Glutathione transferases (
GSTs) are often overexpressed in
tumors and frequently correlated to bad prognosis and resistance against a number of different anticancer drugs. To selectively target these cells and to overcome this resistance we previously have developed
prodrugs that are derivatives of existing anticancer drugs (e.g.,
doxorubicin) incorporating a
sulfonamide moiety. When cleaved by
GSTs, the
prodrug releases the
cytostatic moiety predominantly in GST overexpressing cells, thus sparing normal cells with moderate
enzyme levels. By modifying the
sulfonamide it is possible to control the rate of drug release and specifically target different
GSTs. Here we show that the newly synthesized compounds, 4-acetyl-2-nitro-benzenesulfonyl
etoposide (ANS-
etoposide) and 4-acetyl-2-nitro-benzenesulfonyl
doxorubicin (ANS-DOX), function as
prodrugs for GSTA1 and MGST1 overexpressing cell lines. ANS-DOX, in particular, showed a desirable cytotoxic profile by inducing toxicity and DNA damage in a GST-dependent manner compared to control cells. Its moderate conversion of 500 nmol/min/mg, as catalyzed by GSTA1, seems hereby essential since the more reactive 2,4-dinitrobenzenesulfonyl
doxorubicin (DNS-DOX) (14000 nmol/min/mg) did not display a preference for GSTA1 overexpressing cells. DNS-DOX, however, effectively killed GSTP1 (20 nmol/min/mg) and MGST1 (450 nmol/min/mg) overexpressing cells as did the less reactive 4-mononitrobenzenesulfonyl
doxorubicin (MNS-DOX) in a MGST1-dependent manner (1.5 nmol/min/mg) as shown previously. Furthermore, we show that the mechanism of these
prodrugs involves a reduction in GSH levels as well as inhibition of the redox regulatory
enzyme thioredoxin reductase 1 (TrxR1) by virtue of their electrophilic
sulfonamide moiety. TrxR1 is upregulated in many
tumors and associated with resistance to
chemotherapy and poor patient prognosis. Additionally, the
prodrugs potentially acted as a general shuttle system for DOX, by overcoming resistance mechanisms in cells. Here we propose that GST-dependent
prodrugs require a conversion rate "window" in order to selectively target GST overexpressing cells, while limiting their effects on normal cells.
Prodrugs are furthermore a suitable system to specifically target
GSTs and to overcome various drug resistance mechanisms that apply to the parental drug.