Novel 5-substituted, 2,4-diaminofuro[2,3-d]pyrimidines as multireceptor tyrosine kinase and dihydrofolate reductase inhibitors with antiangiogenic and antitumor activity

One-pot synthesis of highly substituted poly(furopyrimidine)s via catalyst-free multicomponent polymerizations of diisocyanides, N,N′-dimethylbarbituric acid, and dialdehyde

In this work, the catalyst-free multicomponent polymerizations of diisocyanides, N,N′-dimethylbarbituric acid, and dialdehyde for highly substituted poly(furopyrimidine)s has been achieved. All the experimental conditions such as polymerization solvents, temperature and time were investigated in detail. Through the systematic optimization of the polymerization conditions, the obtained polymers could have molecular weights of up to 16 400 g mol⁻¹, and excellent yields (up to 84%) can be achieved. All the polymers were well characterized by gel permeation chromatography (GPC), nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy. The thermal properties of the polymers were investigated and the decomposition temperature (T_d, 5%) was 277 °C.

In this work, the catalyst-free multicomponent polymerizations of diisocyanides, N,N′-dimethylbarbituric acid, and dialdehyde for highly substituted poly(furopyrimidine)s has been achieved.

Hybrid Drugs-A Strategy for Overcoming Anticancer Drug Resistance?

Despite enormous progress in the treatment of many malignancies, the development of cancer resistance is still an important reason for cancer chemotherapy failure. Increasing knowledge of cancers' molecular complexity and mechanisms of their resistance to anticancer drugs, as well as extensive clinical experience, indicate that an effective fight against cancer requires a multidimensional approach. Multi-target chemotherapy may be achieved using drugs combination, co-delivery of medicines, or designing hybrid drugs. Hybrid drugs simultaneously targeting many points of signaling networks and various structures within a cancer cell have been extensively explored in recent years. The single hybrid agent can modulate multiple targets involved in cancer cell proliferation, possesses a simpler pharmacokinetic profile to reduce the possibility of drug interactions occurrence, and facilitates the process of drug development. Moreover, a single medication is expected to enhance patient compliance due to a less complicated treatment regimen, as well as a diminished number of adverse reactions and toxicity in comparison to a combination of drugs. As a consequence, many efforts have been made to design hybrid molecules of different chemical structures and functions as a means to circumvent drug resistance. The enormous number of studies in this field encouraged us to review the available literature and present selected research results highlighting the possible role of hybrid drugs in overcoming cancer drug resistance.

An efficient diastereoselective synthesis of novel fused 5H-furo[2,3-d]thiazolo[3,2-a]pyrimidin-5-ones via one-pot three-component reaction

Herein, a convenient and efficient synthesis of 7-benzoyl-6-(aryl)-6,7-dihydro-5H-furo[2,3-d]thiazolo[3,2-a]pyrimidin-5-one derivatives was achieved from the reaction of isoquinolinium N-ylides, aromatic aldehydes, and heterocyclic 1,3-dicarbonyl compounds via one-pot three-component diastereoselective domino reaction in good-to-excellent yields. The advantages of this protocol are easily available starting materials, operational simplicity, and avoidance of hazardous organic solvents and catalyst. The synthesized products were characterized by IR, ¹H NMR, ¹³C NMR and mass spectra. Additionally, the conclusive structure of target compounds was confirmed by X-Ray diffraction analysis.

Rational Design of Multitarget-Directed Ligands: Strategies and Emerging Paradigms

Due to the complexity of multifactorial diseases, single-target drugs do not always exhibit satisfactory efficacy. Recently, increasing evidence indicates that simultaneous modulation of multiple targets may improve both therapeutic safety and efficacy, compared with single-target drugs. However, few multitarget drugs are on market or in clinical trials, despite the best efforts of medicinal chemists. This article discusses the systematic establishment of target combination, lead generation, and optimization of multitarget-directed ligands (MTDLs). Moreover, we analyze some MTDLs research cases for several complex diseases in recent years and the physicochemical properties of 117 clinical multitarget drugs, with the aim to reveal the trends and insights of the potential use of MTDLs.

Merging catalyst-free synthesis and iodine catalysis: one-pot synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones

A new and efficient one-pot strategy combining catalyst-free synthesis and iodine catalysis has been developed for the synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones. This approach affords products in moderate to high yields (up to 96%) with excellent diastereoselectivities (up to >25 : 1 dr). The reaction is simple to carry out and is metal-free.

