Gastrin-releasing peptide receptor-based targeting using bombesin analogues is superior to metabolism-based targeting using choline for in vivo imaging of human prostate cancer xenografts.

PURPOSE: Prostate cancer (PC) is a major health problem. Overexpression of the gastrin-releasing peptide receptor (GRPR) in PC, but not in the hyperplastic prostate, provides a promising target for staging and monitoring of PC. Based on the assumption that cancer cells have increased metabolic activity, metabolism-based tracers are also being used for PC imaging. We compared GRPR-based targeting using the (68)Ga-labelled bombesin analogue AMBA with metabolism-based targeting using (18)F-methylcholine ((18)F-FCH) in nude mice bearing human prostate VCaP xenografts. METHODS: PET and biodistribution studies were performed with both (68)Ga-AMBA and (18)F-FCH in all VCaP tumour-bearing mice, with PC-3 tumour-bearing mice as reference. Scanning started immediately after injection. Dynamic PET scans were reconstructed and analysed quantitatively. Biodistribution of tracers and tissue uptake was expressed as percent of injected dose per gram tissue (%ID/g). RESULTS: All tumours were clearly visualized using (68)Ga-AMBA. (18)F-FCH showed significantly less contrast due to poor tumour-to-background ratios. Quantitative PET analyses showed fast tumour uptake and high retention for both tracers. VCaP tumour uptake values determined from PET at steady-state were 6.7 ± 1.4%ID/g (20-30 min after injection, N = 8) for (68)Ga-AMBA and 1.6 ± 0.5%ID/g (10-20 min after injection, N = 8) for (18)F-FCH, which were significantly different (p <0.001). The results in PC-3 tumour-bearing mice were comparable. Biodistribution data were in accordance with the PET results showing VCaP tumour uptake values of 9.5 ± 4.8%ID/g (N = 8) for (68)Ga-AMBA and 2.1 ± 0.4%ID/g (N = 8) for (18)F-FCH. Apart from the GRPR-expressing organs, uptake in all organs was lower for (68)Ga-AMBA than for (18)F-FCH. CONCLUSION: Tumour uptake of (68)Ga-AMBA was higher while overall background activity was lower than observed for (18)F-FCH in the same PC-bearing mice. These results suggest that peptide receptor-based targeting using the bombesin analogue AMBA is superior to metabolism-based targeting using choline for scintigraphy of PC.

Molecular imaging of reduced renal uptake of radiolabelled [DOTA0,Tyr3]octreotate by the combination of lysine and Gelofusine in rats.

AIM: In peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues, kidney uptake of radiolabelled compound is the major dose-limiting factor. We studied the effects of Gelofusine (20 mg) and lysine (100 mg) and the combination of both after injection of therapeutic doses of radiolabelled [DOTA0,Tyr3]octreotate (60 MBq 111In or 555 MBq 177Lu labelled to 15 microg peptide) in male Lewis rats. METHODS: Kidney uptake was measured by single photon emission computed tomography (SPECT) scans with a four-headed multi-pinhole camera (NanoSPECT) at 24 h, 5 and 7 days p. i. and was quantified by volume of interest analysis. For validation the activity concentration in the dissected kidneys was also determined ex vivo using a gamma counter and a dose calibrator. RESULTS: Gelofusine and lysine both reduced kidney uptake of [177Lu-DOTA0,Tyr3]octreotate significantly by about 40% at all time points. The combination of Gelofusine and lysine resulted in a 62% inhibition of kidney uptake (p < 0.01 vs. lysine alone). A weak but significant dose-response relationship for Gelofusine, but not for lysine, was found. In a study with [111In-DOTA0,Tyr3]octreotate, conclusions drawn from NanoSPECT data were confirmed by biodistribution data. CONCLUSIONS: We conclude that rat kidney uptake of radiolabelled somatostatin analogues can be monitored for a longer period in the same animal using animal SPECT. Gelofusine and lysine had equal potential to reduce kidney uptake of therapeutic doses of [177Lu-DOTA0,Tyr3]octreotate. The combination of these compounds caused a significantly larger reduction than lysine or Gelofusine alone and may therefore offer new possibilities in PRRT. The NanoSPECT data were validated by standard biodistribution experiments.

Comparison of (111)In-labeled somatostatin analogues for tumor scintigraphy and radionuclide therapy.

