Characterization of Nanoparticle Concentration in Complex Drugs by Microdroplet Deposition and Optical Microscopy

J. Myung, L.C. Elliott, S.M. Stavis, J. Zheng, D. Kozak
United States

Keywords: single nanoparticle counting


Nanoparticles are widely used to improve the performance of drugs, vaccines, diagnostics, and cosmetics. To ensure product quality and sameness from a regulatory prospective, the physiochemical properties of the nanoparticles, including size distribution, morphology, composition, agglomeration, surface charge, and functionalization, are routinely measured and utilized to set product specifications. However, due to technical limitations of traditional measurement techniques, product concentration has only been assessed through bulk measurement of the total amounts of each compound present in the formulation. Such measurements do not take into account potential differences in nanoparticle concentration, which may be a critical product specification, as well as a sensitive measure of product quality. Recent advances in single-nanoparticle measurements have enabled accurate measurement of nanoparticle concentrations in healthcare and personal products, however, their use as a regulatory tool is not yet well established. Here, we introduce a practical and economical single-nanoparticle measurement technique by combining microdroplet deposition and optical microscopy. To our knowledge, this is the first application of this combination of techniques to characterize complex drugs consisting of soft nanoparticles, such as aqueous suspensions of emulsions and liposomes. Based on the known volume of the microdroplet of drug product, the nanoparticle concentration was measured by imaging and counting each nanoparticle on a substrate with optical darkfield microscopy, as depicted in Figure 1. To assess the potential regulatory use of this and other commercially available single-nanoparticle counting instruments, such as tunable resistive pulse sensing (TRPS) and particle tracking analysis (PTA), the accuracy, precision, and robustness of each technique were compared using model suspensions and several emulsion products with different components and compositions. To gain a better understanding of acceptable inter- and intra- variability in complex generic products that are bioequivalent, the lot-to-lot variation and generic-to-innovator concentrations for four propofol emulsion products were measured. It was found that the lot-to-lot particle concentration variation was not statistically significant, indicating reproducible product quality for each manufacturer. In contrast, a much higher variability was observed between drug product manufacturers although all were bioequivalent and had qualitatively (Q1) and quantitatively (Q2) the same formulation as the reference (innovator) product. These findings indicate that nanoparticle concentration may provide a highly sensitive measure to assess product sameness. This study represents the first application of microfluidic deposition and optical microscopy techniques to the characterization of soft, complex nano-scale drug products.