Elucidation of pathways of 5-fluorouracil metabolism in xenografts of human colorectal adenocarcinoma

Hypoxanthine (Hx) and allopurinol (HPP) have been shown experimentally to reduce the conversion of 5-fluorouracil (FUra) to FUMP by orotate phosphoribosyltransferase (OPRTase). This study was designed to elucidate the major pathway by which FU ra was metabolized to ribonucleotides by human colorectal tumors. Consequently, the effect of Hx and HPP on the metabolism of [6-3H]-FUra was examined in 5 human colorectal adenocarcinomas maintained as xenografts in immune-deprived mice. In 2 tumors the formation of ribonucleotides from FUra was depressed by Hx and HPP in combination during the first hour after treatment, while in 3 other lines ribonucleotide concentrations were not reduced. The data suggested that these 5 xenograft lines may be divided into 2 groups: (1) group 1 tumors formed relatively high levels of FUrd and low levels of fluorinated ribonucleotides after the injection of FUra, with no decrease in ribonucleotide concentrations after the administration of Hx and HPP. These tumors possessed high ratios of uridine (Urd) phosphorylase/orotate phosphoribosyltransferase (OPRTase: 7-24) and ribose-1-phosphate (R-1-P)/5-phosphoribosyl-1-pyrophosphate (PRPP;5), and thus appeared to metabolize FUra by the U rd phosphorylase and U rd kinase pathway; (2) group 2 tumors formed low levels of FU rd, higher concentrations of fluorinated ribonucleotides and a reduction in levels of these nucleotides after administration of the purine combination. Group 2 tumors demonstrated a lower enzyme ratio (1-2), higher endogenous levels of PRPP, a lower R-1-P/PRPP ratio (1) and appeared to metabolize FUra predominantly by the activity of OPRTase. Hypoxanthine and HPP, alone or in combination, caused a rapid depletion of PRPP in each tumor line examined. In group 2 tumors this may be responsible for the decreased formation of FUra ribonucleotides observed.

The selectivity of action of methotrexate in combination with 5-fluorouracil in xenografts of human colon adenocarcinomas

This study was designed to examine the possibilities for increasing the therapeutic index in the treatment of human colon adenocarcinomas maintained as xenografts in immune-deprived mice using combinations of methotrexate (MTX) that preceded 5-fluorouracil (FUra). MTX, at a dose level of 100 mg/kg, increased the 5-phosphoribosyl-1-pyrophosphate (PRPP) concentration in three colon xenograft lines to a maximum between 14 and 24 hr after treatment. In murine bone marrow, concentrations of PRPP decreased progressively after MTX treatment, but in ileum there was a dramatic increase such that by 4 hr PRPP was 968% of control. The metabolism of [6-3H]FUra administered 24 hr after MTX was increased in ileum and resulted in an increased rate and a greater level of incorporation of [6-3H]FUra into RNA. Only a slight elevation in the incorporation of [6-3H]FUra into the RNA of one tumor line (HxELC2) was observed. The scheduling of FUra at a dose level of 25 mg/kg 24 hr after a priming dose of MTX (100 mg/kg) was at least as toxic as 100 mg of FUra per kilogram, administered alone. The dose-limiting toxicity was related to gastrointestinal damage; no bone marrow toxicity was detected. At dose levels of MTX up to 100 mg/kg, the increase in PRPP obtained in gastrointestinal tissue was greater than that observed in human colon xenografts 24 hr after treatment. A basis for increasing the therapeutic efficacy of FUra through a selective increase in tumor PRPP using MTX was not obtained in these studies.

