The Schizonticidal Activity of Hydroxynaphthoquinones against Plasmodium berghei Infections in Mice

Two decades ago 2-hydroxy-1,4-naphthoquinones were reported to have considerable suppressive and prophylactic activity against avian malaria. A few compounds were selected for clinical testing but were ineffective in human malarias presumably because of oxidation in the human body into inactive products. Recently homologs with longer n-alkyl side-chains than those of earlier compounds in the series have been synthesized and tested against rodent malarias. In four of the five series of 2-hydroxy-1,4-naphthoquinones reported in this study, peak activity occurred when the methylene groups adjacent to the alicyclic or terminal methyl group numbered 8 or 9. The higher members of the series have been demonstrated to possess not only schizonticidal activity at a dose level lower than that required for the older hydroxynaphthoquinones but also curative activity at high doses. The discovery of strains of Plasmodium falciparum refractory to either suppressive or therapeutic doses of 4-aminoquinolines (chloroquine, hydroxychloroquine, amodiaquine) and a-aminoacridines (quinacrine, propoquine) made reappraisal of malaria chemotherapy mandatory largely because of the known limitations of alternate drugs (chlorguanide, pyrimethamine, quinine, cycloguanil, primaquine, etc.). In an effort to discover new antimalarial drugs our attention was drawn to a group of 2-hydroxy-l,4-naphthoquinone compounds synthesized by Fieser and his associates at Harvard University's Chemical Laboratory as a part of the antimalarial survey conducted on a national scale during World War II. Two of these naphthoquinones (M-1916 and M-285) were shown to possess suppressive and curative effects against avian plasmodia (P. lophurae and P. gallinaceum), but they did not prove to have significant effects against P. vivax and P. falciparum infections of humans (Fieser et al., 1948a). These and other related naphthoquinone antimalarials received scant attention at that time because of the marked suppressive and therapeutic efficacy of 4aminoquinoline antimalarials at well tolerated dose levels. After the discovery of rodent plasmodial infections (P. berghei, P. vinckei, P. chabaudi), it became possible to screen drugs on a wide scale against mammalian rather than avian plasmodia. Several of the old and a few of the more recently synthesized hydroxynaphthoquinone antimalarials (Fieser et al., 1967a, Received for publication 12 March 1968. b) were screened and evaluated against bloodinduced rodent infections and were found to be effective following oral administration. The discovery by Yoeli and Most in 1960 of means for facilitating the transfer of P. berghei through the bites of infected anophelines under laboratory conditions made it possible for us to test the efficacy of the new hydroxynaphthoquinone derivatives against sporozoiteinduced as well as blood-induced P. berghei infections in mice. This paper describes the antimalarial screening procedures used and presents the activity of several compounds belonging to each of the various series of 3-substituted alkyl-2-hydroxy-1,4-naphthoquinones against blood-induced P. berghei infections in mice. A few selected derivatives from the various series of hydroxynaphthoquinones were administered to mice infected by injection of sporozoites of P. berghei. The details of these tests are the basis for a separate paper. MATERIALS AND METHODS Rodent plasmodia Four strains of Plasmodium berghei (NYU-2, NK-65, KSP-11, and subspecies yoelii), two other rodent plasmodia (P. vinckei and P. chabaudi), and a strain of P. berghei maintained under chloroquine pressure were used in our studies. In this paper the results of screening against two strains of P. berghei (NYU-2 and NK-65) will be presented. The NYU-2 strain was isolated on 6 January 1959 by Dr. Van Braeckel from infected salivary glands of Anopheles dureni caught in the vicinity of Kasapa (near Elizabethville, Congo). Dr. Yoeli of New York University obtained the strain from Professor Jadin of Belgium on 20 February 1959. Although originally a vigorous gametocyte producer, it gradually lost this ability through numer