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Dependent on various factors such as pH, temperature and shear forces, therapeutic insulins undergo a continuous process of chemical degradation during manufacturing and storage until administered by patients. Consequently, changes in secondary up to quaternary structures of the protein appear, with the consequence of a decrease in biological activity due to partial misfolding of the monomers and finally their aggregation to fibrils. Infrared spectrometry has been applied for quantifying chemical degradation processes of therapeutic insulins, based on changes in secondary structure. For the determination of insulin potency, the glucose metabolism rate of cells from the human monocytic cell line MONOMAC-6 has been monitored under standardized conditions, providing a measure of biological insulin activity. For cell culture monitoring with a focus on substrate and metabolite concentrations, microdialysis has been used in combination with infrared spectrometry of the continuously sampled dialysates with duration up to 48 h. The dialysate spectra were analyzed by a classical least-squares (CLS) method with appropriate reference spectra, including the determination of microdialysis recovery rates as obtained from perfusate losses of mannitol, which had been used as internal standard. By analysing the time dependent glucose utilization, the potency of tested insulins can be assessed without patient clamp experiments or animal testing.
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