Antiangiogenic and antitumor agents. Design, synthesis, and evaluation of novel 2-amino-4-(3-bromoanilino)-6-benzylsubstituted pyrrolo[2,3-d]pyrimidines as inhibitors of receptor tyrosine kinases

Several different classes of growth factor receptors containing tyrosine kinases (RTK) are directly or indirectly involved in angiogenesis. Inhibition of these RTKs has provided a new paradigm in the treatment of tumors by restricting their growth and metastasis. We have designed, synthesized and evaluated eleven novel 2-amino-4-(3-bromoanilino)-6-substituted benzyl pyrrolo[2,3-d]pyrimidines as the first in a series of RTK inhibitors. These analogues were synthesized from appropriate alpha-bromomethylbenzyl ketones by cyclocondensation with 2,6-diamino-4-pyrimidone to afford the 2-amino-4-oxo-6-substituted benzyl pyrrolo[2,3-d]pyrimidines. Chlorination of the 4-position followed by displacement with 3-bromoaniline afforded the target compounds. In some instances, the 2-amino moiety of the pyrrolo[2,3-d]pyrimidines was protected prior to the chlorination and displacement followed by deprotection. The compounds were evaluated as inhibitors of vascular endothelial growth factor receptors VEGFR-2 (Flk-1, KDR) and VEGFR-1 (Flt-1); epidermal growth factor receptor (EGFR); and platelet-derived growth factor receptor-beta (PDGFR-beta). Selected compounds were also evaluated against the growth of A431 cells (which overexpress EGFR) in culture and as inhibitors of angiogenesis in the chicken embryo chorioallantonic membrane (CAM) assay. In each evaluation, a known standard compound was used as a comparison. Of the 11 analogues, five were more potent or equipotent as compared to standard compounds against the growth factor receptors. Two analogues showed superior inhibition of A431 cells in culture compared to the standard compounds. Three analogues were equipotent with the standard compound in the CAM assay and four of the analogues were dual inhibitors of RTKs. The structure-activity relationship for inhibition of different RTKs was quite distinct and different, and for VEGFR-2 and EGFR diametrically opposite. The inhibitory data against the RTKs in this study demonstrates that variation of the substituent(s) in the benzyl ring of these 2-amino-4-anilino 6-benzyl pyrrolo[2,3-d]pyrimidines does indeed control both the potency and specificity of inhibitory activity against RTKs.     

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 1.4 Å, respectively) with mouse DHFR and NADPH as well as 1b with human DHFR (1.5 Å) 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-β were discovered for saturated C9-homologated analogs 9 and 10 that were absent in the saturated C9-methyl analogs.     

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.     

2,4-Diamino-5-methyl-6-substituted arylthio-furo[2,3-d]pyrimidines as novel classical and nonclassical antifolates as potential dual thymidylate synthase and dihydrofolate reductase inhibitors

A novel classical antifolate N-{4-[(2,4-diamino-5-methyl-furo[2,3-d]pyrimidin-6-yl)thio]-benzoyl}-l-glutamic acid 5 and 11 nonclassical antifolates 6–16 were designed, synthesized, and evaluated as inhibitors of dihydrofolate reductase (DHFR) and thymidylate synthase (TS). The nonclassical compounds 6–16 were synthesized from 20 via oxidative addition of substituted thiophenols using iodine. Peptide coupling of the intermediate acid 21 followed by saponification gave the classical analog 5. Compound 5 is the first example, to our knowledge, of a 2,4-diamino furo[2,3-d]pyrimidine classical antifolate that has inhibitory activity against both human DHFR and human TS. The classical analog 5 was a nanomolar inhibitor and remarkably selective inhibitor of Pneumocystis carinii DHFR and Mycobacterium avium DHFR at 263-fold and 2107-fold, respectively, compared to mammalian DHFR. The nonclassical analogs 6–16 were moderately potent against pathogen DHFR or TS. This study shows that the furo[2,3-d]pyrimidine scaffold is conducive to dual human DHFR-TS inhibitory activity and to high potency and selectivity for pathogen DHFR.     

