(497f) Nano-Hitchhikers for Effective Delivery of Alpha-Particle Radiotherapy to Glioblastomas

Introduction: Brain cancer is the leading cause of cancer death in children and adolescents, surpassing leukemia. Pediatric glioblastoma, a type of brain cancer in children, is an aggressive disease with no defined standard therapy. Pediatric glioblastoma, is quite distinct from its adult counterpart. Despite aggressive treatment with multimodal therapy (surgery, combined with even highly toxic combination of chemotherapies and/or radiotherapy) most children fail to reach a long-lasting response and do not survive. The reported median survival in pediatric glioblastoma, despite multi-modal therapies ranges from 13 to 73 months with a 5-year survival of less than 20 %. We propose a nanotechnology-based delivery strategy that ‘hitchhikes’ an existing biological mechanism: this mechanism enables selective and uniform delivery of a cytotoxic agent (an α-particle emitter) in brain tumors; through our work proposed we expect to demonstrate that it is more effective at controlling pediatric glioblastoma cancers than currently available approved and alternative methods of treating this disease.

Current therapeutic approaches following surgery necessarily resort to combinations of therapeutics and/or external radiotherapy but, unfortunately, even with highly toxic combination regimens, most pediatric patients fail to reach a long-lasting response partly due to resistance to therapies. Additionally, since (external) radiation therapies in pediatrics have severe consequences to the developing brain, there are further limitations to the therapeutic options that are available for adults.

Alpha-particle radiopharmaceutical therapy (α-RPT) is a form of internal radiation and has already been shown to be impervious to most resistance mechanisms. Due to the massive amount of energy deposited as they traverse tissue, α-particles produce orders of magnitude more DNA damage (complex double strand breaks – the most difficult to repair type of DNA damage) than all other forms of cytotoxic therapy. The complexity and level of rapidly induced DNA damage overwhelms cellular repair mechanisms; this inability to repair lethal damage is the reason that α-particles, if optimally delivered, are impervious to resistance. However, the short range of α-particles in tissue (up to 5 to 10 cell diameters) which makes them ideal for precision killing – especially needed in the brain - also limits penetration within the volume of brain tumors. This is because the diffusion-limited penetration depths of traditionally used carriers (antibodies, molecular targeting vectors, liposomes) in tumors, combined with the short range of α-particles, result in only partial tumor irradiation. This is the root cause of current treatment failure of glioblastoma with α-particle therapies. The limitations lowering efficacy are only exacerbated by the additional challenge of the traditional carriers’ crossing of the Blood Brain-Tumor Barrier.

We propose a novel delivery strategy that is grounded in a strikingly simple, powerful, and clinically implementable approach: the systemic administration of α-emitters loaded on unique hydroxyl PAMAM dendrimer-nanoparticles, i.e. dendrimer NPs (sugar- and peptide-functionalized generation-6 hydroxyl PAMAM dendrimers) which are shown (1) to effortlessly and quickly cross the Blood Brain-Tumor Barrier and with minimal transport across the Blood Brain Barrier of the healthy brain, upon systemic administration, (2) to be effectively taken up by the tumor-associated macrophages (non-cancerous cells but strongly correlated with tumor aggressiveness), and (3) to relatively quickly clear from the body and the brain when outside the tumor-associated macrophages which (4) most importantly, retain the dendrimer-nanoparticles for days, uniformly distributed within the brain tumors, and would uniformly irradiate the brain tumors resulting in maximum killing efficacy of tumor cancer cells.

Materials and Methods:

In vitro, the sensitivity of the glioblastoma cells, GL-261 to the free form of drug (in this case Actinium-225 chelated to DOTA, ²²⁵Ac-DOTA) and the ²²⁵Ac-DOTA radiolabeled onto dendrimer nanoparticles was evaluated using radio-sensitivity assays. The survival of immune-competent C57BL/6 mice bearing intracranially GL-261 glioma tumors was evaluated upon intravenous administration of α-particle therapy delivered by the dendrimer carriers for varying total injected radioactivities and was compared to the no treatment condition. The maximum tolerated dose (MTD) and long-term toxicities on tumor-free mice were also studied.

Results and Discussion: A colony survival assay demonstrated the sensitivity of GL-261 cells, in monolayers, to α-particle irradiation (Figure 1A), which were preserved upon dendrimer chelation. On mice with intracranially implanted GL-261 tumors, a single injection of 600 nCi and/or of 700 nCi total administered radioactivity, delivered by hydroxyl PAMAM dendrimers, showed good inhibition of the growth of GL-261 glioma tumors, as indicated by the statistically significant prolonged survival of these tumor bearing mice (p-values <0.05) (Figure 1B). The MTD has still not been reached at 800 nCi per animal. No long-term toxicities (6 months post injection) have yet been observed.

Conclusions: The use of hydroxyl PAMAM dendrimer nanoparticles as a delivery carrier for α-RPT to combat glioblastoma is promising as indicated by the improved survival of tumor-bearing mice and the well-tolerated profile of the radiolabeled carrier.