Combinations of 5-FU, hypoxanthine and allopurinol in chemotherapy for human colon adenocarcinoma xenografts

A series of four human colon adenocarcinomas, growing as xenografts in immune-deprived mice, have been used to evaluate the efficacy of 5-FU in combination with two purines, hypoxanthine (Hx) and allopurinol (HPP), which have reduced the toxicity of 5-FU in host mice. Tumor-bearing mice were treated at 7-day intervals with 5-FU administered simultaneously with the protecting agents (Hx and HPP). Two tumor lines (HxVRC5 and HxGC3), insensitive to 5-FU alone, failed to show any response to this combination. In 5-FU-sensitive HxELC2 tumors, the combination of 5-FU with Hx and HPP did not increase the therapeutic index, and in HxHC1 xenografts, antagonism to 5-FU cytotoxicity was observed. Tumor response in relation to the pathways of 5-FU metabolism is discussed.

The effect of derivatives of folic acid on the fluorodeoxyuridylate-thymidylate synthetase covalent complex in human colon xenografts

This study was designed to examine the endogenous concentrations of 5,10-methylenetetrahydrofolate (CH2FH4) in human colorectal adenocarcinoma xenografts, and to determine the ability of other folate derivatives to increase the formation of the ternary covalent complex between CH2FH4, [6-3H]-5-fluorodeoxyuridylate (FdUMP) and thymidylate synthetase (TS, EC 2.1.1.45). Levels of CH2FH4 were determined by measuring the release of [3H]2O from [5-3H]-dUMP using TS from Lactobacillus casei. The reaction was linear from 1.9 X 10(-13) to 2.4 X 10(-11) mol of CH2FH4 assayed. Concentrations of CH2FH4 were low, ranging from 66 to 233 nM in cell water. Tetrahydrofolate (FH4) and dihydrofolate (FH2) increased complex formation, while 5-formyltetrahydrofolate (5-CHOFH4) and 5-methyltetrahydrofolate (5-CH3FH4) decreased the covalent binding of [6-3H]-FdUMP in vitro. Administration of FH4 or FH2 to tumor-bearing mice reduced subsequent formation of the covalent complex in vitro. Since 5-CH3FH4 is a major derivative of folate in mammalian tissues, its effect on the covalent binding of [6-3H]-FdUMP was examined further; even in the presence of homocysteine and cyanocobalamin (B12), the formation of the covalent complex was not increased. The fate of [5-14CH3]-FH4 was subsequently examined in vivo. In tumors at 1 hr after injection, 72% of the radiolabel remained as [5-14CH3]-FH4, while 17% had been converted to [14C]-methionine or incorporated into protein. By contrast, however, the incorporation of radiolabel into the protein fraction of liver was almost 30-fold greater at this time. At 4 hr, radioactivity in tumors (dpm/g) and in the fraction associated with [5-14CH3]-FH4 was decreased by over 60%, while metabolism was increased by only 13%. No polyglutamate forms of [5-14CH3]-FH4 were detected in tumors at 4 hr after treatment.

Growth and characterization of childhood rhabdomyosarcomas as xenografts

Eleven surgical specimens of childhood rhabdomyosarcoma (RMS), two bone marrow samples, and cells from ascitic fluid from 1 patient were implanted sc into immune-deprived inbred CBA/CaJ mice. From these, seven lines of RMS were obtained as xenografts, each retaining the histologic characteristics of the tumor of origin and human lactate dehydrogenase isozymes; these represented 6 of the 11 surgical specimens, whereas one line originated from the transplanted cells from ascitic fluid. It was concluded that children RMS has a fairly high frequency of heterotransplantability in this system and that such a laboratory model may prove useful in the development of new therapeutic regimens in this disease.

Chemotherapy of childhood rhabdomyosarcomas growing as xenografts in immune-deprived mice

Xenografts derived from the neoplastic tissues of children with rhabdomyosarcoma have been used in immune-deprived mice to examine the efficacy of agents known to be active against this disease, and in others that received either limited or no clinical evaluation. Two models were derived; xenografts were established from tumors obtained from either (a) untreated patients or (b) from patients who had become refractory to conventional therapy. Model a identified as being effective each of these clinically used agents: vincristine, dactinomycin, cyclophosphamide, and doxorubicin; mitomycin C and 5-(3,3-dimethyl-1-triazeno)-2-methylimidazole-4-carboxamide also showed activity, as did busulfan in one tumor line. Tumors derived from refractory patients were significantly less responsive to all agents examined.

