Phytochemistry and Biological Activities of Guarea Genus (Meliaceae)

Guarea is one of the largest genera of the American Meliaceae family, consisting of over 69 species which are widely distributed in Mexico, Argentina, and Africa and are used in traditional medicine for several diseases. Previous studies reported that the Guarea species produce secondary metabolites such as sesquiterpenoid, diterpenoid, triterpenoid, limonoid, steroid, and aromatic compounds. The preliminary chemical investigation commenced by isolating the limonoid compound, dihydrogedunin, in 1962; then, 240 compounds were obtained from the isolation and hydrodistillation process. Meanwhile, sesquiterpenoid is a significant compound with 52% of Guarea species. The extract and compounds were evaluated for their anti-inflammation, antimalarial, antiparasitic, antiprotozoal, antiviral, antimicrobial, insecticidal, antioxidant, phosphorylation inhibitor, and cytotoxic biological activities. The Guarea genus has also been reported as one of the sources of active compounds for medicinal chemistry. This review summarizes some descriptions regarding the types of Guarea species, especially ethnobotany and ethnopharmacology, such as the compounds isolated from the part of this genus, various isolation methods, and their bioactivities. The information can be used in further investigations to obtain more bioactive compounds and their reaction mechanisms.

Guarea is one of the largest genera of the American Meliaceae family consisting of over 69 species widely distributed in Mexico and Argentina [32], while a few species are found in Africa [33]. Initial chemical investigation which commenced in 1962 by Housley et al. [34] isolated a limonoid compound, dihydrogedunin (221), from the ground heartwood of G. thompsonii (Nigerian pearwood). Subsequently, eight classes of secondary metabolites have been identified along with their biological activities, such as cytotoxic, antiinflammation, antimalarial, antiparasitic, antiprotozoal, antiviral, antimicrobial, insecticidal, antioxidant, and phosphorylation inhibitor.

Overview of Isolated Compounds Derived from Guarea Genus
About 240 compounds have been isolated from the stembark, leaves, fruits, bark, seed, flowering branches, and root of this genus, based on the literature from 1962 to 2022 as shown in Table 1. The extract for the isolation process was obtained from various solvents such as n-hexane, chloroform, methanol and n-butanol. The first step of the process is the maceration of the dried sample with solvent, especially methanol or ethanol; after that, MeOH/EtOH extract is diluted with water and partitioned with other solvents for obtaining crude extract. Meanwhile, between the hydrodistillation and isolation process is different. The hydrodistillation process used a fresh sample (part of Guarea) and submitted to a Clevenger-type apparatus for 4 h for the gained crude oil. The crude extract and crude oil were purified with various techniques such as column chromatography on silica gel or RP-18 silica gel, Sephadex LH-20, preparative TLC, and semipreparative HPLC on RP-18 column for crude extract. The compounds were identified by NMR, mass spectrometry, FTIR, UV, and polarimeter. Moreover, the crude oil was analyzed using a combination of the four techniques of GC, GC/MS, 1 H-, and I3 C-NMR. The compounds identified from the isolation and hydrodistillation processes included 52% sesquiterpenoid, 16% diterpenoid, 15% Triterpenoid, 10% limonoid, as well as 7% non-terpenoid and limonoid. The distribution of the compounds is presented in Figure 2 and the biological activities of the identified compounds are shown in Table 2.

Limonoid
Limonoids are classified into many classes based on the type of skeleton [68,69], and about eleven classes have been reported from this genus. The first exploration by Housley et al. [34] reported dihydrogedunin (221) from the heartwood of G. thompsonii.

Limonoid
Limonoids are classified into many classes based on the type of skeleton [68,69], and about eleven classes have been reported from this genus. The first exploration by Housley et al. [34] reported dihydrogedunin (221) from the heartwood of G. thompsonii.

Cytotoxic
The cytotoxic activity of the Guarea genus has been studied in many extracts and compounds (diterpenoids, triterpenoids, limonoids, and steroids) using various test methods. The findings could lead to the development of new antitumor and anticancer drugs. The extract and the compounds of four species from the Guarea genus were evaluated in 1962. Lukacova et al. [73] identified three compounds from G. guidonia, including 14,15βepoxyprieuriani (210), 7-oxo-gedunin (218), and prieurianin (219). The compounds 210 and 219 are active against the leukemia cell line P388 ED 50 0.47-0.74 µg/mL and P388 ED50 4.4-7.8 µg/mL, respectively, while 218 is not active. Furthermore, methylene chloride extract was evaluated against U-937 cell lines; bark and leaf extract of G. polymera each showed a lethal dose (LD 50 ) of 6.1 ± 0.5 µg/mL and 6.1 ± 1.2 µg/mL while the seed of G. guidonia had a LD 50 of 28.8 ± 8.2 µg/mL [90].

