Abhishek Srivastava

Generation of quinolone antimalarials targeting the Plasmodium falciparum mitochondrial respiratory chain for the treatment and prophylaxis of malaria

There is an urgent need for new antimalarial drugs with novel mechanisms of action to deliver effective control and eradication programs. Parasite resistance to all existing antimalarial classes, including the artemisinins, has been reported during their clinical use. A failure to generate new antimalarials with novel mechanisms of action that circumvent the current resistance challenges will contribute to a resurgence in the disease which would represent a global health emergency. Here we present a unique generation of quinolone lead antimalarials with a dual mechanism of action against two respiratory enzymes, NADH:ubiquinone oxidoreductase (Plasmodium falciparum NDH2) and cytochrome bc1. Inhibitor specificity for the two enzymes can be controlled subtly by manipulation of the privileged quinolone core at the 2 or 3 position. Inhibitors display potent (nanomolar) activity against both parasite enzymes and against multidrug-resistant P. falciparum parasites as evidenced by rapid and selective depolarization of the parasite mitochondrial membrane potential, leading to a disruption of pyrimidine metabolism and parasite death. Several analogs also display activity against liver-stage parasites (Plasmodium cynomolgi) as well as transmission-blocking properties. Lead optimized molecules also display potent oral antimalarial activity in the Plasmodium berghei mouse malaria model associated with favorable pharmacokinetic features that are aligned with a single-dose treatment. The ease and low cost of synthesis of these inhibitors fulfill the target product profile for the generation of a potent, safe, and inexpensive drug with the potential for eventual clinical deployment in the control and eradication of falciparum malaria.

Identification, design and biological evaluation of heterocyclic quinolones targeting Plasmodium falciparum type II NADH:quinone oxidoreductase (PfNDH2).

A program was undertaken to identify hit compounds against NADH:ubiquinone oxidoreductase (PfNDH2), a dehydrogenase of the mitochondrial electron transport chain of the malaria parasite Plasmodium falciparum. PfNDH2 has only one known inhibitor, hydroxy-2-dodecyl-4-(1H)-quinolone (HDQ), and this was used along with a range of chemoinformatics methods in the rational selection of 17 000 compounds for high-throughput screening. Twelve distinct chemotypes were identified and briefly examined leading to the selection of the quinolone core as the key target for structure–activity relationship (SAR) development. Extensive structural exploration led to the selection of 2-bisaryl 3-methyl quinolones as a series for further biological evaluation. The lead compound within this series 7-chloro-3-methyl-2-(4-(4-(trifluoromethoxy)benzyl)phenyl)quinolin-4(1H)-one (CK-2-68) has antimalarial activity against the 3D7 strain of P. falciparum of 36 nM, is selective for PfNDH2 over other respiratory enzymes (inhibitory IC50 against PfNDH2 of 16 nM), and demonstrates low cytotoxicity and high metabolic stability in the presence of human liver microsomes. This lead compound and its phosphate pro-drug have potent in vivo antimalarial activity after oral administration, consistent with the target product profile of a drug for the treatment of uncomplicated malaria. Other quinolones presented (e.g., 6d, 6f, 14e) have the capacity to inhibit both PfNDH2 and P. falciparum cytochrome bc1, and studies to determine the potential advantage of this dual-targeting effect are in progress.

Quantification of rifampicin in human plasma and cerebrospinal fluid by a highly sensitive and rapid liquid chromatographic-tandem mass spectrometric method

A highly sensitive and rapid liquid chromatography tandem mass spectrometry (LC–MS/MS) method has been developed to measure the levels of the antitubercular drug rifampicin (RIF) in human plasma and cerebrospinal fluid (CSF). The analyte and internal standard (IS) were isolated from plasma and CSF by a simple organic solvent based precipitation of proteins followed by centrifugation. Detection was carried out by electrospray positive ionization mass spectrometry in the multiple-reaction monitoring (MRM) mode. The assay was linear in the concentration range 25–6400 ng/mL with intra- and inter-day precision of <7% and <8%, respectively. The validated method was applied to the study of RIF pharmacokinetics in human CSF and plasma over 25 h period after a 10 mg/kg oral dose.

N-Aryl-2-aminobenzimidazoles: Novel, Efficacious, Antimalarial Lead Compounds

From the phenotypic screening of the AstraZeneca corporate compound collection, N-aryl-2-aminobenzimidazoles have emerged as novel hits against the asexual blood stage of Plasmodium falciparum (Pf). Medicinal chemistry optimization of the potency against Pf and ADME properties resulted in the identification of 12 as a lead molecule. Compound 12 was efficacious in the P. berghei (Pb) model of malaria. This compound displayed an excellent pharmacokinetic profile with a long half-life (19 h) in rat blood. This profile led to an extended survival of animals for over 30 days following a dose of 50 mg/kg in the Pb malaria model. Compound 12 retains its potency against a panel of Pf isolates with known mechanisms of resistance. The fast killing observed in the in vitro parasite reduction ratio (PRR) assay coupled with the extended survival highlights the promise of this novel chemical class for the treatment of malaria.

