The pump in question is ABCG2, a member of the ATP-binding cassette family of efflux transporters. These proteins are notorious contributors to multidrug resistance in cancer, actively pumping chemotherapeutic agents and other compounds out of cells. Crucially for SDT, ABCG2 is known to transport several porphyrin-based sensitizing agents, including protoporphyrin IX, chlorin e6 and pheophorbide. Previous work in photodynamic therapy has shown that blocking ABCG2 improves cellular retention of sensitizers and enhances treatment efficacy, and studies in ABCG2 knock-out mice have demonstrated greater porphyrin accumulation than in wild-type animals. Lapatinib, a tyrosine kinase inhibitor approved for advanced HER2-positive breast cancer, directly binds to and inhibits ABCG2, much like the experimental inhibitor Ko143, raising the possibility that it could be repurposed to strengthen SDT.

In the new work, researchers investigated whether lapatinib could enhance SDT against prostate cancer cells using a previously developed and well-characterized formulation of hematoporphyrin-containing polymer-based nanoparticles, or HPNPs. Rather than testing a single uniform cell population, the team deliberately examined three biologically distinct states of the human PC3 prostate cancer cell line: cells grown under normal oxygen conditions, cells conditioned to chronic hypoxia for three weeks, and cells driven toward a neuroendocrine phenotype through a validated dovitinib-induced differentiation protocol, confirmed by increased neuron-specific enolase expression. This design matters because prostate tumors are highly heterogeneous, and both hypoxia and prolonged androgen deprivation therapy can generate neuroendocrine-like cell populations with altered molecular profiles and drug-handling behavior.

The first key measurement concerned accumulation. The researchers incubated all three cell types with HPNPs in the presence or absence of lapatinib and tracked intracellular hematoporphyrin fluorescence over 2, 4, 8 and 24 hours. Nanoparticulate hematoporphyrin was taken up rapidly, with a steep rise in the first two hours followed by slower accumulation up to the 24-hour mark. Surprisingly, lapatinib had no significant effect on cellular uptake or accumulation in any of the three cell systems across the entire 24-hour incubation. This contrasts with earlier findings in glioma cells, where lapatinib significantly enhanced the accumulation of protoporphyrin IX generated from 5-aminolevulinic acid, a difference the authors attribute to the distinct cell type and the different sensitizer generation and release pathway involved.

The retention experiments told a more nuanced story. After 24 hours of HPNP exposure, the treatment medium was replaced with sensitizer-free medium, and intracellular fluorescence was followed over time. In all three cell types, intracellular hematoporphyrin levels nearly halved within the first two hours, falling by 60.20 percent in normoxic PC3 cells, 46.87 percent in hypoxic PC3 cells and 69.25 percent in neuroendocrine-differentiated cells, before plateauing at 24 hours with only 12.62, 24.52 and 12.91 percent of the sensitizer remaining, respectively. Lapatinib did not improve retention in normoxic PC3 cells at any time point. However, in hypoxic PC3 cells it significantly improved retention by 9.80 percent at two hours and 7.61 percent at four hours, and in neuroendocrine cells it produced smaller but statistically significant gains of 5.09 percent at two hours and 5.52 percent at eight hours. The authors note that efflux was inferred from the reduction in intracellular fluorescence rather than by directly measuring exported hematoporphyrin in the medium.

These kinetic profiles were then translated into therapy. The team applied ultrasound at 1 MHz with an acoustic intensity of 2.5 W/cm2, a 50 percent duty cycle and a 30-second exposure, testing SDT at three different windows: after three hours of HPNP incubation, two hours after removing the nanoparticles, and immediately after the standard 24-hour incubation. At the three-hour accumulation time point, lapatinib did not significantly enhance SDT efficacy in any cell type, consistent with the accumulation data, although a tendency toward greater SDT-induced cytotoxicity was visible in hypoxic PC3 cells. At the two-hour retention time point, lapatinib again produced no statistically significant improvement in normoxic or hypoxic cells, but in neuroendocrine-differentiated cells it improved SDT efficacy by a statistically significant 11.5 percent, with a combination synergy of no more than 0.28 percent. At the conventional 24-hour time point, SDT performance was overall improved compared with the earlier windows, but lapatinib had no effect in any of the three cell systems.

