ILEX Initiates ILX-651 Study in Prostate Cancer; Third Phase II Trial in Solid Tumors Now Underway.

SAN ANTONIO -- ILEX(tm) Oncology, Inc. (Nasdaq:ILXO) announced today the opening of a multi-center Phase II study of ILX-651 in patients with hormone-refractory prostate cancer whose disease has progressed after first-line treatment with Taxotere(R) (docetaxel). ILX-651 is a novel tubulin-interactive agent that has shown preliminary activity in a range of solid tumors. Two additional Phase II studies with ILX-651 are ongoing in metastatic melanoma and non-small cell lung cancer.

"There are currently no standard treatments for patients with prostate cancer who have relapsed or who have not responded to docetaxel," said Terry Murdock, Sr. Vice President, Clinical Operations, ILEX Oncology. "Preclinical studies suggest that ILX-651 has a unique mechanism of action, which warrants investigating this agent with this kind of resistant tumor."

This non-randomized, open label study will enroll approximately 40 patients at 10 to 12 clinical research sites. Patients will be treated intravenously with ILX-651 daily for 5 consecutive days every 21 days. The trial is designed for patients to receive treatment with ILX-651 for up to one year.

Results of a Phase I study recently presented by clinical investigators at the 2004 American Society of Clinical Oncology annual meeting showed that ILX-651 appears to be biologically active and well-tolerated in patients with advanced refractory solid tumors. Preliminary safety data from the Phase II metastatic melanoma trial were also presented at ASCO and showed that ILX-651 appears to be safe, tolerable and convenient for patients as administered in that trial.

About ILX-651

ILX-651 is a next-generation synthetic pentapeptide analog of the natural substance dolastatin with a unique mechanism of action that potentially differs from that of microtubule-stabilizers (taxanes and epothilones) and tubulin inhibitors (vinca alkaloids and other dolastatins). The drug has been chemically modified to provide improved pharmacological properties and is orally bioavailable with a potentially enhanced therapeutic window over earlier-generation dolastatins. ILX-651 has shown significant anti-tumor activity in preclinical models in a wide range of solid tumors, including cancer cells that are resistant to other tubulin-interactive drugs, such as the commonly prescribed taxanes.

About ILEX

Founded in 1994 as an oncology drug development company, ILEX has a marketed product, CAMPATH(R) in the United States and MABCAMPATH(R) in the European Union, and is advancing an innovative and diversified pipeline of compounds focused on the treatment of cancer. The ILEX pipeline comprises product candidates at various stages of clinical development, including monoclonal antibodies, novel cytotoxic agents, angiogenesis inhibitors and signal transduction inhibitors. Based in San Antonio, Texas, ILEX maintains a core competency in oncology drug development. ILEX also conducts research in angiogenesis inhibition, cell signaling, medicinal chemistry and nuclear receptor biology at its laboratories in Boston, Mass., and Geneva, Switzerland. In February 2004, ILEX announced its agreement to merge with Genzyme Corporation, a global biotechnology company. The merger, subject to ILEX stockholder approval and other customary conditions, is expected to close in mid-2004. Further information about ILEX can be found on the company's Web site at www.ilexonc.com.

Certain statements contained herein are "forward-looking" statements (as such term is defined in the Private Securities Litigation Reform Act of 1995). Because these statements include risks and uncertainties, actual results may differ materially from those expressed or implied by such forward-looking statements. Specifically, factors that could cause actual results to differ materially from those expressed or implied by such forward-looking statements include, but are not limited to: risks associated with preclinical and clinical developments in the biopharmaceutical industry in general and in ILEX's compounds under development in particular; market acceptance of CAMPATH(R); the ability to expand the number and scope of the indications for which CAMPATH(R) is approved; the potential failure of ILEX's compounds under development to prove safe and effective for treatment of disease; uncertainties inherent in the early stage of ILEX's compounds under development; failure to successfully implement or complete clinical trials; failure to receive marketing clearance from regulatory agencies for our compounds under development; acquisitions, divestitures, mergers, licenses or strategic initiatives that change ILEX's business, structure or projections; the ability of ILEX to predict its future expenses and capital needs; the development of competing products; uncertainties related to ILEX's dependence on third parties and partners; and those risks described in ILEX's report on Form 10-K filed on March 12, 2004, and in other filings made by ILEX with the SEC. ILEX disclaims any obligation to update these forward-looking statements.