Novel 2H-isoquinolin-3-ones as antiplasmodial falcipain-2 inhibitors

A series of 1-aryl-6,7-disubstituted-2H-isoquinolin-3-ones (2–10) was synthesized and evaluated for their inhibition against Plasmodium falciparum cysteine protease falcipain-2, as well as against cultured P. falciparum strain FCBR parasites. All compounds displayed inhibitory activity against recombinant falcipain-2 and against in vitro cultured intraerythrocytic P. falciparum, with the exception of 9. The new compounds exhibited no selectivity against human cysteine proteases such as cathepsins B and L. The inhibitory activity of the synthesized compounds was also evaluated against another protozoal cysteine protease, namely rhodesain of Trypanosoma brucei rhodesiense.     

Synthesis,biological evaluation,hydration site thermodynamics,and chemical reactivity analysis of α-keto substituted peptidomimetics for the inhibition of Plasmodium falciparum

A new series of peptidomimetic pseudo-prolyl-homophenylalanylketones were designed, synthesized and evaluated for inhibition of the Plasmodium falciparum cysteine proteases falcipain-2 (FP-2) and falcipain-3 (FP-3). In addition, the parasite killing activity of these compounds in human blood-cultured P. falciparum was examined. Of twenty-two (22) compounds synthesized, one peptidomimetic comprising a homophenylalanine-based α-hydroxyketone linked Cbz-protected hydroxyproline (39) showed the most potency (IC₅₀ 80 nM against FP-2 and 60 nM against FP-3). In silico analysis of these peptidomimetic analogs offered important protein–ligand structural insights including the role, by WaterMap, of water molecules in the active sites of these protease isoforms. The pseudo-dipeptide 39 and related compounds may serve as a promising direction forward in the design of competitive inhibitors of falcipains for the effective treatment of malaria.     

Pyrido[1,2-a]pyrimidin-4-ones as antiplasmodial falcipain-2 inhibitors

Plasmodium falciparum cysteine protease falcipain-2 (FP-2) is a promising target for antimalarial chemotherapy and inhibition of this protease affects the growth of parasite adversely. A series of pyrido[1,2-a]pyrimidin-4-ones were synthesized and evaluated for their in vitro FP-2 inhibitory potential. Compounds (14,17) showed excellent FP-2 inhibition and can serve as lead compounds for further development of potent FP-2 inhibitors as potential antimalarial drugs.     

Aza vinyl sulfones: synthesis and evaluation as antiplasmodial agents

A series of novel aza vinyl sulfones were designed, synthesized in good yields and evaluated as antiplasmodial agents. Tested compounds did not show activity against papain or the Plasmodium falciparum cysteine protease falcipain-2. However, a number of the new compounds effectively inhibited the in vitro development of P. falciparum. Compounds containing a squaramide group were the most active, with IC₅₀ values between 0.95 and 4.5 μM, suggesting that these are potential lead compounds for the development of new antimalarial agents.     

Synthesis,antimalarial activity and cytotoxicity of 4-aminoquinoline–triazine conjugates

A series of 4-aminoquinoline–triazine conjugates with different substitution pattern have been synthesized and evaluated for their in vitro antimalarial activity against chloroquine-sensitive and resistant strains of Plasmodium falciparum. Compounds 16, 19, 28 and 35 exhibited promising antimalarial activity against both strains of P. falciparum. Cytotoxicity of these compounds was tested against three cell lines. Several compounds did not show any cytotoxicity up to a high concentration (48 μM), others exhibited mild toxicities but selective index for antimalarial activity was high for most of these conjugates.     

Design,synthesis and antiplasmodial activity of novel imidazole derivatives based on 7-chloro-4-aminoquinoline

A series of short chain 4-aminoquinoline-imidazole derivatives have been synthesized in one pot two step multicomponent reaction using van leusen standard protocol. The diethylamine function of chloroquine is replaced by substituted imidazole derivatives containing tertiary terminal nitrogen. All the synthesized compounds were screened against the chloroquine sensitive (3D7) and chloroquine resistant (K1) strains of Plasmodium falciparum. Some of the compounds (6, 8, 9 and 17) in the series exhibited comparable activity to CQ against K1 strain of P. falciparum. All the compounds displayed resistance factor between 0.09 and 4.57 as against 51 for CQ. Further, these analogues were found to form a strong complex with hematin and inhibit the β-hematin formation, therefore these compounds act via heme polymerization target.     

