Vion's toxicology data highlights TAPET®'s safety profile

November 17, 1999

TAPET did not cause mortality; Completely cleared from blood in 24 hours and from all organs in less than two months

Vion presents preclinical data at the International Conference On Molecular Targets and Cancer Therapeutics

Vion enrolling patients for novel cancer therapy at The Cleveland Clinic

NEW HAVEN, CONN., NOVEMBER 18, 1999 -- VION PHARMACEUTICALS, INC. (NASDAQ NM: VION) presented toxicology data on VNP20009, the company's first TAPET® candidate, for its potential use as an antitumor agent and/or delivery vector. The abstract, entitled "VNP20009, A Genetically Modified Salmonella Typhimurium: Anti-Tumor Efficacy, Toxicology, and Biodistribution in Preclinical Models" was presented by Caroline Clairmont, Ph.D., Senior Scientist at Vion, at The International Conference on Molecular Targets and Cancer Therapeutics on November 18, 1999 sponsored by The American Association for Cancer Research (AACR), The National Cancer Institute (NCI) and The European Organization for Research and Treatment of Cancer (EORTC).

The presentation, which will be published in the November 1999 Supplement to Clinical Cancer Research, is focused on data gathered from VNP20009 toxicology studies conducted on mice and non-human primates (cynomolgus monkeys). Most notably, the presentation underscores VNP20009's safety profile, indicating that VNP20009, even at significantly high doses, did not cause mortality, and was completely cleared from the blood in 24 hours and from all organs in less than two months. This presentation is one of three poster presentations by Vion scientists at the conference, all focused on the TAPET development program.

VION's presentation highlights VNP20009's genetic stability, attenuated virulence, reduction of septic shock potential and antibiotic sensitivity in animals. VNP20009 (unarmed TAPET organisms), administered systemically to tumor bearing mice, accumulates preferentially in tumors over livers at a ratio of 1000:1 and inhibits tumor growth up to 95% in mouse models. Further preclinical toxicology studies conducted on monkeys demonstrate that VNP20009, at significantly high doses, were safe.

Alan Kessman, president and CEO of Vion, stated, "Vion, collaborating with researchers at Yale University, has performed extensive toxicology and safety studies on the company's first TAPET candidate, VNP20009. All these studies continue to demonstrate that VNP20009 is safe in preclinical models, and most importantly, can be easily cleared from the blood and organs even when administered at proportionally higher doses than we are using in our Phase I human safety study."

TAPET (Tumor Amplified Protein Expression Therapy), Vion's core platform technology, are Salmonella bacteria that have been engineered to colonize and multiply preferentially within the confines of a tumor. Preclinical studies have demonstrated the ability of the TAPET bacteria to inhibit the growth of tumors on their own. At doses well above those that were needed for anti-tumor activity in the mice, the TAPET bacteria were well-tolerated. In preclinical toxicology tests, the TAPET bacteria did not cause septic shock in septic shock models, and remain susceptible to commonly used antibiotics. Vion plans to conduct clinical tests on the unarmed TAPET bacteria and to develop TAPET vectors to deliver a variety of anticancer agents, or prodrug converting enzymes to solid tumors.

The company's Phase I human safety trial of an unarmed TAPET vector is open for enrollment at the Cleveland Clinic Foundation and is expected to be expanded to include additional sites in the near future. TAPET will be administered by injection directly into a surface tumor, a strategy that demonstrated significant anti-tumor effects against the injected lesion and distal metastases in preclinical animal tumor models. To be eligible, patients must be ambulatory, have advanced cancer that is no longer responsive to standard treatment and must have one or more surface tumors that can be injected with TAPET. The company plans to conduct intravenous studies in the future.
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 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 potent, alkylating agents, are currently being evaluated in preclinical studies.

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-1 (file no. 333-83837) and its amendments. This press release shall not constitute an offer to sell or the solicitation of an offer to buy any securities of Vion.

Vion Pharmaceuticals, Inc.

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