Quinazoline antifolates inhibiting thymidylate synthase: synthesis of four oligo(L-gamma-glutamyl) conjugates of N10-propargyl-5,8-dideazafolic acid and their enzyme inhibition

Redirecting raltitrexed from cancer cell thymidylate synthase to Mycobacterium tuberculosis phosphopantetheinyl transferase

There is a compelling need to find drugs active against Mycobacterium tuberculosis (Mtb). 4'-Phosphopantetheinyl transferase (PptT) is an essential enzyme in Mtb that has attracted interest as a potential drug target. We optimized a PptT assay, used it to screen 422,740 compounds, and identified raltitrexed, an antineoplastic antimetabolite, as the most potent PptT inhibitor yet reported. While trying unsuccessfully to improve raltitrexed's ability to kill Mtb and remove its ability to kill human cells, we learned three lessons that may help others developing antibiotics. First, binding of raltitrexed substantially changed the configuration of the PptT active site, complicating molecular modeling of analogs based on the unliganded crystal structure or the structure of cocrystals with inhibitors of another class. Second, minor changes in the raltitrexed molecule changed its target in Mtb from PptT to dihydrofolate reductase (DHFR). Third, the structure-activity relationship for over 800 raltitrexed analogs only became interpretable when we quantified and characterized the compounds' intrabacterial accumulation and transformation.

Acetylenic antifolates as anticancer agents

Folates are crucial cofactors involved in the de novo generation of purine and deoxythymidine monophosphate, which are essential for DNA synthesis. Antifolates are structural analogues of folate derivatives that act as inhibitors of folate-dependent enzymes and constitute the oldest antimetabolite class of anticancer agents. This review focuses on antifolates with remarkable anticancer activities that include a terminal alkyne function in their molecular structure. The properties of CB3717, a tremendous inhibitor of thymidylate synthase, are described, and the development of raltitrexed and pralatrexate, a dihydrofolate reductase inhibitor approved by the U.S. Food and Drug Administration (FDA) as the first drug for the treatment of relapsed and refractory peripheral T cell lymphoma are presented.

Multiple folate enzyme inhibition: mechanism of a novel pyrrolopyrimidine-based antifolate LY231514 (MTA)

Extensive biochemical and pharmacological evidence indicates that LY231514 is a novel antifolate antimetabolite. LY231514 is transported into cells mainly through the reduced folate carrier system and extensively metabolized to polyglutamated forms. The polyglutamates of LY231514 inhibit at least three key folate enzymes: TS, DHFR, and GARFT, and to a lesser extent AICARFT and C1-tetrahydrofolate synthase. The combined effects of the inhibition exerted by LY231514 at each target give rise to an unusual end-product reversal pattern at the cellular level that is distinct from those of other inhibitors such as methotrexate and the quinazoline antifolates. The metabolic effects exerted by LY231514 on the folate and nucleotide pools are also quite distinct from those of MTX and LY309887. The efficient polyglutamation, longer cellular retention and the multiple folate enzyme inhibition mechanism may all have contributed directly to the exciting antitumor responses now observed in Phase I and II studies. The multitargeted inhibition mechanism of LY231514 is particularly intriguing. This new level of mechanistic insight, which has evolved from the study of LY231514, challenges the traditional concept and paradigm of antifolate drug discovery and development which focused on developing very potent and selective inhibitors of single folate enzyme targets, such as DHFR, TS or one of the enzymes along the de novo purine biosynthetic pathway. Given the complex nature of folate metabolism and the critical role of folates in maintaining the physiological functions of living systems, it is completely reasonable to suspect that agents which can interfere at multiple sites in the folate pathway may trigger and cause more biochemical imbalance in the cellular DNA and RNA synthesis of malignant cells than agents that act on a single point (Fig. 5). In conclusion, LY231514 (MTA) is a new generation antifolate antimetabolite demonstrating inhibitory activity against multiple folate enzymes including TS, DHFR and GARFT. In current phase II studies, MTA is broadly active as a single agent and is showing very encouraging antitumor activity in multiple solid tumors including colorectal, breast and non-small cell lung cancers (38-43). The every three week dosing schedule has proven to be convenient and easy to administer and the clinical toxicities of LY231514 seem to be well tolerated. More advanced and extensive clinical trials of LY231514 are currently in progress.

