Towards bioorthogonal exchange targeting

Abstract

One of the most promising radionuclides used in targeted radionuclide therapy is the alpha particle emitter Actinium-225. Next to its high linear energy transfer the multiple alpha particles that are generated in its decay chain are what renders Actinium-225 a particularly effective cytotoxic radionuclide. The problem with these four high-energy daughter nuclides is that they do not stay bound to the initial target due to the high recoil that happens after the first α-decay. It is crucial for the radioactive decay to take place only inside the targeted tumor cells to take advantage of the α-emitters potency thereby greatly increasing therapeutic efficiency as well as preventing any unwanted off-target dose. Although some ligands such as PSMA-617 actively and rapidly internalize and are thus suited for Actinium-225 therapy many potential therapeutics do not readily internalize, causing reduced efficiency and off-target dose. For this reason, we aimed to take steps towards the development of a triggered internalization mechanism based on biorthogonal chemistry, using an activatable cell-penetrating peptide. This approach should allow the use of highly effective therapeutic radionuclide 225Ac with a broad variety of targets and their ligands. Cell-penetrating peptides (CPPs) are short peptides, ranging from 5-40 amino acids, with the ability to gain access to the cell interior by means of different mechanisms and with the capacity to promote the intracellular delivery of covalently or noncovalently conjugated bioactive cargoes. A significant advancement to improve CPP specificity was achieved by the recent development of activatable CPPs (ACPPs) which consist of a polycationic CPP connected via a cleavable linker to a matching polyanion, which reduces the net charge to nearly zero and thereby inhibits adhesion and uptake into cells. Upon cleavage of this linker, the polyanion is released/dissociated, locally unmasking the polycation and restoring its inherent ability to enter cells. Recently, the fastest known bioorthogonal ligation between 1,2,4,5- tetrazines (Tz) and highly strained transcyclooctenes (TCOs) in an inverse electron demand Diels-Alder reaction has been modified to allow bioorthogonal bond cleavage reactions in a click-to-release approach. Complementary to the bioorthogonal reactions that ligate two molecules, these reactions allow the unmasking of molecules, cleavage of linkers, and release of payloads. Using an activatable cell penetrating peptide would allow the use of tumor specific non internalizing receptors to target the tumor while the cell penetrating peptide is able to internalize the molecule after cleavage of the masking agent. We aim towards the development and investigation of new chemical tools and controlled reactions inside living cells to enable intracellular bioorthogonal cleavage thereby allowing the use of α-emitting radionuclides such as 225Ac for a variety of ligands.