$CWNM For optimal effectiveness, plasma concentrations of many drugs must reside for the maximal possible duration within an often narrow therapeutic range, defined at the upper end by the maximally-tolerated level and at the lower end by the minimally-effective level of the drug. Plasma levels above the upper limit of the range will elicit the occurrence of toxic manifestations, whereas levels below the lower limit of the range fall below the targeted therapeutic activity. Conventional drug delivery systems typically provide for a rapid release of the compound into the circulation over a relatively short duration of time, following first-order rate kinetics.