Improved Ultrafiltration Method to Measure Drug Release from Nanomedicines Utilizing a Stable Isotope Tracer

An important step in the early development of a nanomedicine formulation is the evaluation of stability and drug release in biological matrices. Additionally, the measurement of encapsulated and unencapsulated nanomedicine drug fractions is important for the determination of bioequivalence (pharmacokinetic equivalence) of generic nanomedicines. Unfortunately, current methods to measure drug release in plasma are limited, and all have fundamental disadvantages including non-equilibrium conditions and process-induced artifacts. The primary limitation of current ultrafiltration (and equilibrium dialysis) methods for separation of encapsulated and unencapsulated drug and determination of drug release is the difficulty in accurately differentiating protein bound and encapsulated drug. Since the protein binding of most drugs is high (>70%) and can change in a concentration- and time-dependent manner, it is very difficult to accurately account for the fraction of non-filterable drug that is encapsulated within the nanomedicine and how much is bound to protein. The method in this chapter is an improvement of existing ultrafiltration protocols for nanomedicine fractionation in plasma, in which a stable isotope tracer is spiked into a nanomedicine containing plasma sample in order to precisely measure the degree of plasma protein binding. Determination of protein binding then allows for accurate calculation of encapsulated and unencapsulated nanomedicine drug fractions, as well as free and protein-bound fractions.