Primaquine-pyrimidine hybrids: synthesis and dual-stage antiplasmodial activity

Putative Contribution of 8-Aminoquinolines to Preventing Recrudescence of Malaria

Enhanced therapeutic efficacy achieved in treating Plasmodium vivax malaria with an 8-aminoquinoline (8-AQ) drug such as primaquine (PQ) together with a partner drug such as chloroquine (CQ) is usually explained as CQ inhibiting asexual parasites in the bloodstream and PQ acting against liver stages. However, PQ's contribution, if any, to inactivating non-circulating, extra-hepatic asexual forms, which make up the bulk of the parasite biomass in chronic P. vivax infections, remains unclear. In this opinion article, I suggest that, considering its newly described mode of action, PQ might be doing something of which we are currently unaware.

Pyrimidine: An elite heterocyclic leitmotif in drug discovery-synthesis and biological activity

Heterocyclic compounds bearing the pyrimidine core are of tremendous interest as they constitute an important class of natural and synthetic compounds exhibiting diverse useful biological activities that hold attractive potential for clinical translation as therapeutic agents in alleviation of a myriad of diseases. Heterocycles possessing a pyrimidine scaffold have piqued tremendous interest of organic and medicinal chemists owing to their privileged bioactivities. Drugs having the pyrimidine motif have manifested to exhibit gratifying biological activity like anticancer, antiviral, anti-inflammatory, antibacterial, and antihypertensive activities. This heterocycle, being a significant endogenous component of the body, the pyrimidine derivatives can easily interact with enzymes, genetic materials, and bio components within the cell. The landscape of FDA approved drugs, presently marketed incorporating the pyrimidine scaffold continues to evolve in number and diversity. There is a tremendous surge in discovery of new targets across many diseases especially those involving emerging resistance to clinically used battery of drugs. Pyrimidine scaffolds will continue to be explored expanding their chemical space portfolio in an effort to find novel drugs impacting these targets. This review aims to provide an elaborate recapitulation of the recent trends adopted to synthesize propitious pyrimidine incorporated hits and also focuses on the clinical significance reported for functionalized pyrimidine analogues that would quintessentially aid medicinal chemists for new research explorations in this arena.

One-pot synthesis, X-ray crystal structure, and identification of potential molecules against COVID-19 main protease through structure-guided modeling and simulation approach

Although antimicrobial resistance before the Covid-19 pandemic is a top priority for global public health, research is already ongoing on novel organic compounds with antimicrobial and antiviral properties in changing medical environments in connection with Covid 19. Thanks to the Biginelli reaction, which allows the synthesis of pyrimidine compounds, blockers of calcium channels, antibodies, antiviral, antimicrobial, anti-inflammatory, or antioxidant therapeutic compounds were investigated. In this paper, we aim to present Biginelli's synthesis, its therapeutic properties, and the structural-functional relationship in the test compounds that allows the synthesis of antimicrobial compounds. Both the DFT and TD-DFT computations of spectral data, molecular orbitals (HOMO, LUMO) analysis, and electrostatic potential (MEP) surfaces are carried out as an add-on to synthetic research. Hirshfeld surface analysis was also used to segregate the different intermolecular hydrogen bonds involved in the molecular packing strength. Natural Bond Orbital (NBO) investigation endorses the existence of intermolecular interactions mediated by lone pair, bonding, and anti-bonding orbitals. The dipole moment, linear polarizability, and first hyperpolarizabilities have been explored as molecular parameters. All findings based on DFT exhibit the best consistency with experimental findings, implying that synthesized molecules are highly stable. To better understand the binding mechanism of the SARS-CoV-2 Mpro, we performed molecular docking, molecular dynamics (MD) simulations, and binding free energy calculations.

