SOM230: a novel somatostatin peptidomimetic with broad somatotropin release inhibiting factor (SRIF) receptor binding and a unique antisecretory profile

OBJECTIVE

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.

DESIGN

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.

RESULTS

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.

CONCLUSION

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.

Synthesis and characterisation of [90Y]-Bz-DTPA-oct: a yttrium-90-labelled octreotide analogue for radiotherapy of somatostatin receptor-positive tumours

An investigation into the in vitro behaviour of two yttrium-90-labelled somatostatin analogues was performed. Further in vivo characterisation was performed with the most promising agent. A new DTPA-octreotide analogue (Bz-DTPA-oct) was synthesised by coupling a bifunctional DTPA chelator to the N-terminal amine of the D-Phe1 of Tyr3-octreotide. This new SRIF analogue and DTPA-octreotide (OctreoScan) were radiolabelled with 90Y prior to serum stability being evaluated. Receptor binding assays were also performed on the two radioligands using rat cortex membranes. The [90Y]-Bz-DTPA-oct was further evaluated in vivo using tumour-bearing rats. The first conjugate (DTPA-octreotide) bound with a high affinity to SRIF receptors and the 90Y complex was relatively stable in human serum (t1/2 3.8 d for 90Y lost to serum proteins). The second conjugate (Bz-DTPA-oct) also exhibited a high binding affinity to SRIF receptors, but it demonstrated an even slower loss of 90Y to serum proteins (t1/2 12.1 d). The in vivo evaluation of the more stable [90Y]-Bz-DTPA-oct showed a very rapid and high accumulation in somatostatin receptor-positive tumours, which after 1 h resulted in tumour/nontumour ratios of 3.8, 21, and 4.9 (for blood, muscle, and liver, respectively). These tumour/nontumour ratios increased, and were by 24 h postinjection 138, 285, and 6.1 (for blood, muscle, and liver). Yttrium-90-labelled Bz-DTPa-oct is rapidly and selectively accumulated in somatostatin receptor-positive tissue. Octadentate Bz-DTPA-oct could be ligand for 90Y radiotherapy of somatostatin receptor-positive tumours and their metastases.

Somatostatin analogues for somatostatin-receptor-mediated radiotherapy of cancer

The aim of the present study was to selectively target a beta-emitter-labelled octreotide analogue to somatostatin (SRIF)-receptor-expressing tumours and to evaluate the feasibility of SRIF-receptor-mediated radiotherapy by delivering a lethal dose of radiation to the tumour. The most promising compound in a series of DTPA-coupled octreotide analogues was DTPA-benzyl-acetamido-D-Phe1, Tyr3-octreotide (SDZ413). In vitro, SDZ413 binds with nanomolar affinity to SRIF-receptors (IC50 = 4.0 nM) and inhibits growth hormone release from primary cultures of rat pituitary cells with an IC50 of 7.2 nM. Biodistribution studies with [90Y]SDZ413 demonstrated a fast and significant SRIF-receptor-specific accumulation of the labelled conjugate (tumour/muscle ratio after 24 h: 52/1). [90Y]SDZ413 was effective in the radiotherapy of SRIF-receptor-positive tumours in a nude mouse model. A single treatment with [90Y]SDZ413 led to a significant decrease (25%) of tumour mass. This effect was mediated by the intact radioligand, since treatment with [90Y]SDZ978, a derivative of SDZ413 which does not bind with high affinity to SRIF-receptors or with the unlabelled SDZ413 alone, failed to affect tumour growth. These results suggest that receptor-targeted radiotherapy with a 90Y-labelled octreotide analogue represents a new strategy for the treatment of SRIF-receptor-positive tumours that have been previously diagnosed with OctreoScan111 (pentetreotide).

SDZ CO 611: a highly potent glycated analog of somatostatin with improved oral activity

To obtain orally active octreotide (Sandostatin, SMS 201-995) analogs a new class of glycated somatostatin derivatives were synthesized by the Amadori reaction (Maillard reaction). The synthesis, chemical and biological characterization of a series of new compounds is described. These oligopeptides bind with high affinity to somatostatin receptors and retain full biological activity. Whereas generally polypeptide hormones are almost completely inactive after oral administration, we report here for the first time that these analogs show remarkably high activity by the oral route. Thus for example SDZ CO 611, the D(+)-maltose Amadori derivative of octreotide, has about 10 times higher oral effect bioavailability than octreotide while maintaining the selectivity, metabolic stability and long duration of action of the parent compound.

From somatostatin to Sandostatin: pharmacodynamics and pharmacokinetics

Somatostatin and its octapeptide analogue octreotide (Sandostatin) have a similar high affinity for specific receptors with IC50s in the sub-nanomolar range. Hence, the striking superiority of octreotide in vivo, which includes duration of action, specificity, and potency, must originate from its different distribution, metabolism, and excretion behaviour. In animals and humans, investigations on their pharmacodynamic/pharmacokinetic relationship show plasma levels of 0.2-0.5 ng/ml (approximately 0.3 nM) to be therapeutically relevant for both peptides. The much lower clearance and improved metabolic stability in the circulation and in target organs of octreotide compared with somatostatin, result in much longer lasting, therapeutically relevant plasma and tissue levels and therefore in a longer duration of action. Their apparently specific inhibitory action of growth hormone when compared with that on insulin is pharmacodynamically based and may be exaggerated by physiological mechanisms of carbohydrate regulation. In summary, there is a distinct relationship between pharmacokinetic profiles and pharmacodynamic behaviour of somatostatin and its analogue Sandostatin.

