The mechanism of folate transport in rabbit reticulocytes.

Folate transport in phenylhydrazine-induced rabbit reticulocytes was studied with the non-metabolized folate-analog, methotrexate. The time-course of methotrexate uptake into a mixed population of reticulocytes and mature erythrocytes is a two-component process consisting of a small, but rapid, initial uptake phase followed by a much slower uptake component which remains essentially constant over the period of observation. The velocity of the latter uptake component is directly proportional to the per cent reticulocytes and appears to represent a unidirectional influx of methotrexate into these cells. Uptake of methotrexate into reticulocytes was found to have the following characteristics: (a) temperature sensitivity, Q(10) of 4; (b) uptake velocity as a function of the extracellular methotrexate concentration approximated Michaelis-Menten kinetics with a maximum transport velocity of 48 pmoles/min per g dry wt; the extracellular methotrexate level at which the uptake velocity was one-half maximum was 1.4 muM; (c) 5-formyltetrahydrofolate markedly inhibited methotrexate uptake but pteroylglutamic acid inhibition was weak; (d) uptake was stimulated in cells preincubated with 5-formyltetrahydrofolate, indicative of hetero-exchange diffusion; (e) uptake was independent of extracellular sodium but was inhibited by anions including nitrate, phosphate, and glucose-6-phosphate; (f) uptake was enhanced by azide plus iodoacetate. These data indicate that folate transport in rabbit reticulocytes is mediated by a carrier mechanism which disappears with reticulocyte maturation. The mechanism of folate transport in rabbit reticulocytes is qualitatively similar to tumor cells previously studied; both appear to have an energy-dependent mechanism limiting folate uptake, and influx in both is inhibited by structurally unrelated inorganic and organic anions. These studies suggest that circulating pteroylglutamic acid is of little importance in meeting the folate requirements of folate-dependent tissues and raise the possibility that clinical conditions associated with alterations in the anionic composition of the blood may be accompanied by impaired utilization of the folates.

Chromate transport in human leukocytes.

Chromium is a trace metal of importance in human physiology and, in addition, as 51-chromate, has been extensively used as a label in the study of blood cell pool sizes and intravascular kinetics. The transport characteristics of 51-chromate were investigated in normal human leukocytes. Chromate uptake is unidirectional over a 1 hr incubation with extracellular chromate concentrations up to 200 mumoles/liter. Under these conditions, intracellular 51-chromium is in a form in which it is nonexchangeable. Influx is temperature sensitive with a Q(10) of approximately 2 and may be energy dependent since a variety of metabolic poisons strongly inhibit uptake. The unidirectional influx of chromate follows Michaelis-Menten kinetics; the maximum velocity is 52 mmumoles/g dry weight of cells per min and the chromate concentration at which influx velocity is half maximal is 87 mumoles/liter. This transport mechanism is highly specific for chromate; other divalent tetrahedral anions only slightly inhibit influx at concentrations up to 10 times that of chromate. Metavanadate, however, competitively inhibits chromate influx at equimolar concentrations. Exposure of cells to unlabeled chromate leads to inhibition of subsequent influx of 51-chromate. It is suggested that this is due to a primary inhibitory effect of chromate on cellular energy metabolism.

Analysis of the inhibitory effects of VP-16-213 (etoposide) and podophyllotoxin on thymidine transport and metabolism in Ehrlich ascites tumor cells in vitro.

These studies explore the effects of an epipodophyllotoxin on the membrane transport and metabolism of thymidine in Ehrlich ascites tumor cells. Uptake of 3H after exposure of cells to [3H]-thymidine is characterized by a rapid initial velocity that approximates membrane transport followed by a slower rate of uptake that parallels the accumulation of phosphorylated derivatives of thymidine, primarily thymidine triphosphate, within the cell. The high rate of thymidine transport relative to thymidine metabolism to the triphosphate within the cell decreases as the extracellular nucleoside concentration is reduced due to a much greater decrease in membrane transport than the subsequent metabolic step. Hence, as extracellular thymidine is decreased, transport becomes increasingly rate limiting to metabolism within the cell. VP-16-213 (etoposide) or podophyllotoxin inhibits the initial uptake rate for thymidine and, as a consequence, inhibits the intracellular formation of thymidine triphosphate. When extracellular thymidine is high, inhibitory effects on transport are transient, and the net rate of thymidine triphosphate accumulation within drug-treated cells rapidly approaches a velocity comparable to that of control cells, indicating no direct VP-16-213 or podophyllotoxin effect on nucleoside and nucleotide phosphorylation. When extracellular thymidine is reduced so that transport is rate limiting to metabolism, the duration of the inhibitory effects of VP-16-213 on thymidine triphosphate formation is prolonged. A secondary effect of VP-16-213 becomes manifest beyond 10 min of incubation with [3H]thymidine with the virtual complete cessation of thymidine incorporation into the acid precipitate without any change in the thymidine triphosphate level. This late effect is not observed with podophyllotoxin and indicates a direct effect of VP-16-213 on DNA synthesis that is distinct from the earlier inhibitory effect on thymidine phosphorylation, which is secondary to membrane transport.

