Data Indicates Tanox, Inc. HIV Drug Candidate Active In Vitro Against CCR5 and CXCR4 Viruses And Synergistic With Entry Inhibitor; Shares Jump On HIV Drug Promise

WASHINGTON, Dec. 17 /PRNewswire-FirstCall/ -- Tanox, Inc. today presented data on its lead HIV drug candidate, TNX-355, at the American Society for Microbiology's Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC). The presentation highlighted the compound's unique ability to inhibit entry of HIV-1 into healthy CD4-positive cells regardless of the virus' co-receptor tropism.

TNX-355 is a humanized viral-entry inhibitor monoclonal antibody that coats CD4-positive cells - the primary target of HIV infection. By blocking viral entry into the CD4 cell in this manner, TNX-355 creates a unique new hurdle for HIV, different from entry inhibitors that target viral proteins or chemokine co-receptors. Study results show that TNX-355 is equally active in vitro against virus strains that exhibit tropism for CCR5 as well as CXCR4 - making TNX-355 the most advanced entry inhibitor in development with this crucial property of tropism independence.

"TNX-355 shows significant antiviral activity and could be a powerful new advancement in the treatment of HIV," said Dr. Stanley Lewis, Tanox medical director. "These new data confirm that unlike other viral-entry inhibitors that target cellular proteins, TNX-355 is equally effective against viruses regardless of tropism. This is extremely important, as a significant proportion of treatment-experienced patients harbor mixed virus or dual-tropic virus."

As the company announced in October 2005, Week 24 results of the Phase 2 clinical trial demonstrated that, when compared with an optimized background regimen (OBR) alone, TNX-355 in combination with OBR reduces viral loads in HIV-infected patients, without suppressing their immune systems or causing significant side effects.

Tanox's ICAAC abstract also reported results of testing TNX-355 with another entry inhibitor, enfuvirtide. Synergy between the two agents was demonstrated by conventional in vitro techniques. These data support the use of TNX-355 and enfuvirtide together in the clinical setting. Because HIV must be treated with combination therapy, negative drug-drug interactions are a major concern for clinicians treating the disease. These results suggest that co-administration of TNX-355 and enfuvirtide may enhance the activity of both agents.

About TNX-355

TNX-355 is a humanized monoclonal antibody that binds to CD4 receptors on the surface of CD4-positive cells, preventing the entry of HIV particles into lymphocytes. Phase 1 studies demonstrated that TNX-355 is active against multiple strains of HIV and does not deplete CD4-positive cells. The 48-week Phase 2 study, which met its Week 24 primary endpoint, is a double-blind trial to compare the safety and efficacy of two dosages of TNX-355, each combined with an optimized background regimen, to OBR therapy alone among HIV-1 treatment-experienced patients. The drug was fast tracked by the Food and Drug Administration in 2003. The fast-track designation expedites approval of therapies for life-threatening diseases and allows for rolling new drug application (NDA) submissions.

About Tanox, Inc.

Tanox is a biotechnology company specializing in the discovery and development of monoclonal antibodies. The company develops innovative biotherapeutics for the treatment of immune-mediated diseases, inflammation, infectious disease and cancer. Tanox's lead investigational therapy, TNX-355, is viral-entry inhibitor antibody to treat HIV/AIDS. TNX-355 has shown significant antiretroviral activity in Phase 2 clinical testing. Tanox's first-approved drug, Xolair(R) (omalizumab), is the first antibody approved to treat moderate-to-severe confirmed, allergic asthma. Xolair was developed in collaboration with Genentech, Inc. and Novartis Pharma AG and is currently approved for marketing in the United States, Canada and major European countries. Tanox is based in Houston and has a manufacturing facility in San Diego. Additional corporate information is available at www.tanox.com.

This news release contains forward-looking statements regarding the potential for TNX-355 as a treatment for HIV-1-infected patients. These statements are based on Tanox's current beliefs and expectations, and are subject to risks and uncertainties that could cause actual results to differ materially. The therapeutic potential of TNX-355 as a treatment for HIV-1- infected patients is subject to the risks inherent in drug development. Success in early stage clinical trials does not ensure that later-stage or larger-scale clinical trials will be successful, and the results achieved in later stage trials may not be sufficient to meet applicable regulatory standards. Problems or delays may arise during clinical trials or in the course of developing, testing or manufacturing drugs. For more detailed information on the risks and uncertainties associated with Tanox's drug development and other activities, see Tanox's periodic reports filed with the Securities and Exchange Commission.

Photo: NewsCom: http://www.newscom.com/cgi-bin/prnh/20050207/TNOXLOGOPRN Photo Desk, photodesk@prnewswire.comTanox, Inc.

CONTACT: Steve Sievert of Tanox, Inc., +1-713-578-4211, ssievert@tanox.com

Web site: http://www.tanox.com//