Synthesis and In vitro Efficacy of Tetracyclic Benzothiazepines Against Blood-Stage Plasmodium falciparum and Liver-Stage P. berghei

Objective: A series of novel, substituted tetracyclic benzothiazepines were designed and prepared in an effort to optimize the potency of this chemical class against drug-resistant strains of the malaria parasite. Methods: Tetracyclic benzothiazepines bearing structural modification at seven distinct positions within the structure were synthesized in Knoevenagel condensation reactions followed by sequential intermolecular thio-Michael and then intramolecular imine formation reactions. Following purification and chemical characterization, the novel compounds were tested for in vitro efficacy against blood-stage P. falciparum and liver-stage P. berghei and also for in vivo efficacy against P. berghei. Results: Benzothiazepines bearing structural modification at the sulfur atom and at the three carbocycles within the molecule were successfully synthesized. The majority of analogs inhibited bloodstage P. falciparum with submicromolar IC₅₀ values. The potency of an 8-methoxy-substituted analog 12 exceeded that of chloroquine in all three P. falciparum strains tested. The parent benzothiazepine 1 possessed liver-stage activity, inhibiting P. berghei sporozoites infecting HepG2 cells with an IC₅₀ of 106.4 nM and an IC₉₀ of 408.9 nM, but failed to enhance the longevity of P. berghei infected mice compared to the controls. Compounds displayed modest toxicity toward HepG2 cells and were tolerated by mice at the highest dose tested, 640 mg/kg/dose once daily for three days. Conclusion: The tetracyclic benzothiazepine described, which inhibits P. berghei infected hepatic cells with an IC₅₀ of 106.4 nM, would appear to warrant further investigation. Optimization of ADME properties may be required since the most active analogs are probably excessively lipophilic.