Bombesin/gastrin releasing peptide (BB/GRP) and insulin-like growth factor I (IGF-I) are major autocrine growth factors for small cell lung cancer (SCLC). Previously it was shown that the mechanisms of action of BB/GRP and IGF-I were different, as BB/GRP is a G-protein coupled receptor causing phosphatidylinositol (PI) turnover, whereas IGF-I is a tyrosine kinase. In the present period, new GRP receptor antagonists were evaluated. BW2258U89 bound with high affinity to GRP receptors (IC50 = 5 nM) in a reversible manner. The inositol-1,4,5-trisophosphate released when PI was metabolized, elevated cytosolic Ca2+ whereas the diacylglycerol released activated protein kinase C (PKC). BW2258U89 inhibited the ability of BB to elevate cytosolic Ca2+ and translocate PKC. BW2258U89 inhibited the growth of SCLC in vitro and in vivo. Using nude mice bearing SCLC xenografts, 125-I-BW1023U90 localized preferentially to SCLC tumors. Using microspheres which slowly release BW2258U89, the effective plasma concentration of GRP receptor antagonist was 10 nM. Currently, the pharmacokinetics of BW2258U89 is being investigated. Also, (chlorambucil-5) BB-6-13 methylester was synthesized, and this may be an irreversible GRP receptor antagonist. These data suggest that the GRP receptor antagonists may represent therapeutic agents for SCLC. Also, using A/J mice, lung adenomas formed 4 months after treatment with carcinogen (urethane). Because administration of BW2258U89 reduced the number of adenomas by 50%, it may function as a chemopreventive agent in lung cancer. These data suggest that BB/GRP, similar to phorbol ester, may promote lung carcinogenesis. Currently we are investigating if GRP and/or GRP receptors are biomarkers for lung cancer. BB stimulated nuclear oncogene expression in SCLC cells. BB transiently increased c-fos mRNA in SCLC cells in a time and concentration dependent manner. The increase in c-fos mRNA caused by 10 nM BB was reversed by 1 mu M BW2258U89. Similarly, BB transiently increased c-jun gene expression. Because BB increased both c-fos and c-jun mRNAs, BB may activate AP-1 sites, altering gene expression.
