Preclinical Exploration Of A Novel Probe,¹⁸F-5-FPN,for Imaging And Therapy Evaluation Of Malignant Melanoma

Introduction ¹⁸F-5-fluoro-N-?2-?Diethylamino?ethyl?picolinamide(¹⁸F-5-FPN)is a new positron-emission tomography?PET?radiopharmaceutical with potential for the detection of lymph node?LN?and pulmonary metastatic lesions of melanoma.We compared its performance with that of ¹⁸F-deoxyglucose(¹⁸F-FDG).Methods Cervical LN and lung melanoma metastasis models were established in C57BL/6 mice.Primary tumors were created by injection of melanoma cells into the pinna,and the resulting cervical LN metastases were evaluated.Lung metastases were created by intravenous injection of melanoma cells.The mice underwent ¹⁸F-FDG and ¹⁸F-5-FPN positron emission tomography?PET?imaging.A biodistribution study was conducted after imaging.Histopathologic evaluation of the tumors was also performed.Results LN metastases with a diameter < 1 cm were more visible on ¹⁸F-5-FPN PET imaging than ¹⁸F-FDG imaging.Quantitative analysis showed that the uptake of ¹⁸F-5-FPN was significantly higher than that of ¹⁸F-FDG,with values of 13.29 ± 3.80 %ID/g and 7.24 ± 1.95 %ID/g?n = 5,P < 0.05?,respectively.LN-to-muscle ratios were 21.23 ± 6.02 and 4.50 ± 2.11?n = 5,P < 0.01?for ¹⁸F-5-FPN and ¹⁸F-FDG,respectively.Biodistribution results were similar,with high uptake of ¹⁸F-5-FPN in the LN.¹⁸F-5-FPN imaging manifested the pulmonary lesions clearly,while the ¹⁸F-FDG imaging showed no uptake in lesions < 2 mm.The related uptakes of ¹⁸F-5-FPN and ¹⁸F-FDG were 3.12 ± 1.17 %ID/g and 1.48 ± 0.15 %ID/g,respectively?n = 5,P < 0.05?,with lung metastasis-to-muscle ratios of 8.16 ± 3.12 and 1.28 ± 0.¹⁸?n = 5,P < 0.01?,respectively.H&E and Prussian blue staining displayed pluri nucleated or mega nucleus cells and dark brown granules in the metastatic tissues,characteristic of melanoma.Conclusions ¹⁸F-5-FPN targeted small metastatic lesions with a higher target-to-normal ratio of uptake than those of ¹⁸F-FDG,which suggests its ability to detect metastatic lesions earlier than ¹⁸F-FDG.Further studies with a wide range of melanoma cell lines should be needed to confirm the similar performance.Purpose The increasing global burden and the markedly breakthroughs in therapy of malignant melanoma make urgent demands on efficient response evaluation and surveillance of adverse events.Though there have been a few probes explored for early diagnosis or staging of malignant melanoma,but rare for response assessment investigations except for common ¹⁸F-deoxyglucose(¹⁸F–FDG).In our previous researches,¹⁸F-5-fluoro-N-?2-?diethylamino?ethyl?picolinamide(¹⁸F-5-FPN)exhibited excellent and high sensitivity,specificity and affinity to melanin.Photothermal therapy?PTT?exploits melanin to absorb and convert light energy into heat efficiently for treat cancer.Thus,this research would further explore the feasibility and ability of ¹⁸F-5-FPN PET scanning to evaluate PTT response of malignant melanoma,simultaneously comparing with ¹⁸F-FDG.Methods B16F10?one melanic malignant melanoma cell line originated from mouse?and MDA-MB-231?abbreviated as 231,one kind of breast cancer cells without melanin?subcutaneous tumor models were irradiated with an 808 nm laser for PTT.Malignant melanoma cells were killed but local inflammation emerged.Hence we adopted ¹⁸F-5-FPN and ¹⁸F-FDG PET imaging to estimate the therapy response.At 24 h after treatment,B16F10 tumors in the treated and untreated groups were administered to HE staining.The whole research consisted of three sections.First,¹⁸F-5-FPN and ¹⁸F-FDG PET imaging were used for response evaluation.