225Ac-H4py4pa for Targeted Alpha Therapy

Herein, we present the syntheses and characterization of a new undecadendate chelator, H 4 py4pa, and its bifunctional analog H 4 py4pa-phenyl-NCS, conjugated to the monoclonal antibody, Trastuzumab, which targets the HER2+ cancer. H 4 py4pa possesses excellent affinity for 225 Ac (α, t 1/2 = 9.92 d) for targeted alpha therapy (TAT), where quantitative radiolabeling yield was achieved at ambient temperature, pH = 7, in 30 min at 10 –6 M chelator concentration, leading to a complex highly stable in mouse serum for at least 9 d. To investigate the chelation of H 4 py4pa with large metal ions, lanthanum (La 3+ ), which is the largest nonradioactive metal of the lanthanide series, was adopted as a surrogate for 225 Ac to enable a series of nonradioactive chemical studies. In line with the 1 H NMR spectrum, the DFT (density functional theory)-calculated structure of the [La(py4pa)] − anion possessed a high degree of symmetry, and the La 3+ ion was secured by two distinct pairs of picolinate arms. Furthermore, the [La(py4pa)] − complex also demonstrated a superb thermodynamic stability (log K [La(py4pa)] – ∼ 20.33, pLa = 21.0) compared to those of DOTA (log K [La(DOTA)] – ∼ 24.25, pLa = 19.2) or H 2 macropa (log K [La(macropa)] – = 14.99, pLa ∼ 8.5). Moreover, the functional versatility offered by the bifunctional py4pa precursor permits facile incorporation of various linkers for bioconjugation through direct nucleophilic substitution. In this work, a short phenyl-NCS linker was incorporated to tether H 4 py4pa to Trastuzumab. Radiolabeling studies, in vitro serum stability, and animal studies were performed in parallel with the DOTA-benzyl-Trastuzumab. Both displayed excellent in vivo stability and tumor specificity.