Inhibition of phosphoribosylaminoimidazolecarboxamide transformylase by methotrexate and dihydrofolic acid polyglutamates

The human purH gene product, 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase. Cloning, sequencing, expression, purification, kinetic analysis, and domain mapping

We report here the cloning and sequencing of the cDNA, purification, steady state kinetic analysis, and truncation mapping studies of the human 5-aminoimidazole- 4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase (AICARFT/IMPCHase). These steps of de novo purine biosynthesis, respectively. In all species of both prokaryotes and eukaryotes studied, these two activities are present on a single bifunctional polypeptide encoded on the purH gene. The human purH cDNA is 1776 base pairs in length encoding for a 591-amino acid polypeptic (Mr = 64,425). The human and avian purH cDNAs are 75 and 81% similar on the nucleotide and amino acid sequence level, respectively. The Km values for AICAR and (6R,6S)10-formyltetrahydrofolate are 16.8 microM +/- 1.5 and 60.2 microM +/- 5.0, respectively, for the cloned, purified human enzyme. A 10-amino acid sequence within the COOH-terminal portion of human AICARFT/IMPCHase has some degree of homology to a previously noted "folate binding site." Site directed mutagenesis studies indicate that this sequence plays no role in enzymatic activity. We have constructed truncation mutants which demonstrate that each of the two enzyme activities can be expressed independent of the other. IMPCHase and AICARFT activities are located within the NH2-terminal 223 and COOH-terminal 406 amino acids, respectively. The truncation mutant possessing AICARFT activity displays steady state kinetic parameters identical to those of the holoenzyme.

Methotrexate for inflammatory bowel disease: pharmacology and preliminary results

OBJECTIVE

To review the limited published experience with methotrexate treatment for inflammatory bowel disease, to examine the proposed anti-inflammatory and immune-modifying mechanism of action and pharmacologic properties of methotrexate, and to detail its limiting toxicities.

DESIGN

A comprehensive synopsis of methotrexate is presented to aid physicians who treat patients with inflammatory bowel disease.

RESULTS

Methotrexate and its polyglutamate metabolites are folic acid analogues with inhibitory activity against many enzymes in the metabolic pathway of folic acid. Long-term low-dose methotrexate therapy (25 mg or less once a week) inhibits production of thymidylate, purines, and methionine and leads to accumulation of adenosine, a potent anti-inflammatory substance. These actions inhibit cellular proliferation, decrease formation of antibodies, and decrease production of mediators of inflammation such as interleukins and eicosanoids. Three uncontrolled trials and two controlled trials have demonstrated efficacy of low-dose methotrexate therapy for induction of remission in Crohn's disease and have also suggested possible benefit for ulcerative colitis and for remission maintenance indications in both diseases. Although methotrexate is generally well tolerated for long-term use at a low dose, several serious toxicities potentially limit its use.

CONCLUSION

Methotrexate is a promising new agent for the treatment of inflammatory bowel disease.

Accumulation of methotrexate polyglutamates in lymphoblasts is a determinant of antileukemic effects in vivo. A rationale for high-dose methotrexate

Methotrexate (MTX) is one of the most widely used drugs for the treatment of childhood acute lymphoblastic leukemia (ALL) and is commonly given in high doses. However, the rationale for high-dose MTX (HDMTX) has been challenged recently. To determine whether higher MTX polyglutamate (MTXPG) concentrations in ALL blasts translate into greater antileukemic effects, 150 children with newly diagnosed ALL were randomized to initial treatment with either HDMTX (1,000 mg/m2 intravenously over 24 h) or lower-dose MTX (30 mg/m2 by mouth every 6 h x 6). ALL blasts accumulated higher concentrations of MTXPG and long-chain MTXPG (MTXPGLC) after HDMTX (P < 0.00001). Of 101 patients evaluable for peripheral blast cytoreduction, MTXPG concentrations were higher in patients whose blast count decreased within 24 h (P = 0.005) and in those who had no detectable circulating blasts within 4 days (P = 0.004). The extent of inhibition of de novo purine synthesis in ALL blasts was significantly related to the blast concentration of MTXPGLC (IC95% = 483 pmol/10(9) blasts). The percentage of patients with 44-h MTXPGLC exceeding the IC95% was greater after HDMTX (81%) than LDMTX (46%, P < 0.0001). These data indicate that higher blast concentrations of MTXPG are associated with greater antileukemic effects, establishing a strong rationale for HD-MTX in the treatment of childhood ALL.

