Aloysius T. Nchinda

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, 15, was equipotent across the two strains (K1 = 25.0 nM, NF54 = 28.0 nM) and superior to chloroquine in the K1 strain (chloroquine IC(50) K1 = 194.0 nM). Compound 15 completely cured Plasmodium berghei-infected mice with a single oral dose of 30 mg/kg. Dose-response studies generated ED(50) and ED(90) values of 0.83 and 1.74 mg/kg for 15 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 (t(1/2) ∼ 7-8 h).

Novel Orally Active Antimalarial Thiazoles

An aminomethylthiazole pyrazole carboxamide lead 3 with good in vitro antiplasmodial activity [IC(50): 0.08 μM (K1, chloroquine and multidrug resistant strain) and 0.07 μM (NF54, chloroquine sensitive strain)] and microsomal metabolic stability was identified from whole cell screening of a SoftFocus kinase library. Compound 3 also exhibited in vivo activity in the P. berghei mouse model at 4 × 50 mg/kg administration via the oral route, showing 99.5% activity and 9 days survival and showed low in vitro cytotoxicity. Pharmacokinetic studies in rats revealed good oral bioavailability (51% at 22 mg/kg) with a moderate rate of absorption, reasonable half-life (t(1/2) 3 h), and high volume of distribution with moderately high plasma and blood clearance after IV administration. Toward toxicity profiling, 3 exhibited moderate potential to inhibit CYP1A2 (IC(50) = 1.5 μM) and 2D6 (IC(50) = 0.4 μM) as well as having a potential hERG liability (IC(50) = 3.7 μM).

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.

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 35 was highly active (IC50: K1 = 6.3 nM, NF54 = 7.3 nM) and comparable in potency to artesunate, and 35 exhibited 98% activity in the in vivo P. berghei mouse model (4-day test by Peters) at 4 × 50 mg/kg po. Compound 35 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, 35 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.

Development of Domain‐Selective Angiotensin I‐Converting Enzyme Inhibitors

Somatic angiotensin‐converting enzyme (ACE) is an essential component of the renin‐angiotensin system and consequently plays a key role in blood pressure and electrolyte homeostasis. Thus, ACE inhibitors are widely used in the treatment of cardiovascular disease, causing a decrease in the production of angiotensin II and an increase in the circulating vasodilator bradykinin. The ectodomain of ACE consists of two parts (N and C domains), each bearing an active site that differs in substrate and inhibitor specificity. Advances in the elucidation of the functional roles of these two domains and an expanded view of the renin‐angiotensin system underscore the need for the next generation of domain‐selective inhibitors with improved pharmacologic profiles. Moreover, recent breakthroughs in determining the crystal structure of testis ACE (identical to the C domain) and its homologue ACE2 provide new mechanistic insights into the interactions of ACE inhibitors and substrates with active site pockets. This review summarizes the structural basis and recent synthetic chemistry approaches to the development of novel domain‐selective inhibitors.

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.

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, 4, 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 4 was completely curative at an oral dose of 4 × 10 mg/kg.

Novel ketomethylene inhibitors of angiotensin I-converting enzyme (ACE): inhibition and molecular modelling.

Abstract Inhibition of angiotensin I-converting enzyme (ACE) has become an effective strategy in the treatment of hypertension and cardiovascular disease. Keto-ACE, a previously described C-domain selective ACE inhibitor, was used as the basis for the design, synthesis and molecular modelling of a series of novel ketomethylene derivatives for which ACE inhibition profiles and structural characterisation are reported. K i determinations indicated that the introduction of a bulky aromatic tryptophan at the P2′ position of keto-ACE significantly increased selectivity for the C-domain, while an aliphatic P2 Boc group conferred N-domain selectivity. These data were supported by the potential energies of the compounds docked with the C- and N-domains of ACE.

Identification of Fast-Acting 2,6-Disubstituted Imidazopyridines That Are Efficacious in the in Vivo Humanized Plasmodium falciparum NODscidIL2Rγnull Mouse Model of Malaria

Optimization of a chemical series originating from whole-cell phenotypic screening against the human malaria parasite, Plasmodium falciparum, led to the identification of two promising 2,6-disubstituted imidazopyridine compounds, 43 and 74. These compounds exhibited potent activity against asexual blood stage parasites that, together with their in vitro absorption, distribution, metabolism, and excretion (ADME) properties, translated to in vivo efficacy with clearance of parasites in the PfSCID mouse model for malaria within 48 h of treatment.

Characterization of angiotensin I-converting enzyme N-domain selectivity using positional-scanning combinatorial libraries of fluorescence resonance energy transfer peptides.

Abstract Somatic angiotensin I-converting enzyme (ACE) has two homologous active sites (N and C domains) that show differences in various biochemical properties. In a previous study, we described the use of positional-scanning synthetic combinatorial (PS-SC) libraries of fluorescence resonance energy transfer (FRET) peptides to define the ACE C-domain versus N-domain substrate specificity and developed selective substrates for the C-domain (Bersanetti et al., 2004). In the present work, we used the results from the PS-SC libraries to define the N-domain preferences and designed selective substrates for this domain. The peptide Abz-GDDVAK(Dnp)-OH presented the most favorable residues for N-domain selectivity in the P3 to P1′ positions. The fluorogenic analog Abz-DVAK(Dnp)-OH (Abz=ortho-aminobenzoic acid; Dnp=2,4-dinitrophenyl) showed the highest selectivity for ACE N-domain (kcat/Km=1.76 μm-1·s-1). Systematic reduction of the peptide length resulted in a tripeptide that was preferentially hydrolyzed by the C-domain. The binding of Abz-DVAK(Dnp)-OH to the active site of ACE N-domain was examined using a combination of conformational analysis and molecular docking. Our results indicated that the binding energies for the N-domain-substrate complexes were lower than those for the C-domain-substrate, suggesting that the former complexes are more stable.