Alterations in PIK3CA, the gene encoding the p110α subunit of phosphatidylinositol 3-kinase (PI3Kα), are frequent in head and neck squamous cell carcinomas. Inhibitors of PI3Kα show promising activity in various cancer types, but their use is curtailed by dose-limiting side effects such as hyperglycaemia. In the present study, we explore the efficacy, specificity and safety of the targeted delivery of BYL719, a PI3Kα inhibitor currently in clinical development in solid tumours. By encapsulating BYL719 into P-selectin-targeted nanoparticles, we achieve specific accumulation of BYL719 in the tumour milieu. This results in tumour growth inhibition and radiosensitization despite the use of a sevenfold lower dose of BYL719 compared with oral administration. Furthermore, the nanoparticles abrogate acute and chronic metabolic side effects normally observed after BYL719 treatment. These findings offer a novel strategy that could potentially enhance the efficacy of PI3Kα inhibitors while mitigating dose-limiting toxicity in patients with head and neck squamous cell carcinomas.