Physicochemical properties of antileishmanial agents.
Despite past 60 years of extensive research in antileishmanial drug development, the successful therapy of this disease cannot be achieved at full potential. The biological barriers encountered by the therapeutic modalities favor the disseminations of the disease like intramacrophage location of parasite, lack of oral bioavailability, permeability across the cutaneous tissue, and active efflux of the drug. Nanomedicines are specifically engineered nano-sized delivery systems. The goal of designing a nanomedicine is to achieve the specific therapeutic objective via targeting the specific cells and intracellular locations, pharmacological receptors, enzymes and proteins, crossing biological barriers, and navigation through endocytic pathways. This chapter will cover various nanomedicinal approaches like targeting the macrophages, pathological organs, efflux pumps, metabolic enzymes, redox biology of Leishmania by using polymeric and metal nanocarriers to overcome all the biological barriers thus providing a successful alternative over the conventional therapies.
- biological barriers
- macrophage targeting
- photodynamic therapy
- oral bioavailability
The challenges faced by the current antileishmanial therapy include subtherapeutic efficacy, development of resistance, toxicity, and cost-effectiveness . Despite past 60 years of extensive research in antileishmanial drug development, the successful therapy of this disease cannot be achieved at full potential. There is no vaccine available against
Nanomedicine is a specifically engineered nano-sized particulate drug delivery system designed for the improved pharmaceutical formulations. The nanoparticles can achieve the discrete therapeutic objectives which are otherwise impossible with conventional drug delivery systems like targeting a specific cell and organelles, enzymes, proteins, and pharmacological receptors, accumulating in the pathological organs, bypassing the organs prone to the toxic effects, crossing biological membranes, and navigating through endocytic pathways . All these properties of nanoparticles can address the biological barriers encountered in the effective therapy of leishmaniasis. Various polymeric and metal nanocarriers-based strategies like macrophage targeting, organ targeting, improved oral bioavailability, and photodynamic therapy are being explored for their supreme antileishmanial effects.
This chapter will discuss the various biological barriers compromising the effectiveness of antileishmanial therapy and the role of nanomedicine to overcome the problems associated with the conventional therapeutic modalities thus providing a platform for the enhanced antileishmanial therapy.
2. Current medical management of leishmaniasis
For the past six decades, the standard first-line drugs for the treatment of leishmaniasis are antimonial drugs, meglumine antimoniate, and sodium stibogluconate . Antimonial compounds required to be administered IV/IM at the dose of 20 mg/kg of Sb-V for 10 days in case of cutaneous leishmaniasis (CL) and for 28 days in case of visceral leishmaniasis (VL). Antimonial drugs act by inhibiting a thiol metabolic enzyme trypanothione reductase (TR) and thus causing a decreased trypanothione (T[SH]2) levels which results in the decreased ability of the parasite to counteract the oxidative stress . However, the variations in the clinical response and development of resistance from the past several years are a persistent clinical threat. The activity of aqua glycoproteins, trypanothione reductase/trypanothione (TR/T[SH]2) system, and permeability glycoprotein (P-gp) efflux pumps is involved in the development of resistance resulting in decreased intracellular accumulation of antimony in subtherapeutic concentrations thus jeopardizing the effectiveness [8, 9]. Serious toxic effects associated with antimonial therapy like cardiotoxicity, changes in ECG, renal and liver impairment, muscle pain, and severe fatigue further limit the therapeutic potential of antimonial compounds .
