The principal and potentially serious toxic effects of PURINETHOL are bone marrow toxicity and hepatotoxicity (see WARNINGS and PRECAUTIONS).

Hematologic

The most frequent adverse reaction to PURINETHOL is myelosuppression. The induction of complete remission of acute lymphatic leukemia frequently is associated with marrow hypoplasia. Patients without TPMT enzyme activity (homozygous-deficient) are particularly susceptible to hematologic toxicity, and some patients with low or intermediate TPMT enzyme activity are more susceptible to hematologic toxicity than patients with normal TPMT activity (see WARNINGS, Bone Marrow Toxicity), although the latter can also experience severe toxicity. Maintenance of remission generally involves multiple-drug regimens whose component agents cause myelosuppression. Anemia, leukopenia, and thrombocytopenia are frequently observed. Dosages and also schedules are adjusted to prevent life-threatening cytopenias.

Renal

Hyperuricemia and/or hyperuricosuria may occur in patients receiving PURINETHOL as a consequence of rapid cell lysis accompanying the antineoplastic effect. Renal adverse effects can be minimized by increased hydration, urine alkalinization, and the prophylactic administration of a xanthine oxidase inhibitor such as allopurinol. The dosage of PURINETHOL should be reduced to one third to one quarter of the usual dose if allopurinol is given concurrently.

Gastrointestinal

Intestinal ulceration has been reported. Nausea, vomiting, and anorexia are uncommon during initial administration, but may increase with continued administration. Mild diarrhea and sprue-like symptoms have been noted occasionally, but it is difficult at present to attribute these to the medication. Oral lesions are rarely seen, and when they occur they resemble thrush rather than antifolic ulcerations.

Miscellaneous

The administration of PURINETHOL has been associated with skin rashes and hyperpigmentation. Alopecia has been reported.

Drug fever has been very rarely reported with PURINETHOL. Before attributing fever to PURINETHOL, every attempt should be made to exclude more common causes of pyrexia, such as sepsis, in patients with acute leukemia.

Oligospermia has been reported.

Read the Purinethol (mercaptopurine) Side Effects Center for a complete guide to possible side effects »

When allopurinol and mercaptopurine are administered concomitantly, the dose of mercaptopurine must be reduced to one third to one quarter of the usual dose to avoid severe toxicity.

There is usually complete cross-resistance between mercaptopurine and thioguanine.

The dosage of mercaptopurine may need to be reduced when this agent is combined with other drugs whose primary or secondary toxicity is myelosuppression. Enhanced marrow suppression has been noted in some patients also receiving trimethoprim-sulfamethoxazole.

Inhibition of the anticoagulant effect of warfarin, when given with mercaptopurine, has been reported.

As there is in vitro evidence that aminosalicylate derivatives (e.g., olsalazine, mesalazine, or sulphasalazine) inhibit the TPMT enzyme, they should be administered with caution to patients receiving concurrent mercaptopurine therapy (see WARNINGS).

Last reviewed on RxList: 7/1/2011
This monograph has been modified to include the generic and brand name in many instances.