Synthesis and evaluation of a classical 2,4-diamino-5-substituted-furo[2,3-d]pyrimidine and a 2-amino-4-oxo-6-substituted-pyrrolo[2,3-d]pyrimidine as antifolates

Two classical antifolates, a 2,4-diamino-5-substituted furo[2,3-d]pyrimidine and a 2-amino-4-oxo-6-substituted pyrrolo[2,3-d]pyrimidine, were synthesized as potential inhibitors of dihydrofolate reductase (DHFR) and thymidylate synthase (TS). The syntheses were accomplished by condensation of 2,6-diamino-3(H)-4-oxo-pyrimidine with alpha-chloro-ketone 21 to afford two key intermediates 23 and 24, followed by hydrolysis, coupling with l-glutamate diethyl ester and saponification of the diethyl ester to afford the classical antifolates 13 and 14. Compounds 13 and 14 with a single carbon atom bridge are both substrates for folylpoly-gamma-glutamate synthetase (FPGS), the enzyme responsible for forming critical poly-gamma-glutamate antifolate metabolites with increased potency and/or increased cell retention. Compound 14 is a highly efficient FPGS substrate demonstrating that 2,4-diamino-5-substituted furo[2,3-d]pyrimidines are important lead structures for the design of antifolates with FPGS substrate activity. It retains inhibitory potency for DHFR and TS compared to the two atom bridged analog 5. Compound 13 is a poor inhibitor of purified DHFR and TS, and both 13 and 14 are poor inhibitors of the growth of CCRF-CEM human leukemia cells in culture, indicating that single carbon bridged compounds in these series though conducive to FPGS substrate activity were not potent inhibitors.     

N⁴-Aryl-6-substitutedphenylmethyl-7H-pyrrolo[2,3-d]pyrimidine-2,4-diamines as receptor tyrosine kinase inhibitors

Six novel N(4)-substitutedphenyl-6-substitutedphenylmethyl-7H-pyrrolo[2,3-d]pyrimidine-2,4-diamines were synthesized as multiple receptor tyrosine kinase (RTK) inhibitors and antitumor agents. An improvement in the inhibitory potency against epidermal growth factor receptor (EGFR), vascular endothelial growth factor receptor 1 (VEGFR-1) and vascular endothelial growth factor receptor 2 (VEGFR-2) assays and in the A431 cellular proliferation assay was observed for compounds 8-13 over the previously reported 5-7. Three compounds (8, 9 and 13) demonstrated potent, multiple RTK inhibition and were more potent or equipotent compared to the lead compounds 5 and 7 and the standard compounds. Compounds 10 and 12 showed potent inhibition of VEGFR-2 over EGFR, platelet-derived growth factor receptor-β (PDGFR-β) and VEGFR-1. The results indicate that the RTK inhibitory profile could be modulated with slight variations to the N(4)-aryl-6-substitutedphenylmethyl-7H-pyrrolo[2,3-d]pyrimidine-2,4-diamino scaffold.     

Synthesis, dihydrofolate reductase inhibition, antitumor testing, and molecular modeling study of some new 4(3H)-quinazolinone analogs

In order to produce potent new leads for anticancer drugs, a new series of quinazoline analogs was designed to resemble methotrexate (MTX, 1) structure features and fitted with functional groups believed to enhance inhibition of mammalian DHFR activity. Molecular modeling studies were used to assess the fit of these compounds within the active site of human DHFR. The synthesized compounds were evaluated for their ability to inhibit mammalian DHFR in vitro and for their antitumor activity in a standard in vitro tissue culture assay panel. Compounds 28, 30, and 31 were the most active DHFR inhibitors with IC₅₀ values of 0.5, 0.4, and 0.4 μM, respectively. The most active antitumor agents in this study were compounds 19, 31, 41, and 47 with median growth inhibitory concentrations (GI₅₀) of 20.1, 23.5, 26.7, and 9.1 μM, respectively. Of this series of compounds, only compound 31 combined antitumor potency with potent DHFR inhibition; the other active antitumor compounds (19, 41, and 47) all had DHFR IC₅₀ values above 15 μM, suggesting that they might exert their antitumor potency through some other mode of action. Alternatively, the compounds could differ significantly in uptake or concentration within mammalian cells.     

