Bone marrow dosimetry in peptide receptor radionuclide therapy with [177Lu-DOTA(0),Tyr(3)]octreotate.

PURPOSE: Adequate dosimetry is mandatory for effective and safe peptide receptor radionuclide therapy (PRRT). Besides the kidneys, the bone marrow is a potentially dose-limiting organ. The radiation dose to the bone marrow is usually calculated according to the MIRD scheme, where the accumulated activity in the bone marrow is calculated from the accumulated radioactivity of the radiopharmaceutical in the blood. This may underestimate the absorbed dose since stem cells express somatostatin receptors. We verified the blood-based method by comparing the activity in the blood with the radioactivity in bone marrow aspirates. Also, we evaluated the absorbed cross-dose from the source organs (liver, spleen, kidneys and blood), tumours and the so-called "remainder of the body" to the bone marrow. METHODS: Bone marrow aspirates were drawn in 15 patients after treatment with [(177)Lu-DOTA(0),Tyr(3)]octreotate. Radioactivity in the bone marrow was compared with radioactivity in the blood drawn simultaneously. The nucleated cell fraction was isolated from the bone marrow aspirate and radioactivity was measured. The absorbed dose to the bone marrow was calculated. The results were correlated to the change in platelet counts 6 weeks after treatment. RESULTS: A strong linear correlation and high agreement between the measured radioactivities in the bone marrow aspirates and in the blood was found (r=0.914, p<0.001). No correlation between the calculated absorbed dose in the bone marrow and the change in platelets was found. There was a considerable contribution from other organs and the remainder of the body to the bone marrow absorbed dose. CONCLUSION: (1) After PRRT with [(177)Lu-DOTA(0),Tyr(3)]octreotate, the radioactivity concentration in the bone marrow is identical to that in the blood; (2) There is no significant binding of the radiopharmaceutical to bone marrow precursor stem cells; (3) The contribution of the cross dose from source organs and tumours to the bone marrow dose is significant; and (4) There is considerable variation in bone marrow absorbed dose between patients. These findings imply that for individual dose optimization, individual calculation of the bone marrow absorbed dose is necessary.

Renal uptake and retention of radiolabeled somatostatin, bombesin, neurotensin, minigastrin and CCK analogues: species and gender differences.

INTRODUCTION: During therapy with radiolabeled peptides, the kidney is most often the critical organ. Newly developed peptides are evaluated preclinically in different animal models before their application in humans. In this study, the renal retention of several radiolabeled peptides was compared in male and female rats and mice. METHODS: After intravenous injection of radiolabeled peptides [somatostatin, cholecystokinin (CCK), minigastrin, bombesin and neurotensin analogues], renal uptake was determined in both male and female Lewis rats and C57Bl mice. In addition, ex vivo autoradiography of renal sections was performed to localize accumulated radioactivity. RESULTS: An equal distribution pattern of renal radioactivity was found for all peptides: high accumulation in the cortex, lower accumulation in the outer medulla and no radioactivity in the inner medulla of the kidneys. In both male rats and mice, an increasing renal uptake was found: [(111)In-DTPA]CCK8<[(111)In-DTPA-Pro(1),Tyr(4)]bombesin approximately [(111)In-DTPA]neurotensin<[(111)In-DTPA]octreotide<<[(111)In-DTPA]MG0. Renal uptake of [(111)In-DTPA]octreotide in rats showed no gender difference, and renal radioactivity was about constant over time. In mice, however, renal uptake in females was significantly higher than that in males and decreased rapidly over time in both genders. Moreover, renal radioactivity in female mice injected with [(111)In-DTPA]octreotide showed a different localization pattern. CONCLUSIONS: Regarding the renal uptake of different radiolabeled peptides, both species showed the same ranking order. Similar to findings in patients, rats showed comparable and constant renal retention of radioactivity in both genders, in contrast to mice. Therefore, rats appear to be the more favorable species for the study of the renal retention of radioactivity.

From outside to inside? Dose-dependent renal tubular damage after high-dose peptide receptor radionuclide therapy in rats measured with in vivo (99m)Tc-DMSA-SPECT and molecular imaging.

