Lapatinib is a specific HER1 and 2 targeted
tyrosine kinase inhibitor now widely used in combination with
chemotherapy in the clinical setting. In this work, we investigated the interactions between
lapatinib and specific
chemotherapy agents (
cisplatin,
SN-38,
topotecan) in a panel of cell lines [breast (n = 2), lung (n = 2), testis (n = 4)]. A high-sensitivity cell proliferation/cytotoxicity
ATP assay and flow cytometry were used to determine cell viability, apoptosis, and the effect of the drugs on cell-cycle distribution. CalcuSyn analysis was employed to formally identify synergistic interactions between drugs. Intracellular concentrations of
SN-38 were measured using a novel high-performance liquid chromatography (HPLC) technique. Flow cytometry and HPLC techniques were used to identify the effect of
lapatinib on drug influx and efflux pumps, using specific substrates and inhibitors of these pumps. Results showed significant synergy between
SN-38, and
lapatinib in the majority of cell lines (combination index < 0.75), associated with increased apoptosis. This synergy was not universal but, when observed (Susa S/R, H1975, H358, and MDA-MB-231 cell lines), was related to
SN-38 intracellular accumulation (2.2- to 4.8-fold increase, P < 0.05 for each), attributable to the inhibition of the
breast cancer-related
protein (BCRP) efflux pump by
lapatinib. Flow cytometry analysis showed that
lapatinib (10 μmol/L) inhibited the efflux of
mitoxantrone, a specific substrate of the BCRP pump, in a manner similar to
fumitremorgin C, a known BCRP inhibitor, confirming
lapatinib as a BCRP inhibitor. This work shows that
lapatinib has a direct inhibitory effect on BCRP accounting for the synergistic findings. The synergy is cell line dependent and related to the activity of specific efflux pumps.