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 Ac-225 (alpha, 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 (La3+), which is the largest nonradioactive metal of the lanthanide series, was adopted as a surrogate for Ac-225 to enable a series of nonradioactive chemical studies. In line with the H-1 NMR spectrum, the DFT (density functional theory)-calculated structure of the [La(py4pa)](-) anion possessed a high degree of symmetry, and the La3+ 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)](-) similar to 20.33, pLa = 21.0) compared to those of DOTA (log K-[La(DOTA)](-) similar to 24.25, pLa = 19.2) or H2macropa (log K-[La(macropa)](-) = 14.99, pLa similar to 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.

Automated summary

The study focuses on the synthesis, characterization, and stability of a new undecadendate chelator H(4)py4pa and its bifunctional analog H(4)py4pa-phenyl-NCS conjugated with the monoclonal antibody Trastuzumab targeting HER2+ cancer. The complex showed excellent affinity for Ac-225 and a high thermodynamic stability, making it a promising candidate for targeted alpha therapy. Incorporation of a short phenyl-NCS linker allowed for easy attachment to Trastuzumab, resulting in excellent in vivo stability and tumor specificity.