An agent comprises a vector having a functional gene, a prodrug which can be converted into a cytotoxic agent by an expression product of the gene, and another cytotoxic agent, as a combined preparation for simultaneous, sequential or separate use in the therapy of cancer or of a disease characterised by an impaired mismatch repair (MMR) pathway, wherein the dosage regimen comprises beginning another cytotoxic agent therapy no later than 7 days after the prodrug therapy has finished.
