Inhibition of the growth of transplanted rat pancreatic acinar carcinoma with octreotide

Somatostatin receptors in non-neuroendocrine malignancies: the potential role of somatostatin analogs in solid tumors

Somatostatin receptors (sstrs) are G-protein-coupled receptors that mediate various physiological effects when activated by the neuropeptide somatostatin or its synthetic analogs. In addition to the well-documented antisecretory effects of sstr2-preferential somatostatin analogs octreotide and lanreotide, ligand binding to sstr initiates an inhibitory action on tumor growth. This effect may result from both indirect actions (suppression of growth factors and growth-promoting hormones [e.g., GH/IGF-1 axis] and inhibition of angiogenesis) and direct actions (activation of antigrowth activities [e.g., apoptosis]). As solid tumor cells express multiple sstrs, there is a rationale to evaluate the potential antitumor effects of pasireotide (SOM230), a multireceptor-targeted somatostatin analog with high binding affinity for sstr1–3 and sstr5. Pasireotide reduces systemic IGF-1 levels more potently than currently available somatostatin analogs and has been well tolerated in clinical trials.

The significance of somatostatin analogues in the antiproliferative treatment of carcinomas

Somatostatin is a cyclic tetradecapeptide hormone. It was initially isolated from bovine hypothalami. Somatostatin inhibits endocrine and exocrine secretion, as well as tumor cell growth, by binding to specific cell-surface receptors. Its potent inhibitory activity is limited, however, by its rapid enzymatic degradation and the consequently short plasma half-life. Octreotide is a short somatostatin analogue with increased duration of action compared with somatostatin. Preclinical studies have focused on the anticancer effects of octreotide and the related somatostatin analogues. In vitro, at nanomolar concentrations, these analogues inhibit the growth of tumor cells that express high-affinity somatostatin receptors. Accordingly, such analogues potently inhibit the growth of somatostatin receptor-positive tumors in various rodent models. The range of cancers susceptible to octreotide and related somatostatin analogues includes mammary, pancreatic, gastric, colorectal, prostate, thyroid, and lung carcinomas. Moreover, an indirect antiproliferative effect of somatostatin analogues is achievable in somatostatin receptor-negative tumors whose growth is driven by factors (e.g., gastrin, insulin-like growth factor-1) that become down-regulated by somatostatin. The clinical effect of somatostatin analogues in terms of tumor response in cancer patients is a subject of controversy, however. Most responses have been seen in patients with pancreatic cancers.

Effects of cholecystokinin octapeptide on a pancreatic acinar carcinoma in the rat

PURPOSE

To investigate the effects of increasing concentrations of cholecystokinin octapeptide (CCK-8) on a pancreatic acinar adenocarcinoma.

METHODS

Growth of the tumour was estimated in vivo on rats bearing a subcutaneous pancreatic carcinoma, and in vitro on primary cultured tumour cells. CCK receptors were characterized by binding assays.

RESULTS

CCK-8, administered for 12 successive days, exerted a biphasic action on tumour growth: a dose-dependent stimulation with low doses (0.1 and 0.5 microg/kg) and inhibition with high doses (2 and 4 microg/ kg) as shown by respective increases and decreases in tumor volume, protein, RNA and amylase contents. In cell cultures, [3H]thymidine incorporation was dose-dependently increased with 10-(10) to 10(-8) M CCK-8 and inhibited with 10(-7) M. Both effects were completely suppressed by the CCK-receptor antagonists CR 1409 and L 364,718 (10(-4) M). Binding studies showed the overexpression of two classes of CCK-A receptors of low and high affinity when compared to the normal pancreas which was less sensitive to CCK-8.

CONCLUSIONS

CCK-8 exerts a biphasic growth response on the acinar pancreatic carcinoma, mediated by two classes of CCK-A receptors overexpressed in the tumour.

