Callisto Pharmaceuticals´ Interim Phase II Data of Atiprimod in Advanced Carcinoid Cancer to be presented at ASCO Conference

Callisto Pharmaceuticals, Inc. (AMEX: KAL; FWB: CA4), a developer
of new drug treatments in the fight against cancer and
gastrointestinal disorders, announced today that interim data from the
company´s ongoing open-label Phase II clinical trial of Atiprimod in
advanced carcinoid cancer, will be presented at the 44th annual
meeting of the American Society of Clinical Oncology (ASCO), which
will be held in Chicago May 30-June 3, 2008.

The study is an open-label Phase II trial designed to evaluate the
anti-tumor efficacy, effect on symptoms, and safety and tolerability
of Atiprimod in patients with low to intermediate grade neuroendocrine
carcinoma (also called carcinoid cancer) who have metastatic or
unresectable cancer and who have progression of their disease despite
standard therapy (octreotide and others). Forty-six patients have been
enrolled in the study, all of whom had progressing disease in the six
months preceding enrollment. In addition, patients were required to
complete two weeks of a symptoms diary to establish their symptoms
baseline before commencing Atiprimod dosing. Atiprimod has been given
orphan drug designation to treat carcinoid cancer.

"We had previously hoped to release the interim data from this
trial during the first quarter 2008, but this is not possible due to
ASCO media embargo rules," said Dr. Gary S. Jacob, CEO of Callisto.
"We are of course pleased that ASCO views these data to be important
enough for inclusion in the scientific program at the conference. We
are continuing to collect considerable clinical data from a number of
patients participating in the study, and believe these additional data
collected during the next few months are particularly important.
Advanced carcinoid cancer, although aggressive, is a relatively slow
growing cancer, and evaluating patients who have been on drug for over
6 months is extremely useful in further assessing Atiprimod´s
potential."

Callisto has begun to plan for a pivotal phase III study for
Atiprimod, and for an end of Phase II meeting with FDA at the
appropriate time. The Company also continues to assess the potential
of Atiprimod to treat other cancers, such as cancer of the liver and
mantle cell lymphoma.

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, especially the small bowel.
There are about 37,000 patients with carcinoid cancer. About 7,000 new
cases are diagnosed 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. Very few drugs that have been tested in this cancer have
shown clinically meaningful efficacy. Carcinoid tumors often produce a
condition called "carcinoid syndrome" which is caused by the release
of hormones from 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 gastrointestinal diseases and
cancer. Callisto´s drug candidates include a drug for gastrointestinal
disorders that is currently being developed by its wholly-owned
subsidiary, Synergy Pharmaceuticals, as well as two anti-cancer
agents. Synergy´s proprietary drug SP-304 (Guanilib) is planned to
begin clinical development in 2Q2008 for gastro-intestinal disorders.
SP-304 is a synthetic analog of the human gastrointestinal hormone
uroguanylin, and acts by activating the guanylate cyclase C (GC-C)
receptor on epithelial cells of the colon. The Company´s lead drug in
the clinic, Atiprimod, is presently in a Phase II clinical trial in
advanced carcinoid cancer, a neuroendocrine tumor, and in a Phase II
extension trial in advanced carcinoid cancer patients. Callisto´s
second drug in the clinic, L-Annamycin, is currently in a Phase I/II
clinical trial in adult relapsed or refractory acute lymphocytic
leukemia, and in a Phase I clinical trial in children and young adults
with refractory or relapsed acute lymphocytic leukemia or acute
myelogenous leukemia. Callisto has exclusive worldwide licenses from
Genzyme 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 for the year ended December
31, 2007, 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