Automated radiofluorination of HER2 single domain antibody: the road towards the clinical translation of [18F]FB-HER2 sdAb

Background

With the next generation of Human Epidermal Growth Factor Receptor 2 (HER2) -targeting therapies, such as antibody–drug conjugates, showing benefit in “HER2 low” and even “HER2 ultralow” patients, the need for novel methods to quantify HER2 expression accurately becomes even more important for clinical decision making. A HER2 PET/CT imaging assessment could evaluate HER2 positive disease locations while improving patient care, reducing the need for invasive biopsies. A single-domain antibody (sdAb)-based PET tracer could combine the high specificity of sdAbs with short-lived radionuclides such as fluorine-18 (¹⁸F) and gallium-68 (⁶⁸Ga). SdAb-based PET tracers have clinically been used via a ⁶⁸Ga-chelator approach. However, the distribution of ⁶⁸Ga-labelled pharmaceuticals to peripheral PET centres is more challenging to organize due to the short half-life of ⁶⁸Ga, most certainly when the available activity is limited by a generator. Cyclotron produced ⁶⁸Ga has removed this limitation. Distribution of ¹⁸F-labelled pharmaceuticals remains less challenging due to its slightly longer half-life, and radiofluorination of sdAbs via N-succinimidyl-4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) has shown to be a promising strategy for developing sdAb-based PET tracers. Although [¹⁸F]SFB automation has been reported, automating protein conjugation proves challenging. Herein we report the fully automated, cartridge-based production of [¹⁸F]FB-HER2 sdAb on a single synthesis module.

Results

[¹⁸F]FB-HER2 sdAb (> 6 GBq) was obtained after a fully automated production (95 min), with a RCP > 95%, apparent molar activity > 20 GBq/µmol and decay-corrected radiochemical yield (RCY d.c.) of 14 ± 2% (n = 4). Further upscaling amounted to production batches of 16 GBq with an apparent molar activity > 40 GBq/µmol and RCY d.c. of 8 ± 1% (n = 4). Ex vivo biodistribution and PET imaging showed specific HER2-positive tumour targeting and low kidney retention.

Conclusion

The [¹⁸F]FB-HER2 sdAb tracer was produced with clinically relevant activities using a fully automated production method. The automated production method was designed to ease the translation to the clinic and has the potential to be used not only in mono-centre but also multi-centre clinical trials with one central production site. [¹⁸F]FB-HER2 sdAb showed a favourable biodistribution pattern and could be a valuable alternative to ⁶⁸Ga-labelled sdAb-based PET tracers in the clinic.