Rational Optimization of Dihydropyrimidoinone-Quinoline Hybrids as Plasmodium falciparum Glutathione Reductase Inhibitors

A series of dihydropyrimidinone-based antimalarial compounds were designed and synthesised based on the previously identified amide-based quinoline hybrids which showed good resistance reversal ability against the resistant strain of Plasmodium falciparum . The aromatic ring on the dihydropyrimidinone of the original hits was exchanged for a methyl group to bring the molecular weights below 500 Da and also determine the effect of the aromatic ring count on the resistance reversal ability of the hybrids. Apart from the previously used amide bond, the hybrid linker was also extended to the triazole linker. Although the triazole linker is synthetically easier to access, the use of an amide linker seems to have an activity advantage. The synthesised compounds in addition to the previously identified hits were subjected to molecular docking particularly targeting the orthosteric site of Plasmodium falciparum glutathione reductase ( Pf GR) protein. The ligand with the best binding interaction was rationally optimised to increase its suitability as a competitive inhibitor against the cofactor of the Pf GR. Two of the optimised ligands showed better binding affinities than the cofactor while one of the two ligands displayed hydrophobically packed correlated hydrogen-bond which is very important in maintaining the ligand stability within the protein. In silico ADME predictions of the synthesised compounds indicate that these compounds possess good pharmacokinetic properties.

Novel Pyridothienopyrimidine Derivatives: Design, Synthesis and Biological Evaluation as Antimicrobial and Anticancer Agents

The growing risk of antimicrobial resistance besides the continuous increase in the number of cancer patients represents a great threat to global health, which requires intensified efforts to discover new bioactive compounds to use as antimicrobial and anticancer agents. Thus, a new set of pyridothienopyrimidine derivatives 2a,b-9a,b was synthesized via cyclization reactions of 3-amino-thieno[2,3-b]pyridine-2-carboxamides 1a,b with different reagents. All new compounds were evaluated against five bacterial and five fungal strains. Many of the target compounds showed significant antimicrobial activity. In addition, the new derivatives were further subjected to cytotoxicity evaluation against HepG-2 and MCF-7 cancer cell lines. The most potent cytotoxic candidates (3a, 4a, 5a, 6b, 8b and 9b) were examined as EGFR kinase inhibitors. Molecular docking study was also performed to explore the binding modes of these derivatives at the active site of EGFR-PK. Compounds 3a, 5a and 9b displayed broad spectrum antimicrobial activity with MIC ranges of 4-16 µg/mL and potent cytotoxic activity with IC50 ranges of 1.17-2.79 µM. In addition, they provided suppressing activity against EGFR with IC50 ranges of 7.27-17.29 nM, higher than that of erlotinib, IC50 = 27.01 nM.

Synthesis and antiplasmodial activity of regioisomers and epimers of second-generation dual acting ivermectin hybrids

With its strong effect on vector-borne diseases, and insecticidal effect on mosquito vectors of malaria, inhibition of sporogonic and blood-stage development of Plasmodium falciparum, as well as in vitro and in vivo impairment of the P. berghei development inside hepatocytes, ivermectin (IVM) continues to represent an antimalarial therapeutic worthy of investigation. The in vitro activity of the first-generation IVM hybrids synthesized by appending the IVM macrolide with heterocyclic and organometallic antimalarial pharmacophores, against the blood-stage and liver-stage infections by Plasmodium parasites prompted us to design second-generation molecular hybrids of IVM. Here, a structural modification of IVM to produce novel molecular hybrids by using sub-structures of 4- and 8-aminoquinolines, the time-tested antiplasmodial agents used for treating the blood and hepatic stage of Plasmodium infections, respectively, is presented. Successful isolation of regioisomers and epimers has been demonstrated, and the evaluation of their in vitro antiplasmodial activity against both the blood stages of P. falciparum and the hepatic stages of P. berghei have been undertaken. These compounds displayed structure-dependent antiplasmodial activity, in the nM range, which was more potent than that of IVM, its aglycon or primaquine, highlighting the superiority of this hybridization strategy in designing new antiplasmodial agents.

