3.1. Anti-leukemic activity
In 2007, gypsogenic acid, the 4-carboxyl analogue of gypsogenin
(Figure 2), didn't show observable activity against chronic myeloid leukemia (K562) and acute myeloid leukemia (HL-60) where its IC
50 exceeded 100 µM for both cells [
80]. Another study was in accordance with this where gypsogenic acid IC
50 against K562 was 227.6 µM, however HL-60 was more sensitive (IC
50 61.1 µM) [
81]. The latter value is in discrepancy with the previous report by Lee group [
80]. Gypsogenic acid demonstrated low activity against other lymphoid leukemias SKW-3 and BV-173 (IC
50 79.1 and 41.4 µM, respectively) [
81].
Later on, we found that gypsogenin highly outperforms gypsogenic acid with IC
50 12.7 µM against K562 highlighting the crucial role of 4-aldehyde group [
79]. Simultaneously, Emirdag
et al., revealed that gypsogenin has anti-proliferative effect on HL-60 (IC
50 10.4 µM) by inducing apoptosis [
74,
82]
. Acetylation of its 3-OH group by acetic anhydride in presence of pyridine afforded
1b with no significant impact on activity against HL-60 (
1b IC
50 10.77 µM). Gypsogenin activity is increased by oximation of its aldehyde group (compound
1a IC
50 3.9 µM). Mutually, the 3-acetylated oxime analogue
1d surpassed the activity of
1b (IC
50 5.9 µM) [
74]. Gypsogenin benzyl ester
1c has IC
50 8.1 µM, however
1c acetylation product
1h has IC
50 6.7 µM [
74]
(Figure 3).
By virtue of its notable apoptotic effect,
1c was further benchmarked for its effect on K562 cell line where it showed moderate activity (IC
50 9.3 µM) [
83]. However, this study represented a turning point for a better understanding of gypsogenin’s molecular target.
1c inhibited ABL1 tyrosine kinase with IC
50 8.71 µM. This is assumed to be the main target for its cytotoxic effect on K562. It is needless to say that the presence of other off targets cannot be excluded. Concomitantly,
1c inhibited other kinases such as C-terminal Src kinase (CSK) and Lyn kinase isoform B; LYN B (IC
50 1.5 µM and 2.9 µM, respectively) [
83]. It is clear that oximation of
1c is detrimental for its activity on both K562 and HL-60 as the respective IC
50 value of
1f is 21.3 µM and 10.6 µM [
83].
Ciftci
et al. moved forward with a structure activity relationship study of 1c and succeeded in enhancing its activity [
79]. As mentioned above, the free aldehyde group is crucial for activity against leukemia. Therefore, Ciftci
et al. came up with phenyl substituted esters of 1c keeping a free 4-aldehyde group [
79]. Compounds GP2 and GP5 have IC
50 4.7 and 3.1 µM, respectively, against K562 cells. Additionally, IC
50 of GP2 and GP5 for ABL1 tyrosine kinase was 7.1 µM and 6.1 µM, respectively. Both compounds have induced an explicit apoptosis effect, especially GP2 whose apoptosis induction was clearer than imatinib; a gold standard ABL1 kinase inhibitor for CML therapy. Concomitantly, GP2 suppressed the downstream signaling of extracellular signal-regulated kinase (ERK) phosphorylation [
79]. In a similar vein, both compounds exhibited moderate activity on MT-2 and Jurkat cells. Of interest, the IC
50 of GP5 for MT-2 and Jurkat was 7.2 µM and 4.8 µM, respectively. The authors evaluated both compounds for their effect on peripheral blood mononuclear cells (PBMC) and calculated the selectivity index as the ratio of the IC
50 between PBMC and K562 cells. The higher selectivity index of GP2, 11.0, than GP5, 8.0, reflects a more satisfactory safety profile of GP5.
