Pfizer Inc. on Tuesday said its experimental drug for a deadly form of stomach cancer proved safe and effective in a late-stage trial, prompting researchers to halt the study seven months ahead of schedule.

The world’s largest drugmaker said patients who had been receiving a dummy medicine in the trial now have the option of switching to the Pfizer drug, SU11248, given its strong showing in the Phase III study.

The medicine is being developed to treat patients with gastrointestinal stromal tumors, or GIST, who have failed to benefit from Novartis AG’s Gleevec therapy. The cancer is diagnosed each year in 5,000 to 10,000 Americans.

The Pfizer drug inhibits several kinases, a type of enzyme involved in tumor growth.

The New York-based drugmaker acquired SU11248 through its 2003 merger with rival U.S. drugmaker Pharmacia. The drug is also being studied as a treatment for kidney cancer.

Last June, researchers created excitement at a medical meeting with data showing SU11248 shrank kidney and stomach tumors or impressively prevented new tumor growth in two different early-stage trials.

All stomach cancer patients in one of the earlier trials had previously taken Gleevec, which is approved for GIST, but none were benefiting from it any longer. Researchers said more than half the patients getting the once-daily Pfizer pill saw either shrinkage of tumors or no worsening of their condition for six months or more.

In the other small earlier trial, involving patients with kidney cancer who failed to benefit from other treatments, tumors shrank in 33 percent of the patients. Researchers said that was a sixfold improvement compared with typical results seen with the best standard treatments.

Pfizer shares were up 42 cents, or 1.7 percent, to $25.33 in morning trade on the New York Stock Exchange.