UNLABELLED: In peptide receptor radionuclide therapy (PRRT), the dose-limiting organ is, most often, the kidney. However, the precise mechanism as well as the exact localization of kidney damage during PRRT have not been fully elucidated. We studied renal damage in rats after therapy with different amounts of [(177)Lu-DOTA(0), Tyr(3)]octreotate and investigated (99m)Tc-DMSA (dimercaptosuccinic acid) as a tool to quantify renal damage after PRRT. EXPERIMENTAL DESIGN: Twenty-nine (29) rats were divided into 3 groups and injected with either 0, 278, or 555 MBq [(177)Lu-DOTA(0), Tyr(3) ]octreotate, leading to approximately 0, 46, and 92 Gy to the renal cortex. More than 100 days after therapy, kidney damage was investigated using (99m)Tc-DMSA single-photon emission computed tomography (SPECT) autoradiography, histology, and blood analyses. RESULTS: In vivo SPECT with (99m)Tc-DMSA resulted in high-resolution (<1.6-mm) images. The (99m)Tc-DMSA uptake in the rat kidneys was inversely related with the earlier injected activity of [(177)Lu-DOTA(0), Tyr(3)]octreotate and correlated inversely with serum creatinine values. Renal ex vivo autoradiograms showed a dose-dependent distribution pattern of (99m)Tc-DMSA. (99m)Tc-DMSA SPECT could distinguish between the rats that were injected with 278 or 555 MBq [(177)Lu-DOTA(0), Tyr(3) ]octreotate, whereas histologic damage grading of the kidneys was nearly identical for these 2 groups. Histologic analyses indicated that lower amounts of injected radioactivity caused damage mainly in the proximal tubules, whereas as well the distal tubules were damaged after high-dose radioactivity. CONCLUSIONS: Renal damage in rats after PRRT appeared to start in a dose-dependent manner in the proximal tubules and continued to the more distal tubules with increasing amounts of injected activity. In vivo SPECT measurement of (99m)Tc-DMSA uptake was highly accurate to grade renal tubular damage after PRRT.

Diagnostic versus therapeutic doses of [(177)Lu-DOTA-Tyr(3)]-octreotate: uptake and dosimetry in somatostatin receptor-positive tumors and normal organs.

AIM: The aim of this study was to investigate the influence of a diagnostic versus therapeutic dose of [(177)Lu-DOTA-Tyr(3)]octreotate on the uptake, effects, and dosimetry in somatostatin receptor subtype 2(sst2)-positive tumors and normal organs in a rat tumor model. MATERIALS AND METHODS: Lewis rats bearing rat pancreatic CA20948-tumor grafts were injected intravenously with different amounts of radioactivity and peptide of [(177)Lu-DOTA-Tyr(3)]octreotate: 3 MBq/0.5 microg (group A), 3 MBq/15 microg (group B), 300 MBq/15 microg (group C), and 555 MBq/15 microg (group D). Biodistribution studies were performed at several time points between 3 and 13 days post injection. Ex vivo and in vitro autoradiography was performed with frozen tumor sections. RESULTS: Normal sst2-positive tissues showed a significantly higher uptake of radioactivity [%IA/g] when a low peptide amount was injected. On the other hand, the radioactivity concentration [%IA/g] in sst2-negative tissues and organs (blood, muscles, kidney, and liver) were comparable (groups A and B), independent of the injected peptide amount. Initially, this held true for the tumors as well. Yet, over time, we found a decrease in the radioactivity concentration in the tumors of groups A and B, because of tumor growth. On the other hand, therapeutic amounts of radioactivity (groups C and D) resulted in a significant reduction of tumor size, where radioactivity concentration remained higher than in groups A and B, despite the use of the high peptide amounts. Ex vivo autoradiograms of tumor sections confirmed these results. In vitro autoradiography performed on adjacent tumor sections revealed a reduced density of sst2 in tumors from animals that received a therapeutic radioactivity dose. Ki67-antibody immunohistochemistry showed an absence of proliferating tumor cells after therapy. CONCLUSIONS: The differences in radioactivity retention in tumors after diagnostic or therapeutic doses, depending on a change in tumor kinetics, have to be taken into account when calculating tumor-absorbed radiation doses.

