Abstract: A two component system for therapeutic treatment of a host, having a first component comprising a targeting moiety capable of binding with a tumour associated antigen, linked to a mutated enzyme capable of converting a prodrug into an antineoplastic drug, and a second component comprising a prodrug convertible under the influence of the mutated enzyme to the antineoplastic drug. The mutated enzyme is a mutated form of a natural host enzyme which recognizes its natural substrate by an ion pair interaction with the substrate, wherein the mutated enzyme and prodrug have structures such that the polarity of the mutated enzyme/prodrug ion pair interaction is reversed relative to the natural host enzyme/natural substrate ion pair interaction. The first component is substantially non-immunogenic in the host and the prodrug second component is not significantly convertible into antineoplastic drug in the host by natural unmutated host enzyme.