Drug Combination in Polymeric Nanocarriers for Chemotherapy of Cancer: Preclinical Outcomes in the Last Ten Years

<b>Background:</b> Combination chemotherapy using nanotechnology-based delivery is a promising approach to improve cancer treatment, but the added value of co-loaded polymeric nanocarriers has not been comprehensively appraised. This review synthesizes preclinical evidence on polymeric systems co-encapsulating antitumor agents. <b>Methods:</b> A narrative literature review identified 33 preclinical studies (2015-2025) employing polymer-based nanocarriers to co-load at least two antitumor drugs. Study characteristics and in vitro and in vivo outcomes were qualitatively analyzed. <b>Results:</b> Most studies addressed breast, lung, or ovarian cancer and used micelles or nanospheres. Co-loaded formulations consistently enhanced in vitro cytotoxicity and, in vivo, produced marked tumor growth inhibition relative to free drugs or single-loaded systems; in several reports, near-complete or complete tumor regression was achieved. Synergy was frequently suggested but not consistently quantified, more than half of the studies did not report a combination index. Most formulations showed favorable tolerability, with few reports including mild hepatic toxicity, renal, or weight-related effects. Beyond conventional drug pairs, examples of co-delivering chemotherapeutics with resistance modulators, gene therapy agents, or targeted ligands illustrated how tailored release profiles and active targeting can potentiate efficacy. Nevertheless, heterogeneity in models, dosing schedules, endpoints, and limited long-term safety data hamper cross-study comparison and translation. <b>Conclusions:</b> Co-loaded polymeric nanocarriers constitute a promising platform to optimize combination chemotherapy, improving preclinical antitumor efficacy with generally limited toxicity, but more standardized and mechanistically driven studies are required to support clinical development.