Five Stabilized 111In-labeled neurotensin analogs in nude mice bearing HT29 tumors.

Neurotensin (NT) receptors are overexpressed in different human tumors, such as human ductal pancreatic adenocarcinoma. New stable neurotensin analogs with high receptor affinity have been synthesized by replacing arginine residues with lysine and arginine derivatives. The aim of this study was to explore the biodistribution, tumor uptake, kidney localization, and stability characteristics of these new analogs in order to develop new diagnostic tools for exocrine pancreatic cancer. Four (111)In-labeled DTPA-chelated NT analogs and one (111)In-labeled DOTA-chelated NT analog were evaluated in NMRI nude mice bearing NT receptor-positive HT29 tumors. Experiments with a coinjection of unlabeled NT or lysine were performed to investigate receptor-mediated uptake and kidney protection, respectively. In addition, the in vivo serum stability of the most promising analog was analyzed. In the biodistribution study in mice, at 4 hours postinjection, a low percentage of the injected dose per gram (%ID/g) of tissue for all compounds was found in NT receptor-negative organs, such as the blood, spleen, pancreas, liver, muscle, and femur. A high uptake was found in the colon, intestine, kidneys, and in implanted HT29 tumors. The coinjection of excess unlabeled neurotensin significantly reduced tumor uptake, showing tumor uptake to be receptor-mediated. To a lesser extent, this was also observed for the colon, but not for other tissues. We concluded that DTPA-(Pip)Gly-Pro-(PipAm)Gly-Arg-Pro-Tyr-tBuGly-Leu-OH and the DOTA-linked counterpart have the most favorable biodistribution properties regarding tumor uptake.

Long-term toxicity of [(177)Lu-DOTA (0),Tyr (3)]octreotate in rats.

PURPOSE AND METHODS: Studies on peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues have shown promising results with regard to tumour control. The efficacy of PRRT is limited by uptake and retention in the proximal tubules of the kidney, which might lead to radiation nephropathy. We investigated the long-term renal toxicity after different doses of [(177)Lu-DOTA(0),Tyr(3)]octreotate and the effects of dose fractionation and lysine co-injection in two tumour-bearing rat models. RESULTS: Significant renal toxicity was detected beyond 100 days after start of treatment as shown by elevated serum creatinine and proteinuria. Microscopically, tubules were strongly dilated with flat epithelium, containing protein cylinders. Creatinine levels rose significantly after 555 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate, but were significantly lower after 278 MBq (single injection) or two weekly doses of 278 MBq. Renal damage scores were maximal after 555 MBq and significantly lower in the 278 and 2x278 MBq groups. Three doses of 185 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate with intervals of a day, a week or a month significantly influenced serum creatinine (469+/-18, 134+/-70 and 65+/-15 micromol/l, respectively; p<0.001). Renal histological damage scores were not significantly influenced by dose fractionation. Lysine co-administration with three weekly treatments of 185 MBq significantly lowered serum creatinine and proteinuria. CONCLUSION: Injection of high doses of [(177)Lu-DOTA(0),Tyr(3)]octreotate resulted in severe renal damage in rats as indicated by proteinuria, elevated serum creatinine and histological damage. This damage was dose dependent and became overt between 100 and 200 days after treatment. Dose fractionation had significant beneficial effects on kidney function. Also, lysine co-injection successfully prevented functional damage.

Amifostine protects rat kidneys during peptide receptor radionuclide therapy with [177Lu-DOTA0,Tyr3]octreotate.

