Small cell lung cancer (SCLC) is an aggressive type of lung cancer, for which cytotoxic chemotherapy appears to have reached its maximal efficacy. This neoplasm is characterized by the overexpression of several receptor tyrosine kinases (RTKs), especially c-Kit. The ligand for c-Kit is stem cell factor (SCF). In SCLC, SCF can influence c-Kit activation by autocrine or paracrine mechanisms. We have recently shown that the c-Kit/SCF pathway is operational in SCLC and can be inhibited by Glivec (STI571). Because the inhibition of topoisomerase-I (topo-I) is one approach used to treat SCLC, we determined the effects of c-Kit/SCF signaling on topo-I activity. A unique phosphorylation of c-Kit on amino acid 823 and amino acid 703 was identified with the SCF stimulation of H526 cells. We demonstrate that with SCF stimulation over 16 hours (dose response 0-100 ng/mL) in H526 SCLC cells (c-Kit positive, SCF responsive), a decrease in topo-I activity was observed, whereas in H82 SCLC cells (c-Kit negative, SCF unresponsive) there was no modulation of topo-I activity by SCF. Using STI571 (5 microM, 16 hours) to inhibit the c-Kit pathway following stimulation with SCF (100 ng/mL), an upregulation of topo-I activity was observed in H526 cells but not in H82 cells. Performing viability assays, we show that STI571 in combination with topo-I inhibition by camptothecin or SN38, the active metabolite of irinotecan, can cooperatively inhibit H526 cell viability (but not H82 cell viability) for 72 hours. We also show that STI571 does not directly inhibit topo-I activity in SCLC. The combination of STI571 with topo-I inhibition could provide a useful combination in the treatment of SCLC.