Pfizer Inc. (PFE) has temporarily halted enrollment of new patients in a clinical trial of an experimental lung-cancer drug, saying the drug was associated with a higher rate of adverse events, including fatalities, than those who didn't get the drug.

The drug is figitumumab, also known as CP-751,871. It was in a Phase 3, or late-stage, trial in patients with a form of the disease known as non-adenocarcinoma non-small cell lung cancer.

The trial began around March 2008 and was projected to have about 820 patients before ending in 2011, according to a U.S. government clinical-trial database. Some of the patients were given figitumumab in addition to the drugs paclitaxel and carboplatin, while others received paclitaxel and carboplatin only.

Pfizer said in a written statement Friday that an independent committee monitoring drug safety found there were more serious adverse events, including deaths, in people who received the Pfizer drug. Pfizer didn't immediately disclose the number of such events.

It was the latest research setback for New York-based Pfizer, which has struggled to bring successful new drugs to market to offset lost revenue from patent expirations for current best-sellers. Pfizer, maker of blockbuster cholesterol drug Lipitor and erectile-dysfunction drug Viagra, is in the process of acquiring Wyeth (WYE) partly to beef up its research efforts.

Pfizer is temporarily halting new enrollment of patients in the figitumumab trial, but currently enrolled patients may continue their treatment in accordance with the trial design and in consultation with their doctors. The company will work with the drug-safety committee to analyze the data before issuing further guidance.

A separate study, which tests figitumumab with the drug Tarceva, continues to enroll new patients, Pfizer said. Tarceva is co-promoted by OSI Pharmaceuticals Inc. (OSIP) and Roche Holding AG (RHHBY).

Figitumumab belongs to a class of drugs known as IGF-1R inhibitors, which block a substance in the body known as insulin-like growth factor. The mechanism exploits research suggesting that insulin-like growth factor can stimulate cancer cells.

Several other companies are developing drugs with similar mechanisms, including Merck & Co. (MRK) and Eli Lilly & Co. (LLY) and Biogen Idec Inc. (BIIB). Most are biologics, which means they're developed using living cells rather than traditional chemicals.

If they're successful, the IGF-1R drugs could be lucrative. Research firm Leerink Swann previously estimated Pfizer's compound, which is one of the furthest along in patient trials, could generate peak annual sales exceeding $1 billion.

But even before the safety issue emerged in the Pfizer trial, there was a known theoretical risk to this class of drugs: Just as giving insulin therapy to diabetics can help control blood sugar, blocking the effects of insulin or similar substances in cancer patients could lead to excess blood sugar, a marker of diabetes.

An earlier-stage study of figitumumab showed it to be associated with higher rates of hyperglycemia than those not taking the drug. Still, some doctors think this is a manageable side effect, with non-insulin drugs such as metformin able to control high blood sugar.

It wasn't immediately known whether hyperglycemia or related conditions were a factor in the adverse events or deaths in the halted Pfizer trial. It's also not clear whether the safety issues with the Pfizer drug are specific to the drug itself or represent a "class effect" that would spell trouble for other companies' drugs.

A spokeswoman for Biogen, whose IGF-1R inhibitor is in early-stage studies, said the compound has shown a favorable clinical safety profile, with no hyperglycemia or a blood-platelet disorder observed.

The biological basis for a link between insulin and cancer was behind studies published in the summer which suggested that Sanofi-Aventis's (SNY) insulin product Lantus raised the risk of cancer. Sanofi and health authorities have said those studies were inconclusive, but Sanofi recently announced it would conduct research to help resolve the debate.

The Pfizer trial's halt was reported earlier by CNBC.

Pfizer shares rose 24 cents to $16.94 in recent trading Friday.

-By Peter Loftus, Dow Jones Newswires; 215-656-8289; peter.loftus@dowjones.com

(Thomas Gryta in New York contributed to this article.)