Metabolic and Molecular Mechanisms of Gemcitabine Resistance in Urothelial Carcinoma.

Gemcitabine-based chemotherapy has long served as a standard treatment for urothelial carcinoma (UC), particularly in perioperative and metastatic settings. However, therapeutic efficacy is frequently limited by intrinsic or acquired resistance. Gemcitabine functions as a prodrug whose activity depends on coordinated processes involving cellular uptake, intracellular activation, metabolic inactivation, and nucleotide metabolism. Increasing evidence suggests that resistance in UC is mediated by multiple interconnected mechanisms beyond alterations in gemcitabine transport, activation, and inactivation alone. Key molecular determinants include human equilibrative nucleoside transporter 1 (hENT1), deoxycytidine kinase (dCK), cytidine deaminase (CDA), and ribonucleotide reductase regulatory subunit M1 (RRM1), which is involved in nucleotide pool maintenance and DNA synthesis. In addition, replication stress responses, apoptosis evasion pathways, and tumor microenvironment-associated factors also contribute to gemcitabine resistance. Stress-adaptive pathways involving Y-box binding protein 1 (YB-1), hypoxia-inducible factor-1 alpha (HIF-1α), and autophagy-related mechanisms may further promote survival under chemotherapy-induced stress conditions. In addition, extracellular mucin-associated mechanisms may alter intratumoral drug accessibility and contribute to resistance. In this review, we summarize UC-specific evidence regarding gemcitabine resistance and discuss how these pathways collectively shape an integrated resistant phenotype.