1Department of Radiology, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. 2Department of Anatomy, Histology and Embryology, Innsbruck Medical University, Müllerstrasse 59, 6020 Innsbruck, Austria. 3Biozentrum of the Medical University Innsbruck, Section for Clinical Biochemistry, Fritz-Pregl-Straße 3, 6020 Innsbruck, Austria. 4Center for Medical Research, Stiftingtalstrasse 24, 8010 Graz, Austria. 5Current address: Department of Pharmaceutical Organic Chemistry, Faculty of Pharmacy, Assiut University, Assiut, Egypt We are developing a nanoparticulate histochemical reagent designed for histochemistry in living animals (molecular imaging), which should finally be useful in clinical imaging applications. The iterative development procedure employed involves conceptual design of the reagent, synthesis and testing of the reagent, then redesign based on data from the testing; each cycle of testing and development generates a new generation of nanoparticles, and this report describes the synthesis and testing of the third generation. The nanoparticles are based on human serum albumin and the imaging modality selected is magnetic resonance imaging (MRI). Testing the second particle generation with newly introduced techniques revealed the presence of impurities in the final product, therefore we replaced dialysis with diafiltration. We introduced further testing methods including thin layer chromatography, arsenazo III as chromogenic assay for gadolinium, and several versions of polyacrylamide gel electrophoresis, for physicochemical characterisation of the nanoparticles and intermediate synthesis compounds. The high grade of chemical purity achieved by combined application of these methodologies allowed standardised particle sizes to be achieved (low dispersities), and accurate measurement of critical physicochemical parameters influencing particle size and imaging properties. Regression plots confirmed the high purity and standardisation. The good degree of quantitative physicochemical characterisation aided our understanding of the nanoparticles and allowed a conceptual model of them to be prepared. Toxicological screening demonstrated the extremely low toxicity of the particles. The high magnetic resonance relaxivities and enhanced mechanical stability of the particles make them an excellent platform for the further development of MRI molecular imaging.

1Faculty of Health and Society, Malmö University, 205 06 Malmö, Sweden. 2Institute of Inorganic Chemistry, University of Vienna, Währinger Str. 42, 1090 Vienna, Austria. 3Department of Radiology, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. 4Section for Clinical Biochemistry, Biozentrum of the Medical University Innsbruck, Fritz-Pregl-Straße 3, 6020 Innsbruck, Austria. 5Department of Anatomy, Histology and Embryology, Innsbruck Medical University, Mu¨llerstrasse 59, 6020 Innsbruck, Austria. 6Department of Nuclear Medicine, Innsbruck Medical University, Anichstrasse 35, 6020 Innsbruck, Austria. 7Center for Medical Research, Stiftingtalstrasse 24, 8010 Graz, Austria. 8Division of Neuroanatomy, Department of Anatomy, Histology and Embryology, Innsbruck Medical University, Müllerstrasse 59, 6020 Innsbruck, Austria. 9Current address: Department of Pharmaceutical Organic Chemistry, Assiut University, Assiut, Egypt To develop a platform for molecular magnetic resonance imaging, we prepared gadolinium-bearing albumin-polylactic acid nanoparticles in the size range 20–40 nm diameter. Iterative cycles of design and testing upscaled the synthesis procedures to gram amounts for physicochemical characterisation and for pharmacokinetic testing. Morphological analyses showed that the nanoparticles were spheroidal with rough surfaces. Particle sizes were measured by direct transmission electron microscopical measurements from negatively contrasted preparations, and by use of photon correlation spectroscopy; the two methods each documented nanoparticle sizes less than 100 nm and generally 10–40 nm diameter, though with significant intrabatch and interbatch variability. The particles’ charge sufficed to hold them in suspension. HSA retained its tertiary structure in the particles. The nanoparticles were stable against turbulent flow conditions and against heat, though not against detergents. MRI imaging of liquid columns was possible at nanoparticle concentrations below 10 mg/ml. The particles were non-cytotoxic, non-thrombogenic and non-immunogenic in a range of assay systems developed for toxicity testing of nanoparticles. They were micellar prior to lyophilisation, but loosely structured aggregated masses after lyophilisation and subsequent resuspension. These nanoparticles provide a platform for further development, based on non-toxic materials of low immunogenicity already in clinical use, not expensive, and synthesized using methods which can be upscaled for industrial production.