Sustained-Release of Urease from a Poloxamer Gel Matrix

Abstract

With the increase of extremely specific polypeptide drugs arising from advances in recombinant DNA techniques, there exists a need with which to optimally deliver these genetically engineered drugs. This results from the normally short circulating half-life of these macromolecules. A well characterized model enzyme, urease, was formulated in a 20, 30, and 35% w/w poloxamer 407 gel matrix and the release profile determined in a membraneless diffusion system (Area = 11.4 cm²) in vitro at 37° C over 8 hours. Polymer release into a pH = 7.0 phosphate buffer receptor phase due to matrix erosion was constant throughout 8 hours and ranged from 1.07% ± 0.04 cm–2 hr–1 to 0.48% ± 0.02 cm-2 hr-1for the 20% w/w and 35% w/w poloxamer gel matrices, respectively. The predominant mechanism governing release of protein from the semisolid, poloxamer 407 gel matrix in vitro was matrix erosion with the cumulative urease released ranging from 89.5% ± 3.5 after 7 hours (20% w/w, n = 3) to 46.6% ± 0.3 following 8 hours of released (35% w/w, n = 3), respectively. The percent relative biological activity of the enzyme [(Act._poly/ Act._cont)*100] remaining was determined following incubation in a 14% w/w concentration of poloxamer 407 for 8 hours at 4,22, and 37° C. The percent relative enzyme activity remaining following incubation in the 14% w/w poloxamer 407 solution after 8 hours was not significantly different (p > 0.05) between samples incubated at 4°C (94.2% ± 2.4) and 37°C (89.7% ± 1.7). Hydrodynamic properties of dilute urease and poloxamer 407 solutions were assessed using viscometry. The intrinsic viscosity was experimentally determined to be 2.93 cm³/gm and 40.4 cm³/gm for urease and poloxamer 407, respectively. Based on values of intrinsic viscosity determined by viscometry, it appears that urease incubated in a 0.5 or 1.0 gm/dL solution of poloxamer 407 does not undergo a significant change in 3-dimensional structure. Thus, based on preliminary work, it would appear that poloxamer 407 may be useful as a sustained release vehicle for the formulation of proteins designed for extravascular administration.