Synthesis and Evaluation of [ ¹⁷⁷ Lu]Lu-Labeled-NLS-Trastuzumab Conjugate for Targeting of HER2 Positive Metastatic Breast Cancer: Evaluating the Effect on Intracellular Localization

[¹⁷⁷Lu]-Lu-DOTA-Trastuzumab is currently under clinical evaluation for radioimmunotherapy (RIT) for HER2-positive breast cancer. This study aimed to enhance its internalization by modifying Trastuzumab with a nuclear localization signal (NLS: PKKKRKV). The NLS, bearing a terminal azide group, was synthesized via solid-phase peptide synthesis. DOTA, a chelator for ¹⁷⁷Lu, was incorporated at the α-amino group of lysine at the NLS's proline end, allowing UV-vis-based drug-to-antibody ratio (DAR) estimation. Both DOTA-Trastuzumab and DOTA-NLS-Trastuzumab, along with their [¹⁷⁷Lu]-Lu-labeled counterparts, were synthesized. <i>In vitro</i> studies in HER2-positive SK-OV-3 and SK-BR-3 cells showed an enhanced internalization of the NLS-modified conjugate. However, <i>ex vivo</i> and <i>in vivo</i> evaluation in SCID mice revealed lower tumor and nontarget organ accumulation for [¹⁷⁷Lu]-Lu-DOTA-NLS-Trastuzumab compared to [¹⁷⁷Lu]-Lu-DOTA-Trastuzumab, indicating altered biodistribution despite improved cellular uptake.