Recent Advances in Metal Complexes Based on Biomimetic and Biocompatible Organic Ligands against Leishmaniasis Infections: State of the Art and Alternatives

Abstract

Leishmaniasis is a complex disease present in a variety of manifestations listed by the World Health Organization (WHO) as one of the neglected diseases with a worse prognosis if not treated. Medicinal inorganic chemistry has provided a variety of drugs based on metal–organic complexes synthesized with different metal centers and organic ligands to fight against a great number of parasite maladies and specifically Leishmaniasis. Taking advantage of the natural properties that many metals present for biotechnological purposes, nanotechnology has offered, in recent years, a new approach consisting on the application of metal nanoparticles to treat a great number of parasitic diseases, as a drug vehicle or as a treatment themselves. The aim of this review is to gather the most widely used metal complexes and metallic nanoparticles and the most recent strategies proposed as antileishmanial agents.

1. Introduction

The concept of neglected diseases (NTDs) englobes all the maladies that cause serious problems to wide populations, but there is not enough investment or research on them. The World Health Organization (WHO) published a list with the main NTDs: Buruli ulcer; Chagas disease; dengue and chikungunya; dracunculiasis; echinococcosis; foodborne trematodiases; human African trypanosomiasis; leishmaniasis; leprosy; lymphatic filariasis; mycetoma, chromoblastomycosis, and other deep mycoses; noma; onchocerciasis; rabies; scabies and other ectoparasitoses; schistosomiasis; soil-transmitted helminthiases; snakebite envenoming; taeniasis/cysticercosis; trachoma; and yaws [1]. The 20 diseases, increased from the initial 17 in the last decade, were chosen because of their adverse impact, relative obscurity, and the availability of tools to combat them [2,3].

According to recent reports from the WHO, leishmaniasis is considered to be one of the seven primary illnesses that affects all continents, and is broadly located in tropical and subtropical poor regions of the New World, but is also present in Europe and Asia by migration [4], affecting more than 90 countries worldwide. An estimated 700,000 to 1 million new cases occur annually, with an annual mortality rate of 95% in visceral leishmaniasis if untreated, being only surpassed by malaria among parasitic diseases [5]. For survival, the Leishmania parasite needs a mammalian reservoir, which leads to anthroponotic cycles in humans or zoonotic cycles in animals like rabbits or dogs. The transmission vectors are mainly dipteral female insects of the genera Phlebotomus in the Old World and Lutzomyia in the New World (Figure 1). The recent migration crisis in the Mediterranean area and Ukraine conflict obligate the relocation of large populations, sometimes in unhealthy conditions, facilitating the proliferation of the infection vectors. These situations, with the blood transfusions from infected patients in host countries, make leishmaniasis a global illness that is spreading out into the Third World.

More than 20 Leishmania species cause different forms of leishmaniasis that range in severity from cutaneous lesions to systemic infection [6,7,8], but their manifestation in humans can be classified mainly into three different clinical conditions: visceral, cutaneous, and muco-cutaneous. Visceral leishmaniasis (VL) [9], also known as kala-azar, is the most aggressive and mortal form. It is caused by Leishmania donovani in Asia and in Eastern Africa, where humans are the main pathogen reservoir, and by Leishmania infantum in Latin America and in the Mediterranean area, where VL is a zoonotic disease and dogs and rabbits are the main reservoirs. According to the WHO, almost 13,000 cases of VL occurred in 2020, with an estimated at-risk population of 200 million people [10]. Cutaneous leishmaniasis (CL) [11] is a disfiguring disease, especially for women [12], which produce nodules and ulcers in the skin, and often leaves scars on visible body sites, causing psychological, social, and economic problems, but it does not cause death. It is usually caused by Leishmania mexicana, Leishmania (Viannia) braziliensis, or Leishmania panamensis in the Americas, and Leishmania major or Leishmania tropica in all other countries [13]. Mucocutaneous leishmaniasis (ML) [14], also known as “espundia”, is potentially life threatening and requires treatment. It causes permanent lesions in the mucosa (mouth, nose, or genitals), and occurs years after the onset of cutaneous leishmaniasis. ML is a known risk from Leishmania spp. of the Viannia subgenus, typically found in the Americas (Leishmania (Viannia) braziliensis, Leishmania (Viannia) amazonensis, Leishmania (Viannia) panamensis, and Leishmania (Viannia) guyanensis) [13].

Metals are valuable in multiple research areas due to their wide diversity of properties, promoted by their different electronic configurations, with an extraordinary variety of applications, from industrial to biological disciplines. The unique capabilities of metals can be enhanced by their combination with organic ligands, leading to synergistic effects in the resulting metal complexes, improving their versatility [15,16]. Among their technological uses, their popular use as catalysts can be cited [17,18,19], as can their utility in the material synthesis industry [20] or photochemistry [21]. Medicinal inorganic chemistry can exploit the unique properties of metal ions for the design of new drugs. Recent advances in inorganic chemistry areas make the role of transition metal complexes promising therapeutic compounds [22,23,24]. The advances in inorganic chemistry have made possible the synthesis of a wide series of transition metal complexes with organic ligands of interest, which can be used as therapeutic agents in cancer [25], bacterial [26], or virus [27] infections.

