Selective VEGFR-2 inhibitors: Synthesis of pyridine derivatives, cytotoxicity and apoptosis induction profiling

Recent advances and future directions on small molecule VEGFR inhibitors in oncological conditions

"A journey of mixed emotions" is a quote that best describes the progress chart of vascular endothelial growth factor receptor (VEGFR) inhibitors as cancer therapeutics in the last decade. Exhilarated with the Food and Drug Administration (FDA) approvals of numerous VEGFR inhibitors coupled with the annoyance of encountering the complications associated with their use, drug discovery enthusiasts are on their toes with an unswerving determination to enhance the rate of translation of VEGFR inhibitors from preclinical to clinical stage. The recently crafted armory of VEGFR inhibitors is a testament to their growing dominance over other antiangiogenic therapies for cancer treatment. This review perspicuously underscores the earnest attempts of the researchers to extract the antiproliferative potential of VEGFR inhibitors through the design of mechanistically diverse structural assemblages. Moreover, this review encompasses sections on structural/molecular properties and physiological functions of VEGFR, FDA-approved VEGFR inhibitors, and hurdles restricting the activity range/clinical applicability of VEGFR targeting antitumor agents. In addition, tactics to overcome the limitations of VEGFR inhibitors are discussed. A clear-cut viewpoint transmitted through this compilation can provide practical directions to push the cart of VEGFR inhibitors to advanced-stage clinical investigations in diverse malignancies.

Role of heterocycles in inhibition of VEGFR-2 - a recent update (2019-2022)

The literature reveals that oncogenic protein kinase inhibition has been proved to be a successful anticancer approach. The vascular endothelial growth factor receptor (VEGFR) kinase plays an important role in angiogenesis and metastasis. VEGFR-2 has an upper hand in the angiogenesis process. Vascular endothelial growth factor activates VEGFR-2 which initiates tumor angiogenesis. In addition, VEGFRs are associated with numerous other diseases. Hence, inhibition of VEGFRs is an attractive approach for cancer treatment. In view of this, researchers designed and discovered small molecular heterocycle-based VEGFR-2 inhibitors and some of them have been approved by the Food and Drug Administration (FDA). However, these VEGFR-2 inhibitors pose adverse side effects such as cardiovascular problems, diarrhea, and renal function impairment. Research indicates that combination of certain pharmacophores exhibits excellent VEGFR inhibitory activity. In particular, combination of heterocycles paved the way to efficient VEGFR inhibitors. In this review, the research focusing on VEGFR inhibitory activity has been discussed along with the structure-activity relationship. In addition to emphasizing the most potent molecule among the set of designed molecules, structural features responsible for such an activity are described. This review may aid in designing potent VEGFR inhibitors.

Synthesis, molecular modeling Insights, and anticancer assessment of novel polyfunctionalized Pyridine congeners

The present study describes synthesizing a novel series of polyfunctionalized pyridine congeners 1-18 and assessed for cytotoxic efficacies versus HCT-116, MCF-7, and HepG-2 among one non-cancerous BJ-1 human normal cell. Most compounds were precisely potent anticancer candidate drugs. The molecular impact of the most active compounds 9, 10, 11, 13, 15, and 17 was evaluated after MCF-7 treatment. The gene expression of pro- and ant-apoptosis markers P53, Bax, Caspase-3 and Bcl-2 as well as VEGFR-2 and HER2 were determined. Compounds 13 and 15 induced upregulation of pro-apoptosis of P53, Bax, Caspase-3 and downregulation of anti-apoptosis Bcl-2 gene. However, compound 15 showed higher effect compared to 13 and respective control. Moreover, a slight reduction in HER2 gene expression was detected due to compound 15 treatment, while VEGFR-2 gene was upregulated. In agreement, the immunoblotting analysis showed higher accumulation of P53, Bax, Caspase-3 proteins and of decrease the Bcl-2 protein levels. Furthermore, docking studies united with molecular dynamic simulation exposed compounds 13 and 15 fitting in the middle of the active site at the interface linking the ATP binding site and the allosteric hydrophobic binding pocket. Finally, we performed Petra/Osiris/ Molinspiration (POM) analysis for the newly synthesized compounds. The evaluation of primary in silico parameters revealed significant differences among individual polyfunctionalized pyridine compounds, highlighting the most promising candidates. These preliminary results may help in coordinating and initiating other research projects focused on polyfunctionalized pyridine compounds, especially those with predicted bioactivity, low toxicity, optimal ADME parameters, and promising perspectives.

