Light-enhanced cytotoxicity and intracellular trafficking of the PD-L1-targeting photoimmunoconjugate EITC-atezolizumab

Background: Photodynamic therapy (PDT) uses light to generate reactive oxygen species (ROS) that initiate cytotoxic responses in cancer cells. Photoimmunotherapy (PIT) improves PDT specificity by linking photosensitizers (PS) to monoclonal antibodies. A promising monoclonal antibody (mAb) for immunoconjugates is atezolizumab (Tecentriq), an EMA/FDA-approved PD-L1 immune checkpoint inhibitor for treating non-small cell lung cancer (NSCLC). Objective: Our goal was to: Optimize the cytotoxic efficacy of a photoimmunoconjugate of eosin-5-isothiocyanate and atezolizumab (EITC-atezolizumab) in NSCLC cells; Study the uptake and intracellular transport of atezolizumab; Evaluate EITC-atezolizumab as a candidate for photochemical internalization (PCI) of the ribosome-inactivating protein gelonin. Methods: Cytotoxic efficacy of EITC-atezolizumab was evaluated in NSCLC cell lines H1975 (PD-L1-high) and A549 (PD-L1-low) using the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS) assay. Confocal microscopy was employed to observe the uptake and intracellular localization of AlexaFluor647-atezolizumab and its colocalization with either LysoTracker™Blue or an anti-CD63 antibody labelled with Alexa488. Results: EITC-atezolizumab PIT significantly enhanced cytotoxicity, showing much higher sensitivity in H1975 than A549 NSCLC cells. PIT with EITC-atezolizumab did not enhance gelonin efficacy. Confocal microscopy of H1975 cells revealed near-plasmamembrane puncta fluorescence of Alexa647-atezolizumab, with fractions clustering in CD63+ organelles peaking at 6 h. In A549 cells, AlexaFluor647-atezolizumab showed sustained accumulation in LysoTracker™ Blue+ vesicles for up to 24 h. Conclusion: To the best of our knowledge, this is the first documentation demonstrating that atezolizumab is transported to CD63-positive organelles, thereby enhancing our understanding of its intracellular trafficking. Our study also strengthens the concept of PIT and atezolizumab-based targeting of PD-L1+ NSCLCs.