Recent Advances in the Chemistry and Pharmacology of Cryptolepine
197
Cryptolepine (1) was shown not to be a substrate for the related enzyme, xanthine
oxidase. Interestingly, 2-fluorocryptolepine was found to be a better substrate for
aldehyde oxidase than the parent compound but quindoline (16), 11-chloro-, 2,7dibromo- (43), and 2-methoxy analogs of cryptolepine were not readily oxidized.
These results suggest that 2-substitution may block the oxidation of 1 by aldehyde
oxidase and this may be relevant for the design of antiprotozoal analogs of 1, since
the oxidation product, 15/16, has been found to be inactive against malaria parasites
[72].
In vitro studies in which 1 was incubated with rat or human hepatocytes resulted in
the partial identification of nine hydroxylated, di-hydroxylated, and glucuronidated
metabolites [73]. Several metabolites were also identified in the urine and/or plasma
of rats following oral administration of 1. Oral absorption following a single 5 mg/kg
dose was rapid but bioavailability was low, with less than 1% of 1 excreted in the
urine unchanged. Following i.v. administration (1 mg/kg), the volume of distribution
and plasma clearance were high. These data are consistent with the relatively low
efficacy of 1 in animal models for malaria and trypanosomiasis (see above).
10.2 Toxic Effects
The effect of sub-toxic concentrations of 1 (IC 20 , 7.5 μM and IC 40 , 17 μM) on gene
expression in various strains of the budding yeast Saccharomyces cerevisiae has been
studied in an attempt to gain an insight into the mechanisms involved in the cytotoxic
and antiplasmodial effects of 1 [74]. Gene expression profiles of treated and untreated
erg6 yeast cells showed that 117 genes were significantly upregulated, while 232
were downregulated. General stress-related genes formed the only recognized cluster
among the upregulated genes, while analysis of the downregulated genes revealed
specific effects on genes related to iron transport, acid phosphatases and cell wall
components. It was concluded that cryptolepine (1) affects genes involved in iron
uptake in yeasts, especially those related to the siderophore-dependent pathway and
that, in general, the expression profile coincided with the requirement for Aft1p (a
gene involved in the adaptation of iron metabolism to changes in iron availability)
for the transcription of these genes [74]. Further studies will be required to determine
the relevance of these findings, if any, to the antiplasmodial and cytotoxic modes of
action of 1.
The DNA-intercalating properties of 1 have raised concerns that it may be genotoxic, and in earlier studies it was reported that incubation of V79 cells with 1 (2.5
μM, 24 h) increased micronuclei formation (6.4% compared to 0.4% in untreated
controls). This suggests that 1 may carry a genotoxic risk [75]. In contrast, in the
Comet assay, relatively high (100–200 μM) concentrations of 1 did not cause DNA
damage in human lymphocytes but significant increases in both Olive Tail moments
and % Tail DNA were observed at 300 μM [76]. The analog 2,7-dibromocryptolepine
(43) significantly increased DNA damage at 200 and 300 μM while 8-chloro-7nitrocryptolepine was significantly active at 100 and 200 μM but not at 300 μM.
197
Cryptolepine (1) was shown not to be a substrate for the related enzyme, xanthine
oxidase. Interestingly, 2-fluorocryptolepine was found to be a better substrate for
aldehyde oxidase than the parent compound but quindoline (16), 11-chloro-, 2,7dibromo- (43), and 2-methoxy analogs of cryptolepine were not readily oxidized.
These results suggest that 2-substitution may block the oxidation of 1 by aldehyde
oxidase and this may be relevant for the design of antiprotozoal analogs of 1, since
the oxidation product, 15/16, has been found to be inactive against malaria parasites
[72].
In vitro studies in which 1 was incubated with rat or human hepatocytes resulted in
the partial identification of nine hydroxylated, di-hydroxylated, and glucuronidated
metabolites [73]. Several metabolites were also identified in the urine and/or plasma
of rats following oral administration of 1. Oral absorption following a single 5 mg/kg
dose was rapid but bioavailability was low, with less than 1% of 1 excreted in the
urine unchanged. Following i.v. administration (1 mg/kg), the volume of distribution
and plasma clearance were high. These data are consistent with the relatively low
efficacy of 1 in animal models for malaria and trypanosomiasis (see above).
10.2 Toxic Effects
The effect of sub-toxic concentrations of 1 (IC 20 , 7.5 μM and IC 40 , 17 μM) on gene
expression in various strains of the budding yeast Saccharomyces cerevisiae has been
studied in an attempt to gain an insight into the mechanisms involved in the cytotoxic
and antiplasmodial effects of 1 [74]. Gene expression profiles of treated and untreated
erg6 yeast cells showed that 117 genes were significantly upregulated, while 232
were downregulated. General stress-related genes formed the only recognized cluster
among the upregulated genes, while analysis of the downregulated genes revealed
specific effects on genes related to iron transport, acid phosphatases and cell wall
components. It was concluded that cryptolepine (1) affects genes involved in iron
uptake in yeasts, especially those related to the siderophore-dependent pathway and
that, in general, the expression profile coincided with the requirement for Aft1p (a
gene involved in the adaptation of iron metabolism to changes in iron availability)
for the transcription of these genes [74]. Further studies will be required to determine
the relevance of these findings, if any, to the antiplasmodial and cytotoxic modes of
action of 1.
The DNA-intercalating properties of 1 have raised concerns that it may be genotoxic, and in earlier studies it was reported that incubation of V79 cells with 1 (2.5
μM, 24 h) increased micronuclei formation (6.4% compared to 0.4% in untreated
controls). This suggests that 1 may carry a genotoxic risk [75]. In contrast, in the
Comet assay, relatively high (100–200 μM) concentrations of 1 did not cause DNA
damage in human lymphocytes but significant increases in both Olive Tail moments
and % Tail DNA were observed at 300 μM [76]. The analog 2,7-dibromocryptolepine
(43) significantly increased DNA damage at 200 and 300 μM while 8-chloro-7nitrocryptolepine was significantly active at 100 and 200 μM but not at 300 μM.