Barbiturates--structure versus RIA reactivity.

Over 90 different compounds having structural similarities to barbituric acid were analyzed by radioimmunoassay (RIA) using 125I-secobarbital reagents (Roche). Affinities were compared with molecular structures and a number of observations are made. It was found that all definitive components of the 5,5-dialkyl barbituric acid ring structure were essential for reactivity with the Roche RIA reagents; structural changes at any position of the ring reduced reactivity. No compounds studied were found to be more reactive than secobarbital [5-allyl-3-(1-methylbutyl) barbituric acid] and RO-2-1126[5-allyl-5-(1-carbamoylisopropyl) barbituric acid (the hapten used to prepare the RIA antibody)]. Changes in the 5-allyl and/or the 5-(1-methylbutyl) groups of secobarbital resulted in decreased reactivity.