Recent advances in nitrogen-containing heterocyclic compounds as receptor tyrosine kinase inhibitors for the treatment of cancer: Biological activity and structural activity relationship

Erratic cell proliferation is the initial symptom of cancer, which can eventually metastasize to other organs. Before cancer becomes metastatic, its spread is triggered by pro-angiogenic factors including vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), Platelet-derived growth factor receptor (PDGFR), fibroblast growth factor receptor (FGFR) and Platelet Factor (PF4), all of which are part of receptor tyrosine kinase (RTK) family. Receptor tyrosine kinases (RTKs) are cell-surface proteins and aresignaling enzymes that transfer ATP-phosphate to tyrosine residue substrates. Important biological processes like proliferation, differentiation, motility, and cell-cycle regulation are all possessedby these proteins. Unusual RTK expression is typically associated with cell growth abnormalities, which is linked to tumor acquisition, angiogenesis, and cancer progression. In addition to the already available medications, numerous other heterocyclic are being studied for their potential action against a variety of cancers. In the fight against cancer, in particular, these heterocycles have been used for their dynamic core scaffold and their inherent adaptability. In this review article, we have compiled last five years research work including nitrogen containing heterocycles that have targeted RTK. Herein, the SAR and activity of various compounds containing diverse heterocyclic (pyrimidine, indole, pyridine, pyrazole, benzimidazole, and pyrrole) scaffolds are discussed, and they may prove useful in the future for designing new leads against RTKs. Our focus in this manuscript is to comprehensively review the latest research on the biological activity and structural activity relationship of nitrogen compounds as RTK inhibitors. We believe that this may be an important contribution to the field, as it can help guide future research efforts and facilitate the development of more effective cancer therapies.

Angiogenic signaling pathways and anti-angiogenic therapy for cancer

Angiogenesis, the formation of new blood vessels, is a complex and dynamic process regulated by various pro- and anti-angiogenic molecules, which plays a crucial role in tumor growth, invasion, and metastasis. With the advances in molecular and cellular biology, various biomolecules such as growth factors, chemokines, and adhesion factors involved in tumor angiogenesis has gradually been elucidated. Targeted therapeutic research based on these molecules has driven anti-angiogenic treatment to become a promising strategy in anti-tumor therapy. The most widely used anti-angiogenic agents include monoclonal antibodies and tyrosine kinase inhibitors (TKIs) targeting vascular endothelial growth factor (VEGF) pathway. However, the clinical benefit of this modality has still been limited due to several defects such as adverse events, acquired drug resistance, tumor recurrence, and lack of validated biomarkers, which impel further research on mechanisms of tumor angiogenesis, the development of multiple drugs and the combination therapy to figure out how to improve the therapeutic efficacy. Here, we broadly summarize various signaling pathways in tumor angiogenesis and discuss the development and current challenges of anti-angiogenic therapy. We also propose several new promising approaches to improve anti-angiogenic efficacy and provide a perspective for the development and research of anti-angiogenic therapy.

Novel VEGFR-2 Kinase Inhibitors as Anticancer Agents: A Review Focusing on SAR and Molecular Docking Studies (2016-2021)

Cancer growth, annexation, and metastatic spread are all aided by the formation of new blood vessels (angiogenesis). The commencement of the VEGF pathway leads to signal transduction that enhances endothelial cell survival, relocation, and divergence from pre-existing vasculature. The ability of solid malignancies to bloom and spread depends critically on their ability to establish their independent blood circulation (tumor angiogenesis). VEGFR is a major receptor tyrosine kinase that regulates angiogenesis, cell growth, and metastasis, diminishing apoptosis, cytoskeletal function, and other biological processes VEGFR has proven to be a remarkable focus for a variety of anticancer medicines in clinical studies. This Review explores the development of anti-VEGF-based antiangiogenic therapies having different scaffolds. This review had focused on SAR and docking studies of previously reported molecules.