N-Directed Pd-Catalyzed Photoredox-Mediated C–H Arylation for Accessing Phenyl-Extended Analogues of Biginelli/Suzuki-Derived Ethyl 4-Methyl-2,6-diphenylpyrimidine-5-carboxylates

The availability and application of direct, functional group-compatible C–H activation methods for late-stage modification of small-molecule bioactives and other valuable materials remains an ongoing challenge in organic synthesis. In the current study, we demonstrate that a LED-activated, photoredox-mediated, Pd(OAc)2-catalyzed C–H arylation, employing a phenyldiazonium aryl source and either tris(2,2′-bipyridine)ruthenium(II) or (2,2′-bipyridine)bis[3,5-di-fluoro-2-[5-(trifluoromethyl)-2-pyridinyl-kN][phenyl-kC]iridium(III) as photoredox initiator, may successfully produce unprecedented mono- and bis-phenyl derivatives of functionality-rich 2,6-diphenylpyrimidine substrates at room temperature. The series of 19 substrates employed herein, which share the biologically-relevant 4-methyl-2,6-diphenylpyrimidine-5-carboxylate scaffold, were generated via a synthetic route involving (3-component) Biginelli condensation, oxidative dehydrogenation of the obtained 3,4-dihydropyrimidin-2(1H)-one to 2-hydroxypyrimidine, O-sulfonylation, and Suzuki-Miyaura C–C cross-coupling. Submission of these substrates to pyrimidine-N-atom-directed C–H arylation conditions led to regioselective phenylation at the ortho site(s) of the pyrimidine-C2-connected phenyl ring, revealing substituent-dependent electronic and steric effects. A focused library of 18 mono- and 10 bis-phenyl derivatives was generated. Its members exhibit interesting 3D and peripheral substitution features that render them promising for evaluation in drug discovery efforts.

Novel Antiplasmodial Compounds Leveraged with Multistage Potency against the Parasite Plasmodium falciparum: In Vitro and In Vivo Evaluations and Pharmacokinetic Studies

Hydroxyethylamine (HEA)-based novel compounds were synthesized and their activity against Plasmodium falciparum 3D7 was assessed, identifying a few hits without any apparent toxicity. Hits 5c and 5d also exhibited activity against resistant field strains, PfRKL-9 and PfC580Y. A single dose, 50 mg/Kg, of hits administered to the rodent parasite Plasmodium berghei ANKA exhibited up to 70% reduction in the parasite load. Compound 5d tested in combination with artesunate produced an additional antiparasitic effect with a prolonged survival period. Additionally, compound 5d showed 50% inhibition against hepatic P. berghei infection at 1.56 ± 0.56 μM concentration. This compound also considerably delayed the progression of transmission stages, ookinete and oocyst. Furthermore, the toxicity of 5d assessed in mice supported the normal liver and kidney functions. Altogether, HEA analogues (5a-m), particularly 5d, are nontoxic multistage antiplasmodial agents with therapeutic and transmission-blocking efficacy, along with favorable preliminary pharmacokinetic properties.

Novel Pyrimidine Derivatives as Potential Anticancer Agents: Synthesis, Biological Evaluation and Molecular Docking Study

In the present paper, new pyrimidine derivatives were designed, synthesized and analyzed in terms of their anticancer properties. The tested compounds were evaluated in vitro for their antitumor activity. The cytotoxic effect on normal human dermal fibroblasts (NHDF) was also determined. According to the results, all the tested compounds exhibited inhibitory activity on the proliferation of all lines of cancer cells (colon adenocarcinoma (LoVo), resistant colon adenocarcinoma (LoVo/DX), breast cancer (MCF-7), lung cancer (A549), cervical cancer (HeLa), human leukemic lymphoblasts (CCRF-CEM) and human monocytic (THP-1)). In particular, their feature stronger influence on the activity of P-glycoprotein of cell cultures resistant to doxorubicin than doxorubicin. Tested compounds have more lipophilic character than doxorubicin, which determines their affinity for the molecular target and passive transport through biological membranes. Moreover, the inhibitory potential against topoisomerase II and DNA intercalating properties of synthesized compounds were analyzed via molecular docking.

