Native Somatostatin Research Articles

The Effect of Albumin Fusion Patterns on the Production and Bioactivity of the Somatostatin-14 Fusion Protein in Pichia pastoris

Somatostatin is a natural inhibitor of growth hormone, and its analogues are clinically used for the therapy of acromegaly, gigantism, thyrotropinoma, and other carcinoid syndrome. However, natural somatostatin is limited for clinical usage because of its short half-life in vivo. Albumin fusion technology was used to construct long-acting fusion proteins, and Pichia pastoris was used as an expression system. Three fusion proteins, (somatostatin (SS)14)2-human serum albumin (HSA), (SS14)3-HSA, and HSA-(SS14)3, were constructed with different fusion copies of somatostatin-14 and fusion orientations. The expression level of (SS14)3-HSA and HSA-(SS14)3 was much lower than (SS14)2-HSA due to the additional fusion of the somatostatin-14 molecule. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry revealed that severe degradation occurred in the fermentation process. Similar to the standard of somatostatin-14, all three fusion proteins were able to inhibit growth hormone secretion in the blood, with (SS14)2-HSA being the most effective one. On the whole, (SS14)2-HSA was the most effective protein in both production level and bioactivity, and increasing the number of small protein copies fused to HSA may not be a suitable method to improve the protein bioactivity.

The role of somatostatin analogues in treatment of TSH secreting pituitary adenomas

Background TSH-secreting tumours are extremely rare case of hyperthyroidism. Most important clinical feature is preserved TSH level in subjects with apparent thyrotoxicosis. Possible misdiagnosis of primary thyroid hyperfunction could lead to mistreatment with anti-thyroid medications. This worsens disease course and outcome. Neurosurgery success rate is limited by tumour size and its extrasellar expansion. Native somatostatin is key negative regulator of TSH secretion. In most cases tumour cells express somatostatin receptors. This feature creates potential use of somatostatin analogues for medical treatment of TSHsecreting tumours.

The Mechanism of Cytostatic Effect of a New Somatostatin Analog Cifetrelin on In Vitro Cultured MCF-7 Breast Carcinoma Cells

The mechanisms of antitumor effect of cifetrelin have been studied in vitro on cultured MCF-7 breast carcinoma cells. Cifetrelin exhibits higher cytotoxic activity than native somatostatin. In subthreshold concentrations, cifetrelin boosts the apoptotic effects of drugs, e.g. adriamycin. Cifetrelin-induced apoptosis develops by the p53-independent mechanism and is associated with early suppression of NF-kappaB activity. These data attest to high antitumor activity of cifetrelin.

Phosphorylation of Threonine 333 Regulates Trafficking of the Human sst5 Somatostatin Receptor

The frequent overexpression of the somatostatin receptors sst2 and sst5 in neuroendocrine tumors provides the molecular basis for therapeutic application of novel multireceptor somatostatin analogs. Although the phosphorylation of the carboxyl-terminal region of the sst2 receptor has been studied in detail, little is known about the agonist-induced regulation of the human sst5 receptor. Here, we have generated phosphosite-specific antibodies for the carboxyl-terminal threonines 333 (T333) and 347 (T347), which enabled us to selectively detect either the T333-phosphorylated or the T347-phosphorylated form of sst5. We show that agonist-mediated phosphorylation occurs at T333, whereas T347 is constitutively phosphorylated in the absence of agonist. We further demonstrate that the multireceptor somatostatin analog pasireotide and the sst5-selective ligand L-817,818 but not octreotide or KE108 were able to promote a detectable T333 phosphorylation. Interestingly, BIM-23268 was the only sst5 agonist that was able to stimulate T333 phosphorylation to the same extent as natural somatostatin. Agonist-induced T333 phosphorylation was dose-dependent and selectively mediated by G protein-coupled receptor kinase 2. Similar to that observed for the sst2 receptor, phosphorylation of sst5 occurred within seconds. However, unlike that seen for the sst2 receptor, dephosphorylation and recycling of sst5 were rapidly completed within minutes. We also identify protein phosphatase 1γ as G protein-coupled receptor phosphatase for the sst5 receptor. Together, we provide direct evidence for agonist-selective phosphorylation of carboxyl-terminal T333. In addition, we identify G protein-coupled receptor kinase 2-mediated phosphorylation and protein phosphatase 1γ-mediated dephosphorylation of T333 as key regulators of rapid internalization and recycling of the human sst5 receptor.

