Advanced melanoma is hard to cure and causes high mortality in clinical setting due to a high rate of metastasis and invasion. Therefore, early diagnosis is extremely critical to achieve high survival rates and prolong overall survival in melanoma patients. Generally, most melanomas are detected and diagnosed through a clinical history and systemic examinations including both blood tests and imaging inspection. Skin examination tends to be performed by an experienced physician due to the complexity of pigmentation and morphological patterns. Histopathological examination remains the diagnostic gold standard for melanoma cases that are difficult to diagnose in the clinic. However, biopsy-based methods are invasive procedures with inherent limitations. Moreover, a lack of definitive objective, highly repeatable criteria that are suitable for all melanomas and the absence of molecular diagnostics and prognostic stratification factors contribute to the inconsistency in histopathological diagnosis of melanoma, even for experts in the field (Cerroni et al., 2010; Shoo et al., 2010). In recent years, non-invasive imaging techniques, such as dermoscopy and confocal laser microscopy have proven to be of great value in improving the accuracy of diagnosis of melanomas (Figure 1A) (Vestergaard et al., 2008; Schadendorf et al., 2015; Zhang et al., 2018; Agozzino et al., 2019). Notably, nanotechnology based bioimaging has proven to be a beneficial tool for remarkable improvement of the diagnosis of melanoma. Kim et al. demonstrated that the use of gold nanocages (AuNCs) as an excellent contrast agent for photoacoustic tomography (PAT) could be capable of active targeting the diagnosis of melanomas in vivo, increasing the sensitivity and specificity. They demonstrated that the melanocyte-stimulating hormone conjugated AuNCs obtained a 300% imaging contrast enhance in melanoma tumor model when compared to that of the control nanocages (Figure 2; Kim et al., 2010a). Yang et al. (2019) revealed a combination of dual-mode molecular-targeted ultrasound and NIR fluorescence imaging by a novel contrast agent named Nds-IR780 to precisely detect the primary lesions and sentinel lymph nodes (SLN) in cutaneous malignant melanoma. Moreover, Nds-IR780 could also serve as a theranostic probe for targeted therapy in cutaneous malignant melanoma (Figure 1B). Since the tissue autofluorescence and scattering faded with further increase of detection wavelength, imaging in the NIR range can significantly improve the imaging quality in terms of imaging contrast and penetration depth (Tao et al., 2013; Hong et al., 2017; Liu et al., 2019; Zhu et al., 2019). The targeted method can further decrease normal tissue exposure and improve signal at the tumor site. Thus, targeted molecular imaging methods are good approaches to improve the accurate rate of cancer cells detecting.

Surgical resection is the most curative and widely used treatment for cancer patients and is currently considered the optimal choice for majority of patients with resectable solid tumors. Moreover, complete tumor removal during surgery provided the best predictor for patient survival. It has been reported that surgery cured ∼45% of all patients with cancer by removing the whole tumor during the surgical resection (De Grand and Frangioni, 2003). In melanoma, the primary treatment for localized disease (primary tumors) was performed based on the Breslow thickness of the lesions to enable wide local excision with different safety margins (Cancer Genome Atlas Network, 2015). Nevertheless, surgical resection is not the best option for the advanced metastatic melanoma patients, although there are evidences that resection of distant metastases are associated with improved survival rate for the patients with a good state (Gamboa et al., 2020). The identification of surgical margins is a critical surgical task for surgical resection, and the clinicians should ensure a complete resection of the tumor while minimizing damages for surrounding healthy tissue. Thus, it’s necessary and urgent to develop novel technologies and methods that helping in defining the tumor margins, differentiating residual tumor cells, and verifying the complete resection. Blau et al. (2018) developed three polymer-based Turn-ON fluorescent nanoprobes which could be activated by cathepsin enzymatic degradation to generate a fluorescence signal. Their findings showed that the polymeric Turn-ON nanoprobes could delineate tumors boundaries precisely in two orthotopic tumor models of mammary adenocarcinoma and melanoma in vivo, largely benefiting the surgical resection (Figure 3). By using quantum dots (QDs) tagged with antibodies, Gao et al. (2004) demonstrated the antibodies labeled QD have the capacity for tumor localization with both targeting and imaging abilities in vivo. Apart from the wide local excision of the tumor, careful examination of the regional nodes for metastatic disease spread is also essential for accurate staging melanoma in its early phases. Nanoparticles have been used to detect and visualize SLNs by intraoperative NIR fluorescence imaging. Bradbury et al. (2013) used ¹²⁴I-(cyclic arginine-glycine-aspartic acid-tyrosine)-(methoxyterminated polyethylene glycol)-(core–shell silica nanoparticles) (¹²⁴I-cRGDY-PEG-C dots) coupled with optical imaging devices to localize SLNs in melanoma animal models with promising results. As such, QDs were injected around melanoma tumors in a spontaneous melanoma model in small pigs, and it was found that SLNs imaged in real time were identified in all injections. Furthermore, nodal tissue was resected and histologically confirmed after tissue resection (Tanaka et al., 2006). Collectively, these studies suggest that nanoparticles have multilineage potential in surgical excision.