The relationship among transport, intracellular binding, and inhibition of RNA synthesis by actinomycin D in Ehrlich ascites tumor cells in vitro.

Uptake of actinomycin D (AD) was characterized in Ehrlich ascites tumor cells in vitro. The time course of uptake consists of an initial component that represents a unidirectional flux of AD into the cells, following which the net uptake rate falls to achieve a velocity that is constant over a 45-min interval of observation. The velocity of influx exceeds that of the late uptake phase by a factor of 2. Influx is 1st order over an AD concentration range of 0.02 to 14 muM. Influx is highly temperature sensitive (Q10 = 4.5); this is related only in part to the high temperature dependence (Q10 =2) of the oil-adqueous partition coefficient for this agent. Within 4 min after exposure of cells to AD, a component of intracellular AD appears which exceeds the concentration of extracellular AD and rapidly leaves the cells (t 1/2 congruent to 3 min) when the cells are resuspended into an AD-free medium. The level of this component of intracellular drug is unaffected by metabolic poisons, and the apparent chemical gradient is attributed to loose binding. While transport of AD does not appear to be energy-dependent, the data do not clafity whether translocation across the cell membrane is diffusional or carrier mediated. In addition to a loosely bound intracellular component, analysis of the unidirectional net efflux of AD indicates at least 3 other exit components; one with a t 1/2 of 35 min, a very tightly bound component, and another representing drug that is free within the intracellular water, The bulk of AD taken up is tightly bound to intracellular constituents, and the rate of accumulation of this cellular component accounts almost entirely for the net rate of uptake of AD during the late uptake phase. Because influx exceeds the rate of tight binding within the cell by a factor of only 2, it is unlikely that osmotically active AD achieves thermodynamic equilibrium across the cell membrane over the interval of these experiments. Rather, prior to saturation of binding sites within the cell, the level of free AD achieved is probably below the extracellular concentration. Since the rate of binding should be influenced by the level of free intracellular AD, factors that increase influx and bring the intracellular AD level closer to equilibrium should also increase the rate of intracellular binding. Tween 80 augments both influx and AD binding within the cell, an effect that may be related to augmented transport alone, but an additional stimulatory effect on the binding process is also possible. Association of AD with only a small number of the very-high-affinty intracellular binding sites, in the absence of free or loosely bound AD, results in approximately 35% inhibition of [2-14C]uridine incorporation into RNA within 50 sec after exposure of cells to 1 muM AD. Within 10 min. 75% inhibition is achieved indicating that association of AD with only a small fraction of the total high-affinity sites is sufficient to suppress RNA synthesis.

Formation of 7-hydroxymethotrexate polyglutamyl derivatives and their cytotoxicity in human chronic myelogenous leukemia cells, in vitro.

The rapid synthesis of poly-gamma-glutamyl derivatives of 7-hydroxymethotrexate (7-OH-MTX) and their selective intracellular retention are reported in human chronic myelogenous leukemia cells, K-562. After a 30-min exposure to 5 microM [3H]7-OH-MTX, three different polyglutamyl derivatives were detected by high-performance liquid chromatography. When extracellular 7-OH-MTX was removed, the 7-OH-MTX diglutamate level declined slowly in comparison to the monoglutamate, but the higher polyglutamyl derivative levels increased. Within 10 min after exposure of cells to 7-OH-MTX, the level of these polyglutamyl derivatives far exceeds the dihydrofolate reductase binding capacity. Gel filtration or charcoal binding analysis followed by high-performance liquid chromatography analysis of the bound component showed intracellular binding of virtually all 7-OH-MTX tetraglutamate at a level 4-fold higher than that of the dihydrofolate reductase binding capacity. No bound 7-OH-MTX diglutamate or triglutamate could be detected. Treatment of the 7-OH-MTX tetraglutamate: protein complex with 100 microM unlabeled methotrexate (MTX) for 15 min resulted in only a partial dissociation of this complex to an extent compatible with the dihydrofolate reductase level. The residual 7-OH-MTX tetraglutamate remained bound to a site with a molecular weight of approximately 25,000 to 35,000 as assessed by Bio-Gel P-60 analysis and could not be displaced by folic acid, 5-formyltetrahydrofolate, 7-OH-MTX, or the tetraglutamate of MTX. 7-OH-MTX and MTX cytotoxicities were compared by clonogenic assay in agar and by their effects on cell growth. After a 2-hr exposure, the 50% inhibitory concentrations for 7-OH-MTX and MTX in cells growing in agar were 10(-5) and 10(-6) M, respectively. A 10-fold difference in cytotoxicity was also observed in cells growing in suspension. Continuous exposure to glycine: adenosine: thymidine completely protects cells from a sustained exposure to 7-OH-MTX over the entire period of clonal growth. However, even a brief exposure to 7-OH-MTX also requires continuous exposure to glycine: adenosine: thymidine for protection. This suggests that, as observed for MTX, the 7-OH-MTX polyglutamyl derivatives that are retained within the cells have a sustained cytotoxic effect after the monoglutamate is removed.

