Crafting Stable Antibiotic Nanoparticles via Complex Coacervation of Colistin with Block Copolymers

Abstract

To combat the ever-growing increase of multidrugresistant (MDR) bacteria, action must be taken in the development of antibiotic formulations. Colistin, an effective antibiotic, was found to be nephrotoxic and neurotoxic, consequently leading to a ban on its use in the 1980s. A decade later, colistin use was revived and nowadays used as a last-resort treatment against Gram-negative bacterial infections, although highly regulated. If cytotoxicity issues can be resolved, colistin could be an effective option to combat MDR bacteria. Herein, we investigate the complexation of colistin with poly(ethylene oxide)-b-poly(methacrylic acid) (PEO-b-PMAA) block copolymers to form complex coacervate core micelles (C3Ms) to ultimately improve colistin use in therapeutics while maintaining its effectiveness. We show that well-defined and stable micelles can be formed in which the cationic colistin and anionic PMAA form the core while PEO forms a protecting shell. The resulting C3Ms are in a kinetically arrested and stable state, yet they can be made reproducibly using an appropriate experimental protocol. By characterization through dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS), we found that the best C3M formulation, based on long-term stability and complexation efficiency, is at charge-matching conditions. This nanoparticle formulation was compared to noncomplexed colistin on its antimicrobial properties, enzymatic degradation, serum protein binding, and cytotoxicity. The studies indicate that the antimicrobial properties and cytotoxicity of the colistin-C3Ms were maintained while protein binding was limited, and enzymatic degradation decreased after complexation. Since colistin-C3Ms were found to have an equal effectivity but with increased cargo protection, such nanoparticles are promising components for the antibiotic formulation toolbox.

Coacervation in Slow Motion: Kinetics of Complex Micelle Formation Induced by the Hydrolysis of an Antibiotic Prodrug

“…However, it is unclear how many charges are necessary for the hydrolysis products to effectively complex with the partly anionic PEO-b-PMAA. To resolve the composition, shape, size, and structure, all the data sets were analyzed on an absolute scale using a model for spherical polydisperse micellar structures with graded interfaces, ,,− taking into account the mass balance of possible structures (micelles, free polymers/peptides). This CS/CMS complex coacervate model consists of three separate contributions: complex coacervate scattering; free (noncomplexed) polymer and colistin, CMS; and an additional structure factor describing the internal structure (charge correlations between “blobs”) between polyelectrolytes in the core, S ( Q ) internal .…”

“…This CS/CMS complex coacervate model consists of three separate contributions: complex coacervate scattering; free (noncomplexed) polymer and colistin, CMS; and an additional structure factor describing the internal structure (charge correlations between “blobs”) between polyelectrolytes in the core, S ( Q ) internal . The complex coacervate scattering ( I Coa ( Q )) is described by a fuzzy-sphere form factor, ,, including a structure factor for cluster formation (aggregates), S ( Q ) cluster , and a blob contribution, blob( Q ). ,,,, The free scattering contributions ( I Poly,free ( Q ), I Col,free ( Q ), and I CMS ( Q )) make use of the Debye scattering form factor for polyelectrolytes . The complete scattering function is described in eq . I false( Q false) = φ · f Coa true( f clu · S ( Q ) cluster · I Coa ( Q ) + ( 1 − f clu ) · I Coa ( Q ) + blob ( Q ) V Coa true) + φ false( f Poly · I Poly , free ( Q ) · f mix + f Col ...…”

“…To describe the scattering patterns at high Q values, a blob scattering term, i.e., a contribution from an internal polyelectrolyte network, was included characterized by the fraction ( f blob ) and blob correlation length (ξ). Furthermore, to account for blob charge correlations, , we also included a Gaussian peak function described by the width ( W ) correlation peak position ( Q local ).…”

“…The PEO-b-PMAA polymer was dissolved in each buffer to obtain a stock solution of 10 mg/mL. Previously, it was found that charge-matching conditions (equal charges of PMAA and colistin) led to the most stable complex core coacervate micelles (C3Ms) . With the assumption that all CMS would be hydrolyzed to colistin (from an effective charge of −5 to +5), a charge ratio ( f + ) of 0.5 (equal charges in the final state) was used.…”

“…In a previous study in which we investigated CS, we showed that the cationic colistin could be complexed by a partly oppositely charged block copolymer, PEO-b-PMAA (poly­(ethylene oxide)-b-poly­(methacrylic acid)), forming complex coacervate core micelles (C3Ms) . C3Ms generally consist of a charge-dense polyelectrolyte core containing combinations of oppositely charged polymers, , peptides/proteins, − DNA, , or drugs. − The C3M cores are typically sterically stabilized through the presence of polymer brushes conjugated to the charged species present in the core.…”