Tomudex (ZD1694): from concept to care, a programme in rational drug discovery

Folate-based anticancer drugs with specificity for thymidylate synthase (TS) have come of age. Ideas nurtured in the early 1970s led to the first-generation of antifolates with TS and dihydrofolate reductase (DHFR) inhibitory activities. Compounds with increased selectivity for TS followed with the highly specific inhibitor, CB3717 being synthesised in 1979 at the Institute of Cancer Research (ICR). CB3717 had significant clinical activity but its development had to be abandoned because its low aqueous solubility led to occasional nephrotoxicity. Collaborative laboratory studies between the ICR and ICI Pharmaceuticals (later to become Zeneca Pharmaceuticals) led to the discovery of ZD1694 (Tomudex), the first antifolate to be licensed for the treatment of cancer (UK 1995) in nearly 40 years and the first new drug for colorectal cancer in about 35 years. Tomudex belongs to a class of compounds that use the reduced-folate carrier (RFC) for uptake into cells and which are excellent substrates for folylpolyglutamate synthetase (FPGS). This paper reviews the underlying philosophies, and the milestones reached during the development of Tomudex.

Folate-based thymidylate synthase inhibitors as anticancer drugs

The enzyme, thymidylate synthase (TS) is considered an important target for the development of new anticancer agents. Moreover, the folate-binding site in TS is believed to offer better opportunities for the design of highly specific inhibitors than the pyrimidine (dUMP) binding site. This belief led to the design of N10-propargyl-5,8-dideazafolic acid (CB3717), a quinazoline-based drug which had antitumour activity in clinical studies. Occasional, but serious nephrotoxicity led to the withdrawal of CB3717 from further clinical study. More water-soluble and non-nephrotoxic analogues were developed with an interesting diversity in biochemical profile, particularly with respect to interactions with the reduced-folate cell membrane carrier (RFC) and folylpolyglutamate synthetase (FPGS). An example of a compound that uses both of these processes well is the quinazoline, ZD1694 (Tomudex), a drug which is about to complete phase III evaluation for colorectal cancer. High chain length polyglutamates are formed that are up to 70-fold more potent TS inhibitors than the parent drug (Ki tetraglutamate = 1 nM). Furthermore they are retained in cells/tissues for a prolonged period. A number of other novel folate-based TS inhibitors are currently in pre-clinical or clinical study. For example, LY231514 is a pyrrolopyrimidine analogue in phase I study and, although less potent as a TS inhibitor, has biochemical properties similar to ZD1694. Another compound in phase I study is the benzoquinazoline, BW1843U89 which has somewhat different properties. It is a very potent TS inhibitor (Ki = 0.09 nM) and an excellent substrate for the RFC (human) and FPGS, but polyglutamation proceeds to diglutamate only and is not accompanied by increased TS inhibition. Another highly water-soluble compound in pre-clinical development is ZD9331 which was specifically designed to use the RFC but not be a substrate for FPGS. Potent TS inhibition (Ki = 0.4 nM) was achieved through a rational programme of computerised molecular modelling of the active site of TS and a large database of structure-activity relationships. Two lipophilic compounds were designed to be devoid of interactions with either the RFC or FPGS. High resolutions crystal complexes of E. coli TS were central to obtaining potent TS inhibitors and both AG337 (Ki human recombinant TS = 16 nM) and AG331 (Ki = 12 nM) are in clinical studies. This portfolio of novel compounds therefore comprehensively addresses the potential of TS as a target for cancer chemotherapy.

