New Potential Agents for Malignant Melanoma Treatment—Most Recent Studies 2020–2022

Malignant melanoma (MM) is the most lethal skin cancer. Despite a 4% reduction in mortality over the past few years, an increasing number of new diagnosed cases appear each year. Long-term therapy and the development of resistance to the drugs used drive the search for more and more new agents with anti-melanoma activity. This review focuses on the most recent synthesized anti-melanoma agents from 2020–2022. For selected agents, apart from the analysis of biological activity, the structure–activity relationship (SAR) is also discussed. To the best of our knowledge, the following literature review delivers the latest achievements in the field of new anti-melanoma agents.


Introduction
Malignant melanoma (MM) is the most lethal of all skin cancers, although it accounts for only 1% of all skin cancers [1]. Its incidence has increased faster than almost any other cancer in the last 50 years [2]. The American Cancer Society forecasts that by 2022 there will be nearly 100,000 new cases and more than 7500 deaths from MM in the United States alone [3]. The poor prognosis is that the five-year survival rate for patients with stage IV melanoma is only 10% [4]. MM is derived from melanocytes, which are responsible for the production and secretion of the melanin pigment [5]. Overexposure to UV light or radiation leads to damage to the DNA of melanocytes, resulting in uncontrolled proliferation and oncogenesis. Other factors determining the development of this neoplasm include genetic factors, the previous presence of MM, family history and skin phenotype.
The most studied pathway of melanoma development is the mitogen-activated protein kinase (MAPK) pathway, i.e., rat sarcoma virus (RAS)-rapidly accelerated fibrosarcoma (RAF)-mitogen-activated protein kinase (MEK)-extracellular signal-regulated kinase (ERK). The signaling cascade culminates in increased expression of nuclear transcription factors, such as c-MYC and MITF, both responsible for cell proliferation [6].
The second most studied pathway is the phosphatidylinositol-3-kinase (PI3K) pathway, i.e., PI3K-protein kinase B (Akt)-mammalian target of rapamycin (mTOR) [7]. It is mainly responsible for anti-apoptotic signaling [8] and is crucial for the development of melanoma resistance to BRAF/MEK inhibitors [9]. The MAPK pathway is more actively related to proliferation and invasion and the PI3K-AKT pathway is mainly responsible for anabolism [6]. RAS can also activate the PI3K cascade [8]. Two signal pathways are shown in a simplified manner in Figure 1. The early stages of melanoma are operable, but in the instance of metastases, the treatment options become limited. In malignant melanoma, chemotherapy provides little benefit. However; in recent years, mortality has decreased by~4%, which may be related to treatment progress through targeted therapies and immunotherapies [1].
A new approach to melanoma therapy is the role of ion channels. The channel of high-voltage activated potassium (BK) and Ca2 + ions is expressed in many different cells [10,11] and modulates various processes by forming molecular signaling complexes [12]. Liu et al. reported that neoplastic cells are characterized by an overexpression of BK channels [13]. A correlation of changes in membrane potential with the cell cycle is observed [14], which is confirmed by many studies [13,[15][16][17][18][19][20][21]. In other words, they control cancer proliferation, invasion, and metastasis [14]. Since the key role of ion channels in the development of melanoma has been demonstrated [22], the number of studies devoted to this molecular target has been steadily increasing [23]. They are recognized as new potential targets for new therapies for melanoma [24]. A revolutionary study has been published, exhibiting that the BK activator NS-11021 induces apoptosis and inhibits proliferation in the melanoma cell line IGR39 [25]. Therefore, ion channels may be a new potential molecular target in the treatment of melanoma and their dysregulation may serve as a marker for the prediction of melanoma.
The current trend is to develop new molecularly targeted therapies for specific biomarkers responsible for melanoma progression, which will be an alternative or complement to immunotherapy [1]. Thus, there is a need to search for novel potential agents active against melanoma. A new approach to melanoma therapy is the role of ion channels. The channel of highvoltage activated potassium (BK) and Ca2 + ions is expressed in many different cells [10,11] and modulates various processes by forming molecular signaling complexes [12]. Liu et al. reported that neoplastic cells are characterized by an overexpression of BK channels [13]. A correlation of changes in membrane potential with the cell cycle is observed [14], which is confirmed by many studies [13,[15][16][17][18][19][20][21]. In other words, they control cancer proliferation, invasion, and metastasis [14]. Since the key role of ion channels in the development of melanoma has been demonstrated [22], the number of studies devoted to this molecular target has been steadily increasing [23]. They are recognized as new potential targets for new therapies for melanoma [24]. A revolutionary study has been published, exhibiting that the BK activator NS-11021 induces apoptosis and inhibits proliferation in the melanoma cell line IGR39 [25]. Therefore, ion channels may be a new potential molecular target in the treatment of melanoma and their dysregulation may serve as a marker for the prediction of melanoma.
The current trend is to develop new molecularly targeted therapies for specific biomarkers responsible for melanoma progression, which will be an alternative or complement to immunotherapy [1]. Thus, there is a need to search for novel potential agents active against melanoma.

The Most Potent Anti-Melanoma Agent from Most Recent Studies 2020-2022
The most active structures with potential activity against melanoma from a recent study are presented below. A short SAR analysis is discussed for the selected structures. New potential derivatives have been classified according to the function of the main ring.

Imidazole Derivatives
Imidazole is a five-membered valuable aromatic heterocyclic moiety, bearing two nitrogen atoms. It exerts a great influence on the anticancer activity and its derivatives showed, among others, properties of inhibitors of tubulin polymerization or kinases [26]. Imidazole derivatives with anti-melanoma activity are presented in Figure 2. 23,6084 derivatives with the most promising activity bore a terminal methyl or isopropyl an amide linker, and a hydroxyphenyl moiety [35].

