Radioimmunotherapy (RIT) with α-particle-emitting, 225Ac complexed to trastuzumab may offer an alternative treatment for patients who progress on HER2-targeted therapies. Moreover, RIT with [225Ac]Ac-DOTA-trastuzumab could be combined with SPECT/CT imaging with [111In]In-DOTA-trastuzumab in a theranostic approach. In this study, we compared DOTA-conjugated trastuzumab IgG, F(ab')2 or Fab complexed to 111In or 225Ac 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 [111In]In-DOTA-trastuzumab IgG, F(ab')2 or Fab and [225Ac]Ac-DOTA-trastuzumab IgG, F(ab')2 or Fab. Radiation absorbed doses in the tumor and normal organs for [225Ac]Ac-DOTA-trastuzumab IgG, F(ab')2 or Fab were estimated based on the biodistribution of the [111In]In-DOTA-trastuzumab IgG, F(ab')2 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 [225Ac]Ac-DOTA-trastuzumab IgG, F(ab')2 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 [225Ac]Ac-DOTA-trastuzumab IgG, F(ab')2 or Fab separated by 8 d or irrelevant [225Ac]Ac-DOTA-IgG1, 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. [111In]In-DOTA-trastuzumab IgG or F(ab')2 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 [111In]In-DOTA-trastuzumab IgG was 3.3-fold significantly higher than F(ab')2 and 13.2-fold higher than Fab at 24 h p.i. [111In]In-DOTA-trastuzumab F(ab')2 and Fab exhibited higher kidney uptake than IgG. Tumors were imaged by SPECT/CT with [111In]In-DOTA-trastuzumab IgG and F(ab')2 but were not well-visualized with [111In]In-DOTA-trastuzumab Fab. The absorbed dose in the tumor was 2.2-fold greater for [225Ac]Ac-DOTA-trastuzumab F(ab')2 than IgG and 3.4-fold greater than Fab. Hematological toxicity was observed for [225Ac]Ac-DOTA-trastuzumab IgG but not for [225Ac]Ac-DOTA-trastuzumab F(ab')2 or Fab. No kidney or liver toxicity or decreased body weight was observed for any RIT agent. Tumor growth was significantly inhibited by [225Ac]Ac-DOTA-trastuzumab IgG, F(ab')2 or Fab but [225Ac]Ac-DOTA-trastuzumab F(ab')2 was most effective for increasing median survival (46 d vs 22 d for IgG and 29 d for Fab). We conclude that [111In]In- and [225Ac]Ac-DOTA-trastuzumab F(ab')2 exhibited superior properties for theranostic imaging and α-particle RIT of HER2-positive human BC xenografts in NRG mice.
Keywords: 111In; 225Ac; HER2; SPECT/CT; radioimmunotherapy; theranostic; trastuzumab; α-particles.