Thymidylate synthases from Hymenolepis diminuta and regenerating rat liver: purification, properties, and inhibition by substrate and cofactor analogues

Comparative studies of thymidylate synthases, isolated from the tapeworm, Hymenolepis diminuta, and regenerating liver of its host, rat, aimed at a possibility of specific inhibition of the helminthic enzyme, are presented. While similar in structure (dimers with monomer molecular masses of 33.7 kDa and 34.9 kDa, respectively) and parameters describing interactions with substrates and products, the tapeworm and rat enzymes differed in the dependences of reaction velocity on temperature (Arrhenius plots biphasic and linear, respectively). The tapeworm, compared with the host, enzyme was less sensitive to the competitive slow-binding inhibition by 5-fluoro-dUMP and its 2-thio congener, but equally sensitive to inhibition by 4-thio-5-fluoro-dUMP, N4-hydroxy-dCMP and N4-hydroxy-5-fluoro-dCMP, the latter being more potent inhibitor of the parasite enzyme than 5-fluoro-dUMP. alpha-Anomer of 5-fluoro-dUMP behaved as a very weak competitive slow-binding inhibitor of both enzymes. Both enzymes differed markedly in sensitivity to inhibition by 10-propargyl-5,8-dideazafolate and its di- and triglutamates (pddPteGlu1-3), with pddPteGlu1 being stronger inhibitor of the mammalian enzyme, but pddPteGlu3 showing opposite specificity. Sulfonamidobenzoylglutamate analogue of pddPteGlu (pddPteSO2Glu) and 2-desamino-2-methyl derivative of this analogue (CH3pddPteSO2Glu) were weaker inhibitors of both enzymes than the parent compound. Substitution of the glutamyl residue in CH3pddPteSO2Glu with either norvaline or alanine increased inhibition potency, whereas similar substitutions with glycine, valine or phenylglycine were without a distinct effect with the host enzyme but weakened inhibition of the tapeworm enzyme.

Biochemical effects of folate-based inhibitors of thymidylate synthase in MGH-U1 cells

The TS-inhibitory effects induced by a 24-h exposure to the folate-based TS inhibitors CB3717, C2-desamino analogs of CB3717 including D1694, and BW1843U89 were quantitated using the MGH-U1 human bladder carcinoma. The effects of D1694 on the time course of TS inhibition and on intracellular deoxyuridine monophosphate (dUMP) accumulation and deoxyuridine (dUrd) production were evaluated. D1694 and BW1843U89 were the most active TS inhibitors with IC50 values of 2.4 and 0.5 nM, respectively. The C2-desamino C2-methyl dideazafolates were 27-292 times more potent than the parent CB3717 as TS inhibitors. A methyl group at the C2 position of CB3717 had the most dramatic effect, whereas a thiazole substitution for a benzyl added a small benefit and N10 substitution had a limited impact on TS-inhibitory potency and clonogenic survival. There was a significant correlation between the IC50 values for TS inhibition and those for cytotoxic potency obtained for these drugs. LV and thymidine protected cells from these folate-based TS inhibitors. Intracellular dUMP levels following 24 h D1694 (IC50) exposure increased 7-fold. Levels of dUrd effluxing into the media increased up to 4.5 microM following a 24-h exposure to D1694 (IC90). We conclude that (a) C2-desamino C2-methyl dideazafolates are potent TS inhibitors, (b) TS inhibition requires prolonged exposure with these folate TS inhibitors, (c) survival is correlated with inhibition of TS for the folate-based TS inhibitors and (d) the biochemical consequences of TS inhibition include increased dUMP and dUrd levels.