Benzimidazole Derivatives
Benzimidazole is a six-membered benzene ring fused to an imidazole ring 4-position and 5-position of the imidazole ring. Its derivatives exert antitumor a mainly by inhibiting tubulin polymerization, kinase, and topoisomerase inhi apoptosis inducers, and telomerase inhibitors [36]. Benzimidazole derivatives wit melanoma activity are presented in Figure 3.
Kong et al. obtained novel 1-(5-(1H-benzo[d]imidazole-2-yl)-2,4-dimeth pyrrol-3-yl)ethan-1-one derivatives [37]. The main goal was to find new potential tors of the bromodomain family protein and the beyond-end domain (BET). BETs sponsible for the recruitment of protein complexes to promote the initiation and p gation of transcription; therefore, their inhibition is a promising cancer therapy str Compound (9) possessed the highest activity against melanoma cell line UACC-6 GI50 = 274 nM, as compared to the activity of OTX-015 with GI50 = 161.8 nM, whic found in Phase 2 clinical trials as a pan-BET inhibitor [37].
Youssif et al. obtained novel triarylimidazole derivatives [30]. The selection of the triaryl imidazole core was made on previously reported data based on the rational drug design [31,32]. The most promising compound (4) possessed the highest activity against melanoma cell line LOX IMV1 with a concentration of 50% of the maximal inhibition of cell proliferation (GI 50 = 0.17 µM), and inhibited BRAF V600E most effectively with the concentration of a substance that causes the death of 50% of the population (LC 50 = 0.33 ± 0.10 µM). The authors noted that the activity of arylcarboximidamide derivatives increased in the given series of substituents: 4-chlorophenyl < 2-naphthyl < 4-methoxyphenyl ≤ 1,3benzodioxole for the most potent agent [30].
Ali et al. obtained novel (1H-imidazol-5-yl)pyrimidine-based derivatives [33]. The starting point for further syntheses was the imidazole derivative SB203580, a specific inhibitor of p38 mitogen-activated protein kinase (p38 MAPK) [34]. The highest activity was exhibited by (5) with the final 3-fluorophenylsulfonamide, which was about eightfold more active with an IC 50 = 0.9 µM, as compared to standard Staurosporine with an IC 50 = 7.15 µM against melanoma cell line LOX-IMVI. Compound (6), on the other hand, possessed about 12.5-fold more activity against BRAF V600E kinase with an IC 50 = 2.49 nM, as compared to the same standard with an IC 50 = 31.60 nM [33].

Imidazothiazole Derivatives
Imidazo[2 ,1-b]thiazole is a fused heterocycle that is of great importance in med chemistry due to its wide range of biological activity. Its derivatives exert antitumor tivity by inhibiting tubulin polymerization, kinases, or inducing apoptosis [41]. Imid othiazole derivatives with anti-melanoma activity are presented in Figure 4.
Ammar et al. obtained novel imidazo[2,1-b]thiazole derivatives bearing nitrophenyl group at the 6-position [42]. In the present study, the authors modified p viously reported inhibitor structures of 3-fluoro-substituted scaffolds based on im azo[2,1 -b]thiazole [43]. Electron-withdrawing properties were left with concomitant troduction of additional H-bond acceptors to improve binding to the active BRAF V600E . Compound (12) exhibited the highest activity against BRAF V600E with an IC  [37]. The main goal was to find new potential inhibitors of the bromodomain family protein and the beyond-end domain (BET). BETs are responsible for the recruitment of protein complexes to promote the initiation and prolongation of transcription; therefore, their inhibition is a promising cancer therapy strategy. Compound (9) possessed the highest activity against melanoma cell line UACC-62 with GI 50 = 274 nM, as compared to the activity of OTX-015 with GI 50 = 161.8 nM, which was found in Phase 2 clinical trials as a pan-BET inhibitor [37].

Imidazothiazole Derivatives
Imidazo[2,1-b]thiazole is a fused heterocycle that is of great importance in medical chemistry due to its wide range of biological activity. Its derivatives exert antitumor activity by inhibiting tubulin polymerization, kinases, or inducing apoptosis [41]. Imidazothiazole derivatives with anti-melanoma activity are presented in Figure 4.
Ammar et al. obtained novel NH 2 -based imidazothiazole derivatives based on the structural features of Dabrafenib [44]. Dabrafenib is a BRAF serine/threonine kinase inhibitor approved for the treatment of metastatic melanoma as monotherapy or in combination with Trametinib [45]. The novel derivatives bore an amine substitution on the terminal phenyl ring, a sulfonamide group on the side of the chain, and a terminally substituted phenyl ring. The aim of the syntheses was to obtain new BRAF V600E kinase inhibitors. Compound (15) exhibited the highest activity against BRAF V600E with an IC 50 = 1.20 nM. The authors noted that the activity can be related to the propylene linker and the 4-methoxyphenyl ring in the side chain, which provided excellent conditions for high affinity and formation of a solid complex with BRAF. A single dose (10µM) an in vitro cytotoxic assay indicated that, in the melanoma cell line UACC-62, Compound (16) possessed 100% inhibition. Moreover, the SAR study indicated that compounds bearing the aforementioned propyl linker exhibited an overall higher inhibitory activity on the UACC-62 melanoma cell line than compounds bearing an ethylene linker [44]. [46]. The main goal of the syntheses was to discover new inhibitors of BRAFV 600E and BRAF WildType . Compound (17) was the most active against BRAF V600E with an IC 50 = 0.98 ± 0.012 nM and Compound (18) exhibited the highest activity against BRAF WildType with an IC 50 = 2.7 ± 1 nM. Surprisingly, the compounds possessing the highest activity in the BRAF V600E and BRAF WildType kinase enzyme assay did not show the highest activity in the cell assay. Compound Furthermore, the immunoblot test indicated that (20) could inhibit the phosphorylation of MEK and ERK, making it an excellent potential agent for melanoma [46].
1.20 nM. The authors noted that the activity can be related to the propylene linker and the 4-methoxyphenyl ring in the side chain, which provided excellent conditions for high affinity and formation of a solid complex with BRAF. A single dose (10μM) an in vitro cytotoxic assay indicated that, in the melanoma cell line UACC-62, Compound (16) possessed 100% inhibition. Moreover, the SAR study indicated that compounds bearing the aforementioned propyl linker exhibited an overall higher inhibitory activity on the UACC-62 melanoma cell line than compounds bearing an ethylene linker [44].

