Evaluation of radiolabeled stimuli-sensitive nanogels for application in nuclear medicine

• Evaluierung radiomarkierter Stimuli-sensitiver Nanogele für die Anwendung in der Nuklearmedizin

Drude, Natascha Ingrid; Möller, Martin (Thesis advisor); Mottaghy, Felix M. (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020

Abstract

Endogenous radiotherapy allows the direct delivery of radiation to a tumor by means of radiolabeled antibodies or minibodies as well as peptides, proteins or isotopes of halogens (like radioiodine therapy of thyroid cancer). However, still only few of these endogenous radiotherapy approaches are implemented in standard patient treatment. So far and irrespective of the strong research interest in nanomedicine, the efficacy of aqueous nanogels carrying radionuclides for endogenous radiotherapy has not been evaluated systematically and there are only few reports on the diagnostic potential of radiolabeled nanogels. Among the different types of nanocarriers (e.g. liposomes, polymeric micelles) which have been proposed as drug delivery vehicles, nanogels are rather unique due to their predominantly hydrophilic character. Nanogels can be “squeezed” and the fluctuations impede adsorption of proteins for entropic reasons. Hence, their aqueous consistence ensures minimized interaction with cells and proteins in the blood, which results in an extended circulation. The shape compliance can be tailored by the degree of crosslinking and very soft nanogels can squeeze through holes, which are much smaller than their hydrodynamic radius. This is important regarding excretion pathways as well as the passive accumulation of the nanogels into the tumor tissue by the enhanced permeability and retention (EPR) effect. The open structure is, however, in contradiction with a high loading capacity. Different from cytotoxic drugs, the dose-effect relation in diagnostic as well as therapeutic application of radionuclides is not only controlled by the concentration but also by the decay characteristics of the chosen radioisotope. While an auger electron emitter can promote cell death on a single cell level, an -emitter kills the cells only in decay proximity (radiation range 40-80 µm) and a --emitter can efficiently radiate the tumor tissue within a depths of 10-100 cells (long-range penetration with 0.1-10 mm). With respect to e.g. larger solid tumor manifestations and the intratumoral turgor effect, which limits the accessibility of the drugs to the deep tumor layers, the crossfire effect of - emitters can be a major advantage. Overall, their effective concentrations are significantly lower than those of a molecular drug and the radioisotope complexes established for medical application are mostly highly hydrophilic and can thus be incorporated in a hydrophilic nanogel without altering its stealth character. Thus, we designed stimuli-sensitive hydrophilic nanogels as carriers of radiolabeled chelators and tested their suitability in different preclinical settings. The nanogel-radionuclides were diagnostically evaluated by means of positron emitting radionuclides (68Ga and 64Cu), suitable for positron emission tomography (PET). In a general biodistribution study in healthy rodents, the radiolabeled nanoformulations showed a size dependent renal elimination and independent of the size only marginal accumulation in organs associated with the mononuclear phagocyte system (MPS). However, gel electrophoresis and size exclusion experiments indicated premature degradation in the circulation of the redox-sensitive nanogels within the first 5 h post injection (p.i.). This early degradation could be reduced by pre-treatment of the animals with buthionine sulfoximine (BSO) which significantly reduced glutathione (GSH; an antioxidant) levels in vivo (especially in liver/ spleen). This GSH reduction resulted in reduced renal excretion, however, with a slight increase in liver accumulation. The non-targeted nanogels rely on the passive accumulation into the tumor tissue by the EPR effect and upon degradation in the tumor microenvironment; an improved penetration of the released low molecular weight prepolymers was expected to yield a homogenous distribution of radioactivity throughout the tumor. However, the EPR effect has its own limitations, i.e., the tremendous heterogeneity in tumor vessel leakiness over space, time, and different types of tumors. Very crucial are variations in the blood flow to the tumor, i.e., density of vascular structures and permeability of the vasculature, as well as structural barriers imposed by the perivascular tumor cells and the extracellular matrix. Furthermore, increased interstitial fluid pressure limits the transport through the dense collagen matrix surrounding the tumor. Those limitations like the high interstitial fluid pressure in combination with drug efflux pumps resulted in a low retention of the proposed polymeric structures. To tackle this limitation receptor/transporter targeting ligands were introduced to yield radiolabeled degradation products that are suitable to actively target tumor cells. This concept was evaluated in somatostatin receptor positive pancreatic carcinoma xenografted mice and PET images indicated a significant higher retention compared to solely passively targeted nanogels as well as a deep penetration that was additionally confirmed by immunohistochemistry via staining of the radiolabeled targeting moiety. In another proof of concept study, nanogels were equipped with a cross reactive material (CRM-197) to promote receptor mediated transcytosis across the Blood Brain Barrier (BBB). Herein, an auger electron emitting radionuclide-labeled nucleoside analogue was covalently attached via enzyme cleavable substrates. The nucleoside analogue alone is not able to cross the BBB, while the nanogel can promote its delivery. The auger electron emitter reduces the crossfire effect and thus reduces damage to healthy tissue. Evaluation of the delivery of (nano)irradiation was performed in an in vitro model of the BBB in a co-culture of human cerebral endothelial cell line (hCMEC/D3), pericytes and astrocytes and/or different glioblastoma cell lines. Receptor mediated transport was confirmed via competition experiments. Upon cleavage, the nucleoside was still able to target cells and induce cell death. In summary, using different isotopes or radionuclides and different targeting moieties, the same construct of nanogels can be used for a variety of diseases and be tailored according to the scientific question.