Callisto Pharmaceuticals Files IND for SP-304 (Guanilib) in Chronic Constipation and Irritable Bowel Syndrome

Callisto Pharmaceuticals, Inc. (AMEX: KAL; FWB: CA4) announced
today that through its wholly-owned subsidiary, Synergy
Pharmaceuticals, Inc. it filed an IND on April 2, 2008 with the FDA
for SP-304 (also called Guanilib) for the treatment of chronic
constipation and constipation-predominant irritable bowel syndrome.

SP-304 is an analog of uroguanylin, a natural hormone produced in
the gastro-intestinal (GI) tract that is a key regulator of intestinal
function. SP-304 works by activating a unique receptor, the GC-C
receptor, on intestinal epithelial cells, promoting fluid and ion
transport. The drug is administered orally. Nonclinical animal studies
have shown SP-304 to be well tolerated. Importantly, SP-304 shows
almost no absorption systemically into the body, exerting its effect
locally on GC-C receptors within the gut.

"Callisto is pleased that SP-304 is moving forward on schedule
into clinical studies. There is strong clinical evidence of efficacy
and safety for this class of compounds and we are hopeful that this
product may address the significant unmet medical need of patients
suffering from chronic constipation and IBS," said Gary S. Jacob,
Ph.D., CEO of Callisto, and Chairman of Synergy Pharmaceuticals.
"There are only two compounds presently in this class, our drug, and
linaclotide, a drug that is currently being developed by Microbia and
Forest Laboratories to treat GI disorders. We believe that SP-304 has
the potential to be the best in class."

In animal models SP-304 has been shown to produce responses
supporting its potential to reduce symptoms of chronic constipation
and constipation-predominant irritable bowel syndrome. Pending
clearance by the FDA, Callisto intends to initiate an SP-304 Phase I
clinical trial in 2Q2008 to evaluate safety, pharmacokinetic, and
pharmacodynamic properties.

About Chronic Constipation

Chronic constipation is a very common gastrointestinal disorder.
Up to 26 million Americans suffer from the disorder, and of this
population about 5 million have a severe condition necessitating
relief. The prevalence of the disorder is similar in other developed
countries. Patients with chronic constipation often experience hard
stools, straining during bowel movements and not enough bowel
movements during the week. People with chronic constipation can
experience serious discomfort which adversely affects their ability to
work and their quality of life.

Irritable Bowel Syndrome

Up to one sixth of adults experience inflammatory bowel syndrome
(IBS), a condition marked by disturbed bowel function and abdominal
pain. IBS patients can have three different sets of symptoms;
diarrhea-predominant (IBS-D), constipation-predominant (IBS-C) and
mixed or alternating disorder (IBS-M). The split in prevalence between
the forms is about 1/3rd each. In addition, most patients suffering
from the mixed form of IBS (IBS-M) are believed to mainly have
constipation. An estimated 10 M people in the US and an additional 10
M people in the EU suffer from IBS-C. IBS (all forms) accounts for 12%
of adult visits to primary care physicians in the US.

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 SP-304, a proprietary 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 cancer 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.

Web site: http://www.callistopharma.com