Aminophylline for methotrexate-induced neurotoxicity

Methotrexate, a mainstay treatment for children with acute lymphoblastic leukaemia, can cause neurotoxicity, with paralysis, seizures, somnolence, anorexia, and headaches. The pathophysiology of this reaction is unknown. It has been suggested that the anti-inflammatory effect of methotrexate in patients with arthritis is due to adenosine release brought on by inhibition of purine synthesis. Since adenosine is a central nervous system depressant, we wondered whether adenosine release in the central nervous system could account for some of the neurotoxicity due to methotrexate, and whether that toxicity could be lessened by displacement of adenosine from its receptor by aminophylline. 6 patients (age 3-16 years) who had methotrexate-induced neurotoxicity unresponsive to standard treatment received 2.5 mg/kg aminophylline. In addition, the concentration of adenosine in the cerebrospinal fluid (CSF) from 11 children completing a 24-h systemic methotrexate protocol was compared with that in 8 newly diagnosed patients and 12 who had not received any treatment for at least a week. 4 of 6 patients with toxic signs and symptoms attributed to methotrexate and unrelieved by steroids, epidural blood patch, promethazine, 5-hydroytryptamine antagonists, paracetamol, and narcotics, had complete resolution of neurotoxicity after or during a 1-h infusion of aminophylline; 2 others had a pronounced improvement but persistent nausea. CSF adenosine concentrations of patients receiving methotrexate, even when there was very slight or no toxicity, were greatly increased compared with control subjects (mean values of 217 and 51 nmol/L, median 175 and 52 nmol/L). Subacute methotrexate neurotoxicity may be mediated by adenosine and relieved by aminophylline.

The antiinflammatory effects of methotrexate are mediated by adenosine

In summary, intermittent, low dose methotrexate treatment is: 1.) antiinflammatory in the murine air pouch model of inflammation; 2.) selectively increases intracellular AICAR concentration; 3.) increases adenosine concentration in an inflammatory exudate; and, 4.) inhibits leukocyte accumulation at an inflamed site by a mechanism that is specifically reversed by adenosine deaminase and the adenosine A2 receptor antagonist DMPX but not the A1 antagonist DPCPX. In conclusion, we have demonstrated a novel mechanism for the antiinflammatory action of methotrexate; methotrexate is a nonsteroidal antiinflammatory agent that acts by promoting the release of adenosine which engages A2 receptors on inflammatory cells.

5-Aminoimidazole-4-carboxamide ribotide transformylase-IMP cyclohydrolase from human CCRF-CEM leukemia cells: purification, pH dependence, and inhibitors