Design, Synthesis, and Biological Evaluation of Novel Dihydropyridine and Pyridine Analogs as Potent Human Tissue Nonspecific Alkaline Phosphatase Inhibitors with Anticancer Activity: ROS and DNA Damage-Induced Apoptosis

Small molecules with nitrogen-containing scaffolds have gained much attention due to their biological importance in the development of new anticancer agents. The present paper reports the synthesis of a library of new dihydropyridine and pyridine analogs with diverse pharmacophores. All compounds were tested against the human tissue nonspecific alkaline phosphatase (h-TNAP) enzyme. Most of the compounds showed excellent enzyme inhibition against h-TNAP, having IC₅₀ values ranging from 0.49 ± 0.025 to 8.8 ± 0.53 µM, which is multi-fold higher than that of the standard inhibitor (levamisole = 22.65 ± 1.60 µM) of the h-TNAP enzyme. Furthermore, an MTT assay was carried out to evaluate cytotoxicity against the HeLa and MCF-7 cancer cell lines. Among the analogs, the most potent dihydropyridine-based compound 4d was selected to investigate pro-apoptotic behavior. The further analysis demonstrated that compound 4d played a significant role in inducing apoptosis through multiple mechanisms, including overproduction of reactive oxygen species, mitochondrial dysfunction, DNA damaging, and arrest of the cell cycle at the G1 phase by inhibiting CDK4/6. The apoptosis-inducing effect of compound 4d was studied through staining agents, microscopic, and flow cytometry techniques. Detailed structure–activity relationship (SAR) and molecular docking studies were carried out to identify the core structural features responsible for inhibiting the enzymatic activity of the h-TNAP enzyme. Moreover, fluorescence emission studies corroborated the binding interaction of compound 4d with DNA through a fluorescence titration experiment.

Discovery of New VEGFR-2 Inhibitors: Design, Synthesis, Anti-Proliferative Evaluation, Docking, and MD Simulation Studies

Four new nicotinamide-based derivatives were designed as antiangiogenic VEGFR-2 inhibitors. The congeners were synthesized possessing the pharmacophoric essential features to bind correctly with the VEGFR-2 active pocket. All members were evaluated for their cytotoxic and VEGFR-2 inhibitory potentialities. Compound 6 was the most potent showingIC₅₀ values of 9.3 ± 0.02 and 7.8 ± 0.025 µM against HCT-116 and HepG-2 cells, respectively, and IC₅₀ of 60.83 nM regarding VEGFR-2 enzyme inhibition. Compound 6 arrested the growth of HCT-116 cells at the pre-G1 and G2-M phases. Further, it induced both early and late apoptosis. Additionally, compound 6 caused a significant decrease in TNF-α and IL6 by 66.42% and 57.34%, respectively. The considered compounds had similar docking performances to that of sorafenib against the VEGFR-2 (PDB ID: 2OH4). The correct binding of compound 6 with VEGFR-2 was validated using MD simulations, and MM-GPSA calculations.

Nicotinonitrile-derived apoptotic inducers: Design, synthesis, X-ray crystal structure and Pim kinase inhibition

