[(225)Ac]Ac- and [(111)In]In-DOTA-trastuzumab theranostic pair: cellular dosimetry and cytotoxicity in vitro and tumour and normal tissue uptake in vivo in NRG mice with HER2-positive human breast cancer xenografts

BACKGROUND: Trastuzumab (Herceptin) has improved the outcome for patients with HER2-positive breast cancer (BC) but brain metastases (BM) remain a challenge due to poor uptake of trastuzumab into the brain. Radioimmunotherapy (RIT) with trastuzumab labeled with α-particle emitting, (225)Ac may overcome this challenge by increasing the cytotoxic potency on HER2-positive BC cells. Our first aim was to synthesize and characterize [(111)In]In-DOTA-trastuzumab and [(225)Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC, respectively. A second aim was to estimate the cellular dosimetry of [(225)Ac]Ac-DOTA-trastuzumab and determine its cytotoxicity in vitro on HER2-positive BC cells. A third aim was to study the tumour and normal tissue uptake of [(225)Ac]Ac-DOTA-trastuzumab using [(111)In]In-DOTA-trastuzumab as a radiotracer in vivo in NRG mice with s.c. 164/8-1B/H2N.luc(+) human BC tumours that metastasize to the brain. RESULTS: Trastuzumab was conjugated to 12.7 ± 1.2 DOTA chelators and labeled with (111)In or (225)Ac. [(111)In]In-DOTA-trastuzumab exhibited high affinity specific binding to HER2-positive SK-BR-3 human BC cells (K(D) = 1.2 ± 0.3 × 10(–8) mol/L). Treatment with [(225)Ac]Ac-DOTA-trastuzumab decreased the surviving fraction (SF) of SK-BR-3 cells dependent on the specific activity (SA) with SF < 0.001 at SA = 0.74 kBq/µg. No surviving colonies were noted at SA = 1.10 kBq/µg or 1.665 kBq/µg. Multiple DNA double-strand breaks (DSBs) were detected in SK-BR-3 cells exposed to [(225)Ac]Ac-DOTA-trastuzumab by γ-H2AX immunofluorescence microscopy. The time-integrated activity of [(111)In]In-DOTA-trastuzumab in SK-BR-3 cells was measured and used to estimate the absorbed doses from [(225)Ac]Ac-DOTA-trastuzumab by Monte Carlo N-Particle simulation for correlation with the SF. The dose required to decrease the SF of SK-BR-3 cells to 0.10 (D(10)) was 1.10 Gy. Based on the D(10) reported for γ-irradiation of SK-BR-3 cells, we estimate that the relative biological effectiveness of the α-particles emitted by (225)Ac is 4.4. Biodistribution studies in NRG mice with s.c. 164/8-1B/H2N.luc(+) human BC tumours at 48 h post-coinjection of [(111)In]In-DOTA-trastuzumab and [(225)Ac]Ac-DOTA-trastuzumab revealed HER2-specific tumour uptake (10.6 ± 0.6% ID/g) but spleen uptake was high (28.9 ± 7.4% ID/g). Tumours were well-visualized by SPECT/CT imaging using [(111)In]In-DOTA-trastuzumab. CONCLUSION: We conclude that [(225)Ac]Ac-DOTA-trastuzumab exhibited potent and HER2-specific cytotoxicity on SK-BR-3 cells in vitro and HER2-specific uptake in s.c. 164/8-1B/H2N.luc(+) human BC tumours in NRG mice, and these tumours were imaged by SPECT/CT with [(111)In]In-DOTA-trastuzumab. These results are promising for combining [(111)In]In-DOTA-trastuzumab and [(225)Ac]Ac-DOTA-trastuzumab as a theranostic pair for imaging and RIT of HER2-positive BC. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s41181-023-00208-0.