The high antimalarial activity we had found for 2-fluorohistidine (2FHIS) against Plasmodium falciparum in vitro could not be extended because the compound proved too toxic to owl monkeys, despite the fact that mice tolerate up to 500 mg/kg. Therefore, we have screened a large number of other substituted histidines (which we had synthesized for the first time) for in vitro antimalarial activity: 2-iodo showed very strong and 2-azido gave moderate activity. Neither 4-X-histidines nor 2,4-di-X-histidines show any activity, while 2-iodohistamine is somewhat active. To our surprise, the 2-chloro and 2-bromo analogues are inactive. Thus, the role of the iodine atom cannot be electronic but may be just the right size to plug the erythrocyte membrane diffusion hold present in infected cells. In contrast to 2-FHIS, 2-IHIS does not significantly retard protein synthesis while retarding maturation, a result supporting the proposed mechanism of action. Although 2-IHIS proved non-toxic to monkeys, it retarded growth of parasite for only 24 h. We suspected that the compound may be a substrate for a mammalian deiodinase but, in the course of a search for such a deiodinase, found that the iodine is rapidly removed by any synthetic or natural sulfhydryl species under physiological conditions. This loss of activity would not be observed in vitro because of the very low levels of sulfhydryl compounds present in the culture medium. The finding that the deiodination need not be enzyme-mediated greatly reduces the possibility of stabilizing the drug by bulky N-alkylation of the imidazole ring or by use of the D-histidine series. Another approach, therefore, involves efforts to replace the iodine atom by metabolically stable groups of comparable size: iPr, tBu, Ph, Bz, etc. The most direct route to 2-alkylhistidines is based on a procedure we had developed to synthesize 2-perfluoroalkylhistidines - ring closure of 1,2- diacylaminoethylenes. Initial attempts to apply the same scheme for the introduction of alkyl or aryl groups did not succeed because of decomposition at the high temperatures required. We are now utilizing specific catalysts in order to facilitate these ring closures at lower temperatures.