PURPOSE: In peptide receptor radionuclide therapy (PRRT) using radiolabelled somatostatin analogues, the kidneys are the major dose-limiting organs, because of tubular reabsorption and retention of radioactivity. Preventing renal uptake or toxicity will allow for higher tumour radiation doses. We tested the cytoprotective drug amifostine, which selectively protects healthy tissue during chemo- and radiotherapy, for its renoprotective capacities after PRRT with high-dose [(177)Lu-DOTA(0),Tyr(3)]octreotate. METHODS: Male Lewis rats were injected with 278 or 555 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate to create renal damage and were followed up for 130 days. For renoprotection, rats received either amifostine or co-injection with lysine. Kidneys, blood and urine were collected for toxicity measurements. At 130 days after PRRT, a single-photon emission computed tomography (SPECT) scan was performed to quantify tubular uptake of (99m)Tc-dimercaptosuccinic acid (DMSA), a measure of tubular function. RESULTS: Treatment with 555 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate resulted in body weight loss, elevated creatinine and proteinuria. Amifostine and lysine treatment significantly prevented this rise in creatinine and the level of proteinuria, but did not improve the histological damage. In contrast, after 278 MBq [(177)Lu-DOTA(0),Tyr(3)]octreotate, creatinine values were slightly, but not significantly, elevated compared with the control rats. Proteinuria and histological damage were different from controls and were significantly improved by amifostine treatment. Quantification of (99m)Tc-DMSA SPECT scintigrams at 130 days after [(177)Lu-DOTA(0),Tyr(3)]octreotate therapy correlated well with 1/creatinine (r(2)=0.772, p<0.001). CONCLUSION: Amifostine and lysine effectively decreased functional renal damage caused by high-dose [(177)Lu-DOTA(0),Tyr(3)]octreotate. Besides lysine, amifostine might be used in clinical PRRT as well as to maximise anti-tumour efficacy.

[(99m)Tc]Demotate 2 in the detection of sst(2)-positive tumours: a preclinical comparison with [(111)In]DOTA-tate.

PURPOSE: The aim of this study was to evaluate [(99m)Tc]Demotate 2 ([(99m)Tc-N(4) (0-1),Asp(0),Tyr(3)]octreotate) as a candidate for in vivo imaging of sst(2)-positive tumours and to compare it with [(111)In]DOTA-tate ([(111)In-DOTA(0),Tyr(3)]octreotate). METHODS: Labelling of Demotate 2 with (99m)Tc was performed at room temperature using SnCl(2) as reductant in the presence of citrate at alkaline pH. Radiochemical analysis involved ITLC and HPLC methods. Peptide conjugate affinities for sst(2) were determined by receptor autoradiography on rat brain cortex sections using [DOTA(0),(125)I-Tyr(3)]octreotate as the radioligand. The affinity profile of Demotate 2 for human sst(1)-sst(5) was studied by receptor autoradiography in cell preparations using the universal somatostatin radioligand [(125)I][Leu(8),(D: )Trp(22),Tyr(25)]somatostatin-28. The internalisation rates of [(99m)Tc]Demotate 2 and [(111)In]DOTA-tate were compared in sst(2)-positive and -negative control cell lines. Biodistribution of radiopeptides was studied in male Lewis rats bearing CA20948 tumours. RESULTS: Peptide conjugates showed selectivity and a high affinity binding for sst(2) (Demotate 2 IC(50)=3.2 nM and DOTA-tate IC(50)=5.4 nM). [(99m)Tc]Demotate 2, like [(111)In]DOTA-tate, internalised rapidly in all sst(2)-positive cells tested, but not in sst(2)-negative control cells. After injection in CA20948 tumour-bearing rats both radiopeptides showed high and specific uptake in the sst(2)-positive organs and in the implanted tumour and rapid excretion from non-target tissues via the kidneys. CONCLUSION: [(99m)Tc]Demotate 2, similarly to the known sst(2)-targeting agent [(111)In]DOTA-tate, showed promising biological qualities for application in the scintigraphy of sst(2)-positive tumours.

Anticancer activity of targeted proapoptotic peptides.

