Effects of ABCG2 knockdown on therapeutic enhancement of 5‐aminolevulinic acid with lapatinib

5-Aminolevulinic acid (ALA) is administered as a prodrug for tumor fluorescence detection and photodynamic therapy (PDT) after being converted endogenously to the active drug protoporphyrin IX (PpIX). As a substrate of the ABCG2 transporter, PpIX is subject to outward transport in cells with ABCG2 expression, resulting in reduced PpIX fluorescence and PDT effect. Due to the lack of approved ABCG2 inhibitors, lapatinib (Lap), a kinase inhibitor approved for cancer treatment, has been repurposed as an ABCG2 inhibitor for the enhancement of ALA. To provide evidence that Lap enhances ALA by inhibiting ABCG2, we knocked down ABCG2 in four tumor cell lines and evaluated the effects of Lap on enhancing ALA-PpIX fluorescence and PDT in the wild-type and ABCG2-knockdown cell lines. ABCG2 knockdown enhanced ALA-PpIX fluorescence and PDT response, underscoring the importance of ABCG2 in regulating intracellular PpIX accumulation. However, compared with the corresponding wild-type cell lines, Lap induced less enhancement of PpIX fluorescence and PDT in ABCG2-knockdown cell lines, particularly at high concentrations and in cell lines with low ABCG2 activities. By downregulating ABCG2 in tumor cells, we showed that Lap-induced ALA enhancement was reduced, demonstrating that Lap enhances ALA by blocking ABCG2-mediated PpIX efflux.