Callisto Pharmaceuticals Reports Promising Interim Phase II Data forAtiprimod in Advanced Carcinoid Cancer

Callisto Pharmaceuticals, Inc. (AMEX: KAL; FWB: CA4), a developer of new

drug treatments in the fight against cancer and gastrointestinal

disorders, announced today promising interim data from the company´s

ongoing open-label Phase II clinical trial of Atiprimod to treat low to

intermediate grade neuroendocrine carcinoma (advanced carcinoid cancer).

Overall, the interim results suggest that Atiprimod is an active and

well tolerated drug in the treatment of carcinoid cancer.
In this interim analysis, 25 of 46 enrolled patients had sufficient data

available for evaluation. The median follow up of the patients was 6

months (range 2 to over 12 months). All patients enrolled in this study

had evidence of progressing disease in the 6 months preceding

enrollment. Details of this analysis 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.
Interim analysis patient selection: The efficacy reporting includes all patients for which screening

baseline data and at least Cycle 2 RECIST measurements were available

as well as patients who had progressive disease before Cycle 2 at the

cut off time for the analysis. The safety reporting includes all

reported SAE information as of the cut off date for all 46 enrolled

patients.
Tumor response analysis: Of the

evaluable patients, 92% had stable disease as best response per standard

RECIST criteria, with a median duration of 6 months. Actuarial

progression free survival at 6 months is 76% and at 12 months it is 50%.

There were no objective RECIST responses for tumor regression in the

analyzed cohort. To date, 7 patients have completed all 12 planned

cycles of Atiprimod therapy with stable disease and have entered an

extension trial to continue treatment.
Symptom relief analysis: In this

analysis, 17 patients were evaluable for relief of symptoms of carcinoid

cancer (flushing, wheezing, diarrhea). 82% of patients responded with at

least 1 treatment cycle´s daily average

frequency of at least 1 symptom that was reduced by 20% or more from the

screening daily average for that symptom, as recorded by the patients in

a daily diary. The breakdown in responding symptoms were wheezing (2)

flushing (11) and diarrhea (5). (The number of symptom responses exceeds

the number of responding patients because some patients had more than

one symptom response.) Some of these symptom responses were quite

clinically striking and durable; for example, one patient had a steady

reduction in daily flushing frequency from nearly 6 episodes per day to

less than one episode per day by Cycle 7. This patient is continuing in

his 8th treatment cycle.
Side effect analysis: Serious

Adverse Events (SAE´s) from all 46 patients

enrolled in the study and reported to date have been analyzed. 15 SAEs

were reported in 13 unique patients. Nine of the 15 SAEs were assessed

as unrelated to Atiprimod by the treating investigator. Four SAEs were

assessed as possibly related, 1 as probably, and 1 as definitely

related. Of the 6 patients with SAEs assessed as possibly, probably, or

definitely related, all involved up to grade 4 AST/ALT elevations with

or without nausea and vomiting. Grade 1-2 bilirubin elevations were

noted in 3 of these. All of the 6 patients with SAEs of abnormal liver

function have recovered and 2 of these patients were able to

subsequently continue Atiprimod. Detailed dose modification guidelines

for liver function abnormalities are included in the protocol. Two SAEs

had an outcome of death "“ both were assessed

as unrelated to Atiprimod and involved complicated urinary tract

infections in elderly patients. Of the 13 remaining SAEs occurring in 11

unique patients in whom the outcome of the SAE was not death, 6 patients

discontinued study participation because of the SAE.
Study design: The study is an open

label Phase II study designed to evaluate the anti-tumor efficacy

effect on symptoms, safety and tolerability of Atiprimod over 12

treatment cycles (about 12 months) in patients with low to intermediate

grade neuroendocrine carcinoma (also called carcinoid cancer) who have

metastatic or unresectable cancer. Patients in the study must have

either progression of their cancer or symptoms from their carcinoid

tumor which is not controlled with standard octreotide therapy.

Forty-six patients were 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 is given orally according to a pharmacokinetically-driven

dosing regimen in 4-week cycles.
"We are pleased with the results that Atiprimod has shown so far in this

trial," said Dr. Gary S. Jacob, Chief Executive Officer of Callisto. "Importantly

in this slow growing cancer, Atiprimod has shown an ability to stabilize

disease progression and to reduce the symptoms of this disease, with a

side effect profile that is generally well tolerated, with reversible

increases in liver transaminases as the most notable adverse event. This

appears to be a drug that patients can tolerate for long term treatment

in this slowly progressing disease. We hope to be able to meet with the

FDA, sometime in the next twelve months, to discuss the design of a

pivotal Phase III trial."
Dr. Max Sung from the Mount Sinai School of Medicine in New York, the

lead investigator on the study, stated: "In

this interim analysis, Atiprimod has shown a clear trend towards

stabilizing patients who previously had progressive disease. This is a

very important finding and I believe that this drug has the potential to

become a first line therapy for this cancer."
Said Dr. Jacob: "This is the start of

realizing the potential of Atiprimod in various cancers. Carcinoid is an

excellent initial indication for Atiprimod. Importantly, the end point

in this trial, progression free survival, has been accepted by the FDA

as the primary endpoint for other recent phase III trials in this

disease. In addition to a phase III trial in carcinoid cancer, we will

also start to investigate possible designs of exploratory phase 2

programs in other cancers where we believe the drug has potential, such

as liver and colon cancer."
Atiprimod has been given orphan drug designation to treat carcinoid

cancer.
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 7,000 new cases of carcinoid cancer 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 two anti-cancer agents as

well as a drug for gastrointestinal disorders that is currently being

developed by its wholly-owned subsidiary, Synergy Pharmaceuticals. 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. Synergy´s proprietary drug SP-304 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. 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