The use of metal complexes in the field of parasitology is developing promising perspectives [28], with transition metal centers like Cu [29,30], Ni [31,32], or Zn [33,34], precious metals like Au [35], Ag [36], Pt, and Pd centers [37], or organometallic cores [38]. The importance of the metal ion and the nature of the organic ligands is critical to understanding the chemistry between the proposed drugs and the parasite environment.

This review includes the use of the most common metal complexes as treatment, alternatives based on biocompatible ligands, and new possibilities based on metallic nanoparticles.

2. Metal Complexes Used as Conventional Therapies against Leishmaniasis

The use of metal complexes as antiparasitic agents is one of the main therapeutic approaches to fight Leishmania spp. infections, with pentavalent antimonials acting as the most effective and low-cost drugs, but they cause strong side effects due to the toxicity of antimony [39,40]. There are also few commercial alternatives without antimony, but their effectivity is not so high as the antimonial established treatments.

2.1. Antimonial Commercial Drugs

The importance of antimony in early medicine is well documented, even being considered to be a general panacea in the 16th century introduced by Paracelsus. The use of the trivalent antimonial, tarter emetic [41], was first reported for the treatment of CL and VL, but later this drug was found to be highly toxic as well as very unstable in tropical climates [40]. Tartar emetic was considered to be an irritating drug, since it exhibited side effects such as cough, chest pain, and severe depression. This led to the discovery of pentavalent antimonials [42], where meglumine antimoniate and sodium stibogluconate stand out among them.

2.1.1. Meglumine Antimoniate

Meglumine antimoniate [43] (Figure 2), commercially known as Glucantime^®, is still a first-line drug in the treatment of leishmaniasis in several countries [44,45]. This antimony complex presents high water solubility (>300 mg/mL), and its adsorption can be enhanced for oral administrations with secondary molecules like β-cyclodextrin [46]. It can also be used in combination with liposomes, metallic, and polymeric nanoparticles to improve its biodistribution [47,48]. The mode of action of meglumine antimoniate is still unknown [42], but one of the proposed mechanisms is that Sb(V) would behave as a prodrug, being reduced within the organism into more toxic and active Sb(III), which can enter Leishmania cells primarily [49,50]. Sb(III) can interact with low-molecular thiols, cysteine-containing peptides, and, in particular, thiol-dependent enzymes [51].

Despite its commercial use, meglumine antimony causes common adverse effects during treatment, like anorexia, nausea, vomiting, malaise, myalgia, headache, and lethargy. There are many reports of serious side effects in patients treated with Glucantime^®, from electrocardiographic changes [52,53] to pancreatic toxicity [54], or the presence of antimony in plasma and skin [55]. Different studies also put in the spotlight the transplacental transference of antimonies from the mother during the gestation process [56], or the problematic nature of pharmacokinetics in children administration [57].

2.1.2. Sodium Stibogluconate

Sodium stibogluconate, or Pentostam^®, is also an effective drug in the fight against leishmaniasis. Its ionic nature makes it freely soluble in water (Figure 3), and its mechanism of action is well known since four decades ago [58], focusing on the inhibition of the incorporation of nucleobases into the parasite protein synthesis and blocking its energy metabolism through stibogluconate interactions.

Despite no serious toxicity being observed even when a ten-fold increase in the daily recommended concentration for sodium stibogluconate was applied, there are few reports where dose-dependent factors promote histological changes in the liver and kidneys [59], and health problems like pancreatitis, nephrotoxicity, and elevation of liver enzymes were also reported [60]. This evidence, joined to the development of drug resistance of Leishmania spp., make it necessary to attempt more efforts to design and test new metal complexes with antileishmanial activity.

Some other antimonial drugs can be mentioned, like trivalent antimonial stibocaptate [61], but their use is less extensive than the first two.

2.2. Other Commercial Drugs against Leishmania spp. and Trypanosoma cruzi

The antiparasitic metal drugs are not limited to the antimonial ones, and other commercial treatments can be mentioned [62], such as organic molecules like amphotericin B [63,64,65], miltefosine [66], or paromomycin [67], as well as combinations of all of them to improve efficacy [68]. Nevertheless, there are not many examples of metal complexes derived from these organic ligands. One approach from Chudzik et al. in 2013 can be mentioned which uses copper complexes of amphotericin-B [69], but its study was focused in the antifungal activity against C. albicans.

On the other hand, benznidazol and nifurtimox are two nitro derivatives commonly used against trypanosoma infections like Chagas disease, a zoonosis malady comparable to leishmaniasis [70,71]. Benznidazole (2-nitro-N-[phenylmethyl]-1H-imidazole-1-acetamide) is a nitroimidazole derivative that showed efficacy against T. cruzi parasites, and different clinical and experimental studies were published using it as a treatment for acute and chronic Chagas disease. Nifurtimox (3-methyl-N-′[(5-nitro-2 furanyl)-methylene]-4-morpholinamine 1,1 dioxide) is a nitrofuran derivative which generates nitroanion radicals by nitroreductases that, in the presence of oxygen, produce free radicals that damage T. cruzi [72]. But once again, their use in combination with metal ions like copper and silver is just at the initial phase, it being worthwhile to mention the work of Cunha da Souza et al. in 2023 [73].

