= Drug delivery
<Drug delivery> concerns how a therapeutic substance is presented to a biological system: where it becomes available, over what time and in what form. Formulation, a carrier material and the route of administration can change the resulting transport. <Drug release kinetics> describes escape from a formulation or reservoir; <pharmacokinetics> additionally concerns what happens as the substance moves through and is removed from the body. These are related stages rather than interchangeable measurements.
A motivating historical development was the use of <polymers> for sustained release of <proteins> and other <macromolecules>, reported by <Robert S. Langer> and <Judah Folkman> in 1976.<Polymers for the sustained release of proteins and other macromolecules by Robert Langer and Judah Folkman>[Original publication, Nature 263, pp. 797–800]{ref} The engineering question includes whether a carrier can retain a substance initially and then make it available over time while preserving the intended activity.
An elementary reservoir model separates remaining material from cumulative release. If the remaining amount $M(t)$ obeys
$$
\frac{dM}{dt}=-kM,\qquad M(0)=M_0,
$$
then $M(t)=M_0e^{-kt}$ and the released fraction is $1-e^{-kt}$. For an illustrative reservoir-release half-time of six hours, $k=\ln2/(6\,\mathrm h)$. Half has been released at six hours and three quarters at twelve hours. The second six-hour interval releases one quarter of the initial amount, even though the first interval released one half. These are model fractions, not a drug regimen; the reservoir-release half-time is not an elimination half-life measured in a person.
An <in vitro drug release test> can place a formulation in a controlled medium, separate released material from retained material and measure amount against time. The apparatus can influence the answer. The FDA’s technical account explains that passive transport through a dialysis membrane can limit the measured response; its adaptive-perfusion prototype instead uses pressure-driven separation to investigate release from complex formulations.<Adaptive Perfusion: A Novel In Vitro Drug Release Testing Method for Complex Drug Products by the FDA>[Technology Summary and Development Stage]{ref}
A useful experimental record therefore states the medium, temperature, separation method, sampling schedule and material balance. A slowly changing measured concentration could reflect slow carrier release, slow transport through the apparatus or both. Comparing apparatus conditions and accounting for material remaining in the carrier helps distinguish these mechanisms. Evidence about release under those conditions must then be related to the biological environment before making claims about tissue exposure or therapeutic effect.
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