Role of the cellular oxidation-reduction state in methotrexate binding to dihydrofolate reductase and dissociation induced by reduced folates.

5-Formyltetrahydrofolate promotes the net dissociation of methotrexate bound to dihydrofolate reductase in the Ehrlich ascites tumor (L. H. Matherly et al., Cancer Res., 43: 2694-2699, 1983). Treatment of Ehrlich tumor cells with glucose or inhibitors of electron transfer stabilized the association of the antifolate with dihydrofolate reductase as reflected by a 2-fold increased fraction of dihydrofolate reductase-bound methotrexate and an abolition of the 5-formyltetrahydrofolate-induced dissociation of the inhibitor-enzyme complex. Glucose and azide were also found to increase the intracellular ratio of reduced nicotinamide adenine dinucleotide phosphate (NADPH) to oxidized nicotinamide adenine dinucleotide phosphate (NADP+) in the tumor approximately 8- and 11-fold, respectively. However, other agents which enhanced the association between methotrexate and its target enzyme were less effective in increasing the intracellular level of NADPH relative to NADP+. Micromolar concentrations of NADPH promoted methotrexate binding to the purified Ehrlich tumor dihydrofolate reductase. Bound methotrexate could be dissociated from the purified enzyme by 5-methyltetrahydrofolate but less readily by 5-formyltetrahydrofolate and only in the presence of reduced levels of NADPH relative to NADP+. The tetraglutamate derivative of 5-methyltetrahydrofolate was even more effective than the underivatized compound in dissociating methotrexate from dihydrofolate reductase. These findings suggest a critical role for the cellular oxidation-reduction state in determining the affinity of dihydrofolate reductase for methotrexate and thus the cellular sensitivity to the antifolate. In addition, the data are consistent with the possibility that dihydrofolate reductase is a key locus for intracellular competitive interactions between reduced folates and methotrexate during leucovorin rescue from the pharmacological effects of the antifolate.

A basis for fluoropyrimidine-induced antagonism to methotrexate in Ehrlich ascites tumor cells in vitro.

The inhibitory effect of methotrexate on [3H]deoxyuridine incorporation into DNA is reduced as the basal rate of this reaction is inhibited by pretreatment of Ehrlich ascites tumor cells with fluoropyrimidines. This observation is a basis for fluoropyrimidine-methotrexate antagonism in anticancer regimens and supports the concept that the sensitivity of thymidylate synthesis in tumor cells to methotrexate is related, in part, to the basal rate of thymidylate synthesis from deoxyuridylate.

Antifolate polyglutamylation and competitive drug displacement at dihydrofolate reductase as important elements in leucovorin rescue in L1210 cells.

