The rationale for the design of structural analogues of a normal metabolite is that such compounds may interfere in the utilization or function of the metabolite. A compound which is effective in this respect may be called an antimetabolite. To be successful in chemotherapy of bacterial, viral, or tumor growth, an antimetabolite should adversely affect some vital metabolic reactions in the parasite or parasitic tissue without seriously endangering the host tissue. If a metabolic process of the offending growth is different from that of the host, it is likely that the metabolism or activity of...