The Ionic and Hydrophobic Interactions Are Required for the Auto Activation of Cysteine Proteases of Plasmodium falciparum

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 are major hemoglobinases and potential antimalarial drug targets. Our previous studies demonstrated that these enzymes are equipped with specific domains for specific functions. Structural and functional analysis of falcipains showed that they have unique domains including a refolding domain and a hemoglobin binding domain. As with many proteases, falcipain-2 and falcipain-3 are synthesized as inactive zymogens. However, it is not known how these enzymes get activated for hemoglobin hydrolysis. In this study, we are presenting the first evidence that salt bridges and hydrophobic interactions are required for the auto activation of cysteine proteases of P.falciparum. To investigate the mechanism of activation of these enzymes, we expressed the wild type protein as well as different mutants in E.coli. Refolding was assessed by circular dichroism. Both CD and trans activation data showed that the wild type enzymes and mutants are rich in secondary structures with similar folds. Our study revealed that prodomain-mature domain of falcipain-2 and falcipain-3 interacts via salt bridges and hydrophobic interactions. We mutated specific residues of falcipain-2 and falcipain-3, and evaluated their ability to undergo auto processing. Mutagenesis result showed that two salt bridges (Arg ¹⁸⁵ - Glu ²²¹, Glu ²¹⁰ - Lys ⁴⁰³) in falcipain-2, and one salt bridge (Arg ²⁰²-Glu ²³⁸) in falcipain-3, play crucial roles in the activation of these enzymes. Further study revealed that hydrophobic interactions present both in falcipain-2 (Phe^214, Trp⁴⁴⁹ Trp ⁴⁵³) and falcipain-3 (Phe ²³¹ Trp ⁴⁵⁷ Trp ⁴⁶¹) also play important roles in the activation of these enzymes. Our results revealed the interactions involved in auto processing of two major hemoglobinases of malaria parasite.     

Synthesis and potent antiprotozoal activity of mono/di amidino 2-anilinobenzimidazoles versus Plasmodium falciparum and Trypanosoma brucei rhodesiense

A series of mono and dicationic new 2-anilinobenzimidazole carboxamidines were prepared in a four step process starting from 4-amino-3-nitrobenzonitrile and corresponding o-phenylenediamines. Their antiparasitic activity against Plasmodium falciparum (P. falciparum) and Trypanosoma brucei rhodesiense (T.b. rhodesiense) were evaluated in vitro. Some of the dicationic compounds (10,12,14) showed equal or very close activity against T.b. rhodesiense with melarsoprol and also showed promising activity against P. falciparum as compared to chloroquine. Among the monocationic derivatives compound 21 exhibited best inhibitory activity against P. falciparum.     

Antiplasmodial activity of hydroxyethylamine analogs: Synthesis,biological activity and structure activity relationship of plasmepsin inhibitors

Malaria, particularly in endemic countries remains a threat to the human health and is the leading the cause of mortality in the tropical and sub-tropical areas. Herein, we explored new C₂ symmetric hydroxyethylamine analogs as the potential inhibitors of Plasmodium falciparum (P. falciparum; 3D7) in in-vitro cultures. All the listed compounds were also evaluated against crucial drug targets, plasmepsin II (Plm II) and IV (Plm IV), enzymes found in the digestive vacuole of the P. falciparum. Analog 10f showed inhibitory activities against both the enzymes Plm II and Plm IV (K_i, 1.93?±?0.29?µM for Plm II; K_i, 1.99?±?0.05?µM for Plm IV). Among all these analogs, compounds 10g selectively inhibited the activity of Plm IV (K_i, 0.84?±?0.08?µM). In the in vitro screening assay, the growth inhibition of P. falciparum by both the analogs (IC₅₀, 2.27?±?0.95?µM for 10f; IC₅₀, 3.11?±?0.65?µM for 10g) displayed marked killing effect. A significant growth inhibition of the P. falciparum was displayed by analog 12c with IC₅₀ value of 1.35?±?0.85?µM, however, it did not show inhibitory activity against either Plms. The hemolytic assay suggested that the active compounds selectively inhibit the growth of the parasite. Further, potent analogs (10f and 12c) were evaluated for their cytotoxicity towards mammalian HepG2 and vero cells. The selectivity index (SI) values were noticed greater than 10 for both the analogs that suggested their poor toxicity. The present study indicates these analogs as putative lead structures and could serve as crucial for the development of new drug molecules.     

Designing novel inhibitors against falcipain-2 of Plasmodium falciparum

Coumarin containing pyrazoline derivatives have been synthesized and tested as inhibitors of in vitro development of a chloroquine-sensitive (MRC-02) and chloroquine-resistant (RKL-2) strain of Plasmodium falciparum and in vivo Plasmodium berghei malaria. Docking study was also done on cysteine protease falcipain-2 which showed that the binding pose of C-14 molecule and epoxysuccinate, inhibitor of falcipain-2, binds in the similar pattern. The most active antimalarial compound was 3-(1-benzoyl-5-(4-flurophenyl)-4,5-dihydro-1H-pyrazol-3yl)-7-(diethyamino)-2H-chromen-2-one C-14, with an IC₅₀ of 4.21?µg/ml provided complete protection to the infected mice at 24?mg/kg X 4?days respectively.     