Nævus éruptifs sous octréotide

Octreotide is a synthetic octapeptide having properties related to those of natural somatostatin, a hypothalamic hormone. We report a case of eruptive naevus in patients treated with octreotide. A 52-year-old man consulted for multiple achromic naevi of recent onset. He was undergoing treatment with octreotide 30 mg per month for hepatic metastases in a setting of operated colonic carcinoid tumor. Since the start of this treatment, he had presented an efflorescence of diffuse naevus comprising over 200 lesions, certain of which were highly atypical. Annual follow-up was carried out. Ten years later, the patient was still alive and undergoing octreotide therapy at the same dose. The number of naevi had stabilized and there was no evidence of melanoma. Eruptions of naevi are rare; they may occur at the sites of lesions in bullous diseases or during immunodepression (malignant or drug induced, HIV infection). There are questions concerning possible stimulation of naevus cells and the risk of onset of melanoma. Routine monitoring of pigmented lesions is thus necessary in patients treated with octreotide.

Frontiers in Cancer Endocrinology

Frontiers in Cancer Endocrinology

Three Native Somatostatin Isoforms Differentially Affect Membrane Voltage-Sensitive Ion Currents in Goldfish Somatotrophs

Message encoding for three isoforms of somatostatin (SS) peptides, SS-14, goldfish brain (gb)SS-28 and [Pro²]SS-14, are expressed in goldfish hypothalamus and pituitary tissues. All three native goldfish SSs are active in reducing basal and stimulated growth hormone (GH) responses in cultured goldfish pituitary cells, although with different potencies and efficacies. In the present study, we examined the effects of these three endogenous SSs on electrophysiological properties of goldfish somatotrophs and their physiological relevance. Voltage-sensitive K+ , Ca²+ and Na+ channels in identified goldfish somatotrophs in primary culture were isolated using whole-cell, amphotericin B-perforated patch-clamp techniques. None of the three SSs affected Na+ currents but all three SSs increased maximal K+ current magnitude, with SS-14 being the most effective. [Pro²]SS14 did not affect Ba²+ currents through voltage-sensitive Ca²+ channels but SS14 decreased the magnitude of early and late Ba²+ currents, whereas gbSS-28 reduced that of the late Ba²+ current. Under current-clamp conditions, SS14 and gbSS28 attenuated evoked action potential magnitudes by 34% and 18%, respectively, although [Pro²]SS14 had no effects. However, all three SSs decreased basal intracellular Ca²+ levels ([Ca²+ ](i)) and suppressed basal GH release. These data suggest that, although the ability of SS-14 and gbSS-28 to decrease basal [Ca²+](i) and GH release can be explained, at least in part, by their attenuating effects on cell excitability and current flow through voltage-sensitive Ca²+ channels, [Pro²]SS14-induced reduction in GH responses and [Ca²+](i) cannot be explained by changes in Ca²+ channel properties.

Is It Time to Widen the Use of Somatostatin Analogs in Neuroendocrine Tumors?

