[ 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.