Cyclosporin analogues

Cyclosporins are a family of hydrophobic cyclic undecapeptides produced by the fungus Tolypocladium niveum. The main metabolite cyclosporin A (CsA, see Scheme, R = R' = H) is the first line drug currently used to prevent rejection of organ transplants. Its strong immunosuppressive activity is achieved through the formation of a cyclosporin–cyclophilin A (CypA) complex inhibiting the calcium- and calmoduline-dependent protein phosphatase calcineurin (Cn). This trimeric complex prevents translocation of the transcription enhancer NFAT into the nucleus, inhibiting T-cell activation. It was also shown that, in the presence of CsA, chronically infected cells produce non-infectious HIV particles. By binding with the HIV capsid gag polyprotein pr55, CypA is recruited in stoichiometric amounts in the nascent HIV virion. The formation of the CsA-CypA complex disrupts this interaction at the maturation stage of the HIV infection and blocks the replication of HIV in cells. Based on SAR studies, the present thesis aims to develop novel analogues and prodrugs of CsA exhibiting antiviral, non-immunosuppressive activities. The first part of this work describes the regioselective synthesis and the pharmacological properties of a novel series of dimers of cyclosporin analogs modified at position 8 (see Scheme, in blue). Our findings demonstrate that the dimerization at position 8 of cyclosporin derivatives strongly increase their affinity to CypA and drastically alter their binding to Cn, thus representing the first derivatives of cyclosporin exhibiting non-immunosuppressive (loss of Cn binding), anti-viral (CypA binding) activities by the exclusive modification at position 8. By designing linkers of tailored length to permit the cooperative interaction with both immunophilin receptors, bivalent ligands have been developed. As indicated by receptor binding studies, these dimers represent the most potent non-immunosuppressive cyclophilin binders reported up to now, potential anti-HIV-1 inhibitors. The goal of the second part of this thesis was to use various concepts for prodrug design for modulating the pharmacokinetic properties and for increasing the low water-solubility of cyclosporin analogs that is responsible for undesirable pharmaceutical properties of cyclosporins such as erratic oral absorption profile, poor oral bioavailability and complications in formulating. A prodrug is a pharmacologically inactive derivative of a parent drug molecule that requires spontaneous or enzymatic transformation within the body in order to release the active drug, exhibiting improved delivery properties compared to the parent drug molecule. The elaborated chemical modifications of the cyclosporin derivatives allowed us (1) to mask the bioactive conformation of the cyclosporine, leading to the loss of binding to CypA and Cn, and consequently in a loss of the immunosuppressive activity and (2) to increase the solubility of the cyclosporins through the introduction of a solubilizing group (SG). As a first chemical modification, the hydroxyl group of the residue MeBmt-1 of cyclosporins was masked by a chemical unit that can be selectively removed enzymatically (Scheme, in green). The double prodrugs showed an increase in water-solubility of factors up to 1000 compared to their parent drug and a specific enzymatic controlled drug release. Alternatively, a novel strategy for the selective introduction of an acyl-function at different positions of hydroxyl-containing cyclosporins (Scheme, blue and red) was explored. The corresponding O-acyl isopeptides undergo spontaneous O,N-acyl migration (serving as chemical switch), resulting in the recovery of the parent compound under physiologically conditions (pH controlled drug release). The investigated derivatives showed high thermodynamic stability, differential conversion rates and strongly increased water solubility, offering novel chemoreversible prodrugs of cyclosporin analogs. In extending this concept, O-acyl iso-cyclosporins stabilized by a pro-moiety were developed, setting the stage for a novel class of double prodrugs exhibiting improved thermodynamic stability and tailored chemical and enzymatic conversion rates. With the increasing challenges in drug delivery of complex therapeutic agents, the elaborated systems provide versatile, generally applicable strategies for hydroxyl group containing drug candidates.