ccr5

Feb. 5 (Bloomberg) -- Pfizer Inc.'s new AIDS drug and a similar pill from Schering-Plough Corp. may stop working in some patients because a test identifying who should get the medicines is sometimes inaccurate.

The pills, made by Pfizer, of New York, and Schering- Plough, based in Kenilworth, New Jersey, block a chemical entryway known as CCR5 that the virus uses to infect cells. In about 10 percent of cases, a Monogram Biosciences Inc. test incorrectly identifies patients who will benefit from the drug, scientists said this week at an AIDS meeting.

New research on Pfizer's Selzentry and Schering-Plough's vicriviroc, as well as the test's reliability, was presented today at the Conference on Retroviruses and Opportunistic Infections in Boston. While the pills promise to fight HIV in patients who can't take older medicines, the new drugs' effectiveness depends on accurate screening.

``The test is wrong in about 8 to 10 percent of patients initially screened to see if they are candidates for a CCR5 antagonist,'' David Hardy, director of the infectious disease division at Cedars Sinai Medical Center in Los Angeles, said in a telephone interview. ``We're waiting to see if the next- generation test from Monogram will eliminate the errors.''

Selzentry was cleared in August for patients who stopped responding to older medicines. It's the only approved CCR5 inhibitor, the first new family of AIDS medicines in a decade.

$300 Million

Pfizer didn't report revenue for Selzentry last year. Analysts have projected the pill could have peak annual sales of about $300 million. Vicriviroc, a similar drug, is in the third and final stage of testing usually required for U.S. regulatory approval.

As many as 90 percent of previously untreated HIV patients will have a strain of the virus that enters healthy cells through the CCR5 doorway, Howard Mayer, executive director of clinical research and development for Pfizer, said in an interview at the meeting in Boston.

After five years of HIV infection, about half of patients still have that strain, Mayer said. By then, most patients have higher levels of another virus version known as X4 that infects cells through a different route unaffected by drugs such as Selzentry and vicriviroc.

116 Patients

In a study presented today, Monogram used an enhanced model of its test to re-examine virus samples from 116 patients who had been screened with the original version. The newer test caught 25 patients previously identified as candidates for vicriviroc who in fact had a mixture of HIV strains, R5 and X4, making them unlikely to benefit from Selzentry or vicriviroc.

The test ``may further optimize selection of patients who may benefit from CCR5 inhibitors,'' lead study author Jacqueline Reeves, with Monogram, said in a report of the data.

A new test to better determine who can benefit from the Pfizer and Schering-Plough drugs is about six months from reaching the U.S. market, Chris Petropoulos, chief scientific officer for South San Francisco-based Monogram, said in a telephone interview.

``We will improve our ability to identify the patients who shouldn't go on these drugs, but not at the cost of screening patients who should get the drugs and benefit from it,'' Petropoulos said.

HIV Formidable

Monogram dropped by 1 cent to $1.32 at 4 p.m. New York time in Nasdaq Stock Market composite trading. Pfizer fell 61 cents to $23.02 on the New York Stock Exchange, while Schering-Plough declined 82 cents to $20.39.

HIV has proven formidable for drug designers because of its ability to mutate into new strains and shapes to force its way into human cells. The virus's attack begins when it attaches itself to a cell surface molecule, called a receptor, and uses one of its proteins to push its way into the cell.

Monogram's test, called Trofile, is designed to detect which patients have strains of HIV that will latch onto the CCR5 receptor to attack healthy cells. Patients can have more than one strain of the virus at once, at different levels in the blood. Sometimes, the test doesn't detect the presence of a virus strain that uses a different doorway, known as the X4 receptor.

``In patients with X4, Selzentry or another CCR5 antagonist is not likely to have a significant effect on viral load,'' said Pfizer's Mayer. ``It would be helpful to identify those patients who had X4 variants with the first screening so those patients would not inappropriately be put on Selzentry.''