Our research used an 808 nm laser to treat B16F10 tumors engrafted into nude mice. We explored the feasibility of 18F-5-FPN PET imaging for PTT response evaluation, further compared with the detection of inflammation and other tumors to ascertain its specificity for MM. 18F-5-FPN is a promising probe to assess the therapeutic response of MM and is likely to be a new strategy for the clinical response evaluation and follow-up of MM.
18F-5-FPN and 18F-FDG PET acquisitions were performed on B16F10- engrafted models in treated and untreated groups at each time point (Fig. 2a). In the treated group (Fig. 2c), tracer accumulation in tumors after PTT (Day 1, 2) declined from baseline. No recurrence was discovered via gross inspection within 6 days after treatment. Nevertheless, regional uptakes were observed clearly on Day 5 and 6 in mice. Then the tumor relapsed with corresponding tracer accumulation. Hence, 18F-5-FPN PET imaging might help to reveal an occult residual recurrence before observation.
Tumor cells were killed by PTT and tumor size shrank. Residual tumor cells regrew and tumor relapsed later. In the quantitative analysis of images (Fig. 2g, h and Table 1), the mean tumor uptakes of 18F-5-FPN in the treated group declined on Day 2 and 6, then enhanced, which represented the diminution of melanin in tumor tissue after PTT followed by escalation. Serial 18F-5-FPN PET imaging recorded the course accurately with high sensitivity. No difference was found for 18F-FDG uptake before and on Day 1 after therapy, implying less sensitivity compared to 18F-5-FPN. Therefore, 18F-5-FPN might have a higher sensitivity to an early response.
In the untreated group, tumor size was small initially with low 18F-FDG uptake and enlarged gradually with increasing 18F-FDG uptake, while 18F-5-FPN values remained on a high level. Although the mean tumor uptakes attenuated due to local necrosis later, the maximum uptakes of the whole tumor were still high (Day 9, 18F-5-FPN, 35.92 ± 5.61%ID/g. Day 8, 18F-FDG, 29.16 ± 5.53% ID/g).
CT and MRI are generally used to evaluate therapy response in the clinic. However, they sometimes fail to discriminate between real progression and pseudoprogression. One investigation revealed that 22 out of 227 patients responded atypically, and lesions were confirmed to be inflammatory infiltration and necrosis, not tumor proliferation by biopsy . Moreover, conventional techniques cannot monitor tumor response to immunotherapy in a timely and accurate fashion [6, 8]. The previous article reported that the pseudoprogression rate via conventional techniques was as high as 10–15% for malignant melanoma patients treated with immunotherapy . Changes in tumor function and metabolism always precede changes in size. Thus, functional imaging may facilitate the detection of early therapy response [7, 22, 23].
By now, a few probes have been explored for early diagnosis or staging of MM, but few potential agents have been evaluated for response evaluation, except for 18F–FDG and 18F-FLT [6–8, 13, 14]. 18F-FDG PET/CT imaging plays an important role in response evaluation to conventional chemotherapy, and also has some value in targeted therapy and immunotherapy [7, 8]. Geven et al. used 18F-FDG and 18F-FLT PET imaging to assess the response of MM models to BRAF-mutant inhibitor and revealed that tumor uptakes of 18F-FLT had no difference before and after therapy, though the accumulation of 18F-FDG decreased obviously after therapy . Because both MM and immune-related inflammation accumulate 18F-FDG, 18F-FDG PET imaging may make it quite challenging to differentiate between real progression and pseudoprogression . One study investigated 12 metastatic MM patients under anti-CTLA-4 antibody treatment. It concluded that the SUVmax changes of 18F-FDG and 18F-FLT PET images would not offer reliable value to predict late response 3 months after the first therapy . However, another article concluded that early 18F-FDG PET/CT scanning could predict the response of MM to immunotherapy . According to the literature, it is still controversial whether 18F-FDG PET imaging might estimate therapy response of MM well or not, and whether the imaging can exclude the interference of inflammation and detect an occult residual recurrence efficiently in the clinic.
Similarly, in our imaging surveillance, the area of 18F-FDG accumulation was larger than that of 18F-5-FPN on Day 1 vs. Day 2 and Day 5 vs. Day 6 (Fig. 2c). Comparative imaging of B16F10 tumor, inflammation and MDA-MB-231 tumor was subsequently implemented to judge whether inflammation might affect the result or not.
In the comparative imaging, only B16F10 tumor was distinctly visible on 18F-5-FPN PET imaging, while three models were all obvious on 18F-FDG PET images (Fig. 4a-c). Quantitative analysis also corroborated that B16F10 tumor accumulated considerable 18F-5-FPN, and all three showed high uptakes of 18F-FDG (Fig. 4d and Table 2). Moreover, 18F-5-FPN uptakes of inflammatory and healthy sides resembled, while inflammation accumulated much more 18F-FDG than normal muscle. 18F-5-FPN displays an excellent affinity for melanin, but the specificity of 18F-FDG is not good enough to distinguish between tumor and inflammation, nor between pigmented tumors and other carcinomas. Local inflammation may emerge after PTT, and 18F-FDG cannot reveal early response to therapy accurately. This may be correlated to the finding that 18F-5-FPN uptake declined early after PTT, while a reduction in 18F-FDG uptake was not obvious in this research (Fig. 2g, h). Here, the smaller size of the B16F10 tumor may be responsible for its lower uptake of 18F-FDG than that of the other two models. This can also be demonstrated by the value > 13%ID/g over the increasing volume of B16F10 tumor in the imaging surveillance.
The therapy landscape of MM has made profound progress and several drugs for targeted therapy and immunotherapy, including vemurafenib, dabrafenib, ipilimumab, pembrolizumab, and nivolumab, have been approved following clinical trials . Overall, targeted therapy achieves short-term palliation, immunotherapy needs a long time to judge the response, and the two approaches induce different kinds of side effects [2–5, 24]. Thereby, this research did not adopt drug therapy but PTT. Exposed to an 808 nm laser, the temperature of B16F10 tumors increased rapidly [15, 25–28], but only slightly in MDA-MB-231 tumors. Melanin can transform optical energy into heat efficiently. Histological analysis, imaging surveillance, and survival analysis identified that the PTT protocol inhibited tumor growth strikingly over a short time, although all tumors eventually recurred. It said that tumor cells might tolerate hyperthermia via the mediation of heat shock proteins to enhance their survival rate, which might explain why tumors relapsed after PTT . Thus, several reports have explored gold nanomaterials or other materials to improve the efficacy of PTT and even combined PTT with chemotherapy or other treatment strategies [15, 25–27, 30–35].
The melanin in MM successfully transformed light energy into heat for PTT. Tumor growth was suppressed for a short time, and 18F-5-FPN PET imaging was more sensitive to estimate the early response of B16F10 tumor to PTT than 18F-FDG. Unfortunately, tumors recurred later. Prior to observation, 18F-5-FPN and 18F-FDG PET imaging detected occult residual recurrence. Moreover, 18F-5-FPN showed high specificity for MM and overcame the interference of inflammation, suggesting a more accurate response evaluation. Undoubtedly, the superiority of 18F-5-FPN may offer a new approach for MM response assessment and follow-up. This research still has some limitations. 1) PTT can only be used for superficial lesions and cannot work for deep or visceral metastatic lesions. Therefore, our group plans to assess therapy response of a metastatic model of human MM cell line to targeted therapy or immunotherapy via 18F-5-FPN PET imaging. 2) Although > 90% of primary cases are pigmented [36, 37], a minority of MMs are amelanotic, making them unsuitable for 18F-5-FPN PET imaging.