Identification and In-Vitro ADME Assessment of a Series of Novel Anti-Malarial Agents Suitable for Hit-to-Lead Chemistry

Triage of a set of antimalaria hit compounds, identified through high throughput screening against the Chloroquine sensitive (3D7) and resistant (Dd2) parasite <i>Plasmodium falciparum</i> strains identified several novel chemotypes suitable for hit-to-lead chemistry investigation. The set was further refined through investigation of their <i>in vitro</i> ADME properties, which identified templates with good potential to be developed further as antimalarial agents. One example was profiled in an <i>in vivo</i> murine <i>Plasmodium berghei</i> model of malaria infection

Medicinal Chemistry Optimization of Antiplasmodial Imidazopyridazine Hits from High Throughput Screening of a SoftFocus Kinase Library: Part 2

On the basis of our recent results on a novel series of imidazopyridazine-based antimalarials, we focused on identifying compounds with improved aqueous solubility and hERG profile while maintaining metabolic stability and in vitro potency. Toward this objective, 41 compounds were synthesized and evaluated for antiplasmodial activity against NF54 (sensitive) and K1 (multidrug resistant) strains of the malaria parasite <i>Plasmodium falciparum</i> and evaluated for both aqueous solubility and metabolic stability. Selected compounds were tested for in vitro hERG activity and in vivo efficacy in the <i>P. berghei</i> mouse model. Several compounds were identified with significantly improved aqueous solubility, good metabolic stability, and a clean hERG profile relative to a previous frontrunner lead compound. A sulfoxide-based imidazopyridazine analog <b>45</b>, arising from a prodrug-like strategy, was completely curative in the <i>Plasmodium berghei</i> mouse model at 4 × 50 mg/kg po

Isoxazolopyrimidine-Based Inhibitors of Plasmodium falciparum Dihydroorotate Dehydrogenase with Antimalarial Activity

Malaria kills nearly 0.5 million people yearly and impacts the lives of those living in over 90 countries where it is endemic. The current treatment programs are threatened by increasing drug resistance. Dihydroorotate dehydrogenase (DHODH) is now clinically validated as a target for antimalarial drug discovery as a triazolopyrimidine class inhibitor (DSM265) is currently undergoing clinical development. We discovered a related isoxazolopyrimidine series in a phenotypic screen, later determining that it targeted DHODH. To determine if the isoxazolopyrimidines could yield a drug candidate, we initiated hit-to-lead medicinal chemistry. Several potent analogues were identified, including a compound that showed in vivo antimalarial activity. The isoxazolopyrimidines were more rapidly metabolized than their triazolopyrimidine counterparts, and the pharmacokinetic data were not consistent with the goal of a single-dose treatment for malaria

Structure–Activity-Relationship Studies around the 2‑Amino Group and Pyridine Core of Antimalarial 3,5-Diarylaminopyridines Lead to a Novel Series of Pyrazine Analogues with Oral in Vivo Activity

Replacement of the pyridine core of antimalarial 3,5-diaryl-2-aminopyridines led to the identification of a novel series of pyrazine analogues with potent oral antimalarial activity. However, other changes to the pyridine core and replacement or substitution of the 2-amino group led to loss of antimalarial activity. The 3,5-diaryl-2-aminopyrazine series showed impressive in vitro antiplasmodial activity against the K1 (multidrug resistant) and NF54 (sensitive) strains of Plasmodium falciparum in the nanomolar IC₅₀ range of 6–94 nM while also demonstrating good in vitro metabolic stability in human liver microsomes. In the Plasmodium berghei mouse model, this series generally exhibited good efficacy at low oral doses. One of the frontrunner compounds, <b>4</b>, displayed potent in vitro antiplasmodial activity with IC₅₀ values of 8.4 and 10 nM against the K1 and NF54 strains, respectively. When evaluated in P. berghei-infected mice, compound <b>4</b> was completely curative at an oral dose of 4 × 10 mg/kg

Medicinal Chemistry Optimization of Antiplasmodial Imidazopyridazine Hits from High Throughput Screening of a SoftFocus Kinase Library: Part 1

A novel class of imidazopyridazines identified from whole cell screening of a SoftFocus kinase library was synthesized and evaluated for antiplasmodial activity against K1 (multidrug resistant strain) and NF54 (sensitive strain). Structure–activity relationship studies led to the identification of highly potent compounds against both strains. Compound <b>35</b> was highly active (IC₅₀: K1 = 6.3 nM, NF54 = 7.3 nM) and comparable in potency to artesunate, and <b>35</b> exhibited 98% activity in the in vivo P. berghei mouse model (4-day test by Peters) at 4 × 50 mg/kg po. Compound <b>35</b> was also assessed against P. falciparum in the in vivo SCID mouse model where the efficacy was found to be more consistent with the in vitro activity. Furthermore, <b>35</b> displayed high (78%) rat oral bioavailability with good oral exposure and plasma half-life. Mice exposure at the same dose was 10-fold lower than in rat, suggesting lower oral absorption and/or higher metabolic clearance in mice

