Shares in Pfizer dipped almost 4% in a seemingly knee-jerk reaction to reports of four deaths in patients taking part in a clinical trial assessing tofacitinib as a treatment for rheumatoid arthritis.

At first glance data from the study, presented at the European League Against Rheumatism meeting, showed that four patients in the tofacitinib arm of the trial died while a further four developed infections compared to no serious effects in the placebo group, fueling concern among shareholders.

However, Pfizer stressed that investigators have ruled that three of these deaths were not related to its drug, with one being the result of a brain injury following trauma 22 days after treatment ended, one being down to worsening of the disease 42 days after discontinuation of the study drug; and one involving acute heart failure.

According to Pfizer, only one case - in which a patient died of respiratory failure - was reported by the investigator to be study drug related.

It also noted that because of the design of the study, the majority of adverse events would be expected to occur in patients on active treatment, and that the mortality rate from all causes across the drug's development program "is within the range of rates reported for biologic therapies for RA".

Still approvable

Analysts also seem relatively untouched by the news. J.P. Morgan's Chris Schott pointed out in a research note that other RA therapies on the market have also been linked with a higher risk of heart failure, and he believes the product "would still be approvable even if a modest signal [for heart failure] were present," according to media reports.

Last month Pfizer received a boost after it published data from the second of six Phase III trials showing that tofacitinib - a novel, oral Janus kinase (JAK) inhibitor - hit its primary endpoints by showing statistically significant changes versus placebo in reducing the signs and symptoms of RA after six months.

Results of the first of these late-stage trials, published at the end of last year, showed that the drug induced significant response rates and improved physical functionality in patients with the disease.

Pfizer is pinning much of its hopes the success of tofacitinib to help it overcome the loss of sales from generic competition to its top earner Lipitor, and analysts believe that If the drug does make it to market, it could generate sales of $2-$3 billion.