Amphotericin B (AmB), a polyene antibiotic, is the second-line standard drug for the leishmaniasis since the 1960s . Whereas, in India, the AmB is the first-line drug approved for VL due to widespread resistance against antimonial compounds. The standard dose of AmB for VL is 1 mg/kg every other day for 20 days via IV route. It has selective activity against the
Miltefosine (MILT) has been recently tagged as an antileishmanial drug required to be administered at the dose of 50 mg orally three times a day for 28 days. The variation in the clinical response has been observed due to the species variations and the development of resistance . Promastigote-resistant strains of
Pentamidine (PTM) is also being used as the second-line therapy against leishmaniasis; however, the use is limited in zoonotic settings. The recommended dose is 2–3 mg/kg, IV or IM once a day for 4–7 doses in case of CL, while for VL, its dose is 2–4 mg/kg administered every other day via IV or IM for up to 15 doses. The use of PTM in the pentavalent antimonial Sb(V) refractory patients in India resulted in decreased efficacy from 95 to 70% within a short duration suggesting the development of resistant against the PTM. Resistance to PTM in
From above discussion, it is evident that resistance against the antileishmanial agents is rising, compromising the therapeutic efficacy. Apart from the development of resistance, other factors like associated toxic effects, unavailability of oral dosage forms, longer duration of therapy, and high cost also contributing toward the suboptimal control of leishmaniasis. These limitations arise primarily due to the various biological barriers encountered by the antileishmanial agents. Considering the development of parasitic resistance and a limited number of effective antileishmanial drugs, there is an imperative demand to revise the standard medical management of leishmaniasis. Administering the available standard drugs with appropriate delivery systems that help to cross the biological barriers seems to be an encouraging strategy, which requires being given serious consideration.
3. Biological barriers to leishmaniasis therapy
3.1. Intramacrophage location
Mononuclear phagocytes (MP; monocytes, macrophages, and dendritic cells) along with eosinophils and neutrophils constitute the first line of defense against the invading pathogens and are involved in detection and elimination of the foreign bodies . When the sand fly takes the blood meal, it inoculates the promastigotes of
3.2. Activity of P-gp efflux pumps
The activity of P-gp efflux pumps presents another major barrier to the antileishmanial therapy . Active efflux of the drug via the efflux pumps is one of the most common mechanisms for developing multidrug resistance (MDR) in the microorganism . In fact, MDR mediated through efflux pumps has been described in various organisms like fungi, bacteria, and protozoa including
3.3. Lack of oral bioavailability
Oral administration is the most suitable method of delivering the drugs due to the convenience of dosing, noninvasive nature, and high acceptance at patient levels . Most of the therapeutic agents used for systemic and localized GIT effects are administered orally because of the highly absorptive nature of the intestine that provides a large surface of around 300–400 m2. The oral administration is successful only in the case where the drugs have sufficient bioavailability. Many physiochemical and physiological factors determine the oral bioavailability of drugs like solubility, permeability, a mucus layer, partition coefficient, stability, dissolution, pH, enzymatic degradation, and activity of P-gp efflux pumps. Unfortunately, most of the antileishmanial drugs encounter the above-described barriers and exhibit limited oral bioavailability except for MILT.
In fact, solubility and permeability govern the oral bioavailability. Most of the drugs diffuse across cell membrane via passive transport, and for that purpose, the drug should be lipophilic in nature as the unionized form is better to diffuse across the phospholipid bilayer. However, the drug molecules should not be lipophilic enough to remain soluble in the lipid bilayer suggesting a suitable log P value. To maximize the possibilities of passive diffusion, the ideal log P value is considered to be around 2. The molecular weight of the drug also has a role in the passive diffusion of the drug and molecular mass less than 500 Da is considered to be favorable for the absorption across the small intestine . The effect of solubility, permeability, and molecular weight is better explained in Lipinski’ s rule. Lipinski’s rule of five is a very useful tool to predict the drug-like characteristics of a compound and is especially applicable to assess whether a drug is orally active or not . Lipinski’s rule states that in general, an orally active drug has no more than one violation of the following criteria:
No more than five hydrogen bond donors (the total number of nitrogen-hydrogen and oxygen-hydrogen bonds);
No more than 10 hydrogen bond acceptors (all nitrogen or oxygen atoms);
A molecular mass less than 500 Da;
An octanol-water partition coefficient log P not greater than 5.