Comparative cytotoxicity of folate-based inhibitors of thymidylate synthase and 5-fluorouracil +/- leucovorin in MGH-U1 cells

Thymidylate synthase (TS) is a critical enzyme in the synthesis of DNA and an important target for cancer chemotherapy. 5-Fluorouracil (5FU) combined with leucovorin (LV) has been used to inhibit TS, and inhibition is dependent on the formation of a ternary complex between a folate cofactor, TS, and 5-fluorodeoxyuridine monophosphate (FdUMP), a metabolite of FU. The folate-based TS inhibitors CB3717, its analogs, and BW1843U89 have been synthesized as specific inhibitors of TS that do not require activation or the presence of a cofactor. We have compared the cytotoxicity of 5FU +/- LV with that of these folate-based TS inhibitors in human bladder cancer MGH-U1 cells using a colony-forming assay. After a 6-h exposure, FU+LV, CB3717, dCB3717, or C2 methyl dideazafolate analogs demonstrated similar cytotoxic potency that was 0.96 to 2.9 times that of 5FU alone. A 24-h exposure did not increase the potency of 5FU+LV relative to 5FU alone, but there was a marked increase in the cytotoxicity of the dideazafolates as compared with 5FU+LV. Similarly, BW1843U89 was more cytotoxic than 5FU/LV. This was reflected in a 3.2- to 1333-fold decrease in the 50% inhibitory concentration (IC50). Simultaneous exposure to LV and thymidine (TdR) protected MGH-U1 cells from the cytotoxicity of CB3717, its analogs, and BW1843U89. We conclude that (a) the folate-based TS inhibitors are more potent than 5FU+LV after a 24-h exposure, (b) protection by LV and TdR indicates that TS inhibition is the primary site of action, and (c) BW1843U89 is more potent than D1694 in MGH-U1 cells.

The in vivo metabolic stability of dipeptide analogues of the quinazoline antifolate, ICI 198583, in mice

In the search for quinazoline thymidylate synthase inhibitors that are not subject to intracellular polyglutamation, a class of dipeptide analogues of the diglutamate of 2-desamino-2-methyl-N10-propargyl-5,8-dideazafolic acid (ICI 198583-gamma-L-glu) has been evaluated for their stability to in vivo hydrolysis. Replacement of the second glutamate with another amino acid, e.g. alanine, prevented polyglutamation in vitro but such compounds were subject to hydrolysis when injected into mice. The extent of hydrolysis was measured in plasma, liver and kidney by HPLC analysis of tissue removed from mice 1 hr after i.p. injection. The enzyme responsible for this hydrolysis is thought to be a gamma-glutamyl hydrolase which hydrolyses the amide bond, releasing ICI 198583 which may then be polyglutamated. Development of stable dipeptide compounds was achieved by structural modification in two principal ways: either by replacement of the second amino acid (e.g. glutamate or alanine) with its D-enantiomer or removal of the carboxyl on the alpha-carbon of the second amino acid (alpha'-COOH). In this second approach two series of compounds were investigated. Monocarboxylate-derived dipeptides, e.g. ICI 198583-gamma-L-phenylalanine or ICI 198583-gamma-phenylglycine, resulted in stable compounds after removal of the alpha'-COOH (to give -ethylamide and -benzylamide derivatives, respectively). However, for the dicarboxylic amino acids a less clear picture emerged. Although removal of the alpha'-COOH from ICI198583-gamma-L-glutamate to give ICI 198583-gamma-gamma-aminobutyric acid resulted in a stable compound, the corresponding aspartate analogue (-beta-alanine) was subject to hydrolysis.

A human leukemia cell line made resistant to two folate analogues, trimetrexate and N10-propargyl-5,8-dideazafolic acid (CB3717)

