Effects and mechanisms of actions of cyclosporin, FK506 and 15-deoxyspergualin

Immunosuppressive drugs can be used to deal with unwanted or inappropriate immune reactions, such as the rejection of a transplanted organ. However, treatment with these drugs are often associated with serious adverse effects, and therefore less toxic immunosuppressants are desired. The aims of the present study were to evaluate the immunologic effects of cyclosporin-sulphate, FK506 and 15-deoxy-spergualin (DSG). CsA-sulphate, a previously unidentified CsA metabolite, is present in high concentration in the bile and plasma of transplanted patients. In vitro and in vivo studies showed that this metabolite has limited immunosuppressive effects. It is likely that the mother compound is responsible for both the immunosuppressive and adverse effects associated with treatment. FK506 is structurally completely distinct from CsA, but was found to have identical immunosuppressive effects in vitro, although at l0-l00 times lower concentrations.

Based on studies of combinations of the two drugs, the effects CsA and FK506 are stipulated to be additive. The similar and additive effects of the drugs indicate that they share a common mechanism of action. The prevailing theory is that the drugs first bind to their respective immunophilin. The drug-immunophilin complexes inhibit the calcineurin-mediated dephosphorylation and thus the translocation to the nucleus of the cytoplasmic component of the transcription factor NF-AT. DSG have immunosuppressive properties that are distinct from CsA and FK506, but its mechanism of action remains elusive. In a safety-study, where DSG was given in combination with methyl-prednisolone as treatment of acute graft rejection in kidney transplant recipients, the effects of DSG on blood mononuclear cells were studied. DSG decreased the number of leukocytes, and results indicated that DSG may affect B-cells and perhaps also CD4+ cells.

Our in vitro studies confirmed an effect of DSG on the development of cytotoxic T cells. DSG also suppressed the proliterative responses to S. aureus and PHA, if added within 24 hours after stimulation. These effects could be explained by a DSG-dependent reduced Il-6 production. However, addition of recombinant Il-6 only partly restored the development of cytotoxic T-cells, and thus, additional Il-6 independent steps should be DSG-sensitive in the differentiation of CTL. In the transplantation of fetal porcine islet cells, reactivity against the graft is likely to have been induced via the indirect activation pathway of immune recognition. CsA was found to be less efficient in inhibiting the cytotoxicity induced by xenogeneic porcine Iymphocytes via the indirect as compared to the direct pathway. In contrast, DSG inhibited the induction of cytotoxicity equally, irrespective of activation pathway. Although DSG did not suppress antibody production to pig PBL in mice completely, it was more efficient than CsA. DSG may act by modulating the activities of the heat shock proteins Hsc70 and Hsp90. This thesis adds to the knowledge of the effects and mechanisms of action of cyclosporin, FK506 and DSG, knowledge that may be valuable for the optimal treatment of transplant recipients, and in the further development of immuno-suppressive drugs.