A novel ⁸⁹Zr-a-PD-1 immuno-PET-CT may improve pseudoprogression detection in a lung cancer murine model receiving immunotherapy.

e15116 Background: Immune checkpoint inhibitors (ICIs) have revolutionized lung cancer treatment improving survival outcomes of non-small cell lung cancer (NSCLC) patients. Response assessment of patients on ICIs treatment represents a challenge since an increase in tumor size or the appearance of new lesions might not reflect true disease progression (P) but pseudoprogression (PP), which has been reported in a range of 3-6% in NSCLC. Conventional 18 F-FDG-PET scans do not accurately discriminate P from PP. We have recently reported the efficacy of a combined blockade of PD-1 and ID1 in NSCLC mouse model. We aimed to evaluate a novel 89 Zr-labelled radiotracer for micro mPET-CT to detect PP in a NSCLC murine model. Methods: Syngeneic subcutaneous tumors were generated using LLC cells (with constitutive expression of ID1 or ID1-silenced) in C57BL6J and in ID1-deficient mice, treated with PBS or with a commercially available monoclonal antibody against PD-1 (a-PD-1) (RPM1-14, 300 mg, IP, on days 7, 10 and 14). Tumor growth and response to ID1-PD-1 blockade, was measured by 18 F-FDG uptake. Additionally, PP was measured labelling the anti-PD-1 with 89 Zr ( 89 Zr-a-PD-1) and analyzing 89 Zr uptake at day 15, 17 and 20. Tumor microenvironment was analyzed with immunohistochemistry, multiplex immunofluorescence and quantification of relative expression of interleukins by RT-PCR. Results: 18 F-FDG uptake did not show significant differences between groups, underestimating the real metabolic response induced by the treatment. However, 89 Zr-mPET-CT showed a significantly higher 89 Zr uptake when ID1 was inhibited in both, tumor cells and tumor micro-environment and mice were treated with a-PD-1 (p = 0.0075). The analysis of the tumor tissues in those animals by immunohistochemistry and multiplex immunofluorescence disclosed an increase in immune T CD8 + cells infiltration, being responsible for the antitumor response observed and correlating with 89 Zr signal. Moreover, the analysis of interleukins expression showed that tumors with ID1 inhibition in the tumor micro-environment presented an upregulation of pro-inflammatory interleukins, favoring T- cell infiltration. Conclusions: ID1 inhibition in tumor cells and in tumor micro-environment combined with PD-1 blockade enhanced immune cell infiltration through pro-inflammatory interleukins upregulation. 89 Zr-a-PD-1 immuno-PET-CT may improve PP detection in a NSCLC murine model receiving immunotherapy.