Synthesis, characterization and radiolabeling of carborane-functionalized tetrazines for use in inverse electron demand Diels–Alder ligation reactions

Carborane-tetrazine derivatives were developed that can be used to enable boron clusters to bind specific targets in vivo using pretargeting strategies and bioorthogonal inverse electron demand Diels–Alder chemistry. Specifically, 1,12-dicarba-closo-dodecaborate-1-(4-(1,2,4,5-tetrazin-3-yl)phenyl)methanamide and 1,2-dicarba-closo-dodecaborate-1-(4-(1,2,4,5-tetrazin-3-yl)phenyl)methanamide were synthesized in 49% and 78% yield respectively, and X-ray structures of both compounds determined. The ortho-carborane derivative was converted to the corresponding nido-cluster under mild conditions, and the product successfully radiolabelled with 125I and 123I. The carborane-tetrazines react rapidly and in quantitative yields with (E)-cyclooct-4-enol (TCO) via a [4 + 2] inverse-electron demand Diels–Alder type reaction, where the second order rate constant in acetonitrile for the iodinated cluster was 199 ± 26 M−1s−1. The labelled tetrazines were stable in solution for extended periods and they were shown to bind in vitro to H520 cells, labelled with a TCO-modified antibody. Imaging studies on the 123I-labeled carborane-tetrazine were performed in healthy mice, and demonstrated minimal loss of iodine in vivo and high uptake in the liver and gall bladder. The reported compounds offer an alternative means of developing targeted boron neutron capture therapy (BNCT) and boron-based molecular imaging agents.