Vasoactive intestinal peptide–camptothecin conjugates inhibit the proliferation of breast cancer cells

The effects of vasoactive intestinal peptide–camptothecin (VIP–CPT) conjugates were investigated on breast cancer cells and cells transfected with VIP receptors (R). (Ala2,8,9,19,24.25.27, Nle17, Lys28)VIP, (A-NL-K)VIP, was synthesized and Lys28 was coupled to a linker, N-methyl-amino-ethyl-glycine, L2, which formed a carbamate bond with CPT. The resulting (A-NL-K)VIP-L2-CPT was cytotoxic for MCF7 breast cancer cells, which have VPAC1-R, with IC50 values of 380 and 90 nM using the MTT and clonogenic assays, respectively. (A-NL-K)VIP, (A-NL-K)VIP-L2 and (A-NL-K)VIP-L2-CPT inhibited specific binding of 125I-VIP to 3T3 cells transfected with VPAC1-R with IC50 values of 1.9, 56 and 126 nM, respectively. In contrast, (A-NL-K)VIP, (A-NL-K)VIP-L2 and (A-NL-K)VIP-L2-CPT inhibited specific binding of 125I-Ro25-1553 to 3T3 cells transfected with VPAC2-R with IC50 values of 3.9, 3162 and 2690 nM, respectively. (A-NL-K)VIP, (A-NL-K)VIP-L2 and (A-NL-K)VIP-L2-CPT caused increased cAMP after addition to MCF7 cells. 125I-(A-NL-K)VIP-L2-CPT was internalized by MCF7 cells at 37 °C but not 4 °C. These results indicate that (A-NL-K)VIP-L2-CPT is a VPAC1-R agonist which is cytotoxic for breast cancer cells.