CHICAGO (AP) - A relatively new anti-rejection drug for kidney patients is less toxic than the mainstay medicine most transplant recipients use and might reduce the need for another transplant, new research suggests.

The drug sirolimus, sold as Rapamune, was approved by the U.S. Food and Drug Administration in 1999 for use in kidney transplant recipients along with cyclosporine. In the new research, patients given Rapamune were weaned off cyclosporine without increasing their risk of kidney rejection.

The findings are significant because while cyclosporine revolutionized all organ transplants when it was approved in the early 1980s, it paradoxically can cause kidney damage, said Dr. Donald Hricik, medical director of Case Western Reserve University's kidney transplant program. It tends to reduce blood flow to the organ, causing scarring, he said.

Hricik noted that Rapamune is being used experimentally in recipients of other organs to reduce rejection and cyclosporine-linked kidney damage.

Patients in Hricik's yearlong study who were weaned off cyclosporine three months after their transplants had significantly better kidney function several months later than patients who remained on cyclosporine.

Both groups had similarly low rejection rates.

Rapamune, like cyclosporine, inhibits the function of white blood cells that recognize the donated organ as foreign and attack it. Previous studies have shown that when the two drugs are used together - usually along with steroids - rejection is much less likely.

While Rapamune has its own side effects, kidney damage doesn't appear to be one of them.

``The drug is certainly an asset. It does offer the prospects of even better long-term success,'' said Dr. Stephen Jensik, a kidney transplant surgeon at Rush Presbyterian-St. Luke's Medical Center in Chicago. Jensik was not involved in the research, presented Sunday in Chicago at the American Transplantation Society's annual meeting.

Jensik noted that Rapamune can cause significant anemia and, because generic versions aren't available, is more expensive than cyclosporine.

Rapamune, a liquid taken daily, can cost patients $3,000 or more a year, comparable to non-generic versions of cyclosporine pills.

Hricik said anemia occurred in both groups but the study was not designed to evaluate whether it was more severe in the group weaned from cyclosporine.

The study, funded by Rapamune maker Wyeth-Ayerst Laboratories, involved 197 kidney transplant patients, about half of whom were weaned off cyclosporine within three months of their operations.

A year after their operations, kidney function as measured by its ability to filter waste products from the blood was significantly better in the group weaned off cyclosporine. About 93 percent of the transplanted kidneys were functioning in both groups.

Dr. Alan Langnas, chief of transplantation at the University of Nebraska, called the findings preliminary ``but very encouraging.''

``The transplant community is very excited'' to use drugs like Rapamune to improve long-term outcome, Langnas said.

He noted that about 40 percent of patients awaiting kidney transplants nationwide need a second transplant because the first was rejected or failed. Doctors believe cyclosporine use contributes significantly to that problem, he said.

Copyright 2001 The Associated Press. All rights reserved.