**4. Case study: secondary metabolites in the prevention and treatment of dengue**

#### **4.1. Dengue fever**

Gajah (*Clinacanthus nutans* (Burm.f.) Lindau), and Pegaga (*Centella asiatica* (L.) Urb) [10]. It is noteworthy that a dietary supplement combining *Labisia pumila* and *Eurycoma longifolia* has reached the clinical trials investigating the effects on menopausal women and their quality of

Not discounting many other important local plants, **Table 1** listed some of the important secondary metabolites from the agriculture NKEA-EPP1-select plants to illustrate the myriad of secondary metabolites responsible for various biological effects. The secondary metabolites

Overharvesting of medicinal plants for their secondary metabolites may lead to their disappearance from the natural habitats. Due to this, researchers turn to alternative and innovative methods to meet the increased demand for these natural products. In particular, plant tissue culture has become a well-established and attractive alternative for mass production of secondary metabolites through callus, suspension, and organ culture [18, 19]. Tissue culture itself is defined as the technique of maintaining plant tissue in vitro in a synthetic medium under controlled conditions, and it is reported to be extremely useful for commercial produc-

There are many advantages of using plant tissue culture for producing metabolites including the (i) ability to improve the production of certain compounds within the plant cell using elicitors and plant hormones to manipulate the cultured cells [21], (ii) the ability to continuously produce secondary metabolites through propagation in sterile bioreactors independently of growth conditions such as soil content and microclimate [22], (iii) the ability of in vitro plant tissue culture to achieve higher rates of metabolism than the in vivo differentiated intact cells [22], and (iv) the ability to bypass the structural complexity of the plant organism rendering it

> Flavonoid (panduratin, pinocembrin, pinostrobin, cardamonin, and alpinetin)

kaempferol, luteolin, and

acid, madecassic acid, asiaticoside, and madecassoside)

Cell suspension Triterpenoids (asiatic

Cell suspension Flavonoid (quercetin,

rutin)

**Secondary metabolites Biological activities Reference**

activities

activities

Antimicrobial, antiulcer, antiviral, and antitumor

Antibacterial, antimalarial, antiproliferative, and wound-healing properties

Antibacterial, antiviral, antiallergic, antiplatelet, anti-inflammatory, antitumor, and antioxidant [24]

[25]

[26]

life (trial registration number NCT02269891) [11].

104 Secondary Metabolites - Sources and Applications

tion of therapeutically important compounds [20].

to be a convenient tool in research studies [23].

Embryogenic and non-embryogenic

**culture**

callus

**Plant species Type of plant** 

*Boesenbergia rotunda* **(Kencur)**

*Centella asiatica (L.) Urb* **(Pegaga)**

range from flavonoids, quassinoids, phytosterols, and terpenoids.

**3. Secondary metabolites from plant tissue culture**

The dengue prevalence in Malaysia remains overwhelming, with increasing rate of incidence reported annually. According to the latest WHO report, in 2016, more than 375,000 suspected cases of dengue were recorded in the Western Pacific Region, 100,028 of which occurred in Malaysia [34]. The steady surge of cases over the years has prompted serious efforts from the government and the community, including intensive efforts in both vector control and elucidation of potential therapeutic agents. Although evaluation of several vaccines is currently in progress, the ambivalence of such treatment with regard to serotype interference, incomplete protection, and dose sufficiency [35] makes the search for new antiviral agents imperative.

abundance of medicinal plants, should be prioritized, as this is deemed the best strategy for

Secondary Metabolite Research in Malaysia: Current Status and Future Prospects

http://dx.doi.org/10.5772/intechopen.76305

107

Malaysia, due to its prolific nature and wide diversity of plant sources, offers a wide range of pharmaceutical options, with high potential as anti-DENV agents. Most of these plant species have not been studied extensively, and the existing research tends to focus on folklore medications. It is important to emphasize that the characterization of secondary metabolites responsible for anti-DENV activities is still very limited. Consequently, none of these medicinal plants have reached the clinical stage in the drug design pipeline, thus necessitating further verification studies. To date, only several secondary metabolites from a number of local Malaysian plant species have been identified and their chemical structures elucidated (**Table 3**), while many more remain to be explored. For instance, the effects of local plant extracts on in vitro dengue replication were recently documented [36], whereby *Hydrocotyle sibthorpioides* Lam. extracts showed variable effects on dengue viral replication, depending on the treatment, cell lines, and solvent types. In an earlier study, extract from *Phyllanthus* spp. also exhibits antiviral activity against DENV-2, which was supported by differential

A summary of plant secondary metabolites found useful in dengue treatment and prevention

Despite a more concerted efforts and strategic approach to add value to the country's herbal industry, the desired output has not reached its optimum. The main challenges including the lack of good research documentation, monographs, standardization in farming practices (good agricultural practices, GAP), good laboratory practice (GLP), and development and commercialization of products still persist. Further, the increased harvesting of medicinal herbs raises the concerns about the extinction of plant populations and degradation of natural plant habitat causing shortage in plant raw materials which may affect the effort to reach high mass production of secondary metabolites. To this end, biotechnological applications such as plant tissue cultures have been recognized an alternative tool for scaling-up the production of secondary metabolites. However, challenges inherent in plant tissue cultures must be overcome in order for it to contribute to significant cost-effective production of secondary metabolites. This includes the understanding of the secondary metabolites and their metabolic pathways, identification of the highest yielding populations, control of specific gene expression and regulatory enzymes, and the use of economic sterile bioreactors. Leveraging the country's wealth of flora species, a continuous effort from the government, academia, and industry to further nurture and

**4.2. Secondary metabolites from Malaysian plant species used for the prevention** 

the development of new dengue fever treatment regimens.

regulation of various hosts and viral proteins [37].

**5. Challenges and future prospects**

uphold the herbal industry is indispensable.

**and treatment of dengue**

is depicted in **Table 3**.

In Malaysia, the National Dengue Strategic Planning 2015–2020 (the Sixth Strategy: Dengue Research) highlights the importance of research aimed at enhancing the effectiveness, costeffectiveness, sustainability, and the scale of existing interventions, as well as producing ideas and new methods, while promoting collaboration with relevant agencies. In this regard, it is anticipated that comprehensive research focusing on the local natural heritage, including the

**Table 3.** Selected secondary metabolites from plants with effects against dengue or its vector.

abundance of medicinal plants, should be prioritized, as this is deemed the best strategy for the development of new dengue fever treatment regimens.

## **4.2. Secondary metabolites from Malaysian plant species used for the prevention and treatment of dengue**

Malaysia, due to its prolific nature and wide diversity of plant sources, offers a wide range of pharmaceutical options, with high potential as anti-DENV agents. Most of these plant species have not been studied extensively, and the existing research tends to focus on folklore medications. It is important to emphasize that the characterization of secondary metabolites responsible for anti-DENV activities is still very limited. Consequently, none of these medicinal plants have reached the clinical stage in the drug design pipeline, thus necessitating further verification studies. To date, only several secondary metabolites from a number of local Malaysian plant species have been identified and their chemical structures elucidated (**Table 3**), while many more remain to be explored. For instance, the effects of local plant extracts on in vitro dengue replication were recently documented [36], whereby *Hydrocotyle sibthorpioides* Lam. extracts showed variable effects on dengue viral replication, depending on the treatment, cell lines, and solvent types. In an earlier study, extract from *Phyllanthus* spp. also exhibits antiviral activity against DENV-2, which was supported by differential regulation of various hosts and viral proteins [37].

A summary of plant secondary metabolites found useful in dengue treatment and prevention is depicted in **Table 3**.
