**1. Introduction**

This chapter is dedicated to understand the mode of action of specific constituents of licorice, which have been isolated and characterized as antileishmanial agents. Among over 20

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

triterpinoids and 300 flavanoids present and characterized in licorice, only three, namely 18β-glycyrrhetinic acid, glycyrrhizin and licochalcone A, have been extensively studied for their antileishmanial properties. The structures of the compounds are given in **Figure 1**.

**Figure 1.** Structure of the characterized antileishmanial constituents of licorice [1].

#### **1.1. Leishmaniasis**

Before going into the mechanistic details of these compounds, a brief idea about leishmaniasis will help understand the infection biology and the target of action of these molecules. *Leishmania* is a protozoan parasite that causes a spectrum of diseases ranging from the selfhealing cutaneous to fatal visceral leishmaniasis. The visceral leishmaniasis particularly affects the spleen, liver and bone marrow [2]. It is highly endemic in the Indian subcontinent and East Africa. It is second only to malaria in annual worldwide fatalities due to protozoal infections [3–4].

### **1.2. Life cycle**

*Leishmania* parasites remain in nature by transmission between the mammalian hosts through infected female sandfly bite. *Leishmania* lifecycle is characterized by having three developmental stages: procyclic promastigotes, metacyclic promastigotes and the amastigotes (**Figure 2**).

**Figure 2.** Lifecycle stages of *Leishmania sp* [2].

#### **1.3. Transmission**

triterpinoids and 300 flavanoids present and characterized in licorice, only three, namely 18β-glycyrrhetinic acid, glycyrrhizin and licochalcone A, have been extensively studied for their antileishmanial properties. The structures of the compounds are given in **Figure 1**.

Before going into the mechanistic details of these compounds, a brief idea about leishmaniasis will help understand the infection biology and the target of action of these molecules. *Leishmania* is a protozoan parasite that causes a spectrum of diseases ranging from the selfhealing cutaneous to fatal visceral leishmaniasis. The visceral leishmaniasis particularly affects the spleen, liver and bone marrow [2]. It is highly endemic in the Indian subcontinent and East Africa. It is second only to malaria in annual worldwide fatalities due to protozoal infections

*Leishmania* parasites remain in nature by transmission between the mammalian hosts through infected female sandfly bite. *Leishmania* lifecycle is characterized by having three developmental stages: procyclic promastigotes, metacyclic promastigotes and the amasti-

**Figure 1.** Structure of the characterized antileishmanial constituents of licorice [1].

148 Biological Activities and Action Mechanisms of Licorice Ingredients

**1.1. Leishmaniasis**

[3–4].

**1.2. Life cycle**

gotes (**Figure 2**).

**Figure 2.** Lifecycle stages of *Leishmania sp* [2].

In the sandfly (*Phlebotomus* and *Lutzomyia* spp) gut, the promastigote form of the parasite is seen which subsequently transforms to the infective metacyclic form which are regurgitated and injected in the skin of the mammalian host. Infected promastigotes then transforms to the amastigote form in the mammalian host. In a subsequent blood meal of this infected host, the amastigote forms are taken up which then transforms to the promastigotes in the gut and the cycle continues (**Figure 3**).

**Figure 3.** Schematic 3D view of the phases of interaction between the *Leishmania* parasite and sandfly and between the parasite and the vertebrate cells. "(A) Attachment of a promastigote to the macrophage surface. (B) The process of internalization via phagocytosis begins with the formation of pseudopods. (C) Leading to the formation of the parasitophorous vacuole (PV). In the PV, the promastigote transforms into an amastigote. (D) Recruitment and fusion of host cell lysosomes with the PV takes place. (E) In the PV, amastigotes divide several times. (F–G) Intense multiplication generates several hundreds of amastigotes. (H) The host cell bursts, and the parasites reach the extracellular space. (I) Schematic view of female sandfly showing the digestive tract. (J) During a blood meal, a female sandfly ingests infected macrophages with amastigote forms present in the blood of the vertebrate host. (K) Amastigotes form 'nest cells' in the abdominal midgut. (L) Amastigotes transform into procyclic promastigotes. (M) Promastigotes multiply and attach to the midgutepithelium. (N) Parasites migrate toward the anterior midgut, resume replication and start to produce promastigote secretory gel (PSG). (O) Promastigotes transform into infective metacyclic promastigotes. (P) Metacyclic promastigotes infect a new mammalian host via regurgitation during the blood meal. These images are based on micrographs obtained by scanning and transmission electron microscopy and by video microscopy" [5].

#### **1.4. Disease manisfestation**

Visceral leishmaniasis patients show signs of systemic infection which include fever, fatigue, weakness, loss of appetite and weight loss. The parasite invades the blood and reticuloendothelial system, such as enlarged lymph nodes, spleen and liver. Darkening of the skin, popularly known as kala azar, which means black fever in Hindi, is infrequent. Anemia which is worsened by hypersplenism, leucopenia or thrombocytopenia, and hypergammaglobulinemia are characteristic. Untreated disease in any age group in time can produce profound cachexia, multisystem disease, bleeding, susceptibility to secondary infections and death [6].

#### **1.5. Current treatment options**

For visceral leishmaniasis, treatment is always recommended due to the fatal nature of the disease. The only treatment currently available for leishmaniasis relies on chemotherapy [7] as no vaccine has been successfully developed till date for humans. The first line of classical drug for treatment of leishmaniasis includes pentavalent antimonials, stibogluconate or meglumine antimonite. Pentavalent antimonials have been the drug of choice for more than 50 years [8–10]. Other common drugs are pentamidine, allopurinol, amphotericin B, imidazoles, miltefosine and paromomycin among others. Even though these drugs are effective other problems associated with these are (1) there lengthy regimens (weeks to months), (2) invasive modes of administration (intramuscular or intravenous) and (3) high drug-related toxicity along with the high cost of therapy which is beyond the capacity of many as this disease is known to be the 'disease of the poor'[10, 11]. All these factors together may lead to the patients discontinuing there therapies in the mid which have resulted in emergence of drug-resistant parasite strains adding to the list of problems associated with antileishmanial therapies [9, 11, 12].

To add to these overwhelming situations, emergence of coinfection collaterally with increasing incidences of HIV has decreased the number of options available to patients. As we will see in this chapter that immunosuppression is a key for parasite survival drugs, which can modulate host immune defenses, should be considered for effective treatment of leishmaniasis. Development of vaccines has remained a challenge; however, a canine vaccine has been developed, and it is in use in South America [13]. At present, no immunization options are available against leishmaniasis in humans.

#### **1.6. Subversion of host defense**

Like any other successful pathogens, *Leishmania* has also developed strategies to evade host immune mechanisms in order to survive within the host. A significant number of virulence factors discovered in *Leishmania* are directed against circumventing the host immune response. Apart from these, the parasite also has the ability to maintain a chronic infectious state within its host by modulation of regulatory factors, which exemplifies the extent on its immune evasion potential. Indeed, the ongoing battle between the robust immune response mounted by a host and the counter evasion strategies by the parasite ultimately decides the fate of the disease.

All these further establishes the fact that a chemotherapeutic alone is not enough to treat this intelligent parasite, and indeed an immunomodulator which can activate hosts own defense mechanism will be a better adjunct to the current line of treatment, which is relatively better in terms of toxicity, efficacy and mode of administration.
