We assessed the effect of the
protein kinase C inhibitor 2,6-diamino-N-([1-(1-oxotridecyl)-2-piperidinyl]methyl)hexanami de (
NPC 15437) on the action of
anthracyclines, epipodophyllotoxins and
vinca alkaloids in
P-glycoprotein (Pgp)-expressing CH(R)C5 hamster ovary and MCF-7/Adria(R) human
breast cancer cells. Flow microfluorimetry revealed that treatment of CH(R)C5 cells with 75 microM
NPC 15437 for 1 h resulted in a 6- to 10-fold increase in the nuclear accumulation of
daunorubicin. Colony forming assays revealed that treatment with 75 microM
NPC 15437 was associated with a 4-fold decrease in the LD90 for
etoposide and a 2.5-fold decrease in the LD50 for
vincristine. At higher concentrations of
NPC 15437, greater modulation of
anthracycline accumulation was observed; but
NPC 15437 itself inhibited subsequent colony formation. Similar effects on
drug accumulation and cytotoxicity were observed in MCF-7/Adria(R) cells. Experiments designed to investigate the mechanism by which
NPC 15437 exerts these effects revealed that treatment with the
protein kinase C activator
phorbol-12-myristate 12-acetate partially reversed the effect of
NPC 15437, suggesting that
NPC 15437 was exerting an effect through
protein kinase C. Photoaffinity labeling experiments revealed that
NPC 15437 also inhibited the binding of [3H]-
azidopine to Pgp in isolated membrane vesicles. These results identify
NPC 15437 [correction of NPC15437] as the prototype of a new class of potential Pgp modulators but indicate that the effects of this agent as a modulator are potentially limited by its cytotoxicity.