Acylphenols and Dimeric Acylphenols from the Genus Myristica: A Review of Their Phytochemistry and Pharmacology

The genus Myristica is a medicinally important genus belonging to the Myristicaceae. Traditional medicinal systems in Asia have employed plants from the genus Myristica to treat a variety of ailments. Acylphenols and dimeric acylphenols are a rare group of secondary metabolites, which, to date, have only been identified in the Myristicaceae, in particular, in the genus Myristica. The aim of the review would be to provide scientific evidence that the medicinal properties of the genus Myristica could be attributed to the acylphenols and dimeric acylphenols present in the various parts of its plants and highlight the potential in the development of the acylphenols and dimeric acylphenols as pharmaceutical products. SciFinder-n, Web of Science, Scopus, ScienceDirect, and PubMed were used to conduct the literature search between 2013–2022 on the phytochemistry and the pharmacology of acylphenols and dimeric acylphenols from the genus Myristica. The review discusses the distribution of the 25 acylphenols and dimeric acylphenols within the genus Myristica, their extraction, isolation, and characterization from the respective Myristica species, the structural similarities and differences within each group and between the different groups of the acylphenols and dimeric acylphenols, and their in vitro pharmacological activities.


Introduction
The genus Myristica, one of the most common genera within the Myristicaceae, consists of 120 species distributed in Australia, South Asia, from west Polynesia, Oceania, and East India to the Philippines [1][2][3]. The trees are of various sizes, reaching up to about 120 feet in height, with buttresses or stilt roots. The bark is either black or brown in color, brittle, and scaly or fissured. The leaves are alternate, generally long, leathery, sometimes hairy, dark shiny green above and glaucous below. The inflorescences are branching or axillary panicles. The flowers are mostly tiny, sometimes sweetly scented, flask or bell shaped, white or pale yellow in color, with brown hair on the outer side and glabrous on the inner side with three acute reflexed lobes. The fruits are usually large, round-to-oblong usually longer than broad, pointed, yellow, or red upon ripening, sometimes hairy, have a thick fleshy wall, ultimately, splitting into two halves to expose the single large hard seed (nutmeg) [4]. The seed is covered in a pink or red waxy flesh that resembles a lace-like membrane, known as the mace [5].
Nutmeg and mace from certain members of the genus Myristica, for example, M. fragrans Houtt, M. malabarica Lam., and M. argentea Warb., are spices with similar tastes. Nutmeg is sweeter, while mace is known to release more delicate flavors [5]. Nutmeg and mace are added during cooking to enhance the flavor and aroma of savory dishes (e.g., potato dishes, sauces, curries), desserts (e.g., cakes, muffins, pies), and beverages (e.g., tea, mulled wine). Mace at times is preferred in some dishes for the orange color

Phytochemical Investigation
The chemical structures of the acylphenols and dimeric acylphenols and their names in relation to the corresponding Myristica species are illustrated in Figures 1 and 2 and presented in Table 1, respectively. Table 2 summarizes the techniques which were used to extract, isolate, and characterize the acylphenols and dimeric acylphenols from the respective Myristica species.       (Table 1). Though acylphenols were characterized in all of the eight species, it is interesting to note that the dimeric acylphenols were only reported to be present in the Myristica species collected from Malaysia: M. cinnamomea and M. maxima.

Extraction, Isolation, and Characterization of the Acylphenols and Dimeric Acylphenols
The acylphenols and dimeric acylphenols were extracted from either the fruits (rind, seeds, aril, kernel), the bark, or the leaves of the different Myristica species using the solvent extraction method. The extraction of the plant materials was carried out either at room temperature or under reflux using organic solvents or aqueous alcohol ( Table 2).
The resulting crude solvent extracts were either used directly or further partitioned with organic solvents and subjected to various chromatographic techniques mainly silica gel column chromatography, preparative TLC, and preparative HPLC to isolate and purify the acylphenols and dimeric acylphenols ( Table 2).
The structures of these acylphenols and dimeric acylphenols were subsequently elucidated by a combination of various spectroscopic techniques such as infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV-Vis), mass spectrometry (MS), and nuclear magnetic resonance spectroscopy (NMR) ( Table 2).

Type 1 Acylphenols
The basic skeleton of compounds 1-6 was constructed from three substructures: substructure I (ring a), substructure II (ring b), and substructure III (aliphatic chain c). Typically, substructures I and II in each compound were linked together via substructure III. A closer look at the structures of compounds 1-6 revealed that these six acylphenols were structurally related to one another.

