The bioactivities of the metabolites isolated from sample of genus Phorbas mainly focus on the antiproliferative activity. For clarity and better reading purposes, data have been summarized in Table 1.
The alkaloid zarzissine (
13) showed a potent cytotoxic activity against three cell lines: murine leukemia P-388, human nasopharyngeal carcinoma KB, and human lung carcinoma NSCLC-N6
[22].
Macrolides such as phorbasides A (
20), C (
22), D (
23), and E (
24) exert prominent cytotoxic effects against HCT-116 (human colon cancer cell line), demonstrated through in vitro assays. However, phorbaside B (
21) showed no activity. These results suggest that the presence of the free hydroxyl group at C-2 of the sugar moiety may play a key role in maintaining bioactivity
[25]. Muironolide A (
19) and phorboxazole A (
17) are two other representative macrolides that possess cytotoxic activity against colon tumor cells
[24].
Among steroids, phorbasterones A–D (
29–
32) displayed moderate cytotoxicity toward HCT-116 cells
[26]. More recently, the lipid fraction obtained from samples of
P. amaranthus, likely enriched of sterols, was found to possess antiproliferative properties against HCT-116 cells
[27].
The sesterterpenoids phorbaketals A–C (
45–
47) exhibited cytotoxic activity against human colorectal cancer HT-29, hepatoma cancer HepG2, and adenocarcinoma human alveolar basal epithelial cells lines A549, while phorbaketal N (
50) was cytotoxic against human renal cancer cell lines A498 and ACHN and pancreatic cancer cell line PANC-1. Phorbaketal N (
50) showed a better activity than the positive control molecule, fluorouracil. Studies on
50 and derivatives may be a path in the search for new treatments for pancreatic cancer
[28][30]. Phorbaketal H–I (
55–
56), isolated from the sponge
Monanchora sp., showed weak cytotoxicity against the human renal A498 cancer cell line. Considering structure−activity relationships, a ketone group at C-5 of ring A of phorbaketals is much more favorable than a hydroxy group for activity, and the hydroperoxy group in the side chain is harmful to the cytotoxicity
[29].
The compound phorbin A (
59), isolated from
Monanchora sp., a possible precursor of several sesterterpenoids isolated for
Phorbas, also showed moderate activity against renal human cancer cell lines ACHN and A498, and potent cytotoxicity against human pancreatic cancer cell lines PANC-1 and MIA-paca, similar to or better than the positive control, 5-fluorouracil
[29].
In addition, the sesterterpenoids isosuberitenone B (
84) and 19-suberitenone B (
85) unveiled significant grow-inhibitory effects against A549, HepG2, HT-29, and MCF-7 tumor cell lines. In the same study, compounds suberitenone B (
82), oxaspirosuberitenone (
83), and isooxaspirosuberitenone (
86) showed moderate activity against these same cell lines. These sesterterpenoids isolated from
P. areolatus were also tested against Mia-Paca-2 (pancreatic cancer cell line), but showed no activity
[31].
Putative anticancer lead compounds with a diterpenoid backbone were a) phorbasin B–C (
88–
89) and the terpenyl-taurine phorbasin E (
91), tested in a colon cancer model (HCT-116 cell line)
[32] and b) gagunins A–Q (
101–
117) in K-562 cells (leukemia cell line)
[33]. Among the latter, gagunins A and B (
101–
102) turned out to be the less active compounds. The authors suggest that the presence of a bulky group at C-11 of the five-membered ring negatively affects bioactivity, as compounds
107 and
108 are far less active than their congeners featuring either an acetoxyl group or hydrogen at the same position
[33]. However, a synthetic gagunin A-derivative, in which the substituent groups placed on the three rings were replaced by hydroxyl groups, lacks activity
[33].
