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Selectivity:
Selectivity:
The capacity or propensity of a drug to affect one cell population in preference to others, i.e., the ability of a drug to affect one kind of cell, and produce effects, in doses lower than those required to affect other cells. Selectivity can be measured or described by means of such numbers as the Therapeutic Index, or the Standardized Safety Margin: not infrequently one wishes to express selectivity of drug action with respect to two potentially beneficial effects, or two potentially toxic doses, or two toxic doses, instead of one each.
"Selectivity" is not to be confused with " potency"; a potent drug may be non-selective or a selective drug may be impotent. "Selectivity" is however, a measure of the relative potency of a drug in producing different effects.
Selectivity is generally a desirable property in a drug, e.g., it is desirable that an antibacterial agent affect parasites in doses too small to affect host cells. Sometimes, selectivity of action is virtually precluded by the nature of the drug, e.g., in the case of analogs of hormones that have many target cells or tissues. Sometimes selectivity of action for cells within an organism is not necessarily desirable, as in the case of certain economic poisons, i.e., pesticides, herbicides, rodenticides; even in this case, however, it is desirable to have a drug selective for cells of a particular species, and this criterion can most easily be met by drugs selective for certain cell types in the organisms of the target species.
"Selectivity" and "specificity" are, unfortunately, frequently used as synonyms for each other. They describe separate phenomena, each of which deserves an unambiguous name.
Cf. Specificity, Therapeutic Index
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