Study on the thermal stability of green fluorescent protein (GFP) in glucose parenteral formulations

Large volume parenteral solutions (LVPS) that are widely used in the healthcare system must be processed by moist-heat treatment to an assured sterility level in which the efficacy is measured by a bioindicator (BI) that provides fast, accurate and reliable results. This study evaluated the thermal stability of green fluorescent protein (GFP) into glucose-based LVPS (1.5–50%) solutions to determine its utility as a BI for thermal processes. GFP, expressed by Escherichia coli, isolated/purified by TPP/HIC, was diluted in buffered (each 10 mM: Tris–EDTA, pH 8; phosphate, pH 6 and 7; acetate, pH 5) and in water for injection (WFI; pH 6.70 ± 0.40) glucose solutions (1.5–50%) and exposed to constant temperatures from 80 °C to 95 °C. The thermal stability was expressed in decimal reduction time (D-value, time required to reduce 90% of the GFP fluorescence intensity). At 95 °C, the D-values for GFP in 1.5–50% glucose were: (i) 1.63 ± 0.23 min (pH 5); (ii) 2.64 ± 0.26 min (WFI); (iii) 2.50 ± 0.18 min (pH 6); (iv) 3.24 ± 0.28 min (pH 7); (v) 2.89 ± 0.44 min (pH 8). By the convenient measure of fluorescence intensity and its thermal stability, GFP has the potential as a BI to assay the efficacy of moist-heat processing of LVPS at temperatures ≤100 °C.