UPDATE 4-Pfizer arthritis drug shows mixed results

* Study shows oral Pfizer drug matches Humira efficacy

* Greater number of serious side effects with Pfizer drug

* Other studies confirm drug effective, well tolerated

* Deaths across trials deemed similar to standard drugs

* Pfizer shares slip 1 pct to $18.82 (Adds analyst comment, updates shares)

By Ransdell Pierson and Bill Berkrot

NEW YORK, Sept 8 (Reuters) - New data from a trial of Pfizer Inc's (PFE.N) experimental pill to treat rheumatoid arthritis showed the medicine was as effective as Abbott Inc's (ABT.N) blockbuster injectable Humira drug, but more patients taking it had serious side effects.

Summaries of several other late-stage trials to be presented at a major rheumatology meeting in November were released on Thursday and confirmed earlier findings that the Pfizer drug, tofacitinib, was effective and in general well tolerated.

The incidence of overall adverse events was similar for both drugs, but the proportion of patients who developed serious side effects, though small, was numerically higher in the tofacitinib groups.

Serious adverse effects are those deemed to have caused death, life-threatening events, need for hospitalization, or disability or permanent damage.

"Impressive efficacy was demonstrated, with focus now expected to be on safety leading into its regulatory review," Jeffrey Holford, an analyst at Jefferies & Co, said in a research note. "While the market was expecting a similar efficacy between the higher dose of tofacitinib and Humira, the strong performance of the lower dose is impressive."

Pfizer plans to seek approval of the medicine by the end of the year. The world's biggest drugmaker is hoping tofacitinib, which some analysts believe can reach $2 billion in annual sales, and other new medicines can help offset plunging revenue from its Lipitor cholesterol fighter when it faces competition from cheaper generics as early as November.

Pfizer shares fell 1 percent to $18.82 early Thursday afternoon.

Data from a 12-month trial comparing tofacitinib and Humira, called ORAL Standard, involved 717 patients with moderate to severe disease who continued to take a widely used treatment called methotrexate.

One group of the patients took 5 milligram doses of the Pfizer drug twice a day, a second group took 10 mg doses twice a day, while a third group received Humira injections every two weeks. A fourth group of patients received a placebo.

The effectiveness of tofacitinib and Humira was numerically similar by all measures, according to the data, although the study was not designed to definitively assess superiority of one drug over the other.

Full details of the tofacitinib Phase III studies are expected to be unveiled in November at the annual American College of Rheumatology (ACR) scientific meeting in Chicago.

"Based on the totality of the tofacitinib RA clinical development program, Pfizer believes the risk/benefit profile supports regulatory submission for both the 5 and 10 mg dose," Pfizer spokeswoman Victoria Davis said.

NO IMMEDIATE THREAT TO HUMIRA

Wells Fargo analyst Larry Biegelsen predicted both doses of tofacitinib will be approved next year. But he said the drug should not greatly hamper sales of Humira its first few years on the market because doctors will prescribe tofacitinib mainly for patients who have failed to benefit from standard treatments.

"However, we expect tofacitinib to have a larger impact on Humira over time as Pfizer tries to move its use to an earlier stage" of treatment, Biegelsen said in a research note.

He said Humira is on track for sales this year of $7.8 billion, and that they will grow to $10.5 billion in 2015.

Data also became available for the first time on Thursday on the incidence of overall mortality and infections across numerous late-stage trials of the Pfizer medicine, and from longer-term follow-up of patients.

That pooled analysis, involving more than 6,200 patients, showed a mortality rate with tofacitinib of 0.572 deaths per 100 patient years in the Phase III trials, and 0.641 deaths per 100 patient years in the long-term evaluations.

In all, 12 patients died out of more than 3,000 who received tofacitinib in the Phase III trials, with deaths of another 20 patients seen among more than 3,200 who were followed in longer-term studies.

Pfizer said the death rates were consistent with those typically seen with standard treatments meant to slow the progress of arthritis symptoms.

Tofacitinib belongs to a new class of drugs known as JAK inhibitors that block a protein associated with inflammation.

It would compete against a lucrative family of biotech treatments that block a different protein called tumor necrosis factor. These treatments include Humira, one of the world's biggest selling medicines, as well as Johnson & Johnson's (JNJ.N) Remicade and Enbrel, which is sold by Amgen Inc (AMGN.O) and Pfizer.

(Reporting by Ransdell Pierson and Bill Berkrot; Editing by Lisa Von Ahn, John Wallace and Richard Chang)