Thymidylate synthase as an oncogene?

A surprising finding in the report by Rahman et al. in this issue of Cancer Cell is that forced overexpression of human thymidylate synthase transforms immortalized murine cells into a malignant phenotype. We discuss the possibility that elevated levels of thymidylate synthase noted in some human malignancies may contribute to tumor progression and may also reflect increased levels of its transcriptional activator E2F-1.

Correlation of dihydrofolate reductase elevation with gene amplification in a homogeneously staining chromosomal region in L5178Y cells.

A methotrexate (MTX)-resistant murine lymphoblastoid cell line has been obtained by serial passage in increasing concentrations of MTX which is greater than 100,000-fold resistant to MTX (L5178YR) and has dihydrofolate reductase (DHFR) levels 300-fold higher than the parental line. The L5178YR cell line synthesizes approximately 10-11% of its total soluble cell protein as DHFR regardless of growth phase, as measured by direct immunoprecipitation with a monospecific antiserum. Molecular hybridization of a purified [3H]DNA probe complimentary to DHFR specific mRNA with cellular DNA and RNA indicates that DHFR coding sequences are elevated several hundred fold in both nucleic acid species in the mutant cell line. Giemsa-banding studies of the diploid mutant line indicate the presence of a large homogeneously staining region on chromosome No. 2. In situ molecular hybridization studies indicate that the DHFR genes are localized in this homogeneously staining region. The homogeneously staining region probably consists of tandom repeats of a basic segment approximately 800 kilo base pairs long.

Inhibition of growth of leukemia cells by enzymic folate depletion.

A new bacterial enzyme, designated as carboxypeptidase G(1), inhibited growth of the L5178Y and L1210 murine leukemias, as well as the Walker carcinoma and the human lymphoblastoid line (RPMI 4265), propogated in vitro. This enzyme hydrolyzes the glutamate moiety from both oxidized and reduced folate forms, and thus it may prove to be of value in creating rapid folate depletion in vivo.

Treatment of leukemia with large doses of methotrexate and folinic acid: clinical-biochemical correlates.

Patients with acute leukemia were given repeated cycles consisting of infusions of methotrexate followed by "rescue" with folinic acid. Peripheral blood leukemic cells were harvested from patients before cyclical treatment, and the rates of incorporation of thymidine and of deoxyuridine into deoxyribonucleic acid (DNA) were measuared in vitro. There was no relationship between the pretreatment incorporation of either deoxynucleoside into DNA and the clinical response to therapy. Methotrexate suppressed deoxyuridine incorporation into DNA by the leukemic blasts in vitro, but the patients whose cells were most sensitive to this effect did not necessarily go into remission when treated. Leukemic cells were sampled during methotrexate infusions and the deoxynucleoside incorporation rates were determined. Thymidine incorporation into DNA was variably affected. If, by the end of the first infusion, it remained elevated, remission rarely followed, whereas if it was below the pretreatment value, remission was much more likely. In all cases, deoxyuridine incorporation was suppressed during the infusion. The greatest suppression occurred in patients who went on to remission, but the suppression did not correlate with that expected from pretreatment in vitro tests unless due weight was given to the concomitant effects of the methotrexate therapy on thymidine incorporation. Leukemic blasts surviving successive cycles of therapy became progressively more resistant to the suppressing effects of methotrexate in vitro. This resistance became especially marked in the blasts of patients who did not go into remission. During methotrexate infusions, inhibition of leukemic cell dihydrofolate reductase activity was greatest in blasts of patients whose disease subsequently remitted.

Impaired utilization of serum folate in pernicious anemia. A study with radiolabeled 5-methyltetrahydrofolate.

The possible role of cobalamins in the utilization of serum methyltetrahydrofolate has been investigated by means of radiolabeled methyltetrahydrofolate in subjects suffering from pernicious anemia. After intravenous administration, methyltetrahydrofolate-(3)H (SA 11,500 Ci/mole; dose 0.05 mug/kg) was cleared from the serum to tissues of B(12)-deficient subjects half as fast as after the same subjects had received vitamin B(12) therapy. B(12) deficiency was also associated with an increased rate of renal excretion of methyltetrahydrofolate or its derivatives, and a decreased rate of renal metabolism of methyltetrahydrofolate to other urinary folate derivatives.Intravenously administered methyl-(14)C-tetrahydrofolate-(3)H at a higher dose (5 mug/kg) caused a severalfold elevation of the total serum folate concentration and, in B(12)-deficient subjects, it did not disappear from the serum significantly more slowly although its urinary excretion was significantly increased. These results indicate that there is some cobalamin requirement for the utilization of serum methyltetrahydrofolate and verify one prediction of the "methyltetrahydrofolate trap" explanation for the megaloblastosis of B(12) deficiency.