Effect of the gastrin-releasing peptide antagonist BIM 26226 and lanreotide on an acinar pancreatic carcinoma

The effects of a potent specific gastrin-releasing peptide receptor antagonist, BIM 26226 ([D-F5 Phe6, D-Ala11] bombesin (6-13) OMe), and the long-acting somatostatin analogue, lanreotide (BIM 23014), on the growth of an acinar pancreatic adenocarcinoma growing in the rat or cultured in vitro were investigated. Lewis rats bearing a pancreatic carcinoma transplanted s.c. in the scapular region, were treated with gastrin-releasing peptide (30 microg/kg per day), BIM 26226 (30 and 100 microg/kg per day) and lanreotide (100 microg/kg per day) alone or in combination for 14 successive days. Chronic administration of BIM 26226 and lanreotide significantly inhibited the growth of pancreatic tumours stimulated or not by gastrin-releasing peptide (GRP), as shown by a reduction in tumour volume, protein, ribonucleic acid, amylase and chymotrypsin contents. This effect was more pronounced with 100 microg/kg per day BIM 26226 than with 30 microg/kg per day. However, BIM 26226 and lanreotide, given together, did not exert any additive effect on GRP-treated and -untreated tumours. In cell cultures, both BIM 26226 and lanreotide (10(-6) M) inhibited [3H]thymidine incorporation in tumour cells induced or not by GRP, but no increased effect was observed after combined treatment with both agents. Binding studies showed that BIM 26226 had a high affinity for GRP receptors in tumour cell membranes (IC50 = 6 nM). These results from in vivo and in vitro experiments suggest that BIM 26226 and lanreotide are able to reduce the growth of an experimental acinar pancreatic tumour. Thus, these agents represent interesting steps toward the development of new approaches for treatment of pancreatic carcinomas.

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.

Synthesis, biodistribution and renal handling of various chelate-somatostatin conjugates with metabolizable linking groups

A series of DTPA-octreotide conjugates, with various linking groups, were synthesised to investigate the effect of different metabolizable linkers on the renal retention of radioactivity. All these newly synthesised octreotide conjugates retained the high binding affinity of octreotide for the somatostatin (SRIF) receptors either when unlabeled or radiolabeled with 111In. Some of the metabolizable linkers were rapidly degraded in vitro when incubated with a kidney homogenate. However, in vivo, all these conjugates displayed a significantly lower uptake in SRIF receptor-positive tissue compared to two conjugates with short, stable linkers. Additionally, the compounds with a potentially metabolizable linker had a higher whole-body retention of activity as opposed to the three metabolically stable compounds. Several of the linkers gave evidence of cleavage while in circulation in the blood, and it is probable that the lower tumour accumulation of most of the compounds tested was low due to the high enzymatic nature of the exocrine pancreatic tumour model used. In short, no increase in the tumour-to-kidney ratio was achieved with the analogues containing a metabolizable linker. The highest target-to-nontarget tissue ratios were obtained for the DTPA-octreotide conjugates that had short, metabolically stable linkers.

In vivo laser-induced fluorescence imaging of a rat pancreatic cancer with pheophorbide-a

