Biochemical modulation of fluoropyrimidines by antifolates and folates in an in vitro model of human leukemia

C(5) modified uracil derivatives showing antiproliferative and erythroid differentiation inducing activities on human chronic myelogenous leukemia K562 cells

The K562 cell line has been proposed as a useful experimental system to identify anti-tumor compounds acting by inducing terminal erythroid differentiation. K562 cells exhibit a low proportion of hemoglobin-synthesizing cells under standard cell growth conditions, but are able to undergo terminal erythroid differentiation when treated with a variety of anti-tumor compounds. In this paper we report a screening study on a set of different modified C(5) uracil derivatives for the evaluation of their antiproliferative effect in connection with erythroid differentiation pathways, and for defining a new class of drug candidates for the treatment of chronic myelogenous leukemia. Activity of the derivatives tested can be classified in two effect: an antiproliferative effect linked to a high level of erythroid differentiation activity and an antiproliferative effect without activation of gamma globin genes The highest antiproliferative effect and erythroid induction was shown by compound 9, a thymine derivative bearing a n-octyl chain on nitrogen N(1), whereas thymine did not show any effect, suggesting the importance of the linear alkyl chain in position N(1). To our knowledge this compound should be considered among the most efficient inducers of erythroid differentiation of K562 cells. This work is the starting point for the quest of more effective and specific drugs for the induction of terminal erythroid differentiation, for leading new insights in the treatment of neoplastic diseases with molecules acting by inducing differentiation rather than by simply exerting cytotoxic effects.

Phase I Study of Piritrexim and Gemcitabine in Patients With Advanced Solid Tumors

OBJECTIVES

In this phase I study, the combination of piritrexim and gemcitabine was given to establish the maximum tolerated dose and the recommended phase II dose, and to determine a toxicity and efficacy profile.

METHODS

Fifty-two patients with normal and impaired renal function were enrolled on this phase I study. The starting dose was piritrexim 10 mg 3 times daily (5 days of the week for 3 weeks and 1 week off each 28-day cycle) and gemcitabine 1000 mg/m2 on days 1, 8, and 15. The piritrexim was escalated in a stepwise fashion with this dose of gemcitabine and then with gemcitabine 1000 mg/m2 for days 1 and 15.

RESULTS

The recommended phase II dose of this combination was felt to be piritrexim 50 mg/day (10 mg every morning, 20 mg every noon, and 20 mg every evening) with gemcitabine 1000 mg/m2 on days 1, 8, and 15, and piritrexim 75 mg/day (25 mg thrice daily) with gemcitabine 1000 mg/m2 on days 1 and 15. Neutropenia and thrombocytopenia were the most often reported toxicity. Dose-limiting toxicity was thrombocytopenia in both groups. The number of renal-impaired patients enrolled was too small to establish a maximum tolerated dose for this group (piritrexim became unavailable), but the combination was tolerated in the patients with impaired renal dysfunction. There was 1 complete response, 1 partial response, and 1 minimal response.

CONCLUSION

The combination of piritrexim and gemcitabine was determined to be tolerable in heavily pretreated patients for use in solid tumors.

Mechanism of the antimicrobial drug trimethoprim revisited

We tested the hypothesis that the mechanism of action of the antifolate drug trimethoprim is through accumulation of bacterial dihydrofolate resulting in depletion of tetrahydrofolate coenzymes required for purine and pyrimidine biosynthesis. The folate pool of a strain of Escherichia coli (NCIMB 8879) was prelabeled with the folate biosynthetic precursor [(3)H]-p-aminobenzoic acid before treatment with trimethoprim. Folates in untreated E. coli were present as tetrahydrofolate coenzymes. In trimethoprim-treated cells, however, a rapid transient accumulation of dihydrofolate occurred, followed by complete conversion of all forms of folate to cleaved catabolites (pteridines and para-aminobenzoylglutamate) and the stable nonreduced form of the vitamin, folic acid. Both para-aminobenzoylglutamate and folic acid were present in the cell in the form of polyglutamates. Removal of trimethoprim resulted in the reconversion of the accumulated folic acid to tetrahydrofolate cofactors for subsequent participation in the one-carbon cycle. Whereas irreversible catabolism is probably bactericidal, conversion to folic acid may constitute a bacteriostatic mechanism since, as we show, folic acid can be used by the bacteria and proliferation is resumed once trimethoprim is removed. Thus, the clinical effectiveness of this important drug may depend on the extent to which the processes of either catabolism or folic acid production occur in different bacteria or during different therapeutic regimes.

Activities of thymidylate synthetase and thymidine kinase in gastric cancer

In a previous clinical study, sequential methotrexate and 5-fluorouracil has shown improved efficacy for treating advanced gastric cancer of the poorly differentiated type. In this study, we investigated whether difference in the levels of thymidylate synthetase (TS) and thymidine kinase (TK) activities in gastric cancer tissue account for selectivity of the treatment. Activity of TS was higher in 19 cases of the poorly differentiated type than in 16 cases of the well differentiated type (P < 0.02), whereas TK activity was lower in the poorly differentiated type than in the well differentiated type (P < 0.01). Thus, the TS/TK ratio was significantly higher in poorly differentiated gastric cancers than in well differentiated cancers (P < 0.001). These results suggest that a greater dependence upon the de novo pathway of pyrimidine synthesis in poorly differentiated gastric carcinomas may enhance the efficacy of sequential methotrexate and 5-fluorouracil treatment.