Synthesis, crystal structure and antibacterial studies of dihydropyrimidines and their regioselectively oxidized products

The syntheses and investigations of new biologically active derivatives of dihydropyrimidines by Biginelli reaction in the presence of copper triflate are reported. Due to the fact that salicylaldehyde and its derivatives under Biginelli reaction conditions can lead to the formation of 2 types of dihydropyrimidines, the influence of copper triflate on product formation was also investigated. In addition to this, regioselective oxidation of dihydropyrimidines was performed in the presence of cerium ammonium nitrate and novel oxidized dihydropyrimidines were obtained. Single crystals of some of them were obtained and as a result, the structures of them were investigated by X-ray diffraction method, which allows determining the presence of hydrogen bonds in their structures. In addition to this, the presence of hydrogen bonds in their structures affects the formation of the corresponding tautomer during oxidizing of dihydropyrimidines. Since dihydropyrimidines are claimed to be biologically active compounds, activities of the synthesized compounds were studied against Acinetobacter baumanii, Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae and Staphylococcus aureus bacteria.

In silico modeling of tetraoxane-8-aminoquinoline hybrids active against Plasmodium falciparum

Background

Quantitative structure-activity relationships (QSAR) is a technique that is used to produce a model that connects biological activities of compounds to their chemical structures, and molecular docking is a technique that reveals the binding mode and interactions between a drug and its target enzyme. These techniques have been successfully applied in the design and development of many drug candidates and herein were employed to build a model that could help in the development of more potent antimalaria drugs.

Results

Descriptors of the compounds were calculated using the PaDEL-Descriptor software, and Genetic Function Algorithm (GFA) was used to select descriptors and build the model. A robust and reliable model was generated and validated to have internal and external squared correlation coefficient (R2) of 0.9622 and 0.8191, respectively, adjusted squared correlation coefficient (Radj) of 0.9471, and leave-one-out (LOO) cross-validation coefficient (Q2cv) of 0.9223. The model revealed that the antiplasmodial activities of 1,2,4,5-tetraoxane-8-aminoquinoline hybrids depend on MATS3m, GATS8p, GATS8i, and RDF50s descriptors. MATS3m, GATS8i, and RDF50s influenced the antiplasmodial activities of the compounds positively while GATS8p negatively with the greatest influence. The docking result shows strong interactions between 1,2,4,5-tetraoxane-8-aminoquinoline hybrids and Plasmodium falciparum lactate dehydrogenase (pfLDH) with binding affinities ranging from − 6.3 to − 10.9 kcal/mol which were better than that of chloroquine (− 6.1 kcal/mol), suggesting that these compounds could be better inhibitors of pfLDH than chloroquine.

Conclusion

The results of this study could serve as a model for designing new potent 1,2,4,5-tetraoxane-8-aminoquinolines with better antiplasmodial activities for the development of highly active antimalaria drugs.

Molecular Design and Synthesis of Ivermectin Hybrids Targeting Hepatic and Erythrocytic Stages of Plasmodium Parasites

Ivermectin is a powerful endectocide, which reduces the incidence of vector-borne diseases. Besides its strong insecticidal effect on mosquito vectors of the disease, ivermectin inhibits Plasmodium falciparum sporogonic and blood stage development and impairs Plasmodium berghei development inside hepatocytes, both in vitro and in vivo. Herein, we present the first report on structural modification of ivermectin to produce dual-action molecular hybrids with good structure-dependent in vitro activity against both the hepatic and erythrocytic stages of P. berghei and P. falciparum infection, suggesting inclusion of ivermectin antimalarial hybrids in malaria control strategies. The most active hybrid displayed over threefold and 10-fold higher in vitro activity than ivermectin against hepatic and blood stage infections, respectively. Although an overwhelming insecticidal effect against Anopheles stephensi mosquitoes in laboratory conditions was not noticed, in silico docking analysis supports allosteric binding to glutamate-gated chloride channels similar to ivermectin.

Primaquine derivatives: Modifications of the terminal amino group

Numerous modifications of the well-known antimalarial drug primaquine, both at the quinoline ring and at the primary amino group, have been reported, mostly to obtain antimalarial agents with improved bioavailability, reduced toxicity and/or prolonged activity. Modifications of the terminal amino group were made with the main idea to prevent the metabolic pathway leading to inactive and toxic carboxyprimaquine (follow-on strategy), but also to get compounds with different activity (repurposing strategy). The modifications undertaken until 2009 were included in a review published in the same year. The present review covers various classes of primaquine N-derivatives with diverse biological profiles, prepared in the last decade by our research group as well as the others. We have summarized the synthetic procedures applied for their preparation and discussed the main biological results. Several hits for the development of novel antiplasmodial, anticancer, antimycobacterial and antibiofilm agents were identified.

