Spirocyclic chromanes exhibit antiplasmodial activities and inhibit all intraerythrocytic life cycle stages

Identification of new oxospiro chromane quinoline-carboxylate antimalarials that arrest parasite growth at ring stage

Malaria still threatens half the globe population despite successful Artemisinin-based combination therapy. One of the reasons for our inability to eradicate malaria is the emergence of resistance to current antimalarials. Thus, there is a need to develop new antimalarials targeting Plasmodium proteins. The present study reported the design and synthesis of 4, 6 and 7-substituted quinoline-3-carboxylates 9(a-o) and carboxylic acids 10(a-b) for the inhibition of Plasmodium N-Myristoyltransferases (NMTs) using computational biology tools followed by chemical synthesis and functional analysis. The designed compounds exhibited a glide score of -9.241 to -6.960 kcal/mol for PvNMT and -7.538 kcal/mol for PfNMT model proteins. Development of the synthesized compounds was established via NMR, HRMS and single crystal X-ray diffraction study. The synthesized compounds were evaluated for their in vitro antimalarial efficacy against CQ-sensitive Pf3D7 and CQ-resistant PfINDO lines followed by cell toxicity evaluation. In silico results highlighted the compound ethyl 6-methyl-4-(naphthalen-2-yloxy)quinoline-3-carboxylate (9a) as a promising inhibitor with a glide score of -9.084 kcal/mol for PvNMT and -6.975 kcal/mol for PfNMT with IC50 values of 6.58 µM for Pf3D7 line. Furthermore, compounds 9n and 9o exhibited excellent anti-plasmodial activity (Pf3D7 IC50 = 3.96, 6.71 µM, and PfINDO IC50 = 6.38, 2.8 µM, respectively). The conformational stability of 9a with the active site of the target protein was analyzed through MD simulation and was found concordance with in vitro results. Thus, our study provides scaffolds for the development of potent antimalarials targeting both Plasmodium vivax and Plasmodium falciparum.Communicated by Ramaswamy H. Sarma.

Design, Synthesis and Biological Evaluation of Novel Spiro-[chroman-2,4'-piperidin]-4-one Analogues as Anti-Tubercular Agents

A series of novel spiro-[chromane-2,4'-piperidine]-4(3 H )-one derivatives were designed, synthesized and structures were confirmed by analytical methods viz., 1 H NMR, 13 C NMR and Mass spectrometry. Synthesized derivatives were evaluated for their anti-mycobacterial activity against Mycobacterium tuberculosis ( Mtb ) H37Ra strain. Among all the evaluated compounds, PS08 exhibited significant inhibition with MIC value of 3.72 μM while MIC values of the remaining compounds ranged from 7.68 to 230.42 μM in comparison to the standard drug INH (MIC 0.09 μM). The two most active compounds however showed acute cytotoxicity towards the human MRC-5 lung fibroblast cell line. The in-silico ADMET profiles of the titled compounds were predicted and found within the prescribed limits of the Lipinski and Jorgenson rules. Molecular docking study of the significantly active compound ( PS08 ) was also carried out after performing validation in order to understand the putative binding position of the test ligand at the active site of selected target protein Mtb tyrosine phosphatase (PtpB).

Structure-activity and structure-property relationship studies of spirocyclic chromanes with antimalarial activity

Malaria is a prevalent and lethal disease. The fast emergence and spread of resistance to current therapies is a major concern and the development of a novel line of therapy that could overcome, the problem of drug resistance, is imperative. Screening of a set of compounds with drug/natural product-based sub-structural motifs led to the identification of spirocyclic chroman-4-one 1 with promising antimalarial activity against the chloroquine-resistant Dd2 and chloroquine-sensitive 3D7 strains of the parasite. Extensive structure-activity and structure-property relationship studies were conducted to identify the essential features necessary for its activity and properties.

Specific sub fractions from Terminalia mantaly (H. Perrier) extracts potently inhibit Plasmodium falciparum rings, merozoite egress and invasion

ETHNOPHARMACOLOGICAL RELEVANCE

Terminalia mantaly (H. Perrier) and Terminalia superba (Engl. & Diels) are sources of treatment for various diseases, including malaria and/or related symptoms in parts of Southwestern Cameroon. However, there is limited information on the extent of the antiplasmodial potential of their extracts.