Compounds 1-4
Substructures I and III were common in all four acylphenols and were, respectively, identified as a 2-acylresorcinol moiety and an n-octyl chain. Variations were, however, observed in substructure II. Unlike compound 1, whose substructure II was a monosubstituted aromatic ring, those of compounds 2-4 were either 1,4-disubstituted (2) or 1,3,4-trisubstituted (3 and 4) aromatic rings. Furthermore, positions C-12 and C-13 in sub-structure II in compounds 2-4 were each occupied by oxygenated functional groups such as hydroxyl (2 and 3) and methylenedioxy (4) groups in contrast to compound 1, whose substructure II does not have any oxygenated functional groups bonded to it. Based on this review, these acylphenols are ubiquitous in the genus Myristica in particular compounds 2 and 3 (Table 1, Figure 1).

Compound 5
Until present, compound 5 has only been isolated and characterized from the fruits of M. cinnamomea (Table 1, Figure 1), thus making it a potential chemotaxonomic marker for this particular Malaysian Myristica species. The structure of compound 5 closely resembled that of compound 2 with the only difference being in its substructure I. In compound 5, its 2-acylresorcinol moiety has a hydroxyl group at position C-19 in addition to the hydroxyl groups at positions C-17 and C-21, thus making its substructure I a 1, 2, 3, 5-tetrasubstituted symmetrical aromatic ring instead of a 1, 2, 3-trisubstituted symmetrical aromatic ring, as in compound 2.

Compound 6
Though compound 6 is a known acylphenol, it was identified in the bark of M. fatua Houtt. var. magnifica for the first time in 2018 (Table 1, Figure 1). Its structure was almost identical to that of compound 1, with the only difference being in its substructure I. Unlike in compound 1, whose position C-21 bore a hydroxyl group, the corresponding position in compound 6 was occupied by a methoxyl group instead.

Type 2 Acylphenols
Compounds 7 and 8, classified as Type 2 acylphenols, were isolated and characterized from the fruit rinds of M. beddomei subsp. sphaerocarpa and M. malabarica (Table 1, Figure 1). Similar to Type 1 acylphenols, the basic skeleton of Type 2 acylphenols was also constructed from three substructures: substructure I (ring a) and substructure II (ring b), which were linked together via substructure III (aliphatic chain c). It is interesting to note that there was a striking difference between the type of ring which constituted substructure I in Types 1 and 2 acylphenols. It was evident that compounds 7 and 8 were structurally related to one another. Substructure I, an 1-acyl-4-hydroxycyclohexane-2,6-dione ring and substructure III, an n-octyl chain were common in both the acylphenols. Substructure II, however, differed between them. While substructure II was a 1,4-disubstituted aromatic ring with a hydroxyl group at position C-13 in compound 7, the corresponding substructure in compound 8 was a 1,3,4-trisubstituted aromatic ring with positions C-12 and C-13 being occupied by hydroxyl groups.

Type 3 Acylphenols
Compounds 9 and 10, classified as Type 3 acylphenols, were obtained from the bark and the various parts of the fruits of M. beddomei subsp. Sphaerocarpa, M. fatua, M. fatua var. magnifica, and M. malabarica (Table 1, Figure 1). Type 3 acylphenols were significantly different compared to Types 1 and 2 acylphenols. The former was only constructed from two substructures instead of three: substructure I (ring a) and substructure II (aliphatic chain b). Substructures I and II in compound 9 were, respectively, identified as a 2-acylresorcinol moiety and an n-tridecyl chain. Compound 10 was structurally similar to compound 9.
However, position C-20 of its 2-acylresorcinol moiety bore a methoxyl group in place of a hydroxyl group.

Type 4 Acylphenols
Compound 11 was isolated and characterized from the bark of M. cinnamomea (Table 1, Figure 1). It was an acylphenol with a novel skeleton comprising four substructures: substructure I (ring a), substructure II (ring b), substructure III (aliphatic chain c), and substructure IV (ring d). Substructures I and IV were, respectively, identified as a 2-acylresorcinol moiety and a dihydropyran-4-one moiety, and both of these substructures were fused together. Substructure II, on the other hand, was a 1,4-disubstituted aromatic ring with a hydroxyl group bonded to position C-13. These three substructures were linked together via substructure III, a n-hexyl chain.
In compound 12, the neolignan unit was bonded to position C-18 of the acylphenol unit via a carbon-carbon linkage. The monoterpenoid unit, on the other hand, was bonded via an ether linkage to position C-12 of the acylphenol unit.
The structure of compound 13 was almost identical to compound 12. The monoterpenoid unit in compound 13, however, occupied position C-13 of the acylphenol unit, unlike in compound 12.