Radiotherapy is one of the major therapeutic modalities for tumor treatment in the clinical practice. In addition to relying on inducing DNA damage, interrupting the cell cycle, and causing tumor cell death through apoptosis and necrosis, are considered the main mechanisms of radiotherapy with conventionally fractionated radiation. The radiotherapy can also induce neoantigen formation, tumor antigen presentation, and cytokine release to enhance immune responses (Baskar et al., 2012; Takahashi and Nagasawa, 2020). Although radiotherapy often fails to eradicate the tumor completely, it is indeed another adjuvant treatment for the majority melanoma patients who were not suitable for surgical recession, or the advanced nodal disease or primary tumor after definitive resection (Takahashi and Nagasawa, 2020). Adjuvant postoperative radiotherapy may improve local-regional control (LRC) in patients at high-risk of locoregional recurrence after surgery alone; however, overall and disease-free survival rates appear unaffected by this approach (Henderson et al., 2015; Mendenhall et al., 2017). At present, numerous researchers are searching for achieving more efficient and accurate dose delivery to the targeted organs and tissues and thereby reducing the damage to normal tissues and side effects of radiotherapy such as local discomfort.

Currently, the application of high atomic element nanomaterials as radiosensitizers is certainly of significant interest of researchers in radiation oncology and gained widespread attention (Liu et al., 2018). Chang et al. (2008) reported that gold nanoparticles (AuNP) radio-sensitized melanoma cells in a colony formation assay; moreover, AuNP conjugated with ionizing radiation has been found to promote apoptosis of the tumor cells and prolong survival compared to the control group, indicating AuNPs could enhance the effect of radiotherapy. Similarly, Kotb et al. (2016) focused on enhancing radiotherapy utilizing small Gd-based nanoparticles, named AGuIX^®, and found it was useful as an adjuvant for radiation therapy. They demonstrated that combining AGuIX^®, with radiation therapy magnified the effect of radiotherapy in vitro, thus leading to a decrease in the number of tumor cells and enhancing the survival rate in mice presenting with multiple brain metastases of melanoma. Overall, although nanoparticles have opened up new possibilities for the development of cancer radiotherapy and improvement of clinical efficacy of radiotherapy, many challenges still remain in terms of safety concerns, biocompatibility, and stability.

Chemotherapy, as mono- or poly-therapy, has been the backbone of systemic treatment for melanoma for many years. DTIC remains the only FDA- and EMA-approved chemotherapy drug for treating advanced malignant melanoma and has been used as the standard in most trials of newer agents. It has been reported that the response to DTIC was only 7% in a clinical trial of 64 patients (Johnson et al., 2016). In addition, many other drugs have been used off-label, including temozolomide, fotemustine, paclitaxel, docetaxel, cis/carboplatin, and nitrosoureas. However, single-agent chemotherapy has little role in advanced cutaneous melanoma and is not related to survival benefit, although some response may be observed in certain individual patients. Eigentler et al. (2003) undertook a systematic review of 41 randomized studies and showed that polychemotherapeutic schedules are associated with a higher objective response rate, but is also often more toxic; none was proven to prolong overall survival. This could be attributed to several severe limitations such as systemic toxicity, serious side effects, multi-drug resistance. All these limitations hamper the adequate drug accumulation in the tumor, frequently compromising the efficacy and persistence of the treatment.

