[Clinical experience with FK 506]

FK 506 is a superior immunosuppressive agent that should improve patient survival after the commonly performed transplant procedures, make feasible transplantations that have been previously impractical, allow immune intervention for serious autoimmune diseases, and create a better spin-off understanding of basic biologic processes including signal transduction.

Effects of cyclosporin, FK506, and rapamycin on graft-vessel disease

Graft-vessel disease (GVD) limits the long-term survival of heart-transplant patients, and this effect has not been altered by use of cyclosporin for immunosuppression. We compared the effects of the immunosuppressants cyclosporin, FK506, and rapamycin on GVD in a rat-heart transplantation model. Allografted hearts from rats treated with 1 mg/kg FK506 for 50 days showed the same degree of myocardial rejection but a significantly worse (p less than 0.05) grade of GVD compared with grafted hearts from rats treated with 1.5 mg/kg cyclosporin for the same time. 2 mg/kg FK506 for 50 days prevented cellular rejection but GVD was as severe as that found with 1 mg/kg FK506. Moderate GVD was present in two of five allografted hearts after treatment with 4 mg/kg FK506. 1.5 mg/kg rapamycin for 50 days was an effective inhibitor of rejection and GVD. Based on our results in rats, the possibility that GVD may occur in human heart-transplant recipients treated with FK506 cannot be excluded.

The effect of FK506 and cyclosporin A on antigen-induced arthritis

FK506 and cyclosporin A inhibited the development of antigen-induced arthritis in the rat and rabbit. FK506 was five times more potent than cyclosporin A in the rat and approximately 20 times more potent in the rabbit. FK506 was effective in both species if administered either from the day of intra-articular administration of antigen or when the arthritis was established. In the rabbit, arthritis returned when administration of FK506 was stopped. FK506 (10 mg/kg/day) caused renal damage which was not observed at a dose of 2.5 mg/kg/day. Both of these doses were equally effective at inhibiting the arthritis. The conclusion from these studies is that FK506 is a more effective anti-arthritic agent than cyclosporin A and that a pronounced therapeutic effect can be achieved at non-toxic doses of the drug.

Cyclosporine, FK-506 and other drugs in organ transplantation

As experience of the most effective way to use cyclosporine for immunosuppression in organ transplantation grows, new drugs are emerging, which may improve the potency of future immunosuppressive protocols or at least provide alternative drugs in selected situations. These newer agents include FK506, which is undergoing extensive clinical trials, rapamycin, RS-61443 and deoxyspergualin. The increasing understanding of the mechanism of action of some of these drugs on signal transduction pathways in the T cell should allow the development of drugs with perhaps more specific actions.

Prevention of graft-versus-host disease following allogeneic bone marrow transplantation in rats using FK506

FK506 and cyclosporine were used for the prevention of acute graft-versus-host disease. Acute GVHD was induced in Lewis rats by total-body irradiation and subsequent reconstitution with allogeneic (ACI) bone marrow and spleen cells (BMTx). GVHD was assessed by both clinical and histologic parameters during the experiment duration of 60 days, and longer for selected animals. All untreated BM recipients died within 26 days from severe acute GVHD. GVHD was prevented with CsA during the period of immunosuppressive therapy, but it appeared within a few days afterward. FK506-treated BM recipients were also protected, but they had a markedly prolonged GVHD-free period after therapy was discontinued. Most such animals eventually developed GVHD but with notable exceptions. Maintenance therapy with doses of FK506 as low as 0.1 mg/kg every other day (1/20 of daily induction dose) was infallible insurance against delayed GVHD. The relevance of these findings to GVHD caused by lymphoid-containing solid organs such as the intestine was discussed.

Initial studies with FK506 in renal transplantation

FK506 is a novel immunosuppressive agent which is approximately 100 times as potent as cyclosporine in vitro. In this initial trial, 65 renal transplant patients of high complexity received primary FK506 immunosuppression. Overall, graft and patient survival rates are 80% and 98.5%, respectively. A major advantage of FK506 is its potency with relatively few side effects, which has permitted elimination of steroids in 31 (60%) of these patients. Because of these encouraging results, a randomized trial comparing the therapeutic efficacy and toxicity of FK506 and cyclosporine is currently underway at our institution.

FK506 treatment of S-antigen induced uveitis in primates

FK506 is a new immunosuppressive agent which has been found more potent than cyclosporine based on the dosage. FK506 was examined here for its effect on the development of uveitis in primates immunized with S-antigen. FK506 successfully inhibited uveitis in monkeys, even when administered three weeks after the first immunization, at the time when the immunopathogenic mechanism of uveitis is assumed to be developed. All four monkeys injected with 0.5 mg/kg/day of FK506 did not develop uveitis, 2 out of 4 treated with the 0.25 mg and 3 out of 4 of those receiving the 0.125 mg also did not develop disease. FK506 suppressed to some extent the cellular and humoral immune responses to S-antigen. The main side effect of FK506 was weight loss. We consider that this drug may be considered as a new potential therapeutic agent for immune-mediated ocular disease in humans.

Combination drug therapies for immunosuppression in transplantation

Current combination immunosuppression protocols used worldwide include azathioprine/prednisolone, cyclosporine protocols, cyclophosphamide/steroids and FK506/steroids. The fact that many different immunosuppression protocols are currently in use, demonstrates that none is ideal. Major determinants in the choice of any protocol include graft and patient survival, side effects and cost. While most protocols may offer one year graft survival rates of 80% to 90% in renal transplantation, it is the long term results which are becoming increasingly important, but as many protocols have only been recently introduced, it may be sometime before these answers are known. While the most effective regimens include cyclosporine, long term nephrotoxicity remains a problem. Furthermore, the cost of the drug may be prohibitive in many countries worldwide. Azathioprine and low dose steroids still provide acceptable results with lesser expense and where cost and drug availability are critical, cyclophosphamide may even be introduced in place of azathioprine in living-related renal transplant recipients. The role of the newer immunosuppressive agents such as FK506 remains unclear, as the results of prospective randomised studies are not yet available. With excellent results now obtained with many different protocols, it is apparent that the choice of the most suitable immunosuppressive regimen is no longer dictated by graft survival alone.