Most of the antileishmanial drugs do not follow the Lipinski’s rule of five, therefore, not absorbed orally. According to biopharmaceutics classification system (BCS), AmB is class IV drug with the aqueous solubility of <1 mg/L at physiological pH, molecular weight of 924 Da, and log P of 0.95, 17 hydrogen bond acceptors and 12 hydrogen bond donors and in this way does not follow the rule of 5. Similarly, sodium stibogluconate possesses a molecular weight of 910.10 Da and log P of −0.34, 17 hydrogen bond acceptors, and 5 hydrogen bond donors, thus violate the rule of 5. The physiochemical properties of the antileishmanial drugs are presented in Table 1.
|Drug||Molecular weight (Da)||Log P||Hydrogen acceptor||Hydrogen donor||Rule of five|
Thus, the lack of oral bioavailability of most of the antileishmanial agents is the major limitation in the cost-effective and optimum therapy of leishmaniasis. Minimal oral absorption below the minimum effective concentration (MEC) necessitates the formulation of antileishmanial drugs for the parenteral administration. The long-term parenteral administration has its own limitations as it requires the patient to be hospitalized, increased cost of the therapy, and patient compliance.
3.4. Skin as barrier to topical therapy
Skin is the largest organ of the body and protects the organism from the external environment. Histologically, the skin is divided into the superficial layer called the epidermis and a deeper layer, the dermis. The several strata make up the epidermis distinguished by the changes in keratinocytes from dermis-epidermis junction to the outer surface of epidermis, the stratum corneum (SC). SC is designated as the main barrier to the transport of substances across the skin and is formed by the corneocytes characterized as densely packed, dead, and keratinized cells. These cells are surrounded by the intracellular lipid matrix composed of nonpolar lipids in lamellar lipid layers, making SC a hydrophobic layer . Although SC is only 10–20 μm in thickness, it acts a barrier and hampers the penetration of microorganisms, drugs, and other chemicals besides being involved in the transepidermal water loss.
Drug delivery across the skin follows three possible pathways: the intracellular route, between the corneocytes sinuously through the lipid layer; the transcellular route, through the corneocytes and lipid matrix; and through the cutaneous appendices (sweat and sebaceous glands, hair follicles). The drug diffusion across the skin is essentially a passive transport; however, it follows either one or combination of the three pathways. However, it must be noted that the intracellular route is considered to be the most suitable for the drug diffusion as it offers less resistance compared to the transcellular route in which the drug molecules have to move between the intercellular hydrophobic region to intracellular hydrophilic region repeatedly. The skin appendages, although having a small surface area compared to the total skin (0.1%), present an opportunity for the penetration of ions, polar compounds, and large molecules and thus circumvent the low diffusional character of SC .
To deliver the drug across the skin, the choice of the adequate molecule in terms of molecular weight and partition coefficient is very important. A molecular weight less than 600 Da, low melting point, and suitable log P are desired. Thus, the drug with high molecular weight and hydrophilic character will face the maximum resistance and their penetration will be limited . The drugs for the CL-like paromomycin (PA), antimonial compounds, face the problem of skin penetration due to their hydrophilicity and high molecular weight. In case of CL, the lesions are developed and parts of epidermis and dermis are lost; therefore, the barriers provided by the SC are absent and almost any type of drug can be absorbed. However, the formation of scar tissues and keratotic nodules during the healing process restores the functionality of SC thus depriving the drug absorption at the end of the treatment, and complete healing of the lesion is difficult .
4. Nanodrug delivery system for leishmaniasis
The drug delivery systems are crucial in drug development and design, and many active pharmaceutical ingredients result in serious side effects when administered nonspecifically. The lack of appropriate drug delivery system causes the therapeutic modalities to be accumulated in healthy tissue inciting the adverse effects, lower bioavailability, and inefficient targeting of the desired pathological organs. Most of the latest researches in the field of leishmaniasis are focused on addressing the physiological, biological, and biopharmaceutical aspects of the use of nanotechnology. Nanodrug delivery systems provide an attractive opportunity to resolve the drug delivery problems associated with the therapy of leishmaniasis by crossing the above-demonstrated barriers encountered by the antileishmanial drugs. Examples of nanotechnology progress in pharmaceutical products include liposomes , niosomes , nanodisks , nanoemulsions , polymeric nanoparticles , solid lipid nanoparticles, and polymer-drug conjugates  as described in Table 2.
