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A rheumatoid arthritis drug in clinical trials has been linked to the death of at a patient.

In a research report, Wells Fargo analyst Larry Biegelsen stated that an abstract on a clinical trial of of the Pfizer drug tofacitinib outlined the deaths of four patients who had been taking the drug.

As Pfizer shares tumbled, the company rushed out a statement explaining that only one of the four deaths, triggered by respiratory failure, was known to be drug-related.

"The mortality rate from all causes across the tofacitinib RA development program, including the ORAL Sync study, is within the range of rates reported for biologic therapies for RA," Pfizer stated.

The reasons given for the other deaths included a patient who died from acute heart failure, a death caused by brain injury, and a case of worsening rheumatoid arthritis. The brain injury death took place 22 days after the patient had finished taking tofacitinib, and the patient who died of worsening rheumatoid arthritis had finished taking tofacitinib six weeks earlier.

Pfizer's drug, part of a new class of drugs called JAK inhibitors, has demonstrated potential in clinical trials as a possible alternative to frequently-used injectable medications.

Tofacitinib is far ahead of rival drugs currently in development, and some economic experts have predicted multibillion-dollar sales for tofacitinib if it is approved by the FDA.

"Now that you're doing longer trials and you see some deaths, the cardiovascular scrutiny here is going to be really tight because the FDA obviously doesn't want another Vioxx issue," Hapoalim Securities analyst Jon Lecroy told Reuters.

Vioxx is the Merck & Co. pain drug that made headlines when it was pulled from the market after it was found to raise the chances of patients having heart attacks.

Morningstar predicts that the new patient death, combined with the already known side effects of tofacitinib, will probably delay tofacitinib approval until late 2012 at the soonest.