In suicide gene therapy, the introduced therapeutic gene encodes an enzyme capable of transforming a non-toxic prodrug into a cytotoxic drug. Utilizing cancer-specific promoters suicide gene expression can be selectively targeted to the cancer cells of interest. For that purpose we have identified several promotor regions, which are highly promising candidates for transcriptionally targeted gene therapy for small cell lung cancer (SCLC). The suicide gene yeast cytosine deaminase (YCD) converts the prodrug 5-fluorocytosin (5-FC) into the known chemotherapeutic agent 5-fluorouracil (5-FU). YCD was cloned for regulated expression from the SCLC specific promoter Insulinoma-associated 1 (INSM1) (1) and transiently transfected into different cell lines, which were exposed to increasing concentrations of 5-FC. Transfected SCLC cells were greatly sensitised to 5-FC and significant cell death was achieved while cancer cell lines of other origins were unaffected to treatment. Furthermore the YCD gene was fused with the yeast uracil posphoribosyltransferase (YUPRT) gene (2), which augments the conversion of 5-FU into active cytotoxins. The fusion construct (YCD-YUPRT) demonstrated significantly increased sensitivity towards 5-FC in treated SCLC cells resulting in an IC50 value 4 times lower than with YCD. Due to limited efficiency of gene delivery in vivo an important feature of suicide gene therapy is the bystander effect where suicide gene/prodrug-produced toxins diffuse to untransfected neighbouring cells. In the cytosine deaminase-based suicide gene therapy 100 % cell death was achieved after 5-FC treatment when only 50 % of cells expressed the YCD or YUPRT gene. Further it was established that the YCD-YUPRT/5-FC strategy caused extensive cell death when as few as 10 % cells expressed the transgene. This contrast previously obtained results with the suicide gene Herpes simplex virus thymidine kinase (HSVtk) and the prodrug penciclovir (PCV) where cell death was restricted to HSVtk-transfected cells. As succesfull cancer treatment relies on multi-targeting treatment the combination of HSVtk and YCD-YUPRT suicide gene therapy was tested. At low prodrug concentrations an additive effect of the systems was obtained while the YCD-YUPRT mediated toxicity dominated at high 5-FC concentrations. Further testing of these and other suicide systems in vivo will conclude on the significance of combinatorial suicide gene therapy for SCLC. (1) Pedersen N, Pedersen MW, Lan MS, Breslin MB, Poulsen HS. The insulinoma-associated 1: a novel promoter for targeted cancer gene therapy for small-cell lung cancer. Cancer Gene Ther 2005 Jul 29. (2) Graepler F, Lemken ML, Wybranietz WA, Schmidt U, Smirnow I, Gross CD, et al. Bifunctional chimeric SuperCD suicide gene -YCD: YUPRT fusion is highly effective in a rat hepatoma model. World J Gastroenterol 2005 Nov 28;11(44):6910–9.