Although a plethora of targeted anticancer small molecule drugs became available, the low response rate and drug resistance imply the continuous need for expanding the anticancer chemical space. In this study, a novel series of nicotinonitrile derivatives was designed, synthesized and evaluated for cytotoxic activities in HepG2 and MCF-7 cells. All derivatives showed high to moderate cytotoxic activity against both cell lines, with cell-type and chemotype-dependent cytotoxic potential. The normal HEK-293 T cells were ca. 50-fold less susceptible to the cytotoxic effect of the inhibitors. The in vitro enzyme inhibitory activity of selected active cytotoxic derivatives 8c, 8e, 9a, 9e and 12 showed that they have sub- to one digit micromolar 50 % inhibitory concentration (IC₅₀) against the three Pim kinase isoforms, with 8e being the most potent (IC₅₀ ≤ 0.28 μM against three Pim kinases), comparable to the pan kinase inhibitor, Staurosporine. In HepG2, 8e induced cell cycle arrest at the G2/M phase. Apoptotic mechanistic studies with 8c and 8e in HepG2 cells, indicated a significant upregulation in both P53 and caspase-3 relative gene expression, as well as increased Bax/Bcl-2 protein expression level. Further, docking studies combined with molecular dynamic simulation showed a stable complex with high binding affinity of 8e to Pim-1 kinase; exploiting a negative electrostatic potential surface interaction with the added dimethyl amino group in the new compounds. Moreover, in silico ADME profile prediction indicated that all compounds are orally bioavailable and most of them can penetrate the blood-brain barrier. This study presents novel nicotinonitrile derivatives as auspicious hits for further optimization as antiproliferative agents against liver cancer cells and promising pan Pim kinase inhibitors at submicromolar concentrations.

Design, Synthesis and Cytotoxicity Screening of New Thiazole Derivatives as Potential Anticancer Agents through VEGFR-2 Inhibition

Z-configurated isomers are kinetically preferred molecules. Compounds with Z-configuration are contained in many natural products, biologically active compounds and as synthons for organic synthesis. Two series of new thiazole-based analogs were synthesized from appropriate starting materials hydrazinecarbothioamide derivatives (Z)-2a,b to be evaluated for their inhibitory activity towards VEGFR-2. The prepared thiazole compounds 3a-5b were screened for their cytotoxic potency against the MDA-MB-231 breast cancer cell line and their percentage inhibition against VEGFR-2. Compound 4d exhibited good VEGFR-2 inhibitory activity. A DNA flow cytometry analysis was conducted, and compound 4d demonstrated cell cycle arrest at the G1 and G2/M phases of the cell cycle profile and an apoptosis-inducing effect by increasing the percentage of pre-G1 phase. Compound 4d was further evaluated for its apoptosis-inducing effect by studying the effect on mitochondrial membrane potential (MMP) and p53 activation. It was found to boost the level of p53 and reduce the level of MMP compared with the untreated control cells.

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.

Design, synthesis, anticancer, and docking of some S- and/or N-heterocyclic derivatives as VEGFR-2 inhibitors

Novel heterocyclic derivatives (4-22) were designed, synthesized, and evaluated against hepatocellular carcinoma type (HepG2) and breast cancer (MCF-7) cells, targeting the VEGFR-2 enzyme. Compounds 18, 10, 13, 11, and 14 were found to be the most potent derivatives against both the HepG2 and MCF-7 cancer cell lines, with GI₅₀  = 2.11, 2.54 µM, 3.16, 3.64 µM, 3.24, 6.99 µM, 7.41, 6.49 µM and 8.08, 10.46 µM, respectively. Compounds 18 and 10 showed higher activities against both HepG2 and MCF-7 cells than sorafenib (GI₅₀  = 9.18, 5.47 µM, respectively) and doxorubicin (GI₅₀  = 7.94, 8.07 µM, respectively). Compounds 13, 11, and 14 showed higher activities than sorafenib against HepG2 cancer cells, but lower activities against MCF-7 cells. Compounds 18, 13, and 10 were more potent than sorafenib, inhibiting vascular endothelial growth factor receptor-2 (VEGFR-2) at GI₅₀ values of 0.05, 0.06, and 0.08 µM, respectively. Compound 11 inhibited VEGFR-2 at an IC₅₀ value of 0.10 µM, which is equipotent to sorafenib. Compound 14 inhibited VEGFR-2 at an IC₅₀ value of 0.11 µM, which is nearly equipotent to sorafenib. The tested compounds have more selectivity against cancer cell lines. Compounds 18, 10, 13, 11, and 14 are, respectively, 16.76, 9.24, 6.06, 2.78, and 2.85 times more toxic in HePpG2 cancer cells than in VERO normal cells. Also, compounds 18, 10, 13, 11, and 14 are, respectively, 14.07, 8.02, 2.81, 3.18, and 2.20 times more toxic in MCF-7 than in VERO normal cells. The most active compounds, 10, 13, and 18, showed a good ADMET (absorption, distribution, metabolism, excretion, and toxicity) profile.