Development of resistance to vincristine in a childhood rhabdomyosarcoma growing in immune-deprived mice

A cell line of childhood rhabdomyosarcoma (RD) has been grown as a xenograft in immune-deprived mice. The tumor responded to vincristine (VCR), but not to vinblastine, doxorubicin or actinomycin D. The rate and frequency at which resistance developed from administration of VCR once weekly was investigated. Tumor growth could be inhibited for 6 weeks, after which time 11 or 16 xenografts grew progressively despite continued treatment. That this was a tumor-acquired resistance was confirmed by growing both parent and "resistant" lines in the same host. Under these conditions VCR completely inhibited growth of the parent tumor, but not growth of the resistant for 6 weeks, after which time 11 or 16 xenografts grew progressively despite continued treatment. That this was a tumor-acquired resistance was confirmed by growing both parent and "resistant" lines in the same host. Under these conditions VCR completely inhibited growth of the parent tumor, but not growth of the resistant for 6 weeks, after which time 11 or 16 xenografts grew progressively despite continued treatment. That this was a tumor-acquired resistance was confirmed by growing both parent and "resistant" lines in the same host. Under these conditions VCR completely inhibited growth of the parent tumor, but not growth of the resistant line. Continued passage of the resistant line for 10 months in mice either treated with VCR (1.5 mg/kg/wk) or untreated, demonstrated that resistance to VCR was stable in the absence of selection pressure. In addition, the VCR-resistant line acquired a stable change in karyotype with the addition of a number 9 chromosome and an additional, unknown marker.

Biochemical determinants of responsiveness to 5-fluorouracil and its derivatives in xenografts of human colorectal adenocarcinomas in mice

The level of thymidylate synthetase (EC 2.1.1.45) and its activity have been measured in a series of human colorectal adenocarcinomas growing as xenografts in immune-deprived mice. Enzyme activity varied between 8.4 and 124 pmol per mg protein per hr; within each tumor line, this activity correlated with the capacity to bind [6-3H]5-fluoro-2'-deoxyuridine 5'-monophosphate ([6-3H]FdUMP), which varied between 0.16 and 1.68 pmol [6-3H]FdUMP bound per g tissue. Highest and lowest activities were measured in tumor lines that were insensitive to 5-fluorouracil, 5-fluorouridine, and 5-fluoro-2'-deoxyuridine. The ratio of the maximum free FdUMP concentration to thymidine 5'-monophosphate synthetase-binding activity did not differentiate fluorinated pyrimidine-responsive lines from those innately insensitive. Maximum potential binding of [6-3H]FdUMP in vitro was measured without addition of dl-L-5,10-methylenetetrahydrofolate (CH2FH4) in cytosol from two tumor lines, both of which demonstrated some sensitivity to fluorinated pyrimidine therapy. The other four insensitive tumor lines required CH2FH4 to be added in order to attain maximum [6-3H]FdUMP binding. Similar data were obtained using nitrocellulose membrane filtration to isolate both covalent and noncovalent complexes. Direct measurement of thymidine 5'-monophosphate synthetase activity after incubation of tumor cytosols with FdUMP, with or without added CH2FH4 showed that, in nonresponsive tumors, maximum enzyme inhibition was achieved only in the presence of exogenous cofactor. It is suggested that the availability of cofactor may prove important in the formation of the ternary complex CH2FH4:thymidine 5'-monophosphate synthetase:FdUMP when high concentrations of FdUMP are present for only short periods of time.