UNLABELLED: Tumor-induced angiogenesis can be targeted by RGD (Arg-Gly-Asp) peptides, which bind to alpha(v)beta(3)-receptors upregulated on angiogenic endothelial cells. RGD-containing peptides are capable of inducing apoptosis through direct activation of procaspase-3 to caspase-3 in cells. Additionally, tumor cells overexpressing somatostatin receptors can be targeted by somatostatin analogs. Radiolabeled somatostatin analogs are successfully used to image and treat such tumors via receptor-targeted scintigraphy and therapy. We combined these 2 peptides, RGD and somatostatin, to synthesize a new hybrid peptide, RGD-diethylenetriaminepentaacetic acid (DTPA)-octreotate (c(Arg-Gly-Asp-D-Tyr-Asp)-Lys(DTPA)-D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr). An earlier study showed that tumor-bearing rats had high receptor-specific uptake of RGD-(111)In-DTPA-octreotate in somatostatin receptor subtype 2-positive tissues and tumors. Furthermore, RGD-(111)In-DTPA-octreotate showed a pronounced tumoricidal effect, which is probably the result of increased apoptosis, as is shown by an increased caspase-3 activity after incubation with (111)In-labeled RGD-DTPA-octreotate in comparison with the 2 monopeptides (111)In-DTPA-RGD and (111)In-DTPA-Tyr(3)-octreotate. In this study, we evaluated the biodistributions of RGD-(111)In-DTPA-octreotate and (125)I-RGD-octreotate and investigated the caspase-3 activation of the unlabeled compound RGD-DTPA-octreotate in vitro. METHODS: Biodistribution studies on tumor-bearing rats were performed with RGD-(111)In-DTPA-octreotate and (125)I-RGD-octreotate. The apoptotic activity, by activation of caspase-3 with RGD-DTPA-octreotate and RGD-octreotate, was examined using colorimetric and immunocytochemical assays. RESULTS: The radiolabeled compound, RGD-(111)In-DTPA-octreotate, showed high uptake and retention in the rats in which rat pancreatic CA20948 tumor had been implanted. A major drawback was high renal uptake. In vitro, the unlabeled peptide RGD-DTPA-octreotate induced a significant increase in caspase-3 levels in various cell lines in comparison with RGD and Tyr(3)-octreotate (P < 0.01). Caspase-3 activation was time dependent. To alter the elimination route, we examined the biodistribution of radioiodinated RGD-octreotate without DTPA [c(Arg-Gly-Asp-D-Tyr-Asp)-D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr], as a model of unlabeled RGD-octreotate, in tumor-bearing rats. (125)I-RGD-octreotate showed a much lower renal uptake than did RGD-(111)In-DTPA-octreotate. Furthermore, the affinity of RGD-octreotate increased in comparison with RGD-DTPA-octreotate (values of 1.4 x 10(-8) mol/L vs. 9.4 x 10(-8) mol/L, respectively, for inhibitory concentration of 50%). Finally, RGD-octreotate was still able to activate caspase-3, as was indicated with immunocytochemistry. CONCLUSION: Because of the high renal uptake, RGD-(111)In-DTPA-octreotate is unsuitable for radionuclide therapy. However, the unlabeled peptides, RGD-DTPA-octreotate and RGD-octreotate, also induced an increase in caspase-3 levels, indicating the therapeutic potential of this compound. Thus, the development of hybrid molecules can become a new approach in the treatment of cancer.

Megalin is essential for renal proximal tubule reabsorption of (111)In-DTPA-octreotide.

UNLABELLED: Radiolabeled somatostatin analogs have been shown to be important radiopharmaceuticals for tumor diagnosis and radionuclide therapy. The kidney has appeared to be the critical organ during radionuclide therapy because of peptide reabsorption and retention in the proximal tubules after glomerular filtration. The molecular mechanism of renal reabsorption of these analogs has not been clarified. A possible receptor candidate is megalin, a multiligand scavenger receptor in the renal proximal tubules. The objective of this study was to investigate the role of megalin in tubular reabsorption of radiolabeled somatostatin analogs by using kidney-specific megalin-deficient mice versus mice with normal renal megalin expression. [(111)In-Diethylenetriaminepentaacetic acid (DTPA)]octreotide was used as a practical model of peptide. METHODS: Renal uptake of [(111)In-DTPA]octreotide was determined by animal SPECT scintigraphy at different time points after injection of the tracer and by measurement of radioactivity after isolation of the organs. Furthermore, ex vivo autoradiography of renal sections revealed the zonal distribution of radioactivity in the megalin-deficient and megalin-expressing kidneys. RESULTS: SPECT scintigraphy of [(111)In-DTPA]octreotide at 3 and 24 h after injection clearly showed lower renal radioactivity in megalin-deficient kidneys than in megalin-expressing kidneys, both in male and in female mice, in accordance with counts obtained after isolation of the organ (70%-85% reduction of uptake in the megalin-deficient kidneys, P < 0.001). Renal uptake of [(111)In-DTPA]octreotide was significantly higher in female than in male kidneys (P < 0.001). Ex vivo autoradiograms clearly showed that renal radioactivity was not homogeneously distributed in the megalin-expressing kidneys but localized in the renal cortex. Quantification of the autoradiogram data confirmed the reduced radioactivity in the renal cortex of megalin-deficient kidneys. CONCLUSION: This study revealed the molecular mechanism of [(111)In-DTPA]octreotide uptake in renal proximal tubules involving the receptor megalin. Identification of megalin may be crucial for further research into strategies to reduce renal uptake.