Quinoline Derivatives
Quinoline contains a benzene ring fused with pyridine on two adjacent carbon atoms. Its derivatives exert antitumor activity by inhibiting tubulin polymerization, DNA repair, angiogenesis, key enzymes for tumor growth, and the ability to intercalate DNA [47][48][49][50]. Quinoline derivatives with anti-melanoma activity are presented in Figure 5.

Pyrazolopirimidines Derivatives
Among the N-fused heterocycles, pyrazolopyrimidine is one of the favored scaffolds due to its synthesis and pharmacological importance. Its derivatives exert antitumor activity mainly as kinase inhibitors [55]. Pyrazolopirimidines derivatives with anti-melanoma activity are presented in Figure 6.
Li et al. obtained new pyrazolo [1,5-a]pyrimidine derivatives [56]. In their previous research, the authors synthesized derivatives of 4-substituted methoxybenzoylarylthiazoles [57] and 2-aryl-4-benzoyl-imidazoles [58,59] with significant antitumor activity. However, it turned out that the presence of the unstable carbonyl group between the imidazole and phenyl ring is susceptible to metabolic reduction. Therefore, by means of a fusion strategy, this group was incorporated into the pyrimidine ring. The pyrazolo [1,5-a]pyrimidine scaffold plays the role of an imidazole and ketone bioisostere. The 3,4,5-trimethoxyphene group was retained related to its pharmacophoric nature for antitumor activity. Compound (31) showed the highest activity against B16-F10 tumor cells with an IC50 = 0.021 ± 0.002 μM, which is about four-fold more active than the reference standard colchicine with an IC50 = 0.087 ± 0.006 μM and is comparable to paclitaxel with an IC50 = 0.021 ± 0.002 μM. Compound (31) inhibited tubulin polymerization in an in vitro study and induced cell cycle arrest in the G2 / M phase. These results were confirmed by an in vivo study in the B16-F10 murine melanoma model where tumor growth was inhibited without apparent toxicity. The SAR study of various aromatic groups indicated that the most favored for activity are the presence of electron-donating groups in the phenyl ring. The highest activity was shown by the compound substituted with the methyl group in the para position of the phenyl ring. The key to the activity is the presence of 3,4,5-trimethoxyphenyl moiety at the 7-position of the pyrazolo[1,5-a]pyrimidine ring, and for the 2-position, the activity increased in the given series: 3-amino-4methoxyphenyl < 6-indole < 5-(1-methyl-1H-indole) < 4-methylphenyl for the most potential agents. Moreover, the introduced modifications increased the metabolic stability of the compound [56]. El-Damasy et al. obtained novel 2-anilinoquinoline derivatives [51]. The aim of the synthesis was to introduce a phenyl ring with different lipophilic and steric substituents as an alternative for huge aromatic groups of 2-arylamides with anilinoquinolines, while maintaining the amide linker. Compound (21) exhibited the most promising activity against melanoma cell line MDA-MB-435 with GI 50 = 0.878 µM, as compared to the reference standard Imatinib, which possessed about 20-fold less activity with GI 50 = 17.91 µM. Compound (22) showed the highest and the broadest spectrum of activity against many lines of melanoma cells, such as LOX IMVI, MALME-3M, M14, and SK-MEL-28 with GI 50 = 0.870 µM, 0.611 µM, 0.668 µM, and 0.846 µM, respectively, as compared to the previously mentioned standard with GI 50 = 18.11 µM, 16.33 µM, 19.28 µM, and 14.62 µM, respectively. Moreover, it exhibited microtubule polymerization stabilizing effects comparable to paclitaxel. The authors noted that the key to the antitumor activity was the presence of 3-trifluoromethylphenyl group substituted with cyclic amines, which resulted in an enhanced steric effect. Experimental results were confirmed by molecular docking study [51].
Elbadawi et al. synthesized novel 4-alkoxy-2-aryl-6,7-dimethoxyquinolines [52]. Compound (23) exhibited the most promising activity against melanoma cell lines LOX IMVI, MDA-MB-435, and SK-MEL-5 with an IC 50 = 0.116 µM, 0.709 µM, and 0.247 µM, respectively; (24) was active against melanoma cell line M14 with an IC 50 = 0.327 µM; and (25) was active against melanoma cell line UACC-62 with an IC 50 = 0.628 µM. Derivatives bearing a trifluoromethyl group in the para position of the phenyl ring possesses higher activity in regard to the chlorine atom in the same position. The authors noted that for the 4-position of the quinoline ring, the presence of the propyl linker between the oxygen atom and the aryl group is crucial for the activity. Within this aryl group, the activity increased in the given series: 4-hydroxypiperidine > morpholine > piperidine > pyrrolidine, and for the 2-position of the quinoline ring, the activity increased in the given series: 2-thiophenyl < 3-thiophenyl < 2-furanyl < phenyl [52].
Albadari et al. obtained novel derivatives with high anti-melanoma activity, based on the hydroxyquinoline scaffold [53]. The previous study indicated that the 8-hydroxyquinoline moiety is crucial for antiproliferative activity [42] Compound (26) exhibited the most promising activity against the multi-drug resistant melanoma line MDA-MB-435/LCC6MDRI with an IC 50 = 0.2 ± 0.0 µM, as well as (27) against melanoma cell lines A375 and MDA-MB-435 with an IC 50 = 0.5 ± 0.1 µM for both lines. Both compounds (28) and (29) possessed the same activity against melanoma cell line RPMI7951 with an IC 50 = 0.5 ± 0.1 µM. The results indicated that the phenyl, 4-isoquinoline, 3-thiophene, and bromine groups seem to be crucial for the activity [53].
Manikala et al. obtained new chalcone-tethered quinoline derivatives [54]. Compound (30) exhibited the most promising activity against melanoma cell line A375 with an IC 50 = 0.34 µM, as compared to the reference standard Adriamycin, which possessed about 16-fold less activity with an IC 50 = 5.51 µM. The most active compound bore the nitro group in the para position of the phenyl ring. The activity of the derivatives increased in the given series of phenyl substituents: 4-bromo < 3,4,5-trimethoxy < 3-nitro < 4-nitro for the most potential agents [54].