Neuroendocrine tumors are considered rare, although their incidence and prevalence have significantly increased over the last two decades. They are heterogeneous tumors in terms of clinical and biologic features, and they originate from neuroendocrine cells in the thymus, lung, pancreas, and GI tract. They may be associated with specific symptoms because of hormone hypersecretion, which differentiates “functioning” from “nonfunctioning”; in the latter, symptoms are related to mass effect from tumor growth only. Most neuroendocrine tumors ( 80%) express a high density of somatostatin receptors. To date, five subtypes of somatostatin receptors have been identified (SSTR1-5). Each receptor subtype activates multiple intracellular transduction pathways, and the antisecretory effects of somatostatin on various hormones involve particularly SSTR2 and SSTR5. Native somatostatin 14, which was isolated by Brazeau and Guillemin in 1973 from sheep hypothalamic tissue, binds to all five somatostatin receptors with almost equal potencies. Because of its short half-life ( 2 minutes), native somatostatin has not been useful in clinical practice. In 1980, Bauer et al at Sandoz synthesized an analog with the name octreotide, constituting an octapeptide with three unnatural amino acids, whereby the compound became resistant to metabolic degradation and presented a half-life of 3 to 4 hours in circulation. This peptide binds with high affinity to SSTR2 and SSTR5 and is therefore effective in inhibiting the secretion of peptides and amines from neuroendocrine cells. Other octapeptides with similar affinity to SSTR2 and SSTR5 were developed later, such as lanreotide and vapreotide. Both octreotide and lanreotide have long-acting formulations that can be given once per month. These two analogs have been the gold standard in the treatment of acromegaly and different functioning neuroendocrine tumors. Octreotide has been registered worldwide for functioning neuroendocrine tumors such as carcinoid tumors with the carcinoid syndrome, endocrine pancreatic tumors with glucagonoma, or Verner-Morrison syndrome. Possible antitumor effects of somatostatin analogs have always been in question, but octreotide has not been registered in any country as an antitumor agent. However, in some countries, octreotide can be used at the doctor’s discretion in nonfunctioning tumors, and various studies have indicated a low but significant antitumor effect. No randomized trials have been published to date. The antitumor effects of somatostatin can be divided into direct and indirect effects. Direct effects are mediated through binding directly to receptors on the tumor cells whereas indirect effects are via inhibition of growth factors, stimulation of the immune system, or inhibition of angiogenesis. Octreotide has been available in clinical practice for more than two decades and has been the most effective drug to inhibit clinical symptoms related to hypersecretion of amines and peptides in neuroendocrine tumors. Diarrhea and flushing can be controlled in 40% to 50% of patients with the carcinoid syndrome. Other patients benefiting from treatment with octreotide include those with functional neuroendocrine tumors of the pancreas, such as glucagonomas, VIPomas (VIP, vasoactive intestinal polypeptide) and, to a lesser extent, gastrinomas and metastatic insulinomas. Other clinical syndromes for which octreotide may provide benefit include ectopic adrenocorticotropic hormone (ACTH) secretion with Cushing’s syndrome, ectopic gonadotropin-releasing hormone receptor (GHRH) secretion, and oncogenic osteomalacia and hypercalcemia caused by secretion of ectopic parathyroid hormone–related peptide. Octreotide therapy results in remission or stabilization of tumor markers, such as serotonin and chromogranin A, in approximately 60% to 70% of patients. A more controversial area has been the treatment of nonfunctioning neuroendocrine tumors with somatostatin analogs. Nonrandomized studies have so far given disappointing results with regard to significant tumor regression according to criteria from WHO or RECIST (Response Evaluation Criteria in Solid Tumors). Tumor shrinkage is demonstrated in less than 5% of patients. However, stabilization of tumor growth after computed tomography– documented progression before somatostatin analog treatment occurs in up to 50% of patients with neuroendocrine tumors. In this issue of Journal of Clinical Oncology, Rinke et al have reported on the double-blind PROMID study (Placebo-Controlled Prospective Randomized Study on the Antiproliferative Efficacy of Octreotide LAR in Patients with Metastatic Neuroendocrine Midgut Tumors). Treatment-naive patients were randomly assigned to either placebo or octreotide LAR 30 mg intramuscularly every month until tumor progression. The primary efficacy end point of the study was time to progression, and secondary end points were survival time and tumor response. The study was conducted at 18 German academic centers and was planned for enrollment of 162 patients. However, enrollment stopped after 85 patients (43 patients received octreotide and 42 patients received placebo) at the time of planned interim analysis. Inclusion criteria were locally inoperable metastatic neuroendocrine tumors with well-differentiated tumor histology, low Ki-67 index ( 2%), and a Karnofsky performance status of more than 60%. JOURNAL OF CLINICAL ONCOLOGY E D I T O R I A L VOLUME 27 NUMBER 28 OCTOBER 1 2009

