The Kinetic Pathways for the Formation of Complex Coacervate Micelles of Antimicrobial Peptides and Block Copolymers

Abstract

The conceptual application of complex coacervate core micelles (C3Ms) as drug delivery vehicles has obtained significant interest over the past several decades. Although C3Ms show promise in targeting specific body areas and improving drug circulation times, their low reproducibility and limited stability have hindered their progression to clinical trials and commercial viability. One of the major obstacles is the lack of understanding of the formation and growth kinetics of C3Ms. Since their structure is often controlled kinetically rather than thermodynamically, this insight is essential for achieving good reproducibility and stability of C3M drug delivery vehicles. Time-resolved small-angle X-ray scattering (TR-SAXS) offers excellent insights by resolving spatiotemporal kinetic processes on nanometer/millisecond scales. Here, we investigate the formation kinetics of C3Ms comprised of the antimicrobial peptide, colistin, and poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA). Using TR-SAXS coupled to stopped-flow mixing, we have identified three distinct steps in the mechanism of this complex coacervation system: nucleation, fusion, and single chain/ion-pair insertion. Nucleation cannot be experimentally resolved as it is completed during the mixing process. The fusion kinetics proceed as a first-order reaction with relaxation times of approximately 50–85 ms. The driving force for fusion is larger at increased concentrations. Chain/pair insertion occurs at relaxation times of 15–25 s, where larger remaining size mismatches (higher dispersity) drive the final equilibration process. As expected, the structural formation is concentration-dependent, providing a handle to acquire control over the size and dispersity of these kinetically trapped C3Ms. These findings contribute to a deeper understanding of C3M formation and stability, potentially advancing the development of C3M-based drug delivery systems.