Figure 4.

This figure shows conditionally cytotoxic gene therapy, also referred to as enzyme-prodrug activating therapy or suicide gene therapy, which is the most commonly used gene therapy. In conditionally cytotoxic approaches, as described in the text, the transgene for a noncytotoxic enzyme is delivered into tumor cells using a vector, and this enzyme remains noncytotoxic until the administration of a noncytotoxic prodrug. Upon prodrug administration, the enzyme converts the noncytotoxic prodrug into a toxic metabolite that induces tumor cell death