Nothing can stop Pfizer's (NYSE: PFE) oral rheumatoid arthritis treatment tofacitinib. The drugmaker reported two more positive clinical trial successes yesterday on the way to making tofacitinib its Lipitor 2.0.

Well, almost nothing. More on that in a bit.

The trials tested patients at both ends of the disease progression. One trial, dubbed ORAL Standard, tested patients who failed to get an adequate response from methotrexate, the first drug typically given to rheumatoid arthritis patients. In the other, dubbed ORAL Step, patients were farther along having failed to get an adequate response from TNF inhibitors: Merck (NYSE: MRK) and Johnson & Johnson's (NYSE: JNJ) Remicade, Abbott Labs' (NYSE: ABT) Humira, and Enbrel from Amgen (Nasdaq: AMGN) and Pfizer.

The ORAL Step trial would almost seem useless since Pfizer is hoping to grab the patients before they go onto TNF inhibitors. The older drugs all have to be injected or infused, so there's a clear advantage to having an oral medication.

But there were five deaths reported in two previous studies of tofacitinib. One of them, a case of respiratory failure, could be related to the drug. The full safety data for the latest trials weren't disclosed, so we don't know how big the problem is.

If there were no drug-related deaths in the latest trials, the combined rate of death will decrease, which should ensure that tofacitinib can be used after methotrexate but before TNF inhibitors. If however, additional deaths pop up, tofacitinib might be limited to patients for which TNF inhibitors don't work.

It's safe to assume we're talking about a difference of billions of dollars in sales between the two groups, so while the additional efficacy data is nice, the potential of tofacitinib really lies in the yet-to-be released safety data.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Motley Fool has a disclosure policy.