Discovery of new VEGFR-2 inhibitors based on bis([1, 2, 4]triazolo)[4,3-a:3',4'-c]quinoxaline derivatives as anticancer agents and apoptosis inducers

Herein, a new wave of bis([1, 2, 4]triazolo)[4,3-a:3',4'-c]quinoxaline derivatives have been successfully designed and synthesised. The synthesised derivatives were biologically investigated for their cytotoxic activities against HepG2 and MCF-7. Also, the tested compounds were further examined in vitro for their VEGFR-2 inhibitory activity. The most promising derivative 23j was further investigated for its apoptotic behaviour in HepG2 cell lines using flow cytometric and western-plot analyses. Additional in-silico studies were performed to predict how the synthesised compounds can bind to VEGFR-2 and to determine the drug-likeness profiling of these derivatives. The results revealed that compounds 23a, 23i, 23j, 23l, and 23n displayed the highest antiproliferative activities against the two cell lines with IC50 values ranging from 6.4 to 19.4 µM. Furthermore, compounds 23a, 23d, 23h, 23i, 23j, 23l, 23 m, and 23n showed the highest VEGFR-2 inhibitory activities with IC50 values ranging from 3.7 to 11.8 nM, comparing to sorafenib (IC50 = 3.12 nM). Moreover, compound 23j arrested the HepG2 cell growth at the G2/M phase and induced apoptosis by 40.12% compared to the control cells (7.07%). As well, such compound showed a significant increase in the level of caspase-3 (1.36-fold), caspase-9 (2.80-fold), and BAX (1.65-fold), and exhibited a significant decrease in Bcl-2 level (2.63-fold).

Recent advances in, and challenges of, anti-angiogenesis agents for tumor chemotherapy based on vascular normalization

A major problem associated with cancer treatment is resistance-prone chemotherapeutic drugs. An increasing number of studies have documented that the occurrence of resistance tends to be associated with abnormal blood vessels. In 2001, Jain proposed the vascular normalization theory, which was recently applied to the drug-resistant treatment of tumors in the clinic. Through the intervention of angiogenesis inhibitors, remodeling the structure and function of abnormal vessels can maximize the efficacy of chemotherapeutic drugs. In this review, we systematically describe the occurrence and progress of tumor angiogenesis, as well as the pathological characteristics of tumor blood vessels. Moreover, druggable targets for vascular normalization and the development of related inhibitors are also outlined.

Pyridine-derived VEGFR-2 inhibitors: Rational design, synthesis, anticancer evaluations, in silico ADMET profile, and molecular docking

Novel pyridine-derived compounds (5-19) were designed and synthesized, and their anticancer activities were evaluated against HepG2 and MCF-7 cells, targeting the VEGFR-2 enzyme. Compounds 10, 9, 8, and 15 were found to be the most potent derivatives against the two cancer cell lines, HepG2 and MCF-7, respectively, with IC₅₀  = 4.25 and 6.08 µM, 4.68 and 11.06 µM, 4.34 and 10.29 µM, and 6.37 and 12.83 µM. Compound 10 displayed higher activity against HepG2 cells than sorafenib (IC₅₀  = 9.18 and 5.47 µM, respectively) and doxorubicin (IC₅₀  = 7.94 and 8.07 µM, respectively). It also showed higher activity than doxorubicin against MCF-7 cells, but lower activity than sorafenib. Compounds 9, 8, and 15 displayed higher activities than sorafenib and doxorubicin against HepG2 cells but exhibited lower activities against MCF-7 cells. Compound 10 potently inhibited VEGFR-2 at an IC₅₀ value of 0.12 µM, which is nearly equipotent to sorafenib (IC₅₀  = 0.10 µM). Compounds 8 and 9 exhibited very good activity with the same IC₅₀ value of 0.13 µM. The six most potent derivatives, 6, 9, 8, 10, 15, and 18, were tested for their cytotoxicity against normal Vero cells. Compounds 6, 8, 9, 10, 15, and 18 are, respectively, 1.13, 3.74, 4.18, 3.64, 2.81, and 2.00 times more toxic to HepG2 and 2.06, 1.58, 1.76, 2.54, 1.40, and 2.69 times more toxic to MCF-7 breast cancer cells than in normal Vero cells.