We evaluated the following (111)In-labeled somatostatin (SS) analogues (diethylenetriaminepentaacetic acid, DTPA; tetraazacyclododecanetetraacetic acid, DOTA): [DTPA0]octreotide, [DTPA0,Tyr3]octreotide, [DTPA0,D-Tyr1]octreotide, [DTPA0,Tyr3]octreotate [Thr(ol) in octreotide replaced with Thr], and [DOTA0,Tyr3]octreotide, in vitro and in vivo. In vitro, all compounds showed high and specific binding to SS receptors in mouse pituitary AtT20 tumor cell membranes, and IC50s were in the nanomolar range. Furthermore, all compounds showed specific internalization in rat pancreatic tumor cells; uptake of [(111)In-DTPA0,Tyr3]octreotate was the highest of the compounds tested, and that of [(111)In-DTPA0,D-Tyr1]octreotide was the lowest. Biodistribution experiments in rats showed that, 4, 24, and 48 h after injection of [(111)In-DTPA0,Tyr3]octreotide, [(111)In-DTPA0,Tyr3]octreotate, and [(111)In-DOTA0,Tyr3]octreotide, radioactivity in the octreotide-binding, receptor-expressing tissues and tumor-to-blood ratios were significantly higher than those after injection of [(111)In-DTPA0]octreotide. Uptake of [(111)In-DTPA0,Tyr3]octreotate in the target organs was also, in vivo, the highest of the radiolabeled peptides tested, whereas that of [(111)In-DTPA0,D-Tyr1]octreotide was the lowest. Uptake of [(111)In-DTPA0,Tyr3]octreotide, [(111)In-DTPA0,Tyr3]octreotate, and [(111)In-DOTA0,Tyr3]octreotide in target tissues was blocked by >90% by 0.5 mg of unlabeled octreotide, indicating specific binding to the octreotide receptors. Blockade of [(111)In-DTPA0,D-Tyr1]octreotide was >70%. In conclusion, radiolabeled [DTPA0,Tyr3]octreotide and, especially, [DTPA0,Tyr3]octreotate and their DOTA-coupled counterparts are most promising for scintigraphy and radionuclide therapy of SS receptor-positive tumors in humans.

Comparative study of chylomicron and fatty acid utilization in small intestine and heart.

Chylomicrons were isolated from the urine of rats after a surgical procedure in which the cysterna chyli was connected with the right ureter. The fatty acids of the chylomicrons served as a respiratory substrate for rat heart and not for rat small intestine during in vitro vascular perfusions. The reason for the absence of chylomicron utilization in small intestine was found to be the virtual absence of lipoprotein lipase from this organ. Both heart and small intestine oxidized oleate complexed to albumin. Increasing the molar ratio of fatty acid to albumin from 3 to 6 did not affect the rate of fatty acid oxidation in heart, but increased fatty acid oxidation in small intestine.

Internalization of the radioiodinated somatostatin analog [125I-Tyr3]octreotide by mouse and human pituitary tumor cells: increase by unlabeled octreotide.