AIM OF THE STUDY

The present study was designed to investigate the antiplasmodial potential of chromatographic sub fractions (SFs) from promising fractions of Terminalia mantaly (Tm) [Tmsb^wChl, the chloroform fraction from water extract of Tm, IC₅₀ (μg/mL) PfINDO: 0.56, Pf3D7: 1.12; SI > 357 (HEK/PfINDO) & 178 (HEK/Pf3D7)] and Terminalia superba (Ts) [Tsr^mEA, the ethyl acetate fraction from methanolic extract of Ts, IC₅₀ (μg/mL) PfINDO: 1.82, Pf3D7: 1.65; SI > 109 (HEK/PfINDO) & 121 (HEK/Pf3D7)] obtained from previous studies. The SFs were tested against Plasmodium falciparum 3D7 (Pf3D7-chloroquine sensitive) and INDO (PfINDO-chloroquine resistant) strains in culture. Also, the phytochemical profile of potent SFs was determined and finally, the inhibition of the asexual blood stages of Plasmodium falciparum by the SFs with the highest promise was assessed.

MATERIAL AND METHODS

Selected SFs were submitted to a second bio-guided fractionation using silica gel column chromatography. The partial phytochemical composition of potent antiplasmodial SFs was determined using gas chromatography coupled to mass spectrometry (GC-MS). The SYBR Green I-based fluorescence microtiter plate assay was used to monitor the growth of Plasmodium falciparum parasites in culture in the presence or absence of extracts. Microscopy and flow cytometry counting was used to assess the Plasmodium falciparum stage-specific inhibition and post-drug exposure growth suppression by highly potent extracts.

RESULTS

Twenty-one of the 39 SFs afforded from Tmsb^wChl showed activity (IC₅₀: 0.29-4.74 μg/mL) against both Pf3D7 and PfINDO strains. Of note, eight SFs namely, Tm25, Tm28-30, Tm34-36 and Tm38, exerted highly potent antiplasmodial activity (IC₅₀ < 1 μg/mL) with IC₅₀PfINDO: 0.41-0.84 μg/mL and IC₅₀Pf3D7: 0.29-0.68 μg/mL. They also displayed very high selectivity (50 < SI_PfINDO, SI_Pf3D7 > 344) on the two Plasmodial strains. On the other hand, 7 SFs (SFs Ts03, Ts04, Ts06, Ts09, Ts10, Ts12 and Ts13) from Tsr^mEA showed promising inhibitory potential against both parasite strains (IC₅₀: 2.01-5.14 μg/mL). Sub fraction Tm36 (IC₅₀PfINDO: 0.41 μg/mL, SI_PfINDO > 243; IC₅₀Pf3D7: 0.29 μg/mL, SI_Pf3D7 > 344) showed the highest promise. The GC-MS analysis of the 8 selected SFs led to the identification of 99 phytometabolites, with D-limonene (2), benzaldehyde (12), carvone (13), caryophyllene (35), hexadecanoic acid, methyl ester (74) and 9-octadecenoic acid, methyl ester (82) being the main constituents. Sub fractions Tm28, Tm29, Tm30, Tm36 and Tm38 inhibited all the three intraerythrocytic stages of P. falciparum, with strong potency against ring stage development, merozoite egress and invasion processes.

CONCLUSIONS

This study has identified highly potent antiplasmodial SFs from Terminalia mantaly with significant activity on the intraerythrocytic development of Plasmodium falciparum. These SFs qualify as promising sources of novel antiplasmodial lead compounds. Further purification and characterization studies are expected to unravel molecular targets in rings and merozoites.

Ring Distortion of Vincamine Leads to the Identification of Re-Engineered Antiplasmodial Agents

There is a significant need for new agents to combat malaria, which resulted in ∼409,000 deaths globally in 2019. We utilized a ring distortion strategy to create complex and diverse compounds from vincamine with the goal of discovering molecules with re-engineered biological activities. We found compound 8 (V3b) to target chloroquine-resistant Plasmodium falciparum Dd2 parasites (EC₅₀ = 1.81 ± 0.09 μM against Dd2 parasites; EC₅₀ > 40 μM against HepG2 cells) and established structure-activity relationships for 25 related analogues. New analogue 30 (V3ss, Dd2, EC₅₀ = 0.25 ± 0.004 μM; HepG2, EC₅₀ > 25 μM) was found to demonstrate the most potent activity, which prevents exit on the parasite from the schizont stage of intraerythrocytic development and requires >24 h to kill P. falciparum Dd2 cells. These findings demonstrate the potential that vincamine ring distortion has toward the discovery of novel antimalarial agents and other therapies significant to human health.