Intestinal folate absorption. II. Conversion and retention of pteroylmonoglutamate by jejunum.

These studies were designed to determine whether pteroylmonoglutamic acid (PGA) at physiologic concentrations is transported across the small intestine unaltered or is reduced and methylated to the circulating folate form (5-methyltetrahydrofolate [5-MeFH(4)]) during absorption. [(3)H]PGA was incubated in vitro on the mucosal side of rat jejunum. Of the folate transferred to the serosal side, the percent identified as 5-MeFH(4) by DEAE-Sephadex chromtography was inversely related to the initial mucosa PGA concentration: at 7, 20, and 2,000 nM, 44%, 34%, and 2%, respectively, was converted to 5-MeFH(4). In contrast, less than 4% of the folate transferred across ileal mucosa was 5-MeFH(4) when the initial mucosa concentration was 20 nM. Specific activity of dihydrofolate (DHF) reductase, the enzyme responsible for converting PGA to tetrahydrofolic acid, was measured in villus homogenates and was significantly greater in the jejunum than in the ileum. 1,000 nM methotrexate (MTX), a DHF reductase inhibitor, markedly inhibited PGA conversion to 5-MeFH(4) by the jejunum. Studies of transmural flux, initial rate of mucosal entry (influx) and mucosal accumulation (uptake) of folate were also performed. Although MTX did not alter the influx of PGA, MTX decreased jejunal mucosal uptake but increased transmural movement. Transmural folate movement across ileal mucosa was greater than across jejunal mucosa although mucosal uptake was greater in the jejunum than in the ileum. These results could explain previous studies which have failed to identify conversion of PGA to 5-MeFH(4) when intestinal preparations have been exposed to higher and less physiologic concentrations of PGA. Further, these studies suggest that 5-MeFH(4) may be retained by the jejunal mucosa.

Transport characteristics of folates in cerebrospinal fluid; a study utilizing doubly labeled 5-methyltetrahydrofolate and 5-formyltetrahydrofolate.

Serum disappearance curves in dogs after the intravenous injection of radioactive methotrexate, 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, and folic acid followed first-order kinetics with half-disappearance times ranging from 1.3 to 9 hr respectively. Equilibration of spinal fluid tritium levels with those in serum was rapid (3.0 hr) for both of the reduced folates but was not observed at any time after folic acid and methotrexate. The only radioactive folate identified in the spinal fluid after intravenous injection of either 5-formyltetrahydrofolate or folic acid, as well as 5-methyltetrahydrofolate was 5-methyltetrahydrofolate. These findings indicated that 5-methyltetrahydrofolate was taken up preferentially into the spinal fluid and that the other folate congeners were converted to this compound before uptake. Diphenylhydantoin administration did not alter the uptake of 5-methyltetrahydrofolate into the spinal fluid but was associated with reduced renal excretion of this compound.

Maintaining the dose intensity of ICE chemotherapy with a thrombopoietic agent, PEG-rHuMGDF, may confer a survival advantage in relapsed and refractory aggressive non-Hodgkin lymphoma.

INTRODUCTION: HDT/ASCT is standard for relapsed and refractory DLCL patients responding to second-line chemotherapy. We incorporated a thrombopoietic agent into the ICE chemotherapy program to potentially: decrease platelet associated toxicities, augment stem cell collection and maintain dose intensity. METHODS: This randomized, double-blind, placebo-controlled phase I/II trial examines PEG-rHuMGDF versus placebo with ICE chemotherapy. Phase I compared three cohorts and defined a clinically effective dose (CED). Phase II evaluated the CED versus placebo. Outcome measures included safety, hematological end-points, stem cell collection and the impact of dose-intensity on outcome. RESULTS: Forty-one patients with primary refractory (16) or relapsed DLCL (25) were treated; Response rates for evaluable patients are: 75% (12/16) for placebo and 82% (18/22) for PEG-rHuMGDF. PEG-rHuMGDF treated patients had significantly less grade IV thrombocytopenia, higher median platelet nadirs, and less platelet transfusion per cycle. ICE dose intensity was improved with PEG-rHuMGDF versus placebo: 75 versus 42% (P = 0.008). At 8.5 years median follow-up, overall and event-free survival are 47 and 31%, respectively. Patients treated on PEG-rHuMGDF versus placebo had improved survival (59 versus 31%, P = 0.06). CONCLUSION: PEG-rHuMGDF ameliorated thrombocytopenia, improved platelet recovery, and maintained ICE dose intensity. Potential survival advantages conferred by maintaining dose intensity require validation with newer thrombopoietic agents.