Group 2
The structure of compound 14 was significantly different from compound 13. Though the monoterpenoid unit (terpinen-4-ol) in compound 14 was bonded to position C-13 of the acylphenol unit (compound 3), similar to compound 13, position C-18 of the acylphenol unit in compound 14 was, however, occupied by a phenylpropanoid unit instead of a 8-O-4 type neolignan unit.

Group 3
The structures of compounds 15-19 differed notably from those of compounds 12-14.
The former were constructed only from an oxygenated monoterpenoid (either terpinen-4-ol or alpha-terpineol), which was bonded to either position C-12 or C-13 of an acylphenol (either compound 2 or 3) via an ether linkage.

Type 1 Dimeric Acylphenols
Compounds 20-22 were structurally related to each other and were derived from the following acylphenols; compounds 1-3. While compound 20 was identified to be a dimer of compound 3, compound 22 was characterized as a dimer of compounds 2 and 3. As for compound 21, though it was established as a new dimeric acylphenol, which was obtained from the bark of M. cinnamomea, it was constructed from the two known acylphenols: compounds 1 and 3.
The bark of M. maxima also yielded a new dimeric acylphenol, compound 23, which was a positional isomer of compound 20. Compounds 20 and 23 only differed from one another in the position of the hydroxyl groups in their respective ring b. Unlike compound 20, whose ring b bore ortho-dihydroxyl groups with the hydroxyl groups being at positions C-12 and C-13, the corresponding substructure in compound 23 bore meta-dihydroxyl groups, in which the hydroxyl groups were bonded to positions C-12 and C-14 instead. It is interesting to note that compound 23 featured a new acylphenol as one of its monomers that is yet to be identified in the plant kingdom, while its other monomer was compound 3.
Compounds 20-23 were all non-symmetrical in nature. Their monomers (acylphenols) were linked together via a carbon-carbon linkage between the aromatic rings in each monomer.

Type 2 Dimeric Acylphenols
Compounds 24 and 25 were structurally related to each other and were constructed from the following acylphenols: compounds 2 and 3. Compound 24 was derived from compound 3, while compound 25 was derived from compounds 2 and 3.
Compounds 24 and 25 were non-symmetrical in nature. The monomers (acylphenols), which constituted each dimeric acylphenol, were linked together via a carbon-carbon linkage between an aromatic ring of one monomer and the n-octyl chain of the other monomer.

Pharmacological Activities of Myristica spp.
The acylphenols and dimeric acylphenols from this genus have been shown to have a broad spectrum of pharmacological activities, which are summarized in Tables 3-13 and will be discussed in the following sections.  (Table 3). They subsequently concluded that the number of hydroxyl groups present in ring b in the acylphenols and ring b' in the dimeric acylphenols and the position of the hydroxyl groups in ring b in the dimeric acylphenols could have contributed to the differences in their scavenging potentials (Figures 1 and 2). Othman et al. further postulated that dimerization could have increased the scavenging capacity of compound 20 in comparison to its monomer compound 3. Compound 20 possesses double the number of hydroxyl groups as compared to compound 3. Therefore, with a larger conjugated system, the electron withdrawing effects in compound 20 could have led to the easy oxidation of the hydroxyl groups.
Li et al. (2020) revealed that compound 3 was a stronger DPPH, ABTS, and hydroxyl radical scavenger compared to compound 2 (Table 3). Compound 3 was also found to be, respectively, 4.1 folds and 2.0 folds more effective in scavenging the DPPH and ABTS radicals compared to BHT. As for its ability to scavenge the hydroxyl radical, compound 3 was found to be comparable to that of BHT (Table 3). The potency of compound 3 compared to that of compound 2 may have been attributed to the presence of the two hydroxyl groups in its ring b, unlike the latter, which only bore a single hydroxyl group in its corresponding substructure ( Figure 1). Sathya et al. (2020) found compound 3 to have a comparable free radical scavenging potential as ascorbic acid (Table 3).    Table 4). The dose dependent treatment of compound 3 improved the histopathological appearance of indomethacin-induced mucosal lesions and gastric inflammation in male Swiss albino mice. At a dosage of 10 mg/kg, compound 3 decreased gastric inflammation following the exposure to indomethacin more effectively compared to omeprazole (3 mg/kg), after three days of treatment (Table 4). By decreasing oxidative and nitrative stress, treatment with compound 3 prevented NSAID-induced mitochondrial dysfunction and cell death; nuclear factor j-light-chain enhancer of activated B cell induction; the release of proinflammatory cytokines and neutrophil infiltration; and disruptions in the vascular endothelial growth factor/endostatin balance that contributed to mucosal auto healing. Importantly, compound 3 failed to impact the therapeutic anti-inflammatory properties of multiple NSAIDs in a model of acute inflammation. In all assays tested, compound 3 proved as or more efficacious than the current first-line therapy for NSAID-dependent GI complications, the proton pump inhibitor omeprazole.

