Optoacoustic imaging enabled biodistribution study of cationic polymeric biodegradable nanoparticles Susana P. Egusquiaguirre a,b , Nicolas Beziere c , José Luís Pedraz a,b , Rosa M. Hernández a,b , Vasilis Ntziachristos c * † and Manuela Igartua a,b * † Nanosized contrast agents for molecular imaging have attracted widespread interest for diagnostic applications with high resolution in medicine. However, many solid nanoparticles exhibit a great potential to induce toxicity, hin- dering their use for clinical applications. On the other hand, near-infrared (NIR) dyes have also been used for exten- sive biological applications, but show some limitations due to their poor aqueous stability, tendency to aggregation and rapid elimination from the body. An alternative proposed in this work to overcome these limitations is the use of NIR dye-loaded nanoparticles. Here we introduce nanoparticles constructed with poly(D,L-lactide-co-glycolic acid) (PLGA), a biodegradable and biocompatible polymer widely used for biomedical applications, attached to the polycation polyethyleneimine (PEI) to obtain positively charged nanoparticles. The in vivo biodistribution of the cat- ionic PEI–PLGA nanoparticles was investigated after administration through three different routes (intravenous, in- traperitoneal and subcutaneous) using multispectral optoacoustic tomography (MSOT). The prepared nanoparticles exhibited good colloidal stability and adequate optical properties for optoacoustic imaging. The in vivo biodistribution assays indicated a strong accumulation of the particles in the liver and spleen, and retention in these organs for at least 24 h. Therefore, these nanoparticles could find promising applications in MSOT due to a sharp and characteristic optoacoustic spectrum and high optoacoustic signal generation, and become a promising building block for theranostic strategies. Copyright © 2015 John Wiley & Sons, Ltd. Additional supporting information may be found in the online version of this article at the publisher’s web site. Keywords: biodistribution; cationic nanoparticles; imaging; MSOT; optoacoustic; nanoparticles 1. INTRODUCTION Over the last decade, molecular imaging in biological systems has attracted great attention as a promising strategy used in health-care for monitoring processes at cellular and subcellular levels, as well as the diagnosis and tracking of the progress of pathologies, and eventually their treatment. Therefore, it is important to develop suitable imaging platforms that could be used as contrast agents to trace these issues in vivo. Optical imaging techniques have been widely used to attain these purposes, although the low imaging depth, due to photon scattering, limits their use in vivo. However, multispectral optoacoustic tomography (MSOT) (1) is able to overcome these shortcomings, and its outstanding properties, including high res- olution, high penetration depth and detection sensitivity, as well as real-time monitoring and the use of non-ionizing radiation, qualify it as the ideal modality for preclinical and clinical transla- tion investigations. While providing an elegant way to monitor pathophysiological parameters based on light absorbance of ei- ther endogeneous (hemoglobin, melanin, …) or exogenous con- trast agents, it can also enable observation of the behavior of nanosized compounds such as drug delivery platforms in vivo and in real time, provided they display adequate light absorbing properties. Until now, a broad variety of nanoparticles have been studied for enhancing contrast in optoacoustic imaging, the most common being gold nanorods (2,3) and single-walled carbon nanotubes (SWNTs) (4), with a high molar extinction (absorption) coefficient and photostability, making them excellent candidates as contrast agents for optoacoustics (5). However, even though many drug delivery platforms based on this type of nanoparticle are currently being developed (6), issues related to safety and * Correspondence to: V. Ntziachristos, Institute for Biological and Medical Imag- ing, Technische Universität München and Helmholtz Zentrum München, Ingolstädter Landstraße 1, 85764 Neuherberg, Germany. E-mail: v. ntziachristos@helmholtz-muenchen.de M. Igartua, NanoBioCel Group, Laboratory of Pharmaceutics, School of Phar- macy, University of the Basque Country (UPV/EHU), Paseo de la Universidad 7, 01006 Vitoria-Gasteiz, Spain. E-mail: manoli.igartua@ehu.es † M. Igartua (manoli.igartua@ehu.es) and V. Ntziachristos (v.ntziachristos@helmholtz- muenchen.de) equally share credit for senior authorship. a S. P. Egusquiaguirre, J. L. Pedraz, R. M. Hernández, M. Igartua NanoBioCel Group, Laboratory of Pharmaceutics, School of Pharmacy, University of the Basque Country (UPV/EHU), Paseo de la Universidad 7, 01006, Vitoria-Gasteiz, Spain b S. P. Egusquiaguirre, J. L. Pedraz, R. M. Hernández, M. Igartua Biomedical Research Networking Center in Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Paseo de la Universidad 7, 01006, Vitoria-Gasteiz, Spain c N. Beziere, V. Ntziachristos Institute for Biological and Medical Imaging, Technische Universität München and Helmholtz Zentrum München, Ingolstädter Landstraße 1, 85764, Neuherberg, Munich, Germany Full paper Received: 30 September 2014, Revised: 17 March 2015, Accepted: 01 April 2015, Published online in Wiley Online Library: 27 May 2015 (wileyonlinelibrary.com) DOI: 10.1002/cmmi.1644 Contrast Media Mol. Imaging 2015, 10 421–427 Copyright © 2015 John Wiley & Sons, Ltd. 421