Discovery of benzimidazole derivatives as novel multi-target EGFR, VEGFR-2 and PDGFR kinase inhibitors

Multi-target EGFR, VEGFR-2 and PDGFR inhibitors are highly useful anticancer agents with improved therapeutic efficacies. In this work, we used two virtual screening methods, support vector machines (SVM) and molecular docking, to identify a novel series of benzimidazole derivatives, 2-aryl benzimidazole compounds, as multi-target EGFR, VEGFR-2 and PDGFR inhibitors. 2-Aryl benzimidazole compounds were synthesized and their biological activities against a tumor cell line HepG-2 and specific kinases were evaluated. Among these compounds, compounds 5a and 5e exhibited high cytotoxicity against HepG-2 cells with IC?? values at ～2 μM. Further kinase assay study showed that compound 5a have good EGFR inhibitory activity and moderate VEGFR-2 and PDGFR inhibitory activities, while 5e have moderate EGFR inhibitory activity and slightly weaker VEGFR-2 and PDGFR inhibitory activities. Molecular docking analysis suggested that compound 5a more tightly interacts with EGFR and PDGFR than compound 5e. Our study discovered a novel series of benzimidazole derivatives as multi-target EGFR, VEGFR-2 and PDGFR kinases inhibitors.     

Design,synthesis and preclinical evaluation of 5-methyl-N4-aryl-furo[2,3-d]pyrimidines as single agents with combination chemotherapy potential

The design, synthesis and biological evaluation of 4-substituted 5-methyl-furo[2,3-d]pyrimidines is described. The Ullmann coupling of 5-methyl-furo[2,3-d]pyrimidine with aryl iodides was successfully optimized to synthesize these analogs. Compounds 6–10 showed single-digit nanomolar inhibition of EGFR kinase. Compounds 1 and 6–10 inhibited VEGFR-2 kinase better than or equal to sunitinib. Compounds 1 and 3–10 were more potent inhibitors of PDGFR-β kinase than sunitinib. In addition, compounds 4–11 had higher potency in the CAM angiogenesis assay than sunitinib. Compound 1 showed in vivo efficacy in an A498 renal xenograft model in mice. Multiple RTK and tubulin inhibitory attributes of 1, 4, 6 and 8 indicates that these compounds may be valuable preclinical single agents targeting multiple intracellular targets.     

Arylphthalazines. Part 2: 1-(Isoquinolin-5-yl)-4-arylamino phthalazines as potent inhibitors of VEGF receptors I and II

A novel class of 1-(isoquinolin-5-yl)-4-arylamino-phthalazines is described as inhibitors of vascular endothelial growth factor receptor II (VEGFR-2). Many compounds display VEGFR-2 inhibitory activity with an IC(50) as low as 0.017 microM in an HTRF enzymatic assay. The compounds also inhibit VEGFR-1, a related tyrosine kinase.     

Novel non-classical C9-methyl-5-substituted-2,4-diaminopyrrolo[2,3-d]pyrimidines as potential inhibitors of dihydrofolate reductase and as anti-opportunistic agents

Six novel C9-methyl-5-substituted-2,4-diaminopyrrolo[2,3-d]pyrimidines 18-23 were synthesized as potential inhibitors of dihydrofolate reductase (DHFR) and as anti-opportunistic agents. These compounds represent the only examples of 9-methyl substitution in the carbon-carbon bridge of 2,4-diaminopyrrolo[2,3-d]pyrimidines. The analogs 18-23 were synthesized in a concise eight-step procedure starting from the appropriate commercially available aromatic methyl ketones. The key step involved a Michael addition reaction of 2,4,6-triaminopyrimidine to the appropriate 1-nitroalkene, followed by ring closure of the nitro adducts via a Nef reaction. The compounds were evaluated as inhibitors of DHFR from Pneumocystis carinii (pc), Toxoplasma gondii (tg), Mycobacterium avium (ma) and rat liver (rl). The biological result indicated that some of these analogs are potent inhibitors of DHFR and some have selectivity for pathogen DHFR. Compound 23 was a two digit nanomolar inhibitor of tgDHFR with 9.6-fold selectivity for tgDHFR.     

