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

Aims Tumor cells use immune checkpoints, such as Programmed Death Ligand-1 (PD-L1), to escape the anti-tumor 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 tumor 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. 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. Methods The 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. Radiochemical purity (RCP) was assessed by reverse phase chromatography (RPC). 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 8.5-8.7 containing 20% V/V% ethanol. The radiolabeled Nb was purified using a disposable PD-10 size exclusion column (SEC). RCP was assessed by SEC and RPC. 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. Results [18F]-SFB was obtained after the automated production of 48 minutes with RCP > 95% and decay corrected radiochemical yield of 31 ± 4 % (N = 8). [18F]-FB-(hPD-L1) Nb was obtained with > 95% RCP. In vitro characterization 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). Conclusions The anti-hPD-L1 Nb was successfully labelled with 18F and shows a favorable biodistribution pattern which, together with its in vitro characteristics, is attractive for further characterization as a new tracer for imaging PD-L1 overexpression in tumors.