Comparative analysis of folate derived PET imaging agents with [ ¹⁸F]-2-fluoro-2-deoxy-d-glucose using a rodent inflammatory paw model

Activated macrophages play a significant role in initiation and progression of inflammatory diseases and may serve as the basis for the development of targeted diagnostic methods for imaging sites of inflammation. Folate receptor beta (FR-β) is differentially expressed on activated macrophages associated with inflammatory disease states yet is absent in either quiescent or resting macrophages. Because folate binds with high affinity to FR-β, development of folate directed imaging agents has proceeded rapidly in the past decade. However, reports of PET based imaging agents for use in inflammatory conditions remain limited. To investigate whether FR-β expressing macrophages could be exploited for PET based inflammatory imaging, two separate folate-targeted PET imaging agents were developed, 4-[¹⁸F]-fluorophenylfolate and [ ⁶⁸Ga]-DOTA-folate, and their ability to target activated macrophages were examined in a rodent inflammatory paw model. We further compared inflamed tissue uptake with 2-[¹⁸F]fluoro-2-deoxy-d-glucose ([ ¹⁸F]-FDG). microPET analysis demonstrated that both folate-targeted PET tracers had higher uptake in the inflamed paw compared to the control paw. When these radiotracers were compared to [¹⁸F]-FDG, both folate PET tracers had a higher signal-to-noise ratio (SNR) than [¹⁸F]-FDG, suggesting that folate tracers may be superior to [¹⁸F]-FDG in detecting diseases with an inflammatory component. Moreover, both folate-PET imaging agents also bind to FR-α which is overexpressed on multiple human cancers. Therefore, these folate derived PET tracers may also find use for localizing and staging FR⁺ cancers, monitoring response to therapy, and for selecting patients for tandem folate-targeted therapies.