Antiproliferative and antimalarial anthraquinones of Scutia myrtina from the Madagascar forest

Bioassay-guided fractionation of an ethanol extract of a Madagascar collection of the bark of Scutia myrtina led to the isolation of three new anthrone–anthraquinones, scutianthraquinones A, B and C (1–3), one new bisanthrone–anthraquinone, scutianthraquinone D (4), and the known anthraquinone, aloesaponarin I (5). The structures of all compounds were determined using a combination of 1D and 2D NMR experiments, including COSY, TOCSY, HSQC, HMBC, and ROESY sequences, and mass spectrometry. All the isolated compounds were tested against the A2780 human ovarian cancer cell line for antiproliferative activities, and against the chloroquine-resistant Plasmodium falciparum strains Dd2 and FCM29 for antiplasmodial activities. Compounds 1, 2 and 4 showed weak antiproliferative activities against the A2780 ovarian cancer cell line, while compounds 1–4 exhibited moderate antiplasmodial activities against P. falciparum Dd2 and compounds 1, 2, and 4 exhibited moderate antiplasmodial activities against P. falciparum FCM29.     

Synthesis,DNA binding,fluorescence measurements and antiparasitic activity of DAPI related diamidines

A novel series of extended DAPI analogues were prepared by insertion of either a carbon–carbon triple bond (16a–d) or a phenyl group (21a,b and 24) at position-2. The new amidines were evaluated in vitro against both Trypanosoma brucei rhodesiense (T. b. r.) and Plasmodium falciparum (P. f.). Five compounds (16a, 16b, 16d, 21a, 21b) exhibited IC₅₀ values against T. b. r. of 9 nM or less which is two to nine folds more effective than DAPI. The same five compounds exhibited IC₅₀ values against P. f. of 5.9 nM or less which is comparable to that of DAPI. The fluorescence properties of these new molecules were recorded, however; they do not offer any advantage over those of DAPI.     

Structure–activity relationship of antiparasitic and cytotoxic indoloquinoline alkaloids,and their tricyclic and bicyclic analogues

Based on the indoloquinoline alkaloids cryptolepine (1), neocryptolepine (2), isocryptolepine (3) and isoneocryptolepine (4), used as lead compounds for new antimalarial agents, a series of tricyclic and bicyclic analogues, including carbolines, azaindoles, pyrroloquinolines and pyrroloisoquinolines was synthesized and biologically evaluated. None of the bicyclic compounds was significantly active against the chloroquine-resistant strain Plasmodium falciparum K1, in contrast to the tricyclic derivatives. The tricyclic compound 2-methyl-2H-pyrido[3,4-b]indole (9), or 2-methyl-β-carboline, showed the best in vitro activity, with an IC₅₀ value of 0.45 μM against P. falciparum K1, without apparent cytotoxicity against L6 cells (SI > 1000). However, this compound was not active in the Plasmodium berghei mouse model. Structure–activity relationships are discussed and compared with related naturally occurring compounds.     

Antimalarial activity of azadipeptide nitriles

Azadipeptide nitriles—novel cysteine protease inhibitors—display structure-dependent antimalarial activity against both chloroquine-sensitive and chloroquine-resistant lines of cultured Plasmodium falciparum malaria parasites. Inhibition of parasite’s hemoglobin-degrading cysteine proteases was also investigated, revealing the azadipeptide nitriles as potent inhibitors of falcipain-2 and -3. A correlation between the cysteine protease-inhibiting activity and the antimalarial potential of the compounds was observed. These first generation azadipeptide nitriles represent a promising new class of compounds for antimalarial drug development.     

Synthesis and evaluation of 7-chloro-4-(piperazin-1-yl)quinoline-sulfonamide as hybrid antiprotozoal agents

A new series of 4-aminochloroquinoline based sulfonamides were synthesized and evaluated for antiamoebic and antimalarial activities. Out of the eleven compounds evaluated (F1–F11), two of them (F3 and F10) showed good activity against Entamoeba histolytica (IC₅₀ <5 μM). Three of the compounds (F5, F7 and F8) also displayed antimalarial activity against the chloroquine-resistant (FCR-3) strain of Plasmodium falciparum with IC₅₀ values of 2 μM. Compound F7, whose crystal structure was also determined, inhibited β-haematin formation more potently than quinine. To further understand the action of hybrid molecules F7 and F8, molecular docking was carried out against the homology model of P. falciparum enzyme dihydropteroate synthase (PfDHPS). The complexes showed that the inhibitors place themselves nicely into the active site of the enzyme and exhibit interaction energy which is in accordance with our activity profile data. Application of Lipinski ‘rule of five’ on all the compounds (F1–F11) suggested high drug likeness of F7 and F8, similar to quinine.     

