Glycoconjugates represent a recent trend in
cancer chemotherapy that adopts the concept of selective
prodrug/drug targeting of
tumor cells by selectively binding to specific transmembrane
glucose transporters. Following preferential uptake of
sugar conjugates into
cancer cells, they are presumably subject to enzymatic cleavage by specific beta-
glycosidases to liberate the free active cytotoxic aglycones that act selectively on
cancer cells and spare other noncancerous ones. In this sense, the cytotoxicity of an array of newly synthesized
glycoconjugates, including
curcumin beta-
glucoside,
perillyl alcohol beta-
glucoside,
perillyl alcohol beta-galactoside, diethylstilbesterol beta-
glucoside and diethylstilbesterol
beta-galactoside have been investigated over 24-96 h in a panel of human
colon cancer cells namely, Caco-2, HT29 and T84 cells. The role of beta-
glycosidases and
caspases in the bioactivation and cytotoxicity of these compounds has been addressed in the current study. All the
glycoconjugates have proven cytotoxic efficacy in a time-dependent manner.
Curcumin beta-
glucoside was the most potent amongst all
glycoconjugates tested. The sensitivity rank order of
tumor cells towards all beta-
glucosides was Caco-2 > HT29 > T84. This sensitivity ranking was well correlated with
beta-glucosidase activity assessed in these cell lines. Unlike
perillyl alcohol galactoside, the cytotoxicity rank order for diethylstilbesterol
beta-galactoside was not coping with the
beta-galactosidase activity detected. Apoptosis was assessed by fluorometric assay of
caspase-3 and
caspase-9 activities. Initiation and activation of apoptosis were increased in all
colon cancer cells following exposure to most of the
glycoconjugates, and this was well correlated with the cytotoxicity rank order of these
prodrugs. Enzymatic cleavage of
glycoconjugates was accomplished using a host of hydrolytic
enzymes and cleavage kinetics was determined using HPLC. The
glycoconjugates were only cleaved by
beta-glucosidases and
beta-galactosidases, but not by pancreatic
lipase or hepatic
esterase. Taken together, one could conclude that
beta-glucosidases and
beta-galactosidases are crucial for the bioactivation and cytotoxicity of these
glycoconjugates. Also, initiation and activation of apoptosis in
tumor cells may contribute, at least partly, for the cytotoxicity of these
sugar conjugates.