A Versatile Platform for the Development of Radiolabeled Antibody-Recruiting Small Molecules.

Building on clinical case reports of the abscopal effect, there has been considerable interest in the synergistic effects of radiation and immunotherapies for the treatment of cancer. Here, the first radiolabeled antibody-recruiting small molecule that can chelate a variety of cytotoxic radionuclides is described. The platform consists of a tunable antibody-binding domain against a serum antibody of interest (e.g., dinitrophenyl hapten) to recruit endogenous antibodies that activate effector cell function, a chelate capable of binding diagnostic and therapeutic radiometals, and a tetrazine for bioorthogonal coupling with trans-cyclooctene-modified targeting vectors. The dinitrophenyl-tetrazine ligand was shown to both affect dose-dependent antibody recruitment and immune cell function (phagocytosis) in vitro, and the bisphosphonate 177Lu-complex was shown to accumulate at sites of calcium accretion in vivo, which was achieved using both active and pretargeting strategies.

A Bone-Seeking trans-Cyclooctene for Pretargeting and Bioorthogonal Chemistry: A Proof of Concept Study Using 99mTc- and 177Lu-Labeled Tetrazines.

A high yield synthesis of a novel, small molecule, bisphosphonate-modified trans-cyclooctene (TCO-BP, 2) that binds to regions of active bone metabolism and captures functionalized tetrazines in vivo, via the bioorthogonal inverse electron demand Diels-Alder (IEDDA) cycloaddition, was developed. A 99mTc-labeled derivative of 2 demonstrated selective localization to shoulder and knee joints in a biodistribution study in normal mice. Compound 2 reacted rapidly with a 177Lu-labeled tetrazine in vitro, and pretargeting experiments in mice, using 2 and the 177Lu-labeled tetrazine, yielded high activity concentrations in shoulder and knee joints, with minimal uptake in other tissues. Pretargeting experiments with 2 and a novel 99mTc-labeled tetrazine also produced high activity concentrations in the knees and shoulders. Critically, both radiolabeled tetrazines showed negligible uptake in the skeleton and joints when administered in the absence of 2. Compound 2 can be utilized to target functionalized tetrazines to bone and represents a convenient reagent to test novel tetrazines for use with in vivo bioorthogonal pretargeting strategies.

Triazole Appending Agent (TAAG): A New Synthon for Preparing Iodine-Based Molecular Imaging and Radiotherapy Agents.

A new prosthetic group referred to as the triazole appending agent (TAAG) was developed as a means to prepare targeted radioiodine-based molecular imaging and therapy agents. Tributyltin-TAAG and the fluorous analogue were synthesized in high yield using simple click chemistry and the products labeled in greater than 95% RCY with (123)I. A TAAG derivative of an inhibitor of prostate-specific membrane antigen was prepared and radiolabeled with (123)I in 85% yield where biodistribution studies in LNCap prostate cancer tumor models showed rapid clearance of the agent from nontarget tissues and tumor accumulation of 20% injected dose g(-1) at 1 h. The results presented demonstrate that the TAAG group promotes minimal nonspecific binding and that labeled conjugates can achieve high tumor uptake and exquisite target-to-nontarget ratios.