Multidrug resistance (MDR) remains a formidable barrier to successful cancer treatment, driven by mechanisms such as efflux pump overexpression, enhanced DNA repair, evasion of apoptosis and the protective characteristics of the tumour microenvironment. Nanoparticle-based delivery systems have emerged as promising platforms capable of addressing these challenges by enhancing intracellular drug accumulation, enabling targeted delivery and facilitating stimuli-responsive and controlled release. This review provides a comprehensive overview of the molecular and cellular mechanisms underlying MDR and critically examines recent advances in nanoparticle strategies developed to overcome it. Various nanoparticle designs are analysed in terms of their structural and functional features, including surface modifications, active targeting ligands and responsiveness to tumour-specific cues. Particular emphasis is placed on the co-delivery of chemotherapeutic agents with gene regulators, such as siRNA, and the use of nanoparticles to deliver CRISPR/Cas9 gene editing tools as a means of re-sensitising resistant cancer cells. While significant progress has been made in preclinical settings, challenges such as tumour heterogeneity, limited clinical translation and immune clearance remain. Future directions include the integration of precision nanomedicine, scalable manufacturing and non-viral genome editing platforms. Collectively, nanoparticle-based drug delivery systems offer a multifaceted approach to combat MDR and hold great promise for improving therapeutic outcomes in resistant cancers.

CDK4/6 inhibition represents a new generation of cancer therapies, targeting CDK4/6 complexes to induce cell cycle arrest in the G1 phase. These inhibitors have been widely used in combination with hormone receptor antagonists for treating ER+/HER2− breast cancer, achieving significant clinical success. Building on this progress, ongoing research explores novel combination therapies and expands the application of CDK4/6 inhibitors to other diseases. However, challenges remain, including variable cellular responses and the rapid development of drug resistance. Recent studies have uncovered new resistance mechanisms and their unexpected effects on cell metabolism, autophagy, and the tumor microenvironment beyond cell cycle arrest. This review provides a comprehensive overview of the mechanisms by which CDK4/6 inhibitors combat cancer and explores their potential for more effective and personalized treatment strategies.