**1. Introduction**

Photodynamic therapy (PDT) is an oncology treatment based on photochemical reactions involved photosensitizer (PS) and light irradiation of an appropriate wavelength. The interaction of PS and light, producing reactive oxygen species (ROS), can directly induce cellular damage to organelles and cell membranes. PDT's action on cellular organelles, mainly in the endoplasmic reticulum (ER) and Golgi, interferes with several metabolic pathways [1]. The glycosylation process is a post-translational modification that occurs in the ER [2]. It results in the addition of carbohydrate motifs—glycans to proteins that are, in most cases, destined for the cell surface. The resultant glycoprotein structures at the cell surface form a carbohydrate-rich layer, which presents an essential role in the cell's interaction with its surrounding environment. Glycosylation of a given protein is a process catalyzed by glycosyltransferases, localized in the Golgi, leading to the formation of protein-bound glycans with specific and diverse biological functions [3]. These carbohydrate side chains can modulate the protein's interaction with its environment, influencing key factors such as protein half-life, solubility, binding activity, and specificity. Two significant types of glycosylation occur on proteins. (1) O-linked glycosylation refers to the addition of N-acetyl-galactosamine to serine or threonine residues by the enzyme UDP-N-acetyl-D-galactosamine transferase, followed by the addition of other carbohydrates, such as galactose, N-acetyl-D-glucosamine, or sialic acid; (2) N-linked glycosylation occurs in the ER [4, 5]. It refers to the insertion of an oligosaccharide chain enzymatically attached to the amide group of asparagine in the consensus sequence Asn-X-Ser/Thr (where X represents any residue except proline). Alterations in glycosylation of malignant cells can take a variety of forms, including changes in the amount, linkage, and acetylation of sialic acids; changes in the branching of N-glycans mediated by glycosyltransferases; alterations in the expression of glycosaminoglycans such as heparan sulfate; and altered glycosylation of mucins, which are heavily glycosylated epithelial-derived proteins known to be implicated in certain cancers [5, 6].

Changes in glycoprotein glycans are significant in malignant tumor transformation and are closely associated with tumor cell adhesion, invasion, and metastasis [7]. Christiansen et al., 2014 [8] demonstrated changes in cell surface glycosylation in five different types of cancer: breast, colon, liver, skin (melanoma), and ovarian, and how these changes may be associated with carcinogenesis. Synthesis and expression of cell-surface carbohydrates is a highly regulated process that affects several cellcell interactions. The presence of specific oligosaccharides in highly malignant cells is essential for expressing the metastatic phenotype [9]. Elevation of glycoproteins above normal levels reflects local tissue destruction processes with the release of preformed tissue proteins or becomes a local synthesis and releases glycoproteins through tumor cells [10].

Waiting for glycans and glycoconjugates to play a relevant role in various tumor progression stages, biosynthetic cell machinery involved in glycan biosynthesis and modification is a promising target for cancer treatment. Treatments that act on such targets should be researched to act or stimulate specific glycosylation inhibitors that may target a pro-metastatic biological product or interfere with the immune response's modulation.