The bifunctional enzyme 5-aminoimidazole-4-carboxamide ribotide (AICAR) transformylase-IMP cyclohydrolase has been purified 780-fold to apparent homogeneity from human CCRF-CEM leukemia cells, completed with chromatography on Affi-Gel Blue followed by AICAR-Sepharose 4B. Using a sensitive radioassay, IMP cyclohydrolase has a Ks value for 5-formamidoimidazole-4-carboxamide ribotide (FAICAR) at pH 7.4 of 0.87 +/- 0.11 microM. The following purine nucleotide derivatives were potent competitive inhibitors of IMP cyclohydrolase: 2-mercaptoinosine 5'-monophosphate (Ki = 0.094 +/- 0.024 microM), xanthosine 5'-monophosphate (Ki = 0.12 +/- 0.01 microM), 2-fluoroadenine arabinoside 5'-monophosphate (Ki = 0.16 +/- 0.02 microM), 6-mercaptopurine riboside 5'-monophosphate (Ki = 0.20 +/- 0.02 microM), adenosine N1-oxide 5'-monophosphate (Ki = 0.28 +/- 0.03 microM), and N6-(carboxymethyl)adenosine 5'-monophosphate (Ki = 1.7 +/- 0.42 microM). The pH dependencies of Vmax and Vmax/Ks values for IMP cyclohydrolase are consistent with a single ionizable amino acid residue (pKa = 7.57 +/- 0.09) of the enzyme which must be unprotonated for catalysis to occur and a residue (pKa = 7.57 +/- 0.14) which must be unprotonated for FAICAR to bind. The pKa values of 5.81 +/- 0.03 and 9.41 +/- 0.04 determined for FAICAR indicate that ionization of the substrate does not contribute significantly to the pH effects observed. Chemical modification of IMP cyclohydrolase provides evidence for arginine and cysteine residues at the active site, and roles for these residues in the mechanism of catalysis are proposed.

Blast cell methotrexate-polyglutamate accumulation in vivo differs by lineage, ploidy, and methotrexate dose in acute lymphoblastic leukemia

High-dose methotrexate (HDMTX) is a component of most treatment protocols for childhood acute lymphoblastic leukemia (ALL), yet recent studies of receptor-mediated transport and saturable polyglutamylation have questioned its rationale. To investigate this in vivo, methotrexate and its active polyglutamated metabolites (MTX-PG) were measured in bone marrow blasts obtained from 101 children randomized to single-agent therapy with either HDMTX (1 g/m2 per 24 h i.v., n = 47) or low-dose MTX (LDMTX, 30 mg/m2 by mouth every 6 h x 6, n = 54), before remission induction therapy. Blast concentrations of total MTX-PGs (median 460 vs 1380 pmol/10(9) cells) and of long-chain MTX-glu4-6 were both significantly higher after HDMTX (P < 0.001). With either treatment, MTX-PGs were significantly higher in B-lineage blasts than in T-lineage blasts (LDMTX P = 0.001, HDMTX P = 0.03). In a multiple regression analysis of B-lineage ALL, blast MTX-PG was significantly related to MTX dose (or plasma MTX concentration), lymphoblast ploidy (hyperdiploid > nonhyperdiploid), and percentage S-phase. This is the first evidence that HDMTX achieves higher MTX-PG concentrations in ALL blasts in vivo, establishing a rationale for HDMTX in the treatment of childhood ALL, especially T-lineage or nonhyperdiploid B-lineage ALL, disease characteristics associated with a poor prognosis on conventional therapy.

Novel pyrrolo[2,3-d]pyrimidine antifolate TNP-351: cytotoxic effect on methotrexate-resistant CCRF-CEM cells and inhibition of transformylases of de novo purine biosynthesis

N-[4-[3-(2,4-Diamino-7H-pyrrolo[2,3-d]pyrimidin-5- yl)propyl]benzoyl]-L-glutamic acid (TNP-351), characterized by a pyrrolo[2,3-d]pyrimidine ring, is a novel antifolate that exhibits potent antitumor activities against mammalian solid tumors. The mechanism of action of TNP-351 was evaluated using some methotrexate-resistant CCRF-CEM human lymphoblastic leukemia cell lines as well as partially purified enzymes folylpolyglutamate synthetase (FPGS), aminoimidazolecarboxamide ribonucleotide transformylase (AICARTFase), and glycinamide ribonucleotide transformylase (GARTFase) from parent CCRF-CEM cells. TNP-351 was found to inhibit the growth of L1210 and CCRF-CEM cells in culture, with the doses effective against 50% of the cells (ED50 values) being 0.79 and 2.7 nM, respectively. The growth inhibition caused by TNF-351 was reversed by leucovorin or a combination of hypoxanthine and thymidine. The methotrexate-resistant CCRF-CEM cell line, which has an impaired methotrexate transport, showed less resistance to TNP-351 than to methotrexate. TNP-351 was also found to be an excellent substrate for FPGS with a Michaelis constant (Km) of 1.45 microM and a maximum of velocity (Vmax) of 1,925 pmol h-1 mg-1. Inhibitory activities of TNF-351-Gn (n = 1-6) for AICARTFase were found to be significantly enhanced with increasing glutamyl chain length [inhibition constants (Ki): G1, 52 microM; G6, 0.07 microM]. Neither TNP-351 nor its polyglutamates were very strong inhibitors of GARTFase. These findings have significant implications regarding the mechanism of action of TNP-351.

