The importance of a neurologic path may be quite different in a simple metazoan and a vertebrate. Pathogens and hosts may inactivate or activate drugs at widely different rates. Important receptors may be relatively protected by barriers that are absent in the pathogen. An example is the blood brain barrier, in vertebrates. Cell wall synthesis in bacteria and the folic acid system in a variety of organisms are examples of the value of drugs acting on unique enzymes.
Animals have no cell walls.
Therefore, drugs such as the beta-lactam antibiotics, i.e., penicillins and cephalosporins, that inhibit cell wall synthesis have the possibility of powerful actions on bacteria with minimal effects on animals. The folic acid system is less clearcut because it, or a closely related counterpart, is nearly ubiquitous. This system is crucial for inosine synthesis, hence DNA and RNA synthesis are severely curtailed when it is inhibited. Vertebrates must have folic acid in their diet whereas many bacteria must synthesize their own. In contrast to the other two major target categories, this one has no widely known clinically useful examples.