3. Biomimetic Metal Complexes against Leishmaniasis

Due to the high toxicity of antimony [39], there is a need to find novel metal complexes which can substitute the pentavalent antimonials to fight parasite infections, maintaining the advantages of the synergetic effect of the inclusion of metal ions in the structure of the proposed drug, but avoiding the toxicity of heavy metals like antimonies. The variety of metal complexes in the field of parasitology is wide and rich, so this review is focused on recent advances in the derived metal complexes based on biomimetic and well-known commercial drugs against leishmaniasis infections.

3.1. Nucleobase Analogous Active Compounds

Numerous organic ligands with purine or pyridine rings in their structures have been demonstrated to be effective against parasitic infections once they were coordinated with metallic ions.

3.1.1. Azoles

Azoles [74] (Figure 4) represent a broad family of five-membered heterocyclic aromatic compounds whose framework contains from one and up to five nitrogen atom(s), and can also contain at least one S or O atom as a part of the azole conjugated ring (N,S and N,O subclasses of azoles, respectively) [75].

Their importance as biological agents has been demonstrated throughout the last decades, with special emphasis on the antibacterial [76] and antifungal [74] effectivity, but their use as antiparasitic agents is also well defined [77,78], even with specificity to fight malaria [79] or amoeba infections [80]. The use of azoles against Leishmania spp. infections has been studied, with a focus on the 14α-demethylase as a distinct target for antileishmanial therapy [81] or for the inhibition of trypanothione reductase [82]. The assays with ketoconazole [83], indazol [84], or chloro-derived azoles such as miconazole, tioconazole, clotrimazole, and their analogs and metal complexes is to be remarked in this context [85].

3.1.2. Triazolopyrimidines

Triazolopyrimidine ligands have been known since early last century [86], but their complexation properties were not studied until 1952 by Birr [87]. Multiple biological applications have been studied for triazolopyrimidine ligands, as well as for their metal complexes, due to their remarkable pharmacology [88,89,90,91]. For the case of 1,2,4-triazolo[1,5-a]pyrimidines ligands [92] (Figure 5A), which have shown the best antiparasitic activity, their structural similarity with purines makes them an optimal model to study the reactivity with different metal ions [93,94,95,96]. Triazolopyrimidines show great versatility as ligands, because of the presence of nitrogen atoms on their skeleton with available electron pairs. The coordination occurs mainly through N3-monodentate or N3,N4-bridging, but there are descriptions of many other bonding routes being possible when the number of exocyclic substituents in the aromatic ring increases. The interaction between metal cations and triazolopyrimidines exhibit a great potential to act as building blocks with enormous versatility, giving rise to multidimensional systems with useful properties, as well as technological and biological properties [97,98,99,100].

The 1,2,4-triazolo[1,5-a]pyrimidine family includes a wide variety of derived structures with antileishmanial activity, such as the non-substituted simple structure (tp) [101], the 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine with two methyl groups (dmtp) [102] and 7-amine-1,2,4-triazolo[1,5-a]pyrimidine (7-atp) with a very reactive amine entity in the structure [103]. To increase the coordination positions, there are also several triazolopyrimidines containing exocyclic oxygen atoms, which spread out the range of metallic centers that can interact with these ligands (i.e., lanthanide ions). Some of the reported oxygenated triazolopyrimidine derivatives are dihydro-5-oxo-[1,2,4]triazolo-[1,5-a]pyrimidine (5HtpO and 7HtpO), whose structures were described by Abul Haj et al. [104], the commercial 4,7-dihydro-5-methyl-7-oxo[1,2,4]-triazolo[1,5-a]pyrimidine (HmtpO) [105], and the most recently reported, 7-oxo-5-phenyl-1,2,4-triazolo[1,5-a]pyrimidine (HftpO) [106]. The efficacy of [1,2,4]-triazolo[1,5-a]pyrimidines against leishmania spp. have been contrasted in different studies [107,108]. Comparing them with reference drug Glucantime ^® (meglumine antimoniate), methyl, and amino substituted triazolopyrimidines, the organic molecules slightly increase their efficacy against different leishmania strains like L. infantum or L. braziliensis, but the hydroxy-tryazolopyrimidines are the ones which led to a substantial increase in the leishmanicidal effect, with a selectivity index (SI, proportion between toxicity to the host cells and specificity reducing parasite population) up to 40 times better than the drug reference (it is possible to reach even more than 50 time better effect to trypanosomatid parasites) [106]. These results positioned tryazolopyrimidines as an extraordinary potential alternative to antimonial traditional treatments, with fewer side effects and higher biological activity.

3.1.3. Quinolines

Quinoline is an heterocyclic aromatic nitrogen molecule with a double-ring skeleton containing a benzene ring fused to pyridine at two adjacent carbon atoms (Figure 5B) [109]. The extraordinary variety of potential positions to include substituents (from C2 to C8) make this basic core a very versatile basic unit to build numerous series of biologically active families.