The strategy of nanomaterial carrying drugs has been proved to be effective in the field of chemotherapy as they can protect the carried drugs against degradation thus preserving their long-term stability and efficacy. NDDSs can be adjusted to release their cargo only at the tumor site with a response to the specific scenario around the tumors thus reducing cytotoxic side effects on normal tissues (Gmeiner and Ghosh, 2015). Chaudhuri et al. (2010) showed a combination of doxorubicin (DOX) and carbon nanotube in mouse melanoma models could diminish the drug systemic toxicity without sacrificing its therapeutic effect, thus abrogating tumor growth. In addition, Yoncheva et al. (2019) encapsulated DOX into chitosan/alginate nanoparticles and evaluated the therapeutic response in vitro/in vivo in melanoma cells. The results showed that the encapsulated DOX increased intracellular accumulation of the drug and produced a long-lasting cytotoxic effect thus reducing both the viability of melanoma cells and the tumor size in melanoma mouse models. Similar results have been reported in a study where a trinary tumor accumulation of DOX was seen in animals developing melanoma, using polymeric nanoparticles in comparison to the commercially available PEGylated-liposomal DOX (Zou et al., 2016).

Melanoma has been recognized as an immunogenic cancer eliciting striking antitumor responses for long time. Immunotherapy, mainly including vaccines, bio-chemotherapy, and the transfer of adoptive T cells and dendritic cells, aimed at priming the immune system and activating immune responses to the residual tumor cells (Rodríguez-Cerdeira et al., 2017). Immunotherapy can reduce the size and involvement of locally advanced melanoma, converting inoperable tumors to operable status. Patients with advanced unresectable melanoma may receive high-dose interleukin-2 monotherapy approved by FDA in 2011, meanwhile, the PEGylated interferon-α2b could be used as an adjuvant treatment for surgically treated melanoma patients (Herndon et al., 2012; Tarhini et al., 2012). In addition, significant progress has been made in targeting the immune checkpoints of tumors with PD-1 and CTLA-4. But the efficacy of the checkpoint inhibitors of the immune system was not observed for all patients, and their success rates are often less than 50%, suggesting significant room for improvement (Chen and Mellman, 2017).

A recent study reported a magnetic nanomedicine-conjugated anti-PD-L1 and T-cell activators may enter the tumor site by the magnetic navigation, inhibiting tumor growth in vivo and enhancing the tumor microenvironment immunity. This kind of modality may present a novel approach for nanotechnology derived immunotherapy (Chiang et al., 2018). Although this study was not done in melanoma model, it is very promising for future melanoma therapy. Fontana et al. (2019) also revealed that the biohybrid nano-vaccine with a checkpoint inhibitor could enhance the efficacy of the immunotherapy, thereby retarding the tumor growth compared to the standard monotherapy with checkpoint inhibitor.

Approximately 70% of patients with cutaneous melanoma have gene mutations in the key signaling pathways (Flaherty, 2012). Targeted therapy, a crucial systemic treatment, is a novel method through using small molecule inhibitors or antibodies to affect oncogenic sites. In melanoma, these small molecule inhibitors primarily include BRAF inhibitors, MEK inhibitors, CKIT inhibitors, vascular endothelial growth factor inhibitors, P13K-AKT-mTOR pathway inhibitors, cyclin-dependent kinase inhibitors, and receptor ErbB4 inhibitors (Domingues et al., 2018). Approximately 50% of metastatic melanoma lesions have mutations in the serine-threonine kinase BRAF, mostly that of valine to glutamine in codon 600 (Davies et al., 2002). At present, the high and rapid response rates as well as good safety profile of BRAF-targeted agents offer an unprecedented opportunity for a neoadjuvant approach in melanoma treatment.

