Drug resistance is a major impediment to the successful treatment of
breast cancer using
chemotherapy. The photoactivatable
drug calphostin C has shown promise in killing select
drug-resistant
tumor cells lines in vitro. To assess the effectiveness of this agent in killing
doxorubicin- or
paclitaxel-resistant
breast tumor cells and to explore its mode of action, MCF-7 cells were exposed to increasing concentrations of either
doxorubicin or
paclitaxel until maximum resistance was obtained. This resulted in the creation of isogenic
drug-resistant MCF-7TAX and MCF-7DOX cell lines, which were approximately 50- and 65-fold resistant to
paclitaxel and
doxorubicin, respectively. Interestingly,
calphostin C was able to kill MCF-7TAX cells as efficiently as wildtype MCF-7 cells (IC50s were 9.2 and 13.2 nM, respectively), while MCF-7DOX cells required a 5-fold higher concentration of
calphostin C to achieve the same killing (IC50 = 64.2 nM). Consistent with their known mechanisms of action,
paclitaxel killed
tumor cells by inducing mitotic arrest and cell multinucleation, while
doxorubicin induced plasma membrane blebbing and decreased nuclear staining with
propidium iodide. In contrast, cytoplasmic vacuolization accompanied cell killing by
calphostin C in these cell lines, without the induction of
caspase-8 or PARP cleavage or the release of
cytochrome c from mitochondria.
Calphostin C had little effect on the uptake of either
paclitaxel or
doxorubicin by the cells. Taken together, the above data suggests that
calphostin C is able to potently kill
drug-resistant
breast tumor cells through a mechanism that may involve the induction of cytoplasmic vacuolization, without activation of typical apoptotic pathways. Consequently,
calphostin C may prove useful clinically to combat
tumor growth in
breast cancer patients whose
tumors have become unresponsive to
anthracyclines or
taxanes, particularly in association with
photodynamic therapy.