In vivo imiquimod-induced psoriasis-like skin lesion
Topical application with 2 mM of compound 3 significantly ameliorated hyperplasia and inflammatory cell infiltration.  Compound 3 decreases gastric inflammation following indomethacin exposure with improved efficacy over omeprazole.
Compound 3 protects against oxidative stress induced by indomethacin in the stomach. Compound 3 ameliorates indomethacin-induced cell death and inflammation by decreasing oxidative and nitrative stress.
Indomethacin-dependent increases in endostatin and decreases in VEGF levels are reversed by compound 3.
Compound 3 improves stomach ulceration following indomethacin exposure without impacting the anti-inflammatory properties of the drug. [53] Recently, Tsukayama et al. (2022) demonstrated that compound 3 exhibited potent (IC 50 = 0.2 µM) competitive inhibition of the 5-LOX enzyme (Table 4). A concentration between 0-0.5 µM of compound 3 was also found to dose dependently reduce the production of LTB 4 in the RBL-2H3 cells without having any effect on the cell viability. LTB 4 is derived from arachidonic acid via the LOX pathway and is an important proinflammatory mediator. Studies have demonstrated that elevated levels of LTB 4 are linked to the pathogenesis of several inflammatory diseases [58]. Tsukayama et al., subsequently, investigated the effects of compound 3 in ameliorating the imiquimod-induced skin inflammation in BALB/c mice (Table 4). Their study revealed that the topical application of compound 3 (2 mM) significantly ameliorated hyperplasia and inflammatory cell infiltration and suppressed the expression of the psoriasis-associated genes S100a9, Krt1, Il17a, and Il22. Lipid metabolome analysis of these psoriasis-like skin lesions showed that compound 3 markedly decreased the level of LTB 4 but did not significantly increase the levels of other pro-inflammatory lipid mediators.

Antiproliferative and Cytotoxic Activities
Tyagi et al. (2014a) reported the cytotoxicity of compound 3, isolated from the methanol extract of the fruit rinds of M. malabarica, using the photometric enzyme immunoassay to quantify the formation of cytoplasmic histones associated DNA fragments (mono and oligosomes) after the apoptotic cell death. Compound 3 was found to dose dependently inhibit the growth of the MCF-7 cells more effectively compared to curcumin after 48 h of incubation ( Table 5). The pro-apoptotic mechanism of compound 3 with the MCF-7 cells involved the deregulation of multiple targets associated with the mitochondria, leading to the activation of different enzymatic cascades.
Later  (Table 5). Compound 3 activated the ATM-CHK1-p38 MAPK cascade to cause mitochondrial cell death in the lung carcinoma cells. Wu et al. (2014) investigated the cytotoxic effects of acylphenols on the human gastric cancer cell lines (NCIN87 and MGC803) using the MTT assay (Table 5). Compounds 2 and 3 were both able to inhibit the cytotoxic activities of the NCIN87 and MGC803 cell lines. However, compound 2 was more cytotoxic compared to compound 3, and it exhibited a cytotoxic activity almost similar to that of vinorelbine. The molecular mechanism of the cytotoxicity of these acylphenols on these human gastric cancer cell lines, however, is unclear. Manna et al. (2015a) screened the cytotoxic potential of compound 1 against leukemic cell lines (MOLT3, K562 and HL-60) and compared its activity against solid tumor cell lines (MCF7, A549 and HepG2) ( Table 5). Compound 1 demonstrated higher cytotoxicity against all three leukemic cell lines than in the solid tumor cell lines. With regard to the leukemic cell lines, the IC 50 values of compound 1 ranged from 12.70 ± 0.46 µg/mL to 18.10 ± 0.95 µg/mL, whereas for the solid tumor cell lines, the IC 50 values were higher and ranged from 28.10 ± 0.58 µg/mL to 55.26 ± 5.90 µg/mL ( Table 5). The higher degree of cytotoxicity against MOLT3 was due to a higher induction of redox imbalance, evident from an increased generation of ROS and a concomitant depletion of thiols. This was confirmed by pre-incubation with NAC and BSO, wherein NAC decreased compound-1-induced cytotoxicity by 2.04-folds, while BSO enhanced the cytotoxicity and decreased the IC 50 value by 5.60-folds.     Thiol antioxidants modulate pro-survival signaling in compound-3-induced death process.
NAC enhances S-glutathionylation of p65 and p53 proteins in response to treatment with compound 3.  In the same year, Manna et al. (2015b) also evaluated the cytotoxic potential of compound 1 against the T-lymphoblastic leukemic cell line, CCRF CEM, and its multidrug resistance (MDR) counterpart CEM/ADR5000 ( Table 5). The cytotoxicity of compound 1 was 1.8-folds higher against the CEM/ADR5000 cell line than against the CCRF CEM cell line, suggesting that compound 1 demonstrated "collateral sensitivity". This cytotoxicity of compound 1 was attributed to an enhanced generation of oxidative stress, as the IC 50 value increased following the addition of an antioxidant N-acetyl cysteine (NAC). Furthermore, compound 1 depleted glutathione and inhibited the activity of glutathione peroxidase, which also contributed to the generation of a redox imbalance.