The potential biological applications of quinoline derivatives vary from antibacterial efficacy to antitubercular, antimicrobial, or anticancer applications [110,111,112,113,114], but they also can be used as antiparasitic agents [115] with leishmanicidal efficacy [116,117]. The interest of the scientific community in these compounds increased after 2013 [118], with the work of Bringmann et al. [119], who evaluated 49 different quinolines against promastigote and amastigote forms of L. major, obtaining lower values of IC₅₀ than 10 µM in 20 compounds. The higher SI were obtained with 4′-methylphenyl-pentyl substituted. The halogen ones had great activity against the parasites, but also were very toxic to the J774.1 cells used as a host model in that study. But, in general, modified quinolines show a similar or better efficacy against L. major, positioning them as excellent candidates to further studies and building blocks for metal complexes. From that point, the use of quinolines against Leishmania spp. forms increased during the last decade [120,121].

3.1.4. 1,10-Phenanthrolines

The three-aromatic-ringed 1,10-phenanthroline (Figure 5C) is one of the most versatile and used organic bidentate ligands used to synthesize metal complexes [122]. Its two nitrogen atoms in positions 1 and 10 make it an extraordinary strong chelating agent. Its great solubility in organic solvents makes it a good candidate to interact with metal ions in a solution to coordinate with them as a main ligand or as an auxiliary ligand to complete the coordination sphere. There are recent studies of the influence of this organic ligand and some derivatives, like the 1,10-phenanthroline-5,6-dione of Lane et al. in 2021 [123], where both forms can induce a variety of disorders in a variety of biological systems, like a drastic reduction in cell proliferation, prominent morphological changes, the appearance of electron-dense deposits (mainly consisting of calcium ions) in the parasite’s cytoplasm, as well as triggering perturbations of the mitochondrial membrane and the cisternae of the endoplasmic reticulum of the epimastigote forms of T. cruzi [124].

3.2. Metal Complexes Derived from Nucleobase Analogous Active Compounds

Transition metal complexes represent an attractive platform for the development of anti-parasitic therapeutics, as they present an extraordinary chemical diversity and go by different mechanisms of action, such as the induction of the generation of reactive oxygen species (ROS), enzymatic inhibition, or DNA intercalation [125].

Table 1. Summary of metal complexes cited in Section 3.2., classified by the organic ligand, metal, and parasite studied, with comparison between the best selectivity index (IC₅₀ for cells/IC₅₀ for parasites, SI) for the extracellular forms of each family.

Ligand	Metal	Parasite	SI	Reference
Ketoconazole	Ru(II/III)	L. major	150.0	[126]
Clotrimazole	Ru(II/III)	L. major	500.0	[127]
[1,2,4]Triazolo [1,5-a]Pyrimidine	Cd(II), Cu(II) and Zn(II)	L. infantum, L. braziliensis	3.1	[128]
[1,2,4]Triazolo [1,5-a]Pyrimidine	Ni(II)	L. infantum, L. braziliensis	20.0	[129]
[1,2,4]Triazolo [1,5-a]Pyrimidine	Ru(II/III)	L. infantum, L. braziliensis and L. donovani	52.0	[130]
[1,2,4]Triazolo [1,5-a]Pyrimidine	La(III), Nd(III), Eu(III), Gd(III), Tb(III), Dy(III) and Er(III)	L. infantum, L. braziliensis	49.9	[131]
[1,2,4]Triazolo [1,5-a]Pyrimidine	Cu(II)	L. infantum, L. braziliensis, L. peruviana, L. mexicana and L. donovani	54.0	[132]
N-heterocyclic carbene–quinoline	Au(I)	L. infantum	9.8	[133]
1,10-phenanthroline	Cu(II) and Ag(I)	L. amazonensis	43.4	[134]
Ibuprofen	Ni(II), Mn(II) and Pd(II)	L. amazonensis	3.3	[135]
Diclofenac	Mn(II), Ni(II) and Co(II)	L. infantum, L. braziliensis and L. donovani	58.8	[136]
Triethylphosphine	Au(I)	L. infantum, L. braziliensis	5.6	[137]
Bumetanide	Zn(II)	L. infantum, L. braziliensis and L. donovani	22.2	[138]

summarizes the most significant complexes discussed in this point.

The family of azole compounds is extraordinarily diverse, and their possible combinations with metals is increasing every day. One of the most useful ligands against leishmania spp. is ketoconazole, an antifungal agent used to treat localized or systemic fungal infections by inhibiting ergosterol synthesis [139]. Its combination with ruthenium, published by Iniguez et al. in 2013, demonstrated that the metal complex improves the leishmanicidal effect of the organic ligand, from 63 SI to 150 SI [126]. The counter-anion salts used in the synthesis play a crucial role in the activity of the ketoconazole ruthenium complexes, with chloride, etilendiamine (en), or bipyridine (bipy) as the most effective ions. Following the ruthenium compounds, clotrimazole ruthenium complexes also exhibit great leishmanicidal power against L. major and T. cruzi [127], and again the use of bipy, en, and acetylacetonate (acac) in the synthesis reaches the best activity.

