NEW YORK, May 24 /PRNewswire-FirstCall/ — Callisto Pharmaceuticals, Inc. (Amex: KAL; FWB: CA4), a developer of new drugs for the treatment of cancer and inflammatory diseases, announced today the addition of Lahey Clinic in Burlington, MA to its Phase II clinical trial of Atiprimod to treat low to intermediate grade neuroendocrine carcinoma (advanced carcinoid cancer). The Principal Investigator at this clinical site is Keith Stuart, MD. Other major cancer centers are currently reviewing the trial protocol, and the Company anticipates additional sites will be added to the trial.

“The addition of the Lahey Clinic to the Phase II trial of Atiprimod in advanced carcinoid patients signifies our continuing commitment to facilitate rapid enrollment of this trial,” said Dr. Gary S. Jacob, Chief Executive Officer of Callisto. “The Lahey Clinic specializes in the treatment of neuroendocrine cancers, and has built a significant reputation within the carcinoid community, which should substantially enhance the pace of enrollment and increased accessibility to prospective patients interested in participating in this trial.”

The primary objective of the Phase II clinical trial is to evaluate efficacy of Atiprimod in patients with low to intermediate grade neuroendocrine carcinoma who have metastatic or unresectable cancer and who have progression of their disease despite standard therapy (octreotide). Patients, after signing an informed consent, are required to complete two weeks of a symptoms diary to establish their symptoms baseline before commencing Atiprimod dosing. A maximum of 40 evaluable patients will be enrolled in this trial. Efficacy evaluations will include the measure of target lesions (per RECIST), and the quantization of symptom relief. Further details of this trial can be found at http://www.clinicaltrials.gov .

The other three sites currently participating in this Phase II trial are: 1) the Hematology Oncology Services of Arkansas in Little Rock, Arkansas, 2) the Dana-Farber Cancer Institute in Boston, MA, and 3) Mount Sinai in New York, NY.

About Atiprimod

Atiprimod is an orally bio-available small molecule drug that displays multiple mechanisms of action. The drug has been shown to be antiangiogenic, inhibit secretion of VEGF and IL-6, elicit an apoptotic response (programmed cellular death), and inhibit phosphorylation of key kinases involved in tumor progression and survival including Akt and STAT3. The drug is presently in two clinical trials: a Phase II trial in advanced carcinoid cancer patients, and a Phase I/IIa human clinical trial in relapsed or refractory multiple myeloma patients. Callisto earlier announced in June, 2006 interim data from a Phase I trial of Atiprimod in advanced cancer patients. The patients who were entered into this trial had growing tumors and symptoms that were no longer controlled by the standard therapies utilized. During treatment, three of the five advanced carcinoid patients had measurable tumor regressions and loss of many of the debilitating symptoms of this disease.

About Carcinoid Cancer

Carcinoid tumors, or carcinoids, originate in hormone-producing cells of the gastrointestinal (GI) tract, the respiratory tract, the hepatobiliary (liver) system and the reproductive glands. The most common site of origin is the GI tract, with tumors often developing in the rectum, and other sections of the small intestine. Approximately 7,000 cases of carcinoid cancer are diagnosed in the U.S. annually, with the number increasing over the past 20-30 years. Carcinoid tumors that metastasize to the liver have a poor prognosis. Traditionally, chemotherapy relieves symptoms in less than 30% of cases of metastatic carcinoid tumors, usually for less than 1 year. Carcinoid tumors typically produce a condition called “carcinoid syndrome” which is caused by the release of hormones by the tumors into the blood stream. The symptoms vary depending on which hormones are released by the tumors, but typically include diarrhea, facial flushing, wheezing, abdominal pain and valvular heart disease.

About Callisto Pharmaceuticals, Inc.

Callisto is a biopharmaceutical company focused on the development of new drugs to treat various forms of cancer and other serious afflictions. Callisto’s drug candidates in development currently include anti-cancer agents in clinical development, in addition to drugs in pre-clinical development for other significant health care markets, including ulcerative colitis. One of the Company’s lead drug candidates, Atiprimod, is in development to treat advanced carcinoid cancer, a neuroendocrine tumor, and relapsed multiple myeloma, a blood cancer. Atiprimod is presently in a Phase II clinical trial in advanced carcinoid cancer patients, and in Phase I/IIa human clinical trials in relapsed or refractory multiple myeloma patients, and advanced cancer patients, respectively. Another anti-cancer drug, L-Annamycin, is being developed as a treatment for forms of relapsed or refractory acute leukemia, a currently incurable blood cancer. Callisto initiated a clinical trial of L- Annamycin in adult relapsed or refractory acute lymphoblastic leukemia patients in 4Q 2005. L-Annamycin, a new compound from the anthracycline family of proven anti-cancer drugs, has a novel therapeutic profile, including activity against resistant diseases and significantly reduced cardiotoxicity, or damage to the heart, compared to currently available drug alternatives. Callisto also has drugs in preclinical development for gastro-intestinal inflammation, and cancer. Callisto has exclusive worldwide licenses from AnorMED Inc. and M.D. Anderson Cancer Center to develop, manufacture, use and sell Atiprimod and L-Annamycin, respectively. Callisto is also listed on the Frankfurt Stock Exchange under the ticker symbol CA4. More information is available at http://www.callistopharma.com.

Forward-Looking Statements

Certain statements made in this press release are forward-looking. Such statements are indicated by words such as “expect,” “should,” “anticipate” and similar words indicating uncertainty in facts and figures. Although Callisto believes that the expectations reflected in such forward-looking statements are reasonable, it can give no assurance that such expectations reflected in such forward-looking statements will prove to be correct. As discussed in the Callisto Pharmaceuticals Annual Report on Form 10-K/A for the year ended December 31, 2005, and other periodic reports, as filed with the Securities and Exchange Commission, actual results could differ materially from those projected in the forward-looking statements as a result of the following factors, among others: uncertainties associated with product development, the risk that products that appeared promising in early clinical trials do not demonstrate efficacy in larger-scale clinical trials, the risk that Callisto will not obtain approval to market its products, the risks associated with dependence upon key personnel and the need for additional financing.