In vivo tumor growth in mice
The following year, Manna et al. (2016) reported on the cytotoxic activity of compound 1 against two hematopoietic cell lines U937 and MOLT3 (Table 5). Manna and his coworkers demonstrated that the IC 50 values of compound 1 against both cell lines were 15.38 ± 1.91 µg/mL and 17.42 ± 0.47 µg/mL, respectively (Table 5). Compound 1 enhanced the phosphorylation of the components of the pro-apoptotic pathway, namely ASK1, p38, and JNK while decreasing the phosphorylation of AKT and mTOR. The cytotoxicity of compound 1 was attenuated by the inhibitors of p38 and JNK, whereas it was enhanced in the presence of a PI3K/AKT inhibitor.  Table 5). The results from their MTT assay revealed that only compounds 1, 20, and 24 were active against the PC3 cell lines after 48 h of treatment (Table 5). Othman and his coworkers concluded that the increase in the number of hydroxyl groups in ring b in the acylphenols could have decreased their cytotoxic potential (Figure 1). Othman et al. deduced that the potency of compound 20 compared to compound 23 could have been attributed to the presence of the ortho-dihydroxyl groups in the ring b of the former in contrast to the latter, whose ring b bore meta-dihydroxyl groups instead (Figure 2). Othman et al. further postulated that the stronger cytotoxicity of compound 24 compared to that of compound 25 may have resulted from the presence of the two hydroxyl groups in its ring b' unlike the latter which only bore a single hydroxyl group in its ring b (Figure 2). Megawati et al. (2017) assessed the cytotoxic activity of compounds 2 and 3 isolated from M. fatua against the breast carcinoma cancer cell line MCF-7, using the Alamar blue assay. Both compounds exhibited strong to potent cytotoxicity activity with IC 50 values of 0.71 µg/mL and 2.38 µg/mL, respectively (Table 5) Table 5). Compound 2 showed selective toxicity towards the A549, A375, and Jurkat cells without showing toxicity towards the INT407, HEK293, and WI-38 cells. Among the tested cell lines, compound 2 was found to exhibit the strongest cytotoxic activity against the A549 cell line, following which it was found to be 3.2 folds more potent than curcumin (Table 5). Compound-2-induced apoptosis was mediated by an increase in the intracellular reactive oxygen species (ROS), as a result of the cell-permeable antioxidants, N-acetylcysteine (NAC), and PEG-SOD, which strongly inhibited its cytotoxicity towards the A549 cells. Compound 2 increased the BAX level while simultaneously decreasing the BCL-2 and BCL-XL levels in the A549 cells, in turn, triggering the mitochondrial apoptotic pathway as revealed from the release of cytochrome c and the activation of caspase-9 and caspase-3. Pre-treatment of the A549 cells with caspase-9, caspase-3, and pan-caspase inhibitors made them more resistant to the treatment with compound 2. This effect of compound 2 was strongly associated with the concomitant decrease in anti-apoptotic (IAP1, IAP2, and survivin), angiogenic (growth factors), and cancer invasiveness (matrix metalloproteinase-9, COX-2) modulating proteins. Compound-2-induced cytotoxicity was unaffected by the shRNA-mediated depletion of p53 in the A549 cells.
Subsequently, Tyagi et al. (2020) demonstrated that compound 3 with N-acetyl cysteine (NAC) was a promising therapeutic regimen for lung cancer treatment in vitro and in vivo (Table 5) (Table 5). Compounds 1-4 showed promising cytotoxic activity against the MCF-7 and MD-AMB -231 cell lines but showed moderate toxicity against the normal cells. Their IC 50 values were comparable to or better than the standard Doxorubicin.
Recently, Bauri et al. (2022) conducted an in vitro assay to determine the cytotoxic potential of the acylphenols (compounds 1-4 and 7-9) from M. malabarica against the human ovarian cancer cell lines A2780. All six compounds exhibited moderate anti-proliferative activity as compared to paclitaxel. It is noteworthy to mention that compounds 1, 3, 7, 8, and 9 possessed almost similar cytotoxicity (Table 5) (Table 5). p62, the selective autophagy substrate, simultaneously increased, thus indicating that compounds 15 and 16 could have inhibited the HEK293-GFP-LC3 cell autophagy as effectively as bafilomycin A1. Park et al. (2017) reported the antibacterial activity of acylphenols through an S. pneumoniae sialidases inhibition assay (Table 6). Compounds 2 and 3, respectively, showed strong pneumococcal sialidases inhibition for NanA (IC 50 = 0.4 µM and 0.3 µM), NanB (IC 50 = 5.7 µM and 3.6 µM), and NanC (IC 50 = 14.3 µM and 2.9 µM). While compounds 2 and 3 inhibited the activity of NanA by the competitive inhibition mechanism, they, however, were noncompetitive inhibitors of NanB and NanC (Table 6). Since the inhibitory activities of compounds 2 and 3 against NanA and NanB were more potent compared to DANA (Neu5Ac2en), these results therefore suggested that both acylphenols could be potential agents for combating S. pneumoniae infection. Table 6. The antibacterial and anti-quorum sensing activities of acylphenols and dimeric acylphenols from the genus Myristica.  Recently, Rajimol et al. (2022) demonstrated that compound 3 was an excellent antimicrobial agent in smart coating, showing 99.99% efficiency, and the coating exhibited activity against both gram-positive (S. aureus) and negative (P. aeruginosa) bacteria (Table 6).