Previous studies from this laboratory have shown that the addition of leucovorin to tumor cells dissociates methotrexate, but not methotrexate polyglutamates, from dihydrofolate reductase (L. H. Matherly, D. W. Fry, and I. D. Goldman, Cancer Res., 43: 2694-2699, 1983). To further assess the importance of these interactions to leucovorin rescue, antifolate growth inhibition toward L1210 cells in the presence of leucovorin was correlated with the metabolism of (6S)-5-formyl tetrahydrofolate to dihydrofolate as a measure of dihydrofolate reductase activity. Growth inhibition (greater than 95%) by methotrexate (5-10 microM) following its intracellular polyglutamylation during a 3-h preexposure, or by continuous treatment with high levels of the lipophilic antifolate, trimetrexate (1 microM), was only slightly diminished by 10 microM leucovorin (15-25%). High-pressure liquid chromatographic analyses of the derivatives formed from radiolabeled (6S)-5-formyl tetrahydrofolate under these conditions showed an incomplete conversion to dihydrofolate and metabolism to predominantly 10-formyl tetrahydrofolate. Neither of the antifolates interfered appreciably with the metabolism of the folate derivatives to polyglutamates. Growth inhibition in the presence of leucovorin correlated with the accumulation of dihydrofolate (1.5-2.2 nmol) from radiolabeled (6S)-5-formyl tetrahydrofolate, reflecting continued suppression of dihydrofolate reductase activity at these drug concentrations. With lower equitoxic levels of the trimetrexate (7.5 nM), the provision of leucovorin allowed for a restoration of cell growth to a level greater than 90% of control. Under these conditions, control levels of dihydrofolate (0.2 nmol) were formed from radiolabeled cofactor, consistent with sustained dihydrofolate reductase activity. These findings support a role for the activation of dihydrofolate reductase as an important component of the reversal of the effects of diaminoantifolates by leucovorin, presumably by a competitive displacement of drug from the enzyme. Since no displacement occurs in cells which have accumulated methotrexate polyglutamates, or in the presence of high levels of trimetrexate, it appears that the concentration of unbound drug within cells is a significant determinant of the extent of this competitive binding interaction. From these considerations, the high levels of methotrexate polyglutamates that accumulate in sensitive tumors relative to bone marrow and gastrointestinal cells would appear to represent an important factor for the selectivity of leucovorin rescue in vivo.

Polyglutamylation, an important element in methotrexate cytotoxicity and selectivity in tumor versus murine granulocytic progenitor cells in vitro.

Methotrexate (MTX) cytotoxicity was assessed by clonogenic assay in agar with granulocytic progenitor cells from mouse bone marrow and in the Ehrlich ascites tumor, the K562 human chronic myelogenous leukemia, and the P388 murine leukemia. After a 2-hr exposure to MTX, the concentrations necessary to produce 50% inhibition of colony formation were 100, 25, 1.2, and 0.25 microM, respectively. This was inversely related to the ability of the tumor cells to accumulate MTX polyglutamyl derivatives and consistent with the observation that no polyglutamyl derivatives were observed in granulocytic progenitor cells after a 2-hr exposure to 5 micron MTX. Continuous exposure to glycine (200 microM)-adenosine (100 microM)-thymidine (10 microM) (GAT), along with MTX, protected cells from MTX cytotoxicity by circumventing the requirement for tetrahydrofolate cofactors. However, while the presence of GAT during a 2-hr exposure to 5 microM MTX is sufficient to protect granulocyte progenitor cells from MTX cytotoxicity, the presence of GAT, even after MTX is removed, is required to protect tumor cells. Indeed, if, after a 2-hr exposure of tumor cells to MTX and GAT, both MTX and GAT are removed before plating in agar, cytotoxicity to tumor cells was expressed. This sustained antitumor effect of MTX correlates with the rapid build-up of polyglutamyl derivatives that are retained in the cell even after extracellular and intracellular monoglutamate is eliminated. This is in contrast to granulocytic progenitor cells which appear to be susceptible to the drug only during the period of exposure to the monoglutamate under these conditions. The data strongly suggest that the marked differences in the accumulation of MTX polyglutamyl derivatives between the tumor cells studied and the murine bone marrow granulocytic progenitor cells are an important element in MTX selectivity.

Role of methotrexate polyglutamylation and cellular energy metabolism in inhibition of methotrexate binding to dihydrofolate reductase by 5-formyltetrahydrofolate in Ehrlich ascites tumor cells in vitro.

5-Formyltetrahydrofolate was found to reverse the binding of methotrexate to dihydrofolate reductase in the Ehrlich ascites tumor in vitro. When cells pretreated with methotrexate were resuspended in methotrexate-free buffer containing 5-formyltetrahydrofolate (or 5-methyltetrahydrofolate), net dissociation of the antifolate from the enzyme was observed. Methotrexate associated with the enzyme under these conditions was below the enzyme binding capacity. However, glucose or azide increased the fraction of dihydrofolate reductase associated with methotrexate and abolished the effect of tetrahydrofolates on this intracellular component. Addition of 5-fluoro-2'-deoxyuridine had no effect on this response to the reduced folate, thereby precluding a direct role for the thymidylate synthase-dependent generation of dihydrofolate in this dissociation of methotrexate from dihydrofolate reductase. Enzyme-bound methotrexate could also be reduced by exposure to 5-formyltetrahydrofolate prior to uptake and efflux of free methotrexate. When cells were incubated under conditions which favored formation of methotrexate polyglutamate derivatives, subsequent treatment with 5-formyltetrahydrofolate had no effect on the binding of the conjugated antifolate to dihydrofolate reductase. These findings support a role for dihydrofolate reductase as a locus for competitive binding interactions between reduced folates and methotrexate that may be a basis for the ability of 5-formyltetrahydrofolate to prevent the biochemical effects of this antifolate. These data suggest that the presence of methotrexate polyglutamate derivatives and cellular energy metabolism may be critical determinants of the responsiveness of methotrexate-treated cells to reduced folates and may play important roles in the selectivity of 5-formyltetrahydrofolate rescue.

