The invention provides CaO2 nanoparticles having a pH-responsive coating for use in a method of adjuvant therapy of hypoxic tumour cells or tissues. The nanoparticles find particular use in enhancing cancer therapies that depend on oxygen to exert their effect, such as photodynamic therapy (PDT), sonodynamic therapy (SDT), and radiotherapy. The invention also provides pharmaceutical compositions containing the coated CaO2 nanoparticles, together with at least one photosensitiser, sonosensitiser, or radiosensitiser and, optionally, at least one pharmaceutical carrier or excipient.