111In-labelled somatostatin analogues in a rat tumour model: somatostatin receptor status and effects of peptide receptor radionuclide therapy.

PURPOSE: Peptide receptor scintigraphy with the radioactive somatostatin analogue 111In-DTPA-octreotide is a sensitive and specific technique to show in vivo the presence of somatostatin receptors on various tumours. Since 111In emits not only gamma rays but also therapeutic Auger and internal conversion electrons with a medium to short tissue penetration (0.02-10 microm and 200-550 microm, respectively), 111In-DTPA-octreotide is also being used for peptide receptor radionuclide therapy (PRRT). In this study we investigated the therapeutic effects of 111In-DTPA-octreotide in tumours of various sizes. Regrowth of a tumour despite PRRT with 111In-DTPA-octreotide may be due to the lack of crossfire from 111In, whereby any possible receptor-negative tumour cell can multiply. We therefore also investigated the somatostatin receptor status of the tumour before and after PRRT. METHODS: The radiotherapeutic effects of different doses of 111In-DTPA-octreotide in vivo were investigated in Lewis rats bearing small (< or = 1 cm2) or large (> or = 8 cm2) somatostatin receptor-positive rat pancreatic CA20948 tumours expressing the somatostatin receptor subtype 2 (sst2). In addition, the somatostatin receptor density on the tumour after injection of a therapeutic labelled somatostatin analogue was investigated when the tumour was either declining in size or regrowing after an initial reduction in size. To initiate a partial response of the tumour (so that regrowth would follow) and not a complete response, a relatively low dose was administered. RESULTS: Impressive radiotherapeutic effects of 111In-labelled octreotide were observed in this rat tumour model. Complete responses (up to 50%) were found in the animals bearing small (< or 1 cm2) tumours after at least three injections of 111 MBq or a single injection of 370 MBq 111In-DTPA-octreotide, leading to a dose of 6.3-7.8 mGy/MBq (1-10 g tumour). In the rats bearing the larger (> or = 8 cm2) tumours, the effects were much less pronounced and only partial responses were achieved in these groups. Clear sst2 expression was found in the control as well as in the treated tumours. A significantly higher tumour receptor density (p<0.001) was found when the tumours regrew after an initial decline in size after low-dose PRRT in comparison with the untreated tumours. CONCLUSION: Therapy with 111In-labelled somatostatin analogues is feasible but should preferably start as early as possible during tumour development. One might also consider the use of radiolabelled somatostatin analogues in an adjuvant setting after surgery of somatostatin receptor-positive tumours in order to eradicate occult metastases. We showed that PRRT led to an increase in the density of somatostatin receptors when the tumours regrew after an initial decline in size because of PRRT. Upregulation of the somatostatin receptor may lead to higher uptake of radiolabelled peptides in therapeutic applications, which would probably make repeated injections of radiolabelled peptides more effective.

Localisation and mechanism of renal retention of radiolabelled somatostatin analogues.

