A structure function study of C-terminal extensions of bombesin.

Synthetic C-terminal extensions of BN were synthesized and the biological potency evaluated using Swiss 3T3 and small cell lung cancer cells. BN, which has an amidated C-terminal, inhibited specific [125I-Tyr4]BN binding activity to Swiss 3T3 cells with an IC50 value of 1 nM, whereas the IC50 of BN-OH, which has a free C-terminal, was 1800 nM. The IC50 values of BNG, BNGK and BNGKK were 1400, 4700 and 500 nM, respectively. Similar binding data were obtained using SCLC cell line NCI-H345 and the bombesin analogues functioned as agonists based on the ability to elevate cytosolic Ca2+ in Fura-2 AM loaded SCLC cells. Also, the bombesin analogues stimulated 3H-thymidine uptake in Swiss 3T3 cells and the ED50 values for BN, BNG, BNGK and BNGKK were 1, 1300, 3900 and 400 nM. These data suggest that an amidated C-terminal is essential for high affinity binding and potency of BN.