**5. Mechanism of action of select bioactive phytochemicals**

#### **5.1 Bixin**

Bixin constitutes the main pigment of the industrial annatto obtained from the seed coat of *Bixa orellana* [34]. This phytochemical belongs to the relatively small family of apocarotenoids; it was the first cis-carotenoid to be isolated from natural sources [35]. However, it was not until 1961 that its chemical structure and stereochemistry were determined through nuclear magnetic resonance spectroscopy studies [36].

This phytochemical compound shows pleiotropic bioactivities with healthpromoting properties. It was recently demonstrated that bixin caused arrest of Hep3B cell line at G2/M checkpoint of the cell cycle and the molecular mechanism of action was demonstrated by a modeling study, which was based in the favorable

 binding of bixin to domains of Bax BH3 and FasL proteins [37]. Consequently, bixin should be used for developing agents to combat human hepatocellular carcinoma. Bixin is also a potent activator of the transcription factor nuclear factor erythroid 2-related factor 2 (NRF2), which is the master regulator of the cellular antioxidant response protecting the skin against various environmental stressors including UV radiation and electrophilic pollutants [38–40]. The protective effects against solar UV-induced skin damage are due to the NRF2-dependence of bixininduced antioxidant and anti-inflammatory effects [39]. In addition, bixin displays molecular activities as antioxidant, excited-state quencher, peroxisome proliferatoractivated receptor α/γ agonist, and Toll-like receptor 4/nuclear factor kappa-lightchain-enhancer of activated B-cell antagonist. Together, these bioactivities may be important to the improvement of skin barrier function and environmental stress protection [40].

### **5.2 Crofelemer**

 Crofelemer previously known as SP-303 is a large proanthocyanidin oligomer isolated from the bark latex of the plant *Croton lechleri* Müll. Arg. [41]. Initial studies have demonstrated the immense antiviral activity of crofelemer against a gamma of DNA and RNA viruses such as respiratory syncytial virus, influenza A virus, parainfluenza virus, herpesvirus types 1 and 2, and hepatitis A and B viruses. The antiviral mechanism implies the direct interaction of crofelemer to components of the viral envelope, blocking both the viral attachment and the cell invasion [41]. More recently, crofelemer is used as a first-in-class antidiarrheal medication, and its efficacy has been investigated *in vivo* assays [42] and in patients with HIV-associated diarrhea, diarrhea of various infectious etiologies, as well as diarrhea-predominant irritable bowel syndrome [43]. Crofelemer was recently approved by the FDA to treat diarrhea in HIV/AIDS patients on antiretroviral therapy [44].

The mechanism of action as antidiarrheal of this proanthocyanidin oligomer consists in the dual inhibitory action on two structurally unrelated prosecretory intestinal Cl− channels, which are responsible for chloride secretion and subsequent luminal hydration. The first target is an extracellular site of the cystic fibrosis transmembrane regulator (CFTR) Cl− channel (∼60%, IC50 ∼ 7 μM), which produces a voltage-independent block with stabilization of the channel closed state. The second target is the intestinal calcium-activated Cl− channel (CaCC) by a voltage-independent inhibition mechanism (>90%, IC50 ∼ 6.5 μM) [45].

#### **5.3 Linalool**

An abundant (~90%) essential oil of the leaves of *Aniba rosaeodora* [46, 47] that is used in the traditional medicine of the Peruvian and Brazilian Amazon for its effects on the central nervous system, such as sedative, anticonvulsant, and antidepressant [19, 47, 48]. Additionally, linalool has anti-inflammatory [49], anticancer [50–52], antihyperlipidemic, antinociceptive, analgesic, anxiolytic, and neuroprotective properties [53]. Several studies have demonstrated a gamma of anti-infectious activity like antiviral [54], antibacterial [55–57], antifungal [58, 59], and antileishmanial [55, 60, 61].

The anticancer mechanisms of action of linalool in hepatocellular carcinoma (HCC) HepG2 cells were recently revealed by Rodenak-Kladniew et al. [50] (**Figure 4**). According to these researchers, linalool in a dose-dependently blocked cell proliferation by inducing G0/G1 cell cycle arrest, through Cdk4 and cyclin A downregulation, p21 and p27 upregulation, and apoptosis, characterized by

*Medicinal Plants of the Peruvian Amazon: Bioactive Phytochemicals, Mechanisms of Action… DOI: http://dx.doi.org/10.5772/intechopen.82461* 

**Figure 4.** 

*Anticancer mechanisms of action of linalool in hepatocellular carcinoma (HCC) HepG2 cells.* 

mitochondrial membrane potential loss, caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and DNA fragmentation

#### **5.4 Mitraphylline**

A pentacyclic oxindolic alkaloid that was isolated from the alkaloid fraction of the dried inner bark of *Uncaria tomentosa* (Willd. ex Schult.) DC; it represents the most abundant phytochemical (40%) of the alkaloid fraction [62]. Several investigations have demonstrated the immunoregulatory activity of this compound or the pentacyclic oxindolic alkaloid-enriched fraction [63–67].