Malaria Hybrids: A Chronological Evolution

Malaria, an upsetting malaise caused by a diverse class of Plasmodium species affects about 40% of the world's population. The distress associated with it has reached colossal scales owing to the development of resistance to most of the clinically available agents. Hence, the search for newer molecules for malaria treatment and cure is an incessant process. After the era of a single molecule for malaria treatment ended, there was an advent of combination therapy. However, lately there had been reports of the development of resistance to many of these agents as well. Subsequently, at present most of the peer groups working on malaria treatment aim to develop novel molecules, which may act on more than one biological processes of the parasite life cycle, and these scaffolds have been aptly termed as Hybrid Molecules or Double Drugs. These molecules may hold the key to hitherto unknown ways of showing a detrimental effect on the parasite. This review enlists a few of the recent advances made in malaria treatment by these hybrid molecules in a sequential manner.

Novel antimalarial chloroquine- and primaquine-quinoxaline 1,4-di-N-oxide hybrids: Design, synthesis, Plasmodium life cycle stage profile, and preliminary toxicity studies

Emergence of drug resistance and targeting all stages of the parasite life cycle are currently the major challenges in antimalarial chemotherapy. Molecular hybridization combining two scaffolds in a single molecule is an innovative strategy for achieving these goals. In this work, a series of novel quinoxaline 1,4-di-N-oxide hybrids containing either chloroquine or primaquine pharmacophores was designed, synthesized and tested against both chloroquine sensitive and multidrug resistant strains of Plasmodium falciparum. Only chloroquine-based compounds exhibited potent blood stage activity with compounds 4b and 4e being the most active and selective hybrids at this parasite stage. Based on their intraerythrocytic activity and selectivity or their chemical nature, seven hybrids were then evaluated against the liver stage of Plasmodium yoelii, Plasmodium berghei and Plasmodium falciparum infections. Compound 4b was the only chloroquine-quinoxaline 1,4-di-N-oxide hybrid with a moderate liver activity, whereas compound 6a and 6b were identified as the most active primaquine-based hybrids against exoerythrocytic stages, displaying enhanced liver activity against P. yoelii and P. berghei, respectively, and better SI values than primaquine. Although both primaquine-quinoxaline 1,4-di-N-oxide hybrids slightly reduced the infection of mosquitoes, they inhibited sporogony of P. berghei and compound 6a showed 92% blocking of transmission. In vivo liver efficacy assays revealed that compound 6a showed causal prophylactic activity affording parasitaemia reduction of up to 95% on day 4. Absence of genotoxicity and in vivo acute toxicity were also determined. These results suggest the approach of primaquine-quinoxaline 1,4-di-N-oxide hybrids as new potential dual-acting antimalarials for further investigation.

A review on diverse heterocyclic compounds as the privileged scaffolds in antimalarial drug discovery

The upward extend of malaria collectively with the emergence of resistance against predictable drugs has put enormous pressure on public health systems to introduce new malaria treatments. Heterocycles play an important role in the design and discovery of new malaria active compounds. Heterocyclic compounds have attracted significant attention for malaria treatment because of simplicity of parallelization and the examining power with regard to chemical space. Introduction of a variety of heterocyclic compounds have enabled to maintain the high levels of antimalarial potency observed for other more lipophilic analogues whilst improving the solubility and the oral bioavailability in pre-clinical species. In this review, we present an overview of recent literature to provide imminent into the applications of different heterocyclic scaffolds in fighting against malaria.

