Abstract <p>The effectiveness of photodynamic therapy based on stable 4,5,6,7-tetrahydropyrazolo[1,5-<i>a</i>]pyridine-fused chlorins (PX) was explored in endometrial cancer. In vitro, photocytotoxicity of three derivatives of ring-fused chlorins (dihydroxymethyl, dicarboxylic acid, and dimethyl ester moieties) was evaluated in three endometrial cancer cell lines, ECC-1, RL95-2, and HEC-1-A. A comprehensive assessment of chlorins-induced intracellular production of reactive oxygen species, viability and PDT-induced types of cell death, influence on the cell cycle, intracellular localisation, and the protein expression of aldehyde dehydrogenase after photodynamic treatment was carried out. The photodynamic effect of the dihydroxymethyl derivative was then evaluated in vivo by monitoring tumour volume and performing histological and immunohistochemical analyses of the tumours. The dihydroxymethyl and dicarboxylic acid derivatives were the most active photosensitisers, presenting IC₅₀ values in the nanomolar range. These chlorins induced a photodynamic action mediated by the production of peroxides, superoxide anion and singlet oxygen, culminating in cell death predominantly by late apoptosis and necrosis. Within endometrial cancer cells, the dihydroxymethyl derivative accumulates in the mitochondria, endoplasmic reticulum, and plasma membrane, corresponding to the localisation of the aldehyde dehydrogenase, whose expression was diminished upon treatment in ECC-1 and HEC-1-A cells. Nevertheless, cytotoxicity seems to be independent of the expression of this enzyme. In vivo studies using an ECC-1 mouse model highlighted the dihydroxymethyl derivative's anticancer potential in managing endometrial cancer.</p> Graphical abstract <p></p>