Pfizer's new rheumatoid arthritis pill impressed experts at a medical meeting in London today, despite four deaths in a study that spooked investors last month, according to Reuters.

Tofacitinib is Pfizer's biggest pipeline hope and could become the first major new medicine to come from its own labs since Viagra more than a decade ago.

It has the potential to upend the rheumatoid arthritis (RA) market, currently dominated by injections, by offering patients a more convenient twice-daily tablet.

But the experimental drug's prospects have been clouded by safety worries following news that four patients died after taking it in a late-stage phase III study.

Paul Emery, president of the European League Against Rheumatism (EULAR) and a rheumatologist from Leeds Teaching Hospital, said the trial results presented at the organization's annual meeting showed tofacitinib had considerable potential. "It's a very impressive drug," he told reporters. "The way it is priced will have the biggest influence on its market penetration."

Emery said such mortality rates were to be expected among RA patients.

Pfizer said last month three of the deaths were determined by local investigators not to be study related, leaving just one case of a 58-year-old U.S. man with respiratory infection linked to the drug.

But deciding causality is not easy.

Joel Kremer of Albany Medical College, the primary investigator for the study, told Reuters it seemed clear two of the other deaths—one due to brain trauma and one caused by heart failure in a Chinese patient with valvular disease—were not linked to tofacitinib, but the case of a 48-year-old Russian man was less certain. The Russian patient, on the lower 5mg dose of the drug, died 42 days after discontinuing treatment from a respiratory complication.

"It's unlikely that two of the four were associated (with tofacitinib). It's possible that the other two were associated, but based upon the information I have it's impossible to say with absolute certainty," Kremer said.

Results of the study were laid out at a news conference, ahead of presentation to the EULAR meeting.

Headline findings in March already declared the trial a success and Kremer said the drug's effectiveness was "in the same ballpark" as injected biotech drugs, such as Abbott's $6.5 billion-a-year blockbuster Humira.

The 792-patient trial tested two doses of tofacitinib against placebo in people with rheumatoid arthritis who failed to respond to an existing disease modifying anti-rheumatic drug. Significant and sustained responses were seen within a few weeks and, after six months, 58.3% of patients on the higher 10mg dose had a 20% improvement in symptoms, such as swollen joints, pain and disability.

Another EULAR study showed that adding Pfizer's existing drug Lipitor to tofacitinib helped bring down cholesterol.

Atlantic Equities analyst Richard Purkiss said the rate of deaths among elderly RA patients in various studies was very similar to that with other potent treatments.

"My conclusion is that tofacitinib is no different from any of the other RA drugs," he said. What would be worrying was if all the deaths were from the same cause, such as heart attack, but in fact they have been "all over the place," Purkiss said.

Pfizer hopes to capture a large chunk of the market with its so-called JAK inhibitor pill, which works by blocking signals from entering cells that would activate inflammatory responses.

Analysts, on average, have forecast annual tofacitinib sales of $1.1 billion by 2015, according to Thomson Reuters Pharma.

It should be filed for approval by the end of 2011 and Pfizer believes it has a healthy lead over rivals. "We are one phase ahead and that usually translates to about two years," said Saeed Fatenejad, head of inflammation drug development.