Antimalarial Efficacy of Hydroxyethylapoquinine (SN-119) and Its Derivatives

Quinine and other cinchona-derived alkaloids, although recently supplanted by the artemisinins (ARTs), continue to be important for treatment of severe malaria. Quinine and quinidine have narrow therapeutic indices, and a safer quinine analog is desirable, particularly with the continued threat of antimalarial drug resistance. Hydroxyethylapoquinine (HEAQ), used at 8 g a day for dosing in humans in the 1930s and halving mortality from bacterial pneumonias, was shown to cure bird malaria in the 1940s and was also reported as treatment for human malaria cases. Here we describe synthesis of HEAQ and its novel stereoisomer hydroxyethylapoquinidine (HEAQD) along with two intermediates, hydroxyethylquinine (HEQ) and hydroxyethylquinidine (HEQD), and demonstrate comparable but elevated antimalarial 50% inhibitory concentrations (IC₅₀) of 100 to 200 nM against Plasmodium falciparum quinine-sensitive strain 3D7 (IC₅₀, 56 nM). Only HEAQD demonstrated activity against quinine-tolerant P. falciparum strains Dd2 and INDO with IC₅₀s of 300 to 700 nM. HEQD had activity only against Dd2 with an IC₅₀ of 313 nM. In the lethal mouse malaria model Plasmodium berghei ANKA, only HEQD had activity at 20 mg/kg of body weight comparable to that of the parent quinine or quinidine drugs measured by parasite inhibition and 30-day survival. In addition, HEQ, HEQD, and HEAQ (IC₅₀ ≥ 90 μM) have little to no human ether-à-go-go-related gene (hERG) channel inhibition expressed in CHO cells compared to HEAQD, quinine, and quinidine (hERG IC₅₀s of 27, 42, and 4 μM, respectively). HEQD more closely resembled quinine in vitro and in vivo for Plasmodium inhibition and demonstrated little hERG channel inhibition, suggesting that further optimization and preclinical studies are warranted for this molecule.