Synthesis and antimalarial activity of new haemanthamine-type derivatives

Thirty one derivatives were prepared from the natural alkaloids haemanthamine (1), haemanthidine (2) and 11-hydroxyvittatine (3). They were evaluated for their in vitro antimalarial activity against chloroquine-sensitive strains of Plasmodium falciparum and some structure-activity relationships were outlined. For haemanthamine derivatives having a methoxy group at C-3, the presence of a free hydroxyl group at C-11 is important for the activity. The double bond at C-1-C-2 plays also an important role to achieve good inhibitory activity. Compound 35 with two nicotinate groups at C-3 and at C-11 was the most active compound with a IC(50)=0.8±0.06μM.     

2-((1H-Azol-1-yl)methyl)-N-arylbenzamides: novel dual inhibitors of VEGFR-1/2 kinases

Novel potent derivatives of (azol-1-yl)methyl-N-arylbenzamides with improved solubility (>3mM) are described as ATP-competitive inhibitors of vascular endothelial growth factor receptor 2 (VEGFR-2). Many compounds display VEGFR-2 inhibitory activity reaching IC(50)<100 nM in the enzymatic assay. The compounds also inhibit the related tyrosine kinase, VEGFR-1, with similar potencies. Several compounds containing bulky lipophilic substituents at the benzamide pharmacophore yielded 10- to 17-fold selectivity for the VEGFR-2 versus VEGFR-1 kinase.     

Design,synthesis and biological evaluation of type-II VEGFR-2 inhibitors based on quinoxaline scaffold

In an effort to develop ATP-competitive VEGFR-2 selective inhibitors, a series of new quinoxaline-based derivatives was designed and synthesized. The target compounds were biologically evaluated for their inhibitory activity against VEGFR-2. The design of the target compounds was accomplished after a profound study of the structure activity relationship (SAR) of type-II VEGFR-2 inhibitors. Among the synthesized compounds, 1-(2-((4-methoxyphenyl)amino)-3-oxo-3,4 dihydroquinoxalin-6-yl)-3-phenylurea (VIIa) displayed the highest inhibitory activity against VEGFR-2. Molecular modeling study involving molecular docking and field alignment was implemented to interpret the variable inhibitory activity of the newly synthesized compounds.     

Diamino benzo[b]thiophene derivatives as a novel class of active site directed thrombin inhibitors: 3. Enhancing activity by imposing conformational restriction in the C-4" side chain

The preparation and biological evaluation of a series of benzo[b]thiophene diamine thrombin inhibitors possessing conformationally restricted C-4" linkers are reported. Compared to the parent compounds 1a/b, the unsaturated derivatives 3a/b exhibited a modest twofold increase in thrombin inhibitory activity, while the more lipophilic carbocyclic ring containing analogs 4a/b affected an eightfold enhancement in potency.     

Antitumor agents 281. Design, synthesis, and biological activity of substituted 4-amino-7,8,9,10-tetrahydro-2H-benzo[h]chromen-2-one analogs (ATBO) as potent in vitro anticancer agents

In our exploration of new biologically active chemical entities, we designed and synthesized a novel class of antitumor agents, substituted 4-amino-7,8,9,10-tetrahydro-2H-benzo[h]chromen-2-one (ATBO) analogs. We evaluated their cytotoxic activity against seven human tumor cell lines from different tissues, and established preliminary structure-activity relationships (SAR). All analogs, except 8, 9, and 25-27, displayed potent tumor cell growth inhibitory activity. Especially, compounds 15 and 33 with a 4-methoxyphenyl group at position C-4 were extremely potent with ED(50) values of 0.008-0.064 and 0.035-0.32 μM, respectively. Compound 15 was the most potent analog compared with structurally related neo-tanshinlactone (e.g., 1) and 4-amino-2H-benzo[h]chromen-2-one (ABO, e.g., 4) analogs, and thus merits further exploration as an anti-cancer drug candidate.     

