FK506 was initially given to patients who were taking cyclosporine and facing retransplantation because of ongoing rejection. Cyclosporine, usually given with high-dose steroids to buffer its negative side effects, can be extremely toxic to the kidneys. With patients experiencing this complication, the cyclosporine dose is decreased and additional drugs added in an attempt to compensate. This, in turn, increases the possibility of rejection. FK506 affects T-lymphocytes, white blood cells originating in the thymus, which are part of the immune system's defenses against the transplanted organ.
Rejection is the process of the activation and accumulation of T-cells in the organ.
FK506, like cyclosporine and other immunosuppressants, inhibits the secretion of certain proteins which are necessary for the T-cell to function. Unlike patients taking cyclosporine, about 90 percent of whom use steroids more than a year after liver transplantation, many FK506 patients are able to discontinue taking steroids shortly after transplantation, the Pitt team has found. To date, more than 4,000 patients who have received organ transplants at Pitt have been treated with the drug. Research indicates that FK506 is 50 to 100 times more powerful than cyclosporine, thus improving the one-year survival rates. With cyclosporine the survival rate for liver transplant patients after one year was 85 percent.