Non-invasive visualization of tumor infiltrating lymphocytes in patients with metastatic melanoma undergoing immune checkpoint inhibitor therapy: a pilot study

Early in the course of immunotherapy there is frequently a transient enlargement of tumor masses (pseudo-progression) due to tumor infiltration by TILs. Current clinical imaging modalities are not able to distinguished pseudo-progression from true tumor progression. Thus, patients often remain on treatment 4-8 weeks longer to confirm disease progression. Nuclear medicine offers the possibility to image immune cells and potentially discriminate pseudo-progression and progression. We conducted a pilot study in patients with metastatic melanoma receiving ipilimumab (IPI) or pembrolizumab (PEMBRO) to assess safety and feasibility of SPECT/CT imaging with ^99mTc- interleukin-2 (^99mTc-HYNIC-IL2) to detect TILs and distinguish between true progression from pseudo- progression. Scans were performed prior to and after 12w treatment. After labelling,^99mTc-HYNIC-IL2 was purified and diluted in 10 mL of 5% glucose with 0.1% human serum albumin. Of the 5 patients (2 treated with IPI and 3 with PEMBRO) enrolled, two failed to complete the second scan as they discontinued IPI due grade 3 colitis (1 patient) or patient refusal after developing multiple toxicities attributed to IPI (1 patient). Following the first scan, one patient reported to have a grade 1 pruritus with grade 1 pain. No other toxicities attributed to the radiopharmaceutical infusion were reported. Metastatic lesions could be visualized by ^99mTc-IL2 imaging and there was positive correlation between size and ^99mTc-HYNIC-IL2 uptake, both before and after 12 weeks of therapy. The results of this pilot study demonstrate the safety and feasibility of ^99mTc-IL2 imaging and has led to a number of hypotheses to be tested in future studies.