Aptamers and endocytosis: The path into tumor cells

Aptamers are short, single-stranded DNA or RNA oligonucleotides that fold into unique three-dimensional structures, enabling high-affinity and specific binding to target molecules, which underpins their significant antitumor potential. However, their efficacy is often limited by inefficient cellular uptake and trafficking. This review synthesizes current knowledge on the endocytic pathways that govern aptamer internalization into tumor cells. We detail the mechanisms of receptor-mediated endocytosis, such as clathrin- and caveolin-dependent pathways, and receptor-independent pathways, including the CLIC/GEEC (clathrin-independent carrier/GPI-AP-enriched early endosomal compartment) route, macropinocytosis, and phagocytosis. The review highlights key differences in these pathways between normal and tumor tissues, emphasizing how the tumor microenvironment, with its enhanced permeability and retention effect and receptor overexpression, favors selective aptamer accumulation. A critical discussion is dedicated to the intracellular fate of internalized aptamers, whose entrapment in the endosomal-lysosomal system often precludes reaching cytoplasmic targets. We further explore innovative strategies to modulate this fate, including chemical modifications, nanocarriers, and endosomolytic agents, to enhance cytosolic delivery and therapeutic outcomes. Understanding these mechanisms provides a rational basis for designing next-generation aptamer-based platforms with optimized efficacy and specificity in oncology.