Synthesis and Structural Characterization of 1-(E-1-Arylpropenon-3-yl)-3,4,6-tri-O-benzyl-d-glucals and Their Transformation into Pentasubstituted (2R,3S,4R)-Chromanes via Pd-Catalyzed Cross Dehydrogenative Coupling Reaction

We have developed an efficient methodology for the synthesis of (2R,3S,4R)-2-hydroxymethyl-3,4-dihydroxy-6-aryl-7-aroylchromanes in which the chirality at the C-2, C-3, and C-4 positions is being drawn from C-glucopyranosyl aldehyde, which in turn can be efficiently synthesized from d-glucose. Thus, the synthesis starts with the transformation of sugar aldehyde into 1-(E-1-arylpropenon-3-yl)-3,4,6-tri-O-benzyl-d-glucals using Claisen-Schmidt type condensation reaction with different acetophenones and then to 1,2-disubstituted glucals via Pd(II)-catalyzed cross dehydrogenative coupling reaction, which in turn has been efficiently converted into (2R,3S,4R)-chromanes via 6π-electrocyclization and in situ dehydrogenative aromatization.

Antiplasmodial Compounds from Deep-Water Marine Invertebrates

Novel drug leads for malaria therapy are urgently needed because of the widespread emergence of resistance to all available drugs. Screening of the Harbor Branch enriched fraction library against the Plasmodium falciparum chloroquine-resistant strain (Dd2) followed by bioassay-guided fractionation led to the identification of two potent antiplasmodials; a novel diterpene designated as bebrycin A (1) and the known C21 degraded terpene nitenin (2). A SYBR Green I assay was used to establish a Dd2 EC₅₀ of 1.08 ± 0.21 and 0.29 ± 0.02 µM for bebrycin A and nitenin, respectively. Further analysis was then performed to assess the stage specificity of the inhibitors antiplasmodial effects on the Dd2 intraerythrocytic life cycle. Exposure to bebrycin A was found to block parasite maturation at the schizont stage if added any time prior to late schizogony at 42 hours post invasion, (HPI). In contrast, early life cycle exposure to nitenin (prior to 18 HPI) was identified as crucial to parasite inhibition, suggesting nitenin may target the maturation of the parasite during the transition from ring to early trophozoite (6–18 HPI), a novel property among known antimalarials.

Synthesis, Structure-Activity Relationship, and Antimalarial Efficacy of 6-Chloro-2-arylvinylquinolines

There is an urgent need to develop new efficacious antimalarials to address the emerging drug-resistant clinical cases. Our previous phenotypic screening identified styrylquinoline UCF501 as a promising antimalarial compound. To optimize UCF501, we herein report a detailed structure-activity relationship study of 2-arylvinylquinolines, leading to the discovery of potent, low nanomolar antiplasmodial compounds against a Plasmodium falciparum CQ-resistant Dd2 strain, with excellent selectivity profiles (resistance index < 1 and selectivity index > 200). Several metabolically stable 2-arylvinylquinolines are identified as fast-acting agents that kill asexual blood-stage parasites at the trophozoite phase, and the most promising compound 24 also demonstrates transmission blocking potential. Additionally, the monophosphate salt of 24 exhibits excellent in vivo antimalarial efficacy in the murine model without noticeable toxicity. Thus, the 2-arylvinylquinolines represent a promising class of antimalarial drug leads.

Discovery of fast-acting dual-stage antimalarial agents by profiling pyridylvinylquinoline chemical space via copper catalyzed azide-alkyne cycloadditions

To identity fast-acting, multistage antimalarial agents, a series of pyridylvinylquinoline-triazole analogues have been synthesized via CuAAC. Most of the compounds display significant inhibitory effect on the drug-resistant malarial Dd2 strain at low submicromolar concentrations. Among the tested analogues, compound 60 is the most potent molecule with an EC₅₀ value of 0.04 ± 0.01 μM. Our current study indicates that compound 60 is a fast-acting antimalarial compound and it demonstrates stage specific action at the trophozoite phase in the P. falciparum asexual life cycle. In addition, compound 60 is active against both early and late stage P. falciparum gametocytes. From a mechanistic perspective, compound 60 shows good activity as an inhibitor of β-hematin formation. Collectively, our findings suggest that fast-acting agent 60 targets dual life stages of the malarial parasites and warrant further investigation of pyridylvinylquinoline hybrids as new antimalarials.