Anti-Inflamation
Catabolism takes precedence over anabolism in an inflammatory state. It is also a defense mechanism that aids in the elimination of potentially harmful factors and maintains body homeostasis. Because of the increased permeability of capillaries and white blood cells, this causes increased blood flow to the site of inflammation, resulting in symptoms such as redness, swelling, and pain.
Oga et al. [82] reported the anti-inflammation activity from ethanol extract of G. guidonia seeds against male Wistar rats. About an 8.0 mL/kg extract dose provided significant inhibition of carrageenin-induced edema, and the effects increased periodically. Similarly, a 5.0 mL/kg extract dose provided effects amounting to 15% on granuloma tissue formation after 2, 4, and 6 days.

Antimalarial
Four extracts from G. multiflora were obtained using petroleum ether, methanol, water, and chloroform. They were collected from leaves, stem bark, and wood, as well as fruits. The extracts showed no significant results as three, namely, petroleum ether from leaves, methanol of stem bark and fruits, as well as chloroform from stem bark, had an IC50 of 50 µg/mL. Meanwhile, other extracts showed an IC 50 of 500 µg/mL and were not active [83].
Furthermore, Weniger et al. [90] identified methylene chloride extract of bark and leaves of G. polymera which has a selectivity index against Leishmania Viannia panamensis with a lethal dose/effective dose (LD 50 /ED 50 ) of 1.5 µg/mL. The seeds of G. guidonia were also active against Plasmodium falciparum with an LD 50 /IC 50 2.9 µg/mL. Hexane extract obtained from the root of G. kunthiana reportedly had antileishmanial activity on the intracellular parasite, Leishmania donovani. The test was evaluated using the colorimetric method which was an MTT assay and the extract showed an IC 50 of 7.9 ± 1.3 µg/mL [84]. Moreover, the 3β-O-tigloylmelianol (167) was investigated with larvicide and ecydysis tests against the cattle tick of Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae); the compound showed a significant reduction in the number of oocytes [91].

Antiviral
Two water extracts from the fruits and leaves of G. guidonia were identified to have antiviral activity against pseudorabies and mouth disease virus in the IB-RS-2 pig cell lines and against bovine herpesvirus 1 (BHV-1) in the GBK bovine cell line. The result of the fruit extract test was more active than the leaves in the IB-RS-2 cell. Meanwhile, the activity of the two extracts increased with an IC 50 of 62.5 and 125 µg/mL in the GBK cell [85].

Antimicrobial
Several compounds isolated from Guarea have been found to have antimicrobial activity. This activity provides antibiotics against microorganisms that can cause food defects, such as pathogens. A study conducted by Pandini et al. reported the result of antimicrobial activity for essential oil and methanol extracts from G. kunthiana [88]. Methanol extract showed no activity in the MIC or MBC test. Meanwhile, the essential oil evaluated with MIC and MBC against S. infantris, S. tyrphimurium and S. give showed antimicrobial activity amounting to 54.6 µg/mL. The ethyl acetate extract had activity ranging from 100 to 200 µg/mL.

Insecticidal Activity
Four compounds were isolated from G. grandiflora and evaluated against the growth of larva ECB (European corn borer). The results showed that 21-α-acetylmelianone (191) and melianone (184) have the activity to inhibit the growth of ECB larvae using the fed control diet. Meanwhile, the pupal weight was not affected by any of the compounds but the percentage of pupation was significantly reduced by melianodiol (185) [65].

Antioxidant and Phosphorylation Inhibitor
The antioxidant activity is a defense mechanism that protects our bodies from oxidative stress caused by free radicals and reactive oxygen species (ROS). Oxidative stress can occur as a result of ROS formation and the detoxification of elevated levels of ROS, resulting in impaired cellular function. The compounds which have been isolated from this genus have antioxidant activity [88]. The essential oil, alcoholic, aqueous, and ethyl acetate ex-tracts were evaluated. Based on the results, the alcoholic extract showed an IC 50 of 15.3 µg/mL while ethyl acetate had the lowest activity with an IC 50 176.8 µg/mL.

Conclusions
Guarea is one of the largest genera of the Meliaceae family, and about 240 compounds have been obtained through the hydrodistillation and isolation process with the majority of them being sesquiterpenoids. Furthermore, the bioactivity data show that this plant has a variety of activities, specifically for cytotoxic activity.  Data Availability Statement: The study did not report any data.