Lanreotide for the treatment of acromegaly

Lanreotide is an eight-amino acid peptide, which is an analog of the native somatostatin peptide, physiological inhibitor of growth hormone (GH). The drug shows high binding affinity for somatostatin receptors, SSTR2 and SSTR5, which is the primary mechanism considered to be responsible for decreasing GH secretion and GH cell proliferation in acromegaly. Two different formulations of lanreotide are currently available: lanreotide slow release, which requires intramuscular injection every 7-14 days, and lanreotide autogel, which requires deep subcutaneous injection every 4-8 weeks. Several studies have been published to date on the use of lanreotide in acromegaly. Antisecretory efficacy has been reported in 35%-70% of cases; this huge variability is probably explained by different indications (eg, primary or adjunctive postsurgical treatment), or the fact that some studies were based on patients known to be responders to somatostatin analogs. As a primary treatment, antisecretory efficacy was very similar, confirming the possibility of lanreotide as an option in cases of unsuccessful surgery, contraindication, or surgery refusal. Lanreotide also has antitumoral effects as it induces a decrease in tumor volume of [Symbol: see text]25% in 30%-70% of patients. This could be beneficial before transsphenoidal surgery, as a pretreatment, to decrease tumor volume and ease surgery; however, to date, advantages in terms of final remission or uncured status remain a matter of debate. Side effects are rare; the most frequent being gastrointestinal discomfort and increased risk of gallstone formation, and glucose metabolism modifications. Comparison with the other somatostatin analog, octreotide, tends to show identical levels of efficacy between both drugs. Lanreotide thus seems to be an effective treatment in acromegaly. To date, however, lanreotide is still considered as only suspending GH secretion, thus requiring prolonged and costly treatment.

A functional comparison of recombinant and native somatostatin sst2 receptor variants in epithelia

Somatostatin (SRIF-14) exerts broad spectrum antisecretory effects by activating the somatostatin 2 (sst(2)) receptor. The rat (r) sst(2) receptor exists in 'long' (sst(2a)) and 'short' (sst(2b)) forms that differ in their C termini, while a single human (h) sst(2a) exists. This study compares the characteristics of recombinant rsst(2a), rsst(2b) and hsst(2a) activation in human epithelia, and with native sst(2) responses in rat colon. Epithelial layers of each clone or rat colon were placed in Ussing chambers and short-circuit current (I (SC)) measured in response to SRIF-14 and chosen analogues. The relative potencies and ability to cause desensitization to SRIF-14 were assessed, and the affinities of the sst(2) antagonist, D-Tyr(8) CYN154806 for hsst(2a), rsst(2a) and native rat colon sst(2) receptors were established. Basolateral SRIF-14 responses were transient in hsst(2a) and rsst(2a) epithelia, but prolonged in rsst(2b)-expressing cells. Activation of rsst(2a) resulted in significant desensitization to SRIF-14 and receptor phosphorylation, whereas the rsst(2b) receptor did neither. Sst(2)-preferred agonists (BIM23190C and BIM23027) reduced I (sc) with similar potency and both caused complete desensitization to SRIF-14. CYN154806 antagonized hsst(2a) and rsst(2a) receptors with pK (B) values of 7.9 and 7.8, respectively. In rat colon mucosa, CYN154806 blocked SRIF-14 responses with a pA (2) value of 8.2, and BIM23190C responses with a pK (B) of 8.4. SRIF-14 caused rapid rsst(2a) receptor phosphorylation and desensitization of epithelial antisecretory responses, neither of which occurred with the rsst(2b) receptor. These mechanisms are most likely to be a prerequisite for sensitivity to sst(2)-analogues with radiotherapeutic potential.

