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

Potentially nonadentate (N 5 O 4 ) bifunctional chelator p -SCN–Bn–H 4 neunpa and its immunoconjugate H 4 neunpa–trastuzumab for 111 In radiolabeling are synthesized. The ability of p -SCN–Bn–H 4 neunpa and H 4 neunpa–trastuzumab to quantitatively radiolabel 111 InCl 3 at an ambient temperature within 15 or 30 min, respectively, is presented. Thermodynamic stability determination with In 3+, Bi 3+, and La 3+ resulted in high conditional stability constant ( p M) values. In vitro human serum stability assays have demonstrated both 111 In complexes to have high stability over 5 days. Mouse biodistribution of [ 111 In][In( p -NO 2 –Bn–neunpa)] −, compared to that of [ 111 In][In( p -NH 2 –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 111 In-neunpa–trastuzumab and 111 In–CHX-A″–DTPA–trastuzumab demonstrate a similar distribution profile but with slightly lower tumor uptake of 111 In-neunpa–trastuzumab compared to that of 111 In–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–H 4 neunpa to be a promising chelator for 111 In (and other radiometals) with high in vitro stability and also show H 4 neunpa–trastuzumab to be an excellent 111 In chelator with promising biodistribution in mice.