3,5-Diaryl-2-aminopyridines as a Novel Class of Orally Active Antimalarials Demonstrating Single Dose Cure in Mice and Clinical Candidate Potential

A novel class of orally active antimalarial 3,5-diaryl-2-aminopyridines has been identified from phenotypic whole cell high-throughput screening of a commercially available SoftFocus kinase library. The compounds were evaluated in vitro for their antiplasmodial activity against K1 (chloroquine and drug-resistant strain) and NF54 (chloroquine-susceptible strain) as well as for their cytotoxicity. Synthesis and structure–activity studies identified a number of promising compounds with selective antiplasmodial activity. One of these frontrunner compounds, <b>15</b>, was equipotent across the two strains (K1 = 25.0 nM, NF54 = 28.0 nM) and superior to chloroquine in the K1 strain (chloroquine IC₅₀ K1 = 194.0 nM). Compound <b>15</b> completely cured <i>Plasmodium berghei</i>-infected mice with a single oral dose of 30 mg/kg. Dose–response studies generated ED₅₀ and ED₉₀ values of 0.83 and 1.74 mg/kg for <b>15</b> in the standard four-dose Peters test. Pharmacokinetic studies in the rat indicated that this compound has good oral bioavailability (51% at 20 mg/kg) and a reasonable half-life (<i>t</i>_1/2 ∼ 7–8 h)

Structure–Activity Relationship Studies of Orally Active Antimalarial 3,5-Substituted 2‑Aminopyridines

In an effort to address potential cardiotoxicity liabilities identified with earlier frontrunner compounds, a number of new 3,5-diaryl-2-aminopyridine derivatives were synthesized. Several compounds exhibited potent antiplasmodial activity against both the multidrug resistant (K1) and sensitive (NF54) strains in the low nanomolar range. Some compounds displayed a significant reduction in potency in the hERG channel inhibition assay compared to previously reported frontrunner analogues. Several of these new analogues demonstrated promising in vivo efficacy in the Plasmodium berghei mouse model and will be further evaluated as potential clinical candidates. The SAR for in vitro antiplasmodial and hERG activity was delineated

2,4-Diaminothienopyrimidines as Orally Active Antimalarial Agents

A novel series of 2,4-diaminothienopyrimidines with potential as antimalarials was identified from whole-cell high-throughput screening of a SoftFocus ion channel library. Synthesis and structure–activity relationship studies identified compounds with potent antiplasmodial activity and low in vitro cytotoxicity. Several of these analogues exhibited in vivo activity in the Plasmodium berghei mouse model when administered orally. However, inhibition of the hERG potassium channel was identified as a liability for this series

Tetrahydro-2-naphthyl and 2‑Indanyl Triazolopyrimidines Targeting <i>Plasmodium falciparum</i> Dihydroorotate Dehydrogenase Display Potent and Selective Antimalarial Activity

Malaria persists as one of the most devastating global infectious diseases. The pyrimidine biosynthetic enzyme dihydroorotate dehydrogenase (DHODH) has been identified as a new malaria drug target, and a triazolopyrimidine-based DHODH inhibitor <b>1</b> (DSM265) is in clinical development. We sought to identify compounds with higher potency against <i>Plasmodium</i> DHODH while showing greater selectivity toward animal DHODHs. Herein we describe a series of novel triazolopyrimidines wherein the <i>p</i>-SF₅-aniline was replaced with substituted 1,2,3,4-tetrahydro-2-naphthyl or 2-indanyl amines. These compounds showed strong species selectivity, and several highly potent tetrahydro-2-naphthyl derivatives were identified. Compounds with halogen substitutions displayed sustained plasma levels after oral dosing in rodents leading to efficacy in the <i>P. falciparum</i> SCID mouse malaria model. These data suggest that tetrahydro-2-naphthyl derivatives have the potential to be efficacious for the treatment of malaria, but due to higher metabolic clearance than <b>1</b>, they most likely would need to be part of a multidose regimen

<i>N</i>‑Aryl-2-aminobenzimidazoles: Novel, Efficacious, Antimalarial Lead Compounds

From the phenotypic screening of the AstraZeneca corporate compound collection, <i>N</i>-aryl-2-aminobenzimidazoles have emerged as novel hits against the asexual blood stage of <i>Plasmodium falciparum</i> (<i>Pf</i>). Medicinal chemistry optimization of the potency against <i>Pf</i> and ADME properties resulted in the identification of <b>12</b> as a lead molecule. Compound <b>12</b> was efficacious in the <i>P. berghei</i> (<i>Pb</i>) 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 <i>Pb</i> malaria model. Compound <b>12</b> retains its potency against a panel of <i>Pf</i> isolates with known mechanisms of resistance. The fast killing observed in the <i>in vitro</i> parasite reduction ratio (PRR) assay coupled with the extended survival highlights the promise of this novel chemical class for the treatment of malaria