Novel 4-(piperazin-1-yl)quinolin-2(1H)-one bearing thiazoles with antiproliferative activity through VEGFR-2-TK inhibition

A new series of 2-(4-(2-oxo-1,2-dihydroquinolin-4-yl)piperazin-1-yl)-N-(4-phenylthiazol-2-yl)acetamide derivatives were synthesized and evaluated for anticancer activity. All target compounds showed anticancer activity higher than that of their 2-oxo-4-piperazinyl-1,2-dihydroquinolin-2(1H)-one precursors. Multidose testing of target compounds was performed against breast cancer T-47D cell line. Five compounds showed higher cytotoxic activity than Staurosporine. The dihalogenated derivative showed the best cytotoxic activity with IC₅₀ 2.73 ± 0.16 µM. In addition, the VEGFR-2 inhibitory activity of all synthetic compounds was evaluated. Two compounds of 6-fluoro-4-(piperazin-1-yl)quinolin-2(1H)-ones showed inhibitory activity comparable to sorafenib with IC₅₀ 46.83 ± 2.4, 51.09 ± 2.6 and 51.41 ± 2.3 nM, respectively. The cell cycle analysis of two compounds namely, 2-(4-(6-fluoro-2-oxo-1,2-dihydroquinolin-4-yl)piperazin-1-yl)-N-(4-phenylthiazol-2-yl)acetamide and N-(4-(4-chlorophenyl)thiazol-2-yl)-2-(4-(2-oxo-1-phenyl-1,2-dihydroquinolin-4-yl)piperazin-1-yl)acetamide revealed that the arrest of cell cycle occurred at S phase. In apoptosis assay, the same two compounds were able to induce significant levels of early and late apoptosis. In a similar manner to Sorafenib, docking of target compounds with VEGFR-2 protein 4ASD showed HB with Cys919 in hinge region of enzyme and HB with both Glu885 and Asp1046 in gate area. Using SwissADME, all target compounds were predicted to be highly absorbed from gastrointestinal tract with no BBB permeability. It is clear that the two compounds are promising antiproliferative candidates that require further optimization.

Design, synthesis, docking, ADMET profile, and anticancer evaluations of novel thiazolidine-2,4-dione derivatives as VEGFR-2 inhibitors

The anticancer activity of novel thiazolidine-2,4-diones was evaluated against HepG2, HCT-116, and MCF-7 cells. Among the tested cancer cell lines, HCT-116 was the most sensitive one to the cytotoxic effect of the new derivatives. In particular, compounds 18, 11, and 10 were found to be the most potent derivatives among all the tested compounds against the HepG2, HCT-116, and MCF-7 cancer cell lines, with IC₅₀ values ranging from 38.76 to 53.99 µM. The most active antiproliferative derivatives (7-14 and 15-19) were subjected to further biological studies to evaluate their inhibitory potentials against VEGFR-2. The tested compounds displayed a good-to-medium inhibitory activity, with IC₅₀ values ranging from 0.26 to 0.72 µM. Among them, compounds 18, 11, and 10 potently inhibited VEGFR-2 at IC₅₀ values in the range of 0.26-0.29 µM, which are nearly three times that of the sorafenib IC₅₀ value (0.10 µM). Although our derivatives showed lower activities than the reference drug, they could be useful as a template for future design, optimization, adaptation, and investigation to produce more potent and selective VEGFR-2 inhibitors with higher anticancer analogs. The ADMET profile showed that compounds 18, 11, and 10 do not violate any of Lipinski's rules and have a comparable intestinal absorptivity in humans. Also, the new derivatives could not inhibit cytochrome P3A4. Unlike sorafenib and doxorubicin, compounds 18, 11, and 10 are expected to have prolonged dosing intervals. Moreover, compounds 10 and 18 displayed a wide therapeutic index and higher selectivity against cancer cells as compared with their cytotoxicity against normal cells.