Preclinical validation of [18F]-FB-(Anti Human PD-L1) Nanobody for PET imaging

ObjectivesTumour cells use immune checkpoints, such as Programmed Death Ligand-1 (PD-L1), to escape the anti-tumour immune response by limiting the T-cell activity. Cancer immunotherapy and more specifically Programmed Death-1 (PD-1): PD-L1 checkpoint blockade therapy has proven to be effective against multiple types of cancer. The expression of PD-L1 in the tumour microenvironment has proven to be a positive indicator for the responsiveness to therapy but its assessment is limited by the inherent limitations of immunohistochemistry assays.1 Non-invasive imaging techniques, such as Positron Emission Tomography (PET)-imaging could overcome these limitations as they depict the whole picture of PD-L1 expression within the body. This study aims to develop a radiofluorinated Nanobody (Nb)-based tracer targeting human PD-L1 (hPD-L1) for PET-imaging.MethodsThe anti-hPD-L1 Nb was labelled with N-succinimidyl-4-[18F]fluorobenzoate ([18F]-SFB) prosthetic group. [18F]-SFB is produced using disposable cassettes on an AllinOne® synthesizer module (Trasis) in a 3-step, 1-pot reaction, previously described.2 Radiochemical purity (RCP) was assessed by reverse phase chromatography (RPC) on a diisopropylcyanopropylsilane column. The dried [18F]SFB was incubated with the anti-hPD-L1 Nb (4.34 mg/mL) for 30 min at room temperature in 0.2 M phosphate buffer pH 8.5-8 containing 20% V/V% ethanol. The radiolabelled Nb was purified using a disposable PD-10 size exclusion column (SEC). RCP was assessed by SEC and RPC on a dissopropylcyanopropylsilane column. The affinity and specificity towards PD-L1 were assessed on PD-L1-positive (pos.) or -negative (neg.) 624-MEL cells as control and biodistribution was evaluated in female C57BL/6 mice (N = 3, 6 weeks old). The biodistribution study was compared to previously obtained data from [68Ga]-Ga-NOTA-(hPD-L1) Nb.3Results[18F]-SFB was obtained after the automated production with a RCP > 95% and decay corrected radiochemical yield of 31 ± 0.04% (N = 8), in a procedure time of 48 minutes. [18F]-FB-(hPD-L1) Nb was obtained with > 95% RCP. In vitro characterisation showed that the radiofluorinated Nb retained its affinity (KD= 2.5 nM) (figure 1a) and specificity (2.37 ± 0.01% of total well activity on pos. cells vs. 0.32 ± 0.00% on neg cells, p<0.001) (figure 1b). The biodistribution study showed no unspecific organ accumulation and excretion via the kidneys (7.11 ± 0.95%). Kidney retention is 2.7 times lower compared to the [68Ga]-Ga-NOTA-(hPD-L1) Nb (figure 1c).ConclusionsThe anti-hPD-L1 Nb was successfully labelled with 18F and shows a favourable biodistribution pattern which, together with its in vitro characteristics, is attractive for further characterization as a new tracer for imaging PD-L1 overexpression in tumours.AcknowledgementsThe authors like to thank Kevin De Jonghe for handling the animals and Jesse Asiedu, Simon Deheegher and Ludo De Vis for technical help with the cyclotron.References1. Broos, K. et al. Noninvasive imaging of the PD-1:PD-L1 immune checkpoint: Embracing nuclear medicine for the benefit of personalized immunotherapy. Theranostics 8, 3559 (2018).2. Blykers, A. et al. PET imaging of macrophage mannose receptor-expressing macrophages in tumor stroma using 18F-radiolabeled camelid single-domain antibody fragments. J. Nucl. Med. 56, 1265–1271 (2015).3. Bridoux, J. et al. Anti-Human PD-L1 Nanobody for Immuno-PET Imaging: Validation of a Conjugation Strategy for Clinical Translation. Biomol. 2020, Vol. 10, Page 1388 10, 1388 (2020).