Tumour targeting of Auger emitters using DNA ligands conjugated to octreotate

Delivery systems exploiting natural cell transport processes of macromolecules for intracellular targeting of Auger electron emitters

The presence of Auger electrons (AE) among the decay products of a number of radionuclides makes these radionuclides an attractive means for treating cancer because these short-range electrons can cause significant damage in the immediate vicinity of the decomposition site. Moreover, the extreme locality of the effect provides a potential for selective eradication of cancer cells with minimal damage to adjacent normal cells provided that the delivery of the AE emitter to the most vulnerable parts of the cell can be achieved. Few cellular compartments have been regarded as the desired target site for AE emitters, with the cell nucleus generally recognized as the preferred site for AE decay due to the extreme sensitivity of nuclear DNA to direct damage by radiation of high linear energy transfer. Thus, the advantages of AE emitters for cancer therapy are most likely to be realized by their selective delivery into the nucleus of the malignant cells. To achieve this goal, delivery systems must combine a challenging complex of properties that not only provide cancer cell preferential recognition but also cell entry followed by transport into the cell nucleus. A promising strategy for achieving this is the recruitment of natural cell transport processes of macromolecules, involved in each of the aforementioned steps. To date, a number of constructs exploiting intracellular transport systems have been proposed for AE emitter delivery to the nucleus of a targeted cell. An example of such a multifunctional vehicle that provides smart step-by-step delivery is the so-called modular nanotransporter, which accomplishes selective recognition, binding, internalization, and endosomal escape followed by nuclear import of the delivered radionuclide. The current review will focus on delivery systems utilizing various intracellular transport pathways and their combinations in order to provide efficient targeting of AE to the cancer cell nucleus.

Modular Nanotransporters for Nuclear-Targeted Delivery of Auger Electron Emitters

This review describes artificial modular nanotransporters (MNTs) delivering their cargos into target cells and then into the nuclei – the most vulnerable cell compartment for most anticancer agents and especially for radionuclides emitting short-range particles. The MNT strategy uses natural subcellular transport processes inherent in practically all cells including cancer cells. The MNTs use these processes just as a passenger who purchased tickets for a multiple-transfer trip making use of different kinds of public transport to reach the desired destination. The MNTs are fusion polypeptides consisting of several parts, replaceable modules, accomplishing binding to a specific receptor on the cell and subsequent internalization, endosomal escape and transport into the cell nucleus. Radionuclides emitting short-range particles, like Auger electron emitters, acquire cell specificity and significantly higher cytotoxicity both in vitro and in vivo when delivered by the MNTs into the nuclei of cancer cells. MNT modules are interchangeable, allowing replacement of receptor recognition modules, which permits their use for different types of cancer cells and, as a cocktail of several MNTs, for targeting several tumor-specific molecules for personalized medicine.

Imaging the inside of a tumour: a review of radionuclide imaging and theranostics targeting intracellular epitopes

Molecular imaging of tumour tissue focusses mainly on extracellular epitopes such as tumour angiogenesis or signal transduction receptors expressed on the cell membrane. However, most biological processes that define tumour phenotype occur within the cell. In this mini-review, an overview is given of the various techniques to interrogate intracellular events using molecular imaging with radiolabelled compounds. Additionally, similar targeting techniques can be employed for radionuclide therapy using Auger electron emitters, and recent advances in Auger electron therapy are discussed.

PET imaging of tumours with a 64Cu labeled macrobicyclic cage amine ligand tethered to Tyr3-octreotate

The use of copper radioisotopes in cancer diagnosis and radionuclide therapy is possible using chelators that are capable of binding Cu(II) with sufficient stability in vivo to provide high tumour-to-background contrast. Here we report the design and synthesis of a new bifunctional chelator, 5-(8-methyl-3,6,10,13,16,19-hexaaza-bicyclo[6.6.6]icosan-1-ylamino)-5-oxopentanoic acid (MeCOSar), that forms copper complexes of exceptional stability by virtue of a cage amine (sarcophagine) ligand and a new conjugate referred to as SarTATE, obtained by the conjugation of MeCOSar to the tumour-targeting peptide Tyr(3)-octreotate. Radiolabeling of SarTATE with (64)Cu(II), a radioisotope suitable for positron emission tomography (PET), was fast (~20 min), easily performed at room temperature and consistently resulted in high radiochemical purity (>99%). In vitro and in vivo evaluation of (64)CuSarTATE demonstrated its high selectivity for tumour cells expressing somatostatin receptor 2 (sstr2). Biodistribution and PET imaging comparisons were made between (64)CuSarTATE and (64)Cu-labeled DOTA-Tyr(3)-octreotate ((64)CuDOTATATE). Both radiopharmaceuticals showed excellent uptake in sstr2-positive tumours at 2 h post-injection. While tumour uptake of (64)CuDOTATATE decreased significantly at 24 h, (64)CuSarTATE activity was retained, improving contrast at later time points. (64)CuSarTATE accumulated less than (64)CuDOTATATE in the non-target organs, liver and lungs. The uptake of (64)CuSarTATE in the kidneys was high at 2 h but showed significant clearance by 24 h. The new chemistry and pre-clinical evaluation presented here demonstrates that MeCOSar is a promising bifunctional chelator for Tyr(3)-octreotate that could be applied to a combined imaging and therapeutic regimen using a combination of (64)Cu- and (67)CuSarTATE complexes, owing to improved tumour-to-non-target organ ratios compared to (64)CuDOTATATE at longer time points.

Novel radioisotope-based nanomedical approaches

Radioisotope therapy of cancer is on the rise applying mainly β-emitting radionuclides. However, due to exposure of healthy tissues, the maximum achievable radiation dose with these is limited. Auger-electron emitters (AEs) represent a promising alternative because of their mode of decay within a short nanometer range. The challenge is that their therapeutic efficacy relies on a close vicinity to DNA. To overcome this and to minimize toxicity, the construction of smart nanomedical devices is required, which ascertain tumor cell targeting with succeeding cellular uptake and nuclear translocation. In this review we describe the potential of AEs with focus on their delivery down to the DNA level and their cellular effects. Reported efforts comprise different tumor-targeting strategies, including the use of antibodies or peptides with nuclear localizing sequences. Recently, attention has shifted to various nanoparticle formats for overcoming delivery problems. To this end, these approaches have mostly been tested in cell lines in vitro applying AEs more suited for imaging than therapy. This defines a demand for nanomedical formulations with documented in vivo activity, using AEs selected for their therapeutic potential to come closer to real clinical settings.