Progress in medicinal chemistry relies heavily on ligands as pharmacologic tools to evaluate selectivity of action. This is particularly relevant to opioid receptors, as there are at least three major receptor types (p, K, 6) that modulate a variety of physiologic pr0cesses. l Although highly selective opioid antagonists have been reported for p2, 3 and K~ receptors, the fact that antagonist ligands for 6 receptors are peptides5 related to enkephalin limits ...
[Lever, John R.; Kinter, Chris M.; Ravert, Hayden T.; Musachio, John L.; Mathews, William B.; Dannals, Robert F. Journal of Labelled Compounds and Radiopharmaceuticals, 1995 , vol. 36, # 2 p. 137 - 146]