Multimodal nanoparticles for quantitative imaging

The scope of this thesis is research related to applications of nanoparticles in quantitative preclinical imaging. Nanoparticles are a versatile platform that can interact with biological systems at many different length scales and can furthermore be rendered visible for basically any medical imaging technique by modification with appropriate contrast providing moieties. Thus, nanoparticles can be used as a new class of contrast agents for basically all imaging modalities, e.g. as long circulating blood pool agents in CT, or as MRI contrast agents. Vice versa, non-invasive imaging techniques can be used to for example follow the biodistribution of nanoparticles in vivo and apply nanoparticles as a tool to investigate biological processes related to disease processes. Dual modal imaging applying multifunctional and dual-labeled nanoparticles offer new approaches to quantitative imaging, giving new insights into technology development on one side and biological read-outs on the other. For instance, quantification of biological processes that lie at the basis in the development of disease may lead to earlier detection and better disease diagnosis and treatment. Results and concepts presented in this thesis have high impact on therapeutic application of nanoparticles, for example when they are used as drug delivery systems. Imaging can provide valuable information on drug delivery and biodistribution in a quantitative manner, which may help in development of new therapeutic strategies. Nanoparticles are promising structures for quantitative imaging. Its surface can be utilized to attach almost any desirable molecule. Nanoparticles are relatively large in size (typically 10-200 nm) and can for instance accommodate a high payload of contrast agent per particle on its surface or inside the particle, thereby increasing the signal/particle by five orders of magnitude. In addition, also multiple imaging probes for different imaging modalities can be incorporated providing a double read-out. For the understanding of biological processes, targeting ligands such as antibodies, proteins and peptides can be attached to its surface. Despite the wide variety of possibilities with nanoparticles, they have hardly been studies for quantitative imaging purposes. Therefore, the aim of the research described in this thesis was to explore and develop several nanoparticles for quantitative imaging by using existing or newly developed imaging techniques. Chapter 1 gives a general introduction in the field of nanoparticles for quantitative imaging. Several imaging techniques are described such as CT, Spectral CT, SPECT and MRI, and how nanoparticles can play an important role in research. Chapter 2 describes the development of a novel nanoparticulate CT contrast agent. Several amphiphilic molecules were investigated in this chapter in the combination with different iodinated oils for their influence on the size stability of the nanoparticles. In Chapter 3, the dose dependent biodistribution of the nanoparticles is investigated as well as strategies to vary the biodistribution. The effect of a co-injection with liposomes and soy bean oil emulsions was investigated using CT, SPECT and ? -counting. The final optimized blood pool CT contrast agent from chapter 2 and 3 can be used for qualitative imaging in CT as well as in quantitative imaging in Spectral CT. Chapter 4 describes the very first use of this novel imaging technique Spectral CT in quantitative imaging. For this, the nanoparticles of chapter 2 were extended to a multimodal nanoparticulate contrast agent for CT, Spectral CT and SPECT. Spectral CT quantification was compared to quantification using SPECT and ICP-MS to demonstrate the correlations and accuracy of the techniques. In Chapter 5, the development is described of a dual-isotope SPECT imaging protocol as a tool for pre-clinical testing of new molecular imaging tracers. New molecular targeting probes are consistently investigated as a tool to enable target specific binding of nanoparticles to cellular surfaces of interest. Dual-isotope SPECT can be used in which the biodistribution of two different ligands labelled with two different radionuclides can be studied in the same animal, thereby excluding experimental and physiological inter-animal variations. The developed dual-isotope protocol was tested using a known angiogenesis specific ligand (cRGD peptide) in comparison to a potential non-specific control (cRAD peptide). Chapter 6 describes the use of a multimodal radiolabeled paramagnetic liposomal contrast agent that allows simultaneous imaging with SPECT and MRI. A double read-out is then possible and demonstrates the additional advantages of the combination of the two techniques. SPECT can for instance quantify the nanoparticle concentration and MRI can spatially localize the nanoparticle. The combination however gives an indirect read-out of the water exchange, which in return reveals insights in biological processes and environments. Chapter 7 describes a study that investigates the use of nanoparticles in the quantitative imaging technique fluorine MRI. The use of gadolinium-complexes as signal modulating ingredients into the nanoparticle formulation has emerged as a promising approach towards improvement of the fluorine signal. Paramagnetic lipids based on gadolinium complexes can be incorporated to increase the 19F MR signal per particle. Here, 3 different paramagnetic lipids were investigated on its influence at five different field strengths. This furthermore also provides important insights in the dependency of the magnetic field on fluorine signal intensity. The final Chapter 8 describes the future perspectives of the use of multimodal nanoparticles for quantitative imaging.