Peptidyl-prolyl isomerases (PPIases) are a well conserved class of enzymes found throughout nature in microorganisms, plants and animals. They are characterized by their ability to catalyze the conversion of cis- and trans- peptidyl-proline bonds in proteins and consequently are able to exert control over target protein structure and function.
PPIases are widely expressed across cell types with isoforms demonstrating subcellular specificity and trafficking between compartments. Certain PPIases may be secreted and it is now recognized that, in addition to being chaperones that control the spatial-temporal function of target proteins, they may also be able to act as secreted cytokines acting at cell surface receptors.
PPIases also interact with other post-translational control mechanisms, elegantly exemplified by the PPIase Pin1. It recognises target proteins containing a phosphoSer/Thr-Pro motif and its activity is therefore able to control events governed by proline-directed phosphorylation and dephosphorylation. As a result, Pin1 has become a focus of attention for oncology and Alzheimer’s disease research.
Original studies on the immunosuppressive properties of cyclosporine identified the cyclophilin subfamily of PPIases as molecules important in controlling inflammatory responses, either intracellularly or via interaction with cell surface receptors such as CD147 and probably others. A large body of work has focused on the immunomodulatory activity of cyclophilin and FKBP inhibitors and their use in transplantation has formed the basis of their commercial exploitation.
As the wider role of PPIases has become better understood, additional uses have arisen. In particular, several cyclophilin inhibitors are in clinical development for the treatment of viral infections as the host cell cyclophilin A is an important co-factor in the replication of certain viruses such as HCV and HIV.
Read the white paper, Prolyl Isomerases – Old Proteins as New Therapeutics Targets, co-written by Selcia and Cypralis scientists.
Sub-type selective cyclophilin inhibitors
The cyclophilin inhibitors on the market or in development are non-selective between the four common cyclophilin isoforms A, B, C and D. Knockout animals have been used to identify roles specific to the individual isoforms and highlight their use as targets for treating diseases. This has elevated the interest in obtaining drugs selective for particular isoforms, in particular cyclophilin D for degenerative diseases involving mitochondrial dysfunction.
Cyclophilin D has been recognized as an excellent molecular target for several years  but until recently, achieving sub-type selectivity has been elusive. Cypralis is at the forefront of developing sub-type selective cyclophilin inhibitors.
Neurodegenerative diseases: The potential of cyclophilin inhibition
Drug Target Review, 21 January 2016