Recently, we developed a technique that allows the in vivo visualization in man of somatostatin receptor-positive neuroendocrine tumors after i.v. injection of [125I-Tyr3]octreotide or [111In-DTPA-D-Phe1]octreotide. Radiotherapy of such tumors using somatostatin analogs coupled to alpha- or beta-emitting radionuclides has been proposed as an application for radiolabeled somatostatin analogs. To develop this concept further, it is of importance to know whether the above-mentioned radiolabeled somatostatin analogs are internalized by the tumor cells, and whether it might be possible to manipulate the degree of internalization. In the present study we investigated the internalization of a stable somatostatin analog, [125I-Tyr3]octreotide, by mouse AtT20/D16V pituitary tumor cells and primary cultures of human GH-secreting pituitary tumor cells. Treatment of the cells with low pH was used to distinguish between membrane-bound (acid-releasable) and internalize (acid-resistant) radioligand. [125I-Tyr3]octreotide showed a time-dependent increasing accumulation in AtT20 cells; after 4 h of incubation, values up to 6-8% of the dose of radioligand added were obtained. Binding and internalization of [125I-Tyr3]octreotide were temperature dependent and inhibited by pertussis toxin. Inhibitors of lysosomal degradation did not increase the amount of internalized radioligand. After 4 h of incubation, 88% of the radioactivity present in the cells was still peptide bound, suggesting a low intracellular breakdown of this radioligand. Six of seven human GH-secreting adenoma cell cultures also internalized [125I-Tyr3]octreotide (variation between 0.24-4.98% of the dose radioligand added). Displacement of binding and internalization of [125I-Tyr3]octreotide by unlabeled octreotide showed a bell-shaped curve in AtT20 cells. At low concentrations (0.1 and 1 nM), binding and internalization were increased, whereas at higher concentrations, saturation occurred. In contrast to this, binding of [125I-Tyr3]octreotide to a broken cell preparation of AtT20 cells was displaced in a dose-dependent manner by unlabeled octreotide, with an IC50 of 0.1 nM. Similar observations were made in the human GH-secreting adenoma cell cultures. In conclusion, a high amount of [125I-Tyr3]octreotide is internalized in a specific-, time-, temperature-, and pertussis toxin-sensitive GTP-binding protein-dependent manner by mouse AtT20 and human GH-secreting pituitary tumor cells. In the presence of a low concentration of unlabeled octreotide, a rapid increase in the amount of [125I-Tyr3]octreotide internalized by AtT20 cells and by the majority of the human GH-secreting adenoma cell cultures was found.(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatobiliary handling of iodine-125-Tyr3-octreotide and indium-111-DTPA-D-Phe1-octreotide by isolated perfused rat liver.

Radiolabeled bioactive peptides may show receptor-mediated binding to tumors, making them suitable for scintigraphic imaging. The liver is an important organ for peptide clearance. To gain insight into the uptake and intracellular processing of somatostatin analogs, we compared the hepatobiliary handling of 125I-Tyr3-octreotide and 111In-DTPA-D-Phe1-octreotide, which are successfully used to image somatostatin receptor-positive tumors in vivo in isolated recirculating perfused rat livers. Sixty minutes following administration of the radiolabeled peptides, perfusion medium and biliary radioactivity were analyzed. Radioiodinated Tyr3-octreotide was rapidly cleared by the liver and 60% of the dose was excreted intact into the bile after 60 min. In contrast, 111In-DTPA-D-Phe1-octreotide was not cleared by the liver; medium radioactivity levels remained about constant and only 2% of the dose was found in the bile. These results are in agreement with in vivo findings in rats and humans. We concluded that isolated rat liver perfusion is a good system to rapidly gain insight into the hepatic handling of radiopharmaceuticals.

Inhibition of renal uptake of indium-111-DTPA-octreotide in vivo.

UNLABELLED: Indium-111-DTPA-octreotide has been successfully used for imaging of somatostatin receptor-positive lesions. However, significant renal uptake of 111In-DTPA-octreotide exists, reducing the scintigraphic sensitivity for detection of small tumors in the perirenal region and the possibilities for radiotherapy. The aim of the present study was to determine whether renal uptake of 111In-DTPA-octreotide could be reduced in vivo in rats. METHODS: Male Wistar rats (200-250 g) were placed in metabolic cages and injected with 111In-DTPA-octreotide (0.2 MBq and 0.5 microgram octreotide), in the presence or absence of re-uptake blockers. At time t = 20 hr after injection, rats were sacrificed and organs were isolated and counted for radioactivity. RESULTS: Adding NH4Cl or NaHCO3 to the food, resulting in the production of more acid or alkaline urine respectively, resulted in less radioactivity in the kidneys 20 hr after injection compared to controls. Lysine in a single dose of 400 mg/kg resulted in an inhibition of kidney uptake of 40%. When lysine was injected 30 min before 111In-DTPA-octreotide, the inhibition was 25%. Arginine had less effect on tubular uptake of 111In-DTPA-octreotide than lysine (20% inhibition). Sodium maleate inhibited kidney uptake of 111In-DTPA-octreotide most successfully. Acetazolamide (100 mg/kg), succinylacetone (100 mg/kg), cystine dimethylester (340 mg/kg) and increase in urinary flow did not influence 111In-DTPA-octreotide retention in the kidneys. CONCLUSION: It appeared possible to reduce re-uptake of 111In-DTPA-octreotide in the rat kidney in vivo. The most pronounced effects were seen after administration of sodium maleate or lysine but, because of the described toxic effects of maleate, we will study further only the effects of lysine in a clinical setting.

D-lysine reduction of indium-111 octreotide and yttrium-90 octreotide renal uptake.