Pyrazolopirimidines Derivatives
Among the N-fused heterocycles, pyrazolopyrimidine is one of the favored scaffolds due to its synthesis and pharmacological importance. Its derivatives exert antitumor activity mainly as kinase inhibitors [55]. Pyrazolopirimidines derivatives with anti-melanoma activity are presented in Figure 6. Ibrutinib is an FDA-approved Bruton's tyrosine kinase (BTK) inhibitor used in the treatment of leukemias [60]. Moreover, its promising activity against solid tumors has been shown [61][62][63][64]. Related to the widespread drug resistance and significant side effects [65][66][67], it is necessary to search for new drugs with increased effectiveness and lower toxicity. As a result, modifications to the structure of ibrutinib are of great interest to scientists looking for anticancer drugs [68]. Based on the literature data, the thiourea moiety was introduced, which ensures selectivity, effectiveness, and favorable physicochemical parameters [69][70][71][72][73]. Compound (32) exhibited the highest activity against melanoma cell line B16 with an IC50 = 6.07 ± 1.06 μM [74]. Furthermore, it arrested cells in the G1 phase and promoted cell apoptosis and autophagy in melanoma cell line B16. In an in vivo study, (32) significantly inhibited the growth of melanoma in mice [74].

Indole Derivatives
Indole consists of a benzene ring fused at the 2-position and 3-position of the pyrrole ring. Its derivatives exert antitumor activity as inhibitors of tubulin polymerization and topoisomerase and as inducers of apoptosis, aromatase, and kinase inhibitors [75]. Indole derivatives with anti-melanoma activity are presented in Figure 7.
Dinavahi et al. synthesized new 2,3-dioxo-2,3-dihydroindol-1-yl derivatives [76]. The aim of the syntheses was to obtain inhibitors of aldehyde dehydrogenase (ALDH), which is overexpressed in various neoplasms [77][78][79][80]. Compound (33) showed the highest activity against melanoma cell line UACC 903 with an IC50 = 3 ± 0.6 μM, and (34) against melanoma cell line 1205 Lu with an IC50 = 2.1 ± 0.6 μM. One of the mechanisms of action of the proposed compounds is to induce the activity of reactive oxygen species, lipid peroxidation, and accumulation of toxic aldehydes, as well as cell cycle arrest in the G2 / M phase and apoptosis [76].  [56]. In their previous research, the authors synthesized derivatives of 4-substituted methoxybenzoylarylthiazoles [57] and 2-aryl-4-benzoyl-imidazoles [58,59] with significant antitumor activity. However, it turned out that the presence of the unstable carbonyl group between the imidazole and phenyl ring is susceptible to metabolic reduction. Therefore, by means of a fusion strategy, this group was incorporated into the pyrimidine ring. The pyrazolo [1,5-a]pyrimidine scaffold plays the role of an imidazole and ketone bioisostere. The 3,4,5trimethoxyphene group was retained related to its pharmacophoric nature for antitumor activity. Compound (31) showed the highest activity against B16-F10 tumor cells with an IC 50 = 0.021 ± 0.002 µM, which is about four-fold more active than the reference standard colchicine with an IC 50 = 0.087 ± 0.006 µM and is comparable to paclitaxel with an IC 50 = 0.021 ± 0.002 µM. Compound (31) inhibited tubulin polymerization in an in vitro study and induced cell cycle arrest in the G2/M phase. These results were confirmed by an in vivo study in the B16-F10 murine melanoma model where tumor growth was inhibited without apparent toxicity. The SAR study of various aromatic groups indicated that the most favored for activity are the presence of electron-donating groups in the phenyl ring. The highest activity was shown by the compound substituted with the methyl group in the para position of the phenyl ring. The key to the activity is the presence of 3,4,5trimethoxyphenyl moiety at the 7-position of the pyrazolo[1,5-a]pyrimidine ring, and for the 2-position, the activity increased in the given series: 3-amino-4-methoxyphenyl < 6indole < 5-(1-methyl-1H-indole) < 4-methylphenyl for the most potential agents. Moreover, the introduced modifications increased the metabolic stability of the compound [56].
Ibrutinib is an FDA-approved Bruton's tyrosine kinase (BTK) inhibitor used in the treatment of leukemias [60]. Moreover, its promising activity against solid tumors has been shown [61][62][63][64]. Related to the widespread drug resistance and significant side effects [65][66][67], it is necessary to search for new drugs with increased effectiveness and lower toxicity. As a result, modifications to the structure of ibrutinib are of great interest to scientists looking for anticancer drugs [68]. Based on the literature data, the thiourea moiety was introduced, which ensures selectivity, effectiveness, and favorable physicochemical parameters [69][70][71][72][73]. Compound (32) exhibited the highest activity against melanoma cell line B16 with an IC 50 = 6.07 ± 1.06 µM [74]. Furthermore, it arrested cells in the G1 phase and promoted cell apoptosis and autophagy in melanoma cell line B16. In an in vivo study, (32) significantly inhibited the growth of melanoma in mice [74].

Indole Derivatives
Indole consists of a benzene ring fused at the 2-position and 3-position of the pyrrole ring. Its derivatives exert antitumor activity as inhibitors of tubulin polymerization and topoisomerase and as inducers of apoptosis, aromatase, and kinase inhibitors [75]. Indole derivatives with anti-melanoma activity are presented in Figure 7.