The tetraterpenoid gukulenin B (
119) exhibited significant cytotoxicity against human pharynx cell carcinoma line FaDu, gastric carcinoma cell MKN45, colon carcinoma cell line HCT-116, and renal carcinoma cell SN12C, and gukulenins C−F (
120–
123) showed potent cytotoxicity against K-562 and A549 cells
[35]. Interestingly, gukulenin F (
123) exhibited cytotoxicity against K-562 that was 17-fold more potent than doxorubicin, a positive control
[35]. Moreover, gukulenin A (
118) was shown to be a promising antitumor agent that (a) inhibited tumor growth in an ovarian cancer xenograft mouse model without any considerable adverse effect on their body weights, and (b) markedly reduced cell viability through apoptosis induction via the activation of caspases in four ovarian cancer cell lines. The cytotoxic activity of gukulenin A (
118) is more potent than the positive control, cisplatin, in all ovarian cancer cells tested. This is the first report of an in vivo activity among compounds isolated from
Phorbas [36].
Although several compounds showed promising results, cytotoxic studies on compounds from the genus
Phorbas are, in most cases, at the initial phase. Only as recently as 2019 was there a study with gukulenin A (
118) that advanced to in vivo studies using mouse models
[36].
2.2. Other Biological Activities
Secondary metabolites isolated from sponges of the genus Phorbas displayed a large array of biological activities other than cytotoxicity (Table 2).
Anchinopeptolides B–D (
2–
4), peptide alkaloids from
P. tenacior, exhibited high efficacy in displacing specific ligands from their relevant receptors: human B2 bradykinin, which has a high correlation with inflammation mediators by causing vasodilation, increasing vascular permeability, and stimulating the synthesis of prostaglandins; neuropeptide Y, which is involved in physiological and homeostatic processes such as vasoconstriction and growth of fat tissue; and somatostatin receptors, which belong to the G protein class and have a wide expression pattern in both normal tissues and solid tumors
[38][56][27]. On the other hand, anchinopeptolide A (
1) was found to have weaker bioactivity in these binding assays
[37]. The alkaloids zarzissine (
13) and
p-hydroxybenzaldehyde (
14) showed slight antimicrobial activity against
Staphylococcus aureus (gram-positive bacterium) and
C. albicans and
C. tropicalis (yeasts)
[22].
The crude extract of
Phorbas topsenti was reported to have high antioxidant activity in oxygen radical absorbance capacity (ORAC) assay, thereby leading to the isolation of phorbatopsins A–C (
14–
16), i.e., the compounds responsible for the observed radical scavenging activity. The antioxidant capacity of the isolated compounds was also evaluated with ORAC assay, measuring the loss of fluorescence of fluorescein in the presence of the oxidative species AAPH [2,2′-azobis(2-amidino-propane) dihydrochloride] and compared with Trolox
®, used as the positive control. Phorbatopsin A (
14) was the most active substance, with an ORAC value comparable to Trolox®. These data clearly indicate the importance of the C5–C6 double bond in compound
14 in improving the antioxidant properties of the phorbatopsin scaffold
[39].
Macrolides phorboxazoles A–B (
17–
18) exhibited antifungal activity in the agar disc diffusion inhibition assay against
Candida albicans and
Saccharomyces carlsbergensis [23]. Another example is the macrolide muironolide A (
19), which was reported to have antifungal activity against strains of
Cryptococcus neoformans [57].
The genus
Phorbas is also a source of other bioactive compounds, such the steroids amaroxocanes A–B (
37–
38), which were isolated and tested for chemical defense of the Caribbean coral reef sponge
Phorbas amaranthus from fish predators. Amaroxocane B (
38) showed significant deterrent activity (3/10 pellets eaten), while amaroxocane A (
37) elicited little feeding deterrence (8/10 pellets eaten) against a common reef predator, namely the bluehead wrasse. This study suggests that structural differences in the heterocycle moiety or the degree of sulfation may be responsible for differential anti-predatory activity
[41].
