Label-Free Analysis Reveals In Vivo Premature Drug Release Governed by Diffusion from Polymeric Micelles

Area of Science:

• Biomaterials Science
• Nanotechnology
• Pharmacokinetics

Background:

• Premature drug release from polymeric micelles (PMs) limits their clinical application.
• Current methods cannot simultaneously track drugs and carriers, obscuring release mechanisms.
• Distinguishing drug diffusion from micelle dissociation is crucial for optimizing PMs.

Purpose of the Study:

• To develop a label-free analytical platform for simultaneous quantification of drugs and PMs in circulation.
• To elucidate the *in vivo* kinetics of drug release and micelle dissociation.
• To determine the primary mechanism responsible for premature drug release from PMs.

Main Methods:

• Development of an integrated platform combining stable isotope-assisted size-exclusion chromatography, ultrafiltration, and mass spectrometry.
• Absolute quantification of micelle-encapsulated drug, free drug, protein-bound drug, intact micelles, and unimers.
• Application to paclitaxel-loaded methoxypoly(ethylene glycol)-block-poly(d,l-lactide) micelles for *in vivo* pharmacokinetic studies.

Main Results:

• The developed platform successfully quantified five critical circulatory species without labels.
• Systematic analysis of paclitaxel-loaded PMs revealed drug release kinetics and micelle dissociation.
• Results indicate that drug diffusion is the predominant mechanism for premature drug leakage from PMs.

Conclusions:

• Drug diffusion, rather than micelle dissociation, is the primary cause of reduced circulatory efficacy of PMs under tested conditions.
• The established label-free platform provides a robust framework for future pharmacokinetic studies of nanocarriers.
• This research offers critical insights into the *in vivo* behavior of polymeric micelles, guiding the design of more effective drug delivery systems.