host tissues and of the parasites showed that the radioactivity was concentrated largely in the mature flukes and in the eggs. Females absorbed more of the radioactive substance than did males. The level of radioactivity was appreciably higher in the deposited eggs than in the surrounding tissues, as is shown in the micro-autoradiogram of a liver section reproduced in Figure 2. There was a high concentration also in the mucous lining of the stomach and in the intestines. In the light of our findings, it may be assumed that the mode of action is as follows. Owing to the presence of the imidazolidinone ring [or of the perhydrodiazinone ring in (24)], the nitrothiazole derivative is selectively absorbed by the schistosomes and their eggs; the nitro group then exerts its influence on metabolism, perhaps by blocking a redox enzyme. This assumption is all the more plausible in view of the fact that metabolites and synthetic substances without the nitro group displayed no activity.
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