Effect of native somatostatin on Sphincter of Oddi motility in patients with acute recurrent pancreatitis: A pilot study with Ultrasound-Secretin test

Background The effect of native somatostatin on Sphincter of Oddi motility still remains controversial. Sphincter of Oddi inhibition was demonstrated at manometry in patients in the acute phase of alcoholic pancreatitis. Other investigators showed marked somatostatin-induced impairment of bile flow by hepato-biliary scintigraphy. Aim Aim of the study was to determine the effects of therapeutical doses of exogenous somatostatin on Sphincter of Oddi motility. Patients and methods We studied eight patients (two men, six women, age 18–42), in the quiescent phase of idiopathic recurrent pancreatitis. We directly studied Sphincter of Oddi motility by perendoscopic manometry and, indirectly, secretin-stimulated pancreatic juice outflow by Ultrasound-Secretin test. The two tests were repeated before and after somatostatin infusion. Results Manometry was performed in two patients. After 250 μg somatostatin bolus the sphincter showed an increase of motor activity. At Ultrasound-Secretin test mean diameters were significantly larger at 40–60 min evaluation intervals during 250 μg/h somatostatin infusion as compared to saline infusion, showing a delayed pancreatic duct emptying. Conclusions Acute administration of somatostatin seems to induce an excitatory effect on Sphincter of Oddi motility, with impaired pancreatic outflow in patients in the quiescent phase of recurrent pancreatitis.

Somatostatin receptor subtypes mediate contractility on human colonic smooth muscle cells

This study demonstrates the expression of functional somatostatin receptor (sstr) subtypes in human circular and longitudinal colonic smooth muscle cells (SMC). Native somatostatin (SS) and sstr subtype-specific analogues were used to characterize the sstr subtypes present in both cell types by contraction/relaxation studies. Qualitative and quantitative mRNA analysis and immunohistochemistry of sstr subtypes were also carried out. sstr subtype 2 mRNA was expressed in circular SMC, and various levels of subtypes 1, 2 and 3 mRNA were expressed in longitudinal colonic SMC. Native SS and each subtype-specific analogue exerted a modest, but significant, contraction, although inhibition of carbachol-induced contraction (relaxation) was the main effect on SMC from both layers. CH-288, a sstr subtype 1-specific analogue, and octreotide, a sstr subtype 2-specific analogue, were the most effective relaxant analogues on longitudinal and circular SMC, respectively. sstr subtypes display a distinct expression pattern on human colonic SMC; on circular SMC, subtype 2 is the only sstr, whereas sstr subtypes 1, 2 and 3 are expressed on human SMC isolated from the longitudinal layer. The contractile effects of SS are mediated through sstr subtype 2 and sstr subtype 1 on circular and longitudinal human colonic SMC, respectively.

Preparation, quality control and stability of 131I-Lanreotide

Lanreotide, a synthetic octapeptide analog of a native hormone somatostatin, was labeled with 131I, the most widely used therapeutic and easily available radionuclide. Radioiodination of Lanreotide was carried out by Chloramine-T and Iodogen methods. Chloramine-T and Iodogen were used as oxidizing agents to form an electrophilic iodine species, which then labeled the tyrosine of Lanreotide. The maximum radiolabeling yield was ~80%. Chloramine-T was found more suitable than the Iodogen method, because nearly 25% of the initial iodine activity was lost/adsorbed on the Iodogen coating. Thin layer and high performance liquid chromatographies were used for monitoring the reaction of 131I with Lanreotide, the stability and purity of 131I-Lanreotide.

Radiolabelled somatostatin analog therapy in prostate cancer: current status and future directions

Natural somatostatin is characterized as an inhibitory peptide and had anti-neoplastic actions. The effect of somatostatin is mediated via specific somatostatin receptors (SSTRs). Clinical trials have demonstrated therapeutic applications of radionuclide labeled somatostatin analogues in many tumors bearing the receptors. In prostate cancer, significance of neuroendocrine cells remains unclear, there were conflicting data about occurrence and localization of SSTRs. Therefore, radionuclide labeled somatostatin analog therapy is not applied to prostate cancer clinically, experimental trials with non-radioactive analogues have been disappointing. More efforts need to be made for evaluation of potential applications of somatostatin analog therapy in prostate cancer, for example, transfection of the receptor gene to increase binding of analogues to the tumor cells, improvement of radionuclide delivery and combinations of different therapeutic modalities.