Platinum(ii)-chloroquine complexes are antimalarial agents against blood and liver stages by impairing mitochondrial function

Chloroquine is an antimalarial agent with strong activity against the blood stage of Plasmodium infection, but with low activity against the parasite's liver stage. In addition, the resistance to chloroquine limits its clinical use. The discovery of new molecules possessing multistage activity and overcoming drug resistance is needed. One possible strategy to achieve this lies in combining antimalarial quinolones with the pharmacological effects of transition metals. We investigated the antimalarial activity of four platinum(ii) complexes composed of chloroquine and phosphine ligands, denoted as WV-90, WV-92, WV-93 and WV-94. In comparison with chloroquine, the complexes were less potent against the chloroquine-sensitive 3D7 strain but they were as active as chloroquine in inhibiting the chloroquine-resistant W2 strain of P. falciparum. Regarding selectivity, the complexes WV-90 and WV-93 displayed higher indexes. Unlike chloroquine, the complexes act as irreversible parasiticidal agents against trophozoites and the WV-93 complex displayed activity against the hepatic stage of P. berghei. The in vivo suppression activity against P. berghei in the Peters 4 day test displayed by the complexes was similar to that of chloroquine. However, the efficacy in an established P. berghei infection in the Thompson test was superior for the WV-93 complex compared to chloroquine. The complexes' antimalarial mechanism of action is initiated by inhibiting the hemozoin formation. While chloroquine efficiently inhibits hemozoin, parasites treated with the platinum complexes display residual hemozoin crystals. This is explained since the interaction of the platinum complexes with ferriprotoporphyrin is weaker than that of chloroquine. However, the complexes caused a loss of mitochondrial integrity and subsequent reduction in mitochondrial activity, and their effects on mitochondria were more pronounced than those in the chloroquine-treated parasites. The dual effect of the platinum complexes may explain their activity against the hemozoin-lacking parasites (hepatic stage), where chloroquine has no activity. Our findings indicate that the platinum(ii)-chloroquine complexes are multifunctional antimalarial compounds and reinforce the importance of metal complexes in antimalarial drug discovery.

Endoperoxide-8-aminoquinoline hybrids as dual-stage antimalarial agents with enhanced metabolic stability

Hybrid compounds may play a critical role in the context of the malaria eradication agenda, which will benefit from therapeutic tools active against the symptomatic erythrocytic stage of Plasmodium infection, and also capable of eliminating liver stage parasites. To address the need for efficient multistage antiplasmodial compounds, a small library of 1,2,4,5-tetraoxane-8- aminoquinoline hybrids, with the metabolically labile C-5 position of the 8-aminoquinoline moiety blocked with aryl groups, was synthesized and screened for antiplasmodial activity and metabolic stability. The hybrid compounds inhibited development of intra-erythrocytic forms of the multidrug-resistant Plasmodium falciparum W2 strain, with EC₅₀ values in the nM range, and with low cytotoxicity against mammalian cells. The compounds also inhibited the development of P. berghei liver stage parasites, with the most potent compounds displaying EC₅₀ values in the low μM range. SAR analysis revealed that unbranched linkers between the endoperoxide and 8-aminoquinoline pharmacophores are most beneficial for dual antiplasmodial activity. Importantly, hybrids were significantly more potent than a 1:1 mixture of 8-aminoquinoline-tetraoxane, highlighting the superiority of the hybrid approach over the combination therapy. Furthermore, aryl substituents at C-5 of the 8-aminoquinoline moiety improve the compounds' metabolic stability when compared with their primaquine (i.e. C-5 unsubstituted) counterparts. Overall, this study reveals that blocking the quinoline C-5 position does not result in loss of dual-stage antimalarial activity, and that tetraoxane-8- aminoquinoline hybrids are an attractive approach to achieve elimination of exo- and intraerythrocytic parasites, thus with the potential to be used in malaria eradication campaigns.

