Public Release: 

VION presents preclinical Tapet® data demonstrating Tapet's ability to effectively deliver anticancer proteins

Vion Pharmaceuticals, Inc.

NEW HAVEN, CT -- APRIL 5, 2000 -- VION PHARMACEUTICALS, INC. (NASDAQ NM: VION) presented preclinical data at the 2000 Annual Meeting of the American Association for Cancer Research (AACR) on the ability of its TAPET® technology to preferentially accumulate and to express anticancer agents within tumors. TAPET are genetically altered Salmonella bacteria that colonize and multiply preferentially within solid tumors, achieving very high concentrations of bacteria within tumors as compared to normal tissues. Four abstracts were presented, two during the general sessions and two as poster presentations, all of which focused on the production, tumor accumulation and antitumor activity of TAPET engineered to deliver therapeutic anticancer proteins.

Vion scientists presented preclinical data on the following armed TAPET vectors, all of which demonstrated preferential replication in tumors with tumor to normal tissue ratios of greater than 130-1000:1 TAPET expressing Colicin E3 (ColE3), a bacterial toxin that increased the natural antitumor activity of the parental ("unarmed") TAPET in several murine tumor models.

• TAPET expressing the prodrug converting enzyme cytosine deaminase, which converts the prodrug 5-Flucytosine into the antitumor agent 5-Fluorouracil (5FU), produced high levels of 5FU selectively within the tumor. Data were also presented on TAPET expressing DT-Diaphorase (DTD), an enzyme that activates the anticancer drug Mitomycin C, and therefore has the potential to increase the amount of activated drug and overall cytotoxicity within the tumor. Conversion of pro-drugs to their active species at the tumor, combined with TAPET's ability to preferentially accumulate within the tumor, is expected to increase the antitumor activity of the drug while avoiding systemic toxicities.

• TAPET expressing the antiangiogenic protein Endostatin. TAPET expressing Endostatin were shown to increase the natural antitumor activity of the parental TAPET.

• TAPET expressing TNF (tumor necrosis factor-alpha), a known anticancer agent, produced complete tumor regression in 40% to 75% of mice bearing a colon carcinoma.

Alan Kessman, president and CEO of Vion, commented, "The preclinical data presented at this important and prestigious conference continue to demonstrate TAPET's great versatility in expressing a variety of anticancer agents. It is important to note that some of the anticancer agents expressed by TAPET, such as TNF (alpha), have potent anticancer activity but are extremely toxic when given systemically. The novel ability of TAPET bacteria to selectively replicate within a tumor as compared to normal tissue and to express anticancer agents within the confines of the tumor potentially solves this important treatment challenge. As a result, TAPET may prove to be an effective method of addressing the delivery and toxicity challenges associated with many current cancer treatments. This promise, backed by extensive preclinical data, continues to encourage the aggressive pursuit of both the preclinical and clinical development of what we believe to be a true platform technology."

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TAPET® (Tumor Amplified Protein Expression Therapy), Vion's core platform technology, are highly attenuated bacteria that, in preclinical studies, have demonstrated preferential replication in tumors compared to normal tissues. The bioengineered bacteria have demonstrated an excellent safety profile in preclinical toxicology studies. Preferential replication allows the bacteria to produce and deliver a variety of anticancer therapeutic products at high concentrations to tumors while minimizing toxicity to normal tissues. By bringing the "drug factory" preferentially to the tumor, Vion believes that TAPET may result in a cancer therapy that is more concentrated, more effective and less toxic to normal tissue. Furthermore, the unarmed bacteria by themselves have shown good antitumor activity in animal models. Vion plans to develop the unarmed TAPET alone as an antitumor agent and to develop second-generation products that produce and deliver potent therapeutic anticancer agents.

Vion Pharmaceuticals, Inc. is a biopharmaceutical company engaged in the research, development and commercialization of cancer treatment technologies. Vion's product portfolio consists of TAPET, a drug delivery platform, and three cancer therapeutics (Promycin®, Triapine® and Sulfonyl Hydrazine Prodrugs). TAPET has been shown in preclinical models to effectively deliver anticancer agents while having a minimal toxic effect on healthy normal tissues. TAPET uses genetically altered strains of Salmonella as a bacterial vector, or vehicle, for delivering cancer-fighting drugs preferentially to solid tumors. Promycin, which attacks oxygen depleted cancer cells, is currently being evaluated with radiation in a multicenter Phase III clinical trial for the treatment of head and neck cancer. Triapine, which is designed to prevent the replication of tumor cells by blocking a critical step in the synthesis of DNA, is currently being evaluated for its safety in a Phase I clinical trial. Sulfonyl Hydrazine Prodrugs, compounds that are designed to be converted to unique potent, alkylating agents, are currently being evaluated in preclinical studies. For additional information on Vion and its research and product development programs, visit the company's Internet web site at http://www.vionpharm.com.

Statements included in this press release, which are not historical in nature, are forward-looking statements made pursuant to the safe-harbor provisions of the Private Securities Litigation Reform Act of 1995. Forward-looking statements regarding the company's future business prospects, plans, objectives, expectations and intentions are subject to certain risks, uncertainties and other factors that could cause actual results to differ materially from those projected or suggested in the forward-looking statements, including, but not limited to those contained in the company's Registration Statement filed on Form S-3/A (file no. 333-95671). The shares of Common Stock, described above, have not been registered under the Securities Act of 1933, as amended, and may not be offered and sold in the United States absent registration under such Act or an applicable exemption from registration. This press release shall not constitute an offer to sell or the solicitation of an offer to buy its securities nor shall there be any sale of these securities in any state in which such offer, solicitation or sale would be unlawful prior to registration or qualification under the securities laws of any such state.

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