A new and efficient one-pot strategy combining catalyst-free synthesis and iodine catalysis has been developed for the synthesis of dihydrofuropyrimidines and spirodihydrofuropyrimidine pyrazolones.

Synthesis of Trans-6-(4-Chlorobenzoyl)-7-(Aryl)-1,3-Dimethyl-6,7-Dihydrofuro[3,2-d]Pyrimidine-2,4-Diones Using Choline Hydroxide as an Efficient Catalyst in an Aqueous Medium

A green and efficient synthesis of the trans-6-(4-chlorobenzoyl)-7-(aryl)-1,3-dimethyl-6,7-dihydrofuro[3,2-d]pyrimidine-2,4-diones has been achieved via a three-component, one-pot condensation of 2-[2-(4-chlorophenyl)-2-oxoethyl)]isoquinolinium bromide with 1,3-dimethylbarbituric acid and an aromatic aldehyde in the presence of catalytic amounts of choline hydroxide in water under reflux conditions. This gives trans-6-(4-chlorobenzoyl)-7-(aryl)-1,3-dimethyl-6,7-dihydrofuro[3,2-d]pyrimidine-2,4-diones in excellent yield and in short reaction times.

Discovery and preclinical evaluation of 7-benzyl-N-(substituted)-pyrrolo[3,2-d]pyrimidin-4-amines as single agents with microtubule targeting effects along with triple-acting angiokinase inhibition as antitumor agents

The utility of cytostatic antiangiogenic agents (AA) in cancer chemotherapy lies in their combination with cytotoxic chemotherapeutic agents. Clinical combinations of AA with microtubule targeting agents (MTAs) have been particularly successful. The discovery, synthesis and biological evaluations of a series of 7-benzyl-N-substituted-pyrrolo[3,2-d]pyrimidin-4-amines are reported. Novel compounds which inhibit proangiogenic receptor tyrosine kinases (RTKs) including vascular endothelial growth factor receptor-2 (VEGFR-2), platelet-derived growth factor receptor-β (PDGFR-β) and epidermal growth factor receptor (EGFR), along with microtubule targeting in single molecules are described. These compounds also inhibited blood vessel formation in the chicken chorioallantoic membrane (CAM) assay, and some potently inhibited tubulin assembly (with activity comparable to that of combretastatin A-4 (CA)). In addition, some of the analogs circumvent the most clinically relevant tumor resistance mechanisms (P-glycoprotein and β-III tubulin expression) to microtubule targeting agents (MTA). These MTAs bind at the colchicine site on tubulin. Two analogs displayed two to three digit nanomolar GI50 values across the entire NCI 60 tumor cell panel and one of these, compound 7, freely water soluble as its HCl salt, afforded excellent in vivo antitumor activity against an orthotopic triple negative 4T1 breast cancer model and was superior to doxorubicin.

Stereoselective synthesis of medicinally relevant furo[2,3-d]pyrimidine framework by thermal rearrangement of spirocyclic barbiturates

The thermally initiated transformation of substituted spirocyclic barbiturates in DMSO at 100 °C results in the stereoselective formation of medicinally relevant furo[2,3-d]pyrimidines in 50–75% yields.

The design and discovery of water soluble 4-substituted-2,6-dimethylfuro[2,3-d]pyrimidines as multitargeted receptor tyrosine kinase inhibitors and microtubule targeting antitumor agents

The design, synthesis and biological evaluations of fourteen 4-substituted 2,6-dimethylfuro[2,3-d]pyrimidines are reported. Four compounds (11-13, 15) inhibit vascular endothelial growth factor receptor-2 (VEGFR-2), platelet-derived growth factor receptor β (PDGFR-β), and target tubulin leading to cytotoxicity. Compound 11 has nanomolar potency, comparable to sunitinib and semaxinib, against tumor cell lines overexpressing VEGFR-2 and PDGFR-β. Further, 11 binds at the colchicine site on tubulin, depolymerizes cellular microtubules and inhibits purified tubulin assembly and overcomes both βIII-tubulin and P-glycoprotein-mediated drug resistance, and initiates mitotic arrest leading to apoptosis. In vivo, its HCl salt, 21, reduced tumor size and vascularity in xenograft and allograft murine models and was superior to docetaxel and sunitinib, without overt toxicity. Thus 21 affords potential combination chemotherapy in a single agent.

