Purpose: The human epidermal growth factor receptors EGFR (erbB1) and HER2
(erbB2/neu) are involved in mediating resistance to chemotherapy and ionising radiation
(IR). In vitro studies demonstrate that small molecule tyrosine kinase inhibitors (TKIs) which
target these receptors can increase the effectiveness of DNA damaging agents. However,
these combinations have failed to produce the clinical results anticipated and one potential
explanation is that the inhibition of EGFR and HER2 cell signalling pathways by TKIs is short
lived, with cells able to switch to alternative mechanisms of signalling through HER3. The
purpose of this study was to examine whether the duration of exposure to TKIs modulates
the induction and repair of DNA damage produced by chemotherapy or IR and describes
attempts to elucidate the role of HER2 in mediating resistance to chemotherapy.
Experimental design: Two HER targeting TKIs, lapatinib and gefitinib were investigated. The
effect of lapatinib in combination with cisplatin and doxorubicin on the inhibition of cell
proliferation and the role of schedule were examined in drug combination assays. The
influence of the duration of exposure to TKIs on the induction and repair of DNA lesions
induced by cisplatin, IR, doxorubicin, etoposide and m-AMSA were investigated using the
alkaline and neutral Comet assays and measurement of γH2AX and RAD51 foci. DNA
expression arrays were used to identify the potential mechanisms through which HER2
produces resistance to cisplatin in cells transfected with HER2.
Results: Lapatinib is able to synergistically inhibit cell proliferation in combination with
cisplatin or doxorubicin in a schedule dependent manner. Duration of exposure to TKIs has
no effect on the induction of DNA lesions by cisplatin or IR, but significantly reduces the
production of DNA double strand breaks by doxorubicin, etoposide and m-AMSA in part
through the down-regulation of the expression of topoisomerase IIα (Topo IIα), increasing
resistance to these drugs.
Conclusions: These results indicate the scheduling of small molecule TKIs targeted against
EGFR and HER2 is important and continuous exposure to these drugs induces resistance to
doxorubicin, etoposide and m-AMSA, through reduced expression of their target, Topo IIα.
The importance of schedule should be considered when combining TKIs with chemotherapy
in clinical practice