In advanced-stage cancer, autophagy plays a crucial role in tumour cell survival, making it a potential target for cancer treatment. Photodynamic therapy (PDT) can induce autophagy, leading to cancer cell death when apoptosis is compromised. Conversely, at low-dose PDT, autophagy serves as a protective mechanism in cancer cells, allowing them to repair damage caused by oxidative stress. In this presentation, I will discuss cellular resistance mechanisms against PDT, focusing on autophagy. Our previous studies demonstrated that PDT using photochemical internalization (PCI)-photosensitizers localized in lysosomal membranes directly damages both lysosomes and the key autophagy regulator mTOR. Recently, we found that low-dose PDT with the PCI photosensitizer TPCS2a (also known as fimaporfin or Amphinex) induces autophagy while at high-dose PDT inhibits autophagic flux. After TPCS2a-PDT, the activation of the 4EBP1 protein (a substrate of mTOR) significantly decreased in PDT-treated cells compared to controls. The autophagic response post low-dose TPCS2a-PDT involves recruitment of ubiquitin (Ubq), autophagic adaptor protein p62, and microtubule-associated protein 1A/1B-light chain 3 (LC3) to damaged vesicles, marked by Galectin 3 (Gal3). Ultrastructural analysis revealed a thick p62-positive layer surrounding these permeabilized vesicles. Although p62 appears crucial during selective autophagic sequestration, its presence isn’t essential for effective removal of damaged vesicles or lysosomal content recovery. Interestingly, an active autophagic response and p62 presence are vital for cancer cells to survive low-dose TPCS2a-PDT. Thus, targeting of p62 may constitute a rational strategy to improve the cytotoxic efficacy of PDT. Hence, to overcome PDT resistance, we can either inhibit autophagic flux by targeting lysosomes and/or directly address proteins crucial for autophagy and cell survival. Alternatively, we propose using PCI to disrupt endosomes and lysosomes, releasing entrapped cancer drugs into cancer cell cytosol, where autophagy provides strong cytoprotection.