It is possible to find many examples of metallic complexes with antiparasitic activity with triazolopyrimidines [107,140], especially those with transition metals like copper [141], zinc [128], nickel [129], or ruthenium [130], as well as lanthanide cations [131]. All of these complexes show better IC₅₀ values and higher selectivity indexes than reference drugs against Leishmania spp. and Trypanosoma cruzi. The proposed drugs were assayed in a solution for a cytotoxicity test and for amastigote and promastigote screenings in vitro, but the different solubility of some triazolopyrimidine derivatives make them difficult to manipulate is some cases, especially in that ligands with organic cycles as substituents in positions 5 or 7 [107]. Recently, Martin-Montes et al. [132] reported a novel methodology to deliver the triazolopyrimidine complexes, with the first example of the use of silica nanoparticles as carriers of the ligand and metal complexes in parasitology (Figure 6).

The metallic complexes presented similar acceptable values for IC₅₀ in the promotion of cytotoxicity assays, but they still did not exceed the performance of commercial drugs in some cases, like that of L. braziliensis. Nevertheless, for the triazolopyrimidine ligand, as well as its metal complex [132], the cytotoxicity results were better than those of the reference drugs. Comparing Glucantime and triazolopyrimidine ligands, the concentration needed to consider the organic molecule toxic for the host cells was more than sixty-fold higher than that of the commercial leishmanicidal drug. The results for the antiparasitic activity of the nanomaterials evaluated indicate a great efficacy of the copper complex, with higher values of SI in comparison with free Glucantime, while keeping a similar efficacy range, even though silica nanoparticles were modified with only 20% of the ligand or metal complex, relative to the weight percentage. The release profile shows a complete liberation of the drug in two hours, confirming the complete administration of the complex during the screening experiment.

Despite the increasing studies of quinolines derivatives against Leishmania spp. infections, there are not many examples of quinoline metal complexes against leishmaniasis. The only significant test is the one published by Paloque et al. in 2015 [133], where silver (I) and gold(I) N-heterocyclic carbene–quinoline complexes proved to be promising metallodrugs with selective action against the pathological relevant form of Leishmania spp. From these studies, gold(I) N-heterocyclic carbene-quinoline, and, in particular, a neutral complex, proved to be promising metallodrugs with potent and selective action against the pathologically relevant form of Leishmania infantum, but the standard treatments with amphotericine B or miltefosfine are still more effective. Nevertheless, there is a rich chemistry of antimony (III) hidroxyquinoline complexes with activity against trypanosomatid parasites as an analogous treatment to tri- and pentavalent antimonial standard drugs [142].

The chemistry of 1,10-phenanthroline metal complexes is a well-studied field, mainly with copper ions, where multiple applications are studied, such as the following technological [143] and biological applications: anticancer [144], antibacterial [145], or antifungal [146]. It is also common to observe mixed ligand complexes between one subunit of 1,10-phenanthroline and another biocompatible organic ligand, such as nucleobases [147], quinolines [148], or commercial drugs [149], for example. But, once again, the studies of metal complexes derived from 1,10-phenanthrolines against parasite infections are in low proportion in comparison with other diseases. There are some works about malaria and antiplasmodic activity [150,151] or T. vaginalis [152], but it can be considered to be anecdotic in comparison with anticancer or antibacterial studies. In the last decade, vanadium phenanthroline complexes against Leishmania spp. and Trypanosoma cruzi [153,154], and copper and silver complexes with 1,10-phenanthroline modified ligands published by Oliveira et al. from 2021, are the few examples that can be found in the literature for leishmaniasis [123,134,155]. These compounds have shown significant activity against L. braziliensis, influencing metallopeptidases produced by the parasites and being able to block the interaction process of the parasite with host macrophage cells. Figure 7 shows the morphological changes produced by the use of metal complexes against leishmania extracellular forms [134], playing a critical role in normal parasite population growth and leading to great SI values.

3.3. Commercial Drug Metal Complexes

One of the trends in the development of new metal complexes with leishmanicidal activity is to use accepted drugs for other treatments and derivatize them to metallic complexes to test against the parasite infection.

The most extended family of drug employed against leishmaniasis are the analgesic and anti-inflamatory compounds. Ni(II), Mn(II), and Pd(II) ibuprofen complexes were tested against Leishmania amazonensis [135]. Mn(II), Ni(II), and Co(II) diclofenac complexes were tested against the following three different strains of Leishmania spp., Leishmania infantum, Leishmania braziliensis, and Leishmania donovani, to study the three clinical forms of leishmaniasis [136]. In both cases, the efficacy and specificity increased due to the synergetic effect of the metal ion. There are also some studies with paracetamol and aspirin metal complexes, but which only focused on antibacterial activity [156].

Several drugs for hyperuricemia and gout were tested against parasitic infections. Allopurinol [157] and colchicine [158,159,160] are two of the gout-tested recommended drugs used against Leishmania spp. infections, but no metal complex was tested to date. Other examples of active drug complexes tested as antileishmanial agents are anticancer miltefosine in combination with gold or vanadium complexes [137,161]; vomit-inducer bumetanide with a Zn synergetic effect against Leishmania braziliensis [138]; or antidiabetic metformin, lowering the blood sugar level to the minimum physiological limit and destroying parasites [162].

