Cytogen Corporation (Nasdaq: CYTO) today announced the presentation of data demonstrating the expression of prostate-specific membrane antigen (PSMA) in cancer of the kidney. The differential expression of PSMA in renal malignancies was presented during the 101st American Urological Association Annual Meeting held May 20-25, 2006, in Atlanta, GA.

In the study, independent investigators at the Cleveland Clinic evaluated PSMA expression in normal kidneys and tissue samples from both benign and malignant renal lesions using a tissue microarray. Specimens from 60 normal kidneys and 99 renal neoplasms of various cell types were immunostained with the vascular endothelial marker CD34 and an antibody to PSMA. PSMA expression in tumor-associated neovasculature was tested in triplicate and scored for staining intensity and percentage of vessels. PSMA was not expressed in any vascular structures of normal kidneys though it was detected in the brush border of proximal tubules. In renal neoplasms, PSMA was only detected in the tumor neovasculature.

"PSMA represents an excellent target for prostate and potentially other cancers," said Warren D.W. Heston, Ph.D., director, research program in prostate cancer at The Cleveland Clinic Foundation and one of the authors of the study. "In addition to being abundantly and preferentially expressed on the surface of prostate cancer cells, PSMA is also present on endothelial cells of new blood vessels that supply most other solid tumors. Our study results demonstrating the expression of PSMA in kidney cancer justify continued exploration of both imaging and therapeutic approaches that recognize this unique target."

Differential PSMA staining was noted among the various pathological types of lesions. PSMA staining was positive in 16 of 21 (76%) clear cell carcinomas, 5 of 16 (31%) of chromophobe renal cell carcinomas, 0 of 20 papillary renal cell carcinomas, 10 of 19 (53%) oncocytomas, 3 of 14 (21%) transitional cell carcinomas, and 0 of 19 angiomyolipomas. Only clear cell carcinoma lesions demonstrated diffuse strong staining while weak or focal staining was noted in half of the oncocytomas, less than a third of chromophobe renal cell carcinomas, one fifth of the transitional cell carcinomas, and none of the specimens of papillary renal carcinoma or angiomyolipoma. Clear cell carcinoma is the most common type of malignant renal tumor and oncocytoma is a rare benign lesion.

"This data emphasizes the growing relevance of PSMA as a tumor marker and suggests that it may have wider application than just prostate cancer," noted Michael J. Manyak, M.D., vice president of medical affairs for Cytogen. "These findings may provide a means to differentiate renal tumors and also a method to evaluate possible recurrence in patients who undergo renal ablative procedures or partial nephrectomy for malignancy."

A copy of the following abstract presented at the AUA annual meeting is currently available on the AUA website at auanet, which is not part of this press release:

"Expression of Prostate-Specific Membrane Antigen in Tumor-Associated Vasculatures of Renal Neoplasms" presented on Sunday, May 21 from 1:00 p.m. to 4:00 p.m. during a Poster Session on Kidney and Ureteral Cancer: Basic Research (Abstract # 382).

About Kidney Cancer

In the United States, kidney cancer accounts for about three percent of all adult cancers. In 2006, the American Cancer Society estimates that there will be about 38,890 new cases of kidney cancer in the United States and that about 12,840 people will die from the disease. Kidney cancer occurs most often in people between the ages of 55 and 84 and affects men almost twice as often as women.

About PROSTASCIN

Cytogen's PROSTASCINT molecular imaging agent is the first and only commercial product targeting PSMA. PROSTASCINT consists of a monoclonal antibody (7E11.C5.3) directed against PSMA that is linked to the imaging radioisotope Indium-111. By targeting PSMA, the PROSTASCINT molecular imaging procedure can detect the extent and spread of prostate cancer using a standard gamma camera.

Cytogen is also developing CYT-500, a therapeutic product candidate using the same monoclonal antibody from PROSTASCINT combined with a higher affinity linker to attach a therapeutic as opposed to an imaging radionuclide. CYT-500 is designed to enable targeted delivery of a cytotoxic agent to PSMA- expressing cells. Cytogen expects to begin the first U.S. Phase I clinical trial of CYT-500 in patients with hormone-refractory prostate cancer during 2006.