Design, synthesis and antitumor activity of 4-aminoquinazoline derivatives targeting VEGFR-2 tyrosine kinase

We report herein the design and synthesis of novel 4-aminoquinazoline derivatives based on the inhibitors of VEGFR-2 tyrosine kinases. The VEGFR-2 inhibitory activities of these newly synthesized compounds were also evaluated and compared with that of ZD6474. We found that most of target compounds had good inhibitory potency. In particular, compounds 1h, 1n and 1o were found to be 6, 2 and 2-fold more potent than the positive control ZD6474. The leading compound 1h also showed an in vivo activity against HepG2 human tumor xenograft model in BALB/c-nu mice.     

Understanding topoisomerase I and II in terms of QSAR

A variety of antitumor agents currently used in chemotherapy or evaluated in clinical trials are known to inhibit DNA topoisomerase I or II. We have developed sixteen quantitative structure-activity relationships (QSAR) for different sets of compounds that are camptothecin analogs, 1,4-naphthoquinones, unsaturated acids, benzimidazoles, quinolones, and miscellaneous fused heterocycles to understand chemical-biological interactions governing their inhibitory activities toward topoisomerase I and II.     

Synthesis and structure-activity relationships of taxuyunnanine C derivatives as multidrug resistance modulator in MDR cancer cells

A series of new generation taxoids bearing a bulky group on different positions such as C-2, C-5, C-7, C-9, C-10 or C-14 were obtained by chemical modifications and biotransformation of taxuyunnanine C (1) and its analogs, 4, 5, and 10. Compounds 3, 5, 6, 8, and 9a showed significant activity toward calcein accumulation in MDR 2780AD cells. The most effective compound 9a with a cinnamoyloxy group at C-14 and a hydroxyl group at C-10 was actually efficient for the cellular accumulation of the anticancer agent, vincristine, in MDR 2780AD cells. The enhancing effects of 6 and 9a for taxol, adriamycin, and vincristine were at the same levels as those of verapamil toward MDR 2780AD cells. Thus, compounds 6 and 9a can modulate the multidrug resistance of cancer cells. The cytotoxicity (IC(50)) of the compounds was examined against human normal cell line, WI-38, and cancer model cell lines, VA-13 and HepG2. Since compounds 6 and 8 had no cytotoxicity, they were expected to be lead compounds of MDR cancer reversal agents. On the contrary, compounds 3, 5, and 9a showed cell growth inhibitory activity toward VA-13 and/or HepG2 as well as accumulation activity of calcein and/or vincristine in MDR 2780AD and they were expected to be lead compounds of new-type anticancer agents.     

Discovery of new nicotinamides as apoptotic VEGFR-2 inhibitors: virtual screening,synthesis, anti-proliferative,immunomodulatory, ADMET,toxicity, and molecular dynamic simulation studies

A library of modified VEGFR-2 inhibitors was designed as VEGFR-2 inhibitors. Virtual screening was conducted for the hypothetical library using in silico docking, ADMET, and toxicity studies. Four compounds exhibited high in silico affinity against VEGFR-2 and an acceptable range of the drug-likeness. These compounds were synthesised and subjected to in vitro cytotoxicity assay against two cancer cell lines besides VEGFR-2 inhibitory determination. Compound D-1 showed cytotoxic activity against HCT-116 cells almost double that of sorafenib. Compounds A-1, C-6, and D-1 showed good IC₅₀ values against VEGFR-2. Compound D-1 markedly increased the levels of caspase-8 and BAX expression and decreased the anti-apoptotic Bcl-2 level. Additionally, compound D-1 caused cell cycle arrest at pre-G1 and G2-M phases in HCT-116 cells and induced apoptosis at both early and late apoptotic stages. Compound D-1 decreased the level of TNF-α and IL6 and inhibited TNF-α and IL6. MD simulations studies were performed over 100 ns.