3.4. Mechanisms of Action between Metal Complexes and Parasites

The use of metal complexes derived from leishmanicidal drugs led to a decrease in the infections through a synergetic effect between the organic ligands and metal centers. There are different mechanisms of action proposed for the interaction between metal complexes and parasite forms, like the following: DNA interaction and damage [163], DNA structure disruption, and replication and transcription inhibition, or by the generation of reactive oxygen species (ROS) that cause DNA strands to break; inhibition of metalloenzymes, such as ribonucleotide reductase or topoisomerase, or proteases involved in the parasite’s life cycle, can disrupt protein processing and the metabolism [164]; membrane disruption [165] through ROS generated by metallic complexes that can initiate lipid peroxidation, compromising the integrity of cellular membranes or by directly binding to membrane components, disrupting the membrane’s structure and function, leading to cell lysis; or metabolic disruption [166] by the inhibition of key metabolic pathways targeting essential enzymes for the parasite’s energy production and biosynthesis due to metallic complexes that can impair mitochondrial function, ATP production disruption, and apoptosis induction.

ROS play a critical role in most of the mechanisms proposed by diverse authors to explain the benefits of using metal complexes as alternative leishmanicidal drugs. Many metallic complexes can undergo redox cycling, producing ROS that damage cellular components like lipids, proteins, and DNA. In addition, metal complexes that participate in Fenton-like reactions can catalyze the production of hydroxyl radicals, leading to oxidative stress.

4. Alternative Use of Metals: Metallic Nanoparticles as Antiparasitic Agents

In recent years, nanotechnology approaches have arisen as a promising solution to traditional therapies’ limitations against parasites, namely reduced efficacy and poor penetration (which is especially worrying in the case of intracellular parasites), leading to the administration of higher doses that provoke severe and undesired side-effects. Additional noteworthy improvements that nanoparticle (NP)-based strategies against parasitic infections bring are higher biocompatibility and membrane penetration, the possibility to selectively target parasites (protecting from side toxicity to the host), an improvement in drug delivery and drug stability, or absence of drug resistance (which is a remarkable limitation for many traditional therapies). Although the specific mechanism of action depends on the precise parasite species and on the type of nanoparticle used, general modes of operation are interaction with biological membranes or some parasite’s proteins, impact on the proliferation of the microorganism, or the production of reactive oxygen species (ROS). The generation of ROS, which can be triggered by the excitation of metallic nanoparticles with the appropriate wavelength (visible and infrared), results in the release of free electrons able to hinder cells’ homeostasis and osmotic potential [167,168,169,170].

Moreover, nanoparticle use in the fight against parasitic infections is not only limited to treatment, but also enables the prevention, control, and detection of several infections [171]. Regarding the latter, fluorescent, magnetic, and metal NP (especially gold and silver NP [172]) are promising tools, owing to the possibility of conjugation with different probes or signal enhancers, improving the detection process [173].

One of the main advantages that nanomaterials bring is their versatility in numerous aspects such as application fields, morphologies, compositions, or synthetic procedures. In this context, it is remarkable to highlight the current trend in developing metal nanoparticles for antiparasitic purposes by green methods using natural resources. Examples of natural products used for the green synthesis of nanoparticle-based materials with antiparasitic features are fig, olive [174,175], ginger [176], curcumin [177,178], or onion [179]. Some of the most remarkable advantages that green synthesis pose compared to traditional procedures are cheap precursors, waste conversion into valuable matter, environmental care, and non-toxic products.

Despite the great variety of metallic nanoparticles that can be synthesized, the most numerous research articles found in the literature for antiparasitic purposes are mainly silver and gold, followed by metallic oxides, which could be attributed to their efficacy [180] (Figure 8). Therefore, some of the most outstanding results using these nanoparticles will be reviewed as follows.

4.1. Silver Nanoparticles

Silver nanoparticles (AgNPs) have been found to present several properties which make them excellent candidates for biomedical purposes. AgNPs present low toxicity, an anti-inflammatory effect, antimicrobial activity against numerous microorganisms like bacteria, fungi, protozoa, and some types of viruses, accumulation in tissues, and production of ROS [181]. Despite the fact that AgNPs can be synthesized using a great number of physical (laser ablation, irradiation, evaporation…) or chemical (chemical reduction, sol–gel, pyrolysis…) procedures, bio-based methodologies are currently widespread [182]. The use of biological reactants is crucial due to their capacity to reduce and stabilize Ag ions together with their proven therapeutic effectiveness. Examples of green synthesis methods are proposed by Almayouf and coworkers (using fig and olive extracts as phenolic and flavonoid sources in addition to their chemical reductive capacity) [174], Mohammadi et al. (used ginger rhizome extract) [176], or Javed and colleagues (used an aqueous extract of Mentha Arvensis) [167].

The action of metal nanoparticles is not limited to the activity of the noble metal itself, but can also be used to improve certain properties of other active compounds or to provide synergistic effects to other elements. For example, Badirzadeh et al. designed curcumin-coated silver nanoparticles to enhance curcumin’s water solubility and bioavailability for the treatment of cutaneous leishmaniasis (CL) [178], while Shakeel et al. synthesized thiolated AgNPs to provide muco-adhesion and muco-permeation to the nanosystem and mannosylated chitosan to enhance the uptake of macrophages [183].