The antiinflammatory mechanism of methotrexate. Increased adenosine release at inflamed sites diminishes leukocyte accumulation in an in vivo model of inflammation

Methotrexate, a folate antagonist, is a potent antiinflammatory agent when used weekly in low concentrations. We examined the hypothesis that the antiphlogistic effects of methotrexate result from its capacity to promote intracellular accumulation of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) that, under conditions of cell injury, increases local adenosine release. We now present the first evidence to establish this mechanism of action in an in vivo model of inflammation, the murine air pouch model. Mice were injected intraperitoneally with either methotrexate or saline for 3-4 wk during induction of air pouches. Pharmacologically relevant doses of methotrexate increased splenocyte AICAR content, raised adenosine concentrations in exudates from carrageenan-inflamed air pouches, and markedly inhibited leukocyte accumulation in inflamed air pouches. The methotrexate-mediated reduction in leukocyte accumulation was partially reversed by injection of adenosine deaminase (ADA) into the air pouch, completely reversed by a specific adenosine A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX), but not affected by an adenosine A1 receptor antagonist, 8-cyclopentyl-dipropylxanthine. Neither ADA nor DMPX affected leukocyte accumulation in the inflamed pouches of animals treated with either saline or the potent antiinflammatory steroid dexamethasone. These results indicate that methotrexate is a nonsteroidal antiinflammatory agent, the antiphlogistic action of which is due to increased adenosine release at inflamed sites.

Differences in purine metabolism in patients with Down's syndrome

Three enzymes intervening in de novo purine synthesis, as well as cystathionine B-synthetase, have been mapped to chromosome 21. In order to gain a better understanding of purine synthesis anomalies in Down's syndrome, the present authors studied the variations in mitotic index of lymphocyte cultures to which various inhibitors or metabolites of purine synthesis had been added. In spite of common gene dosage effects, unexpected and highly significant differences were noted between Down's syndrome patients without complications and those presenting with additional psychotic features. In Down's syndrome patients without complications, a highly significant decrease in mitotic index was noted in the presence of exogenous inosine. A significant decrease in the presence of adenosine and guanosine was also noted. These findings are in keeping with the expected metabolic repercussions of genes mapped to chromosome 21. In Down's syndrome patients with psychotic complications, the in vitro reactions were quite different. A paradoxal increase in mitotic index was noted in the presence of inosine and of adenosine, but the response to guanosine did not differ from that observed in normal controls. These findings were unexpected and seem to indicate that, in spite of the gene dosage effect, psychotic Down's syndrome patients are unable to compensate abnormal purine synthesis and resulting imbalances. Furthermore, a marked difference in purine metabolic reactions was noted between Down's syndrome patients receiving supplemental folic/folinic acid and those on no therapy. This suggests that some modulation of the gene dosage effect may be possible.

