Using the site-specific radiolabeling of a PD-L1 nanobody via maleimide–cysteine chemistry to develop a PET-tracer as screening tool for personalized oncology treatment

Description

Aim. Immune checkpoints inhibitors of programmed cell death-1 (PD-1) and its ligand PD-L1 have proven to successfully treat cancer in a subset of patients. Typically, from all the patients with various cancer types, 20% have a positive response. Cancer cells expressing PD-L1 are more likely to be responsive to the treatment. Therefore, distinguishing oncology patients that do express PD-1/PD-L1 from patients that do not allows a more personalized and efficient treatment. PD-1 and PD-L1 expression in cancer cells is often assessed through immunohistochemical detection in a tumor lesion biopsy. Besides being invasive, taking a tumor biopsy to assess PD-1/PD-L1 expression does not take into account the heterogeneity of cancer cells in a tumor lesion. We aim to develop a nanobody (Nb)-based radio-immunotracer for PET-imaging targeting human PD-L1 to efficiently screen oncology patients in a non-invasive manner for a more personalized treatment.Methods. Maleimide (mal)-cysteine chemistry was used to site-specifically conjugate the cysteine-tagged Nb with a mal-NOTA chelator. The radiolabeling of the conjugated Nb with gallium-68 (68Ga) was tested at different Nb concentrations and temperatures. Affinity and specificity were assayed by surface plasmon resonance and on hPD-L1POS or hPD-L1NEG 624-MEL cells. Xenografted athymic nude mice bearing 624-MEL PD-L1POS or PD-L1NEG tumors were injected with the tracer and imaging was performed 1 h post-injection (p.i.). Ex vivo biodistribution studies were performed 1 h 20 min p.i.Results. Ideal 68Ga-labeling conditions were found at 50 °C for 15 min and at a concentration of 3.6 μM of NOTA-mal-hPD-L1 Nb. [68Ga]Ga-NOTA-mal-hPD-L1 Nb was obtained in 80 ± 5% DC-RCY with a RCP > 99%, and was stable in injection buffer and human serum up to 3 h. The in vitro characterization confirmed the affinity and selectivity of [68Ga]Ga-NOTA-mal-hPD-L1 Nb for hPD-L1POS cells. 1 h p.i. PET-imaging allowed the visualization of the tumor lesions with PD-L1 expressing cells. Ex vivo biodistribution revealed a tracer uptake of 1.86 ± 0.67% IA/g in the positive tumors compared with 0.42 ± 0.04% IA/g in the negative tumors. Low background uptake was measured in the other organs and tissues, except for the kidneys and bladder, due to the expected excretion route of Nbs.Conclusion. The data obtained show that 68Ga-labeled NOTA-mal-hPD-L1 Nb is a promising PET-tracer targeting tumor lesions with PD-L1 expressing cells. Thanks to its ease of production, stability, and specificity for PD-L1, the developed tracer could routinely be used for non-invasive screening of oncology patients.

Period	7 May 2022
Event title	Belgian Society of Nuclear Medicine 2022 Antwerp Symposium
Event type	Conference
Location	Antwerpen, Belgium