Augmentation of the intracellular levels of polyglutamyl derivatives of methotrexate by vincristine and probenecid in Ehrlich ascites tumor cells.

The intracellular accumulation of poly-gamma-glutamyl derivatives of methotrexate was evaluated in the presence of vincristine or probenecid (agents which raise the intracellular level of free methotrexate) in Ehrlich ascites tumor cells. The results show that both intracellular methotrexate and its metabolites are increased by these agents and that, in the presence of L-glutamine, polyglutamate derivatives are increased by a higher percentage than is methotrexate. From 1 to 50 microM vincristine increased the levels of polyglutamate derivatives from 25 to over 300%, whereas methotrexate was raised from 25 to 80%. Similarly, 50 to 200 microM probenecid increased methotrexate polyglutamate derivatives from 31 to 88%, whereas methotrexate was raised from 0 to 30%. A determination of the bound fraction of drug indicated that the proportion of dihydrofolate reductase bound with methotrexate polyglutamates increased in the presence of these agents. Efflux studies showed that over 90% of the large pools of intracellular methotrexate polyglutamates produced by these agents was retained for at least 1 hr in the absence of extracellular methotrexate, whereas the majority of intracellular methotrexate exited the cell. These studies (a) indicate that vincristine and probenecid may be potentially useful for selectively increasing methotrexate polyglutamates in tumor cells and (b) introduce another basis for synergism observed between alkaloids and methotrexate.

Exchangeable intracellular methotrexate levels in the presence and absence of vincristine at extracellular drug concentrations relevant to those achieved in high-dose methotrexate-folinic acid "rescue" protocols.

Studies were undertaken to (a) assess intracellular methotrexate (MTX) levels at extracellular drug concentrations comparable to those achieved in high-dose MTX-folinic acid rescue protocols and (b) establish whether there is a rationale for the use of vincristine in these regimens. The data indicate that only low levels of exchangeable MTX (intracellular MTX in excess of the tightly bound fraction) accumulated in Ehrlich ascites tumor cells at high extracellular MTX concentration. For instance, the exchangeable steady-state intracellular MTX level was approximately 6.5 muM when the extracellular drug concentration was 85 muM. Over the interval of these experiments, exchangeable intracellular MTX did not exceed approximately 10 muM even when extracellular MTX was raised to 250 muM. These exchangeable intracellular MTX concentrations are comparable to those levels required experimentally to suppress (a) tetrahydrofolate synthesis from dihydrofolate and (b) tetrahydrofolate-dependent purine, pyrimidine, and amino acid synthesis in these cells in vitro. Vincristine (10 muM) augmented net MTX accumulation when the extracellular MTX level was 10, 100, or 250 muM. The limited capacity of cells to accumulate exchangeable intracellular MTX and the apparent role for this intracellular MTX component in achieving the metabolic effects of this agent may account for the necessity for high MTX blood levels in the treatment of some tumors and may be the basis, in part, for the enhanced chemotherapeutic efficacy of high-dose MTX regimens. These studies provide a rationale for the combined use of vincristine and MTX in high-dose MTX protocols. The addition of vincristine may permit the achievement of the level of exchangeable intracellular MTX that is required to critically inhibit tetrahydrofolate synthesis without an increase in the extracellular MTX concentration. This may permit a reduced MTX dose, diminishing the excretory load on the kidney and minimizing nephrotoxicity due to deposition of MTX in the renal tubule and interstitium. While the data indicate that the ratio of the concentration of exchangeable intracellular MTX to the extracellular drug concentration may be very low under steady-state conditions at high extracellular drug levels, further studies are required to establish that these steady-state gradients for MTX represent nonequilibrium conditions.

Formation of methotrexate polyglutamates in rat hepatocytes.