An Endoperoxide-Based Hybrid Approach to Deliver Falcipain Inhibitors Inside Malaria Parasites

The emergence of artemisinin‐resistant Plasmodium falciparum malaria in Southeast Asia has reinforced the urgent need to discover novel chemotherapeutic strategies to treat and control malaria. To address this problem, we prepared a set of dual‐acting tetraoxane‐based hybrid molecules designed to deliver a falcipain‐2 (FP‐2) inhibitor upon activation by iron(II) in the parasite digestive vacuole. These hybrids are active in the low nanomolar range against chloroquine‐sensitive and chloroquine‐resistant P. falciparum strains. We also demonstrate that in the presence of FeBr2 or within infected red blood cells, these molecules fragment to release falcipain inhibitors with nanomolar protease inhibitory activity. Molecular docking studies were performed to better understand the molecular interactions established between the tetraoxane‐based hybrids and the cysteine protease binding pocket residues. Our results further indicate that the intrinsic activity of the tetraoxane partner compound can be masked, suggesting that a tetraoxane‐based delivery system offers the potential to attenuate the off‐target effects of known drugs.

Triaminopyrimidine is a fast-killing and long-acting antimalarial clinical candidate

The widespread emergence of Plasmodium falciparum (Pf) strains resistant to frontline agents has fuelled the search for fast-acting agents with novel mechanism of action. Here, we report the discovery and optimization of novel antimalarial compounds, the triaminopyrimidines (TAPs), which emerged from a phenotypic screen against the blood stages of Pf. The clinical candidate (compound 12) is efficacious in a mouse model of Pf malaria with an ED99 <30 mg kg−1 and displays good in vivo safety margins in guinea pigs and rats. With a predicted half-life of 36 h in humans, a single dose of 260 mg might be sufficient to maintain therapeutic blood concentration for 4–5 days. Whole-genome sequencing of resistant mutants implicates the vacuolar ATP synthase as a genetic determinant of resistance to TAPs. Our studies highlight the potential of TAPs for single-dose treatment of Pf malaria in combination with other agents in clinical development.

Aminoazabenzimidazoles, a Novel Class of Orally Active Antimalarial Agents

Whole-cell high-throughput screening of the AstraZeneca compound library against the asexual blood stage of Plasmodium falciparum (Pf) led to the identification of amino imidazoles, a robust starting point for initiating a hit-to-lead medicinal chemistry effort. Structure-activity relationship studies followed by pharmacokinetics optimization resulted in the identification of 23 as an attractive lead with good oral bioavailability. Compound 23 was found to be efficacious (ED90 of 28.6 mg·kg(-1)) in the humanized P. falciparum mouse model of malaria (Pf/SCID model). Representative compounds displayed a moderate to fast killing profile that is comparable to that of chloroquine. This series demonstrates no cross-resistance against a panel of Pf strains with mutations to known antimalarial drugs, thereby suggesting a novel mechanism of action for this chemical class.

Second generation analogues of RKA182: synthetic tetraoxanes with outstanding in vitro and in vivo antimalarial activities

A series of polar dispiro-1,2,4,5-tetraoxanes have been designed and synthesized by parallel synthesis. From this series, endoperoxides with activity as low as 0.2 nM have been obtained and representatives of this group have excellent oral activities in the P. bergheiANKA mouse model of malaria.

Identification and Mitigation of a Reactive Metabolite Liability Associated with Aminoimidazoles

Reactive metabolites (RMs) have been implicated as causal factors in many drug-associated idiosyncratic toxicities. This study aims at identification and mitigation of an RM liability associated with aminoimidazole and amino(aza)benzimidazole structural motifs from an antimalarial project. Nineteen compounds with different structural modifications were studied in rat and human liver microsomes using glutathione (GSH) and N-acetyl cysteine (NAC) as trapping agents for RM. Metabolite profiling of aminoimidazole compounds in initial studies revealed the presence of dihydrodiol metabolites suggestive of reactive epoxide precursors, confirmed by the identification of a dihydrohydroxy GSH conjugate in GSH supplemented incubations. Substitution of methyl group at a potential site of metabolism blocked the epoxidation; however, formation of an imine-methide RM was suspected. Masking the site of metabolism via benzimidazole and 4/7-azabenzimidazole resulted in the possible formation of quinone-imine intermediates as a product of bioactivation. Further, substitutions with electron withdrawing groups and steric crowding did not address this liability. Mitigation of bioactivation was achieved with 5/6-azabenzimidazole and with CF3 substitution at the 6-position of the 7-azabenzimidazole ring. Moreover, compounds devoid of imidazole -NH2 do not undergo bioactivation. This study, therefore, establishes aminoimidazole and amino(aza)benzimidazoles as potential toxicophores and describes ways to mitigate this bioactivation liability by chemical modification.