 In vitro drug release test

An in vitro drug release test measures the amount or rate at which a substance leaves a formulation under specified laboratory conditions. The medium, temperature, agitation or flow, separation method and sampling schedule define the experiment. A measured release curve is therefore a property of a formulation–apparatus system under those conditions, rather than a direct measurement of the concentration in a person.

A typical design places the formulation in contact with a receiving medium, separates released material from the carrier and measures concentration at several times. The primary FDA technical account describes pressure-driven separation as an alternative to passive dialysis-membrane transport for complex formulations. It emphasizes that the separation step can itself limit the observed rate.^[ref]

Repeated sampling requires a material balance. Suppose a well-mixed receiver has constant volume $V$. At each observation a volume $v$ is removed and replaced with blank medium. If $C_j$ is the concentration immediately before the $j$th withdrawal, the cumulative amount transferred into the receiver by time $t_n$ isThe first term is material still in the receiver; the sum is material removed at earlier observations. This sampling correction in drug release testing assumes matched replacement, a well-mixed receiver and no unmeasured loss or chemical transformation.

For illustrative $V=100\mathrm{mL}$, $v=5\mathrm{mL}$, $C_1=0.10\mathrm{mg}\mathrm{m}{\mathrm{L}}^{-1}$ and $C_2=0.18\mathrm{mg}\mathrm{m}{\mathrm{L}}^{-1}$, the amount at the second observation is $18+0.5=18.5\mathrm{mg}$, not just $18\mathrm{mg}$. These chosen values demonstrate accounting; they are not a formulation specification or dosing instruction.

An informative experiment also accounts for material retained in the formulation and material lost to the apparatus. Adsorption, incomplete separation or degradation can change a measured concentration without the same change in carrier release. A control without the carrier and a recovery check address different parts of this problem. Comparing the measured curve under several justified apparatus conditions helps distinguish drug release kinetics from transport and analytical limitations.