Development and evaluation of interleukin-2 derived radiotracers for PET imaging of T-cells in mice

Abstract

Recently, N-(4-¹⁸F-fluorobenzoyl)-interleukin-2 (¹⁸F-FB-IL2) was introduced as PET tracer for T-cell imaging. However, production is complex and time-consuming. Therefore, we developed two radiolabeled interleukin-2 (IL-2) variants, namely aluminum ¹⁸F-fluoride-(restrained complexing agent)-IL-2 (¹⁸F-AlF-RESCA-IL2) and ⁶⁸Ga-gallium-(1,4,7-triazacyclononane-4,7-diacetic acid-1-glutaric acid)-IL-2 (⁶⁸Ga-Ga-NODAGA-IL2) and compared their in-vitro and in-vivo characteristics with ¹⁸F-FB-IL2. Methods: Radiolabeling of ¹⁸F-AlF-RESCA-IL2 and ⁶⁸Ga-Ga-NODAGA-IL2 was optimized and stability was evaluated in human serum. Receptor binding was studied with activated human peripheral blood mononuclear cells (hPBMCs). Ex-vivo tracer biodistribution in immunocompetent BALB/cOlaHsd (BALB/c) mice was performed at 15, 60 and 90 min after tracer injection. In-vivo binding characteristics were studied in severe combined immune-deficient (SCID) mice inoculated with activated hPBMCs in Matrigel. Tracer was injected 15 min after hPBMCs inoculation and a 60-min dynamic PET scan was acquired, followed by ex-vivo biodistribution studies. Specific uptake was determined by co-injection of tracer with unlabeled IL2 and by evaluating uptake in a control group inoculated with Matrigel only. Results: ⁶⁸Ga-Ga-NODAGA-IL2 and ¹⁸F-AlF-RESCA-IL2 were produced with radiochemical purity >95% and radiochemical yield of 13.1±4.7% and 2.4±1.6% within 60 and 90 min, respectively. Both tracers were stable in serum, with >90% being intact tracer after 1h. In-vitro, both tracers displayed preferential binding to activated hPBMCs. Ex-vivo biodistribution studies in BALB/c mice showed higher uptake of ¹⁸F-AlF-RESCA-IL2 than ¹⁸F-FB-IL2 in liver, kidney, spleen, bone and bone marrow. ⁶⁸Ga-Ga-NODAGA-IL2 uptake in liver and kidney was higher than ¹⁸F-FB-IL2 uptake. In-vivo, all tracers revealed uptake in activated hPBMCs in SCID mice. Low uptake was seen after a blocking dose of IL2 or in the Matrigel control group. In addition, ¹⁸F-AlF-RESCA-IL2 yielded highest contrast PET images of target lymph nodes. Conclusion: Production of ¹⁸F-AlF-RESCA-IL2 and ⁶⁸Ga-Ga-NODAGA-IL2 is simpler and faster than ¹⁸F-FB-IL2. Both tracers showed good in-vitro and in-vivo characteristics with high uptake in lymphoid tissue and hPBMC xenografts.