|Type of nanocarrier||Active moiety||Targeting approach||Strain tested||Model||Ref|
|Thiolated chitosan NPs||Amphotericin B||Macrophage targeting||J774.1 macrophages/BALB/c mice|||
|Gelatin nanoparticles||Amphotericin B||J774.1 macrophages/Hamster|||
|GCPQ chitosan||Amphotericin B||Organ targeting via oral route|||
|MT-chitosan||Amphotericin B||J774.1 macrophages/BALB/c mice||—|
|Liposomes||Zinc phthalocyanine||Photodynamic therapy|||
|Liposomes||Paromomycin||Skin permeating nanocarriers||BALB/c mice|||
Liposomes are the lipid bilayer systems described in 1965 and rapidly taken as drug delivery systems . In 1977, Ward and Hanson, first time reported the encapsulation of Sb-V into liposomes for targeted delivery to liver and spleen in VL. After intravenous administration, the Sb levels of liver and spleen were found to be 20-fold higher compared to the free drugs . However, due to the toxic effects in monkeys, the interest in liposomal Sb-V was declined . The same concept was also applied to AmB in order to avoid its toxicity by encapsulating into multilamellar liposomes. The liposomal AmB got a little bit more attention than Sb-V and initiated model for the development of three-lipid-based AmB drug delivery systems licensed for clinical use (Ambisome®, Amphocil®, and Abelcet®) . However, the only true liposomal formulation, Ambisome®, is recommended for treating patients with leishmaniasis who are resistant to antimonials. The efficacy of liposomal AmB was further enhanced by decorating the liposomal surface with specific ligands like polysaccharides, peptides, antibodies, and glycolipids. The decorated liposomes were able to specifically target the macrophages to avoid the exposure of AmB to healthy tissues [53, 54]. The detail of macrophage-targeted liposomes will be discussed under the section macrophage targeting.
Niosomes are the attractive alternatives over liposomes due to their increased stability, low cost, and biodegradability [55, 56]. Niosomes are the vesicles consisting of nonionic surfactants. Niosomal formulations of sodium stibogluconate were more efficacious compared to the liposomes and free drugs against experimental murine VL . More recently, in vivo studies demonstrated that the niosomes containing autoclaved
4.3. Polymeric nanoparticles
Polymeric nanoparticles are very valuable in the treatment of infectious diseases like leishmaniasis owing to the small size and abilities to enhance the cellular uptake, cross the biological barriers, and deliver drugs at the site of infection [59, 60]. The use of polymer for the development of nanocarriers provides us the opportunity of modifying the functional groups with various chemical methods to incorporate the desired ligands for better penetration and enhanced endocytosis by the active or passive targeting. The ability of polymeric nanocarriers to bear the physiological strains and tunable surface properties provides an edge over liposomes and niosomes. While utilizing the polymeric nanoparticles for leishmaniasis, the category of polymers is of considerable importance as the hydrophobicity of the polymer will facilitate the internalization by macrophages, the core target in leishmaniasis. For example, polymethylmethacrylate-based nanoparticles indicated a superior macrophage uptake compared to the polycyanoacrylate . Various studies reported the potential of polymeric nanocarriers in leishmaniasis. Primaquine-loaded polymeric nanoparticles were found to be 21-fold more efficient compared to the free primaquine . β-aescin-loaded polylactide-co-glycolide nanoparticles showed twofold increase in efficacy against J744.1 macrophage-infected
4.4. Polymer drug conjugate
The advances in the field of polymer engineering have opened new dimensions for the drug delivery. One example is the polymer therapeutics in which the drug molecules are attached to the polymer backbone by using a suitable chemical method. In this way, the efficacy of the drug can be increased significantly with the reduction in the toxicity. The hydrophobic drug encounters a problem of free circulation in the blood. The hydrophilicity of these drugs can be increased by conjugation of these hydrophobic drugs with the hydrophilic polymer. These polymer-drug conjugates provide increased plasma half-life and retention in the infectious tissue with the minimum toxicity. The conjugation of AmB with the N-2-(hydroxypropyl) methacrylamide resulted in the increased efficacy as compared to the free drug (fungizone) .
