Cyclosporin D, a metabolite of Cyclosporin A, is a weak immunosuppressant. Cyclosporin D is used as internal standard for quantification of Cyclosporin A. Cyclosporin A is a potent immunosuppressant agent, suppress T cell activation by inhibiting calcineurin and the calcineurin-dependent transcription factors nuclear factor of activated T cells (NFAc).