Differences may be in pharmacodynamic or pharmacokinetic properties. The majority of drugs used clinically achieve whatever specificity they exhibit by a mechanism in this category. Conceiving of new drugs that achieve specificity by a pharmacodynamic or pharmacokinetic difference between host and parasite is extremely difficult and tends to be done on an empirical basis through drug screening. This is because key differences between organisms cannot be always be predicted with current levels of knowledge. However, once a drug is found that makes a difference known, we can exploit the knowledge with rational attempts to design a more effective drug that exploits that difference.
Thiamine is an important coenzyme in carbohydrate metabolism, the main energy source of coccidia.
Amprolium, a thiamine analog, inhibits the transport of thiamine into the protozoan, competitively, resulting in its death. Specificity is achieved by the fact that cell division occurs in a higher proportion of cancer cells than of normal cells. Tubulin has both alpha and beta subunits. The beta subunits are highly conserved and are similar in all species. In contrast, the alpha subunits vary dramatically across species lines.