What is the mechanism of action of cyclosporine?

The mechanism of action of cyclosporine is accurately described as that of a calcineurin inhibitor. It functions by binding to a cytosolic protein known as cyclophilin. This binding forms a complex that subsequently inhibits the activity of calcineurin, a serine-threonine phosphatase. The inhibition of calcineurin prevents the dephosphorylation of nuclear factor of activated T-cells (NFAT), which is crucial for T-cell activation and the production of interleukin-2 (IL-2). As a result, cyclosporine effectively suppresses the immune response.

This action is particularly relevant in the context of organ transplantation, where the drug is used to prevent rejection by dampening the activity of T cells that respond to foreign antigens. The other options provided refer to different mechanisms of action associated with other drug classes and do not apply to cyclosporine specifically. For instance, the interference with DNA synthesis is more aligned with cytotoxic agents, while the inhibition of ergosterol synthesis pertains to antifungal medications, and the inhibition of protein disulfide isomerase is related to specific biochemical pathways not influenced by cyclosporine.