Five peptides related to the δ-receptor agonist, dermorphin gene associated peptide (DGAP), having structural modifications at the positions 2, 6 and 7 were synthesised by the solid phase method using 9-fluorenylmethoxycarbonyl amino acid trichlorophenyl esters as coupling agents and p-alkoxybenzyl alcohol resin as the solid support. The biological activities of these peptides were determined in vitro using guinea pig ileum and mouse vas deferens assays and in vivo employing the analgesic and antidiarrhoeal assays. The δ-receptor selectivity was determined by the ratio of IC50 values in the MVD assay to that of the GPI assay and all the synthetic peptides were found to possess δ-receptor selectivity; [Nva6]DGAP being the most selective in the present series. [Eth6]DGAP exhibits a higher selectivity with regard to analgesic to antidiarrhoeal activity in the in vivo assay.