Pharmacokinetic optimisation of anticancer therapy

It is obvious that there are great problems with pharmacokinetic individualization of anticancer therapy. The strong relationship between dose intensity (total dose/unit time) and response revealed in clinical trials with some tumours provides a strong support for studies seeking relationships between the individual plasma pharmacokinetic profile and response to treatment. Unfortunately, studies that define a therapeutic window are sparse, and trials that prospectively test such models are even rarer. Thus, for most cancer drugs, it is not possible to give any definite advice on how to use pharmacokinetic determinations to establish individualised therapy, and there is therefore a definite need for such studies. It is important, however, that attempts to establish relationships between drug concentrations and therapeutic effects be founded on a sound theoretical base. When drugs, mainly antimetabolites, are extensively metabolised intracellularly and interact with intracellular processes about which there are data showing a strong interindividual heterogeneity, such data must be considered when designing pharmacokinetic investigations. Cytarabine and fluorouracil are good examples of this. The monitoring of intracellular drug/metabolite concentrations or of the direct biochemical events in the tumour cells seems to be a promising approach with such drugs. It also needs to be emphasised that pharmacokinetically guided individualization cannot be achieved before a therapeutic window is established, i.e. a knowledge of the relationship between drug concentration and clinical effects. The investigators in this field accept a great responsibility when clinical studies are undertaken: a poorly designed study showing no benefit from pharmacokinetically guided individualization can impair the possibilities of performing more adequate studies in the future.

Methotrexate inhibits neutrophil function by stimulating adenosine release from connective tissue cells

Although commonly used to control a variety of inflammatory diseases, the mechanism of action of a low dose of methotrexate remains a mystery. Methotrexate accumulates intracellularly where it may interfere with purine metabolism. Therefore, we determined whether a 48-hr pretreatment with methotrexate affected adenosine release from [14C]adenine-labeled human fibroblasts and umbilical vein endothelial cells. Methotrexate significantly increased adenosine release by fibroblasts from 4 +/- 1% to 31 +/- 6% of total purine released (EC50, 1 nM) and by endothelial cells from 24 +/- 4% to 42 +/- 7%. Methotrexate-enhanced adenosine release from fibroblasts was further increased to 51 +/- 4% (EC50, 6 nM) and from endothelial cells was increased to 58 +/- 5% of total purine released by exposure to stimulated (fMet-Leu-Phe at 0.1 microM) neutrophils. The effect of methotrexate on adenosine release was not due to cytotoxicity since cells treated with maximal concentrations of methotrexate took up [14C]adenine and released 14C-labeled purine (a measure of cell injury) in a manner identical to control cells. Methotrexate treatment of fibroblasts dramatically inhibited adherence to fibroblasts by both unstimulated neutrophils (IC50, 9 nM) and stimulated neutrophils (IC50, 13 nM). Methotrexate treatment inhibited neutrophil adherence by enhancing adenosine release from fibroblasts since digestion of extracellular adenosine by added adenosine deaminase completely abrogated the effect of methotrexate on neutrophil adherence without, itself, affecting adherence. One hypothesis that explains the effect of methotrexate on adenosine release is that, by inhibition of 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) transformylase, methotrexate induces the accumulation of AICAR, the nucleoside precursor of which (5-aminoimidazole-4-carboxamide ribonucleoside referred to hereafter as acadesine) has previously been shown to cause adenosine release from ischemic cardiac tissue. We found that acadesine also promotes adenosine release from and inhibits neutrophil adherence to connective tissue cells. The observation that the antiinflammatory actions of methotrexate are due to the capacity of methotrexate to induce adenosine release may form the basis for the development of an additional class of antiinflammatory drugs.