UNLABELLED: Indium-111-DTPA-octreotide (111In-DTPAOC) is used successfully for imaging somatostatin receptor-positive lesions. A new and promising application is its use in peptide-receptor radionuclide therapy (PRRT). For the latter purpose, [DOTA0,D-Phe1,Tyr3]octreotide (DOTATOC), which is suitable for stable radiolabeling with 90Y, is probably even more promising. Significant renal uptake of these octreotide analogs exists, however, reducing the scintigraphic sensitivity for detection of small tumors in the perirenal region and limiting the possibilities for PRRT. We showed that the renal uptake of 111In-DTPAOC could be reduced to about 50% of control by L-lysine administration in vivo in rats. This study compares the effects of several doses and different methods of administration of D- and L-lysine, in addition to time-related effects of D-lysine, on kidney uptake of 111In-DTPAOC and 90Y-DOTATOC. METHODS: Male Wistar rats (200-250 g) were given 111In-DTPAOC (0.2 MBq, 0.5 microg-0.5 mg intravenously, intraperitoneally or orally) in the presence or absence of D- or L-lysine. At 1, 4 or 24 hr, the rats were killed, and the organs were isolated and counted for radioactivity. In different experiments, male Wistar rats (200-250 g) were given 90Y-DOTATOC (1 MBq, 0.5 microg intravenously) in the presence or absence of D-lysine. At 24 hr, the rats were killed, and the organs were isolated and counted for radioactivity. RESULTS: Administration of D- or L-lysine in a single intravenous dose of 400 mg/kg, resulted in more than 50% inhibition of kidney uptake of 111In-DTPAOC at all time points tested, independently of the mass of 111In-DTPAOC used. Higher or repeated doses of lysine did not give a significantly higher percentage inhibition. D-lysine, given orally in a dose of 400 mg/kg at 30 or 15 min before 111In-DTPAOC injection, resulted in 30% and 20% inhibition of kidney uptake, respectively. L-lysine, given orally 30 min before 111In-DTPAOC administration, resulted in 30% inhibition as well. Inhibition of kidney uptake of 111In-DTPAOC by L-lysine after intraperitoneal administration was 40%. After L-lysine administration, 111In-DTPAOC was decreased in the kidneys and in somatostatin receptor-positive organs such as the pancreas and adrenal glands. In contrast, D-lysine did not have a significant effect on uptake in octreotide receptor-positive organs. Renal uptake of 90Y-DOTATOC was reduced by 65% by intravenous D-lysine, whereas radioactivity in blood, pancreas and adrenal glands was not affected. CONCLUSION: D-lysine may be preferred to L-lysine for reduction of renal uptake of radioactivity during scintigraphy and PRRT because of its lower toxicity and because it should not interfere with the natural amino acid metabolic balance.

Tumor response after [(90)Y-DOTA(0),Tyr(3)]octreotide radionuclide therapy in a transplantable rat tumor model is dependent on tumor size.

UNLABELLED: A promising application of radiolabeled somatostatin analogs is peptide receptor-targeted radionuclide therapy of somatostatin receptor-expressing tumors. A suitable radionuclide is (90)Y, which emits high-energy beta-particles with a pathlength of several millimeters in tissue, making it especially promising for treatment of large tumors. METHODS: We investigated the radiotherapeutic effect of different activities (111 and 370 MBq) of [(90)Y-1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid (DOTA)(0),Tyr(3)]octreotide in Lewis rats bearing somatostatin receptor-positive rat pancreatic CA20948 tumors of different size (0.08-15 cm(2)) in their flank. RESULTS: Dose-dependent radiotherapeutic effects of (90)Y-labeled octreotide in this rat tumor model were found. Tumor control (100% complete response) was found in animals bearing tumors of 3-9 cm(2) (mean, 7.8 cm(2)) after intravenous injection of the highest activity (370 MBq [(90)Y-DOTA(0),Tyr(3)]octreotide). In rats bearing tumors of < or =1 cm(2) or > or =14 cm(2), the effects were less pronounced (50% and 0% complete response, respectively). In tumors of < or =1 cm(2) the (90)Y radiation energy will not be absorbed completely in the tumor, whereas in tumors of > or =14 cm(2) the increased number of clonogenic and probably hypoxic tumor cells may explain the failure to reach a cure. CONCLUSION: This study shows the ability of [(90)Y-DOTA(0),Tyr(3)]octreotide to control tumor growth, especially in medium-sized tumors. The effect of radionuclide therapy appeared to be dependent on tumor size at the onset of therapy.