Polyglutamate derivatives of [3H]methotrexate (MTX) were detected in freshly isolated rat hepatocytes in suspension within 15 min after exposure to the folate analog. The rate of polyglutamate synthesis remained constant for at least one hr, and the polyglutamate derivatives accounted for an increasing proportion of the intracellular radiolabel with time. After initial exposure to 1 micron [3H]MTX, polyglutamate derivatives of Mtx continued to be synthesized even after the extracellular [3H]-MTX concentration had been reduced 20-fold. Prolonged exposure of hepatocytes in primary culture to 1 micron [3H]MTX resulted in the formation of longer-chain polyglutamate derivatives of MTX. The present studies demonstrate another important biosynthetic capacity of the freshly isolated hepatocyte and suggest the usefulness of this system for studying the mechanism of, and controlling factors in, the synthesis of polyglutamate derivatives of MTX. The ramifications of the formation of MTX polyglutamates on drug cytotoxicity in general and hepatotoxicity in particular are considered.

Characteristics of the formation and membrane transport of 7-hydroxymethotrexate in freshly isolated rabbit hepatocytes.

The cellular pharmacology of methotrexate was evaluated in freshly isolated rabbit hepatocytes in suspension with an analysis of drug metabolism by high-performance liquid chromatography. After exposure of hepatocytes at a cytocrit of 5% to 5 microM [3H]-methotrexate, intracellular 7-hydroxymethotrexate appears rapidly within the cell; within 15 sec, the level of 7-hydroxymethotrexate exceeds the level of intracellular methotrexate, although the latter has not achieved the dihydrofolate reductase binding capacity. Within 20 min, virtually all methotrexate is hydroxylated. There is minimal formation of methotrexate polyglutamyl derivatives even after exposure of cells to very high levels of methotrexate, and 7-hydroxymethotrexate polyglutamates do not accumulate in the cell at all after incubation with [3H]-7-hydroxymethotrexate. Because of the rapidity of the hydroxylation of methotrexate, transport of this agent could not be characterized. However, some aspects of the transport properties of 7-hydroxymethotrexate could be studied since the catabolite is neither bound nor metabolized in this system. Net 7-hydroxymethotrexate transport was reduced by the addition of 5-formyltetrahydrofolate. As observed for 4-aminoantifolate transport in other cell systems, net 7-hydroxymethotrexate transport was markedly stimulated by sodium azide, an inhibitor of energy metabolism. The data suggest that hydroxylation of methotrexate proceeds at a rate at least comparable to the rate of association of the drug with dihydrofolate reductase and that transport of methotrexate into rabbit hepatocytes is slow relative to the rate of catabolism to the 7-hydroxy derivative. Rabbit hepatocytes may be a useful model for exploring methotrexate catabolism at the cellular level and may provide insights into the interaction between methotrexate and/or other 4-aminoantifolates and the human liver.

Relationship between membrane transport of methotrexate and endogenous cyclic adenosine 3':5'-monophosphate in the Ehrlich ascites tumor.

Transport of methotrexate in mammalian cells is a complex process. Although the drug is pumped uphill into cells, metabolic poisons enhance influx and transmembrane gradients for this agent. Structurally unrelated inorganic and organic anions nonspecifically depress these parameters in a variety of tumor cells. It has been suggested that cell cyclic adenosine 3':5'-Monophosphate (cyclic AMP) plays a regulatory role in the transport of this agent. To evaluate this further, a number of different experimental approaches were used to analyze the relationship between methotrexate transport and cell cyclic AMP in the Ehrlich ascites tumor. The following results indicate that for the Ehrlich ascites tumor, at least, there is no evidence for a regulatory role for cyclic AMP in the transport of methotrexate. (a) A marked increase in cell cyclic AMP by cholera-toxin or reduction in cyclic AMP by ascorbate was unaccompanied by changes in methotrexate influx. (b) Instantaneous and constant inhibition of methotrexate influx by isobutylmethylxanthine was temporally dissociated from the slower rise in cell cyclic AMP that was induced by this agent. (c) Although isobutylmethylxanthine and dibutyryl cyclic AMP augment cell cyclic AMP and decrease methotrexate influx, they have different effects on efflux and net transport of methotrexate. Isobutylmethylxanthine stimulates net methotrexate uptake and slows methotrexate efflux but dibutyryl cyclic AMP depresses net methotrexate uptake without an effect on methotrexate efflux. (d) Azide markedly stimulates methotrexate influx and net transport without a change in cell cyclic AMP. (d) Dinitrophenol inhibits methotrexate influx and augments net methotrexate accumulation but does not alter cell cyclic AMP.

Transport, binding, and polyglutamation of methotrexate in freshly isolated rat hepatocytes.

