Preclinical Comparison of [In]In- and [Ac]Ac-DOTA-Trastuzumab IgG, F(ab') and Fab for Theranostic SPECT/CT Imaging and α-Particle Radioimmunotherapy of HER2-Positive Human Breast Cancer.

Radioimmunotherapy (RIT) with α-particle-emitting, Ac complexed to trastuzumab may offer an alternative treatment for patients who progress on HER2-targeted therapies. Moreover, RIT with [Ac]Ac-DOTA-trastuzumab could be combined with SPECT/CT imaging with [In]In-DOTA-trastuzumab in a theranostic approach. In this study, we compared DOTA-conjugated trastuzumab IgG, F(ab') or Fab complexed to In or Ac for SPECT/CT imaging and α-particle RIT of subcutaneous (s.c.) HER2-positive 164/8-1B/H2N.luc human BC tumors in NRG mice. SPECT/CT imaging and tumor and normal tissue uptake were compared in NRG or NOD-SCID mice coinjected i.v. with [In]In-DOTA-trastuzumab IgG, F(ab') or Fab and [Ac]Ac-DOTA-trastuzumab IgG, F(ab') or Fab. Radiation absorbed doses in the tumor and normal organs for [Ac]Ac-DOTA-trastuzumab IgG, F(ab') or Fab were estimated based on the biodistribution of the [In]In-DOTA-trastuzumab IgG, F(ab') or Fab. Normal tissue toxicity was assessed by hematology and blood biochemistry analyses and monitoring body weight in NRG mice injected i.v. with 2 and 4 kBq of [Ac]Ac-DOTA-trastuzumab IgG, F(ab') or Fab separated by 8 d. RIT studies were performed in NRG mice with s.c. 164/8-1B/H2N.luc tumors injected i.v. with 2 kBq and 4 kBq of [Ac]Ac-DOTA-trastuzumab IgG, F(ab') or Fab separated by 8 d or irrelevant [Ac]Ac-DOTA-IgG, two doses of unlabeled trastuzumab IgG or 0.9% NaCl. A tumor growth index (TGI) was plotted vs time (d) and Kaplan-Meier median survival estimated. [In]In-DOTA-trastuzumab IgG or F(ab') exhibited 4.1-fold and 3.3-fold significantly greater tumor uptake at 2 d postinjection (p.i.) than Fab at 24 h p.i. However, spleen uptake at 2 d p.i. for [In]In-DOTA-trastuzumab IgG was 3.3-fold significantly higher than F(ab') and 13.2-fold higher than Fab at 24 h p.i. [In]In-DOTA-trastuzumab F(ab') and Fab exhibited higher kidney uptake than IgG. Tumors were imaged by SPECT/CT with [In]In-DOTA-trastuzumab IgG and F(ab') but were not well-visualized with [In]In-DOTA-trastuzumab Fab. The absorbed dose in the tumor was 2.2-fold greater for [Ac]Ac-DOTA-trastuzumab F(ab') than IgG and 3.4-fold greater than Fab. Hematological toxicity was observed for [Ac]Ac-DOTA-trastuzumab IgG but not for [Ac]Ac-DOTA-trastuzumab F(ab') or Fab. No kidney or liver toxicity or decreased body weight was observed for any RIT agent. Tumor growth was significantly inhibited by [Ac]Ac-DOTA-trastuzumab IgG, F(ab') or Fab but [Ac]Ac-DOTA-trastuzumab F(ab') was most effective for increasing median survival (46 d vs 22 d for IgG and 29 d for Fab). We conclude that [In]In- and [Ac]Ac-DOTA-trastuzumab F(ab') exhibited superior properties for theranostic imaging and α-particle RIT of HER2-positive human BC xenografts in NRG mice.