On the mechanism of cytotoxicity of fluorinated pyrimidines in four human colon adenocarcinoma xenografts maintained in immune-deprived mice

Three fluorinated pyrimidines, 5-fluorouracil (FUra), 5-fluorouridine (FUrd), and 5-fluoro-2'-dexoyuridine (FdUrd), have been studied in four human colonic tumor xenograft lines. The preliminary findings may be summarized as follows: 1) after equimolar dosages, the agent reaching the highest concentration in the tumor produced the highest level of fluorodeoxyuridylate (FdUMP); 2) within a tumor line, the order of response to the three agents is related to the order of FdUMP-generation; 3) the tumor-response did not correspond to the level of analogue-incorporation into RNA; 4) the measurement of levels of free FdUMP in tumors is a poor prognostic indicator of drug-response; and 5) the levels of FdUMP in the tumors are maintained for considerable periods and appear to be dependent upon the maintenance of the levels of either FUra or FUrd, irrespective of the parent agent administered.

Evaluation of single-agent therapy in human colorectal tumour xenografts

The response of 6 human colorectal tumour xenografts to 7 cytotoxic agents have been established. Tumour responses have been quantified by growth inhibition, and the time taken for 3H-thymidine fractional incorporation (TFI) to recover to the control value after treatment. The chemosensitivity of each tumour line to a spectrum of agents was individual, and no pattern of response which would allow prediction of individual agent efficacy was apparent. Cyclophosphamide, methyl-CCNU and 5-fluorouracil produced marked growth inhibition in individual tumour lines, whereas actinomycin-D, cis-dichlorodiammine platinum, doxorubicin and pentamethylmelamine showed little activity. Data presented agree with clinical evaluation for single-agent therapy. The uptake and incorporation of radiolabelled 5-fluorouracil into 4 tumour lines is reported. No marked differences between 3 FU-insensitive lines and 1 sensitive line have been observed.

Antimetastatic action of some triazene derivatives against the Lewis lung carcinoma in mice

Two dimethyltriazenoimidazoles, DTIC and BRL 51308, and a benzenoid dimethyltriazene, CB 10286, have been examined for their effects in mice bearing Lewis lung carcinoma. A slight reduction of primary tumor growth was found after treatment with DTIC and BRL 51308, whereas CB 10286 caused no significant effect. On the contrary, all the tested compounds sharply reduced the number of lung metastases and also resulted in a high proportion of animals free of metastases at death. No significant cytotoxic effect of the triazenes was observed in small established pulmonary tumors, as determined by evaluating the effects of treatment on the fractional incorporation of 3H-TdR into DNA of the lung colonies. These results are in contrast to those obtained with a purely cytotoxic agent, cyclophosphamide, and indicate that all three triazene derivatives tested have selective antimetastatic properties.

Effects of cytotoxic agents on TdR incorporation and growth delay in human colonic tumour xenografts

The relationship between the utilization of 3H-thymidine in situ ([3H]-TdR fractional incorporation or TFI) and tumour growth delay after treatment with various cytotoxic agents has been examined. It is shown that (a) it is not possible to predict tumour growth delay, or to select the most effective agent, from changes in TFI 1 day after treatment; (b) there is a good correlation between tumour growth delay and the time for recovery of TFI to the pretreatment level; (c) there is a relationship within a tumour line between the depression of TFI 4 days after treatment and growth dealy induced by the same treatment. This relationship appears to be independent of the mechanism by which the agent exerts its cytotoxic effect.

Fractional incorporation of [3H]thymidine and DNA specific activity as assays of inhibition of tumour growth

The Fractional Incorporation (FI) of [3H] thymidine ([3H]TdR) has been examined in small lung tumours after cyclophosphamide (CY) treatment in vivo and compared to the DNA specific activity (SA) at different times after treatment. FI was found to correlate with the incidence of labelled cells after treatment, whereas SA did not, due to the loss of DNA from drug-killed cells 72 h after treatment. The FI is independent of the precursor concentration in the tissue, and therefore may give a better index of DNA synthesis in irregularly perfused tissues than SA. Following either CY or 60Co radiation treatment, the time necessary for FI to reach the pretreatment level is quite similar to the growth delay measured for the FI depression 45 h after treatment and growth delay has been established in the Lewis lung tumour, which would allow the prediction of growth delay induced by another agent to be made within 2 days of treatment.