We established a novel human acute lymphoblastic leukemia cell line made resistant to two folate analogues, trimetrexate (TMQ) and N10-propargyl-5,8-dideazafolic acid (CB3717), by sequential exposure of the 200-fold TMQ-resistant cells (MOLT-3/TMQ200) to CB3717. A 30-fold-resistant subline to CB3717 was selected from the TMQ-resistant cells and designated as MOLT-3/TMQ200-CB371730. This double-folate-resistant cell line was 15-fold more resistant to methotrexate (MTX) than MOLT-3/TMQ200; however, TMQ resistance was decreased to 10-fold as compared to MOLT-3/TMQ200. The doubly resistant cells also showed 2-fold cross-resistance to 5-fluorouracil (5-FU). Equimolar concentrations of leucovorin almost completely reversed the inhibitory effect of MTX on the doubly resistant cells and partially that of CB3717 and TMQ; on the other hand, leucovorin enhanced the inhibitory effect of 5-FU. Thymidylate synthase activities demonstrated little or no difference among these three cell lines, being consistent with no overexpression of mRNA for this enzyme in the doubly resistant cells. MOLT-3/TMQ200 cells displayed classical multidrug resistance; sequential development of CB3717 resistance in the TMQ-resistant cells resulted in an enhancement of the multidrug-resistance phenotype and a concomitant increase of MDR1 mRNA. The development of a complex resistance pattern seen in this double-folate-resistant subline indicates intricacy in the study of drug resistance after multidrug chemotherapy.

The biochemical pharmacology of the thymidylate synthase inhibitor, 2-desamino-2-methyl-N10-propargyl-5,8-dideazafolic acid (ICI 198583)

2-Desamino-2-methyl-N10-propargyl-5,8-dideazafolic acid (ICI 198583) is a more water-soluble analogue of the quinazoline-based thymidylate synthase (TS) inhibitor, N10-propargyl-5,8-dideazafolic acid (CB3717). A 3-fold loss in TS inhibitory activity (murine and human TS, Ki = 10 nM) was accompanied by a 40-fold increase in growth inhibitory potency against L1210 and W1L2 cells in vitro (IC50 = 0.085 and 0.05 microM, respectively) when compared with CB3717. In L1210 cells a concentrative uptake mechanism was demonstrated for [3H]ICI 198583 (Kt = 2.9 microM). The L1210:1565 cell line, with an impaired ability to transport reduced folates or methotrexate (MTX), was resistant (100-fold relative to the wild-type L1210 line) to ICI 198583 (but not CB3717) and did not take up [3H]ICI 198583 significantly. The measurement of folylpolyglutamate synthetase (FPGS) substrate activity demonstrated a Km of 40 microM for ICI 198583 and a Vmax/Km (relative to folic acid) of 3.5. The formation of intracellular polyglutamate derivatives was demonstrated in both L1210 (mouse) and WIL2 (human) cells grown in vitro after exposure to 1 microM [3H]ICI 198583. In L1210 cells, by 4 hr, approximately 50% of the intracellular 3H(approximately 1 microM) was found as polyglutamate forms of ICI 198583, principally as tri- and tetraglutamates. After 24 hr the ICI 198583 polyglutamate pool had expanded, the tetraglutamate metabolite predominated and there was significant formation of the pentaglutamate. Upon resuspension of L1210 cells in drug free medium, ICI 198583 was largely lost from the cells but the polyglutamates were preferentially retained, after 24 hr approximately 70% remained. Synthetic ICI 198583 polyglutamates were shown to be up to 100-fold more potent as inhibitors of isolated TS than the parent compound. Following in vivo administration (500 mg/kg i.v.) ICI 198583 was cleared rapidly from the plasma of mice (T1/2 beta = 16 min, clearance = 42 mL/min/kg). Despite this clearance there was prolonged, dose-dependent inhibition of TS in L1210:NCI cells in vivo. Thus, following 500 mg/kg i.v. the flux through TS was inhibited by greater than 80% for at least 24 hr. Administration of five doses at 5 mg/kg daily of ICI 198583 to L1210:ICR tumour-bearing mice resulted in greater than 60% of the mice being cured, a 10-fold improvement in potency over CB3717. The maximum tolerated dose (MTD) for ICI 198583 using this schedule was greater than 500 mg/kg/day compared with 200 mg/kg/day of CB3717.(ABSTRACT TRUNCATED AT 250 WORDS)

Thymidylate synthase inhibitors: the in vitro activity of a series of heterocyclic benzoyl ring modified 2-desamino-2-methyl-N10-substituted-5,8-dideazafolates