To understand why the drug’s benefit was so selective, the researchers examined ABCG2 itself. Western blot analysis detected ABCG2 bands at approximately 65 and 72 kDa in all three cell types, with the highest average protein levels in normoxic PC3 cells, intermediate levels in neuroendocrine cells and the lowest in hypoxic cells, though none of these differences reached statistical significance. Immunofluorescence revealed a striking difference in localization: ABCG2 was distributed throughout the cytoplasm in normoxic and neuroendocrine cells, whereas in hypoxic PC3 cells it accumulated preferentially at the cytoplasmic membrane, the position from which efflux actually occurs. The authors suggest this redistribution may reflect a functional adaptation to hypoxia, potentially mediated by hypoxia-inducible factors such as HIF-1α, which are known to modulate the trafficking and transcription of efflux transporters.

Functional assays using the ABCG2 substrate pheophorbide a added a further layer. Although transporter activity did not differ significantly among the three cell systems, hypoxic PC3 cells showed the highest activity and normoxic PC3 cells the lowest, a pattern inversely related to the Western blot protein measurements. The authors propose that strong membrane association of the transporter in hypoxic cells may reduce extraction efficiency during blotting, explaining the apparent discrepancy. Importantly, the tendency of lapatinib to improve SDT response appeared to track with ABCG2 activity rather than total protein level, corroborating previous findings that lapatinib’s effect stems from direct binding to the transporter rather than from altered gene or protein expression. This underscores a broader methodological point: measuring transporter activity, not just abundance, may be essential for predicting how cells will respond to efflux-inhibiting combination strategies.

The study’s authors are careful about the scope of their conclusions. The work was conducted in two-dimensional monolayer cultures, a simplified system that bypasses the diffusion and lymphatic drainage variables of real tumors, where sensitizer accumulation and retention are far less sharply defined. Lapatinib was used at the maximum concentration associated with minimal cytotoxicity to limit confounding by its kinase-inhibitory effects, and PC3 cells themselves express only low to moderate levels of ABCG2. Overall, lapatinib treatment improved the mean SDT response compared with HPNPs alone, but this difference was not statistically significant, and the authors emphasize that any benefit is likely concentrated in cellular populations with higher ABCG2 activity, such as hypoxic regions of a tumor, rather than producing a uniform enhancement across all prostate cancer cell states. Advanced tumors are expected to contain spatially and temporally variable mixtures of normoxic, hypoxic and, after prolonged androgen deprivation therapy, neuroendocrine-like populations, all of which will influence how much ABCG2-mediated efflux limits sensitizer retention.

Even with those caveats, the findings carry real weight for the field. They demonstrate that lapatinib can prolong the intracellular presence of a nanoparticle-delivered sonosensitizer, effectively widening the therapeutic window during which ultrasound can be applied, and that this effect is most pronounced under hypoxic conditions, precisely where sensitizer supply is expected to be lowest due to poor vascularization. They also highlight how subcellular processing of nanoparticulate sensitizers, which may take hours after uptake, could explain why only early treatment windows showed a tendency toward improvement. The authors position the work as a foundation for repurposing lapatinib, already a clinically approved oncology drug, in rationalized combination with SDT, while stressing that further studies are needed to refine dosing for optimal efflux inhibition. For a therapy whose central challenge has always been getting enough sensitizer to stay inside the right cells at the right moment, selectively disarming the cellular pumps that fight back may prove to be an important piece of the puzzle.

Subject of Research: Inhibition of the ABCG2 efflux transporter by lapatinib to improve sonodynamic therapy against prostate cancer cells

Article Title: Investigating the Effect of Lapatinib on the Performance of Sonodynamic Therapy Against Prostate Cancer via Inhibition of the ABCG2 Efflux System

Article References: Wang, Y., Masood, H., & Nomikou, N. (2026). Investigating the Effect of Lapatinib on the Performance of Sonodynamic Therapy Against Prostate Cancer via Inhibition of the ABCG2 Efflux System. Pharmacology Research & Perspectives, 14(6), Article e70340. https://doi.org/10.1002/prp2.70340

Image Credits: AI Generated

DOI: 10.1002/prp2.70340

Keywords: sonodynamic therapy, lapatinib, ABCG2, prostate cancer, hematoporphyrin nanoparticles, hypoxia, drug efflux, reactive oxygen species, ultrasound, neuroendocrine differentiation, Investigating, Effect