Complete Remissions of HER2-Positive Trastuzumab-Resistant Xenografts Using a Potent [225Ac]Ac-Labeled Anti-HER2 Antibody–Drug Radioconjugate

Abstract Purpose: There is overwhelming interest to use actinium-225 ([225Ac]Ac) to develop targeted α therapies. Antibody–drug conjugates (ADC) are highly cytotoxic. Combining [225Ac]Ac with an ADC to develop an antibody–drug radioconjugate [225Ac]Ac–macropa–trastuzumab(T)–PEG6–emtansine (DM1), is expected to be more effective than its ADC (T–PEG6–DM1) against breast cancer. Experimental Design: [89Zr]Zr–p-isothiocyanatobenzyl desferrioxamine (DFO)–T–PEG6–DM1 (imaging) and [225Ac]Ac–macropa–T–PEG6–DM1 (radiotherapy) were developed. Biodistribution and safety evaluations of [225Ac]Ac–macropa–T-PEG6–DM1 were carried out in non–tumor-bearing BALB/c mice. MicroPET imaging and biodistribution were done using [89Zr]Zr-DFO–T–PEG6–DM1, and radiotherapy using [225Ac]Ac–macropa–T–PEG6–DM1 was carried out in athymic BALB/c nude mice bearing trastuzumab-resistant HCC1954 and trastuzumab-DM1 (T-DM1)/trastuzumab-resistant JIMT-1 tumor-bearing mice. Results: After 7 days of incubation at 37°C, [225Ac]Ac–macropa–T–PEG6–DM1 was stable in both human serum (89.2% ± 0.9%) and PBS (82.8% ± 0.4%). T–PEG6–DM1 (8 mg/kg) and [225Ac]Ac–macropa–T–PEG6–DM1 (3 × 18 kBq) administered separately in non–tumor-bearing mice 10 days apart were well tolerated biochemically and hematologically. Imaging and biodistribution showed high tumor uptake of [89Zr]Zr-DFO–T–PEG6–DM1 in tumor-bearing mice at 120 hours after injection: 38.1% ± 2.8% IA/g (HCC1954) and 14.6% ± 1% IA/g (JIMT-1). In HCC1954 tumor-bearing mice, all treatment groups had complete remission (8/8), indicative of the responsiveness of the xenograft to T-DM1–based treatments, whereas for JIMT-1 xenografts (having 1/8 complete remission) at 23 days after treatment, tumor volumes were 332.1 ± 77.5 vs. 244.6 ± 63 vs. 417.9 ± 62.1 vs. 102.4 ± 18.5 for the saline (negative control), T-DM1 (positive control), T–PEG6–DM1, and [225Ac]Ac–macropa–T–PEG6–DM1, respectively. Conclusions: [225Ac]Ac–macropa–T–PEG6–DM1 is more potent than ADC against trastuzumab-resistant breast cancer and necessitates clinical translation.