PURPOSE: Radiolabelled somatostatin analogues, such as octreotide and octreotate, are used for tumour scintigraphy and radionuclide therapy. The kidney is the most important critical organ during such therapy owing to the reabsorption and retention of radiolabelled peptides. The aim of this study was to investigate in a rat model both the localisation and the mechanism of renal uptake after intravenous injection of radiolabelled somatostatin analogues. The multi-ligand megalin/cubilin receptor complex, responsible for reabsorption of many peptides and proteins in the kidney, is an interesting candidate for renal endocytosis of these peptide analogues. METHODS: For localisation studies, ex vivo autoradiography and micro-autoradiography of rat kidneys were performed 1-24 h after injection of radiolabelled somatostatin analogues and compared with the renal anti-megalin immunohistochemical staining pattern. To confirm a role of megalin in the mechanism of renal retention of [111In-DTPA]octreotide, the effects of three inhibitory substances were explored in rats. RESULTS: Renal ex vivo autoradiography showed high cortical radioactivity and lower radioactivity in the outer medulla. The distribution of cortical radioactivity was inhomogeneous. Micro-autoradiography indicated that radioactivity was only retained in the proximal tubules. The anti-megalin immunohistochemical staining pattern showed a strong similarity with the renal [111In-DTPA]octreotide ex vivo autoradiograms. Biodistribution studies showed that co-injection of positively charged D-lysine reduced renal uptake to 60% of control. Sodium maleate reduced renal [111In-DTPA]octreotide uptake to 15% of control. Finally, cisplatin pre-treatment of rats reduced kidney uptake to 70% of control. CONCLUSION: Renal retention of [111In-DTPA]octreotide is confined to proximal tubules in the rat kidney, in which megalin-mediated endocytosis may play an important part.

Radiolabelling DOTA-peptides with 68Ga.

PURPOSE: A new field of interest is the application of 68Ga-labelled DOTA-conjugated peptides for positron emission tomography (PET). The commercially available or house-made generators require time-consuming and tedious handling of the eluate. Radiolabelling at high specific activities without further purification is not possible, while high specific activities are necessary for peptides that potentially display pharmacological side-effects. Here we present the practical aspects and the results of radiolabelling DOTA-peptides with a TiO2-based commercially available 68Ge/68Ga generator. METHODS: Reaction kinetics and parameters influencing the incorporation of the radionuclide at the highest achievable specific activity were investigated. Since high finger doses were anticipated during handling of the high beta-energy emitter 68Ga, finger dosimetric measurements were performed during radiolabelling and in vivo administration. RESULTS: Fractionated elution of the generator revealed that 80% of the radioactivity was recovered in 1 ml. Bi- and trivalent ionic contaminants that compete for the incorporation of the radionuclide were below 50 nM; thus further tedious and time-consuming purification was avoided. Radiolabelling was performed at pH 3.5-4. Plastic shielding (> or =7-mm wall thickness) around the syringe during administration effectively eliminated the positrons. In rats 68GaCl3 had slow clearance from blood, while 68Ga-EDTA was rapidly cleared via the kidneys. Uptake of 68Ga-DOTATOC in somatostatin receptor-positive tissues was high, with no significant difference between 1 and 4 h post injection. CONCLUSION: DOTA-peptides for PET imaging can be labelled with 68Ga up to specific activities of 1 GBq per nmol within 20 min, enabling the clinical application of peptides that display potential pharmacological side-effects.

Combination radionuclide therapy using 177Lu- and 90Y-labeled somatostatin analogs.

UNLABELLED: Peptide receptor-targeted radionuclide therapy of somatostatin receptor-expressing tumors is a promising application of radiolabeled somatostatin analogs. Suitable radionuclides are (90)Y, a pure, high-energy beta-emitter (2.27 MeV), and (177)Lu, a medium-energy beta-emitter (0.5 MeV) with a low-abundance gamma. METHODS: Lewis rats, each bearing both a small (approximately 0.5 cm(2)) and a large (7-9 cm(2)) somatostatin receptor-positive rat pancreatic CA20948 tumor in their flanks, were used. We investigated the radiotherapeutic effects of [(90)Y-tetraazacyclododecanetetraacetic acid (DOTA),Tyr(3)]octreotide, [(90)Y-DOTA,Tyr(3)]octreotate, [(177)Lu-DOTA,Tyr(3)]octreotate, and the combination of (90)Y- and (177)Lu-labeled analogs at the same tumor radiation dose (60 Gy). RESULTS: Radiotherapeutic effects of the (90)Y- and (177)Lu-labeled analogs were found in the rat tumor model. In these animals bearing tumors of different sizes, the antitumor effects of the combination of 50% (177)Lu- plus 50% (90)Y-analogs were superior to those in animals treated with either (90)Y- or (177)Lu- analog alone. In smaller tumors, the (90)Y radiation energy was not completely absorbed in the tumor, whereas in larger tumors the increased number of clonogenic tumor cells at the fixed level of absorbed dose may account for the failure of (177)Lu alone to go completely into remission. CONCLUSION: This study shows the superior antitumor effects of the combination of (177)Lu- and (90)Y-somatostatin analogs when compared with either (90)Y- or (177)Lu-analog alone in animals bearing tumors of various sizes.

