Bin Chen

5‐Aminolevulinic acid (ALA) is a prodrug for protoporphyrin IX (PpIX)‐mediated photodynamic therapy (PDT) and fluorescence‐guided tumor surgery. We previously reported that lapatinib, a repurposed ABCG2 inhibitor, enhanced ALA‐induced PpIX fluorescence and PDT by blocking ABCG2‐mediated PpIX efflux. In the present study, we evaluated how the variation in ABCG2 activities/protein levels affected tumor cell response to the enhancement of PpIX/PDT by lapatinib and Ko143, an ABCG2 tool inhibitor. ABCG2 activities and protein levels were determined in a panel of human cancer cell lines. Effects of lapatinib and Ko143 on enhancing ALA‐PpIX fluorescence and PDT were evaluated and correlated with tumor cell ABCG2 activities. We found that both lapatinib and Ko143 enhanced ALA‐PpIX fluorescence and PDT in a dose‐dependent manner, although lapatinib exhibited lower efficacy and potency than Ko143 in nearly all cancer cell lines. The EC 50 of ABCG2 inhibitors for enhancing ALA‐PpIX and PDT had a positive correlation with tumor cell ABCG2 activities, indicating that tumor cell lines with lower ABCG2 activities were more sensitive to ABCG2 inhibitors for PpIX/PDT enhancement. Our results suggest that, for optimal therapeutic enhancement, the dose of ABCG2 inhibitors needs to be tailored based on the ABCG2 expression/activity in tumors.