Discovery of antitubulin agents with antiangiogenic activity as single entities with multitarget chemotherapy potential

Antiangiogenic agents (AA) are cytostatic, and their utility in cancer chemotherapy lies in their combination with cytotoxic chemotherapeutic agents. Clinical combinations of vascular endothelial growth factor receptor-2 (VEGFR2) inhibitors with antitubulin agents have been particularly successful. We have discovered a novel, potentially important analogue, that combines potent VEGFR2 inhibitory activity (comparable to that of sunitinib) with potent antitubulin activity (comparable to that of combretastatin A-4 (CA)) in a single molecule, with GI50 values of 10(-7) M across the entire NCI 60 tumor cell panel. It potently inhibited tubulin assembly and circumvented the most clinically relevant tumor resistance mechanisms (P-glycoprotein and β-III tubulin expression) to antimicrotubule agents. The compound is freely water-soluble as its HCl salt and afforded excellent antitumor activity in vivo, superior to docetaxel, sunitinib, or Temozolomide, without any toxicity.

Preferential selection of isomer binding from chiral mixtures: alternate binding modes observed for the E and Z isomers of a series of 5-substituted 2,4-diaminofuro[2,3-d]pyrimidines as ternary complexes with NADPH and human dihydrofolate reductase

The crystal structures of six human dihydrofolate reductase (hDHFR) ternary complexes with NADPH and a series of mixed E/Z isomers of 5-substituted 5-[2-(2-methoxyphenyl)-prop-1-en-1-yl]furo[2,3-d]pyrimidine-2,4-diamines substituted at the C9 position with propyl, isopropyl, cyclopropyl, butyl, isobutyl and sec-butyl (E2-E7, Z3) were determined and the results were compared with the resolved E and Z isomers of the C9-methyl parent compound. The configuration of all of the inhibitors, save one, was observed as the E isomer, in which the binding of the furopyrimidine ring is flipped such that the 4-amino group binds in the 4-oxo site of folate. The Z3 isomer of the C9-isopropyl analog has the normal 2,4-diaminopyrimidine ring binding geometry, with the furo oxygen near Glu30 and the 4-amino group interacting near the cofactor nicotinamide ring. Electron-density maps for these structures revealed the binding of only one isomer to hDHFR, despite the fact that chiral mixtures (E:Z ratios of 2:1, 3:1 and 3:2) of the inhibitors were incubated with hDHFR prior to crystallization. Superposition of the hDHFR complexes with E2 and Z3 shows that the 2'-methoxyphenyl ring of E2 is perpendicular to that of Z3. The most potent inhibitor in this series is the isopropyl analog Z3 and the least potent is the isobutyl analog E6, consistent with data that show that the Z isomer makes the most favorable interactions with the active-site residues. The isobutyl moiety of E6 is observed in two orientations and the resultant steric crowding of the E6 analog is consistent with its weaker activity. The alternative binding modes observed for the furopyrimidine ring in these E/Z isomers suggest that new templates can be designed to probe these binding regions of the DHFR active site.

Single agents with designed combination chemotherapy potential: synthesis and evaluation of substituted pyrimido[4,5-b]indoles as receptor tyrosine kinase and thymidylate synthase inhibitors and as antitumor agents

Combinations of antiangiogenic agents (AAs) with cytotoxic agents have shown significant promise in cancer treatment, and several such clinical trials are currently underway. We have designed, synthesized, and evaluated two compounds that each inhibit vascular endothelial growth factor receptor-2 (VEGFR-2) and platelet-derived growth factor receptor-beta (PDGFR-beta) for antiangiogenic effects and also inhibit human thymidylate synthase (hTS) for cytotoxic effects in single agents. The synthesis of these compounds involved the nucleophilic displacement of the common intermediate 5-chloro-9H-pyrimido[4,5-b]indole-2,4-diamine with appropriate benzenethiols. The inhibitory potency of both these single agents against VEGFR-2, PDGFR-beta, and hTS is better than or close to standards. In a COLO-205 xenograft mouse model, one of the analogs significantly decreased tumor growth (tumor growth inhibition (TGI) = 76% at 35 mg/kg), liver metastases, and tumor blood vessels compared with a standard drug and with control and thus demonstrated potent tumor growth inhibition, inhibition of metastasis, and antiangiogenic effects in vivo. These compounds afford combination chemotherapeutic potential in single agents.

