[¹⁸F]FDG-PET/CT Imaging for Response Characterisation of Experimental Melanomas to Anti-PD-L1/Anti-CTLA-4...

Immune checkpoint inhibition has shown promising results in malignant melanoma, but not all patients respond equally well, necessitating early, accurate monitoring of immunotherapy response. [¹⁸F]FDG-PET/CT aids in characterising therapy response beyond morphology, but validated imaging biomarkers for immunotherapy response remain scarce. This study investigated three-time point [¹⁸F]FDG-PET/CT to monitor combined anti-PD-L1/anti-CTLA-4 immunotherapy in murine melanoma allografts and compared quantitative in vivo imaging biomarkers with ex vivo biomarkers from multiparametric immunohistochemistry at each time point.

Melanoma cells (B16-F10) were injected subcutaneously into C57BL/6 mice (n = 40). Seven days post-inoculation, baseline [¹⁸F]FDG-PET/CT was conducted. Animals were randomized into two groups; the therapy group received 5 i.p.-injections of anti-PD-L1/anti-CTLA-4 (20 µg/kg) on days 7, 9, 11, 13 and 15 after tumor cell inoculation. The control group received sham treatment. PET/CT was performed at baseline (day 7 post inoculation), follow-up 1(day 13; FU-1) and follow-up 2 (day 19; FU-2). Tumor allografts were harvested at each time point for immunohistochemistry (CD8, Ki-67, TUNEL) to validate imaging parameters (MTV, SUVmax).

At FU-1, the therapy group exhibited significantly lower MTV than the control group (p = 0.004). At FU-2, MTV and SUVmax were significantly lower (MTV: p = 0.008; SUVmax: p = 0.0003) compared to controls. Ex vivo analysis revealed significant anti-tumor effects in the therapy group, with higher apoptosis rates (FU-1: p = 0.012; FU-2: p = 0.001), more CD8-positive T-cells (FU-2: p = 0.003) and lower tumor cell proliferation (FU-1: p = 0.012; FU-2: p = 0.012).

Multi-time point [¹⁸F]FDG-PET/CT allowed for early non-invasive monitoring of combined anti-PD-L1/anti-CTLA-4 immunotherapy in experimental melanomas, validated by multiparametric immunohistochemistry with significant pro-immunogenic, pro-apoptotic and anti-proliferative effects.