Regarding the application of AgNPs, most research articles focus their work on the treatment of different manifestations of leishmaniasis, but are not limited to this parasite. Different strains of Trypanosoma (cruzi [184] or brucei [185,186] respectively) or Toxoplasma gondii [181] are examples of alternative parasites to be approached using AgNPs.

4.2. Gold Nanoparticles

Gold nanoparticles (AuNPs) are widely used for a variety of applications in the healthcare industry such as imaging, targeting, drug delivery, or treatment [187,188]. Additionally, they have shown interesting features for parasite treatment.

Regarding gold nanoparticles synthesis, they can be obtained using both chemical methods and green procedures as well. For example, Want et al. propose gold nanoparticle synthesis by microwave-assisted heating in the presence of a reductant [189], while Raj et al. synthesized chrysin (a non-toxic flavonoid with therapeutic properties) conjugated gold nanoparticles [190]. It is remarkable to highlight the potential of gold nanoparticles as carriers of relevant bioactive molecules able to perform a variety of tasks as enzyme substrates (like glutathione [185]) or inhibitors (as apoferritin [186]).

Gold nanoparticles have already shown significant activity towards both amastigotes and promastigotes, offering a promising alternative against L. donovani, as proven by Muzamil Yaqub et al. [189]. Nevertheless, antileishmanial activity could be improved if gold and silver nanoparticles were combined. Santanu Sasidharan and Prakash Saudagar synthesized 4′,7-dihydroxyflavone modified gold and silver nanoparticles against L. donovani [191]. In a different study, Dayakar Alti et al. obtained gold–silver bimetallic nanoparticles [192]. IC₅₀ values obtained for the bimetallic nanomaterial were an order of magnitude lower than the individual gold nanoparticle designed by Santanu Sasidhara and coworkers (0.03 µg/mL and 0.1226 µg/mL, respectively).

The interest in gold nanoparticles for antiparasitic purposes is not only limited to the development of effective treatments, but can be used in the design of sensitive biosensors for the detection of leishmania. In this regard, gold nanoparticles usually do not have an active role in the recognition event, but are used as support for the biomolecules responsible for the biorecognition process [193] or to improve electron transfer in the case of electrochemical sensors [194].

4.3. Copper Nanoparticles

Copper is a metallic element that also presents interesting features for biomedical purposes like anti-inflammatory, antinociceptive, or antioxidant effects, immune protection, or antimicrobial activity. Therefore, in recent years, taking advantage of the benefits that nanomaterials provide, copper nanoparticles have been explored for health-related applications. Green synthesis procedures have been published based on the use of plant-based raw materials for the synthesis of copper nanoparticles. Caesalpinia Spinosa [47], Lupinus Arcticus [195], or Allium Tuncelianum [179] are examples of vegetable resources described in the literature for such purposes. These eco-friendly procedures have shown to be not only more sustainable, but also to be able to provide high effectivity in their duty; pristine copper nanoparticles, especially when combined with meglumine antimoniate, are able to significantly inhibit the growth rate of L. major [47].

Not only green synthesis procedures have been proposed for the obtention of copper nanoparticles. Welearegay et al. used an advanced gas deposition technique followed by ligand deposition, and the metal nanoparticles synthesized were used for the development of a gas sensor for the detection of cutaneous leishmaniasis. Interestingly, despite gold nanoparticles being the most widely used in sensing films, copper nanoparticles showed better accuracy, sensitivity, and specificity [195].

4.4. Metal Oxide Nanoparticles

4.4.1. Copper Oxide Nanoparticles

A significant advantage that metal oxide nanoparticles have over metallic nanoparticles is that they do not trigger resistance in microorganisms or pathogenic cells. In addition to this, the cost associated with the precursors is much more competitive compared to silver or gold, which are the prevalent metals used for the synthesis of nanoparticles for anti-parasitic purposes. Nevertheless, copper oxide (CuO) nanoparticles must be modified by a ligand coating or sulfidation to be safe for therapeutic use. Few articles have been published describing copper oxide nanoparticles for anti-parasitic purposes, to the best of our knowledge. As an example, Dunia A. Ruda et al. propose the synthesis of copper oxide nanoparticles using the precipitation method as an anti-leishmania agent [196]. Shah Faisal et al. published a green synthesis based on the use of plant-based precursors of multi-purpose copper oxide nanoparticles with high activity against leishmaniasis, observing a high mortality rate of 49.1 and 56.2% inhibition at 400 µg/mL for promastigote and amastigote, respectively [177].

4.4.2. Nickel Oxide Nanoparticles

Nickel oxide (NiO) presents astonishing physico-chemical properties owing to its electronic behavior. It is a p-type semiconductor transition metal oxide with a broad band gap, which provides remarkable ferromagnetic properties and presents thermal and chemical robustness. In addition, nickel oxide also possesses remarkable antibacterial and antifungal properties. To avoid the use of hazardous precursors, green methods are also widely used for the synthesis of nickel oxide nanoparticles. Rhamnus triquetra and olive leaves have been used to obtain nickel oxide nanoparticles for activity against Leishmania spp. (IC₅₀ of 27.32 µg/mL and 37.4 µg/mL for promastigotes and amastigotes, respectively) [197] and the viruses transmitted by Hyalomma dromedarii [175]. Ali Talha Khalil et al. proposed a synthesis using Sageretia thea, which rendered 24.13 µg/mL and 26.74 µg/mL as IC₅₀ values for the promastigote and amastigote cultures of Leishmania tropica [198]. As for other metal-based nanoparticles, nanostructured nickel oxide might also be useful for sensing purposes, in addition to its biomedical application against leishmania. In this regard, Swati Mohan and coworkers developed a sensitive biosensor for visceral leishmaniasis [199].