A recent report by Ulusoy
et al. showed that reductive amination of the 4-aldehyde group with different aromatic and alicyclic amines has either reduction or complete abrogation of anti-K562 activity [
78]. The hit compound in this study,
6l, has IC
50 11.3 µM which is even less active than the parent compound, gypsogenin [
78]. Furthermore,
6l inhibited ABL1 kinase in a moderate fashion (IC
50 value of 13.0 µM). This is another evidence of the crucial role of the 4-aldehyde group for anti-K562 activity (
Figure 4). In addition,
6l has less effect on MT-2 and Jurkat than those of
GP-2 and
GP-5. Compound
6l has moderate effect on a panel of kinases at 30 µM of drug concentration specially BRK, BTK, LYN B, and SRC. Compound
6j with the more hydrophobic 4-isopropyl substitution exhibited less activity (IC
50 23.8 µM), whereas presence of a bulky
N-piperazinyl benzyl moiety abolished activity as for
6c (IC
50 > 100 µM). The activity is also abolished in presence of electron donating substitution as for
6i (IC
50 > 100 µM).
Surprisingly, there is no report linking between gypsogenin or gypsogenic acid carboxamides and leukemia till now. This is the same case for gypsogenin derivatives with modified ring C (
Figure 3). In a word, gypsogenin benzyl esters are the most active derivatives against K562 and HL-60 leukemias till now. The SAR pertaining to activity against K562 and HL-60 is afforded in
Figure 4.
3.2. Anti-breast cancer activity
Gypsogenin has moderate cytotoxic activity on MCF-7 (IC
50 9.0 µM) however its benzyl ester derivative 1c has IC
50 5.1 µM [
74]. Surprisingly, substituted benzyl esters such as GP2 and GP2 showed less activity than gypsogenin with respective IC
50 51.58 µM and 15.3 µM. Notably, the 3-acetyl analogues of gypsogenin and 1c, namely 1b and 1h, possess less activity (IC
50 20.5 µM and 65.1 µM, respectively). However, oximation of gypsogenin and 1c slightly improves their cytotoxic effect as seen for 1a and 1f. The exact mechanism of action is not elucidated [
74].
Wu
et al. found that gypsogenic acid has weak antiproliferative effect on MCF-7 (IC
50 26.8 µM) which also highlight the role of 4-aldehyde group. The authors highly enhanced gypsogenin and gypsogenic acid activity through mono-and bisamidation [
76]. Gypsogenin carboxamide with imidazole, compound
9, has IC
50 3.7 µM which is similar to gypsogenic acid mono-amide of C28 with pyrazole, compound
14c, with IC
50 3.8 µM. Gypsogenic acid bisamide of both C23 and C28, compound,
8f demonstrated a pronounced activity (IC
50 4.1 µM). The favorable safety profile of those carboxamides is shown through measuring their activity on human umbilical vein endothelial cells (HUVEC cells).
8f possesses the highest selectivity index (24.0) among the mentioned active compounds.
Another evidence of the efficiency of gypsogenin amides was disclosed this year by Sun
et al [
75]. Two strong amides namely, 5 and 8 possess IC
50 5.7 µM and 13.8 µM, respectively, towards MCF-7. They also synthesized compound 9a which is afforded by reductive amination using methylamine; its IC
50 is 11.3 µM which is more than gypsogenin (IC
50 9.0 µM). The selectivity index of 5, 8, and 9a exceeds 30 when related to their effect on HUVEC.
Ring C oxidized gypsogenin derivatives have been recently developed (
Figure 3) [
75]. The epoxide derivative (
2) has IC
50 26.6 µM on MCF-7. In parallel with this, the 11-keto derivative (
3) has a similar activity (IC
50 25.3 µM) implying that modification of this ring by oxidation reduces MCF-7 sensitivity. Conclusively, gypsogenin carboxamides are the best cytotoxic entities against MCF-7 when compared to other derivatives
(Figure 5).
3.3. Anti-lung cancer activity
Gypsogenin can inhibit the growth and metastasis of Lewis lung cancer through inhibition of tumor angiogenesis and induction of apoptosis [
84]. Different molecular targets were implicated in this mechanism. Gypsogenin downregulated mutant P53 and vascular endothelial growth factor (VEGF). It reduces the expression of Bcl-2 protein and raises Bax expression, promoting tumor apoptosis. The anti-proliferative effect of gypsogenin, 3-acetyl gypsogenin, (
1b), and 3-acetyl gypsogenic acid against A549 lung cancer cells is moderate (IC
50 19.6, 30.8, and 23.7 µM) [
68,
76]. Oximation of gypsogenin and
1b keeps the activity without significant change [
68]. 2,4-dinitrophenyl)hydrazono derivative of gypsogenin (
4) demonstrated strong cytotoxic effect on A549 cells (IC
50 3.1 µM) [
68]. In accordance, the amino product (
9a) exhibited stronger cytotoxic effect (IC
50 1.5 µM) [
75].
