Calistoga Pharmaceuticals Presents Preclinical Data on CAL-120 at The AACR-EORTC-NCI Molecular Targets & Cancer Therapeutics Conference.

Data Support the Emerging Role of the Beta Isoform of PI3K in Solid Tumors and the Potent Antitumor Activity of CAL-120 in Solid Tumor Preclinical Models

SEATTLE -- Calistoga Pharmaceuticals, Inc., the leader in the development of isoform-selective phosphatidylinositol 3 kinase (PI3K) inhibitors for the treatment of cancer and inflammatory diseases, today announced the presentation of results from preclinical studies of CAL-120, a novel oral beta/delta PI3K inhibitor, at the AACR-EORTC-NCI International Molecular Targets and Cancer Therapeutics Conference in Boston, Mass.

The PI3K pathway is a target of significant interest for the treatment of patients with cancer. CAL-101, a delta PI3K inhibitor, has demonstrated the value of employing an isoform-selective approach to the treatment of hematologic malignancies. To date the alpha isoform has been the primary target for the treatment of patients with solid tumors, but emerging data has indicated that the beta isoform may be equally important, especially in the common setting of PTEN loss. Therefore, PI3K inhibitors targeting beta may have therapeutic efficacy while avoiding the potential toxicities associated with PI3K alpha inhibition.

Calistoga Pharmaceuticals presented observations of consistent overexpression of PI3K beta and a high level of PI3K pathway activation in the presence of PTEN loss in a screen of solid tumor cell lines. In all cases CAL-120 blocked the PI3K pathway resulting in inhibition of tumor cell proliferation and in a number of cases cell death. In mice bearing xenografts, oral administration of CAL-120 significantly inhibited tumor growth or caused tumor regression.

"These data support our hypothesis that inhibition of the PI3K beta isoform, without inhibition of the alpha isoform, has potential as an effective strategy for treatment of patients with solid tumors," said Neill Giese, Ph.D., Chief Scientific Officer of Calistoga Pharmaceuticals. "We look forward to advancing CAL-120 into clinical development to evaluate this novel approach."

CAL-120 is currently in preclinical development. Calistoga Pharmaceuticals plans to file an Investigational New Drug application for evaluation of CAL-120 in patients with solid tumor malignancies in 2010.

In addition to CAL-120, Calistoga Pharmaceuticals has a pipeline of isoform-selective PI3K inhibitors. This includes CAL-101, an oral, delta isoform-selective PI3K inhibitor under clinical evaluation in patients with hematologic malignancies, and CAL-263, an oral delta isoform-selective PI3K inhibitor for treatment of patients with inflammatory diseases such as asthma, chronic obstructive pulmonary disease and rheumatoid arthritis. In 2010, the Company plans to advance CAL-101 into registration studies and CAL-263 into Phase 2 clinical studies.

About Calistoga Pharmaceuticals

Calistoga Pharmaceuticals is the leader in developing innovative medicines targeting selected isoforms of the PI3 kinase pathway to improve the health of patients with cancer and inflammatory diseases. Calistoga Pharmaceuticals has a portfolio of proprietary compounds. The Company's most advanced compound, CAL-101, a delta-selective PI3K inhibitor, is under clinical evaluation in patients with hematologic malignancies. CAL-263, also a delta-selective PI3K inhibitor, is under Phase 1 evaluation and is being developed for patients with inflammatory diseases. Calistoga is a private company headquartered in Seattle, Washington. For more information, visit the Company's website at: www.calistogapharma.com.