -- the first high resolution X-ray crystal structure of the 50S ribosomal subunit of a Gram-positive bacterium. This is the first time ever that a scientific group has succeeded in obtaining a high resolution structure of the 50S from prototypic Gram-positive organisms. This new structure will enable Rib-X scientists to design new classes of antibiotics based on atomic resolution knowledge of how known antibiotics and Rib-X proprietary compounds interact with this and other pathogenic bacteria.

"The issuance of these patents and our ability to complete a high resolution ribosomal structure from Gram-positive bacteria are two significant achievements that attest to the strength of Rib-X's approach to creating novel and potent antibiotics that overcome resistant bacteria," said Susan Froshauer, Ph.D., President and Chief Executive Officer of Rib-X. "These patents, which further consolidate our proprietary position on the use of the 50S ribosome structure, will enhance the commercial opportunities for the company."

Many existing classes of antibiotics, including those used to treat both community- and hospital-acquired infections, are known to function by targeting the 50S. Among the classes of antibiotics that act this way are macrolides, the ketolides and the oxazolidinones that contribute more than $3 billion to the annual antibiotic marketplace. Zithromax and Zyvox are examples of two such antibiotics. The in-depth knowledge of the 50S structure provides Rib-X scientists an unprecedented, proprietary understanding of exactly how these important classes of antibiotics bind to the ribosome and why they block its function. Rib-X is using these insights to design novel molecules that will overcome the resistance of pathogenic bacteria to existing classes of antibiotics, which is a serious threat to public health.

"We doctors have waited a long time for a new approach to antibiotic development," said Rib-X Chief Clinical Officer, Scott Hopkins, M.D. "Our ribosome technology has already enabled Rib-X to build a pipeline of compounds of distinct structural classes for both hospital- and community-based infections. This new crystal structure puts us in a unique position to target multi-drug-resistant Gram-positive bacteria like MRSA, VRE and PRSP that cause life-threatening infections. The use of the new structure will enhance our accuracy in designing new antibiotics, putting us in an even stronger position to rapidly produce drugs with increased efficacy against infections caused by drug-resistant bacteria."

About Rib-X Pharmaceuticals, Inc. Rib-X Pharmaceuticals is a product-driven small molecule drug discovery and development company focused on the structure-based design of new classes of antibiotics. The Company's drug discovery strategy is focused on the exploitation of its proprietary high-resolution crystal structure of the 50S subunit of the ribosome, which performs an essential role in the fundamental process of protein synthesis, and to which many known antibiotics bind. The 50S ribosomal subunit is the target for many clinically important classes of antibiotics, including those used to treat both community-acquired and hospital-acquired pathogens. The Company's integrated research approach, which combines state of the art, proprietary computational analysis, x-ray crystallography, medicinal chemistry, microbiology and biochemistry, allows it to rapidly synthesize new agents designed to avoid typical antibiotic resistance mechanisms. Its program has already yielded many attractive compounds with attributes that make them appropriate for either the hospital Gram-positive or community respiratory tract infection marketplace. During 2005, Rib-X intends to initiate Phase I studies for at least one clinical candidate from its most advanced program.

For more information on the ribosome and the Rib-X mission, please visit the Company website at http://www.rib-x.com.

Rx Communications Group
Rhonda Chiger (investors)

Pat Garrison (media)

Tony Loke (media)

SOURCE Rib-X Pharmaceuticals, Inc.
-0- 10/05/2005

CONTACT: Investors: Rhonda Chiger, +1-917-322-2569, Media: Pat Garrison,
+1-917-322-2567, or Tony Loke, +1-917-322-2164, all of Rx Communications Group

/Web site: http://www.rib-x.com