The cyclosporins comprise a family of cyclic 11-peptides containing seven N-methylated, hydrophobic amino acids. Cyclosporine A (CsA) representing the major member of this family is widely used as immunosuppressive agent in organ transplantations. The biological activity resides in the cells of lymphocytes T forming in a first step a binary complex with cyclophilin A (CyPA), which transforms to a ternary complex in binding to a second receptor molecule, i.e calcineurin (CN). The formation of a ternary complex results in an inhibition of a dephosphorylation step of transcription factor NFAT, preventing the translocation of NFAT needed for T-cell activation. More recently it was found that CyPA acts in addition as ligand for polyprotein gag pr55 liberated by the capsid of the virus HIV, stimulating the production of the virus. In complexing to CsA, this step is prevented, blocking the bio-synthetic pathway for the HIV reproduction. In previous research work of the Mutter group, CsA has been transformed by chemical synthesis to non-immunosuppressive, anti-viral analogues as potential drugs against AIDS. Based on these studies, the present PhD thesis aims to overcome some major limitations in the application and further exploration of structure-activity-relationship (SAR) studies of pharmaceutically interesting CsA analogues. The low bioavaibility of CsA results from its hydrophobic character combined with low water solubility. For overcoming this problem, the first part of the present thesis elaborates novel types of prodrug systems based on the recently developed concept of "switch-peptides" (M.Mutter et al., Angew.Chem., 2004). To this end, CsA is transformed to O-isoacyl-CsA derivatives featuring a selectively cleavable amino-protecting group Y (figure 1), resulting in a stable, highly soluble prodrug devoid of any biological activity (Soff state). The transformation into the active compound (Son state) can be triggered at physiological conditions by the photolytic or enzymatic cleavage of Y, followed by spontaneous O to N acyl migration. By the introduction of linkers in the enzymatically cleavable system, the transformation of the prodrug into the native compound can be modulated over a broad time interval, i.e between minutes and several hours. Interestingly, the use of photocleavable groups results in completely stable prodrug systems (Soff state), whereas the structurally more demanding enzymatically cleavable systems show strongly increased water solubility, but reduced thermodynamic stability. Figure 1: CsA as prodrug applying the concept of "switch-peptides" The elaborated systems can be transferred to analogues of CsA containing Ser, Thr or Cys residues at specific positions. As a most promising candidate for exploring antiviral, non-immunosuppressive properties, we focus in the second part of the thesis on the synthesis of functional CsA analogues at position 8 by N-alkylation reactions. Previous studies by O.Turpin (PhD thesis 2004, EPFL) have shown, that the differential reactivity of the non N-methylated sites in CsA allows for a selective introduction of functional groups. Scheme 1: Strategy for the synthesis of linear precursor molecules of (Axx8)-CsA By applying optimized methodologies as outlined in Scheme 1, i.e selective N-alkylation at position 8 (step 1), rearrangement reaction (2), ring opening step and Edman degradation (3), we succeeded in overall yields of the linear decapeptide up to 25 % as versatile precursor molecule. Most notably, key step 1 proceeded in high yields, allowing for direct chemical derivatisations within the CN-binding site. In conclusion, the elaboration of synthetic pathways to novel prodrug systems ("switch-peptides") in combination with a selective access to CsA analogues modified at the calcineurin-binding site offers new perspectives in the design of cyclophilin inhibitors of high therapeutical relevance.