The present invention encompasses novel analogs of vasoactive intestinal peptide (VIP), containing substitutions at appropriately selected amino acids. The invention particularly relates to the design and synthesis of novel biologically active VIP analogs containing .alpha., .alpha.-dialkylated amino acids in a site-specific manner. Specifically, the invention relates to the synthesis of VIP peptide derivatives, which bind selectively to VIP receptors on target cells. The invention encompasses methods for the generation of these peptides, compositions containing the peptides and the pharmacological applications of these peptides especially in the treatment and prevention of cancer.