Antibacterial and Anti-Quorum Sensing Activities
Sivasothy et al. (2016a) examined the quorum sensing inhibitory activity of acylphenols and dimeric acylphenols ( Table 6). Compound 20 was identified as a potential antiquorum sensing agent. Increasing the concentration of compound 20 was found to significantly inhibit the bioluminescence produced by both E. coli (pSB401) and E. coli (pSB1075), respectively (Table 6). Therefore, compound 20 has the potential to prohibit bacterial pathogenicity.

Antidiabetic and Antiglycation Activities
Sivasothy et al. (2016b) reported compound 21 to be a potent mixed-type α-glucosidase enzyme inhibitor with a 3-folds higher affinity towards the free enzymes (Table 7). Although the ability of compound 11 to inhibit the activity of the α-glucosidase enzyme was not as significant as that of compound 21, nevertheless, the inhibitory potential of the former was four times greater compared to acarbose (Table 7). Sivasothy et al. (2022) later identified compounds 1, 2, 3, 5, and 20 to be more effective in inhibiting the activity of the α-glucosidase enzyme compared to acarbose. Compounds 3 and 5 were stronger α-glucosidase enzyme inhibitors than compounds 1 and 2 (Table 7). Sivasothy et al. (2022) deduced that the higher number of hydroxyl groups in the structure of the former two acylphenols could have resulted in its lower IC 50 values compared to the latter two acylphenols (Figure 1). Sivasothy et al. (2022) further postulated that the inhibiting potential of compound 20, which was almost double that of compound 3, could have resulted from the dimeric acylphenol possessing double the number of hydroxyl groups compared to its monomeric acylphenol (Figures 1 and 2).
Based on the findings by Prabha et al. (2018), compounds 2 and 3 were determined to be more effective α-glucosidase enzyme inhibitors compared to acarbose (Table 7). Though compounds 2 and 3 exhibited a slightly weaker potential in inhibiting the activity of the α-amylase enzyme compared to acarbose, their IC 50 values were comparable to that of acarbose. Compound 1 on the other hand can only be considered a moderate α-amylase enzyme inhibitor compared to acarbose (Table 7). When the α-glucosidase and α-amylase enzyme inhibitory activities of compounds 1-3 and 6 were compared to those of compounds 9 and 10, the weaker activities of compounds 9 and 10 enabled the researchers to deduce that the presence of two aromatic rings with hydroxyl groups bonded to them was a prerequisite for the respective activities ( Figure 1). Prabha et al. (2018) also concluded that the presence of a methoxyl group at position C-21 in compound 6 could have decreased its α-amylase enzyme inhibitory activity compared to compound 1, whose corresponding position bore a hydroxyl group instead (Figure 1). According to Prabha et al. (2018), compounds 1-3, 6, and 9 also displayed antiglycation properties (Table 7). These five acylphenols effectively inhibited the formation of AGEs compared to ascorbic acid. Compounds 1-3 were 2.5 to 8 folds more potent in inhibiting the formation of AGEs compared to compounds 6 and 9. Though all of the compounds at concentrations of 10, 25, and 50 µM exhibited moderate to good glucose uptake in a dose dependent manner, only