Enzymes of purine metabolism in lymphoid neoplasms, clinical relevance for treatment with enzyme inhibitors

Pentostatin for the Treatment of Indolent Lymphoproliferative Disorders

Purine analogues have been shown to be active in a variety of B- and T-cell malignancies. Among them, pentostatin is also a tight binding inhibitor of adenosine deaminase (ADA), a key enzyme of purine metabolism. ADA is present in all human tissues, with the highest levels in the lymphoid system. Early clinical trials with pentostatin used high doses for acute lymphoblastic leukemias, which were characterized by high levels of ADA. Through the efforts of a few investigators, low-dose regimens that are active and well tolerated for indolent lymphoid malignancies have been developed. Myelosuppressive adverse effects have been shown to be minimal using these schedules. Lymphoplasmacytic lymphoma (LL) is an indolent chronic B-cell lymphoproliferative disorder moderately responsive to alkylating agents. All of the purine analogues have shown activity in LL. However, the advantage of pentostatin over the other agents is the relatively specific toxicity to lymphoid cells and the paucity of myelosuppression as a single agent. No direct comparisons of the agents have been investigated, although pentostatin may be considered to be preferred since it has not been associated with toxicity to myeloid progenitors in colony assays. This is of significance for patients who might benefit from high-dose chemotherapy with autologous stem cell transplantation.

Pentostatin and purine analogs for indolent lymphoid malignancies

Pentostatin has been shown to be active in a variety of B- and T-cell malignancies. The drug is a tight inhibitor of adenosine deaminase, a key degradative enzyme of purine metabolism present in all human tissues, with the highest levels found in the lymphoid system. Early clinical trials indicated that this agent was highly active in acute lymphoblastic leukemias with high intracellular adenosine deaminase levels. Relatively high doses of the drug were needed, which was associated with severe adverse events. Through the efforts of a few investigators, better tolerated, low-dose regimens have been shown to be extremely active in lymphoproliferative diseases with very low intracellular adenosine deaminase levels such as hairy cell leukemia, B- and T-cell chronic leukemias, T-cell cutaneous lymphomas and low-grade non-Hodgkin lymphomas. Clinical as well as experimental data have indicated that this drug induces lymphocyte-specific cytotoxicity, and myelosuppressive adverse events have been minimal. Although all the purine analogs have shown similar activity, the advantage of pentostatin is the relatively specific cytotoxicity against lymphocytes, which permits treatment even in patients with severe cytopenias. Although no direct comparisons of the purine analogs have been performed, pentostatin may be preferred due to this property.

Plasma purine nucleoside phosphorylase in cancer patients

BACKGROUND

Purine nucleoside phosphorylase (PNP) is the purine salvage enzyme that converts guanosine to guanine and inosine to hypoxanthine.

METHODS

279 samples from patients with differing cancers were collected during treatment at both pre- and post-dose stages for plasma PNP activity and compared with a normal population.

RESULTS

Normal plasma PNP activity was found to be 3.2+/-1.4 U/l (n=55) as compared with the cancer patients (pre-dose 12.3+/-7.4 U/l [n=215] and post-dose 11.2+/-5.9 U/l [n=64]). Levels of plasma PNP did not differ greatly between the different cancer types but were on average four times greater than that found in the reference population.