Pharmacology and phase I trial of high-dose oral leucovorin plus 5-fluorouracil in children with refractory cancer: a report from the Children's Cancer Study Group.

Because of the synergy seen in adult trials when 5-fluorouracil is combined with leucovorin, we initiated a Phase I trial of this combination in children's refractory cancer. Leucovorin, an equal mixture of the (6R,S)-diastereoisomers, was administered p.o. for 6 consecutive days as 4 equal doses at 0, 1, 2, and 3 h totaling 500 mg/m2/day. 5-Fluorouracil was given daily on days 2 to 6 as an i.v. bolus immediately following the last dose of leucovorin. The leucovorin dose was held constant while the 5-fluorouracil dose was escalated in cohorts of patients from 300 mg/m2/day to its maximally tolerated dose. Thirty-five patients (19 with acute leukemia and 16 with solid tumors) were evaluable for toxicity. The maximally tolerated dose of FUra was 450 mg/m2/day for 5 treatments for patients with solid tumors and 650 mg/m2/day for 5 treatments for the children with leukemia. The dose-limiting toxicities were myelosuppression and stomatitis. Other side effects included transient, mild elevations of serum transaminases, mild nausea, vomiting, and diarrhea. The pharmacokinetics of high-dose p.o. leucovorin was studied in 23 children. There was considerable interpatient variability in the plasma concentrations of total bioactive folates (TBAF), (6S)-leucovorin, and (6S)-5-methyltetrahydrofolic acid. The maximum plasma concentration (Cmax) of TBAF was 821 +/- 97 (SE) nM, occurring at a median of 8 h; the Cmax of (6S)-leucovorin was 77 +/- 11 nM, occurring at 4 h. The TBAF concentration fell to 146 +/- 42 nM by 24 h. (6S)-5-Methyltetrahydrofolic acid accounted for 90 +/- 7% of the TBAF at the Cmax. The plasma concentration of (6R)-leucovorin, the unnatural isomer, was equal to that of TBAF. Thus, p.o. leucovorin reduced the 5-fold excess of (6R)-leucovorin over TBAF seen after i.v. doses. The relative amounts of the three major plasma species were approximately the same as in adults, even though the Cmax of each compound was lower.

Biochemical interactions between methotrexate and 1-beta-D-arabinofuranosylcytosine in hematopoietic cells of children: a Pediatric Oncology Group study.

Children with acute lymphocytic leukemia (ALL) in remission were treated with overlapping sequential infusions of methotrexate (MTX) and 1-beta-D-arabinofuranosylcytosine (araC) as part of continuation therapy. The doses and the sequence were chosen to mimic conditions that produced greater than additive antineoplastic activity with these two drugs in preclinical studies. To assess the potential for the drug combination to exhibit greater than additive effect in vivo, we investigated several biochemical parameters that had been associated with synergism in vitro. Because the patients were in remission, the intracellular parameters could only be measured in cytologically normal hematopoietic cells. We observed that (1) the mean plasma concentrations of MTX and araC were above those required to obtain a greater than additive cytotoxicity with the two drugs in tissue culture; (2) MTX did not have a significant antipurine effect in bone marrow mononuclear cells; (3) the mean intracellular concentration of deoxycytidine triphosphate (dCTP) was significantly lower after treatment with the drug combination than after therapy with araC alone; and (4) the ratio of araC triphosphate (araCTP) to dCTP was 2.6 times higher after treatment with the combination than after araC alone. These results indicate that it is possible to achieve in patients the biochemical conditions associated with the greater than additive antineoplastic activity of MTX and araC in vitro.

Sensitive radiochemical assay for inosine 5'-monophosphate dehydrogenase and determination of activity in murine tumor and tissue extracts.

Crude tissue or tumor extracts either do not contain sufficient inosine 5'-monophosphate dehydrogenase (IMPD) activity to be measured spectrophotometrically, or interfering enzyme activities prevent the use of a more sensitive radiochemical assay. A modified assay system which incorporates alpha, beta-methylene adenosine 5'-diphosphate, an inhibitor of 5'-nucleotidase; allopurinol, an inhibitor of xanthine oxidase; and ethylenediaminetetraacetate, an inhibitor of alkaline phosphatase, has been developed. [14C]Xanthine monophosphate produced during the assay was separated from [14C]hypoxanthine monophosphate by thin-layer chromatography on flexible diethylaminoethyl-cellulose sheets. Xanthine monophosphate formation was linear for at least 40 min and was inhibited by greater than 95% in the presence of mycophenolic acid, a specific IMPD inhibitor. Partial purified IMPD from murine EMT6 tumors was used to compare assay rates obtained with the radiochemical and spectrophotometric assays under identical conditions. The reaction rate of the radiochemical assay was 0.92 +/- 0.07 (S.E.) of the rate of xanthine monophosphate formation as determined spectrophotometrically at 290 nm, indicating that both assays are measuring product formation with an equal degree of accuracy. The improved radiochemical assay was used to determine IMPD specific activity in supernatants from EMT6 tumors and several normal mouse tissues. The observed activities (nmol/min/mg protein) were: EMT6 tumor, 0.303; spleen, 0.029; brain, 0.022; kidney, 0.015; lung, 0.009; liver, 0.008; and heart and skeletal muscle, less than 0.004.