Laser-induced fluorescence (LIF) of pheophorbide-a (Ph-a) was used for imaging of a rat pancreatic tumor. Using a dimensionless function (the ratio of Ph-a fluorescence by bluish autofluorescence), the fluorescence contrasts between excised tumors and their paired pancreas were investigated up to 48 h after a 9 mg kg-1 Ph-a intravenous administration. Among five tested excitation wavelengths, 355 and 610 nm excitations gave the best distinctive contrasts, both 48 h after dye injection. The LIF imaging of six intrapancreatic tumors and six healthy pancreas was carried out in vivo using two laser excitations: 355 nm (Nd:YAG + tripling) for bluish autofluorescence and 610 nm (rhodamine 6G dye) for reddish autofluorescence and dye emission. Images were recorded through bandpass filters at 470 and 640 nm (autofluorescence) and at 680 nm (dye + autofluorescence) with an intensified charged-coupled device camera. Autofluorescence as Ph-a fluorescence images did not allow accurate LIF diagnosis of pancreatic carcinoma. An image processing, including for each pixel a computed division of Ph-a fluorescence (after subtraction of reddish autofluorescence) by bluish autofluorescence intensity generated poorly contrasted tumor images in five of six and false tumor localization in one of three of the tumor-bearing pancreas. A fitting of the digital 640 nm autofluorescence up to the mean 680 nm fluorescence intensity in pancreas prior to subtraction allowed a safe diagnosis to be made with well-contrasted tumor images. To assess automation ability of the processing, a same fitting coefficient (mean of individual values) was applied. In this way, false-negative (one of six) and false-positive (two of six) images were present in tumor-bearing animals as false-positive in one-half of the controls. A successful standardized procedure was then applied with a normalization of 640 and 680 nm pancreas intensities to a same set threshold prior processing. In opposition to thin-layered hollow organs, such as bronchial tube or digestive tract, LIF imaging of carcinoma inserted in a compact organ is exhausting. The use of a dye excitable in the red wavelength range (610 nm for Ph-a) may partly solve this problem, rendering LIF imaging more accurate and potentially automated.

PET-pharmacokinetics of 18F-octreotide: a comparison with 67Ga-DFO- and 86Y-DTPA-octreotide

The quantitative uptake kinetics of (2-[18F]fluoropropionyl-(D)phe1)-octreotide (I), a somatostatin (SRIF) receptor-specific tracer, was measured by PET. Conventional organ biodistribution and in vivo stabilities of the tracer as well as in vivo displacement and SRIF receptor blocking were determined. The 18F-fluorinated octreotide was compared with ([67Ga]-DFO-B-succinyl-(D)phe1)-octreotide (II) and ([86Y]-DTPA-(D)phe1)-octreotide (III). Initially, 2-10 MBq of the labeled tracers were injected into male Lewis rats bearing an exocrine pancreatic islet cell tumor. PET measurements were performed dynamically between 0 and 120 min postinjection. Organ distributions were determined 5, 15, 30, 60, and 120 min postinjection. The extent of metabolic degradation was analyzed in serial blood and urine samples as well as in homogenized samples of tumor, liver, and kidney. The uptake of (I) by the tumor was rapid (maximum accumulation at 1-2 min postinjection) and high (about 0.5 +/- 0.2% ID/g), followed by a fast and continuous release with koff = 10 +/- 2. 10(-5) s-1. The tracer was found to remain intact in vivo up to 120 min postinjection. Specific binding of (I) to SRIF receptors in the adrenals, the pancreas, and the pituitary gland was demonstrated in vivo by pretreatment and displacement experiments. Compound (II) also showed a fast uptake by the tumor. Its tumor residence half-life was longer (koff = 3.0 +/- 0.5 . 10(-5) s-1). Compound (II) was also predominantly excreted intact. One hour postinjection, the remaining activity in the blood pool was found to be bound to serum proteins. Early uptake kinetics for compound (III) were also rapid but reached only half the tumor uptake of (II). Compared to (I), the release of 86Y-activity from the tumor was slower (koff = 3.1 +/- 1.3 . 10(-5) s-1). Compared to (II), compound (III) was considerably less stable in vivo. The main critical organs for (II) and (III) are kidneys and bones, whereas (I) is predominantly accumulated in the liver. The in vivo behavior of (I) closely resembles 14C-labeled octreotide. Thus, 18F-labeled octreotide may be of interest in the quantitation and investigation of in vivo properties of somatostatin receptors by PET. However, the short residence of (2-[18F]fluoropropionyl-(D)phe1)-octreotide in tumors and its hepatobiliary excretion may complicate the interpretation of abdominal tumors.