compound 2 demonstrated a significantly better stimulation of glucose uptake as compared to metformin under identical experimental conditions (Table 7). Prabha et al. (2021) revealed compound 7 to possess an α-glucosidase enzyme inhibitory activity which was 2.5 folds stronger compared to its potential in inhibiting the activity of the α-amylase enzyme. When compared to acarbose, compound 7 can only be regarded as a strong α-glucosidase enzyme inhibitor (Table 7). Prabha et al. (2021) also considered compound 7 to possess moderate antiglycation effects compared to ascorbic acid and a greater potential compared to metformin in increasing glucose uptake in cells under identical operating conditions. Furthermore, compound 7 was also able to enhance the translocation and expression of GLUT4 and upregulate the AMPK pathway (Table 7).  According to Li et al. (2020), compound 3 inhibited the activity of the α-glucosidase enzyme in a dose dependent manner with an IC 50 value of 20.97 ± 0.17 µg/mL ( Table 7). The inhibition of the enzyme increased from around 15% to around 90% when the concentration of compound 3 was increased from 0.6 µg/mL to 2.2 µg/mL.  (Table 8). On the other hand, Sathya et al. (2020) described that compound 3 was found to show potent AChE inhibitory activity with an IC 50 value of 2.06 ± 0.04 µg/mL (Table 8). Abdul  reported that the acylphenols (1-3 and 5) and the dimeric acylphenols (24 and 25) isolated and characterized from the fruits of M. cinnamomea had the potential to treat Alzheimer's Disease via the inhibition of the activities of the cholinesterase enzymes; AChE and BChE (Table 8). Compounds 2 and 3 were identified as strong mixed mode dual inhibitors, with almost equal AChE and BChE inhibiting potentials. Compound 5, though it was also identified as a dual inhibitor, was only moderate in its ability to inhibit the activities of both the enzymes. Compound 1, on the other hand, was an AChE selective inhibitor. As for compounds 24 and 25, they were moderate AChE and BChE inhibitors ( Table 8). The researchers concluded that the AChE inhibiting potential of compounds 1-3 may have decreased with the increase in the number of hydroxyl groups in their ring b (Figure 1). The lower AChE inhibiting potential of compound 5 upon comparison to compound 2 could have resulted from the additional hydroxyl group in its ring a (Figure 1). With regard to the BChE inhibitory activity, compound 1 showed the weakest activity, which could have been due to the absence of hydroxyl groups in its ring b compared to compounds 2, 3, and 5, which bore one or two hydroxyl groups in their ring b (Figure 1). The researchers further deduced that dimerization, which in turn resulted in the bulkiness of compounds 24 and 25, could have contributed to the decrease in their activities compared to their monomers (compounds 2 and 3) (Figures 1 and 2). Rastegari et al. (2022) in their investigation revealed that compounds 1 and 3 had the potential in inhibiting the activities of the AChE and BChE. While compound 3 was identified as a non-competitive dual AChE and BChE inhibitor with almost equal enzyme-inhibiting strength, compound 1 was a more effective inhibitor of the activity of the BChE compared to the activity of the AChE (Table 8).

