Abstract: The invention provides endoplasmic reticulum-targeted chemiluminescent agents and their use in methods of photodynamic therapy (PDT). In particular, the invention provides compounds of general formula (I), and their pharmaceutically acceptable salts: in which A represents a chemiluminescent moiety; each L, which may be the same or different, is either a direct bond or a linker; each B, which may be the same or different, represents an endoplasmic reticulum-targeting moiety; n is an integer from 1 to 3, preferably 1; and x is an integer from 1 to 3, preferably 1. Such compounds find particular use in the treatment of deeply-sited tumours, e.g. glioblastoma multiforme (GBM), when used in combination with a photosensitizer or photosensitizer precursor.