Here are my two cents:
The words selectivity, specificity, and sensitivity (derived from
Latin seligere, specificus, sensitivus), can be confusing terms as
they are often used synonymously in the medical literature. However,
they should not be used interchangeably as each represents a different
phenomenon:
The word selectivity describes a drug's ability to affect a particular
cell populationin preference to others. Selectivity is generally a
worthy property in a drug because a drug having high selectivity may
have a dramatic effect when there is a single agent that can be
targeted against the appropriate molecular-driver involved in the
pathogenesis of a disease. Selectivity is used to describe the
ability of a drug to affect a particular population, i.e., gene,
protein, signaling pathway, or cell, in preference to others. For
example a selective drug would have the ability to discriminate
between cell or receptor populations, and so affect only one cell
population, and thereby produce an event.
Specificity, a term most often confused with selectivity, is used to
describe the capacity of a drug to cause a particular action in a
population. For example, a drug of absolute specificity of action
might decrease or increase, a specific function of a given gene or
protein or cell type, but it must do either, not both.
Sensitivity is used to describe the capacity of a population, to
respond to a drug's ability, to stimulate that entity at a specified
dose. The smaller the dose required producing an effect, the more
sensitive is the responding system. (The word used to describe this
activity in the drug which is the cause of the population sensitivity,
is potency).