The effect of protease inhibitors and gastrin releasing peptide receptor antagonists on human small cell lung cancer growth

The effect of the protease inhibitors BBI and aprotinin on the in vitro clonal growth of two human small cell lung cancer (SCLC) cell lines was investigated. Also, the effect of BBI on processing of proGRP and on mRNA levels for prohormone convertase 1 and 2 (PC1 and PC2) in SCLC cells was examined. Finally, the in vivo distribution of a BN/GRP receptor antagonist in mice was studied. In vitro growth of NCI-H345 and NCI-N592 SCLC cells was inhibited by the protease inhibitors BBI and aprotinin. When NCI-H345 cells were incubated with BBI and BN in the same plates, the addition of BN appeared to overcome growth suppression by BBI. The BN/GRP receptor antagonists [Psi¹³,¹⁴] BN and [FA]BN(6-13)oME significantly inhibited growth of SCLC cells in vitro at high concentrations but significantly increased growth of the NCI-H345 cell line at lower concentrations. These conflicting data may be due to non-specific binding or mechanisms unrelated to receptor binding. Treatment of NCI-H345 cells with protease inhibitors increased both proGRP and GRP levels. The present study of PC1 and PC2 in two SCLC cell lines indicates that mRNA for prohormone convertases is present and that the cells contain higher levels of PC2 mRNA than PC1 mRNA. When these cells were incubated with 100 μg/ml BBI, mRNA for PC1 and PC2 decreased by approximately 50%. The ¹⁴C radiolabeled BN/GRP receptor antagonist [FA] BN(6-13)oME was absorbed by one hour after subcutaneous administration and distributed to many organs in Balb/c nude mice. The radiolabeled peptide appeared to be eliminated mainly in the feces and urine. The effect of protease inhibitors to increase proGRP and GRP levels in NCI-H345 cells after two-day treatment may be due to several factors. There may be up-regulation of GRP production by these cells after an initial decrease in processing of proGRP. Furthermore, BBI may inhibit transcription or posttranscriptional processing of mRNA for PC1 and PC2. Finally, the GRP receptor antagonist (¹⁴C-Trp⁸)[FA] BN(6-13)oME is quickly distributed to tumor tissue. These results indicate that protease inhibitors inhibit SCLC growth and have potential as novel therapeutic agents in human SCLC