Previously, we successfully synthesized a 18F-labeled positron-emission tomography (PET) tracer, termed 18F-5-fluoro-N-(2-[diethylamino]ethyl)picolinamide (18F-5-FPN), with high specificity for melanin. In this study, we sought to investigate the value of 18F-5-FPN in assessing the response to photothermal therapy (PTT) in melanoma via comparison with 18F-fluorodeoxyglucose (18F-FDG) to reveal an early response, recognize early recurrence, and distinguish the inflammatory response during the treatment. B16F10, inflammatory, and MDA-MB-231 models were subjected to 18F-FDG PET and 18F-5-FPN PET static acquisitions. We compared quantitative data to assess the specificity of different agents for different diseases. B16F10 and MDA-MB-231subcutaneous tumor models were irradiated with an 808 nm laser for PTT. Their survival was documented to observe the efficacy of and response to PTT, using 18F-5-FPN and 18F-FDG PET. 18F-5-FPN accumulated in B16F10 cell xenografts only, whereas 18F-FDG accumulated in all three models. Melanin in B16F10 cell xenografts successfully transformed the optical energy into heat. Hematoxylin and eosin (H&E) staining at 24 h revealed destruction and extensive necrosis of tumor tissue. PTT rapidly inhibited the growth of B16F10 cell xenografts and prolonged the median survival. The mean tumor uptakes of 18F-5-FPN on day 2 (7.52 ± 3.65 %ID/g) and day 6 (10.22 ± 6.00 %ID/g) were much lower than that before treatment (18.33 ± 4.98 %ID/g, p < 0.01). However, a significant difference in 18F-FDG uptakes was not found between day 1 after PTT and before treatment. Compared with 18F-FDG, 18F-5-FPN PET could estimate PTT efficacy in melanoma, monitor minimal recurrence, and distinguish melanoma from inflammation and other carcinoma types, thanks to its high affinity to melanin. 18F-5-FPN may provide a new approach for precise and accurate evaluation of response, timely management of therapeutic regimens, and sensitive follow-up.
Keywords: 18F-5-FPN; 18F-FDG; malignant melanoma; photothermal therapy; positron-emission tomography; response evaluation.