The Chick Embryo Chorioallantoic Membrane as an In Vivo Assay to Study Antiangiogenesis

Antiangiogenesis, e.g., inhibition of blood vessel growth, is being investigated as a way to prevent the growth of tumors and other angiogenesis-dependent diseases. Pharmacological inhibition interferes with the angiogenic cascade or the immature neovasculature with synthetic or semi-synthetic substances, endogenous inhibitors or biological antagonists.The chick embryo chorioallantoic membrane (CAM) is an extraembryonic membrane, which serves as a gas exchange surface and its function is supported by a dense capillary network. Because its extensive vascularization and easy accessibility, CAM has been used to study morphofunctional aspects of the angiogenesis process in vivo and to study the efficacy and mechanism of action of pro- and anti-angiogenic molecules. The fields of application of CAM in the study of antiangiogenesis, including our personal experience, are illustrated in this review article.

CoMFA analysis of tgDHFR and rlDHFR based on antifolates with 6-5 fused ring system using the all-orientation search (AOS) routine and a modified cross-validated r(2)-guided region selection (q(2)-GRS) routine and its initial application

We report the development of CoMFA analysis models that correlate the 3D chemical structures of 80 compounds with 6-5 fused ring system synthesized in our laboratory and their inhibitory potencies against tgDHFR and rlDHFR. In addition to conventional CoMFA analysis, we used two routines available in the literature aimed at the optimization of CoMFA: all-orientation search (AOS) and cross-validated r(2)-guided region selection (q(2)-GRS) to further optimize the models. During this process, we identified a problem associated with q(2)-GRS routine and modified using two strategies. Thus, for the inhibitory activity against each enzyme (tgDHFR and rlDHFR), five CoMFA models were developed using the conventional CoMFA, AOS optimized CoMFA, the original q(2)-GRS optimized CoMFA and the modified q(2)-GRS optimized CoMFA using the first and the second strategy. In this study, we demonstrate that the modified q(2)-GRS routines are superior to the original routine. On the basis of the steric contour maps of the models, we designed four new compounds in the 2,4-diamino-5-methyl-6-phenylsulfanyl-substituted pyrrolo[2,3-d]pyrimidine series. As predicted, the new compounds were potent and selective inhibitors of tgDHFR. One of them, 2,4-diamino-5-methyl-6-(2',6'-dimethylphenylthio)pyrrolo[2,3-d]pyrimidine, is the first 6-5 fused ring system compound with nanomolar tgDHFR inhibitory activity. The HCl salt of this compound was also prepared to increase solubility. Both forms of the drug were tested in vivo in a Toxoplasma gondii infection mouse model. The results indicate that both forms were active with the HCl salt significantly more potent than the free base.

Design, synthesis, and X-ray crystal structures of 2,4-diaminofuro[2,3-d]pyrimidines as multireceptor tyrosine kinase and dihydrofolate reductase inhibitors

To optimize dual receptor tyrosine kinase (RTK) and dihydrofolate reductase (DHFR) inhibition, the E- and Z-isomers of 5-[2-(2-methoxyphenyl)prop-1-en-1-yl]furo[2,3-d]pyrimidine-2,4-diamines (1a and 1b) were separated by HPLC and the X-ray crystal structures (2.0 and 1.4A, respectively) with mouse DHFR and NADPH as well as 1b with human DHFR (1.5A) were determined. The E- and Z-isomers adopt different binding modes when bound to mouse DHFR. A series of 2,4-diaminofuro[2,3-d]pyrimidines 2-13 were designed and synthesized using the X-ray crystal structures of 1a and 1b with DHFR to increase their DHFR inhibitory activity. Wittig reactions of appropriate 2-methoxyphenyl ketones with 2,4-diamino-6-chloromethyl furo[2,3-d]pyrimidine afforded the C8-C9 unsaturated compounds 2-7 and catalytic reduction gave the saturated 8-13. Homologation of the C9-methyl analog maintains DHFR inhibitory activity. In addition, inhibition of EGFR and PDGFR-beta were discovered for saturated C9-homologated analogs 9 and 10 that were absent in the saturated C9-methyl analogs.