¹⁸F-FDG PET scanning on Day-2?before therapy,the day of therapy termed as Day 0?,1,5,8,16 and ¹⁸F-5-FPN PET imaging on Day-1?before therapy?,2,6,9,17 were performed for the treated and untreated groups of B16F10 models.All models were dissected to find lung or hepatic metastases if died during the surveillance or the last detection finished,and lung,liver and kidney organs were performed histological analysis.Second,B16F10,inflammatory and 231 models were subjected to ¹⁸F-FDG and ¹⁸F-5-FPN PET static acquisitions and compared by quantitative data for assessing the specificity of different agents to different diseases.Third,B16F10 and 231 models were exploited for survival analysis aiming to observe the efficacy and response feature of PTT.Kaplan-Meier plots were adopted to exhibit survival data and a log-rank test to analyze them.Two group comparisons were conducted by paired or independent t-test and multiple group comparison by one-way analysis of variance.Results Melanin in B16F10 tumors successfully transformed the optical energy into heat for PTT.HE staining of B16F10 treated group at 24 h discovered apparently framework destruction of tumor tissue,extensive necrosis and a number of pyknosis cells.In the imaging surveillance of first section,¹⁸F-5-FPN and ¹⁸F-FDG PET scanning had different capability to estimate the response of B16F10 tumor to PTT.The mean tumor uptakes of ¹⁸F-5-FPN on Day 2 and Day 6 were far less than that before treatment?Day-1?with values of 7.52 ± 3.65 %ID/g vs.¹⁸.33 ± 4.98 %ID/g?P < 0.001?and 10.22 ± 6.00 %ID/g vs.¹⁸.33 ± 4.98 %ID/g?P < 0.01?.No significance existed between the mean ¹⁸F-FDG uptakes on Day 1 and Day-2(6.¹⁸ ± 1.¹⁸ %ID/g vs.6.54 ± 0.84 %ID/g,P > 0.05),and the value on Day 5 was quite higher than that before treatment?8.69 ± 2.75 %ID/g vs.6.54 ± 0.84 %ID/g,P < 0.05?.¹⁸F-5-FPN and ¹⁸ FFDG PET scanning could shed light on regional accumulation of primary tumor sites while no relapses were visual in B16F10 treated group in 6 days initially after therapy.Then,tumors did relapse due to remained live cells.No any metastasis in lung or liver and any impairment in liver or kidney was observed in all models.Imaging comparison of B16F10 tumor,inflammatory model and 231 tumor in the second section revealed that ¹⁸F-5-FPN PET scanning only manifested B16F10 tumor strikingly?B16F10 vs.inflammation and 231,20.36 ± 4.38 %ID/g vs.2.72 ± 0.49 %ID/g and 1.14 ± 0.30 %ID/g,respectively,P < 0.001?,while they all accumulated ¹⁸F-FDG highly.In the third,PTT actually contributed to suppressing B16F10 tumors' growth rapidly in short time.Whereas,both of the temperature and growth of 231 tumors were not distinctly influenced.PTT prolonged the median survival of B16F10 models in some extent?suppressed vs.relapse vs.untreated groups,34 vs.14.5 vs.9.5 days,respectively?.But,over time,tumor recurrence compromised the efficacy substantially later which reflected that PTT with some response was difficult to annihilate tumor cells thoroughly and was not likely to be one unique regimen.Conclusion Compared with ¹⁸F-FDG PET scanning,¹⁸F-5-FPN PET imaging was capable of estimating PTT efficacy in malignant melanoma,successfully monitored the occult recurrence after therapy,and distinguished malignant melanoma from inflammation and other carcinomas well by high affinity to melanin.This potential probe may provide a new approach for precise and effective response evaluation,timely management of therapeutic regimen and sensitive follow-up.Moreover,¹⁸F-FDG PET imaging can efficiently present inflammatory lesions.Hence,the combination of ¹⁸F-5-FPN and ¹⁸F-FDG PET imaging will monitor the therapy response and adverse events simultaneously,facilitating to offer much more comprehensive and reliable information for the clinic diagnosis and therapy.Further efforts are needed for the clinical translation of ¹⁸F-5-FPN.