Is azathioprine a better immunosuppressive than 6-mercaptopurine?

The toxicities and immunosuppressive potencies of single doses of 6-mercaptopurine and azathioprine have been compared in mice, using the 30-day mortality as a measure of toxicity, and reduction in spleen plaque-forming cells in response to sheep erythrocytes as a measure of immunosuppression. When compared on the basis of equivalent toxicity, 6-mercaptopurine was consistently the more effective agent by the intraperitoneal route. By the subcutaneous route, 6-mercaptopurine was more effective at doses above the LD-30; at lower doses, azathioprine was marginally better, but the difference was probably not significant. For the same cost in toxicity, azathioprine was six to seven times more effective as an immunosuppressive by the subcutaneous as by the intraperitoneal route.