Leucovorin enhances cytotoxicity of trimetrexate/fluorouracil, but not methotrexate/fluorouracil, in CCRF-CEM cells.

BACKGROUND: Increased response rates in studies of patients with colon cancer have indicated that the cytotoxic effects of fluorouracil (5-FU) are potentiated by leucovorin (LV) and by methotrexate (MTX). However, preliminary studies using a sequential combination of MTX, LV, and 5-FU showed no additional potentiation. PURPOSE: We hypothesized that the lack of additional cell kill with this combination could be due to competition of LV with MTX for cellular uptake and reduced folate polyglutamylation. We have tested this possibility by comparing the cytotoxicity of drug combinations containing MTX with that of drug combinations containing trimetrexate (TMTX), an antifolate that does not compete with LV for uptake or polyglutamylation. METHODS: Human lymphocytic leukemia CCRF-CEM cells were exposed to MTX or TMTX for 24 hours and to 5-FU during the last 4 hours of antifolate exposure. LV was administered 30 minutes before 5-FU. RESULTS: After 20 hours of exposure to TMTX or MTX, intracellular levels of phosphoribosyl pyrophosphate were elevated to a similar degree, and these levels did not decrease after a 30-minute exposure to LV. No additional cell kill was observed when LV was added to the MTX/5-FU combination, but cytotoxicity was enhanced when LV was added to the TMTX/5-FU combination. CONCLUSIONS: This study supports the hypothesis that the lack of additional cell kill when high-dose LV is added to the MTX/5-FU combination may be due to competition of MTX with LV for cellular uptake and/or competition of MTX or its polyglutamates with polyglutamylation of reduced folates. Inasmuch as TMTX does not compete with LV and reduced folates for uptake and polyglutamylation, the synergy obtained with the combination of TMTX plus 5-FU and high-dose LV further supports this hypothesis.

Rapid development of resistance to antifolates in vitro: possible clinical implication.

Within three repeated 7-day incubation periods with either methotrexate (MTX) or trimetrexate (TMTX), human colon adenocarcinoma cells (HCT-8) developed high levels of resistance to these drugs, as evidenced by approximately 20- and 50-fold increases, respectively, in the median effective doses. Similarly, within six short-term exposures (4 hours) to the same drugs, a high degree of resistance developed in the cells. Alternating 4-hour treatment cycles with MTX and TMTX did not delay the onset of resistance to these antimetabolites in the HCT-8 cells. The same strategy produced no better results than giving either MTX or TMTX alone to (C57BL/6 x DBA/2)F1 mice bearing murine leukemia P388 cells. Furthermore, HCT-8 cells resistant to short-term (4-hour) exposure to MTX were cross-resistant to the same drug given for 7 days continuously, and cells resistant to MTX given continuously for 7 days were cross-resistant to the same drug given for 4 hours. Analogous results were obtained with TMTX, indicating that, under these circumstances, changing the schedule of administration of the same agent does not overcome resistance to it. The clinical relevance of these data to prolonged adjuvant chemotherapy, as well as loco-regional and continuous-infusion chemotherapy, is discussed.

Synergism and lack of cross-resistance between short-term and continuous exposure to fluorouracil in human colon adenocarcinoma cells.