4.4.3. Zinc Oxide Nanoparticles

Zinc is a biocompatible and biodegradable element recommended for both tissue regeneration and treatment. When combined with oxygen as zinc oxide (ZnO), it is considered to be a safe substance by the US Food and Drug Administration, and has been proven to be an effective antibacterial owing to its capacity to produce ROS and perturb bacterial DNA amplification mechanisms, as well as its capacity to induce lipid membrane oxidation, making these nanoparticles a good alternative to fight against resistant organisms [200,201].

Among zinc oxide nanoparticles, the most outstanding features, in addition to low prize and stability, that make them interesting materials are their wide band gap and photocatalytic behaviors. It is precisely these latter properties which provide one of the main cytotoxic pathways to fight against leishmaniasis; zinc oxide nanoparticles could act as photosensitizers, which are useful in photodynamic therapy against Leishmania, as Akhtar Nadhman et al. established [202].

Different approaches have been found in the literature that use zinc oxide nanoparticles as antiparasitic agents, highlighting nude green-synthesized zinc oxide nanoparticles [203], metal-doped zinc oxide nanoparticles [204], and drug-loaded zinc oxide nanoparticles [200,201,205,206].

In a recent study, Fatemeh Saleh et al. compared the cytotoxic effects of biosynthetic zinc oxide nanoparticles and glucantime towards L. major. Their findings evidence the potential of zinc oxide nanoparticles as antileishmanial agents, since in spite of inducing apoptosis, the toxicity associated with the nanoparticles was lower than glucantime [200]. Nevertheless, in a different study, Samina Nazir et al. evidenced the importance of deeper study about the toxicological behavior of zinc oxide nanoparticles against leishmania, since not only its physiological properties might exert an impact, but also the administration route [207].

Table 2. Summary of the different kinds of metal nanoparticles’ main applications against leishmaniasis infections.

Type	Applications and Advantages against Leishmania spp.	References
Silver nanoparticles	Improve water solubility of leishmanicidal molecules; provide muco-adhesion and muco-permeation.	[178,183]
Gold nanoparticles	Direct leishmanicidal effect; lesihmaniasis biorecognition molecules carrier.	[189,191,192,193,194]
Copper nanoparticles	Leishmanicidal organic molecules vehicle; gas sensor for the detection of cutaneous leishmaniasis.	[195]
Copper oxide nanoparticles	Increase mortality rate of intra and extracellular forms of leishmania.	[177,196]
Nickel oxide nanoparticles	Low IC50 values; biosensor for visceral leishmaniasis.	[198,199]
Zinc oxide nanoparticles	Photodynamic therapy; lower toxicity versus commercial drug Glucantime.	[200,202,207]

shows a list of the most significant metal nanoparticle applications for leishmaniasis, from having a direct leishmanicidal effect to sensing and detecting the disease.

5. Conclusions

Despite the use of metal complexes in medicine being one of the research areas of bioinorganic discipline of greater interest to the scientific community, there is still a gap to fill in the research for neglected diseases, with a special focus needed on parasitic illnesses such as leishmaniasis. The use of metal complexes derived from biomimetic and biocompatible organic ligands to enhance their applications in cancer, bacterial, or fungal therapies is one of the most prospective for the future, and one with the best perspectives, but contributions to leishmaniasis eradication have experienced a significant slowdown in the last decade.

There are few examples in recent years of the synergetic effects of the combination between leishmanicidal ligands and metal ions, with a significant prevalence of copper complexes due their capacity to generate ROS that can affect multiple vital processes in parasite life cycles. Azole derivatives show the best selectivity indexes in comparison with the rest of the emerging synthetic organic leishmanicidal ligands, while triazolopyrimidine metal complexes are the most versatile due their application to more different types of leishmania strains. The use of novel nanoplatforms as an effective vehicle to transport the metal complexes to the infected organ is a promising new way to explore the possibilities of metal-complex therapy against leishmaniasis. The use of metal elements is not limited to be part of complex structures, due to metallic nanoparticles offering an attractive alternative to metallic compounds with a wide variety of applications, ranging from the direct leishmanicidal effect to the development of biosensors for leishmaniasis or for improving the solubility or biodisponibility of leishmanicidal molecules.

But despite all of the mentioned encouraging results obtained in the last decade, there is still much to be carried out in order to find novel metal complexes with good activity and low toxicity, and an appropriate relationship between their efficacy and their production costs, to develop long term sustainable solutions for the problematic derived from parasitic neglected diseases. Increase funding associated with these types of diseases is an urgent need, in order to reveal the mechanisms of action and the best targets to use the novel metal complexes in the fight against neglected diseases such as leishmaniasis.