Antiplasmodial and antitumor activity of dihydroartemisinin analogs derived via the aza-Michael addition reaction

A series of dihydroartemisinin derivatives were synthesized via an aza-Michael addition reaction to a dihydroartemisinin-based acrylate and were evaluated for antiplasmodial and antitumor activity. The target compounds showed excellent antiplasmodial activity, with dihydroartemisinin derivatives 5, 7, 9 and 13 exhibiting IC₅₀ values of ?10 nM against both D10 and Dd2 strains of Plasmodium falciparum. Derivative 4d was the most active against the HeLa cancer cell line, with an IC₅₀ of 0.37 μM and the highest tumor specificity.     

Selected gold compounds cause pronounced inhibition of Falcipain 2 and effectively block P. falciparum growth in vitro

A number of structurally diverse gold compounds were evaluated as possible inhibitors of Falcipain 2 (Fp2), a cysteine protease from P. falciparum that is a validated target for the development of novel antimalarial drugs. Remarkably, most tested compounds caused pronounced but reversible inhibition of Fp2 with K_i values falling in the micromolar range. Enzyme inhibition is basically ascribed to gold binding to catalytic active site cysteine. The same gold compounds were then tested for their ability to inhibit P. falciparum growth in vitro; important parasite growth inhibition was indeed observed. However, careful analysis of the two sets of data failed to establish any direct correlation between enzyme inhibition and reduction of P. falciparum growth suggesting that Fp2 inhibition represents just one of the various mechanisms through which gold compounds effectively antagonize P. falciparum replication.     

Novel spiro-sesquiterpene from the mushroom Anthracophyllum sp. BCC18695

A novel spiro-sesquiterpene, anthracophyllic acid (1), and a new aristolane sesquiterpene, anthracophyllone (2), were isolated from the mushroom Anthracophyllum sp. BCC18695, together with seven known compounds including aurisins A (3), G (4), K (5), nambinones A, C, axinysones A, and B. The relative configuration of 1 and the hitherto unknown absolute stereochemistry of 3 were determined based on X-ray spectroscopic data. Biological activities including antimalarial activity against Plasmodium falciparum K1 strain, antibacterial property against Bacillus cereus, and cytotoxicity against MCF-7, KB, NCI-H187, and Vero cells of the isolated compounds were also evaluated     

Functionalized hydroxyethylamine based peptide nanostructures as potential inhibitors of falcipain-3, an essential proteases of Plasmodium falciparum

Self-assembled peptide based nanostructures gained enough popularity due to their easy biocompatibility and numerous potential applications. An excellent model of self-assembly of hydroxyethylamine based peptide nanostructures was synthesized and characterized by DLS and TEM. Spherical nano structures of I and III were observed with particle size ～50 and ～80 nm, respectively. Further, I and III were screened against anti-malarial target, falcipain-3 (FP3), a crucial cysteine protease involved as a major hemoglobinase of Plasmodium falciparum. Interestingly, compound III completely inhibited the activity of FP3. The effective concentration (1.5 μM) of III found to be more potent than I. This biochemical result was substantiated by molecular-docking studies indicating III to be best inhibitor of FP3. This is the first report showing that bis hydroxethylamine based peptide nanostructures could be very effective inhibitor of malarial cysteine proteases.     

Antiplasmodial activity of new 4-aminoquinoline derivatives against chloroquine resistant strain

Emergence and spread of multidrug resistant strains of Plasmodium falciparum has severely limited the antimalarial chemotherapeutic options. In order to overcome the obstacle, a set of new side-chain modified 4-aminoquinolines were synthesized and screened against chloroquine-sensitive (3D7) and chloroquine-resistant (K1) strains of P. falciparum. The key feature of the designed molecules is the use of methylpiperazine linked α, β³- and γ-amino acids to generate novel side chain modified 4-aminoquinoline analogues. Among the evaluated compounds, 20c and 30 were found more potent than CQ against K1 and displayed a four-fold and a three-fold higher activity respectively, with a good selectivity index (SI = 5846 and 11,350). All synthesized compounds had resistance index between 1.06 and >14.13 as against 47.2 for chloroquine. Biophysical studies suggested that this series of compounds act on heme polymerization target.