BACKGROUND: Our recent findings in vitro in the human colon adenocarcinoma cell line HCT-8 suggest that resistance to fluorouracil (5-FU) in patients with advanced colorectal cancer might be overcome by use of a different treatment schedule. PURPOSE: We tested the hypothesis that HCT-8 cells resistant to short-term 5-FU exposure retain sensitivity to continuous exposure and studied interactions between the two schedules. METHODS: HCT-8 cell lines resistant to short-term (pulse) treatment with 5-FU or to continuous exposure were obtained by six exposures to different concentrations of 5-FU for 4 hours or 7 days. We used a monolayer clonogenic assay to determine 5-FU-induced cell kill in resistant HCT-8 cells and sensitive parent cells. Parent cells were exposed to different concentrations of 5-FU for 1, 4, or 24 hours (short term), for 7 days (continuous exposure), or in a combination of both types of schedules. In a study of the mechanism of interaction between short-term and continuous exposure in parent cells, we performed flow cytometric DNA analysis to determine the percentage of cells in S phase and assays of thymidylate synthase inhibition in intact cells and of incorporation of [6-3H)]5-FU nucleotides into nucleic acids. RESULTS: Sensitive HCT-8 cells became fully resistant to 5-FU within five or six treatments, and low-dose continuous exposure almost immediately produced resistant clones. HCT-8 cells resistant to 5-FU given every 4 hours retained full sensitivity to continuous exposure, suggesting lack of cross-resistance between the two schedules, but cells resistant to continuous exposure were cross-resistant to short-term treatment. Parent cells showed a statistically significant (synergistic) enhancement of the cytotoxic activity for 5-FU exposure for 1 hour (100, 300, or 500 microM) followed by continuous exposure (0.5, 1, or 2 microM) or 4 hours (10, 30, or 60 microM) followed by continuous exposure (1 or 2 microM). Short-term plus continuous exposure produced a marked increase in percentage of S-phase cells, compared with the percentage for each schedule alone. The combination of 1-hour exposure and continuous exposure (1000 and 2 microM, respectively) produced a marked accumulation of cells in S phase at 24 hours (59%), which lasted up to 96 hours (53%). The combination of the two schedules produced only additive enhancement of thymidylate synthase inhibition as well as incorporation of [6-3H]5-FU nucleotides into nucleic acids of HCT-8 cells. CONCLUSIONS: Our findings provide a rationale for the use of bolus 5-FU and continuous infusion 5-FU in sequence. IMPLICATION: We are conducting a clinical trial of bolus methotrexate followed by continuous-infusion 5-FU plus leucovorin.

Effect of cyclin D1 overexpression on drug sensitivity in a human fibrosarcoma cell line.

BACKGROUND: Alterations in the expression of genes that control the cell cycle may be of critical importance in determining the sensitivity of cells and tumors to drugs (chemosensitivity) and radiation. Mutations and deletions of the p53 tumor suppressor gene in cell lines and tumors are associated with resistance to a variety of DNA-damaging agents. The effects of alterations in the cyclin genes and their products on drug action have not been studied. One of these genes, cyclin D1, is expressed in early G1 phase, and its protein product, together with the cyclin-dependent kinases CDK4 and CDK6, mediates the phosphorylation and functional inactivation of the retinoblastoma protein (pRb). Elevated levels of expression of cyclin D1 protein have been found in a variety of cancers, including breast cancer, head and neck cancer, non-small-cell lung cancer, and mantle cell lymphomas. PURPOSE: This study was conducted to investigate the effect of increased expression of cyclin D1 protein on the chemosensitivity profile of a human fibrosarcoma cell line. METHODS: Expression plasmids containing either the neomycin-resistance gene and the complementary DNA sequence encoding human cyclin D1 or the neomycin-resistance gene only (control) were transfected by lipofection into the human HT1080 fibrosarcoma cell line, and cell colonies resistant to the antibiotic neomycin (G418) were isolated. Cyclin D1 messenger RNA (mRNA) and protein levels were measured by ribonuclease protection and western blot analyses, respectively. Dihydrofolate reductase (DHFR) mRNA and protein levels were measured by northern blot and western blot analyses, respectively. The phosphorylation status of pRb was assessed by western blot analysis. Cell cycle analysis was performed by use of the technique of fluorescence-activated cell sorting. Cytotoxicity assays were carried out by use of the sulforhodamine blue assay. RESULTS: Of the 16 cyclin D1-transfected cell clones that were isolated, four were randomly selected for further study. Two cell clones expressed high levels of cyclin D1 mRNA and protein as compared with control cells transfected with plasmids containing the neomycin-resistance gene only. A relative increase in the phosphorylated form of pRb in cells expressing high versus low levels of cyclin D1 was also revealed by western blot analysis. There was an increased fraction of cells in the S and G2 phases of the cell cycle among cells expressing higher levels of cyclin D1. Transfectants with increased cyclin D1 expression also had increased DHFR mRNA and protein expression. Cytotoxicity assays revealed a statistically significant (P < .01) increase in resistance to methotrexate in cells expressing high levels of cyclin D1 compared with cells expressing lower levels. There was no difference in resistance to doxorubicin, paclitaxel (Taxol), and cytarabine. CONCLUSION: Alterations in the expression of cyclin D1 led to altered cell cycle distribution in a human sarcoma cell line. The associated increase in DHFR expression resulted in increased resistance to methotrexate but had no effect on other classes of anticancer agents. IMPLICATIONS: These results indicate that alterations in cell cycle genes may differ in their effects on cytotoxicity. It will be important to determine the effects of alterations of other cell cycle regulatory genes on the responses of cells to specific classes of drugs. Tumors with overexpression of cyclin D1 may be relatively refractory to methotrexate treatment.