Influx of [3H]methotrexate into freshly isolated hepatocytes in suspension is mediated by two routes, one with a high affinity (Km = 5.9 microM) and another with a low affinity for methotrexate. Both transport routes are equally sensitive to the sulfhydryl group inhibitor, p-chloromercuriphenylsulfonic acid, alterations in temperature, substitution of extracellular Na+ with choline, and inhibition by ouabain or azide. The high-affinity pathway for methotrexate shows specificity for the 4-amino group of the pteridine moiety as methotrexate and aminopterin similarly inhibit influx of the labeled drug. On the other hand, 100 microM concentrations of the naturally occurring folates, folic acid, 5-methyltetrahydrofolate, and 5-formyltetrahydrofolate, are not inhibitory to influx of 1 microM methotrexate. Once in the cell, methotrexate rapidly reaches molar equivalence with dihydrofolate reductase following which both exchangeable and nonexchangeable intracellular methotrexate accumulates. The exchangeable component reaches steady state within 0.5 hr while the nonexchangeable component increases for at least 1 hr. The nonexchangeable component represents both bound methotrexate and methotrexate polyglutamates. Polyglutamates of methotrexate are a trivial component of total 3H within the cell until about 15 min, but thereafter, their rate of accumulation is constant so that by 1 hr they represent approximately 30% of total intracellular 3H. At steady state, there is a transmembrane chemical gradient for exchangeable methotrexate of 2.4:1; this is 24 times greater than the chemical gradient predicted for equilibrium when the transcellular membrane potential is considered. These results indicate that there are multiple routes for methotrexate transport in the rat hepatocyte that appear to be, at least in part, distinct from the routes for folic acid and the tetrahydrofolate cofactors. The data suggest that transport is energy and Na+ dependent and that the transport carrier requires intact sulfhydryl groups. Net association of methotrexate with the cells is a complex process determined by transport and binding to multiple sites within the cell and metabolism to polyglutamate derivatives that are retained within the cell.

Interactions between 7-hydroxymethotrexate and methotrexate at the cellular level in the Ehrlich ascites tumor in vitro.

Studies were undertaken to characterize the cellular pharmacology of 7-hydroxymethotrexate (7-OH-MTX) in Ehrlich ascites tumor cells, compare it to that of methotrexate (MTX), and define the interactions between the parent compound and its catabolite. Transport of 7-OH-MTX is mediated by the MTX-tetrahydrofolate cofactor carrier, with a Km of 9 microM in comparison to the MTX Km of 5 microM. Both compounds mutually inhibit their influx and steady-state levels of free drug accumulated. While influx of 7-OH-MTX is slower than influx of MTX, 7-OH-MTX efflux is likewise slower, so that the steady-state level of 7-OH-MTX achieved is comparable to that of MTX. Influx of 7-OH-MTX is inhibited by extracellular 5-formyltetrahydrofolate and trans-stimulated in cells preloaded with this tetrahydrofolate cofactor. The energetics of 7-OH-MTX transport is similar to that of MTX in the influx and net transport are stimulated by sodium azide, while net transport is reduced by glucose. As observed for MTX, 7-OH-MTX transport is sensitive to the anionic composition of the extracellular compartment and was shown to be inhibited by organic and inorganic phosphates. 7-OH-MTX does not, alone, inhibit [3H]deoxyuridine incorporation into DNA at concentrations of up to 50 microM. However, the catabolite reduces MTX inhibition of deoxyuridine metabolism, presumably due to the reduction in the free level of intracellular MTX achieved. These findings support the possibility that when 7-OH-MTX accumulates to high levels relative to MTX in clinical regimens, it may modulate the pharmacological effects of MTX.

Enhanced polyglutamylation of aminopterin relative to methotrexate in the Ehrlich ascites tumor cell in vitro.

The polyglutamylation of aminopterin and methotrexate (N10-methylaminopterin) was compared in the Ehrlich ascites tumor in vitro. Three poly-gamma-glutamyl conjugates of methotrexate and aminopterin were detected, although at an equal (1 microM) extracellular drug concentration, the net accumulation of aminopterin polyglutamates exceeded that for the methotrexate polyglutamyl derivatives by a factor of 9. When compensation was made for transport differences between these compounds by adjusting the extracellular drug concentrations to achieve equivalent intracellular monoglutamyl substrate levels, the polyglutamylation of aminopterin was still 2.8-fold greater than that for methotrexate, suggesting that aminopterin is a better substrate for the folylpolyglutamate synthetase as well as the transport carrier. An additional metabolite of aminopterin was detected within seconds following drug exposure. This derivative did not bind tightly to dihydrofolate reductase, yet it was rapidly converted to a polyglutamate. The formation of both aminopterin polyglutamates and these novel derivatives was enhanced by increases in the free intracellular level of aminopterin. Aminopterin polyglutamates were bound tightly to dihydrofolate reductase and were retained intracellularly relative to unaltered aminopterin when Ehrlich cells containing these forms were suspended in drug-free medium. These findings support a role for the polyglutamylation of aminopterin as a critical element in drug action and as a factor in addition to membrane transport in the disparate antifolate potencies of aminopterin and methotrexate.