Nanometer scale, a lipoprotein-stabilized phospholipid bilayer disk complexes termed nanodisks (NDs) are novel transport vehicles different from liposomes because they do not hold an aqueous core and are completely soluble in aqueous phase media . NDs harboring poorly soluble antileishmanial agent AmB-nanodisks demonstrate an effective therapy for experimental CL (
5. Nanomedicinal targeting approaches for leishmaniasis
Paul Ehrlich in 1891 was the first to theorize the concept of “magic bullets” providing the first description of drug targeting paradigm. The aim of drug targeting is delivering the drug at the right concentration at the right time and at the right place. The evolution of this “magic bullet” concept revolutionized the drug delivery systems and provided a vast platform, known as nanomedicine, to achieve the very specific and highly desirable therapeutic outcomes that are otherwise impossible to achieve with conventional drug delivery systems . Their small size at nanoscale dictates the very unique properties like the interaction with the biological entities, penetration across the membrane, intracellular trafficking, accumulation at the target area, improved blood circulation, and biodistribution. For example, in case of VL, the major organs representing the parasitic burden are liver, spleen, and bone marrow, and the drug has to target the parasite inside the macrophage in these organs . In CL, the drug must reach the parasite inside the macrophages at the inner layers of skin by crossing the skin barrier, SC. To maximize the potential of nanocarriers, a suitable strategy is required to target the pathological area via a patient-friendly route of administration while avoiding the healthy tissues. In view of this, various nanomedicinal targeting approaches have been explored for the therapy of leishmaniasis like macrophage targeting, organ targeting via the oral route, use of permeability enhancers, and photodynamic therapy (PDT).
5.1. Macrophage-targeted drug delivery
The niche in which
Targeting the macrophages via these receptors with surface-decorated nanocarriers leads to the accumulation of appreciable amounts of drug at the same niche where the parasite resides inside the macrophages. Various studies conducted on the macrophage-targeted drug delivery are summarized in Table 2. Recently, our research group utilized the MRs for macrophage-targeted delivery of mannose-anchored thiolated nanocarriers loaded with AmB. The uptake studies by using the J744.1 macrophages indicated that macrophage-targeted nanocarriers provided AmB concentration of 28.6 ± 1.4 μg/106 cells as compared to 0.4 ± 0.01 μg/106 cells of the free AmB. These results provided the evidence that macrophage-targeted nanocarriers were 71-fold more efficient than the nontargeted ones. Also, the macrophage-targeted nanocarriers were having superior antileishmanial activities against
Kansal et al.  utilized scavenger receptors for the macrophage-targeted delivery of doxorubicin via phosphatidylserine-decorated nanocapsules (PS-NCs-DOX) for the therapy of leishmaniasis. Flow cytometry analysis indicated 1.75-fold increased uptake of PS-NCs-DOX compared with nonmodified nanocarriers (NCs-DOX). PS-NCs-DOX also accumulated in liver and spleen at higher concentration against NCs-DOX confirmed via
5.2. Organ targeting via oral route
One of the limitations associated with the conventional antileishmanial therapy is the free systemic circulation of the drug and distribution into different body organs including pathological and nonpathological. The exposure of nonpathological organs to the drugs is associated with the severe toxicity of the antileishmanial drugs thus limiting its therapeutic potential . In this regard, the specific organ targeting is a promising strategy that reduces the toxic effects by minimizing the exposure to nondesired organs and improves therapeutic efficacy by increasing the drug accumulation at the desired organs. One such example of the nanoliposomal formulation of AmB is Ambisome® for VL, when administered is taken up by MP cells and transported to the liver and spleen via passive targeting [52, 75]. Although this strategy greatly improves the safety of AmB, the macrophage-targeted nanocarriers described above can be of more potential in this regard. The surface modification with the ligands actively targets the infected macrophages because of the high expression of endocytic receptors like MRs. However, one factor, the parenteral delivery of these systems, limits their wide application and acceptance at the patient level due to the hazards and high cost associated with and needs to be addressed yet. In pursuit of the solution to this limitation, Serrano et al.  provided the concept of organ targeting via the oral route and introduced nanomedicine in which nanoparticles were taken up by the intestinal epithelia and are transported to liver, spleen, and lungs as shown in Figure 3, thus enhancing the bioavailability of these pathological organs of VL and bypassing the organs of potential toxicity . This technique utilized specifically engineered polymeric excipients with the potential to interact with specific proteins in the intestinal epithelium thus enhancing the permeation and absorption of the constituted nanocarriers.