Heterocyclic para-aminobenzoate modifications of 2-desamino-2-methyl-5,8-dideazafolic acid and a series of its N10-substituted analogs have produced a number of interesting compounds that have enabled a deeper understanding of the biochemical events required for activity in this class of antimetabolite. There is a relationship that has become apparent between compound potency and both uptake via the reduced-folate carrier and FPGS substrate activity. Rapid cellular uptake and metabolism of polyglutamate forms that are approximately 100-fold more potent as inhibitors of TS can translate a modest TS inhibitor such as ICI D1694 into a very potent inhibitor of cell growth (approximately 500- and approximately 10-fold more potent than CB3717 or ICI 198583, respectively). Polyglutamation may therefore act as an almost essential activation step and ICI D1694 may be highly specific for tumors expressing both the reduced-folate carrier and FPGS. Polyglutamation of folate analogs also leads to drug retention which may play a major role in the pharmacodynamics of TS inhibition by ICI D1694 in vivo. Current studies with 3H-ICI D1694 are aimed at demonstrating metabolism to polyglutamates in tumor cells. The serious toxic limitations of CB3717, i.e., liver and kidney toxicities, are not seen with ICI D1694 reflecting the good water solubility of the drug compared with CB3717. The toxicities observed in mice are however to hematological tissues and are due to its TS inhibitory effects. Thus ICI D1694 may elicit toxicities in man more typical of an antimetabolite than of CB3717. The clinical evaluation of ICI D1694 may further our understanding of the role that metabolism to polyglutamates may have in therapeutic activity.

Thymidylate synthase: a target for anticancer drug design

N10-Propargyl-5,8-dideazafolic acid (CB3717) has proved to be an interesting recent addition to the spectrum of antifolate drugs. Its sole biochemical locus of action appears to be thymidylate synthase, an inhibitory effect which is potentiated by intracellular polyglutamation. The drug has shown a spectrum of clinical activity and toxicity which is unusual for an antimetabolite. It seems likely that the former is attributable to its inhibition of TS, whilst the latter relates to the drug's poor aqueous solubility at physiological pH. Seminal to the discovery of a new generation of more selective thymidylate synthase inhibitors has been the observation that the C2 desamino derivative (CB3804) retains the useful TS-inhibitory and cytotoxic properties of CB3717. It is some two orders of magnitude more water soluble than CB3717 at physiological pH and appears not to produce, in the mouse, the liver and kidney toxicities which have restricted the wider use of CB3717. Thus, in desamino CB3717, it has proved possible to separate the structural features determining antitumor activity from those which are responsible for its systemic toxicities. These encouraging results prompted systematic structure-activity studies of other C2-modified quinazolines, which revealed that the desirable properties of the desamino compound are not unique. Results with two other CB3717 analogues, the C2-methyl (CB3819) and C2-methoxy (CB3828), have been discussed in the present paper. All three CB3717 analogues exhibit TS-inhibitory activities which are broadly comparable to those of the parent drug. In continuous culture CB3828 is as cytotoxic as CB3717, while CB3804 and CB3819 are at least an order of magnitude more potent. As with the desamino derivative (CB3804), so CB3819 is substantially more water soluble than CB3717 and is apparently devoid of its major toxicities. However, the effects of CB3828 on whole cell TS inhibition, both in vitro and in vivo, are rapidly reversible upon removal of exogenous compound, while the inhibition is sustained in similar experiments with the other three compounds. It is likely that these effects relate to the extent to which the various derivatives are converted to polyglutamate species and retained intracellularly. With the exception of CB3828, all are good substrates for FPGS, and the polyglutamate derivatives of CB3717, CB3804 and CB3819 are better TS inhibitors than the corresponding monoglutamates. CB3804 and CB3819 are less toxic and are cleared from the plasma much more rapidly than CB3717, so that the rate and extent of their polyglutamation may be an essential prerequisite of pharmacological activity.(ABSTRACT TRUNCATED AT 400 WORDS)