Cancer nanotherapeutics have demonstrated remarkable potential in improving drug solubility, targeting specificity, and pharmacokinetics; however, their clinical translation has remained disproportionately limited. A major yet insufficiently addressed contributor to this translational failure is endosomal entrapment, a process by which nanocarrier-drug complexes, despite efficient cellular internalization, become sequestered within endo-lysosomal compartments, preventing effective cytosolic or nuclear drug delivery. This paper critically examines endosomal entrapment as a central intracellular bottleneck that compromises therapeutic efficacy across multiple nanocarrier platforms, including liposomes, polymeric nanoparticles, lipid nanoparticles, and hybrid systems. The intracellular trafficking routes governing nanoparticle uptake and their convergence toward lysosomal degradation are reviewed, followed by an analysis of the mechanistic determinants of endosomal retention, such as pH-dependent ion trapping, enzymatic degradation, limited membrane permeability, and rapid endosomal maturation. The limitations of conventional nanocarrier evaluation metrics-including particle size, zeta potential, and cellular uptake assays-in predicting functional intracellular drug release are highlighted. Furthermore, the paper evaluates pharmaceutical strategies designed to overcome endosomal entrapment, including pH-responsive materials, proton sponge polymers, fusogenic lipids, membrane-disruptive peptides, ionizable lipids, and prodrug-based approaches, with emphasis on their translational feasibility and safety considerations. Finally, a pharmaceutics-oriented design framework is proposed that positions endosomal escape as a critical quality attribute alongside scalability, regulatory readiness, and clinical relevance. By shifting the focus from cellular uptake to productive intracellular drug delivery, this work provides a rational roadmap for improving the translational success of cancer nanotherapeutics.
