* Uterine cancer drug shows effectiveness in trial
* Side effects significantly worse
* Ariad shares rise 5 pct to $4.14
* Shares of partner Merck rise 0.8 pct (Adds details on side effects, byline, updates shares)
By Ransdell Pierson
NEW YORK, Oct 25 (Reuters) - Ariad Pharmaceuticals ARIA.O said its experimental treatment for advanced uterine cancer was more effective than standard treatment in a mid-stage trial, but side effects were considerably worse.
Patients taking stand-alone treatment with ridaforolimus, which Ariad is developing in partnership with Merck & Co (MRK.N), went significantly longer without a worsening of cancer symptoms than did patients receiving standard medicines, Ariad said.
Half the 114 patients in the study received oral ridaforolimus, while others received either oral progestin or chemotherapy.
Those receiving ridaforolimus went 3.6 months without a worsening of cancer symptoms, a statistically significant 1.7 months longer than those receiving the standard treatments, Ariad said.
But almost one-fourth of patients taking the Ariad drug experienced serious side effects, compared with only about 4 percent receiving standard treatment.
About 38 percent of patients receiving ridaforolimus developed mucositis, or inflammation of digestive tract tissues, while 22 percent experienced inflamed tissues of the mouth. About 27 percent developed elevated blood sugar levels.
Ariad said those side effects have been seen in previous studies and are considered to be common among all drugs in the same class of medicines, called mTOR inhibitors.
Based on the favorable efficacy results, Merck has stopped further enrollment in the study, while continuing to follow surviving patients.
Merck is testing the oral drug against a number of cancers under an exclusive license agreement with Ariad, including bone cancer and breast cancer. Cowen and Co has estimated the medicine could generate sales of $250 million in 2015 if it is approved.
The drug blocks a protein called mTOR that is involved with cell growth and cell survival.
Ariad shares were up 23 cents, or 5 percent, to $4.14 in morning trading on the Nasdaq. Merck shares rose 33 cents to $37.43 on the New York Stock Exchange. (Reporting by Ransdell Pierson, editing by Maureen Bavdek and John Wallace)