Foreword

According to the 2016 review by Newman and Cragg, natural products continue to be an important source of clinical trial drugs and as drug candidates: approximately 65% of small molecule drugs approved from 1981 to 2014 are directly or indirectly related to natural compounds.¹⁾ Among various natural resources, including plants, microbials, and marine organisms, plants have a long history of medicinal use going back to the ancient records of Mesopotamia, which chronicle their use in the treatment of various diseases.²⁾ The total number of higher plants species in the world was previously estimated to be around 250000, and as many as 15% had been studied phytochemically by 2000.³⁾ Although the number of phytochemically studied species is increasing due to ongoing research by natural product chemists, new species are being identified more even today, and a recent paper estimated the total number of angiosperm species at approximately 308000.⁴⁾ In general, species diversity is highest in the tropics and lowest at the poles, with the more temperate regions typically home to an intermediate number of species. Two-thirds of angiosperm species are native to the tropics. This species richness is explained by the Metabolic Theory of Ecology (MTE): the interrelationship between a richness of plant biodiversity and elevated biochemical kinetics of metabolism caused by higher environmental temperature.^5,6) Therefore, our planet’s tropical and subtropical regions provide an abundance of attractive and diverse plant resources, many of which remain to be characterized phytochemically. In this Current Topics, we have assembled five reviews of natural product chemistry on plants collected in five richly biodiverse tropical and subtropical locales—Madagascar, Indonesia, Thailand, Vietnam and Okinawa in Japan,—to provide useful information for researchers in this field.

The first review, entitled “Novel Bioactive Natural Products Isolated from Madagascar Plants and Marine Organisms (2009–2017)” is by Dr. Yumin Dai, Dr. Yixi Liu, and Dr. L. Harinantenaina Rakotondraibe from Ohio State University. Madagascar’s rain forests and tropical dry forests are home to more than 80% of endemic plant species. This high endemism is due to Madagascar’s long separation and isolation from neighboring continents, a seclusion that began approximately 100 million years ago. About 80% of the Madagascan (Malagasy) population relies on their traditional herbal medicines. This review focuses on recent studies of the isolation, structural elucidation and biological activities of Madagascan plants and marine organisms published in the last seven years (2009–2017).

The second review is reported by Dr. Pharkpoom Panichayupakaranant and colleagues of Prince of Songkla University in Thailand, under the title “Antidiabetic Naphthoquinones and Their Plant Resources in Thailand.” Ninety percent of diabetes is type-2, a disease still in search of an effective, safe and economical oral therapy. More than 4 million cases of diabetes mellitus were reported in Thailand in 2015. Thailand’s rich natural biodiversity is due to its unique geographical location. It is home to almost 10250 unique plants, corresponding to 5% of species in the entire world. Thai traditional herbal medicines play an important role in the country’s public health. This review presents antidiabetic compounds from plant resources in Thailand, especially naphthoquinones due to their structural uniqueness and various distinct activities.

The third review, presented by Dr. Phan Minh Giang from Vietnam National University, is entitled “New Compounds and Potential Candidates for Drug Discovery from Medicinal Plants of Vietnam.” Vietnam is among the privileged countries in terms of its high biodiversity of plant species and its long history of medicinal use of native herbs to treat numerous diseases. The flora of Vietnam comprises more than 12000 plant species, with about one-third of these (3948 species) used medicinally. Thus, Vietnam has a vast treasure of traditional medicinal plants. Study of the constituents of these plants, with a focus on their bioactivities, is an important theme for pharmaceutical researchers. This review introduces unique new compounds from systematic studies of medicinal plants used in Vietnam, with more in-depth discussion of the biological activities of six important plant genera: Croton, Mallotus, Artemisia, Goniothalamus, Garcinia, and Ficus of Vietnam.

The fourth review provides recent scientific knowledge of Zingiberaceae, especially for Curcuma, Zingiber, Alpinia, and Justicia, under the title “Chemical Constituents and Bioactivities of Several Indonesian Plants Typically Used in Jamu” by Dr. Retno Widyowati and Prof. Mangestuti Agil from Airlangga University, Indonesia. Indonesia features the world’s greatest biodiversity, with more than 30000 species, of which 6000 species are estimated to possess biological activity; of these, approximately 1000 species have been used in Indonesian traditional medicine, Jamu. These traditional medicines have been transferred from generation to generation, and practiced for hundreds of years. Jamu consists of either a single ingredient or a combination of medicinal plants, and this review focuses on the phytochemical study of Zingiberaceae as the most popular medicinal component in Jamu.

The final of the five reviews was performed by our group, and is entitled “Okinawan Subtropical Plants as a Promising Resource for Novel Chemical Treasury.” Prof. Hideaki Otsuka began research on Okinawan subtropical plants around 30 years ago, because Japan’s Okinawa islands are home to a huge variety of unique subtropical plants species, with relatively high endemism. Okinawa Islands hold 286 families, 1961 genera and 5793 plant species, representing 81.7% of the total plant species (7087) in Japan, although the area of Okinawa is only 0.6% of the total geographical area of Japan. We have phytochemically studied Okinawan plants for many years, and published more than 300 studies to date. These results substantiate the usefulness and richness of unique chemical structures of Okinawan plants as a source of candidate drugs. In this featured review, we provide our recent research progress on Microtropis japonica, Croton cascarilloides and Macaranga tanarius.

The above reviews have been provided by the researchers from the five countries we identified as having rich plant diversity. There are many plant resources throughout the world which remain to be subjected to scientific and chemical investigation. Considering the success in the discovery of a great number of drugs and drug candidates from plant materials to date, natural product chemistry will continue to play a significant role in future drug development.

References

• 1) Newman D. J., Cragg G. M., J. Nat. Prod., 79, 629–661 (2016).
• 2) “Natural Product Chemistry for Drug Discovery.” eds. by Buss A. D., Butler M. S., the Royal Society of Chemistry, 2010. ISBN: 978-0-85404-193-0, ISSN: 1757-7136
• 3) Verpoorte R., J. Pharm. Pharmacol., 52, 253–262 (2000).
• 4) Christenhusz M. J. M., Byng J. W., Phytotaxa, 261, 201–217 (2016).
• 5) Allen A. P., Brown J. H., Gillooly J. F., Science, 297, 1545–1548 (2002).
• 6) Wang Z., Brown J. H., Tang Z., Fang J., Proc. Natl. Acad. Sci. U.S.A., 106, 13388–13392 (2009).