Clinical RationaleTumor hypoxia occurs when a cancerous tumor rapidly outgrows its blood supply, leaving portions of the tumor with regions where the oxygen concentration is significantly lower than in healthy tissues. Several studies have shown that higher levels of tumor hypoxia correlate with poor treatment outcomes for a variety of solid tumors. However, the Company’s first Hypoxia-Activated Prodrug (HAP), TH-302, has the potential to treat slowly dividing tumor cells within hypoxic regions that generally evade traditional chemotherapeutic agents and ultimately contribute to relapse. Translational results in preclinical models demonstrating that TH-302 enhances the antitumor activity of anti-angiogenics provide a rationale for combining TH-302 with anti-angiogenics to increase the treatment benefit in not only the approved indications (RCC, HCC, GIST) but also other indications, such as NSCLC and melanoma. These data were presented at the American Association for Cancer Research (AACR) annual meeting held in April, 2011. Renal Cancer The American Cancer Society estimates that in 2011 there will be more than 60,000 new cases of kidney cancer and over 13,000 deaths from the disease in the United States. About Threshold Pharmaceuticals Threshold is a biotechnology company focused on the discovery and development of drugs targeting Tumor Hypoxia, the low oxygen condition found in microenvironments of most solid tumors. This approach offers broad potential to treat most solid tumors. By selectively targeting tumor cells, we are building a pipeline of drugs that hold promise to be more effective and less toxic to healthy tissues than conventional anticancer drugs. For additional information, please visit our website ( www.thresholdpharm.com). Forward-Looking Statements Except for statements of historical fact, the statements in this press release are forward-looking statements, including statements regarding TH-302 and its potential therapeutic uses and benefits. These statements involve risks and uncertainties that can cause actual results to differ materially from those in such forward-looking statements. Potential risks and uncertainties include, but are not limited to, the ability of Threshold or clinical investigators to enroll patients in clinical trials of TH-302, whether this trial or any others that Threshold undertakes will confirm the results of earlier trials or animal experiments, the time and expense required to conduct such clinical trials and analyze data, Threshold’s ability to develop a clinical formulation of TH-302 and secure and adequate supply, issues arising in the regulatory process and any unanticipated or increased side-effects observed in patients receiving TH-302. Further information regarding these and other risks is included under the heading "Risk Factors" in Threshold's Quarterly Report on Form 10-Q, which was filed with the Securities Exchange Commission on May 12, 2011 and is available from the SEC's website ( www.sec.gov) and on our website ( www.thresholdpharm.com) under the heading "Investors." We do not intend to update any forward-looking statement made in this news release.