Eight analogues of oxytocin and arginine-vasopressin were synthesized, in which the proline residue in position 7 was replaced by either sarcosine or N-methylalanine; some of the pharmacological properties of these analogues were evaluated. In peptides containing a beta-mercaptopropionic acid residue in position 1, the additivity of the effects of deletion of the amino group in position 1 and of the above-noted replacements in position 7 on biological properties of these analogues was ascertained. All of the analogues were found to be potent in either antidiuretic or uterine activity and also selective in action. From the point of view of pharmacological properties, substitution of sarcosine in position 7 of oxytocin gave analogues with higher oxytocic and milk-ejecting activities than did the substitution of N-methylalanine. The opposite structure-activity relationship was observed with arginine-vasopressin, where the N-methylalanine-containing analogues were more potent than the sarcosine-containing analogues with respect to pressor activity and also, if not deaminated, with respect to antidiuretic activity.

Although many workers have measured the plasma half-lives of oxytocin and vasopressin in rats (l), there have been few studies on the half-lives of structural analogs of vasopressin and oxytocin. This is largely due to the lack of suitable radioimmunoassay methods and the difficulties inherent in measuring peptides in plasma by bioassays. Studies on the rates of elimination from plasma of analogs of the neurohypophysial hormones could provide information on the manner in which the endogenous hormones are metabolized. I have, therefore, applied a new method for estimating plasma half-lives from vasopressor responses (2) to a series of analogs of vasopressin and oxytocin in an effort to determine which structural changes can delay elimination from the plasma. From such information one may speculate on the types of enzymatic inactivation that are important for the elimination of arginine-vasopressin and oxytocin and whether these are different for these two hormones. Materials and methods.To screen for possible long-acting analogs, I first looked through several years' records of vasopressor assays done on vasopressin and oxytocin analogs in Dr. W. H. Sawyer's laboratory. If I could mask identification of the injections and consistently distinguish the responses to the test analog as longer than the responses to the standard, I considered the analog as a candidate for half-life studies. The analogs which could be so distinguished were [8-ornithinel-oxytocin, [8-arginine]-vasotocin, and deaminated analogs of vasopressin. Guided largely by this survey, I decided to concentrate on measuring the half-lives of analogs modified in the one position, the eight position, and the one and six positions.Female Sherman rats (from Camm Research) were used throughout. They were prepared as for vasopressor assay, and the pressor responses to a series of intravenous injections of each analog were analyzed to obtain a plasma half-life by the curve-fit method, as previously described (2). The series of injections was 1, 2,4, 8, and 32 pressor mU of peptide, given in ascending order of dose, in all instances. A new injection was given only after blood pressure had returned to normal for 10-15 min. Each rat was used to study two peptides. In the instances in which two peptides were found to have significantly different mean half-lives as determined from separate groups of rats, the longer-acting peptide was also longer-acting when the two peptides were compared in an individual rat.Peptides used for half-life determinations were arginine-vasopressin, lysine-vasopressin, arginine-vasotocin, oxytocin, " 1 -hydroxy-arginine-vasopressin"([ 1 -~-2 -h ydroxy-3-mercaptopropionic acidl-argininevasopressin), " 1 -hydroxy-oxytocin", [ 8-ornithinel-oxytocin, [8-glutamine]-oxytocin, [ 8proline] -oxytocin, "deamino -dicarba-arginine-vasopressin" ([ 1,6-aminosuberic acid]arginine-vasopressin), and deamino-dicarbaarginine-vasotocin. Some of the data on halflives of arginine-vasopressin, arginine-vasotocin, and deamino-arginine-vasopressin have been re...

Several new synthetic analogs of the oxytocin antagonist [1-deaminopenicillamine]oxytocin have been prepared and tested for their abilities to inhibit responses to oxytocin by the isolated rat uterus in the absence and presence of Mg++, by the rat uterus in situ, and by the rat mammary gland in situ. Substituting 2-O-methyltyrosine in [1-deaminopenicillamine]oxytocin strikingly enhances antagonism of all uterin responses, and [1-deaminopenicillamine, 2-O-methyltyrosine]oxytocin and its 4-threonine analog are also potent inhibitors of the milk ejection response. Substituting 2-phenylalanine in [1-deaminopenicillamine]oxytocin also enhances antagonistic activities in all uterine assays, but [1-deaminopenicillamine, 2-phenylalanine]oxytocin retains agonistic activity on milk ejection assays. From these studies we can conclude that changes in the 1-position (1-deaminopenicillamine substitution) and the 2-position (2-O-methyltyrosine or 2-phenylalanine substitution) can have additive effects on antagonistic activities. Substitution of an 8-ornithine also enhances inhibitory potency in vivo, and this effect may also be additive to those of the substitutions in 1- and 2-positions. These findings provide many clues that may lead to the design of even more effective antagonists; several of the analogs reported here appear to the most effective antagonists of oxytocin in vivo yet reported and may be useful agents in further studies on the physiological functions of endogenous oxytocin.

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