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Pyrazole Bioisosteres of Leflunomide as B-Cell Immunosuppressants for Xenotransplantation and Chronic Rejection:  Scope and Limitations

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posted on 12.08.1998 by Christos Papageorgiou, Rainer Albert, Philipp Floersheim, Michel Lemaire, Francis Bitch, Hans-Peter Weber, Elsebeth Andersen, Valerie Hungerford, Max H. Schreier
T-cell immunosuppressant-based therapies efficiently control early graft rejection in allotransplantation settings. They fail, however, to prevent those rejection events which are mediated by transplant-induced antibody (Ab) responses such as those involved in xenograft and chronic allograft rejection. This is mainly due to their inability to block T-cell-independent Ab production against the transplanted organs. The bioactive metabolite 2(Z) of leflunomide (1) inhibits the formation of such Ab, but the drug has pharmacokinetic properties and a therapeutic window incompatible with transplantation indications. Pyrazole 3, a constrained analogue of 2(Z), was designed and shown to be conformationally and biologically similar to 2(Z). Further investigations with derivatives of 3 demonstrated that the pyrazoles had very tight structure−activity relationships, the only equipotent compound being 3o. However, in contrast to 2(Z), both 3 and 3o were inactive in vivo due to short half-life and drug concentrations lower than the in vitro obtained IC50 values. Compound 3o inhibits T-cell-independent Ab production by a different biochemical mechanism from that of 2(Z) and 3 and may therefore represent a valuable tool for the identification of new targets for B-cell inhibition.

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