Effect of dose and specific activity on tissue distribution of indium-111-pentetreotide in rats.

UNLABELLED: To increase the target-to-background ratio in receptor scintigraphy, we hypothesized that receptor scintigraphy is best performed using the lowest possible mass with the highest possible specific radioactivity of the radioligand. METHODS: Rats were injected with 2 or 10 micrograms of unlabeled octreotide or 2 or 10 micrograms of 111In-pentetreotide. Scintigraphic images were then obtained from 10 min before to 20 min after the 111In injection. RESULTS: In some instances, there was a significant increase in 111In uptake in somatostatin receptor-positive organs. In others, there was a significant decrease. Since no significant differences were found in background radioactivity in the percent dose uptake of 111In in receptor-negative organs, these data indicate that target-to-background ratios can be increased by the administration of nonradiolabeled peptides under select conditions. CONCLUSION: The uptake of 111In-pentetreotide in somatostatin receptor-positive organs results in a tissue-specific bell-shaped function of the injected mass of the radiopharmaceutical. This curve may also apply to somatostatin receptor-positive tumors, the visualization of which may be enhanced by optimizing the mass of 111In-pentetreotide.

Somatostatin receptor scintigraphy with indium-111-DTPA-D-Phe-1-octreotide in man: metabolism, dosimetry and comparison with iodine-123-Tyr-3-octreotide.

Scintigraphy with 123I-Tyr-3-octreotide has several major drawbacks as regards its metabolic behavior, its cumbersome preparation and the short physical half-life of the radionuclide. The use of another radiolabeled analog of somatostatin, 111In-DTPA-D-Phe-1-octreotide, has consequently been proposed. DTPA-D-Phe-1-octreotide can be radiolabeled with 111In in an easy single-step procedure. DTPA-D-Phe-1-octreotide is cleared predominantly via the kidneys. Fecal excretion of radioactivity amounts to only a few percent of the administered radioactivity. For the radiation dose to normal tissues, the most important organs are the kidneys, the spleen, the urinary bladder, the liver and the remainder of the body. The calculated effective dose equivalent is 0.08 mSv/MBq. Optimal 111In-DTPA-D-Phe-1-octreotide scintigraphic imaging of various somatostatin receptor-positive tumors was obtained 24 hr after injection. In the six patients studied, tumor localization with 123I-Tyr-3-octreotide and with 111In-DTPA-D-Phe-1-octreotide were found to be similar. However, the normal pituitary is more frequently visualized with the latter radiopharmaceutical. In conclusion, 111In-DTPA-D-Phe-1-octreotide appears to be a sensitive somatostatin receptor-positive tissue-seeking radiopharmaceutical with some remarkable advantages: easy preparation, general availability, appropriate half-life and absence of major interference in the upper abdominal region, because of its renal clearance. Therefore, 111In-DTPA-D-Phe-1-octreotide may be suitable for use in SPECT of the abdomen, which is important in the localization of small endocrine gastroenteropancreatic tumors.

In vivo use of a radioiodinated somatostatin analogue: dynamics, metabolism, and binding to somatostatin receptor-positive tumors in man.

Somatostatin analogues, labeled with gamma-emitting radionuclides, are of potential value in the localization of somatostatin receptor-positive tumors with gamma camera imaging. We investigated the application in man of a radioiodinated analogue of somatostatin, 123I-Tyr-3-octreotide, which has similar biologic characteristics as the native peptide. The radiopharmaceutical is cleared rapidly from the circulation (up to 85% of the dose after 10 min) mainly by the liver. Liver radioactivity is rapidly excreted into the biliary system. Until 3 hr after injection, radioactivity in the circulation is mainly in the form of 123I-Tyr-3-octreotide. Thereafter, plasma samples contain increasing proportions of free iodide. Similarly, during the first hours after injection, radioactivity in the urine exists mainly in the form of the unchanged peptide. Thereafter, a progressive increase in radioiodide excretion is observed, indicating degradation of the radiopharmaceutical in vivo. Fecal excretion of radioactivity amounts to only a few percent of the dose. The calculated median effective dose equivalent is comparable with values for applications of other 123I-radiopharmaceuticals (0.019 mSv/MBq).