Siu et al. (2014) designed non-covalently functionalized nanotubes as a delivery vector for small interfering RNA (siRNA) targeting BRAF gene silencing. The uptake of BRAF siRNA in melanoma tissue has been observed, resulting in regressing the tumor growth in mouse melanoma models. Similarly, Tran et al. (2008) used a unique nano-liposomal ultrasound to deliver siRNA specifically targeting ^V600EB-Raf and Akt3 into cutaneous melanoma, suppressing the tumor development and preventing the metastasis.

Daneshvar et al. demonstrated the use of platinum nanoparticles (PtNPs) could be served as the absorber for laser light and X-rays to perform energy conversion. Moreover, it could also be a sensitizer that could be applied for the treatment of cancer. Conventional radiotherapy shows low efficacy due to the administration of low radiation doses for safety and the weak absorption of radiation beams by soft tissues tumors. Hence, the authors demonstrated PtNPs provided a deeper treatment and producing more ROS with laser light radiation in the B16/F10 cells, compared to those with sole laser radiation or X-rays (Daneshvar et al., 2020). Following a similar approach, Salehi et al. (2019) utilized platinum mesoporous as a novel NIR photosensitizer for PTT and X-ray absorbing agent for X-ray-induced photodynamic therapy (X-PDT) for melanoma cancer cells. The combined exposure (PTT/X-PDT) led to deeper treatment and very low survival rate of melanoma cells and higher effectiveness and cooperativity compared to that with PTT or X-PDT alone.

A combination of the mechanical wounding, cytotoxic and photothermal effects associated with, respectively, ultrasound contrast agents, chemotherapy drugs, and melanin were used for a sono-chemo-photothermal cancer therapy. Hu et al. (2020) developed self-assembling perfluoropentane nanodroplets that exhibited excellent biocompatibility for melanoma therapy. Polyvinyl alcohol (PVA)-shelled and melanin−cored nanodroplets were spontaneously generated by using the Ouzo effect, and then anti-cancer drug DOX was loaded into PVA shells of the nanodroplets (Lepeltier et al., 2014). Then, following the loading process, opto-acoustic irradiation was administrated to manufacture the nano-cavity structure through the nanodroplets vaporization. The results showed that the tumor structures were mechanically disrupted in in vivo animal experiments, and the nanodroplets, combined with opto-acoustic synergistic irradiation treatment and thereafter by laser irradiation could efficiently eliminate melanoma tumors in both in vitro and in vivo experiments (Figure 5).

Qin et al. presented a combination therapy, based on a α-cyclodextrin (CD)-based gel system, that combined DOX, with indocyanine green (ICG), a photothermal agent, and CpG, a molecular adjuvant capable of inducing DC mutation. The as-prepared chemo/immuno/photothermal-therapy strategy can effectively inhibit the primary tumor growth. Unexpectedly, the inclusion of PD-L1 blocking obstructed tumor growth in both primary and metastatic melanoma and reduced the tumor size and inhibited the tumor recurrence (Qin et al., 2020). The combinatorial therapy process is very effective due to the sustainable release of tumor-specific antigens. Moreover, compared with other immunotherapy strategies, this novel therapy developed a powerful tumor-specific immunity which will potentially cover a wide range of solid tumors.

Zhu et al. described a novel nano-system [Al-bovine serum albumin (BSA)-Ce6 NPs] that combines immunotherapy with PDT using chlorin e6 as photosensitizer and the aluminum hydroxide as immunoadjuvant. They demonstrated that the nanoparticles not only effectively killed melanoma cells but also prevented tumor relapse or metastasis by inducing potent antitumor immune response, thereby enhancing the immune response indicators levels such as serum antibody, the levels of cytokine and increasing the proportions of immune cells (Figure 6; Zhu et al., 2020). Collectively, melanoma treatments have ushered a new era, as the combination therapy could boost the therapeutic effects with a never-before-achieved effectiveness.

Nanomedicine-based combination therapy for melanoma may reduce high drug resistance, long-term toxicity and increase the efficacy associated with conventional treatments and monotherapies. Therefore, the combination therapy based on nanomedicine could provide the respective advantages of monotherapy and reduce recurrence and metastasis, improving the survival rate to some extent.