Indole Derivatives
Indole consists of a benzene ring fused at the 2-position and 3-position of the pyrrole ring. Its derivatives exert antitumor activity as inhibitors of tubulin polymerization and topoisomerase and as inducers of apoptosis, aromatase, and kinase inhibitors [75]. Indole derivatives with anti-melanoma activity are presented in Figure 7.
Dinavahi et al. synthesized new 2,3-dioxo-2,3-dihydroindol-1-yl derivatives [76]. The aim of the syntheses was to obtain inhibitors of aldehyde dehydrogenase (ALDH), which is overexpressed in various neoplasms [77][78][79][80]. Compound (33) showed the highest activity against melanoma cell line UACC 903 with an IC50 = 3 ± 0.6 μM, and (34) against melanoma cell line 1205 Lu with an IC50 = 2.1 ± 0.6 μM. One of the mechanisms of action of the proposed compounds is to induce the activity of reactive oxygen species, lipid peroxidation, and accumulation of toxic aldehydes, as well as cell cycle arrest in the G2 / M phase and apoptosis [76].

Sesquiterpene Lactone Derivatives
Sesquiterpenes are natural terpenoids consisting of a 15-carbon skeleton, isolated from plants of the Asteraceae family. The sesquiterpene may be hydrocarbon-based, or it Dinavahi et al. synthesized new 2,3-dioxo-2,3-dihydroindol-1-yl derivatives [76]. The aim of the syntheses was to obtain inhibitors of aldehyde dehydrogenase (ALDH), which is overexpressed in various neoplasms [77][78][79][80]. Compound (33) showed the highest activity against melanoma cell line UACC 903 with an IC 50 = 3 ± 0.6 µM, and (34) against melanoma cell line 1205 Lu with an IC 50 = 2.1 ± 0.6 µM. One of the mechanisms of action of the proposed compounds is to induce the activity of reactive oxygen species, lipid peroxidation, and accumulation of toxic aldehydes, as well as cell cycle arrest in the G2/M phase and apoptosis [76].

Sesquiterpene Lactone Derivatives
Sesquiterpenes are natural terpenoids consisting of a 15-carbon skeleton, isolated from plants of the Asteraceae family. The sesquiterpene may be hydrocarbon-based, or it may contain functional moieties, such as a lactone group. Sesquiterpenes have well-established anticancer activity [81,82]. Sesquiterpene lactone derivatives with anti-melanoma activity are presented in Figure 8.

Heterotricyclic Derivatives
Three-ring condensed heterocycles are a valuable scaffold with antitumor activity in medical chemistry [84,85]. Its derivatives with anti-melanoma activity are presented in Figure 9.
Gomaa et al. obtained the novel 2,3-dihydropyrazino[1,2-a]indole-1,4-dione derivatives, as a potential dual BRAF / EGFR inhibitors [97]. The key factor for the therapy seems to be the simultaneous inhibition of the epidermal growth factor receptor (EGFR), which may be activated as a result of BRAF inhibition. This leads to progressive tumor proliferation [98]. The previously obtained compounds [99] were modified in the C10 atom by extending the alkyl group in order to improve their binding affinity. Compound (38) exhibited the most promising activity against BRAF V600E with an IC50 = 45 ± 5 nM and melanoma cell line LOX-IMVI with an IC50 = 1.02 ± 0.02 μM. The SAR study indicated that the presence of phenethyl in C2 is crucial for the activity. This modification provided flexibility and allowed the benzyl group to expand hydrophobic interactions. Oxygen atom in C4 was responsible for hydrogen bond formation with Cys532 in BRAF V600E . The

Heterotricyclic Derivatives
Three-ring condensed heterocycles are a valuable scaffold with antitumor activity in medical chemistry [84,85]. Its derivatives with anti-melanoma activity are presented in Figure 9.
Gomaa et al. obtained the novel 2,3-dihydropyrazino[1,2-a]indole-1,4-dione derivatives, as a potential dual BRAF/EGFR inhibitors [97]. The key factor for the therapy seems to be the simultaneous inhibition of the epidermal growth factor receptor (EGFR), which may be activated as a result of BRAF inhibition. This leads to progressive tumor proliferation [98]. The previously obtained compounds [99] were modified in the C10 atom by extending the alkyl group in order to improve their binding affinity. Compound (38) exhibited the most promising activity against BRAF V600E with an IC 50 = 45 ± 5 nM and melanoma cell line LOX-IMVI with an IC 50 = 1.02 ± 0.02 µM. The SAR study indicated that the presence of phenethyl in C2 is crucial for the activity. This modification provided flexibility and allowed the benzyl group to expand hydrophobic interactions. Oxygen atom in C4 was responsible for hydrogen bond formation with Cys532 in BRAF V600E . The proliferative activity increased in the given series: morpholinyl > piperidyl > 2-methylpyrrolidinyl > dimethylamino for the para position of the phenylethyl tail. The derivative bearing the chlorine atom in C8 and an ethoxymethyl group at C10 shows the highest activity [97]. [100]. The aim of the syntheses was to optimize the previously reported structure [101,102] so as to separate the antitumor effect from the interactions of tubulin, which is also present in normal cells. The results of molecular modeling allowed the design of the second generation of imiqualine bearing the 3,4-dihydroxyphenyl moiety in the 1-position. Compound (39) exhibited the highest activity against melanoma cell lines A375, ME WO, A2058, and IPC 298 with an IC 50 = 3 nM, 3 nM, 4 nM, and 40 nM, respectively, as compared to the reference standard Vemurafenib with an IC 50 = 139 nM, 283 nM, 425 nM, and 13,000 nM, respectively. In the heterotopic mouse model of the heterotopic human melanoma xenograft, A375 cells were inoculated subcutaneously and the proposed antitumor compound was shown to be strongly correlated with a low mitotic index and lower invasion capacity [100].