Serrano et al.  illustrated this concept by utilizing N-palmitoyl-N-methyl N,N-dimethyl-N,N,N-trimethyl-6-O-glycol chitosan (GCPQ) nanoparticles loaded with AmB. Such modification of chitosan will provide the mucoadhesive character to the nanocarriers. As the mucus is a negatively charged glycoprotein, the positively charged polymer will provide increased electrostatic interaction and bind with proteins thus better chances to be taken up by the enterocytes. Single-dose oral pharmacokinetic studies in CD-1 mice were carried out by utilizing AmB-GCPQ, Amb-sodium deoxycholate (Amb-d), and AmB in dextrose solution at the dose of 5 mg/kg. The nanoparticulate formulations, AmB-GCPQ, and Amb-d exhibited higher plasma drug levels compared to the AmB in dextrose. These results indicate that the particulate formulations were able to cross the intestinal membrane. Furthermore, significantly higher levels of Amb-GCPQ were found in target organs, i.e., liver and spleen as compared to the Amb-d. The target organ to kidney ratio was also determined and provided very encouraging results. As AmB is a nephrotoxic drug, target organ:kidney ratios are crucial. Lung:kidney AUC0-24 ratios for AmB-GCPQ and Amb-d were 1.44 and 0.86, respectively, while the corresponding spleen:kidney ratios were 1.22 and 0.81, respectively, and the corresponding liver:kidney ratios were 0.88 and 0.40, respectively. These data demonstrate that when compared to the deoxycholate micelles, GCPQ nanoparticles delivered relatively more drug to the target organs (liver, lung, and spleen) rather than kidney. These findings were also supported by the low urinary excretion of Amb-GCPQ, while AmB in dextrose delivered most of the drug to the kidney, a fact that contributes to the nephrotoxicity associated with AmB and reduced drug levels in target organs. Also, the oral particle location to major organs was studied by coherent anti-Stokes Raman spectroscopy. The results located the GCPQ nanocarriers within the hepatocytes in the liver, intracellular spaces in the hepatocytes. The reason for their location in the hepatocytes and lungs is their uptake by the intestinal villi from where they are transported to the liver via the hepatic portal vein. GCPQ nanocarriers were also taken up by the M cells of Peyer’s patches from where they are carried to the systemic circulation via the lymphatic system. The
Recently, our research group utilized thiolated polymer-based mannose-anchored nanocarriers to target the visceral organs via oral route for the delivery of AmB against VL (unpublished data). Thiolated polymers, the so-called thiomers, are well known for their mucoadhesion, permeation enhancing, and P-gp inhibition properties with great impact on the nanodrug delivery. Thiomer contains thiol group (─SH) covalently attached to the polymer chain, and by the virtue of –SH, the thiolated polymer can interact with the proteins and receptors via disulfide bond formation (─S─S─) in disulfide exchange mechanism . Mucus in the intestine acts as the physical barrier for the diffusion of drugs across the intestinal membrane. The structure of mucus is complex, which arises from the properties of mucins. Mucins are large glycoproteins composed of more than 800 amino acids, also containing cysteine- and disulfide-rich domains. Mucins have long flexible proline, threonine, and serine (PTS) domains that are glycosylated. The glycans terminate with negatively charged carboxylic groups. Diffusion in the mucus structure depends on the charge of the molecules. Mucus contains pores that are 200–400 nm in diameter, thus allowing diffusion of many APIs [78, 79]. If APIs are encapsulated in nano- or microcarriers, the size of the carrier can preclude diffusion in mucus. Thiomer-based nanocarriers will remain adhered to the mucus by making disulfide bond with the cysteine-rich units of mucin, and by the virtue of small size of nanocarriers, they can easily pass through the