Re-Engineering of Yohimbine's Biological Activity through Ring Distortion: Identification and Structure-Activity Relationships of a New Class of Antiplasmodial Agents

Select natural products are ideal starting points for ring distortion, or the dramatic altering of inherently complex molecules through short synthetic pathways, to generate an array of novel compounds with diverse skeletal architectures. A major goal of our ring distortion approach is to re-engineer the biological activity of indole alkaloids to identify new compounds with diverse biological activities in areas of significance to human health and medicine. In this study, we re-engineered the biological activity of the indole alkaloid yohimbine through ring rearrangement and ring cleavage synthesis pathways to discover new series of antiplasmodial agents. One new compound, Y7j, was found to demonstrate good potency against chloroquine-resistant Plasmodium falciparum Dd2 cells (EC₅₀ = 0.33 μM) without eliciting cytotoxicity against HepG2 cells (EC₅₀ > 40 μM). Y7j demonstrated stage-specific action against parasites at the late ring/trophozoite stage. A series of analogues was synthesized to gain structure-activity relationship insights, and we learned that both benzyl groups of Y7j are required for activity and fine-tuning of antiplasmodial activities could be accomplished by changing substitution patterns on the benzyl moieties. This study demonstrates the potential for ring distortion to drive new discoveries and change paradigms in chemical biology and drug discovery.

DeepMalaria: Artificial Intelligence Driven Discovery of Potent Antiplasmodials

Antimalarial drugs are becoming less effective due to the emergence of drug resistance. Resistance has been reported for all available malaria drugs, including artemisinin, thus creating a perpetual need for alternative drug candidates. The traditional drug discovery approach of high throughput screening (HTS) of large compound libraries for identification of new drug leads is time-consuming and resource intensive. While virtual in silico screening is a solution to this problem, however, the generalization of the models is not ideal. Artificial intelligence (AI), utilizing either structure-based or ligand-based approaches, has demonstrated highly accurate performances in the field of chemical property prediction. Leveraging the existing data, AI would be a suitable alternative to blind-search HTS or fingerprint-based virtual screening. The AI model would learn patterns within the data and help to search for hit compounds efficiently. In this work, we introduce DeepMalaria, a deep-learning based process capable of predicting the anti-Plasmodium falciparum inhibitory properties of compounds using their SMILES. A graph-based model is trained on 13,446 publicly available antiplasmodial hit compounds from GlaxoSmithKline (GSK) dataset that are currently being used to find novel drug candidates for malaria. We validated this model by predicting hit compounds from a macrocyclic compound library and already approved drugs that are used for repurposing. We have chosen macrocyclic compounds as these ligand-binding structures are underexplored in malaria drug discovery. The in silico pipeline for this process also consists of additional validation of an in-house independent dataset consisting mostly of natural product compounds. Transfer learning from a large dataset was leveraged to improve the performance of the deep learning model. To validate the DeepMalaria generated hits, we used a commonly used SYBR Green I fluorescence assay based phenotypic screening. DeepMalaria was able to detect all the compounds with nanomolar activity and 87.5% of the compounds with greater than 50% inhibition. Further experiments to reveal the compounds’ mechanism of action have shown that not only does one of the hit compounds, DC-9237, inhibits all asexual stages of Plasmodium falciparum, but is a fast-acting compound which makes it a strong candidate for further optimization.

4-Nitro styrylquinoline is an antimalarial inhibiting multiple stages of Plasmodium falciparum asexual life cycle

Drugs against malaria are losing their effectiveness because of emerging drug resistance. This underscores the need for novel therapeutic options for malaria with mechanism of actions distinct from current antimalarials. To identify novel pharmacophores against malaria we have screened compounds containing structural features of natural products that are pharmacologically relevant. This screening has identified a 4-nitro styrylquinoline (SQ) compound with submicromolar antiplasmodial activity and excellent selectivity. SQ exhibits a cellular action distinct from current antimalarials, acting early on malaria parasite's intraerythrocytic life cycle including merozoite invasion. The compound is a fast-acting parasitocidal agent and also exhibits curative property in the rodent malaria model when administered orally. In this report, we describe the synthesis, preliminary structure-function analysis, and the parasite developmental stage specific action of the SQ scaffold.