From somatostatin to sandostatin: pharmacodynamics and pharmacokinetics

Somatostatin (SRIF) and its octapeptide analogue, octreotide (Sandostatin), have a similar high affinity for specific receptors with 50% inhibitory concentrations (IC50s) in the subnanomolar range. Hence, the striking superiority of octreotide in vivo, which includes duration of action, specificity, and potency, must originate from its different distribution, metabolism, and excretion behavior. In animals and humans, investigations of their pharmacodynamic/pharmacokinetic relationship show plasma levels of 0.2 to 0.5 ng/mL (approximately 0.3 nmol/L) to be therapeutically relevant for both peptides. The much lower clearance rates and improved metabolic stability in the circulation and in target organs of octreotide, compared with SRIF, result in much longer-lasting, therapeutically relevant plasma and tissue levels and therefore in a longer duration of action. Their apparently specific inhibitory action on growth hormone when compared with that on insulin is pharmacodynamically based, and may be exaggerated by physiological mechanisms of carbohydrate regulation. In summary, there is a distinct relationship between the pharmacokinetic profiles and pharmacodynamic behavior of SRIF and its analogue. Sandostatin.

Intestinal absorption of the octapeptide SMS 201-995 visualized by fluorescence derivatization

The absorption of an intact oligopeptide was investigated in rat and dog small intestine using the metabolically stable somatostatin analogue SMS 201-995. The synthetic octapeptide was coupled to 4-nitrobenzo-2-oxa-1,3-diazol to have a fluorescent label for the direct visualization. The 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide was active in displacing the corresponding hormone 125I-Tyr3-SMS 201-995 (Sandostatin; Sandoz Pharmaceuticals, Basel, Switzerland) from its high-affinity binding site in rat cortex membranes with an IC50 = 4.6 x 10(-10) mol/L. The release of growth hormone from cultured anterior pituitary cells was inhibited by the fluorescent somatostatin analogue with the same potency as by somatostatin 14 (IC50 = 6 x 10(-10) mol/L). Incubation with mucosal scrapings followed by high-performance thin-layer chromatography analysis showed that the peptide was stable against proteolysis. 4-Nitrobenzo-2-oxa-1,3-diazol SMS 201-995 was well absorbed from enterocytes of rat small intestine. The absorption was highest into jejunal cells and it could be inhibited by an excess of unlabeled peptide. A significantly lower absorption was detected in crypts compared with villus tips. No fluorescence could be seen in intestinal mucin and goblet cells. After oral administration, the 4-nitrobenzo-2-oxa-1,3-diazol-labeled peptide rapidly appeared in the blood of rats and dogs, reaching a bioavailability of 4.3% and maintaining pharmacological activity. This suggests that enterocytes are able to absorb intact oligopeptides being stabilized against proteolytic degradation through a transcellular mechanism.

Quantitative magnetic resonance imaging of estradiol-induced pituitary hyperplasia in rats

Magnetic resonance imaging (MRI) has been used for the determination in vivo of rat pituitary size. In midsagittal T2-weighted sections the pituitary, having a lower T2 value than the surrounding tissue, was visible with pronounced contrast. The size has been estimated by pixel counting. A close correlation (r = 0.96) with the pituitary weights determined postmortem has been obtained, demonstrating the reliability of the in vivo method. Using MRI the effects of Sandostatin, a somatostatin analog, on the pituitary size have been monitored in a rat model of prolactinoma (estradiol-induced hyperplasia of the pituitary). Treatment with Sandostatin over 4 weeks resulted in a 40% reduction of the hyperplastic pituitaries. These results have been confirmed by determination of pituitary weights postmortem. However, due to a large interindividual variation in size of hyperplastic pituitaries, more animals are required to reach statistical significance when only endpoints of treatment can be measured. In contrast, MRI allows one to monitor individually the drug effects over a long period of time, eliminating interindividual variations.

Pharmacodynamic profile of CQP 201-403, a novel 8 alpha-amino-ergoline

The profile of action in animals of CQP 201-403, a novel 8 alpha-amino-ergoline, is in most aspects that of a very potent dopaminomimetic, both as a prolactin secretion inhibitor, and at the levels of the CNS and the cardiovascular system. Qualitatively CQP 201-403 differs slightly from bromocriptine and apomorphine in its effects on the CNS (no influence on serotonin metabolism in the rat cortex; induction of masculine mounting behavior in rats) and the cardiovascular system of the dog (reflex tachycardia in response to a blood-pressure fall). In man the new compound proved to be highly active in lowering prolactin serum levels and be more potent than bromocriptine (Parlodel).