Molecular Events as Targets of Anticancer Drug Therapy

The aim of this review is to introduce some molecular targets for cancer chemotherapy, with comments on their mode of action, preclinical and clinical results. The representatives of the following groups are covered: phosphorylation inhibitors, protein kinase modulators, receptor antagonists, immunomodulators, differentiating agents, multidrug resistance modulation, telomerase inhibitors, and bioreductive agents.

Insulin, transforming growth factors, and substrates modulate growth of guinea pig pancreatic duct cells in vitro

BACKGROUND & AIMS

Little is known of the physiological mechanisms that control cellular renewal in the pancreatic excretory duct system. This study investigated the effects of potential regulatory substances on the growth of cultured guinea pig pancreatic duct epithelial monolayers.

METHODS

Pancreatic duct explants were cultured for 3 days on plastic and on permeable filters in the presence and absence of different substances. Growth of epithelial monolayers from these explants was measured by 5-bromo-2'-deoxyuridine incorporation and morphometric procedures.

RESULTS

Epidermal growth factor and insulin both enhanced monolayer growth and together had an additive effect. Transforming growth factor alpha enhanced and transforming growth factor beta inhibited growth, whereas glucagon, somatostatin, pancreatic polypeptide, secretin, cerulein, bombesin, and dexamethasone had no significant effects. Monolayer growth on type 1 collagen-coated filters was enhanced when compared with that of monolayers grown on tissue culture plastic. Cell growth from explants on filters coated with type IV collagen and fibronectin was comparable with that on plastic, whereas growth on Matrigel- or laminin-coated filters was reduced.

CONCLUSIONS

Insulin, transforming growth factors, and substrate components modulate growth of pancreatic duct epithelial cells in vitro, suggesting that they are important regulators of cell division in the excretory duct system of the intact pancreas.

Effects of chronic octreotide treatment on GH secretory dynamics and tumor growth in rats bearing an ectopic somatotroph (GC) tumor

The effects of octreotide, a long-acting somatostatin agonist selective of the sstr2/sstr3/sstr5 receptor subtypes, on ectopic GH secretion and tumor growth were investigated in Wistar-Furth female rats implanted with GH secreting (GC) cells which express mostly somatostatin receptors of the sstr1 and sstr2 subtypes. Octreotide dose dependently inhibited thymidine incorporation (-57%) and GH secretion (-41%) from GC cells in culture. In vivo, 6 weeks after GC cell implantation, plasma GH, IGF-1 and insulin levels were highly elevated. Cluster analysis of GH secretory dynamics revealed that GH secretion was less pulsatile in GC-implanted than in control animals. Furthermore, in GC-implanted animals, passive immunization either with SRIH or GHRH antisera, did not affect GH plasma levels. Three weeks after GC cell implantation, when tumors became palpable, octreotide (1 micrograms/h/kg BW) or saline was infused constantly for three weeks by osmotic minipumps. In octreotide treated rats, GH, IGF-1 and insulin levels were not different from sham-implanted animals and tumors weight were reduced by 80%. High affinity somatostatin binding sites were found in equivalent amounts on tumors from octreotide-treated or saline-treated animals. These findings indicate that GH secretion in GC-rats is mainly derived from the tumors and independent of hypothalamic control and that octreotide reduces both GH secretion and tumor growth. We conclude that the GC-implanted rat represents a good animal model to test the antisecretory and antitrophic properties of somatostatin analogs in vivo.