PROSTASCINT is indicated as a diagnostic imaging agent in newly diagnosed patients with biopsy-proven prostate cancer, thought to be clinically localized after standard diagnostic evaluation and who are thought to be at high risk for pelvic lymph node metastases. PROSTASCINT is also indicated as a diagnostic imaging agent in post-prostatectomy patients with a rising PSA and a negative or equivocal standard metastatic evaluation in whom there is a high clinical suspicion of occult metastatic disease.

A copy of the full prescribing information for PROSTASCINT, including warnings, precautions, adverse events and other safety information, may be obtained in the U.S. from Cytogen Corporation.

About PSMA

PSMA is a protein abundantly expressed on the surface of prostate cancer cells, with an increased expression in high-grade cancers, metastatic disease and hormone-refractory prostate cancer. PSMA is also present at high levels on the newly formed blood vessels, or neovasculature, needed for the growth and survival of many solid tumors. In contrast to other prostate-related antigens such as prostate-specific antigen (PSA), prostatic acid phosphatase (PAP) and prostate secretory protein, PSMA is a membrane glycoprotein that is not secreted. These unique attributes make PSMA an excellent target for monoclonal antibody diagnostic and therapeutic options in prostate and potentially other cancers. Clinical studies have also demonstrated that overexpression of PSMA determined by immunohistochemical staining using the 7E11.C5.3 antibody in primary prostate cancer not only correlates with other adverse traditional prognostic factors, but can independently predict disease recurrence.

ABOUT CYTOGEN CORPORATION

Founded in 1980, Cytogen Corporation of Princeton, NJ, is a biopharmaceutical company dedicated to improving the lives of patients with cancer by acquiring, developing and commercializing innovative molecules targeting the sites and stages of cancer progression. Cytogen's marketed products include QUADRAMET(R) (samarium Sm-153 lexidronam injection), PROSTASCINT(R) (capromab pendetide) kit for the preparation of Indium In-111 capromab pendetide, and SOLTAMOX(TM) (tamoxifen citrate, oral solution 10mg/5mL) in the United States. Cytogen's development pipeline consists of CYT-500, a therapeutic radiolabeled antibody targeting prostate-specific membrane antigen (PSMA), a protein highly expressed on the surface of prostate cancer cells and the neovasculature of solid tumors. Cytogen also has exclusive United States marketing rights to COMBIDEX(R) (ferumoxtran-10) for all applications, and the exclusive right to market and sell ferumoxytol (previously Code 7228) for oncology applications in the United States. Full prescribing information for the Company's products is available at cytogen.

cytogen, which is not part of this press release.

This press release contains certain "forward-looking" statements within the meaning of the Private Securities Litigation Reform Act of 1995 and Section 21E of the Securities Exchange Act of 1934, as amended. All statements, other than statements of historical facts, included in this press release regarding our strategy, future operations, financial position, future revenues, projected costs, prospects, plans and objectives of management are forward-looking statements. Such forward-looking statements involve a number of risks and uncertainties and investors are cautioned not to put any undue reliance on any forward-looking statement. There are a number of important factors that could cause Cytogen's results to differ materially from those indicated by such forward-looking statements. In particular, Cytogen's business is subject to a number of significant risks, which include, but are not limited to: the risk of obtaining additional capital; the risk of obtaining the necessary regulatory approvals; the risk of whether products result from development activities; the risk of shifts in the regulatory environment affecting sales of Cytogen's products such as third-party payor reimbursement issues; the risk associated with Cytogen's dependence on its partners for development of certain projects, as well as other factors expressed from time to time in Cytogen's periodic filings with the Securities and Exchange Commission (the "SEC"). As a result, this press release should be read in conjunction with Cytogen's periodic filings with the SEC. The forward-looking statements contained herein are made only as of the date of this press release, and Cytogen undertakes no obligation to publicly update such forward-looking statements to reflect subsequent events or circumstances.

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