Precinical data on antibody presented at IDSA Meeting showed potential effectiveness against a new epidemic strain of the bacteria 

October 11, 2005 

Princeton, N.J. and Jamaica Plain, MA - Medarex, Inc. (Nasdaq: MEDX) and The Massachusetts Biologic Laboratories (MBL) of the University of Massachusetts Medical School (UMMS) today announced the initiation of a randomized, placebo-controlled Phase II clinical trial of CDA-1 (also referred to as MDX-066), a novel fully human monoclonal antibody they developed to Clostridium difficile (C. difficile)  Toxin A, for hospitalized patients diagnosed with C. difficile associated diarrhea (CDAD).

The single-dose Phase II clinical trial will be conducted at hospitals in Massachusetts and California and is expected to enroll up to 150 patients with CDAD. The study is designed to assess the safety and tolerability of CDA-1 when added to standard of care and to measure and compare the duration of CDAD after treatment, the length of hospital stay and recurrence of CDAD after treatment between the placebo and treatment arms. "The number of people getting sick, and in some cases dying of this condition has increased dramatically. So we remain committed to this new approach and hope it will continue to show promise," said Donna Ambrosino, MD, Director of the MBL and a professor of pediatrics at the medical school.  "Now we'll see how effective the antibody is in reducing severity of the disease and or preventing relapse."

Donald L. Drakeman, President and CEO of Medarex noted, "We look forward to exploring the potential of CDA-1 as a new treatment option for the 300,000 patients who suffer from CDAD each year."

In a related matter, on October 7, 2005, at the annual meeting of the Infectious Disease Society of America (IDSA) held in San Francisco, scientists from MBL and Medarex, and Dr. Dale Gerding from the Hines VA Hospital in Chicago presented preclinical data on CDA-1 (MDX-066) and a related fully human monoclonal antibody called MDX-1388, which has been shown to bind to Toxin B, the second toxin produced by C. difficile.  The preclinical data also showed that CDA-1 (MDX-066) and MDX-1388 effectively bound to the toxins from a newly identified epidemic strain of C. difficile, which is now implicated in severe outbreaks of CDAD in the United States, Canada and the United Kingdom.

"This epidemic strain has raised great concern among infectious disease experts," Dr. Ambrosino said. "The new strain produces significantly higher quantities of both toxins A and B, and is implicated in hundreds, if not thousands of deaths in the past two years. So we immediately conducted studies to see if our antibodies could be used against the new strain." Additional testing is now underway to evaluate the effectiveness of both CDA-1 and MDX-1388 against the new epidemic strain of C. difficile in hamsters.

About CDA-1 (MDX-066) and MDX-1388 
CDA-1 (MDX-066) and MDX-1388 are novel, fully human antibodies that were developed by MBL and Medarex to target and neutralize the effects of Toxin A and Toxin B, respectively, the toxins produced by the bacterium Clostridium difficile (C. difficile) and which are associated with a serious and sometimes deadly form of diarrhea called C.  difficile associated diarrhea (CDAD). Published epidemiologic studies of hospitalized patients at risk for CDAD have shown a positive correlation between detectable levels of antibody in the blood to toxin A and protection from disease or relapse.

Public health officials estimate that there are approximately 300,000 cases of CDAD among hospitalized patients each year in the United States with approximately two percent deaths. CDAD is typically treated with antibiotics; however, prolonged illness can occur, and between 20-25 percent of those affected by CDAD suffer a relapse of the disease.

About MBL
The Massachusetts Biologic Laboratories is the only non-profit FDA-licensed manufacturer of vaccines and other biologic products in the United States. The laboratory was established in 1894 and since then the MBL's mission has been to improve public health through applied research, development and production of biologic products. MBL has been a part of the UMass Medical School since1997.

About UMass Medical School 
The University of Massachusetts Medical School is one of the fastest growing academic health centers in the country and has built a reputation as a world-class research institution, consistently producing noteworthy advances in clinical and basic research. The Medical School attracts more than $174 million in extramural research funding annually. Research dollars enable UMMS scientists to explore human disease from the molecular level to large-scale clinical trials. Basic and clinical research has led to new approaches for diagnosis, treatment and prevention of disease. Visit www.umassmed.edufor additional information.

About Medarex 
Medarex is a biopharmaceutical company focused on the discovery, development and potential commercialization of fully human antibody-based therapeutics to treat life-threatening and debilitating diseases, including cancer, inflammation, autoimmune disorders and infectious diseases. Medarex applies its UltiMAb® technology and product development and clinical manufacturing experience to generate, support and potentially commercialize a broad range of fully human antibody product candidates for itself

and its partners. Twenty-four of these therapeutic product candidates derived from Medarex technology are in human clinical testing, with two of the most advanced product candidates currently in Phase III clinical trials. Medarex is committed to building value by developing a diverse pipeline of antibody products to address the world's unmet healthcare needs. For more information about Medarex, visit its website at

Statement on Cautionary Factors
For Medarex: Except for the historical information presented herein, matters discussed herein contain forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, that are subject to certain risks and uncertainties that could cause actual results to differ materially from any future results, performance or achievements expressed or implied by such statements.  Statements that are not historical facts, including statements preceded by, followed by, or that include the words "potential"; "believe"; "anticipate"; "intend"; "plan"; "expect"; "estimate"; "could"; "may"; or similar statements are forward-looking statements. Medarex disclaims, however, any intent or obligation to update these forward-looking statements. Risks and uncertainties include, among other things, risks associated with our goals, plans and projections regarding our financial position and results of operations, including any financial guidance, risks associated with our product discovery and development, uncertainties related to the outcome of clinical trials, slower than expected rates of patient recruitment, unforeseen safety issues resulting from the administration of MDX-066 in patients, uncertainties associated with the collaborative process, uncertainties related to product manufacturing as well as risks detailed from time to time in Medarex's public disclosure filings with the U.S. Securities and Exchange Commission (SEC), including its Annual Report on Form 10-K for the fiscal year ended December 31, 2004 and subsequent Quarterly Reports on Form 10-Q.  There can be no assurance that such development efforts will succeed, that such products will receive required regulatory clearance or that, even if such regulatory clearance were received, such products would ultimately achieve commercial success. Copies of Medarex's public disclosure filings are available from its investor relations department.


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