The Z isomer of 2,4-diaminofuro[2,3-d]pyrimidine antifolate promotes unusual crystal packing in a human dihydrofolate reductase ternary complex

The crystal structure of the ternary complex of human dihydrofolate reductase (hDHFR) with NADPH and the Z isomer of 2,4-diamino-5-[2-(2'-methoxyphenyl)propenyl]-furo[2,3-d]pyrimidine (Z1) shows that the Z isomer binds in the normal antifolate orientation in which the furo oxygen occupies the 8-amino position observed in the binding of 2,4-diaminopteridine antifolates such as methotrexate and with the methoxyphenyl moiety cis to and coplanar with the furo[2,3-d]pyrimidine ring. The hDHFR ternary complex crystallized in the orthorhombic space group P2(1)2(1)2(1) and its structure was refined to 1.7 A resolution. Although other hDHFR complexes crystallize in this space group, these data provide only the second example of an unusual packing arrangement in which the conserved active-site Arg70 forms a salt bridge to the side chain of Glu44 from a symmetry-related molecule. As a result, the conformations of Phe31 and Gln35 shift with respect to those observed in the structure of mouse DHFR bound to Z1, which crystallizes in the monoclinic space group P2(1) and shows that Gln35 interacts with Arg70.

Synthesis and activity of novel 5-substituted pyrrolo[2,3-d]pyrimidine analogues as pp60(c-Src) tyrosine kinase inhibitors

Therapy with receptor tyrosine kinase inhibitors provides an improved treatment option in a number of diseases such as cancer, myocardial infection, osteoporosis, stroke, and neurodegeneration. We have designed, synthesized, and evaluated a series of novel 2-amino-5-[(benzyl)imino]methyl-3,7-dihydro-4H-pyrrolo[2,3-d]pyrimidine-4-one 7a and 2-amino-5-[(substituted-benzyl)imino]methyl-3,7-dihydro-4H-pyrrolo[2,3-d]pyrimidine-4-one 7b-e derivatives as potential tyrosine kinase inhibitors. These compounds were synthesized by condensation reaction using 2-tritylamino-4-oxo-4,7-dihydro-3H-pyrrolo[2,3-d]pyrimidine-5-carbaldehyde 5 and appropriate benzylamines followed by detritylation. Compounds were evaluated for their inhibitory activity toward tyrosine phosphorylation for the pp60c-Src tyrosine kinase. Compounds 7a, 7d, and 7e demonstrated potent inhibitory activities against pp60c-Src tyrosine kinase with IC50 values of 13.9, 34.5, and 78.4 microM, respectively. Dihalogenated compounds 7d and 7e have 3 to 7-times lower IC50 values than that of the parent compound 7a.

Design, synthesis and biological evaluation of substituted pyrrolo[2,3-d]pyrimidines as multiple receptor tyrosine kinase inhibitors and antiangiogenic agents

Direct and indirect involvement of receptor tyrosine kinases (RTKs) in tumor growth and metastasis makes them ideal targets for anticancer therapy. A paradigm shift from inhibition of single RTK to inhibition of multiple RTKs has been recently demonstrated. We designed and synthesized eight N(4)-phenylsubstituted-6-(2-phenylethylsubstituted)-7H-pyrrolo[2,3-d]pyrimidine-2,4-diamines as homologated series of our previously published RTK inhibitors. We reasoned that increased flexibility of the side chain, which determines potency and selectivity, would improve the spectrum of RTK inhibition. These compounds were synthesized using a bis-electrophilic cyclization to afford substituted pyrrolo[2,3-d]pyrimidines followed by chlorination and substitution at the 4-position with various anilines. Five additional compounds of this series were previously reported by Gangjee et al.(1) with activities against IGFR only. Their synthesis, characterization and biological activities against a variety of other RTKs are reported in this study for the first time. The biological evaluation, in whole cell assays, showed several analogs had remarkable inhibitory activity against epithelial growth factor receptor (EGFR), vascular endothelial growth factor receptor-1 (VEGFR-1), platelet-derived growth factor receptor-beta (PDGFR-beta), the growth of A431 cells in culture, and in the chicken embryo chorioallantoic membrane (CAM) angiogenesis assay. The inhibitory data against the RTKs in this study demonstrate that variation of the 6-ethylaryl substituents as well as the N(4)-phenyl substituents of these analogs does indeed control both the potency and specificity of inhibitory activity against RTKs. In addition, homologation of the chain length of the 6-substituent from a methylene to an ethyl increases the spectrum of RTK inhibition. New multi-RTK inhibitors (8, 12) and potent inhibitors of angiogenesis (15, 19) were identified with the best compound, N(4)-(3-trifluromethylphenyl)-6-(2-phenylethyl)-7H-pyrrolo[2,3-d]pyrimidine-2,4-diamine (15), with an IC(50) value of 30nM in the CAM angiogenesis inhibition assay.