Receptor scintigraphy with a radioiodinated somatostatin analogue: radiolabeling, purification, biologic activity, and in vivo application in animals.

Radioiodinated Tyr-3-octreotide, a somatostatin analogue, is a useful ligand for the in vitro detection of somatostatin receptors. In this study, we have investigated the possible in vivo application of this radioligand in the detection of somatostatin receptor-bearing tumors by scintigraphy. The specific somatostatin-like biologic activity of radioiodinated Tyr-3-octreotide was confirmed in vitro: (a) radioiodinated Tyr-3-octreotide competes in the nanomolar range with specific receptor binding of somatostatin to suspended human meningioma membranes and (b) the secretion of growth hormone by cultured rat pituitary cells was similarly inhibited by iodinated Tyr-3-octreotide and somatostatin. In rats, intravenously injected 123I-Tyr-3-octreotide is rapidly cleared from the circulation mainly by the liver. Although this rapid clearance limits the amount of tracer available for somatostatin receptor-bearing tumors, the advantage of this rapid clearance is that the background level is rapidly reduced in favor of scintigraphic imaging of these tumors. Pancreatic tumors in rats were localized by scintigraphy after intravenous injection of 123I-Tyr-3-octreotide.

In vitro and in vivo studies of substance P receptor expression in rats with the new analog [indium-111-DTPA-Arg1]substance P.

UNLABELLED: We evaluated the potential usefulness of a new radiolabeled substance P (SP) analog, [111In-DTPA-Arg1]SP, as a radiopharmaceutical for the in vivo detection of SP receptor-positive (SPR+) immunologic disorders (i.e., inflammatory bowel disease and arthritis) and tumors (i.e., carcinoid). METHODS: Substance P, [DTPA-Arg1]SP and [3-(p-hydroxyphenyl)propionyl-Arg1]SP (Bolton-Hunter-SP, [BH-SP]) were tested as competitors for 125I-BH-SP to SPR in rat brain cortex membranes. An autoradiographic displacement study of the submandibular gland of the rat with the 125I-BH-SP as radioligand and [DTPA-Arg1]SP as competitor was performed. Tissue distribution and ex vivo autoradiography were studied in rats, with and without pretreatment with the selective nonpeptide antagonist CP96,345 to quantify specific binding. In vivo metabolism of [111In-DTPA-Arg1]SP was performed in control rats. Gamma-camera scintigraphic studies were carried out with control rats to visualize the SPR+ salivary glands in rats bearing the SPR+ transplantable pancreatic tumor CA20948 and in rats with SPR+ adjuvant arthritic joints, which was induced after injection of a homogenate of Mycobacterium tuberculosis. RESULTS: Substance P, [DTPA-Arg1]SP and BH-SP dose-dependently inhibited binding of 125I-BH-SP to SPR in rat brain cortex membranes with IC50 values of 0.2, 4 and 2 nM, respectively. In an autoradiographic displacement study of the submandibular gland with 125I-BH-SP as radioligand, an IC50 of 2.7 nM was found for [DTPA-Arg1]SP. In vivo metabolism of the radiopharmaceutical in the rat revealed a renal clearance rate of 50% of the injected radioactive dose in 30 min and a rapid enzymatic degradation of the radiopharmaceutical, resulting in an effective half-life of the intact radiopharmaceutical in blood of approximately 3 min. Tissue distribution and ex vivo autoradiographic studies in rats showed uptake and specific binding of radioactivity in isolated tumors and submandibular and parotid glands. Optimum SPR+ target-to-background ratios were found 24 hr after injection of [111In-DTPA-Arg1]SP. Visualization of normal SPR+ tissues, such as the salivary glands by gamma camera scintigraphy, after administration of [111In-DTPA-Arg1]SP was demonstrated in untreated rats. Pathological SPR+ processes were visualized both in rats bearing the transplantable pancreatic tumor CA20948 and in those with adjuvant mycobacteria tuberculosis-induced arthritic joints. CONCLUSION: [Indium-111-DTPA-Arg1]SP can be used successfully to visualize SPR+ processes in vivo by gamma camera scintigraphy.

The role of radioactive somatostatin and its analogues in the control of tumor growth.

