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Sunesis Presents Data on Anti-Cancer Therapy, SNS-595 at American Association of Cancer Research Meeting

04/18/05

SNS-595 Demonstrates Unique Mechanism of Action and Consistent In Vitro and In Vivo Activity

South San Francisco, California, April 18, 2005 – Sunesis Pharmaceuticals, Inc. today presented data on the company’s lead anti-cancer therapy, SNS-595, that provide new insights into the drug’s mechanism of action. Findings were presented in three posters at the 96th American Association of Cancer Research (AACR) annual meeting in Anaheim, California.

SNS-595 is a novel cytotoxic agent currently in Phase I clinical studies, and expected to enter multiple Phase II studies in various tumor settings in the second half of 2005. Data presented today showed that SNS-595 selectively targets cells undergoing DNA synthesis to induce the rapid onset of cell death and G2 cell cycle arrest. Researchers found that the onset of apoptosis by SNS-595 occurs faster than other cytotoxic drugs tested. In addition, SNS-595 is unaffected by the loss of the p53 tumor suppressor gene, but signals through p73, which is uncommon among cytotoxic compounds. Collectively these data suggest that SNS-595 has a novel mechanism of action, distinct from other commonly used cytotoxic drugs. In models of human cancers, SNS-595 accumulates in tumor tissues and shows significant activity in animal models, including the induction of complete regressions and cures even when administered on a once every three-week dosing schedule. Sunesis researchers also showed that the same biological events observed in cell culture (rapid onset of apoptosis, G2 arrest signals and p73 activation) are also observed in pre-clinical tumor models at active doses of SNS-595. This indicates that the novel biological activities of SNS-595 are relevant for the antitumor activity of the molecule in preclinical models.

“We are pleased to report our most recent finding on SNS-595’s distinctive mechanism of action which has contributed to the design of our ongoing clinical development program,” said Daniel Adelman, M.D., Senior Vice President of Drug Discovery and Development for Sunesis. “SNS-595 continues to demonstrate consistent and promising anti-cancer properties. We have shown that SNS-595 selectively targets cells undergoing DNA synthesis to induce a rapid onset of cell death and G2 cell cycle arrest. The S-phase specificity, signaling through p73 and the rapid onset of apoptosis induced by SNS-595 is distinct from other cytotoxic drugs we have tested and indicates that SNS-595 has a novel mechanism of action. These data support and expand our knowledge of SNS-595’s activity and we look forward to reporting on this compound’s clinical progress in the future.”

Sunesis scientists and academic collaborators presented three posters describing SNS-595’s biological and in vivo activity at the AACR meeting today, Monday April 18, from 8:00 a.m. to 12 noon.

  • SNS-595, a novel cell-cycle inhibitor in Phase I clinical trials, causes tumor regressions, cell cycle arrest, and apoptosis in murine models of colon cancer
    Abstract # 2277
  • The Phase I clinical compound SNS-595 acts during S-phase and causes sustained G2 arrest
    Abstract # 2293
  • The potent cytotoxic agent SNS-595 causes a rapid onset of apoptosis during the S-phase of
    the cell cycle
    Abstract # 2285

About Sunesis’ Oncology Programs
Sunesis has built a portfolio of preclinical and development stage product candidates in oncology focused on novel pathways and targets, including inhibiting cell cycle and survival signaling. Sunesis is currently conducting Phase I clinical trials for its lead compound, SNS-595, being developed to address certain solid tumor cancers. In addition, Sunesis’ Aurora kinase inhibitor program is undergoing preclinical evaluation and, in cooperation with Biogen Idec, Sunesis is developing novel small molecule inhibitors of Raf and other cancer-related kinases.

About Sunesis Pharmaceuticals
Sunesis is a clinical-stage biopharmaceutical company focused on the discovery, development and commercialization of small molecule therapeutics for oncology, inflammatory diseases and other unmet medical needs. Sunesis has built a product candidate portfolio through internal discovery and acquisition of novel cancer therapeutics. Sunesis is advancing its product candidates through in-house research and development efforts and strategic collaborations with leading pharmaceutical and biopharmaceutical companies. For further information, visit www.sunesis.com.

Forward-Looking Statements
This press release contains forward-looking statements that involve substantial risks and uncertainties. The company may not actually achieve the plans, intentions or expectations contained in such forward-looking statements. Actual results or events could differ materially from the plans, intentions and expectations contained in such forward-looking statements. The company does not assume any obligation to update any such forward-looking statements.

CONTACT:
Eric Bjerkholt,
CFO of Sunesis Pharmaceuticals, Inc.,
+1-650-266-3717

Media and Investors
BCC Partners
Karen L. Bergman or Michelle Coral
650.575.1509 or 415.794.8662