Discovery of novel antitumor antimitotic agents that also reverse tumor resistance

We have discovered a novel series of 7-benzyl-4-methyl-5-[(2-substituted phenyl)ethyl]-7H-pyrrolo[2,3-d]pyrimidin-2-amines, which possess antimitotic and antitumor activities against antimitotic-sensitive as well as resistant tumor cells. These agents bind to a site on tubulin that is distinct from the colchicine, vinca alkaloid, and paclitaxel binding sites and some, in addition to their antitumor activity, remarkably also reverse tumor resistance to antimitotic agents mediated via the P-glycoprotein efflux pump. The compounds were synthesized from N-(7-benzyl-5-ethynyl-4-methyl-7H-pyrrolo[2,3-d]pyrimidin-2-yl)-2,2-dimethylpropanamide 11 or the corresponding 5-iodo analog 14 via Sonogashira couplings with appropriate iodobenzenes or phenylacetylene followed by reduction and deprotection to afford the target analogs. Sodium and liquid NH3 afforded the debenzylated analogs. The most potent analog 1 was one to three digit nanomolar against the growth of both sensitive and resistant tumor cells in culture. Compounds of this series are promising novel antimitotic agents that have the potential for treating both sensitive and resistant tumors.

The role of multidrug resistance efflux transporters in antifolate resistance and folate homeostasis

Members of the ATP-binding cassette (ABC) transporters including P-glycoprotein (Pgp/ABCB1), multidrug resistance proteins (MRPs/ABCC) as well as breast cancer resistance protein (BCRP/ABCG2) function as ATP-dependent drug efflux transporters, which form a unique defense network against multiple chemotherapeutic drugs and cellular toxins. Among antitumor agents is the important group of folic acid antimetabolites known as antifolates. Antifolates such as methotrexate (MTX), pemetrexed and raltitrexed exert their cytotoxic activity via potent inhibition of folate-dependent enzymes essential for purine and pyrimidine nucleotide biosynthesis and thereby block DNA replication. Overexpression of MRPs and BCRP confers resistance upon malignant cells to various hydrophilic and lipophilic antifolates. Apart from their central role in mediating resistance to antifolates and other anticancer drugs, MRPs and BCRP have been recently shown to transport naturally occurring reduced folates. This was inferred from various complementary systems as follows: (a) Cell-free systems including ATP-dependent uptake of radiolabeled folate/MTX into purified inside-out membrane vesicles from stable transfectants and/or cells overexpressing these transporters, (b) Decreased accumulation of radiolabeled folate/MTX in cultured tumor cells overexpressing these transporters, as well as (c) In vivo rodent models such as Eisi hyperbillirubinemic rats (EHBR) that hereditarily lack MRP2 in their canalicular membrane and thereby display a bile that is highly deficient in various reduced folate cofactors and MTX, when compared with wild type Sprague-Dawley (SD) rats. In all cases, these folate/antifolate transporters functioned as high capacity, low affinity ATP-driven exporters. While the mechanism of cellular retention of (anti)folates is mediated via (anti)folylpolyglutamylation, certain efflux transporters including MRP5 (ABCC5) and BCRP were shown to transport both mono-, di- as well as triglutamate derivatives of MTX and folic acid. Furthermore, overexpression of MRPs and BCRP has been shown to result in decreased cellular folate pools, whereas loss of ABC transporter expression brought about a significant expansion in the intracellular reduced folate pool. The latter finding has important implications to antifolate-based chemotherapy as an augmented cellular folate pool results in a significant level of resistance to certain antifolates. Hence, the aims of the present review are: (a) To summarize and discuss the cumulative evidence supporting a functional role for various multidrug resistance efflux transporters of the ABC superfamily which mediate resistance to hydrophilic and lipophilic antifolates, (b) To describe and evaluate the recent data suggesting a role for these efflux transporters in regulation of cellular folate homeostasis under folate replete and deplete conditions. Furthermore, novel developments and future perspectives regarding the identification of novel antifolate target proteins and mechanisms of action, as well as rationally designed emerging drug combinations containing antifolates along with receptor tyrosine kinase inhibitors are being discussed.