The mechanism of action as immunoregulator of mitraphylline consists in both to protect cells against oxidative stress and to elicit a response via an NF-kβdependent mechanism. The first mechanism is based on the inhibition of the inducible nitric oxide synthase gene expression; consequently, nitrite formation and programmed cell death are avoided. Finally, in the second mechanism, the inhibition of NF-kβ signaling permits the abrogation of the release of pro-inflammatory cytokines such as TNFα, IL-6, IL-1 α, IL-1β, IL-4, IL-17, and IFN-α [63–67].

#### **5.5 Quercetin**

A polyphenol categorized as a flavonol, one of the five subclasses of flavonoid compounds. This bioactive phytochemical is biosynthesized and accumulated in

tissues and organs of several medicinal plants of the Peruvian Amazon such as *Annona montana*, *Bauhinia longifolia*, *Bertholletia excelsa*, *Genipa americana*, *Inga edulis*, *Mauritia flexuosa*, *Myrciaria dubia*, *Oenocarpus bataua*, *Solanum sessiliflorum*, *Theobroma bicolor*, *T. cacao*, and *T. grandiflorum* [68–70]. Quercetin exhibits multifaceted therapeutic applications for multiplicity of unrelated acute and chronic human ailments like allergy, arthritis, asthma, bacterial and viral infections, cancer, cardiovascular diseases, inflammation, obesity, diabetes, mood disorders, neuropathologies, and other health problems [71–76].

 This multiple health beneficial properties of quercetin are attributed to their particular mechanism of action based on inhibition of several key proteins and enzymes (**Figure 5**). For example, a recent research showed that this compound is a potent inhibitor of 25 human serine/threonine kinases [77]. The multitarget inhibitor explains its beneficial pleiotropic effects on humans. This flavonoid-type inhibitor is effective against xanthine oxidase, appropriate for the treatment of hyperuricemia, gout, and inflammatory disease states. The inhibitory mechanism is based on the favorable steric complementarity of the conjugated three-ring structure of quercetin with the active site of xanthine oxidase. The enzyme-quercetin binary complex is stabilized by van der Waals forces and hydrogen-bonding interactions with both binding and catalytic amino acid residues, respectively [78, 79]. Recently, Hamilton et al. [80] have demonstrated that quercetin is a competitive inhibitor of glucose uptake by GLUT1. These researchers showed that the inhibitory effect is simply by binding of quercetin to the surface of GLUT1 [80]. Finally, several structural studies by X-ray diffraction have corroborated the inhibitory complex of quercetin with several human protein kinases [81–83].

## **5.6 Taspine**

An alkaloid isolated for the first time by Vaisberg et al. [84] from the bark latex of the plant species *Croton lechleri* Müll. Arg. Previous *in vitro* and *in vivo* investigations have demonstrated that taspine promotes early phases of wound healing in a dose-dependent manner [84, 85]. Taspine was also demonstrated to activate the pro-apoptotic cascade, which oligomerizes Bak/Bax into pores that result in the release of cytochrome c and consequently apoptosis in HCT116 colon carcinoma cells [86]. Similar results were reported for an *in vivo* study conducted with ZR-75- 30 human breast cancer xenografts in athymic mice [87].

The mechanism of action of taspine as a topoisomerase inhibitor was revealed recently. Initially, using *in vitro* assays, Fayad et al. [86] observed the inhibition of both topoisomerases I and II by taspine. Castelli et al. [88] corroborated the

**Figure 5.**  *Inhibitory complex of quercetin with selected human kinase targets.* 

*Medicinal Plants of the Peruvian Amazon: Bioactive Phytochemicals, Mechanisms of Action… DOI: http://dx.doi.org/10.5772/intechopen.82461* 

inhibitory action of taspine on purified topoisomerase I and provided the molecular details of the inhibitory action. These researchers showed that taspine inhibits the catalytic process (cleavage and religation), and molecular docking simulations showed that the formation of the complex enzyme-taspine is accomplished by the interaction in the proximity of the active site preventing the cleavage reaction. While, that the religation inhibition is explained by DNA intercalation of the inhibitor with the enzyme-DNA-binary complex.