The two carboxamides
9 and
14c showed a bit higher activity than compound
4 (IC
50 2.5 and 2.8 µM, respectively) [
76]. Both compounds destroyed the cell membranes and increase their permeability, which led to the outflow of intracellular nucleic acid, but they weakly induce apoptosis and arrest the cell cycle [
76]. Another anti-lung cancer hit derivative is gypsogenic acid bisamidation product of (
8f) whose IC
50 is 2.0 µM. However, it is noteworthy that mono-amidation products
9 and
14c surpass (
8f) activity but with lower selectivity index for HUVEC.
Concomitantly, compounds 5 and 8 showed sub-micromolar effect on A549 (IC50 0.5 µM and 0.9 µM, respectively), induced both apoptosis through damaging the cell membrane and cell cycle arrest. Combining in silico and in vitro tools defined VEGF1 as gypsogenin target. Remarkably, compound 5 showed higher binding affinity to VEGF1 than the parent compound which is in accordance with the cytotoxicity results. Gypsogenin esters showed disappointing results as for GP2 whose IC50 exceeds 100 µM and GP5 which is less active than the parent compound (IC50 24.5 µM).
The epoxide analogue
(2) has almost same activity as the parent compound (IC
50.18.7 µM) whereas the 11-keto derivative
(3) has a slightly better activity (IC
50.13.5 µM)). In conclusion, gypsogenin carboxamides are the most active anti-proliferative entities against A549
(Figure 6).
3.4. Other anti-cancer activities
A batch of gypsogenin derivatives demonstrated other notable anti-cancer effects. In this regard, we will focus only on compounds with at least single digit micromolar IC
50. Compound
1a has a remarkable anti-proliferative activity against SaoS-2 cells (osteosarcoma) and HeLa cells (cervical cancer). Its 3-acetylated derivative (
1d) also has a similar effect on SaoS-2 but not on HeLa. It is noteworthy that gypsogenin has IC
50 7.8 against SaoS-2 which is better than
1a and
1d, and 3-acetyl gypsogenin
(1b). On the other hand,
1d is distinguished by its high activity against HT-29 cells (colorectal adenocarcinoma
) [74] (Table 1).
Other study showed that gypsogenin suppressed gastric cancer cells NCI-N87 proliferation via targeting VEGF and MM-9 and promoting the expression of caspase-3 and Bax proteins [
84]. Compounds
4 and
7g were reported mainly for targeting colon cancer cells (LOVO) through strong induction of apoptosis and dose-dependent S-phase arrest in cells pre-treated with either of them. Both compounds exhibited moderate effect on SKOV3 (ovarian cancer) and HepG2 cells (Hepatocellular carcinoma) [
68]. The amino compound
9a also exhibited notable activity against LOVO. Compounds 2 and 3 showed no or moderate activity towards LOVO [
75]. The most active compound against LOVO cells is compound
8 with submicromolar cytotoxicity implying that gypsogenin carboxamides are usually on the top among other derivatives [
75] (
Table 1).
Three amides were reported by Wu
et al.,
9,
14c, and
8f with outstanding activities against HepG2, TE-1 (esophageal cancer), and MC3-8 (colon cancer) cells [
76]. Gypsogenin 28-COOH ester
GP5 showed better activity on HeLa cells than
GP2 [79]. Ciftci
et al., revealed new derivatives that suppress glioma proliferation through EGFR inhibition. The amino derivative compound
10 has the strongest effect against EGFR and glioma cells U251, T98G, and U87, consequently (
Table 1). The titled compound clearly induced apoptosis in U251 in a comparable fashion to cisplatin. The study revealed that gypsogenin benzyl esters were less effective than
10 on glioma cells [
77]
(Table 1). Furthermore, at 30 µM concentration, compound
10 showed moderate inhibition for a panel of other kinases including ABL1 tyrosine kinase.