Phorbaketal A (
45), which also showed cytotoxic activity, can promote osteogenic differentiation of human mesenchymal stem cells, which exhibited increased levels of differentiation markers such as osteocalcin, Dlx5, ALP, Runx2, and TAZ after drug exposure. This compound showed potential for bone reformation processes and new anabolic therapeutics in bone diseases. Moreover, as inhibiting mesenchymal stem cells differentiate into adipocytes through a transcriptional coactivator with PDZ-binding motif, compound
45 may be a promising lead in designing novel drugs to treat obesity. In addition, this compound showed a promising dose dependent inhibition of inflammatory mediators via down-regulation of the NF-κB pathway and up-regulation of the HO-1 pathway
[43][44][42]. The sesterterpenoids phorbasones A–B (
76–
77) promote calcium deposition in mensenchymal C3H10T1/2 cells, thus inducing osteoblast differentiation. The authors concluded that phorbasone A (
76) showed a distinct calcium deposition effect as compared to phorbasone B (
77). Particularly, gene expression analysis of osteoblast differentiation markers revealed that compound
76 increases Runx2 (a Runt protein), ALP (alkaline phosphatase), OSX (osterix), PTH (parathyroid hormone), and PTHrP (PTHrelated peptide) mRNA
[51]. Another study reported on the potent inhibitory activity on nitric oxide (NO) production in RAW 264.7 LPS-activated mouse macrophage cells by phorbasone A acetate (
79). This result indicated that effective suppression of NO production is a valuable strategy for the discovery of anti-inflammatory compounds
[51].
Among sesterterpenoids, suberitenones A and B, oxaspirosuberitenone, isosuberitenone B, 19-episuberitenone B, and isooxaspirosuberitenone (
81–
86), isolated from
Phorbas areolatus (non-polar fraction), were tested against gram positive (methicillin resistant and methicillin sensitive
Staphylococcus aureus, MRSA, and MSSA) and gram negative (
Escherichia coli, and
Klebsiella pneumoniae) bacteria. This study reported oxaspirosuberitenone (
83) as a significant antimicrobial compound against MRSA at the highest concentration tested
[31][58].
Ansellone A (
65) can activate cAMP signaling in HEK293 cells, derived from human embryonic kidney cells grown in a tissue culture, which is a very important technique for the development of treatments for several diseases such as heart failure, cancer, and neurodegenerative diseases. cAMP signaling activation by ansellone A (
65) was comparable to that of forskolin, a natural product used for the treatment of cancer, obesity, and allergies
[46]. The latency reversal activity (LRA) of
65, which has the function of reactivating the virus production in infected cells and producing an immune response or cell death, was also reported and determined by quantification of the changes in intracellular GFP expression in microplate
[59]. The sesterterpenoid ansellone B (
66) was reported as a potent inhibitor on nitric oxide production in RAW 264.7 LPS-activated mouse macrophage cells
[60].
Alotaketals A and B (
60–
61) have also been reported for the activation of the cAMP cell signaling pathway. In addition, the compounds alotaketal C (
62) and D (
63) and anvilone A (
74) were reported to activate the latent proviral HIV-1 gene expression. Notably, alotaketal C (
62) was more potent and gave a stronger effect than the control compound prostratin at the same concentration, while alotaketal D (
63) and anvilone A (
74) elicited similar responses as prostratin
[28][29][61][48]. The diterpen phorbasin H (
94) was reported as an inhibitor of the yeast-to-hypha transition in
Candida albicans. Growth experiments suggested that this compound does not inhibit yeast cell growth but inhibits filamentous growth in
C. albicans, which means that the phorbasin H (
94) induces a change in
C. albicans morphology
[52]. Another study reported the ethanolic extract rich in phorbasins (
87–
89) from the
Phorbas sp. could exert growth inhibitory activity against gram positive bacteria, such as
Staphylococcus aureus and
Micrococcus luteus. It was not possible to test pure compounds due to the low amount available and their instability
[62][53].
One study concerning the cosmetic use of gagunin D (
104) identified this compound as a potential anti-melanogenic agent. Gagunin D (
104) inhibited the synthesis of melanin in both mouse melan-a cells and a reconstructed human skin model. Suppression of tyrosinase expression and increased rate of tyrosinase degradation as well as inhibition of its enzymatic activity are putative mechanisms underlying the anti-melanogenic activity exhibited by gagunin D (
104). These studies highlight the potential use of gagunin D (
104) for skin lightening cosmetic formulations
[55].
The summary of these biological activities reported for compounds isolated from extracts of the genus Phorbas sp. are found in Table 2.