Synthesis and some pharmacological properties (4-beta-(2-thienyl)-L-alanine)oxytocin

The synthesis and some biological activities of [4-beta-(2-thienyl)-L-alanine]oxytocin are reported. This analogue has been studied in an ongoing exploration of the biological effects of introducing amino acid residues with bulky hydrophobic side chains into the second corner position of the beta turn present in the conformation of the 20-membered ring portion of oxytocin. The analogue was synthesized in stepwise manner by solution techniques utilizing ethylcarbamoyl protection for cysteine side chains. The presence of thienylalanine in position 4 evokes a drastic reduction in both affinity and intrinsic activity; the reduction in intrinsic activity was greater than that found for [Leu4]oxytocin or [Phe4]oxytocin. The analogue possesses 0.51 +/- 0.03 unit/mg of rat uterotonic potency and less than 0.05 unit/mg of rat pressor and rat antidiuretic potency and behaves as a competitive inhibitor of the response to oxytocin in the avian vasodepressor assay with a pA2 value of 7.44 +/- 0.19.