Jaspine Derivatives
Jaspine B is a cyclic anhydrophytosphingosine isolated from sea sponges, Pachas samina sp., and Jaspis sp. It is a natural sphingolipid with a broad spectrum of activ including anticancer. Its derivatives with anti-melanoma activity are presented in Fig  10. Yang et al. obtained 2-epi-jaspine B analogs [106]. Compound (42) turned out to about two-fold more active against melanoma cell line A375 with an IC50 = 0.70 ± 0 μM, as compared to standard Cisplatin with an IC50 = 1.3 ± 0.11 μM. The proposed co pound induced apoptosis of the A375 melanoma cell line by arresting the cell cycle the G1 phase. The most active derivative bore the tetrahydro-2H-pyran-4-yl mo [106]. Novotná [103]. The spirochromanone core was selected on the basis of previously reported derivatives [104]. Compound (40) did not exhibit the highest activity against melanoma cell line B16F10 with an IC 50 = 13.15 ± 1.9 µM, but it possessed the highest selectivity index (SI = 13.37) for them, which made it the one possessing the highest potential. The purulent compound induced apoptosis by arresting the cell cycle in the G2 phase. The proposed compound was the only one to have a simple methylamine moiety; the remaining compounds had longer and more branched or cyclic chains. This may indicate that the methylamine group was crucial for the selectivity of spirochromanone derivatives for melanoma cell line B16F10 [103]. [105]. Compound (41) showed the most promising activity against melanoma cell line SK-MEL-5 with an IC 50 = 0.81 ± 0.03 µM, as compared to the reference standard Dinaciclib, which exhibited about seven-fold less activity with an IC 50 = 5.97 ± 0.25 µM). Paracyclophane/thiazole conjugates bearing the naphthoquinone moiety with the allyl substituent in the thiazole ring exhibited the highest activity, which may be related to the improvement in binding affinity to the target protein and to the increase in flexibility [105].

Jaspine Derivatives
Jaspine B is a cyclic anhydrophytosphingosine isolated from sea sponges, Pachastissamina sp., and Jaspis sp. It is a natural sphingolipid with a broad spectrum of activity, including anticancer. Its derivatives with anti-melanoma activity are presented in Figure 10.

Cinnamic Acid Derivatives
Cinnamic acid exerts a broad spectrum of biological activity [109]. There are many studies on anti-melanoma activity [110][111][112]; new derivatives are presented in the Figure  11.
Vale et al. obtained new cinnamic acid ester derivatives [113]. The basis for the syntheses was the confirmed anticancer activity of cinnamic acid and its derivatives against melanoma [110][111][112]. The results of the melanoma cell line B16-F10 viability assay exhibited that compound (44) possessed the highest activity with an IC50 = 60.28 μM. Moreover, at a concentration of 25 μM, the proposed compound significantly inhibited the migration of the B16-F10 melanoma cell line. The trypan blue method showed that (44) reduced the proliferation of the B16-F10 melanoma cell line at all concentrations tested (6.25, 12.5, 25, and 50 μM) after 24h of treatment. The authors suggested that the probable mechanism may be related to the function of p53, a transcription factor responsible, among others, for inducing cell sugar arrest, promoting apoptosis, activating oncogenes or inhibiting cell proliferation [114]. Biological studies also indicated that (44) disturbed cell invasion, adhesion, colonization, and polymerization of actin. The high activity of the proposed compound is probably related to the addition of two bromine atoms to the double bond of the cinnamic acid structure, which had a favorable effect on bioavailability and interaction with the biological target [113].
Romagnoli et al. obtained new cinnamic acid derivatives linked to arylpiperazines [115]. Compound (45) showed the most promising activity against melanoma cell line A375 with GI50 = 16.1 ± 1.5 μM. The most active compound bore a chlorine atom and a fluorine atom at the 3-position and 4-position, respectively, of the phenyl ring connected to piperazine, and the amino and methoxy groups also at the 3-position and 4-position of the second phenyl ring, respectively [115]. Yang et al. obtained 2-epi-jaspine B analogs [106]. Compound (42) turned out to be about two-fold more active against melanoma cell line A375 with an IC 50 = 0.70 ± 0.09 µM, as compared to standard Cisplatin with an IC 50 = 1.3 ± 0.11 µM. The proposed compound induced apoptosis of the A375 melanoma cell line by arresting the cell cycle in the G1 phase. The most active derivative bore the tetrahydro-2H-pyran-4-yl moiety [106].
Novotná et al. obtained new aza-derivatives of Jaspine B [107]. The basis for the use of the Jaspine B scaffold was its demonstrated ability to induce apoptosis in melanoma cell line SK-MEL-28 [108]. Compound (43) exhibited the highest activity against melanoma cell line BLM with an IC 50 = 31.01 ± 0.89 µM/L, as compared to standard Cisplatin with an IC 50 = 32.18 ± 1.29 µM/L [107].

Cinnamic Acid Derivatives
Cinnamic acid exerts a broad spectrum of biological activity [109]. There are many studies on anti-melanoma activity [110][111][112]; new derivatives are presented in the Figure 11.

Thiosemicarbazone and -zide Derivatives
Thiosemicarbazones and thiosemicarbazides are scaffolds with a privileged place in medical chemistry. Their derivatives exert a wide spectrum of activity, including anticancer activity [116,117]. Its derivatives with anti-melanoma activity are presented in Figure 12.
In our lab, we also obtained a series of compounds as potential agents for melanoma. Pitucha et al. synthesized the copper complexes of thiosemicarbazones [118] based on their confirmed anticancer activity [119][120][121]. Compound Kozyra et al. obtained new 2,4-dichlorophenoxyacetic acid hydrazide thiosemicarbazide derivatives [122]. We based our current research on previously confirmed anticancer activity of this scaffold [123]. Compound (49) turned out to be the most active against melanoma cell line G-361 with an IC50 = 99±4 μM. The proposed compound bore an iodine atom in the para position of the second phenyl ring [122]. The activity of the derivatives increased in the given series of substituents: 2-bromophenyl < napht-1-yl < 2iodophenyl < 4-methylthiophenyl < 4-iodophenyl for the most potential agents. Vale et al. obtained new cinnamic acid ester derivatives [113]. The basis for the syntheses was the confirmed anticancer activity of cinnamic acid and its derivatives against melanoma [110][111][112]. The results of the melanoma cell line B16-F10 viability assay exhibited that compound (44) possessed the highest activity with an IC 50 = 60.28 µM. Moreover, at a concentration of 25 µM, the proposed compound significantly inhibited the migration of the B16-F10 melanoma cell line. The trypan blue method showed that (44) reduced the proliferation of the B16-F10 melanoma cell line at all concentrations tested (6.25, 12.5, 25, and 50 µM) after 24h of treatment. The authors suggested that the probable mechanism may be related to the function of p53, a transcription factor responsible, among others, for inducing cell sugar arrest, promoting apoptosis, activating oncogenes or inhibiting cell proliferation [114]. Biological studies also indicated that (44) disturbed cell invasion, adhesion, colonization, and polymerization of actin. The high activity of the proposed compound is probably related to the addition of two bromine atoms to the double bond of the cinnamic acid structure, which had a favorable effect on bioavailability and interaction with the biological target [113].
Romagnoli et al. obtained new cinnamic acid derivatives linked to arylpiperazines [115]. Compound (45) showed the most promising activity against melanoma cell line A375 with GI 50 = 16.1 ± 1.5 µM. The most active compound bore a chlorine atom and a fluorine atom at the 3-position and 4-position, respectively, of the phenyl ring connected to piperazine, and the amino and methoxy groups also at the 3-position and 4-position of the second phenyl ring, respectively [115].