Distinct functional properties of native somatostatin receptor subtype 5 compared with subtype 2 in the regulation of ACTH release by corticotroph tumor cells

In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst5). After 72 h, the multiligand SS analog SOM230, which has a very high sst5 binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst5 in the regulation of ACTH release, we conducted additional studies with mouse AtT-20 cells. SOM230 showed a 7-fold higher ligand binding affinity and a 19-fold higher potency in stimulating guanosine 5'-O-(3-thiotriphosphate) binding in AtT-20 cell membranes compared with OCT. SOM230 potently suppressed CRH-induced ACTH release, which was not affected by 48-h dexamethasone (DEX) pretreatment. However, DEX attenuated the inhibitory effects of OCT on ACTH release, whereas it increased the inhibitory potency of BIM-23268, an sst5-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst5 mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B(max)) by 72% when 125I-Tyr3-labeled OCT was used as radioligand, whereas B(max) declined only by 17% when AtT-20 cells were treated with [125I-Tyr11]SS-14. These data suggest that the sst5 protein, compared with sst2, is more resistant to glucocorticoids. Finally, after SS analog preincubation, compared with OCT both SOM230 and BIM-23268 showed a significantly higher inhibitory effect on CRH-induced ACTH release. In conclusion, our data support the concept that the sst5 receptor might be a target for new therapeutic agents to treat Cushing's disease.

TT-232

The treatment of the neurogenic component of inflammatory diseases and neuropathic pain remains an unresolved issue in pharmacotherapy. A novel endogenous antiinflammatory/analgesic mechanism mediated by somatostatin released from capsaicin-sensitive sensory nerve terminals has been identified and reported by our research group. These findings raised the possibility of targeting somatostatin receptors to decrease inflammation and nociception. Native somatostatin cannot be used in clinical practice due to its short plasma half-life and broad range of actions, but stable and selective synthetic analogues might open new perspectives for drug development. A cyclic heptapeptide analogue, TT-232, showing the greatest affinity for the somatostatin sst 4 receptor subtype was synthesized at the Peptide Chemistry Research Group of the Hungarian Academy of Sciences and proved to have no endocrine activity. On the other hand, it exerted significant inhibitory effects in several acute and chronic inflammatory and nociceptive animal models, including chronic arthritis, mono- and polyneuropathy conditions. TT-232 is therefore considered a promising lead molecule for the development of a completely new type of antiinflammatory/analgesic agents.

Primary Therapy for Acromegaly with Somatostatin Analogs and a Discussion of Novel Peptide Analogs

Surgery, radiation therapy, and dopamine agonists were the only treatments available for acromegaly until the late 1980s. With those tools, and the fact that the majority of patients with acromegaly have tumors that were too large or invasive to be completely resected, the disease was curable or controllable in only a minority of patients. The introduction of the somatostatin analogs, octreotide and lanreotide, changed the face of acromegaly making it a disease that was controllable in up to 2/3 of cases. These analogs have the advantages of lowering growth hormone for longer periods of time (45×) and are also less potent than somatostatin inhibitors of insulin and other pancreatic hormones as compared to native somatostatin [1]. Both short acting and long acting octreotide lower growth hormone sufficiently to bring IGF-I into the normal age and gender matched norms in up to 65% of patients with acromegaly [2‐17]. Similar results are achieved with lanreotide [18‐23]. A number of studies have shown that significant reduction in GH [24‐26] and IGF-I [27] improves survival. In addition to improvement in biochemical abnormalities and survival, these medications also improve signs and symptoms. Because of a direct action on somatotrope SSTR2, they also shrink tumors [18, 28]. Tumor growth is rarely reported while patients are on medication. The one exception is a study in which acromegaly patients received at least 100 mcg of octreotide for at least 3 months prior to surgery (71% of them received 300 mcg daily). Tumor shrinkage was noted in 31% and tumor growth was reported in 6.7% of patients [29].

