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We have characterized the biodegradable material poly(epsilon-caprolactone) (PCL) as a delivery system for recombinant human growth hormone (hGH). Two contrasting methods for the manufacture of the biomaterial were investigated: namely, solvent casting and solvent casting particulate leaching; the latter yielded porous PCL discs. The degree of porosity, which was assessed by scanning electron microscopy, could be controlled by incorporating selected concentrations of particulate sodium chloride during the manufacturing process. Bioactive hGH released from the PCL preparations was quantified with a highly sensitive and precise bioassay which was based upon hGH activation of rat lymphoma Nb2 cells. Eluates obtained from control discs of PCL which had not been loaded with hGH proved to be nontoxic when tested on these cells. The release of bioactive hGH from hormone-loaded nonporous discs of PCL was found to be a direct function of the initial hormone loading dose. Increased porosity of the discs manufactured by solvent casting particulate leaching increased the delivery of hGH from discs which had been immersion loaded. However, hGH release after surface loading was independent of porosity. Hormone concentrations were also assessed by immunoassay so that the ratios of bio- to immunoactivity (B:I ratio) of the hormone release could be determined. We found that the B:I ratio of the hormone after release from unstored discs was identical to that of the hormone prior to its incorporation into the PCL, demonstrating that the mild incorporation procedures utilized had not adversely affected the structural integrity of the hormone. However, if the hormone-loaded discs were stored at 37 degrees C prior to elution, the B:I ratios of the hGH released decreased indicating that this compromised the bioactive site.

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Previous studies showed that recombinant human growth hormone (hGH) released from hormone-loaded poly(methylmethacrylate) (PMMA) cement stimulated osteoid formation in a rabbit model. Local delivery of hGH from cemented hip arthroplasties may thereby provide a means of reducing the problem of aseptic loosening. We have investigated two different formulations of PMMA as delivery systems for bioactive hGH. The bioactivity of the hormone release in vitro was monitored with an eluted stain assay (ESTA). The hGH was also measured by an immunoassay, which provides an alternative assessment of structural integrity of the hormone released. In addition, we adapted the ESTA bioassay to assess the in vitro cytotoxicity of the cements. Using unloaded cements, the undiluted eluates from both types of PMMA proved cytotoxic. This cytotoxicity could be diluted out, and the procedure allowed us to measure the bioactivity of hGH in the eluates from hormone-loaded cements independent of their cytotoxicity. The major fraction of the bioactivity was released from both of the PMMA cements during the first 24 h, but the hormone remained detectable in eluates collected after 36 days of elution. Comparison of the bio- and immunoactivity of the hGH released showed that the ratio of these two activities (i.e., the B:I ratio) was constant over this time period. However in parallel studies in which hormone-loaded discs were stored under dry conditions prior to elution, we found that the B:I ratio then declined markedly. This suggests that fully hydrated conditions, such as when the discs are bathed in assay medium, are necessary to maintain the bioactivity of the hGH. Both cements released only approximately 1% of the hormone originally incorporated, but the hGH concentration which accumulated in the eluates were high in physiologic terms (approximately 1000 mU/L).