p-NO2–Bn–H4neunpa and H4neunpa–Trastuzumab: Bifunctional Chelator for Radiometalpharmaceuticals and 111In Immuno-Single Photon Emission Computed Tomography Imaging

Potentially nonadentate (N5O4) bifunctional chelator p-SCN–Bn–H4neunpa and its immunoconjugate H4neunpa–trastuzumab for 111In radiolabeling are synthesized. The ability of p-SCN–Bn–H4neunpa and H4neunpa–trastuzumab to quantitatively radiolabel 111InCl3at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In3+, Bi3+, and La3+resulted in high conditional stability constant (pM) values. In vitro human serum stability assays have demonstrated both 111In complexes to have high stability over 5 days. Mouse biodistribution of [111In][In(p-NO2–Bn–neunpa)]−, compared to that of [111In][In(p-NH2–Bn–CHX-A″–diethylenetriamine pentaacetic acid (DTPA))]2–, at 1, 4, and 24 h shows fast clearance of both complexes from the mice within 24 h. In a second mouse biodistribution study, the immunoconjugates 111In-neunpa–trastuzumab and 111In–CHX-A″–DTPA–trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of 111In-neunpa–trastuzumab compared to that of 111In–CHX-A″–DTPA–trastuzumab. These results were also confirmed by immuno-single photon emission computed tomography (immuno-SPECT) imaging in vivo. These initial investigations reveal the acyclic bifunctional chelator p-SCN–Bn–H4neunpa to be a promising chelator for 111In (and other radiometals) with high in vitro stability and also show H4neunpa–trastuzumab to be an excellent 111In chelator with promising biodistribution in mice.