Structural features of 5,10-dideaza-5,6,7,8-tetrahydrofolate that determine inhibition of mammalian glycinamide ribonucleotide formyltransferase

We have investigated the structural features of 5,10-dideaza-5,6,7,8-tetrahydrofolate (DDATHF) that determine the activity of this compound as an inhibitor of glycinamide ribonucleotide formyltransferase (GARFT) purified from mouse L1210 cells. 5-Deazatetrahydrofolate was as good an inhibitor of GARFT as DDATHF, indicating that isosteric replacement of nitrogen by carbon at the 5-position of tetrahydrofolate is sufficient for inhibition of GARFT. 5,10-Dideazafolic acid, 5,8,10-trideazatetrahydrofolate, and 2-desamino-5,10-dideazatetrahydrofolate were poor inhibitors of GARFT, indicating that a reduced pyridopyrimidine ring, N-8, and the 2-amino group of DDATHF, respectively, play an important role in the binding of tetrahydrofolate analogues to this enzyme. DDATHF analogues in which the phenyl ring was replaced either by a cyclohexyl ring or by methylene groups retained activity as inhibitors. 5,10-Dideazatetrahydrohomofolate was about 6 times more potent as an inhibitor of GARFT than DDATHF, but 5,10-dideazatetrahydronorfolate had about one-fifth of the activity of DDATHF. An analogue of DDATHF in which the glutamic acid side chain was replaced by aspartic acid (which was not a substrate for polyglutamation and was only weakly cytotoxic) was equiactive with DDATHF as an inhibitor of purified GARFT. Surprisingly, 5,10-dideazatetrahydropteroic acid was about as active as DDATHF as an inhibitor of GARFT, an indication that the glutamic acid in the side chain of DDATHF does not play a role in this ligand-enzyme interaction. The polyglutamate derivatives of DDATHF bound up to 100 times tighter to GARFT than DDATHF itself; longer chain polyglutamates conformed to Goldstein's zone B behavior under experimental conditions and were projected to be in zone C, i.e., stoichiometric inhibition, in vivo. We conclude that the presence of carbon at the 5-position of tetrahydrofolate analogues is sufficient for inhibition of GARFT, that N-8 and the 2-amino group are involved in binding of DDATHF to GARFT, probably through hydrogen bonds, and that the structures of the phenyl ring and amino acid side chain of DDATHF analogues are not primary determinants of GARFT inhibition by monoglutamate forms of these compounds. We also conclude that polyglutamation plays a major role in the potent cytotoxicity of DDATHF.

Induction of HL-60 leukemia cell differentiation by tetrahydrofolate inhibitors of de novo purine nucleotide biosynthesis

5,10-Dideazatetrahydrofolic acid (DDATHF) is a folate antimetabolite that shows activity against glycinamide ribonucleotide (GAR) transformylase, a folate-requiring enzyme in the de novo purine nucleotide biosynthetic pathway. Previous studies from our laboratory have shown that DDATHF is an effective inducer of the maturation of HL-60 promyelocytic leukemia. In solution, DDATHF is a mixture of two diastereomers due to an asymmetric configuration at carbon 6. Incubation of HL-60 cells with 1 microM of each diastereomer resulted in an inhibition of cellular proliferation after 48 h that preceded an increase in the number of differentiated myeloid cells, as determined by the ability of cells to reduce nitroblue tetrazolium (NBT) and by the binding of the myeloid-specific antibody Mo 1. Several analogs of DDATHF were also tested as inducers of the differentiation of HL-60 cells. With the exception of the 10-acetyl analog of 5-deazatetrahydrofolic acid, all compounds displayed similar activities as inducers of maturation. The finding that both stereoisomers of DDATHF, as well as the analogs tested, could selectively reduce intracellular purine nucleotide levels suggested that these compounds inhibited purine nucleotide biosynthesis de novo. This possibility was confirmed by the finding that hypoxanthine completely prevented the reduction of intracellular purine nucleotide levels, as well as the induction of differentiation and the inhibition of cellular growth, by these folate analogs. The results suggest that GAR transformylase is a target for a series of compounds whose structures resemble that of tetrahydrofolate and indicate that the inhibition of GAR transformylase by these compounds is sufficient to induce the maturation of HL-60 leukemia cells.

