Insulin particle formation in supersaturated aqueous solutions of poly(ethylene glycol

ABSTRACT Protein microspheres are of particular utility in the field of drug delivery. A novel, completely aqueous, process of microsphere fabrication has been devised based on controlled phase separation of protein from water-soluble polymers such as polyethylene glycols. The fabrication process results in the formation of spherical microparticles with narrow particle size distributions. Cooling of preheated human insulin-poly(ethylene glycol)-water solutions results in the facile formation of insulin particles. Tomap out the supersaturation conditions conducive to particle nucleation and growth, we determined the temperature-andconcentration-dependent boundariesof anequilibrium liquid-solid phaseseparation. Thekineticsof formation ofmicrospheres were followed by dynamic and continuous-angle static light scattering techniques. The presence of PEG at a pH that was close to the protein’s isoelectric point resulted in rapid nucleation and growth. The time elapsed from the moment of creation of a super-saturated solution and the detection of a solid phase in the system (the induction period, tind) ranged from tens to several hundreds of seconds. The dependence of tind on supersaturation could be describedwithin the framework of classical nucleation theory, with the time needed for the formation of a critical nucleus (size,10 nm) beingmuch longer than the time of the onset of particle growth. The growth was limited by cluster diffusion kinetics. The interfacial energies of the insulin particles were determined to be 3.2–3.4 and2.2mJ/m2 at equilibrium temperatures of 25 and37C, respectively. The insulin particles formedasa result of theprocesswere monodisperse and uniformly spherical, in clear distinction to previously reported processes of microcrystalline insulin particle formation