Thiosemicarbazone and -zide Derivatives
Thiosemicarbazones and thiosemicarbazides are scaffolds with a privileged place in medical chemistry. Their derivatives exert a wide spectrum of activity, including anticancer activity [116,117]. Its derivatives with anti-melanoma activity are presented in Figure 12.

Other Derivatives
The remaining derivatives, which could not be easily divided depending on the chemical structure, are shown in the Figure 13. In our lab, we also obtained a series of compounds as potential agents for melanoma. Pitucha et al. synthesized the copper complexes of thiosemicarbazones [118] based on their confirmed anticancer activity [119][120][121]. Compound (46) showed the most promising activity against melanoma cell line G361 an IC 50 = 135.64 ± 7.01µM, making it about three-fold more potent than the standard Dacarbazine (DAC) with an IC 50  Kozyra et al. obtained new 2,4-dichlorophenoxyacetic acid hydrazide thiosemicarbazide derivatives [122]. We based our current research on previously confirmed anticancer activity of this scaffold [123]. Compound (49) turned out to be the most active against melanoma cell line G-361 with an IC 50 = 99±4 µM. The proposed compound bore an iodine atom in the para position of the second phenyl ring [122]. The activity of the derivatives increased in the given series of substituents: 2-bromophenyl < napht-1-yl < 2-iodophenyl < 4-methylthiophenyl < 4-iodophenyl for the most potential agents.

