The mechanism of action of
ZR2002, a chimeric amino
quinazoline designed to possess mixed EGFR
tyrosine kinase (TK) inhibitory and
DNA targeting properties, was compared to those of ZR01, a reversible inhibitor of the same class and
PD168393, a known irreversible inhibitor of EGFR.
ZR2002 exhibited 4-fold stronger EGFR TK inhibitory activity than its structural homologue ZR01 but was approximately 3-fold less active than the 6-acrylamidoquinazoline
PD168393. It preferentially blocked
EGF and
TGFalpha-induced cell growth over PDGF and serum. It also inhibited signal transduction in
heregulin-stimulated breast tumour cells, indicating that it does not only block EGFR but also its closely related erbB2 gene product. In contrast to its structural homologues,
ZR2002 was capable of inducing significant levels of
DNA strand breaks in MDA-MB-468 cells after a short 2 hr
drug exposure at a concentration as low as 10 microM. Reversibility studies using whole cell autophosphorylation and growth assays in human breast cell lines showed that in contrast to its reversible inhibitor counterpart ZR01,
ZR2002 induced irreversible inhibition of
EGF-stimulated autophosphorylation in MDA-MB-468 cells and irreversible inhibition of cell growth. Moreover despite possessing a weaker binding affinity than
PD168393, it induced a significantly more sustained antiproliferative effect than the latter after a pulse 2 hr exposure. More importantly, in contrast to ZR01 and
PD168393,
ZR2002 was capable of inducing significant levels of cell death by apoptosis in MDA-MB-468 cells. The results in
toto suggest that the superior antiproliferative potency of
ZR2002 may be due to its ability to induce a protracted blockade of
receptor tyrosine kinase-mediated signaling while damaging cellular
DNA, a combination of events that may trigger cell-killing by apoptosis.