Colloidal particles prepared through the ionic crosslinking of chitosan with tripolyphosphate (TPP) have attracted widespread interest as carriers for protein drugs and vaccines. The literature on these particles, however, is filled with conflicting reports on their properties. To this end, here we describe several studies that shed light on these opposing findings. In examining protein uptake by chitosan/TPP particles, we hypothesized that some of the apparent discrepancies in the literature might stem from the often overlooked variations in XAgg, i.e., the fraction of chitosan molecules that aggregates into particles. Spectroscopic analysis of how XAgg correlates with association efficiency, AE (which is the fraction of added protein that is taken up by the particles), reveals the AE of anionic, chitosan-binding proteins to scale linearly with XAgg and, for each model protein (at variable TPP and protein concentrations), to collapse onto a single AE vs. XAgg curve. This scaling confirms XAgg to be a key determinant of affinity-driven protein uptake into chitosan/TPP particles and reflects: (1) an increase in the particulate binding sites; and (2) a decrease in competitive protein binding to unaggregated chitosan (which competes with the chitosan/TPP particles for the protein). Once the protein is loaded into the particles, its delivery requires control over chitosan/TPP particle stability. Yet, while some have reported these particles to be stable under model physiological conditions, other studies have shown them to either agglomerate or dissolve. Through several experimental techniques, we show these seeming discrepancies to arise from: (1) variations in the chitosan degree of deacetylation; (2) differences in dispersion compositions; and (3) occasional misinterpretation of light scattering data. Finally, we discuss pitfalls in the experimental methods used to analyze release from chitosan/TPP particles, which appear to cause many of the opposing findings on their release performance. These include incomplete particle separation from the release media, particle agglomeration during separation/sampling steps, and failure to maintain sink (and constant pH) conditions. Collectively, the above analyses of protein uptake/release and particle stability reconcile many conflicting reports and highlight the critical experimental parameters that must be controlled when designing and characterizing ionically crosslinked micro- and nanocarriers.

Chitosan/tripolyphosphate particles as colloidal protein carriers: Resolving the opposing findings

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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