Reversal of methotrexate cytotoxicity to human bone marrow cells and leukemic K562 cells by leucovorin: methotrexate polyglutamates formation as a possible important factor

Methotrexate (MTX) is metabolized intracellularly to MTX-polyglutamates (MTX-PGs), which markedly inhibit several folate-dependent enzymes. Polyglutamation defect, therefore, is one of the important factors in drug resistance. In this study, reversal of MTX cytotoxicity by l-leucovorin (l-LV) was investigated using normal human bone marrow granulocyte progenitor cells (G-CFCs), and MTX-sensitive and -resistant leukemic K562 cell lines; the latter showed diminished polyglutamation. Cytotoxicity of 10(-7)M MTX to G-CFCs was completely reversed by an equimolar concentration of l-LV, but with higher MTX concentrations, relatively more l-LV was required. The reversal of MTX cytotoxicity by l-LV was more effective against bone marrow cells than MTX-sensitive K562 cells; this reversal seemed to be correlated to the total intracellular MTX levels as well as MTX-PG formation (low in bone marrow cells and high in K562 cells). When MTX-sensitive and -resistant K562 cells were incubated with MTX under conditions in which the total intracellular MTX levels of both cells were similar, successful reversal of MTX toxicity by l-LV was demonstrated in MTX-resistant cells, but not in MTX-sensitive cells, suggesting that an increase of MTX-PG formation in MTX-sensitive cells may explain the failure of l-LV to overcome MTX cytotoxicity. In addition to competitive reversal of MTX cytotoxicity by LV, noncompetitive reversal relating to variable formation of MTX-PGs is suggested to be another important factor in the mechanism of the reversal of MTX cytotoxicity by LV.

New vistas for leucovorin in cancer chemotherapy

With a resurgence of interest in leucovorin calcium (LV), many questions about its role in cancer chemotherapy are being asked again. How does it modulate 5-fluorouracil (5-FU) therapy? Can it potentiate the cytotoxicity of 5-FU more in malignant than in normal cells? Does it rescue tumor cells from high-dose methotrexate (MTX) more than it does normal cells? How should LV rescue be individualized to maximize the therapeutic index? When can LV be administered orally, and how can LV rescue be conducted at home? When is LV rescue necessary, and what toxicities occur when it is inadequate? Some answers to these questions are provided in this review.

Mutants of human colon adenocarcinoma, selected for thymidylate synthase deficiency

GC3/c1 human colon adenocarcinoma cells were treated with the mutagen ethyl methanesulfonate, and three clones deficient in thymidylate synthase (5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1. 45) activity were selected and characterized. Growth in medium deficient in thymidine caused cell death in two clones (TS- c1 and TS- c3), whereas one clone (TS- c2) showed limited growth. Growth correlated with thymidine synthase activity and 5-fluoro-2'-deoxyuridine 5'-monophosphate-binding capacity and with incorporation of 2'-deoxy[6-3H]uridine into DNA. In the presence of optimal thymidine, growth rates were only 5-18% that of the parental clone (GC3/c1), which grew equally well in thymidine-deficient or -replete medium. Analysis of poly(A)+ RNA showed normal levels of a 1.6-kilobase transcript in TS- c1 and TS- c2 but decreased levels (approximately 6% control) in TS- c3. Clone TS- c3 was 32-, 750-, and greater than 100,000-fold more resistant than the parental clone to 5-fluorouracil, 5-fluoro-2'-deoxyuridine, and methotrexate, respectively. When inoculated into athymic nude mice, each TS- clone produced tumors, demonstrating continued ability to proliferate in vivo.