Inability of leucovorin to rescue a naturally methotrexate-resistant human soft tissue sarcoma cell line from trimetrexate cytotoxicity.

A human lymphoblastoid line (RPMI-1788), a methotrexate-sensitive human fibrosarcoma cell line (HT-1080), and a naturally resistant mixed mesodermal human sarcoma cell line with impaired methotrexate polyglutamylation (HS-42), recently established in our laboratory, were used to compare the ability of leucovorin to prevent trimetrexate cytotoxicity. Growth inhibition and an in situ thymidylate synthesis activity assay showed that inhibitory effects of trimetrexate (1 to 10 microM), 24-h exposure, were prevented by 10 microM leucovorin in the RPMI-1788 and HT-1080 cell lines but not in the HS-42 cell line. Total intracellular reduced folates increased about 2-fold in the three cell lines after exposure to leucovorin (10 microM) for 4 h, and after a 6-hour efflux remained elevated (1.5- and 1.3-fold of control levels) in RPMI-1788 and HT-1080 cells but decreased to 80% of control levels in HS-42 cells. Although uptake of leucovorin and levels of N5,N10-methylenetetrahydrofolate achieved after leucovorin administration were similar in RP-MI-1788 and HS-42 cells, polyglutamylate forms of this coenzyme were less in the HS-42 cells as compared to RPMI-1788 cells. Based on these studies, the combination of trimetrexate with leucovorin should be further investigated as a way to increase the therapeutic index in some patients with soft tissue sarcomas.

Sequence-dependent synergism between dichloromethotrexate and 5-fluorouracil in a human colon carcinoma cell line.

Prior to their use in patients with hepatic metastasis of colon cancer, we studied the cell-killing effects of 5-fluorouracil, 5-fluoro-2'-deoxyuridine, 3',5'-dichloromethotrexate (DCM), and 5-fluoro-2'-deoxyuridine, 3',5'-dichloromethotrexate (DCM), and combinations of these fluoropyrimidines with DCM on a human colon carcinoma cell line (HCT-8). Sequential exposure of these cells to DCM followed by 5-fluorouracil resulted in significant synergistic cell killing at every dose and time studied, and the synergy was more evident with increasing doses of both drugs. In contrast, simple additive effects were observed when DCM was given together with or following 5-fluorouracil. When the antifolate was given before 5-fluoro-2'-deoxyuridine, simple additive effects, rather than synergy, were observed. Mild antagonism and even strong antagonism were found when 5-fluoro-2'-deoxyuridine preceded DCM administration or when the cells were exposed to the drugs simultaneously, respectively.

Possible benefits of hyperthermia to chemotherapy.

The advantages of hyperthermia for chemotherapy are discussed in detail. These advantages are (a) synergy with chemotherapeutic drugs and also with ionizing radiation, (b) low host toxicity, (c) ease of control (heating precisions in the range of +/- 0.1 degrees C and specific definable localized areas), and (d) low resistance (chemotherapeutic resistance and hyperthermic resistance). The potential for hyperthermia and chemotherapy in the treatment of specific human cancers, both disseminated and solid tumors, are discussed with respect to current therapy and the possible benefit of hyperthermia treatment.