Peptide receptor scintigraphy with the radioactive somatostatin analogue [111In-DTPA-D-Phe1]octreotide is a sensitive and specific technique to show in vivo the presence and abundance of somatostatin receptors on various tumors. With this technique primary tumors and metastases of neuroendocrine cancers as well as of many other cancer types can be localized. This technique is currently used to assess the possibility of peptide receptor radionuclide therapy with repeated administration of high doses of [111In-DTPA-D-Phe1]octreotide. 111In emits Auger and conversion electrons, having a tissue penetration of 0.02-10 microns and 200-500 microns, respectively. Thirty end-stage patients with mostly neuroendocrine progressing tumors were treated with [111In-DTPA-D-Phe1]octreotide, up to a maximal cumulative patient dose of about 74 GBq, in a phase-I trial. There were no major clinical side effects after up to 2 years of treatment, except that in a few patients a transient decline in platelet counts and lymphocyte subsets occurred. Promising beneficial effects on clinical symptoms, hormone production, and tumor proliferation were found. Of the 21 patients who received a cumulative dose of more than 20 GBq, eight showed stabilization of disease and six others a reduction in tumor size. There is a tendency towards better results in patients whose tumors have a higher accumulation of the radioligand. Peptide receptor radionuclide therapy is also feasible with 111In as the radionuclide. Theoretically, depending on the homogeneity of distribution of tumor cells expressing peptide receptors and the size of the tumor, beta-emitting radionuclides, e.g., 90Y, labeled to DOTA-chelated peptides may be more effective than 111In for peptide receptor radionuclide therapy. The first peptide receptor radionuclide therapy trials with [90Y-DOTA-Tyr3]octreotide started recently.

99mTc-MIBI, 99mTc-tetrofosmin and 99mTc-Q12 in vitro and in vivo.

The aim of this study was to compare uptake of 99mTc-MIBI, 99mTc-tetrofosmin and 99mTc-Q12 in vitro and biodistribution in vivo in rats. In vitro, uptake decreased in the order MIBI-->tetrofosmin-->Q12. Uptake of MIBI and tetrofosmin, but not of Q12, in cultured tumor cells was dependent on the plasma membrane and mitochondrial potential. In vivo, heart uptake of all three compounds was high and stable. Tumor uptake decreased in the order MIBI-->Q12-->tetrofosmin and the tumor/blood ratio in the order MIBI-->tetrofosmin-->Q12.

Effects of ligand priming and multiple-dose injection on tissue uptake of 111In-pentetreotide in rats.

In patients undergoing somatostatin receptor scintigraphy, treatment with octreotide (Sandostatin) is usually discontinued 24-48 h before and after injection with the radioligand 111In-pentetreotide ([111In-DTPA(O)]octreotide) (Octreoscan) because octreotide competes with radioligand for the same receptors. However, Dörr et al. and Soresi et al. reported improved visualization of carcinoid and small cell lung cancer lesions, respectively, during continued octreotide treatment. We found that intravenous administration of unlabeled octreotide to rats inhibited the binding of an optimal dose (0.5 microg) of 111In-pentetreotide to somatostatin receptors in pancreas and adrenals in a mass- and time-dependent way. Pretreatment with unlabeled octreotide never increased receptor binding of 111In-pentetreotide. Administration of 100 microg of octreotide decreased receptor-bound radioactivity if given simultaneously with or 10 or 20 min after injection of the radioligand, but had no effect if given 30 min after the radioligand. These findings indicate rapid processing of receptor-bound octreotide and suggest that octreotide treatment of patients undergoing 111In-pentetreotide scintigraphy may be reinitiated as soon as 1 h after radioligand administration.

Intracavitary brachytherapy of cystic craniopharyngiomas.

Visual function, endocrinological status, and radiological outcome are reported in 31 patients harboring a cystic craniopharyngioma, who underwent 35 intracavitary brachytherapy procedures with yttrium-90. In 26 of these patients intracavitary brachytherapy was the primary treatment. The follow-up period ranged from 2 to 80 months (41 +/- 22 months, mean +/- standard deviation). Five patients died from tumor-related causes. Endocrine recovery was modest. Prior to brachytherapy, visual acuity was diminished in 38 eyes and field defects were present in 46. Funduscopy before treatment revealed optic atrophy in 47% of the eyes. Visual acuity improved in 29% of the eyes studied, remained stable in 13%, and deteriorated in 58%. Visual field defects improved in 28% of the eyes studied, remained stable in 20%, and deteriorated in 52%. The possible causes for deterioration in visual function are discussed. Complete resolution of 10 cysts was noted. In 12 patients the size of the cyst decreased; however, in three of these patients new cyst formation took place. The cyst size stabilized in six cases and increased in three. Although there is still a substantial degree of visual function deterioration following intracavitary brachytherapy, morbidity is otherwise low, making this treatment modality a reasonable alternative to craniotomy.

