Recent Advances in the Chemistry and Pharmacology of Cryptolepine
193
the phosphorylation of ATM/ATR, BRCA1, Chk1/Chk2, and γH2AX, activation of
the p53 signaling cascade, including enhanced protein expression of p16 and p21,
and down-regulation of cyclin-dependent kinases, cyclin D1, cyclin A, cyclin E and
other proteins involved in cell division (e.g. Cdc25a and Cdc25b), producing cell
cycle arrest at the S-phase.
9.2 Antineoplastic Agents: In Vitro Studies
Given the early potential demonstrated by 1 as a lead molecule for developing an anticancer agent, there are several reports of structural modifications that have improved
upon the antineoplastic activity of 1, while also giving further insight into its mechanism of action. As reviewed in 2008, although 1 itself has only weak affinity for
telomeric G-quadruplex structures in DNA, structural modifications have been made
that have enhanced the affinity of these structures [1].
To improve the binding selectivity of 1 for G-quadruplex structures over duplex
DNA, building on findings that a combination of DNA-binding scaffolds with specific
peptide sequences enhances selectivity [63], a series of peptidyl-benzofuroquinoline
conjugates were developed, with modification at the C-11 position [64]. Following
confirmation of G-quadruplex binding activity, 47 (Fig. 12) was evaluated by
exposing human HL60 leukemia cells to sub-toxic levels for a prolonged time,
resulting in induced cell senescence through telomerase inhibition due to binding
to telomeric G-quadruplex DNA [64]. Another research group also investigated
modifying the C-11 position, synthesizing a series of C-11 alkyl, cycloalkyl, and
arylamine derivatives [65]. Representative molecules from each series were selected
and screened in vitro by the U.S. National Cancer Institute in their 60 human
tumor cell line panel. All three compound types demonstrated good activity in
the panel (IC 50 values in the range 0.32–0.78 μM), but when these compounds
were further evaluated in the in vivo hollow fiber assay against melanoma, breast,
colon, ovarian, lung and CNS cancer cell lines, the only compound to demonstrate
growth inhibitory activity similar to standard anticancer agents was one with a linear
alkylamine side chain (48). A clear mechanism of action was not seen through
COMPARE analysis applied, with compound 48 not showing as strong a selective targeting of G-quadruplexes as the less active NSC748394. Following on from
this work, further modifications were made based on molecular modeling studies of
the binding site of ligands to G-quadruplex DNA, which improved the selectivity for
G-quadruplex over duplex DNA, and showed high differential cytotoxicity in vitro
in the HCT-116 colon adenocarcinoma cell line versus primary rat hepatocytes [66].
The two best compounds, 49 and 50, were trisubstituted indolo[3,2-b]quinolines with
7-(aminoalkyl)carboxylate side chains.
Shen et al. in 2013 demonstrated that a derivative of 1, N
-(7-fluoro-5-N-methyl10H-indolo[3,2-b]quinolin-5-ium)-N,N-dimethylpropane-1,3-diamine iodide (51),
suppressed transcription of the c-kit gene, and expression of the c-kit protein in
the human K562 chronic myelocytic leukemia cell line [67]. The gene that encodes
193
the phosphorylation of ATM/ATR, BRCA1, Chk1/Chk2, and γH2AX, activation of
the p53 signaling cascade, including enhanced protein expression of p16 and p21,
and down-regulation of cyclin-dependent kinases, cyclin D1, cyclin A, cyclin E and
other proteins involved in cell division (e.g. Cdc25a and Cdc25b), producing cell
cycle arrest at the S-phase.
9.2 Antineoplastic Agents: In Vitro Studies
Given the early potential demonstrated by 1 as a lead molecule for developing an anticancer agent, there are several reports of structural modifications that have improved
upon the antineoplastic activity of 1, while also giving further insight into its mechanism of action. As reviewed in 2008, although 1 itself has only weak affinity for
telomeric G-quadruplex structures in DNA, structural modifications have been made
that have enhanced the affinity of these structures [1].
To improve the binding selectivity of 1 for G-quadruplex structures over duplex
DNA, building on findings that a combination of DNA-binding scaffolds with specific
peptide sequences enhances selectivity [63], a series of peptidyl-benzofuroquinoline
conjugates were developed, with modification at the C-11 position [64]. Following
confirmation of G-quadruplex binding activity, 47 (Fig. 12) was evaluated by
exposing human HL60 leukemia cells to sub-toxic levels for a prolonged time,
resulting in induced cell senescence through telomerase inhibition due to binding
to telomeric G-quadruplex DNA [64]. Another research group also investigated
modifying the C-11 position, synthesizing a series of C-11 alkyl, cycloalkyl, and
arylamine derivatives [65]. Representative molecules from each series were selected
and screened in vitro by the U.S. National Cancer Institute in their 60 human
tumor cell line panel. All three compound types demonstrated good activity in
the panel (IC 50 values in the range 0.32–0.78 μM), but when these compounds
were further evaluated in the in vivo hollow fiber assay against melanoma, breast,
colon, ovarian, lung and CNS cancer cell lines, the only compound to demonstrate
growth inhibitory activity similar to standard anticancer agents was one with a linear
alkylamine side chain (48). A clear mechanism of action was not seen through
COMPARE analysis applied, with compound 48 not showing as strong a selective targeting of G-quadruplexes as the less active NSC748394. Following on from
this work, further modifications were made based on molecular modeling studies of
the binding site of ligands to G-quadruplex DNA, which improved the selectivity for
G-quadruplex over duplex DNA, and showed high differential cytotoxicity in vitro
in the HCT-116 colon adenocarcinoma cell line versus primary rat hepatocytes [66].
The two best compounds, 49 and 50, were trisubstituted indolo[3,2-b]quinolines with
7-(aminoalkyl)carboxylate side chains.
Shen et al. in 2013 demonstrated that a derivative of 1, N
-(7-fluoro-5-N-methyl10H-indolo[3,2-b]quinolin-5-ium)-N,N-dimethylpropane-1,3-diamine iodide (51),
suppressed transcription of the c-kit gene, and expression of the c-kit protein in
the human K562 chronic myelocytic leukemia cell line [67]. The gene that encodes