Selective sensitization of retinoblastoma protein-deficient sarcoma cells to doxorubicin by flavopiridol-mediated inhibition of cyclin-dependent kinase 2 kinase activity.

We examined the effects of flavopiridol (FP), a cyclin-dependent kinase inhibitor, on doxorubicin (DOX)-induced cell killing in an osteosarcoma cell line (SaOs-2) that lacks functional retinoblastoma protein (pRb). The IC50 value for DOX was 7-fold lower when combined with a low dose (100 nM) FP in pRb-deficient SaOs-2 cells than in the absence of FP. In contrast, the IC50 value for DOX was not decreased in the presence of 100 nM FP in pRb-restored SaOs-2 cells. Consistent with this, FP enhanced DOX-induced activation of caspase-3, which correlates with apoptosis, in pRb-deficient cells but not in pRb-restored cells. Additional studies showed that FP decreased DOX-induced cell accumulation in S phase in retinoblastoma-restored cells but not in pRb-deficient cells. An increased expression of p21 and inhibition of cyclin-dependent kinase 2 kinase activity by FP was also observed in pRb-deficient cells but not in retinoblastoma-restored SaOs-2 cells. We conclude that pRb plays a key role in determining whether FP selectively sensitizes DOX-induced cell killing in human sarcoma cells. Because lack of functional pRb is a common abnormality in human cancers, the combination of FP with DOX in tumors lacking pRb would be worthy of further investigation.

Highly selective drug combinations for human colon cancer cells resistant in vitro to 5-fluoro-2'-deoxyuridine.

In the preceding companion paper, we describe a human colon carcinoma cell line that is resistant in vitro to 5-fluoro-2'-deoxyuridine by virtue of impaired nucleoside transport. Two drug combinations, methotrexate: hypoxanthine: thymidine (dThd) and 5,8-dideazaisofolic acid: dThd, selectively kill these resistant cells with no effect on the sensitive cell population. As little as 0.3 microM dThd was sufficient to completely protect sensitive cells from 5-fluoro-2'-deoxyuridine, and 5,8-dideazaisofolic acid at concentrations that produced over 80% lethality in unprotected cells and the same concentration of dThd in combination with 100 microM hypoxanthine fully protected sensitive cells from greater than 99% methotrexate-induced cell lethality. In contrast, when resistant cells were exposed to these drugs, they were not protected by dThd, or by the combination dThd: hypoxanthine, in concentrations up to 300 times higher than those necessary to prevent sensitive cell kill. Thus, it may be possible to protect normal renewal tissues while obtaining selective tumor cell kill with these two drug combinations in patients whose colon carcinoma cells are resistant to 5-fluoro-2'-deoxyuridine by virtue of defective transport.

Mechanism of action of 5,8-dideazaisofolic acid and other quinazoline antifols in human colon carcinoma cells.

The clonal cytotoxic effects and mechanism of action of a new series of 2-amino-4-hydroxyquinazoline folate analogues (5,8-dideazafolates) have been assessed using the human colon tumor cell line HCT-8. Of these compounds only 5-methyl-5,8-dideazafolate was potentially more effective than a compound previously identified, 5,8-dideazaisofolate (H-338, NSC 289517). HCT-8 sublines resistant to methotrexate, 5-fluorodeoxyuridine, and H-338 were either minimally or not cross-resistant to the other agents. The cytotoxicity of H-338 was strongly dependent on the time of exposure; at exposure times shorter than 8 h it was essentially nontoxic. Thymidine alone, as well as leucovorin or folic acid, protected against the cytotoxic effects of H-338. This is consistent with thymidylate synthase (TS) as its only locus of action. Studies with dihydrofolate reductase and TS isolated from HCT-8 cells indicated that these quinazolines were weaker inhibitors of dihydrofolate reductase than was methotrexate, but they were not particularly potent TS inhibitors. However, synthetic poly-gamma-glutamate derivatives of quinazolines showed dramatically increased TS, but not dihydrofolate reductase, inhibition. TS inhibition increased as the polyglutamate chain length increased. Using isolated HCT-8 folylpolyglutamate synthetase, all the parent quinazolines containing L-glutamate were found to be substrates. With H-338, the results indicated that tetraglutamate or longer derivatives could be synthesized intracellularly. These results are consistent with our hypothesis that cytotoxicity by such quinazolines necessarily involves "lethal synthesis" from a prodrug; i.e., the nontoxic parent drug must be converted to polyglutamates before TS inhibition and subsequent cytotoxicity can occur.

