Abstract: The invention provides polymeric particles comprising a matrix of a biocompatible polymer and polyethylene imine, said matrix having incorporated therein an anionic or hydrophobic sonosensitiser and, optionally, an immunomodulatory agent and/or an imaging agent. Such particles find use in methods of sonodynamic therapy, in particular in methods of combined sonodynamic therapy and immunotherapy, for example in the treatment of cancer, metastasis or micrometastasis derived from cancer. The invention is particularly suitable for the treatment of deep-sited, hard to treat tumours such as pancreatic cancer.