Porous bone morphogenetic protein-2 microspheres: Polymer binding and in vitro release

Abstract

This research compared the binding and release of recombinant human bone morphogenetic protein 2 (rhBMP-2) with a series of hydrophobic and hydrophilic poly-lactide-co-glycolide (PLGA) copolymers. Porous microspheres were produced via a double emulsion process. Binding and incorporation of protein were achieved by soaking microspheres in buffered protein solutions, filtering, and comparing protein concentration remaining to nonmicrosphere-containing samples. Protein release was determined by soaking bound microspheres in a physiological buffer and measuring protein concentration (by reversed-phase high-performance liquid chromatography) in solution over time. Normalized for specific surface area and paired by polymer molecular weight, microspheres made from hydrophilic 50:50 or 75:25 PLGA bound significantly more protein than microspheres made from the corresponding hydrophobic PLGA. Increased binding capacity correlated with higher polymer acid values. With certain polymers, rhBMP-2 adsorption was decreased or inhibited at high protein concentration, but protein loading could be enhanced by increasing the protein solution:PLGA (volume:mass) ratio or by repetitive soaking. Microspheres of various PLGAs released unbound protein in 3 days, whereas the subsequent bound protein release corresponded to mass loss. RhBMP-2 binding to PLGA was controlled by the acid value, protein concentration, and adsorption technique. The protein released in 2 phases; the first occurred over 3 days regardless of PLGA used and emanated from unbound, incorporated protein, while the second was controlled by mass loss and therefore was dependent on the polymer molecular weight. Overall, control of rhBMP-2 delivery is achievable by selection of PLGA microsphere carriers.

Influence of morphology and drug distribution on the release process of FITC-dextran-loaded microspheres prepared with different types of PLGA

“…Higher molecular weight polymers (RG503 and RG503H) yield a more viscous solution, which leads to bigger emulsion droplets and a relatively stable double emulsion, which corresponds to the increased size and more porous structure. Simultaneously, the uncapped polymer (RG503H) is more hydrophilic than the capped polymer, 20 which facilitates improved convection between the internal aqueous phase and the outer aqueous phase, which leads to the lower internal porosity and more pores on the surface. However, the less viscous solutions of lower molecular weight PLGAs reduce the stability of the double emulsion.…”

Segmental bone regeneration using an rhBMP-2-loaded gelatin/nanohydroxyapatite/fibrin scaffold in a rabbit model

“…However, a number of studies have confirmed that rhBMP-2 released from gelatin alone or in combination with other ceramic scaffolds had a large initial burst release and resulted in imbalanced release of rhBMP-2. Recently, poly(lactic-co-glycolic acid) polymer has been commonly used for controlled delivery of rhBMP-2 and displayed a good release profile [35][36][37], but in general, the system was prepared with the growth factor incorporated within. This system may expose the protein to various organic solvents and temperature variations that may denature the growth factor completely or significantly reduce its bioactivity in vivo.…”

Positive charge of “sticky” peptides and proteins impedes release from negatively charged PLGA matrices

“…Previous studies have noted inverse relationships between “acid number” (a measure of carboxylic acid content of a polymer) and PLGA molecular weight or end-group chemistry. 30, 38 Here, we show that initial polymer chemistry also dictates evolution of bulk intraparticle pH during MP degradation (Fig 4A). Specifically, higher molecular weight (43 kDa) and ester-capped PLGA MPs have higher initial intraparticle pH and more gradual decreases in pH than lower molecular weight uncapped PLGA (Fig 4A).…”

“…Additionally, these studies investigated the effects of polymer matrix charge density on “release”, rather than the influence of the amount of positive charge on the peptide or protein. 15, 30 …”