Development of an assay system for the detection and classification of methotrexate resistance in fresh human leukemic cells.

An assay system was developed for the detection and classification of methotrexate resistance in fresh human leukemic cells. Mechanisms of resistance to be identified were: overexpression of dihydrofolate reductase, decreased cellular uptake of methotrexate, decreased affinity of dihydrofolate reductase for methotrexate, decreased polyglutamylation of methotrexate, and low thymidylate synthase activity. The initial screening procedure utilizes 3H release after addition of [5-3H]-2'-deoxyuridine as a measure of intracellular activity of thymidylate synthase and of DNA synthesis; 3H release is assayed after 3-h incubations with methotrexate, trimetrexate, or gamma-fluoromethotrexate and after 4-h incubations with these agents followed by a 6-h incubation in drug free medium. The pattern of DNA synthesis inhibition and recovery under these two sets of conditions establishes the presence or absence of methotrexate resistance and allows a tentative classification of the resistance mechanism involved. In combination with determinations of dihydrofolate reductase activity, methotrexate titration studies, and the determination of intracellular drug accumulations in vitro, the system is readily able to classify CCRF-CEM human leukemia cell lines possessing well defined mechanisms of resistance. The findings in seven leukemia patients are also reported. Applying tentative reference values, four patients showed biochemical evidence of methotrexate resistance: two patients had only a transport defect, one patient had evidence of both a transport defect and low thymidylate synthase activity, and one patient appeared to have decreased methotrexate polyglutamylation. Application of the assay system in larger numbers of patients is feasible and is required to establish adequate reference values for the evaluation of biochemical-clinical correlates.

Novel mechanism(s) of resistance to 5-fluorouracil in human colon cancer (HCT-8) sublines following exposure to two different clinically relevant dose schedules.

Mechanisms of resistance to 5-fluorouracil (FUra) were compared between a cell line resistant to a short-term exposure (4 h) to this agent (HCT-8/FU4hR) and a cell line resistant to a prolonged exposure (7 days) to the fluoropyrimidine (HCT-8/FU7dR). The two cell lines were obtained by repeatedly exposing 2 x 10(5) cells to a constant concentration of FUra (1000 microM for 4 h or 15 microM for 7 days), able to produce 3-4 logs of cell kill. HCT-8/FU4hR cells were still sensitive to FUra given as a 7-day exposure, suggesting different mechanisms of resistance. In addition, HCT-8/FU7dR cells were cross-resistant to fluorodeoxyuridine and, to a lesser degree, methotrexate; while HCT-8/FU4dR cells were not. Both HCT-8/FU4hR and HCT-8/FU7dR cells were similar to parental HCT-8 cells with regard to uptake of FUra as well as the pattern of FUra-metabolizing and FUra target enzymes. Although neither in situ thymidylate synthase (TS) activity nor the degree of its inhibition by FUra showed any evidence of alteration in HCT-8/FU7dR cells, a rapid recovery of TS activity after drug removal was evident in this cell line. The addition of as much as 100 microM leucovorin did not completely inhibit the recovery of thymidylate synthesis after FUra exposure. No differences were detected in the kinetic properties (Km for 2'-deoxyuridylate and 5,10-methylenetetrahydrofolate, concentration producing 50% inhibition for fluorodeoxyuridylate) or TS from HCT-8/FU7dR cells as compared to parental HCT-8 TS. Baseline levels of 5,10 methylenetetrahydrofolate were decreased in HCT-8/FU7dR cells, and analysis of the chain length distribution of the polyglutamylated form of the folate cofactor showed that in this cell line the defect in 5,10-methylenetetrahydrofolate levels is accompanied by, and possibly due to, a defect in the polyglutamylation of this cofactor. In contrast, HCT-8/FU4hR cells were similar to the parental cell line with regard to both the degree of in situ TS inhibition by FUra and duration of inhibition after FUra removal. Labeling studies with [3H-6]FUra (4 h exposure, 100 microM) showed that the incorporation of the fluoropyrimidine into RNA is significantly decreased in HCT-8/FU4hR cells as compared to parental HCT-8 cells.(ABSTRACT TRUNCATED AT 400 WORDS)