Which immunosuppressive drug is considered an antiproliferative (antimetabolite)?

Azathioprine is classified as an antiproliferative drug, specifically an antimetabolite, which means it interferes with the synthesis of nucleic acids and thereby inhibits the proliferation of cells, particularly lymphocytes that are involved in the immune response. This mechanism makes azathioprine effective in preventing rejection of transplanted organs by suppressing the immune system's ability to mount a response against the transplanted tissue.

Other options like cyclosporine and FK506 (tacrolimus) are calcineurin inhibitors, which primarily work by inhibiting T-cell activation and interleukin-2 transcription, but they do not fall into the category of antiproliferative agents. Prednisolone is a corticosteroid that has broad immunosuppressive effects, including anti-inflammatory actions, but it is not categorized specifically as an antimetabolite. Therefore, azathioprine is unique among the choices for its specific role as an antimetabolite in immunosuppressive therapy.