Phase I Trial on sms‐D70 Somatostatin Analogue in Advanced Prostate and Renal Cell Cancer

Plasma concentrations and tolerability of a novel somatostatin analogue sms-D70 were studied in patients with metastatic hormone-resistant prostate cancer (HRPC) or metastatic renal cell cancer. To overcome the limitations of the octapeptides having affinity only to somatostatin receptor subtypes 2 and 5, HRPC expressing mainly somatostatin receptors 1 and 4, a somatostatin derivative based on the natural somatostatin having affinity to all five somatostatin receptor subtypes, was developed. The in vivo stability of this dextran-conjugated derivative, somatostatin-D70, was confirmed previously in animal studies, and the nanomolar "panaffinity" has been shown in in vitro receptor binding studies on cell lines transfected with the somatostatin receptor genes. Sms-D70 was given with subcutaneous injection once a week at dose levels of 5, 10, 20, 35, and 50 mg. For pharmacokinetic studies, sms-D70 was labeled with 131I. Fourteen patients were treated, of whom 10 had prostate and 4 renal cell cancer. The kinetic data revealed high stability with a long half-life in the blood. The drug was well tolerated, and no grade 4 (WHO) toxicity was observed. The maximal tolerated dose could not be established due to the lack of dose-limiting toxicities. Objective PSA responses were not recorded in these heavily treated patients, but subjective stabilization of pain was observed and urinary symptoms were alleviated in four patients. Three patients with metastatic HRPC received 5-10-mg intravenous injections of sms-D70 once weekly for 4-14 months on a compassionate use basis. In all cases, serum PSA values decreased more than 50% from the pretreatment level, but these results are difficult to interpret due to concomitant treatments given to these patients. In conclusion, sms-D70 was well tolerated in the treatment of metastatic prostate and renal cell cancer, but no responses were found in these heavily treated patients.

The role of somatostatin receptors in the medical treatment of acromegaly

Somatostatin is a hypothalamic inhibitor of pituitary growth hormone secretion and cell proliferation, binding to five distinct receptor subtypes (sstr1–5). Since native somatostatin has a short half-life, somatostatin analogues with a longer half-life have been developed for therapeutic purposes. Octreotide and lanreotide are currently available for treatment of acromegaly, binding with high-affinity sstr2 and sstr5. Octreotide, the first somatostatin analogue used in the medical therapy of acromegaly, was initially given subcutaneously at doses of 100–500 μg three times daily. The introduction of new depot formulations, such as octreotide long-acting release, slow-release lanreotide and lanreotide-autogel, improved patients compliance of long-term therapy, overcoming the inconvenience of multiple daily administration. The treatment with somatostatin analogues induces biochemical control and tumour shrinkage in about 50–70% and 30–60% of patients with acromegaly, respectively. However, the efficacy of this therapy lies on an adequate expression of sstr2 and sstr5 on tumor cells. In the past, somatostatin receptor expression was tested in vivo by 111 In -diethylenetriaminepentaacetate- d-Phe-octreotide scintigraphy: this method has been abandoned since normal pituitary tissue can be visualised by 111 In -diethylenetriaminepentaacetate- d-Phe-octreotide scintigraphy. Currently, the somatostatin receptorial profile can be characterised by autoradiography, molecular biology techniques and immunohistochemistry on surgically removed tumor tissue. These methods may offer an individualised approach sparing patients from unnecessary treatment.

Evolving concepts in the quest for advanced therapeutic analogues of somatostatin

A major challenge in developing somatostatin-based therapies, as well as an important question of basic physiology, is how to achieve functional specificity with an agent that has widespread actions. The natural somatostatin system achieves functional specificity in part through local somatostatin production at the site of action. Further selectivity may be achieved through the recently elucidated somatostatin receptor subtypes. To explore the relationship between the receptor subtypes and somatostatin-mediated functions, we have tested panels of selective somatostatin analogs in specific biological assays. Our studies have demonstrated complex, functional interactions between the somatostatin receptor subtypes. Specific combinations of somatostatin receptor subtypes in specific tissues may be either synergistic or antagonistic. By altering the expression ratio of the interacting receptors, the biological response to somatostatin can be influenced by environmental, hormonal and physiological status. In addition, inappropriate or unbalanced receptor expression may provide a novel mechanism of disease. These concepts take on an even broader context with the demonstration that somatostatin receptor subtypes also interact with other G-protein-coupled receptors. The insight gained from these studies has already resulted in several novel approaches to acromegaly therapy. Our further understanding of these complex cellular codes will provide the conceptual basis for future therapeutics with greatly enhanced efficacy and specificity.