Evidence from rat hepatocytes of an unrecognized pathway of 5-fluorouracil metabolism with the formation of a glucuronide derivative.

Isolated rat hepatocytes in suspension were exposed to [3H]-5-fluorouracil for intervals over 2 h, following which the cells were removed from the media and sonicated, and the cytoplasm was sampled. High-performance liquid chromatography was used to separate 5-fluorouracil (FUra) from its known anabolites and catabolites, with subsequent quantitation of these metabolites by measurement of radioactivity. As the extracellular concentration of FUra was increased above 30 microM, the intracellular levels of FUra increased, with detection of a new peak of radioactivity distinct from any of the known anabolites or catabolites. This new metabolite, "G," increased in concentration as the extracellular concentration of FUra was raised above 1 mM. Inhibition of FUra catabolism by 2 mM thymine resulted in a further increase in intracellular FUra (approaching the extracellular FUra concentration) and was accompanied by a further increase in the intracellular concentration of "G," demonstrating that "G" was not formed via the catabolic pathway. The increase in intracellular FUra and "G" was not accompanied by an increase in intracellular anabolites, suggesting that "G" was formed via a novel metabolic pathway. "G" was retained within the hepatocytes, although it was not bound to intracellular macromolecules. "G" was converted to FUra in the presence of beta-D-glucuronidase; this reaction was inhibited with the addition of saccharo-1,4-beta-lactone, a specific inhibitor of the beta-D-glucuronidase. This data, together with evidence from hepatocyte homogenates in which formation of "G" was shown to be dependent on the concentration of uridine-5'-diphosphoglucuronic acid, demonstrates that "G" is a glucuronide of FUra. The formation of "G" suggests that FUra is metabolized via a previously unrecognized metabolic pathway.

Modulation of 5-fluorouracil catabolism in isolated rat hepatocytes with enhancement of 5-fluorouracil glucuronide formation.

The catabolism of 5-fluorouracil (FUra), which accounts for 90% of the elimination of this antimetabolite in vivo, has recently been characterized in freshly isolated rat hepatocytes in suspension using a highly specific high-performance liquid chromatographic methodology. The present study evaluates the effect of thymine and uracil, which are thought to be catabolized by the same enzymes as FUra, on the metabolism and transmembrane distribution of FUra in isolated rat hepatocytes. Following simulataneous exposure of cells for 5 min to 30 microM [6-3H]FUra and increasing concentrations of either thymine or uracil, dihydrofluorouracil (FUH2) levels decreased in a concentration-dependent manner, and the concentration determined for 50% inhibition of FUra catabolism was 8.0 +/- 0.3 (S.D.) and 67.8 +/- 15.6 microM for thymine and uracil, respectively. Analysis of intracellular and extracellular 3H from 1 min to 2 hr after simultaneous incubation of the hepatocytes with 30 microM FUra and thymine (or uracil) in a 1:7 molar ratio resulted in a decrease of intracellular and extracellular FUH2 and alpha-fluoro-beta-alanine (FBAL), while alpha-fluoro-beta-ureidopropionic acid (FUPA) was enhanced. Unmetabolized FUra (not detected in the absence of thymine or uracil) was detected intracellularly in the presence of thymine or uracil and was accompanied by the appearance of a novel metabolite, preliminarily identified as a glucuronide of the FUra base which reached intracellular levels of 44 +/- 9.76 and 27.45 +/- 1.35 microM in the presence of thymine or uracil, respectively, within 1 hr. This metabolite, which penetrates the cell membrane only slowly, accounted for approximately 60% of the intracellular 3H in the presence of 300 microM FUra and 2 mM thymine, whereas FUra catabolism was inhibited by more than 99% under these conditions. The formation of FUra anabolites was insignificant in the presence of thymine and uracil, and incorporation of FUra into RNA was not enhanced. The lack of anabolism of FUra in isolated hepatocytes exposed to either high initial concentrations of FUra or high intracellular FUra concentrations resulting from modulation (inhibition) of FUra catabolism is consistent with the clinical observation of minimal hepatotoxicity with FUra, despite exposure of the liver to high blood levels. These studies indicate that thymine is a more potent modulator of FUra catabolism in hepatocytes than is uracil. Further studies are needed to clarify the biological importance of the glucuronide of the base FUra which accumulates intracellularly as the concentration of FUra increases within the hepatocytes.