pores in the mucus. After crossing the mucus barrier, they are taken up by the enterocytes, M-cells of Peyer’s patches and also cross the membrane via paracellular route owing to permeation-enhancing capabilities of thiomers. The primary mechanism of the permeation enhancing by thiomers is the inhibition of PTP. The inhibition of PTP is accomplished by the disulfide (─S─S) bond formation by thiomer with cysteine-rich units and consequently increased phosphorylation of membrane proteins thus leading to the opening of tight junction . Furthermore, the mannose anchoring to the thiolated polymer enables the nanoparticles to target the macrophages via mannose receptors. Thus, the combined effect of mucoadhesion, permeation enhancing, and macrophage targeting successfully target the pathological organs of VL, i.e., liver, spleen, and lungs via the oral route.
5.3. Photodynamic therapy
The survival of
Nanoparticles have been extensively explored to improve the efficacy of PDT against CL, due to the ability to penetrate the skin by crossing SC barrier and also protect the PS from aggregation and subsequent inactivation. The current PDT against the CL involves the indirect destruction of the parasites either by enhancing the immune response or by killing the macrophages. Montanari et al.  conducted a study, in which they delivered ZnPc, loaded in liposomes, to treat the infection induced by
Metallic nanoparticles have found their application in the PDT due to their surface localized plasmon response, and they enhance the effectiveness of PDT by producing ROS. Also, they are not involved in the immune system activation. Several studies have been reported in which the effectiveness of the metal nanoparticles in the PDT has been established. PEGylated silver-doped zinc oxide nanoparticles (DSNs) for the PDT of leishmaniasis have been reported by the Nadhman et al. . They indicated DSNs were highly efficacious in providing the photodynamic effect than nondoped zinc oxide nanocarriers (NDSNs). Doping of zinc oxide with silver enhanced the band gap and thus excitation at the visible light source. The IC50 of DSNs was in the range of 0.009 (±0.0012) to 0.02 μg/ml (±0.0023), while that of NDSN was 0.1 μg/ml (±0.016). The DSNs were 10 times more active than the NDSN. Free radical scavenger studies indicated 77–83% cell death occurs due to singlet oxygen, while 18–27% due to the production of hydroxyl ions .
5.4. Skin-permeating nanocarriers
The role of the skin as barriers to the drug delivery has been discussed above in detail. The nanomedicine is a promising strategy to cross the skin barrier since they offer several advantages over the conventional drug delivery systems, and skin permeation and follicular targeting are the most significant regarding the topical treatment of CL. The nanoparticles larger than 20 nm and lesser than 200 nm can be accumulated in the hair follicles where they are retained for longer period of time for up to 10 days, thus providing the continuous supply of the drug for the absorption [88, 89]. Various types of nanocarriers have been utilized for this purpose but the lipid-based nanocarriers such as liposomes, solid lipid nanoparticles, and nanoemulsions are most extensively studied for skin permeation.
Ferriera et al.  first time reported the encapsulation of PA into liposomes and evaluated their permeation across the stripped and intact mouse skin. The results exhibited significantly increased PA penetration into and across the intact skin compared to the PA in solution. However, this model was based on the hairless skin and cannot be extrapolated for human due to the presence of hairs. Topical treatment of
Frankenburg et al.  evaluated the effectiveness of AmB-based lipid nanoformulations applied topically to
The conventional therapy of leishmaniasis failed to provide the satisfactory control over the progression of disease due to the involvement of certain biological barriers.