Other Derivatives
The remaining derivatives, which could not be easily divided depending on the chemical structure, are shown in the Figure 13.   Figure 13. Other derivatives with anti-melanoma activity. Based on the previously described approach, Chang et al. designed agent (50), which possessed activity against melanoma cell lines SK-MEL-28 and A375 with an IC50 = 3.30 ± 0.06 μM and 1.98 ± 0.10μM, respectively [82]. Moreover, compound (50) inhibited G2 / M phase cell arrest. Immunofluorescent staining possessed a disruption of the microtubule network upon exposure to (50), leading to apoptosis and cell arrest, and indicated that tubulin could be a potential molecular target for (50). This was confirmed by a molecular docking study [86].
Razmienė et al. obtained the series of N-aryl-2,6-diphenyl-2H-pyrazolo[4,3c]pyridin-7-amines [124]. The aim of the syntheses was to obtain compounds whose activity increased after irradiation with visible blue light (414 nm). Compound (51) was Based on the previously described approach, Chang et al. designed agent (50), which possessed activity against melanoma cell lines SK-MEL-28 and A375 with an IC 50 = 3.30 ± 0.06 µM and 1.98 ± 0.10µM, respectively [82]. Moreover, compound (50) inhibited G2/M phase cell arrest. Immunofluorescent staining possessed a disruption of the microtubule network upon exposure to (50), leading to apoptosis and cell arrest, and indicated that tubulin could be a potential molecular target for (50). This was confirmed by a molecular docking study [86].
Razmienė et al. obtained the series of N-aryl-2,6-diphenyl-2H-pyrazolo[4,3-c]pyridin-7-amines [124]. The aim of the syntheses was to obtain compounds whose activity increased after irradiation with visible blue light (414 nm). Compound (51) was most active against melanoma cell line G361, possessing an EC 50 (the concentration of a drug that gives halfmaximal response) = 3.5 µM, 1.6 µM, and 0.9 µM in cells irradiated with 1,5, and 10 J/cm 2 , respectively. The photodynamic potential of the proposed compound was also confirmed by the study on human HaCaT keratinocytes, which were about two-fold less sensitive to it. The likely mechanism of action is to induce the formation of reactive oxygen species in cells and damage DNA, leading to apoptosis [124]. Cui [125]. The basis for the research was to obtain dual inhibitors, both targeting snail and histone deacetylase (HDAC). Snail is a transcription factor [126] and HDAC is involved in histone modification [127]. Related to the strong connection between the two pathways, the creation of double inhibitors enable a more effective therapy against cancer. With the highest activity against the above-mentioned targets, Compound (52) was also examined against melanoma cell line MDA-MB-435 with GI 50 = 0.0361 µM. The most active derivative bore a fluorine atom in the para position of the phenyl ring beyond the amine group in the ortho position and a 5-methyl-1H-pyrazol-3-ylamino moiety in the 4-position pyrrolo[2,1-f] [1,2,4]triazin-2yl [125].
Hassan et al. obtained new pyrazoline derivatives [128]. An in vitro study of a single dose test (10-5 µM), determining the percent growth inhibition, indicated that the proposed Compound (53) possessed 98.46% growth inhibition of the SK-MEL-5 melanoma cell line. The most active derivative bore the 4-methoxyphenoxy and 3,4,5-trimethoxyphenyl moiety, which indicated the high potential of the electron-withdrawing group [128].
Gagné-Boulet et al. obtained novel derivatives of 1-(4-(phenylthio)phenyl)imidazolidin-2-one [129]. The imidazolidin-2-one scaffold was selected based on previous studies, and the sulfonate moiety from previous studies was replaced with a thioether linker [130]. Compound (54) exhibited the highest activity against melanoma cell line M21 with an IC 50 = 0.2 ± 0.1 µM. The proposed compound arrested the progression of the cell cycle in the G2/M phase and induced the disruption of the cytoskeleton. The N,N'-ethylenebis(iodoacetamide) (EBI) detection assay indicated the interference of the colchicine binding site on the microtubules by (54). Moreover, (54) showed low toxicity of chicken embryo. The SAR study indicated that the key to the activity is the substituent on the terminal phenyl group. The most active compound bore the chlorine atom substituted in the meta position. Furthermore, the presence of a thioether linker as well as the imidazolidin-2-one moiety is also important for the activity [129].
Zhang et al. obtained novel nitric oxide-releasing derivatives of triptolide [131]. Triptolide was first isolated from Tripterygium wilfordii Hak. f. It exhibited a broad antitumor spectrum, inter alia, in a mouse model, it inhibited the growth of open-bone xenografts, among others, by melanoma cell line B16, and it inhibited the metastasis of the B16F10 melanoma cell line to the spleen and lungs [132]. The interest in nitric oxide as an anticancer factor stems from its ability to induce apoptosis, sensitize cancer cells to chemotherapy, or inhibit metastasis [133][134][135]. Therefore, furoxan, as a nitric oxide donor, was introduced at the 14-position of tryptolide. Compound (55) possessed the most promising activity against murine B16F10 melanoma cell with an IC 50 = 0.021 ± 0.03 µM/L, which is comparable to triptolide with an IC 50 = 0.005 ± 0.002 µM/L. Further studies involving a xenograft mouse model of melanoma established by the subcutaneous inoculation of B16F10 showed that the proposed compound inhibited the growth of melanoma at low doses (0.3 mg/kg). Moreover, the SAR study indicated that the length of the linker connecting the furoxan ring with the triptolide scaffold is inversely proportional to the activity [131].
Abdel-Maksoud et al. obtained novel derivatives based on the pyrrolo[2,3-b]pyridine scaffold [136]. It has a privileged nature and is present in the approved and preclinically tested inhibitors of B-RAF [137][138][139][140]. Compound (56) showed the most promising activity against BRAF V600E with an IC 50 = 0.080 ± 0.003 µM. The derivative bearing a bromine atom in the para position of the phenyl ring possessed the highest activity. For the most potential agents, activity increased in a given series of substituents 3-(4-substituted-phenyl)urea: trifluoromethyl < methyl < bromine [136].
Pałkowski et al. obtained novel 1-methyl 3-octyloxymethylimidazolium derivatives carrying various anionic moieties [141]. The basis for the syntheses were the few scientific reports on the anticancer activity of ionic liquids [142,143]. Compound (57) exhibited the most promising activity against melanoma cell line B16F10 with IC 50 = 1.01 × 10 −2 µM/L. The most active derivative bore the thymoloxyacetic anion [141]. Barbarossa et al. obtained new thalidomide derivatives [144]. Despite its infamous history, thalidomide has been successfully repositioned for cancer treatment [145]. Currently, efforts are focused on optimizing the phthalimide core of thalidomide, which is a pharmacophore, in order to reduce side effects [146,147]. Based on the high antitumor activity of compounds correlated with thalidomide from previous studies [148], a series of derivatives were obtained based on the privileged phthalimide core connected directly to a differently substituted aromatic ring with the N-terminus of the phthalimide moiety. The most promising compound turned out to be (58), against melanoma cell line A2058 with an IC 50 = 15.37 ± 0.7 µM. The immunofluorescence and the tubulin polymerization test indicated that it interfered with the dynamics of microtubules by inhibiting tubulin polymerization. This was confirmed by the results of molecular docking. The TUNEL assay showed the potential to induce apoptosis in melanoma cell line A2058. For the most potential agents, the activity increased in a given series of substituents: 4-phenoxyphenyl < 2-chlorophenoxyphenyl < 2-methylphenoxyphenyl [144].
Geng et al. obtained novel diaryl derivatives [149]. Compound (59), bearing 3 bromine atoms in the phenyl ring, turned out to be the most potent agent against melanoma cell line A375 with an IC 50 = 2.95 ± 0.23 µM, as compared to standard Cisplatin with an IC 50 = 3.75 ± 0.62 µM. The proposed compound leads to apoptosis through the inducing G2/M phase arrest of the A375 melanoma cell line. The results were confirmed by a transplanted tumor nude mouse model [149].
Piechowska et al. obtained a series of hybrid compounds with tropinone and thiazole rings [150]. The choice of such a combination of both groups was based on previous research results [151]. The most promising activity against melanoma cell line B16-F10 with an IC 50 = 1.51 ± 0.14 µM was exhibited by (60), as compared to standard Chlorambucil with an IC 50 = 2.92 ± 0.10 µM. The most active derivative bore the 4-iodophenyl substituent [150].
Remigante et al. investigated the effect of the known BK activator NS-11021 (61) on the viability, proliferation, and migration of IGR39 cells of the melanoma cell line [25]. The authors maintained that the above-mentioned effects are not specifically dependent on the opening of the BK channels. Compound (61), in addition to direct activation of BK channels, also resulted in an increase in intracellular calcium levels. The latter effect is likely due to the activation of the calcium-permeable conductivity and it is this elevation in intracellular calcium levels that may be responsible for inhibiting the viability, proliferation, and migration of IGR39 cells of the melanoma cell line. The latter discovery provides a new potential direction for new potential agents with anti-melanoma activity [25].

Conclusions
This article provides a summary overview of the latest potential melanoma agents from 2020-2022. SAR analysis was discussed for selected structures. Therefore, the present review may be useful for rational drug design with anti-melanoma activity.
Author Contributions: P.K. contributed to conceptualization, analysis, research, writing, review, and editing. D.K., M.P., and P.K. contributed to writing-original draft preparation. D.K. and P.K. contributed to literature gathering. D.K., M.P., and P.K. contributed to visualization. M.P. contributed to supervision. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare no conflict of interest.