A mini-review on the phytochemistry and biological activities of selected Apocynaceae plants

Introduction The plants used for medicinal purposes are known to be potential sources of therapeutics and play important role in our daily life (1,2). In developing nations, medicinal plants are used as alternative source of treatments (3) whereby the extracts and phytochemicals isolated from them have shown in vitro and in vivo biological activities (4). Mankind has used herbal products to treat pains and to even cure different forms of ailments (4) for the past 60 000 years (5). Presently, over 50 % of natural drugs that are being used for medication, are derived from plants (5,6). The World Health Organization (WHO) has reviewed that about 80% of the population all over the world use herbs to treat various ailments (7) as well as for their other healthcare needs (2). Herbals are natural products that have been found to be safe because of the limited side-effects. Apart from the medicinal uses of these herbals, they are also used as dietary supplements assisting the human body as defensive mechanism against these diseases. These days, herbal products are being sold in powder form, tablets, capsules and tea extracts (4). Traditionally, herbal products are seen to be harmless to people; they do not hesitate to take them without any prescription and in any dosage. However, some of these plats may be toxic and can cause grave health problems. However, some that are taken may not be effective and some may even interact with other drugs used. Assessing the quality and standard of these herbal products is very important so as to determine the bioactive ingredients that confers on them these huge medicinal potentials (8). This review provides s detailed insight into the pharmacological activities of these selected members of this family. The Apocynaceae family Apocynaceae family (also called the dogbane family) includes flowering plants which are made up of herbs, shrubs, trees, stem succulents and vines (5). The origin of members of this family were traced to the Europe, Asia, Africa, Australia, and American tropics or subtropics, with some temperate members (9). The former family called Asclepiadaceae (and now Asclepiadoideae) is considered to be a subfamily of the Apocynaceae which contains 348 http://www.herbmedpharmacol.com doi: 10.15171/jhp.2019.xx


Introduction
The plants used for medicinal purposes are known to be potential sources of therapeutics and play important role in our daily life (1,2).In developing nations, medicinal plants are used as alternative source of treatments (3) whereby the extracts and phytochemicals isolated from them have shown in vitro and in vivo biological activities (4).
Mankind has used herbal products to treat pains and to even cure different forms of ailments (4) for the past 60 000 years (5).Presently, over 50 % of natural drugs that are being used for medication, are derived from plants (5,6).
The World Health Organization (WHO) has reviewed that about 80% of the population all over the world use herbs to treat various ailments (7) as well as for their other healthcare needs (2).
Herbals are natural products that have been found to be safe because of the limited side-effects.Apart from the medicinal uses of these herbals, they are also used as dietary supplements assisting the human body as defensive mechanism against these diseases.These days, herbal products are being sold in powder form, tablets, capsules and tea extracts (4).Traditionally, herbal products are seen to be harmless to people; they do not hesitate to take them without any prescription and in any dosage.However, some of these plats may be toxic and can cause grave health problems.However, some that are taken may not be effective and some may even interact with other drugs used.Assessing the quality and standard of these herbal products is very important so as to determine the bioactive ingredients that confers on them these huge medicinal potentials (8).This review provides s detailed insight into the pharmacological activities of these selected members of this family.

The Apocynaceae family
Apocynaceae family (also called the dogbane family) includes flowering plants which are made up of herbs, shrubs, trees, stem succulents and vines (5).The origin of members of this family were traced to the Europe, Asia, Africa, Australia, and American tropics or subtropics, with some temperate members (9).The former family called Asclepiadaceae (and now Asclepiadoideae) is considered to be a subfamily of the Apocynaceae which contains 348 genera (10).Many of these species are mainly the tall trees found in tropical forests.However, some members grow in tropical dry areas while some members are perennial herbs found in temperate zones (5).Many of these plants are known to have milky latex while some species of the family are known to be poisonous if ingested.For instance, Adenium a genera of the Apocynaceae family have been reported to have a milky latex while Pachypodium have only clear sap without latex (3).
Major characteristic features of this family are that almost all species are known to produce milky sap (10).Ethnomedicinally, the Apocynaceae family is being used to treat malaria, gastrointestinal ailments, fever, pain and diabetes as well as skin infections (10).Some of the Apocynaceae plants have been reported for the anticancer and antimalarial properties (1).This family is known as one of the largest and useful families in angiosperm (6).Because of the huge medicinal values of this family, many of them have been used in the treatment of various ailments (11).They are also consumed by many tribes as food and used as poisons (1).The Apocynaceae plants are reported to be rich in triterpenoids, flavonoids, alkaloids, steroids, glycosides, phenols and lactones, (4,11).Phytochemicals like sugars, sterols and lignans as well as lactones have been reported in this family (4).Antimicrobial, anti-inflammatory, antioxidant and cytotoxic activities of crude extracts and the compounds isolated from various members of the Apocynaceae family have been reported (6,11).The summary of phytochemical and pharmacological activities of selected Apocynaceae
The phytochemical investigation of the ethanol extract of the stem bark of Secondatia floribunda A. DC (Apocynaceae) showed antibacterial activity clinically significant against strains of Staphylococcus aureus and Escherichia coli (13).The compounds isolated from this extract were gallic acid, cafeic acid, cianidin, apigenin, catechin and quercetin (7,13).
Also, rutin, epicatechin, quercetin and kaempferol were isolated from Carissa carandas fruits as principal flavonoids with the total flavonoid content of 4.8 mg/100 g (1).The root and stem extracts of Tabernaemontana divaricata were also reported to contain novel flavonoids, calycosin and formononetin (1).All of these flavonoids exhibited anti-inflammatory potential against croton oilinduced edema in an albino mouse model with IC 50 of 500 μg/mL (1,11).
A cardiac glycoside, kaneroside was isolated from the leaves of Nerium oleander (21).Cardiac glycosides were also isolated from the roots of Carissa carandas (4) and identified as odoroside and evomoside (19).Thevefolin was reported as a cardenolide of Thevetia peruviana.Thevefolin was reported to be effective for TRAIL resistance in human gastric adenocarcinoma cells (20).

Conclusion
The Apocynaceae plants have been reported for their vast medicinal potentials.In this study, eleven members of the Apocynaceae plants were reviewed for phytochemical constituent and biological properties.The most important phytochemicals commonly isolated from these plants were triterpenoids, flavonoids, phytosterols, cardiac glycosides and lignans.These phytochemicals were reported to exhibit similar biological activities.We also noted that all of them possess a wide range of biological properties.The ethnomedicinal uses of the Apocynaceae plants in treatment of various kinds of ailments might probably be due to these bioactive constituents.

Figure 1 .
Figure 1.Chemical structure of some compounds isolated from selected Apocynaceae plants.

Table 1 .
Summary of the most important phytochemicals and pharmaceutical activities of selected Apocynaceae plants