The aim of the present study was to identify a small, metabolically stable somatotropin release inhibiting factor (SRIF) analog with a more universal binding profile similar to that of natural somatostatin, resulting in improved pharmacological properties and hence new therapeutic uses. A rational drug design approach was followed by synthesizing alanine-substituted SRIF-14 analogs to determine the importance of single amino acids in SRIF-14 for SRIF receptor subtype binding. The incorporation of structural elements of SRIF-14 in a stable cyclohexapeptide template in the form of modified unnatural amino acids resulted in the identification of the novel cyclohexapeptide SOM230. SOM230 binds with high affinity to SRIF receptor subtypes sst1, sst2, sst3 and sst5 and displays a 30- to 40-fold higher affinity for sst1 and sst5 than Sandostatin (octreotide; SMS 201-995) or Somatuline (BIM 23014). In vitro, SOM230 effectively inhibited the growth hormone releasing hormone (GHRH)-induced growth hormone (GH) release in primary cultures of rat pituitary cells with an IC(50) of 0.4+/-0.1 nmol/l (n=5). In vivo, SOM230 also potently suppressed GH secretion in rats. The ED(50) values determined at 1 h and 6 h post injection of SOM230 indicated its very long duration of action in vivo. This property was also reflected in pharmacokinetic studies comparing plasma levels of SMS 201-995 and SOM230 after subcutaneous application. Whereas SMS 201-995 had a terminal elimination half life of 2 h, this was markedly prolonged in SOM230-treated animals (t(1/2)=23 h). Furthermore, in rats SOM230 demonstrated a much higher efficacy in lowering plasma insulin-like growth factor-I (IGF-I) levels compared with SMS 201-995. The infusion of 10 microg/kg/h of SOM230 using subcutaneously implanted minipumps decreased plasma IGF-I levels far more effectively than SMS 201-995. After 126 days of continuous infusion of SOM230 plasma IGF-I levels were decreased by 75% of placebo-treated control animals. For comparison SMS 201-995, when used under the same experimental conditions, resulted in only a 28% reduction of plasma IGF-I levels, indicating a much higher efficacy for SOM230 in this animal model. It is important to note that the inhibitory effect of SOM230 was relatively selective for GH and IGF-I in that insulin and glucagon secretion was inhibited only at higher doses of SOM230. This lack of potent inhibition of insulin and glucagon release was also reflected in the lack of effect on plasma glucose levels. Even after high dose treatment over 126 days no obvious adverse side effects were noticed, including changes in plasma glucose levels. We have identified a novel short synthetic SRIF peptidomimetic, which exhibits high affinity binding to four of the five human SRIF receptor subtypes and has potent, long lasting inhibitory effects on GH and IGF-I release. Therefore SOM230 is a promising development candidate for effective GH and IGF-I inhibition and is currently under evaluation in phase 1 clinical trials.