Structure-function relationships of somatostatin analogs

Objective of peptide chemistry has always been the production of analogues for clinical application. Advantages sought over natural peptides are (a) reduced molecular size; (b) prolonged biological half-life, and (c) enhanced specificity. After elucidation of the active core of somatostatin a number of analogues have been synthetized. Among them SMS 201-995, an octapeptide, was selected for further development because of its high potency and prolonged plasma clearance. Procedures extending the duration of action of somatostatin derivatives such as enhancement of lipophilicity and amino acid substitution are described, and factors influencing the specificity of such substances are succinctly analyzed.

Biochemical, behavioural, and endocrine effects of CK 204-933, a novel 8 beta-ergolene

CK 204-933 displaces [3H]dopamine and [3H]spiperone with high affinity from D-1 and D-2 recognition sites in membranes of calf caudate. Results from functional in vitro tests suggest that it is a partial agonist at D-1 receptors and an antagonist at D-2 receptors. These opposite effects at dopamine receptor subtypes are also expressed in vivo. For instance, in 6-hydroxydopamine lesioned rats, CK 204-933 induces contralateral rotations which are antagonised by SCH 23390 but not by sulpiride. On the other hand, CK 204-933 induces a long lasting increase of dopamine turnover in rat striatum and antagonises apomorphine-induced gnawing behaviour in rats. CK 204-933 increases prolactin serum levels in rats after subcutaneous administration, whereas after oral administration a moderate decrease of prolactin serum levels was seen. The latter effect is probably due to the formation of active metabolites. CK 204-933 exhibits also a high affinity to [3H]prazosin binding sites and antagonises serotonin-mediated stimulation of adenylate cyclase in rat hippocampus. On the other hand, CK 204-933 has no effect of only very weak effects on noradrenaline and serotonin release from rat cerebral cortex slices, which is consistent with its weak effects on noradrenaline- and serotonin-turnover in rat brain. Based on these properties it is suggested that CK 204-933 could be of therapeutic value in brain diseases associated with disturbances of monoaminergic neurotransmission.

Chemistry and pharmacology of SMS 201-995, a long-acting octapeptide analogue of somatostatin

Starting from a hypothetical conformation of natural somatostatin and a knowledge of the minimal fragment needed for biological activity, a process of rational design and lead optimization has led to the potent, selective, and long-acting analogue SMS 201-995, (formula: see text) which selectively inhibits growth hormone secretion in several animal species for up to 6 h after subcutaneous application. In the rat, SMS inhibits GH, insulin, and glucagon 70, 3, and 23 times more potently than SRIF, resulting in GH/insulin and GH/glucagon selectivities of 20 and 3, respectively. The compound has been shown to inhibit growth of transplantable insulinomas in hamsters and to label selectively a subset of somatostatin receptors in the rat cortex. A radioactively labelled analogue has been used to visualize somatostatin receptors in a GRF-secreting human tumour. The stability and duration of action of SMS 201-995 after subcutaneous injection enable for the first time extended investigations of the clinical utility of somatostatin in various diseases.

SMS 201-995: a very potent and selective octapeptide analogue of somatostatin with prolonged action

Stepwise modification of a conformationally stabilised analogue of the fragment of somatostatin which had been thought to be essential biologically active moiety has enabled us to synthesise the analogue H-(D) Phe-Cys-Phe-(D) Trp-Lys-Thr-Cys-Thr(ol) code-named SMS 201-995, which in vitro is three times more potent than the native hormone in inhibiting the secretion of growth hormone, which is highly resistant to degradation by pure enzymes and by tissue homogenates, which in vivo in rat and rhesus monkey is (depending on test system) at least 20 times more active than somatostatin, which is much longer acting, and which moreover in both species is much more selective in inhibiting the secretion of growth hormone than that of insulin. The compound is active by several routes of administration including the oral, is well tolerated both in laboratory animals and in man, and is currently undergoing preliminary clinical trial.

Two novel prolactin release-inhibiting 8 alpha-amino-ergolines

Prolactin secretion inhibition and changes in striatal dopamine metabolism in rats were compared after the administration of 8 alpha-amino-ergoline CH 29-717 and 2 derivates. CQ 32-084 was similar to but less potent than CH 29-717, while 32-085, the l-methyl derivative, showed delayed dopaminomimetic effects.

[The primary ovarian carcinoid. Case report]

A rare primary carcinoid tumor of the ovary, found incidentally in a 68-year-old woman, is reported. There had been no symptoms of carcinoid syndrome. Grossly, the unilateral tumor was cystic, measuring 13 cm in greatest diameter. Microscopically it consisted mainly of solid epithelial cords. The demonstration of argentaffin cells and the absence of a concomitant teratoma, dermoid or pseudomucinous cyst led to the diagnosis of a pure primary ovarian carcinoid tumor. On average the prognosis of such lesions is good and recurrence or fatal metastases are rare. Treatment consists of salpingo-oophorectomy. To rule out a metastasizing carcinoid with similar histologic features, the contralateral ovary should be biopsied since metastases almost invariably occur bilaterally.