Results of Phase 2 Study of Tolevamer for C. difficile-Associated Diarrhea Published in Clinical Infectious Diseases

International Phase 3 Trials of This Non-Antibiotic Therapy Progressing

August 15, 2006 Genzyme Corporation (Nasdaq: GENZ) announced today that results from its phase 2 trial of tolevamer, an investigational polymer therapy for patients with Clostridium difficile-associated diarrhea (CDAD), have been published in the August 15 issue of Clinical Infectious Diseases.

Tolevamer is a novel therapy that could be the first non-antibiotic treatment approved for CDAD, a sometimes deadly form of infectious diarrhea caused by the bacterium C. difficile. Tolevamer is designed to bind and remove from the body toxins released by C. difficile that damage the large intestine. In an era of increasing concern about the overuse of antibiotics and the emergence of antibiotic resistance and “superbugs,” tolevamer has the potential not only to treat CDAD, but also to reduce its rate of recurrence through a non-antibiotic mechanism of action that does not harm the normal intestinal bacteria that provide protection against C. difficile.

“We are pleased to have these tolevamer study results featured in a leading, international infectious disease journal,” said John P. Butler, president and general manager of Genzyme’s renal business unit, which is developing tolevamer. “Our phase 3 trials of tolevamer are making excellent progress, and we believe this non-antibiotic therapy holds great promise in addressing a large and growing unmet medical need for a new treatment approach to CDAD.”

The phase 2 trial enrolled nearly 300 patients at 58 sites throughout the United States, Canada and the United Kingdom in a randomized, double-blind, active-controlled study to determine the safety and efficacy of tolevamer versus the standard prescribed dose of the antibiotic vancomycin. Vancomycin is the only therapy approved in the United States and Europe for treatment of CDAD. In the phase 2 study, tolevamer demonstrated similar treatment outcomes as vancomycin in terms of time to resolution of diarrhea, and a strong trend toward a reduced recurrence rate of CDAD.

C. difficile is a bacterium found widely in the environment, and is a frequent contaminant in hospitals and long-term care facilities. It is the most common cause of infectious diarrhea among hospitalized patients, and data published recently in the Centers for Disease Control and Prevention journal Emerging Infectious Diseases show that the rate of reported C. difficile cases in the United States nearly doubled between 2000 and 2003. C. difficile is estimated to affect approximately one percent of all hospitalized patients, and to result in more than 400,000 cases of diarrhea and colitis, and more than 5,000 deaths annually in the United States. C. difficile is a worldwide problem. The U.K. Health Protection Agency last month published a report showing that cases of C. difficile infection in patients aged 65 and older increased by 17 percent in England over the last year.

Patients are at risk of developing CDAD when they are treated with antibiotics that alter the normal, protective bacteria that reside in the colon. Virtually all antibiotics have been implicated in causing CDAD. Several deadly outbreaks of CDAD have occurred in the United States, Canada and Europe in recent years, and a new, more virulent strain of C. difficile recently was identified and tied to these outbreaks.

Phase 3 Studies Ongoing

Tolevamer is currently being studied in two concurrent clinical trials, the largest of their kind ever to be conducted for CDAD. One trial is taking place in North America, and the other in Europe and Australia. These studies, which began in April 2005, will involve more than 1,000 patients at approximately 300 sites.

In these trials, tolevamer is being compared to two antibiotic therapies, vancomycin and metronidazole, which are the most commonly prescribed treatments for CDAD. Because tolevamer is a non-antibiotic and will not harm the normal protective intestinal bacteria that prevent C. difficile proliferation, it is expected to reduce the rate of recurrent CDAD. Aside from testing tolevamer’s efficacy, these are also the first large, rigorously designed, multi-center clinical studies comparing the efficacy of the current standards of care.

Genzyme expects to complete the tolevamer trials in 2007 and, pending regulatory review, the first commercial approval is anticipated in 2008. Tolevamer has been given fast-track designation by the U.S. Food and Drug Administration for the treatment of CDAD.

Genzyme is a worldwide leader in developing polymer-based therapies for serious diseases, with two proven products already on the market. Renagel® (sevelamer hydrochloride) controls serum phosphorus in patients with chronic kidney disease on dialysis, and WelChol® (colesevelam hydrochloride) reduces LDL cholesterol in certain patients with elevated LDL levels. In each case, the polymers are designed to provide clinical benefit by binding and removing unwanted substances from the body.

Tolevamer has been evaluated in several clinical studies to date, including the phase 2 study, two open-label clinical trials in CDAD patients, a phase 1 trial in healthy volunteers and an additional phase 1 tolerability study evaluating a new liquid formulation.

About Clostridium difficile

C. difficile proliferates in the setting of altered normal colonic bacterial flora, most commonly due to the administration of broad-spectrum antibiotics. Once established in the colon, C. difficile produces toxins that disrupt the intestinal lining, causing cell death and inflammation that result in diarrhea and colitis.

Even after successful treatment with the current standard of care, approximately 20 percent of patients experience a recurrence of CDAD which may require repeat hospitalization. In addition, a subset of patients with CDAD develop multiple recurrences of the disease, with symptoms that may persist for years.

About Genzyme Corporation

One of the world’s leading biotechnology companies, Genzyme is dedicated to making a major positive impact on the lives of people with serious diseases. This year marks the 25th anniversary of Genzyme’s founding. Since 1981, the company has grown from a small start-up to a diversified enterprise with more than 8,500 employees in locations spanning the globe and 2005 revenues of $2.7 billion. Genzyme has been selected by FORTUNE as one of the “100 Best Companies to Work for” in the United States.

With many established products and services helping patients in more than 80 countries, Genzyme is a leader in the effort to develop and apply the most advanced technologies in the life sciences. The company’s products and services are focused on rare inherited disorders, kidney disease, orthopaedics, cancer, transplant and immune diseases, and diagnostic testing. Genzyme’s commitment to innovation continues today with a substantial development program focused on these fields, as well as heart disease and other areas of unmet medical need.

This press release contains forward-looking statements, including without limitation statements regarding: the potential benefits of tolevamer for CDAD; the size of the CDAD patient population; clinical trial plans for tolevamer; and the anticipated timing of completion of clinical trials for tolevamer and regulatory approval of tolevamer. These statements are subject to risks and uncertainties that could cause actual results to differ materially from those projected in these forward-looking statements. These risks and uncertainties include, among others: the accuracy of Genzyme’s information concerning the CDAD patient population; additional analysis of the tolevamer phase 2 data; the results of other studies and whether such results are consistent with the phase 2 tolevamer trial data; the actual efficacy and safety of tolevamer for CDAD; the actual timing and results of phase 3 clinical trials; the outcome of discussions with the FDA and the EMEA regarding clinical studies and the approval of tolevamer and the timing of such discussions; the timing and content of submissions to and decisions made by regulatory authorities; and the risks and uncertainties described in reports filed by Genzyme with the Securities and Exchange Commission under the Securities Exchange Act of 1934, as amended, including without limitation the information under the heading “Factors Affecting Future Operating Results” in Genzyme’s Quarterly Report on Form 10-Q for the period ended June 30, 2006. Genzyme cautions investors not to place substantial reliance on the forward-looking statements contained in this press release. These statements speak only as of the date of this press release, and Genzyme undertakes no obligation to update or revise these statements.

Renagel® is a registered trademark of Genzyme. WelChol® is a registered trademark of Sankyo Pharma, Inc.

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Erin Emlock

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Genzyme Corporation

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