Anti-Allergic Activity
According to Morikawa et al. (2018), compound 3 inhibited the release of β-hexosaminidase, a biomarker of the anti-allergic activity through antigen-IgE-stimulated degranulation in rat basophilic leukemia cells (RBL-2H3) ( Table 9). The inhibitory activity of compound 3 was greater than that of the antiallergic medicines: tranilast and ketotifen fumalate. Compound 3 also inhibited the production of the antigen-stimulated tumor necrosis factor-α, an important process in the late phase of type I allergic reactions (Table 9). Table 9. The anti-allergic activity of acylphenols from the genus Myristica.

Anti-Anxiety Activity
Modulatory effects in the cannabinoid system through the inhibition of the activities of the fatty acid amide hydrolase (FAAH) and the monoacylglycerol lipase (MAGL) enzymes may serve as therapeutic potentials in treating disorders such as mood and anxiety [11]. El-Alfy et al. (2019) demonstrated that compound 3 was only able to inhibit the activity of the FAAH enzyme, though not as effectively as JZL 195 (Table 10). 4.2.9. Antihypertensive Activity Rathee et al. (2016) reported that the chronic oral administration of compound 3, a natural antioxidant, could reduce blood pressure (BP) and attenuate cardiovascular remodeling in deoxycorticosterone acetate (DOCA)-salt hypertensive rats (Table 11). DOCA-salt hypertensive rats had significantly high systolic BP (SBP), which was related to organ hypertrophy, collagen depositions, inflammatory infiltrations in cardiac and aortic sections, reduction in plasma, total antioxidant status and NO level, and increase in TBARS, PGI2, and vasoconstrictors (AVP, Big ET, and ET-1). DOCA-salt also decreased vascular relaxation caused by smooth muscles and endothelium in rats. Compound 3 reversed all these changes in the DOCA-salt hypertensive rats and improved their vascular reactivity. Compound 3 was found to exert anti-hypertensive properties in DOCA-salt hypertensive rats by reducing oxidative stress and organ hypertrophy and by improving endothelial and vascular functions (Table 11). Sphingomyelin synthase (SMS) is a membrane protein family that includes two isoforms: sphingomyelin synthase 1 (SMS1) and sphingomyelin synthase 2 (SMS2) [62]. The increase in SM and DAG which is produced by the SMSs will lead to obesity and insulin resistance [63,64]. Othman et al. (2019) reported that compounds 1-3 and 5 exhibited strong inhibitory activities against SMS 1 and SMS 2 (Table 12). Compound 3 was highly efficacious in preventing oleic acid uptake across the membrane, by reducing lipid droplet formation in vitro. The potential of compound 3 against diet-induced obesity and lipid metabolism in vivo was also investigated. Compound 3 was found to reduce body weight gain, improve glucose tolerance, and decrease lipid accumulation in the liver in vivo.  Compound 3 was highly efficacious in preventing oleic acid uptake across the membrane, which, in turn, reduced lipid droplet formation in vitro.
Compound 3 was able to reduce body weight gain, improve glucose tolerance, and decrease lipid accumulation in the liver in vivo.
[ 65] 4.2.11. Anti-Dengue Activity According to Sivasothy et al. (2021), compounds 3 and 5 demonstrated antiviral activity against the DENV-2 NS2B/NS3 protease (Table 13). Compounds 3 and 5 both inhibited the activity of the DENV-2 NS2B/NS3 protease, with the latter being the stronger inhibitor among the two and around 1.4 times more potent than that of quercetin itself.

Conclusions
The results from this review were summarized and synthesized from a pool of 35 individual studies that investigated the acylphenol and dimeric acylphenol composition present in the fruits, leaves, and bark of eight different species within the genus Myristica, along with their in vitro and in vivo pharmacological properties. This review provides a comprehensive overview of the existing research from 2013 to 2022. Thus, the results summarized within this review provide a broad up to date representation of the pharmacological activities of this class of secondary metabolites. The review, therefore, provides scientific evidence that the medicinal properties of the genus Myristica could have been attributed to the bioactive acylphenols and dimeric acylphenols present in various parts of its plants. Furthermore, this review also highlights the potential for the development of the acylphenols and dimeric acylphenols from the genus Myristica as pharmaceutical products. Nevertheless, additional research on their pharmacokinetics, metabolism, side effects, and toxicity is needed to show the effectiveness and safety of these compounds.

Conflicts of Interest:
The authors declare no conflict of interest.