Somatostatin analogs for diagnosis and treatment of cancer

Somatostatin (SRIF) is a cyclic tetradecapeptide hormone initially isolated from ovine hypothalami. It inhibits endocrine and exocrine secretion, as well as tumor cell growth, by binding to specific cell surface receptors. Its potent inhibitory activity, however, is limited by its rapid enzymatic degradation and the consequent short plasma half-life. Octreotide is a short SRIF analog with increased duration of action compared to SRIF. Octreotide is approved for the treatment of acromegaly, amine precursor uptake and decarboxylation-omas, complications of pancreatic surgery and severe forms of diarrhea. Preclinical studies have focussed on the anticancer effects of octreotide and the related SRIF analogs BIM 23014 and RC-160. In vitro at nanomolar concentrations, these analogs inhibit the growth of tumor cells that express high affinity SRIF receptors. Accordingly, SRIF analogs, such as octreotide, potently inhibit the growth of SRIF receptor-positive tumors in various rodent models, and, in particular, xenotransplanted human tumors in nude mice. The range of cancers susceptible to octreotide and related SRIF analogs includes mammary, pancreatic, colorectal and lung malignancies. Moreover, an indirect antiproliferative effect of SRIF analogs is achievable in SRIF receptor-negative tumors, whose growth is driven by factors (gastrin, insulin-like growth factor-1, etc.) that are downregulated by SRIF. The use of radiolabeled somatostatin analogs represents a new diagnostic approach. [111In-DTPA]octreotide was developed for gamma camera imaging of SRIF receptor-positive malignancies, such as gasteroenteropancreatic tumors. Visualization of SRIF receptor-positive tumors in humans is emerging as an important methodology, both in tumor staging and predicting therapeutic response to octreotide. Recently, five SRIF receptor subtypes (SSTR1-5) have been cloned, all of which bind SRIF with high affinity. In contrast, SRIF receptor subtypes 1-5 have different binding profiles for short SRIF analogs. Octreotide, SSTR5, show moderate affinity for SSTR3 and fail to bind with high affinity to the other subtypes (SSTR1 and 4). Accordingly, the oncological profile of these three analogs is apparently similar. In conclusion, somatostatin analogs are a promising class of compounds for diagnosis and treatment of cancer. Current work is focussed on the identification of further SRIF receptor subtype-selective analogs with potential in oncology.

Preclinical studies on the anticancer activity of the somatostatin analogue octreotide (SMS 201-995)

The antiproliferative effect of somatostatin-14 and its analogue, octreotide, on in vitro pancreatic and breast tumor cells has led to the suggestion that octreotide may have further oncological indications in addition to its use in the treatment of gastroentero-pancreatic (GEP) tumors. To extend these in vitro observations, we evaluated the effect of octreotide in rodent models of pancreatic and breast tumors. Octreotide at a dose of 5 micrograms or 50 micrograms twice a day in nude mice bearing solid MiaPaCa pancreatic tumors (subline 21) or ZR-75-1 breast tumors induced a significant inhibition of tumor growth from week 2 until the end of treatment at week 5. After 5 weeks, the mean volume of ZR-75-1 tumors in animals treated with the 50-micrograms regimen was 48% of that in controls. Autoradiographic studies showed that a high percentage (71%) of ZR-75-1 tumors were somatostatin receptor-positive. In addition, the growth of ZR-75-1 cells in vitro was significantly inhibited by octreotide. The drug was also tested in a second breast cancer model, 7,12-dimethylbenzanthracene (DMBA)-induced mammary tumors in rats, and continuous administration of 10 micrograms/kg/h over 6 weeks led to an approximate 50% reduction in the number of tumors arising in the rat mammary gland. These data suggest that pancreatic and breast cancer may be among the malignant diseases clinically susceptible to octreotide.

Expression cloning of a rat brain somatostatin receptor cDNA

We have used an expression-cloning strategy to isolate a cDNA encoding a somatostatin (somatotropin release-inhibiting factor, SRIF) receptor from rat cortex and hippocampus. A positive clone was identified by autoradiography after binding of radiolabeled SRIF to COS-1 cells previously transfected with pools of cDNA clones. The deduced amino acid sequence of the receptor displays sequence and structural homology to the family of G-protein-coupled receptors. The affinity of various SRIF analogs to the expressed receptor resembles their effects on growth hormone release from pituitary cells. In addition, the distribution of the mRNA in various tissues corresponds to that described for native SRIF receptors. Therefore, we conclude that we have isolated a rat brain SRIF receptor cDNA.