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 Group1

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 (6/i,,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 évaluablefor 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 (65)-5-methyltetrahydrofolic acid. The maximum plasma concentration (C„„) of TBAF was 821 ±97 (SE) nM, occurring at a median of 8 h; the Câ„¢,of (65>leucovorin was 77 ±11 nM, occurring at 4 h. The TBAF concentration fell to 146 ±42 HMby 24 h. (65>5-Methyltetrahydrofolic acid accounted for 90 ±7% of the TBAF at the ( „,,x. The plasma concentration of (6/?)-leucovorin, the unnatural isomer, was equal to that of TBAF. Thus, p.o. leucovorin reduced the 5-fold excess of (6W)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 ('„,„, of each compound was lower. INTRODUCTION The fluorinated pyrimidine analogue FUra3 is frequently used for the treatment of many adult carcinomas. FUra exerts its antitumor effects through the metabolic conversion to 5-fluorouridine-5'-triphosphate with subsequent incorporation into RNA and through the formation of FdUMP and inhibition of thymidylate synthase (1, 2). FdUMP is a competitive inhibitor of thymidylate synthase by formation of a ternary complex with the active site of the enzyme and 5,10-methylenetetrahydroReceived 3/13/91; accepted 7/8/91. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. ' Supported by the Burroughs Wellcome Company and the Division of Cancer Treatment, National Cancer Institute, NIH, Department of Health and Human Services (Support Grants CA38053 and CCSG). Contributing Children's Cancer Study Group investigators, institutions, and grants are given in Appendix Table 1. 2To whom requests for reprints should be addressed, at Children's Cancer Study Group, 440 East Huntington Drive, Suite 300, P.O. Box 60012, Arcadia, CA 91006-6012. 3The abbreviations used are: FUra, 5-fluorouracil; AUC, area under the plasma concentration versus time curve; CH3-THF, (6S)-5-methyltetrahydrofolate; Cm„, maximum plasma concentration; Cmi„. minimum plasma concentration; Cp, plasma concentration; FdUMP, 5-fluorodeoxyuridine monophosphate; MTD, maximally tolerated dose; TBAF, total bioactive folates. folate. The presence of 5,10-methylenetetrahydrofolate in creases the binding constant for FdUMP by seven or eight orders of magnitude (3, 4). Intracellular levels of reduced folate cofactors can be a critical determinant of the cytotoxic effects of FUra. Should folate cofactors be depleted, administration of leucovorin with fluorinated pyrimidines would be expected to enhance the inhibition of thymidylate synthase by FdUMP. This hypothesis has been verified using LI210 cells and human cells in tissue cultures (4-6). Leucovorin is the soluble calcium salt of diastereoisomers, (oA^J-folinic acid. It is generally accepted that only the natural isomer (6S)-leucovorin is active as a cofactor. The natural isomer is extensively metabolized in the body to its active metabolite 5,10-methylenetetrahydrofolate. Clinical trials using FUra and leucovorin in adult patients with colorectal carcinoma have shown objective tumor re sponses of up to 22% in previously treated patients and 26 to 44% in untreated patients (7-9). FUra showed minimal activity against childhood acute leu kemia and solid tumors in early clinical trials (10, 11). Because of the potentiation seen in animal and adult trials, the Chil dren's Cancer Study Group decided to conduct a Phase I trial of combination chemotherapy (CCG-8603) with high-dose leu covorin and FUra in children with refractory cancer. Adminis tration of leucovorin p.o. was chosen for outpatient convenience and because absorption differs for (65)and (6/?)-leucovorin. The ratio of plasma concentration of the unnatural isomer, (6Ä)-leucovorin, to the TBAF is substantially lower after p.o. than after i.v. doses (12). The unnatural isomer could interfere with transport, binding, or metabolism of (65)-leucovorin or its metabolites, although recent evidence suggests that it may not interfere with the enhancement of FUra toxicity by the (6S)-isomer (13). The absorption of leucovorin is saturable, so the 500-mg/m2/day dose of leucovorin was divided into four doses given at 0, 1, 2, and 3 h (12, 14, 15). FUra was adminis tered as an i.v. bolus daily for 5 days. In order to achieve the maximal effect of leucovorin on folate pools and thus potentiate the effect on ternary complex formation, leucovorin was started 1 day before the FUra and continued daily for a total of 6 days. This paper reports the maximally tolerated dose of FUra when given with leucovorin p.o. and the toxicities and the concentrations of (6A)-leucovorin, (6S>leucovorin, and its ma jor metabolite CH3-THF during the 6 days of daily treatment. MATERIALS AND METHODS Patient Eligibility. This study enrolled patients less than 21 yr old from the selected institutions of the Children's Cancer Study Group and the Pediatrie Branch of the National Cancer Institute. All patients had histologically confirmed cancer refractory to conventional forms of therapy or for which no other therapy with established efficacy was available. Patients had recovered from the toxic effects of prior therapy, 4871 on April 15, 2017. © 1991 American Association for Cancer Research. cancerres.aacrjournals.org Downloaded from LEUCOVORIN AND 5-FLUOROURACIL p.o. IN CHILDREN Table 1 FUra doses in pediatrìepatients