Folylpolyglutamate synthesis and role in the regulation of one-carbon metabolism

The physiological importance of folylpolyglutamates is now well established. These derivatives are the intracellular substrates and regulators of one-carbon metabolism, and their synthesis is required for normal folate retention by tissues. Over the last few years, a considerable amount of information has been obtained on the mechanism by which these compounds are synthesized, on how this synthesis is regulated, and on the effects of the polyglutamate chain on the interaction of folate substrates and inhibitors with folate-dependent enzymes. Many regulatory implications have been suggested by these studies, but the physiological relevance of some of these observations remains to be explored. Folates in mammalian tissues are metabolized to polyglutamates of chain lengths considerably longer than that required for folate retention, but the metabolic advantages of this are not entirely clear. Several in vivo model systems have been developed to explore the functioning of specific folylpolyglutamate chain lengths in metabolic cycles of one-carbon metabolism, and these are likely to shed further light on this point. The role of folate-binding proteins in folate transport, the metabolic role of glutamylhydrolases, and the role of folylpolyglutamates in putative multifunctional protein complexes are also areas that are being actively pursued at present and are likely to produce new insights in the future. Recent studies on the retention of antifolates by cells and on their substrate efficacy for folylpolyglutamate synthetases have also suggested mechanisms for the differential cytotoxicity of these agents for different tissues.

Impairment of methotrexate (MTX)-polyglutamate formation of MTX-resistant K562 cell lines

We examined the mechanism of methotrexate (MTX) resistance in five K562 cell subclones resistant to MTX. Based on a clonogenic assay, the IC50S of these MTX-resistant clones were 10 to 40 microM MTX, indicating 2,000 to 5,000-fold resistance as compared to that of the parent cell line. The doubling times of these MTX-resistant K562 cell lines are longer (27-60 hr) than that of the parent K562 cell line (24 hr). One-hour MTX accumulation in the resistant cells cells was 70-80% of that in parent cells. To investigate the formation of MTX-polyglutamates (MTX-PGs), resistant cells were incubated with 3H-MTX (1 or 10 microM) for 24 hr in the presence of thymidine and deoxyinosine to prevent cytotoxicity. MTX (-Glu1) and the polyglutamate metabolites (MTX-Glu2, -Glu3, -Glu4 and -Glu5) were analyzed by a high-pressure liquid chromatography (HPLC) technique. After a 24-hr incubation with 10 microM MTX, the total concentration of intracellular MTX reached 39 to 89 nmol/g protein, only 20 to 40% of the MTX level of the parent K562 cells. The HPLC analysis revealed that less than 2% of intracellular MTX was in the form of high-molecular MTX-PGs (MTX-Glu3, -Glu4 and -Glu5) in the five MTX-resistant K562 cell lines, while in the parent cells MTX-Glu3-5 comprised 46% of the total intracellular MTX. These data indicate the possibility that impairment of MTX-PG formation, with transport alteration, may be a special mechanism for the high level of resistance to this agent in human leukemic cells.

Effect of methotrexate on intracellular folate pools in purified myeloid precursor cells from normal human bone marrow

We investigated the effects of the antifolate methotrexate on intracellular folate pools of human myeloid precursor cells (MPCs). Immature MPCs, representing 3.2% of the original marrow population, were selected from normal human bone marrow by immune rosetting. The intracellular folate pools were labeled by incubation with 5 X 10(-8) M [3H]5-formyl-FH4 and were quantitated by high performance liquid chromatography. The predominant folates were 5-methyl-tetrahydrofolate (5-methyl-FH4) (36%), 10-formyl-FH4 (41.4%), 5-formyl-FH4 (12.3%), and FH4 (10.3%). A 12-h exposure to 1 microM methotrexate (MTX) resulted in a 34% reduction in the intracellular concentration of 10-formyl-FH4, a 61% decrease in 5-formyl-FH4, and a 62% decrease in 5-methyl-FH4, as well as the appearance and progressive expansion of the FH2 and 10-formyl-FH2 pools. These changes were maximal after 4 h of incubation with MTX. Paralleling the changes in folates, particularly the increase in FH2, were a 64% reduction in myeloid colony formation and a 77% depression of de novo purine synthesis after 4 h of MTX. We conclude that MTX does not produce quantitative depletion of 10-formyl-FH4 and that its antipurine effect may be mediated by direct inhibition of de novo purine synthesis by FH2 and, at later time points, by MTX polyglutamates.