Tzu-Shean Feng

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).

Antiplasmodial and cytotoxicity evaluation of 3-functionalized 2-azetidinone derivatives

3-Azido-, 3-amino- and 3-(1,2,3-triazol-1-yl)-β-lactams were synthesized and evaluated for their antiplasmodial activity against four strains of Plasmodium falciparum and KB cells for their cytotoxicity profiles. The presence of a cyclohexyl substituent at N-1 and a phenyl group on the triazole ring markedly improved the activity profiles of triazole-tethered β-lactam exhibiting IC(50) values of 1.13, 1.21 and 1.00 μM against 3D7, K1 and W2 strains respectively.

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.

Effects of highly active novel artemisinin–chloroquinoline hybrid compounds on β-hematin formation, parasite morphology and endocytosis in Plasmodium falciparum

4-Aminoquinolines were hybridized with artemisinin and 1,4-naphthoquinone derivatives via the Ugi-four-component condensation reaction, and their biological activities investigated. The artemisinin-containing compounds 6a-c and its salt 6c-citrate were the most active target compounds in the antiplasmodial assays. However, despite the potent in vitro activities, they also displayed cytotoxicity against a mammalian cell-line, and had lower therapeutic indices than chloroquine. Morphological changes in parasites treated with these artemisinin-containing hybrid compounds were similar to those observed after addition of artemisinin. These hybrid compounds appeared to share mechanism(s) of action with both chloroquine and artemisinin: they exhibited potent β-hematin inhibitory activities; they caused an increase in accumulation of hemoglobin within the parasites that was intermediate between the increase observed with artesunate and chloroquine; and they also appeared to inhibit endocytosis as suggested by the decrease in the number of transport vesicles in the parasites. No cross-resistance with chloroquine was observed for these hybrid compounds, despite the fact that they contained the chloroquinoline moiety. The hybridization strategy therefore appeared to be borrowing the best from both classes of antimalarials.

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.

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 IC(50) 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 IC(50) of 0.37 μM and the highest tumor specificity.

Aminopyrazolo[1,5-a]pyrimidines as potential inhibitors of Mycobacterium tuberculosis: Structure activity relationships and ADME characterization

Whole-cell high-throughput screening of a diverse SoftFocus library against Mycobacterium tuberculosis (Mtb) generated a novel aminopyrazolo[1,5-a]pyrimidine hit series. The synthesis and structure activity relationship studies identified compounds with potent antimycobacterial activity. The SAR of over 140 compounds shows that the 2-pyridylmethylamine moiety at the C-7 position of the pyrazolopyrimidine scaffold was important for Mtb activity, whereas the C-3 position offered a higher degree of flexibility. The series was also profiled for in vitro cytotoxicity and microsomal metabolic stability as well as physicochemical properties. Consequently liabilities to be addressed in a future lead optimization campaign have been identified.

2-Mercapto-Quinazolinones as Inhibitors of Type II NADH Dehydrogenase and Mycobacterium tuberculosis: Structure–Activity Relationships, Mechanism of Action and Absorption, Distribution, Metabolism, and Excretion Characterization

Mycobacterium tuberculosis (MTb) possesses two nonproton pumping type II NADH dehydrogenase (NDH-2) enzymes which are predicted to be jointly essential for respiratory metabolism. Furthermore, the structure of a closely related bacterial NDH-2 has been reported recently, allowing for the structure-based design of small-molecule inhibitors. Herein, we disclose MTb whole-cell structure–activity relationships (SARs) for a series of 2-mercapto-quinazolinones which target the ndh encoded NDH-2 with nanomolar potencies. The compounds were inactivated by glutathione-dependent adduct formation as well as quinazolinone oxidation in microsomes. Pharmacokinetic studies demonstrated modest bioavailability and compound exposures. Resistance to the compounds in MTb was conferred by promoter mutations in the alternative nonessential NDH-2 encoded by ndhA in MTb. Bioenergetic analyses revealed a decrease in oxygen consumption rates in response to inhibitor in cells in which membrane potential was uncoupled from ATP production, while inverted membrane vesicles showed mercapto-quinazolinone-dependent inhibition of ATP production when NADH was the electron donor to the respiratory chain. Enzyme kinetic studies further demonstrated noncompetitive inhibition, suggesting binding of this scaffold to an allosteric site. In summary, while the initial MTb SAR showed limited improvement in potency, these results, combined with structural information on the bacterial protein, will aid in the future discovery of new and improved NDH-2 inhibitors.