Mutagenic and cytotoxic effects of immunosuppressive drugs on human lymphocyte cultures.

Objectives: Immunosuppressive drugs such as cyclosporine A, mycophenolate mofetil, tacrolimus, and the immunosuppressive agent sirolimus are used effectively to prevent immunologic rejection after solid-organ transplantation. The most serious complication among patients undergoing immunosuppressive therapy is the risk of developing cancer. The question is whether the drugs used have mutagenic properties and so contribute to increased cancer risk.

Materials And Methods: We evaluated the mutagenic and cytotoxic effects of the above-mentioned drugs in human lymphocyte cultures with special consideration given to clinically relevant blood-drug concentrations. Mutagenicity was tested by analyzing micronuclei using the well-established cytokinesis-block micronucleus assay with cytochalasin B. To evaluate cytotoxicity, the cytokinesis-block proliferation index was calculated. Concentrations used ranged from 0.1-2 mug/mL for cyclosporine A, 1-20 microg/mL for mycophenolate mofetil, 5-40 ng/mL for tacrolimus, and 2.5-50 ng/mL for sirolimus. We also estimated mutagenicity and cytotoxicity in the blood of kidney transplanted patients using the above-mentioned techniques.

Results: Cultures supplemented with mycophenolate mofetil or tacrolimus showed higher amounts of micronuclei when compared with solvent controls in all concentrations tested. Addition of cyclosporine A to cultures also led to a rise in the number of micronuclei at concentrations of 0.2 mug/mL and 0.4 mug/mL. In contrast with the other immunosuppressive drugs, sirolimus induced only weak mutagenic activity in the micronuclei test at its highest concentration (50 ng/mL). Cytotoxic effects were seen only in mycophenolatemofetil-supplemented cultures at all concentrations tested (P<0.01). In comparison with healthy persons, those with kidney transplants under immunosuppression displayed a broad reduction in the cytokinesis-block proliferation index (P<0.001) and a significant increase in the frequency of micronuclei (P<0.001).

Conclusions: Our results indicate that mycophenolate mofetil and tacrolimus display more mutagenic effects in vitro than do cyclosporine A or sirolimus, and that transplanted patients exhibit higher amounts of micronuclei and a noteworthy reduction in the cytokinesis-block proliferation index compared with healthy persons.