Disclosed are conjugates, compositions, and methods that include or use one or more gold(III) porphyrin complexes. Preferred conjugates are gold(III) porphyrin-poly(ethylene glycol) (PEG) conjugates. Preferred compositions and methods include or use such gold(III) porphyrin-PEG conjugates. Disclosed is a method for treatment of cancer comprising administering to a human in need of such treatment a composition containing a therapeutically effective amount of gold(III) porphyrin-poly(ethylene glycol) (PEG) conjugates. The gold(III) porphyrin-PEG conjugates can be in the form of nanostructures formed by self-assembly of the gold(III) porphyrin-PEG conjugates; or nanocomposites formed by encapsulation of other therapeutic agents by the nanostructures of gold(III) porphyrin-PEG conjugates.