Primaquine hybrids as promising antimycobacterial and antimalarial agents

Four series of primaquine (PQ) derivatives were screened for antitubercular and antiplasmodial activity: amides 1a-k, ureas 2a-s, semicarbazides 3a-c and bis-ureas 4a-u. Antimycobacterial activity of PQ derivatives against Mycobacterium tuberculosis (MTB), M. avium complex (MAC) and M. avium subsp. paratuberculosis (MAP) were evaluated in vitro and compared with PQ and the standard antitubercular drugs. In general, the PQ derivatives showed higher potency than the parent compound. Most of the compounds of series 1 and 2 showed high activity against MAP, comparable or even higher than the relevant drug ciprofloxacin, and weak or no activity against MTB and MAC. bis-Trifluoromethylated cinnamamide 1k showed low cytotoxicity and high activity against all three Mycobacterium species and their activities were comparable or slightly higher than those of the reference drugs. PQ urea derivatives with hydroxyl, halogen and trifluoromethyl substituents on benzene ring 2f-p exerted very strong antimycobacterial activity towards all tested mycobacteria, stronger than PQ and the relevant standard drug(s). Unfortunately, these compounds had relatively high cytotoxicity, except bromo 2l and trifluoromethyl 2m, 2n derivatives. In general, meta-substituted derivatives were more active than analogues para-derivatives. Phenyl ureas were also more active than cycloalkyl or hydroxyalkyl ureas. Semicarbazide 3a showed similar activity as PQ, while the other two semicarbazides were inactive. Bis-urea derivatives 4 were generally less active than the urea derivatives sharing the same scaffold, differing only in the spacer type. Out of 21 evaluated bis-urea derivatives, only p-Cl/m-CF₃ phenyl derivative 4p, benzhydryl derivatives 4t and 4u and bis-PQ derivative 4s showed high activity, higher than all three reference drugs. After comparison of activity and cytotoxicity, urea 2m and bis-urea 4u could be considered as the most promising agents. Antimalarial potential of PQ derivatives in vitro against the liver stage of P. berghei was evaluated as well. 3-(4-Chlorophenyl)-1-[({4-[(6-methoxyquinolin-8-yl)amino]pentyl}carbamoyl)amino]urea (4l) was the most active compound (IC₅₀ = 42 nM; cytotoxicity/activity ratio >2000). Our results bring new insights into development of novel anti-TB and antimalarial compounds.

Are Antimalarial Hybrid Molecules a Close Reality or a Distant Dream?

Emergence of drug-resistant Plasmodium falciparum strains has led to a situation of haste in the scientific and pharmaceutical communities. Hence, all their efforts are redirected toward finding alternative chemotherapeutic agents that are capable of combating multidrug-resistant parasite strains. In light of this situation, scientists have come up with the concept of hybridization of two or more active pharmacophores into a single chemical entity, resulting in "antimalarial hybrids." The approach has been applied widely for generation of lead compounds against deadly diseases such as cancer and AIDS, with a proven potential for use as novel drugs, but is comparatively new in the sphere of antimalarial drug discovery. A sudden surge has been evidenced in the number of studies on the design and synthesis of hybrids for treating malaria and may be regarded as proof of their potential advantages over artemisinin-based combination therapy (ACT). However, it is evident from recent studies that most of the potential advantages of antimalarial hybrids, such as lower toxicity, better pharmacokinetics, and easier formulation, have yet to be realized. A number of questions left unaddressed at present need to be answered before this approach can progress to the late stages of clinical development and prove their worth in the clinic. To the best of our knowledge, this compilation is the first attempt to shed light on the shortcomings that are surfacing as more and more studies on molecular hybridization of the active pharmacophores of known antimalarials are being published.

Hybrid molecules: The privileged scaffolds for various pharmaceuticals

The practice of polypharmacology is not a new concept but the approaches which are being adopted for administering the two or more drugs together are varied from time to time. Taking two or more drugs simultaneously, co-formulation of two or more active agents in a single tablet and development of hybrid molecular entities capable to modulate multiple targets are the three popular approaches for multidrug therapy. The simultaneous use of more than one drug for the chemotherapy of a single disease demands a lot of patient compliance. Hence the present form of polypharmacology is gaining popularity in the form of hybrid molecules (multiple ligand approach). From the last 1-2 decades, the synthesis of hybrid molecules by the combination of different biologically relevant moieties has been under constant escalation along with their evaluation as diverse range of pharmacological agents and as potent drugs. This review is focused on the biological potential of hybrid molecules with particular mention of those exhibiting anti-fungal, anti-tuberculosis, anti-malarial, anti-inflammatory and anti-cancer activities. A comparison of the drug potency of the hybrid molecules with their individual counterparts is discussed for quantifying the significance of the concept of molecular hybridisation.

Pyrimidine-based antimalarials: design strategies and antiplasmodial effects

The versatility in the design strategies of pyrimidine scaffold offer considerable opportunity for developing antimalarials capable of hitting different biological targets.