[111In-DTPA-D-Phe1]-octreotide, a potential radiopharmaceutical for imaging of somatostatin receptor-positive tumors: synthesis, radiolabeling and in vitro validation.

Somatostatin receptor-positive human tumors can be detected using radioiodinated analogues of somatostatin, both in vitro and in vivo. [123I-Tyr3]-octreotide has been successfully used in the visualization of somatostatin receptor-positive tumors by gamma camera scintigraphy, but this radiopharmaceutical has some major drawbacks, which can be overcome with other radionuclides such as 111In. As starting material for a potentially convenient radiopharmaceutical, a diethylenetriaminopentaacetic acid (DTPA) conjugated derivative of octreotide (SMS 201-995) was prepared. This peptide, [DTPA-D-Phe1]-octreotide (SDZ 215-811) binds more than 95% of added 111In in an easy, single-step labeling procedure without necessity of further purification. The specific somatostatin-like biologic effect of these analogues was proven by the inhibition of growth hormone secretion by cultured rat pituitary cells in a dose-dependent fashion by octreotide, [DTPA-D-Phe1]-octreotide and non-radioactive [115In-DTPA-D-Phe1]-octreotide. The binding of [111In-DTPA-D-Phe1]-octreotide to rat brain cortex membranes proved to be displaced similarly by natural somatostatin as well as by octreotide, suggesting specific binding of [111In-DTPA-D-Phe1]-octreotide to somatostatin receptors. The binding of the indium-labeled compound showed a somewhat lower affinity when compared with the iodinated [Tyr3]-octreotide, but indium-labeled [DTPA-D-Phe1]-octreotide still binds with nanomolar affinity. In conjunction with in vivo studies, these results suggest that [111In-DTPA-D-Phe1]-octreotide is a promising radiopharmaceutical for scintigraphic imaging of somatostatin receptor-positive tumors.

In vivo application of [111In-DTPA-D-Phe1]-octreotide for detection of somatostatin receptor-positive tumors in rats.

Radioiodinated somatostatin analogues are useful ligands for the in vitro and in vivo detection of somatostatin receptors. [111In-DTPA-D-Phe1]-octreotide, a somatostatin analogue labeled with a different radionuclide, also binds specifically to somatostatin receptors in vitro. In this study we investigated its in vivo application in the visualization of somatostatin receptor-positive tumors in rats. The distribution of the radiopharmaceutical was investigated after intravenous injection in normal rats and in rats bearing the somatostatin receptor-positive rat pancreatic carcinoma CA 20948. After injection the radiopharmaceutical was rapidly cleared (50% decrease in maximal blood radioactivity in 4 min), predominantly by the kidneys. Excreted radioactivity was mainly in the form of the intact radiopharmaceutical. Ex vivo autoradiographic studies showed that specific accumulation of radioactivity occurred in somatostatin receptor-containing tissue (anterior pituitary gland). However, in contrast to the adrenals and pituitary, the tracer accumulation in the kidneys was not mediated by somatostatin receptors. Increasing radioactivity over the somatostatin receptor-positive tumors was measured rapidly after injection and the tumors were clearly visualized by gamma camera scintigraphy. In rats pretreated with 1 mg octreotide accumulation of [111In-DTPA-D-Phe1]-octreotide in the tumors was prevented. Because of its relatively long effective half-life, [111In-DTPA-D-Phe1]-octreotide is a radionuclide-coupled somatostatin analogue which can be used to visualize somatostatin receptor-bearing tumors efficiently after 24 hr, when interfering background radioactivity is minimized by renal clearance. This is an advantage over the previously used [123I-Tyr3]-octreotide which has a shorter effective half-life and shows high abdominal interference due to its hepato-biliary clearance. Therefore, [111In-DTPA-D-Phe1]-octreotide seems a better alternative for scintigraphic imaging of somatostatin receptor-bearing tumors.