Increased cell death after therapy with an Arg-Gly-Asp-linked somatostatin analog.

UNLABELLED: Receptor-targeted scintigraphy and radionuclide therapy with radiolabeled somatostatin analogs are successfully applied for somatostatin receptor-positive tumors. The synergistic effects of an apoptosis-inducing factor, for example, the Arg-Gly-Asp (RGD) motif, can increase the radiotherapeutic efficacy of these peptides. Hence, the tumoricidal effects of the hybrid peptide RGD-diethylaminetriaminepentaacetic acid (DTPA)-Tyr3-octreotate (cyclic[c](Arg-Gly-Asp-D-Tyr-Asp)-Lys(DTPA)-D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr), hereafter referred to as RGD-DTPA-octreotate, were evaluated in comparison with those of RGD (c(Arg-Gly-Asp-D-Tyr-Asp)) and Tyr3-octreotate (D-Phe-c(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr). METHODS: The therapeutic effects of RGD-111In-DTPA-octreotate, 111In-DTPA-RGD, and 111In-DTPA-Tyr3-octreotate were investigated with various cell lines by use of a colony-forming assay, and caspase-3 activity was also determined. RESULTS: Tumoricidal effects were found with 111In-DTPA-RGD, 111In-DTPA-Tyr3-octreotate, and RGD-111In-DTPA-octreotate, in order from least effective to most effective. Also, the largest increase in caspase-3 levels was found with RGD-111In-DTPA-octreotate. CONCLUSION: RGD-111In-DTPA-octreotate has more pronounced tumoricidal effects than 111In-DTPA-RGD and 111In-DTPA-Tyr3-octreotate, because of increased apoptosis, as indicated by increased caspase-3 activity.

Norharman and alcohol-dependency in male Wistar rats.

We examined the effects of ethanol ingestion to rats on levels of the beta-carboline norharman in plasma, brain and liver at the end of ethanol ingestion and 10 h after withdrawal. We also investigated the effect of exogenously administered norharman on the behavioural signs of alcohol withdrawal. Ethanol was given by a liquid diet for 21 days. Norharman plasma levels in alcohol fed rats were significantly elevated compared to both control rats and to rats 10 h after withdrawal. Norharman levels in brains and livers showed a similar pattern. The capacity of the livers of both alcohol-dependent and withdrawal rats to catabolise norharman was significantly reduced compared to control rats. Norharman injected intraperitoneally (6.3 mg/kg) attenuated the behavioural signs of alcohol withdrawal significantly. The mechanism behind the increased norharman levels in alcohol-dependent rats may be inhibition of the synthesis and/or activity of liver enzyme(s) responsible for the breakdown of norharman.

Low-dose-rate irradiation by 131I versus high-dose-rate external-beam irradiation in the rat pancreatic tumor cell line CA20948.

AIM: The rat pancreatic CA20948 tumor cell line is widely used in receptor-targeted preclinical studies because many different peptide receptors are expressed on the cell membrane. The response of the tumor cells to peptide radionuclide therapy, however, is dependent on the cell line's radiosensitivity. Therefore, we measured the radiosensitivity of the CA20948 tumor cells by using clonogenic survival assays after high-energy external-beam radiotherapy (XRT) in vitro. It can, however, be expected that results of high-dose-rate XRT are not representative for those after low-dose-rate radionuclide therapy (RT), such as peptide-receptor radionuclide therapy. Therefore, we compared clonogenic survival in vitro in CA20948 tumor cells after increasing doses of XRT or RT, the latter using (131)I. METHODS: Survival of CA20948 cells was investigated using a clonogenic survival assay after RT by incubation with increasing amounts of (131)I, leading to doses of 1-10 Gy after 12 days of incubation (maximum dose rate, 0.92 mGy/min), or with doses of 1-10 Gy using an X-ray machine (dose rate, 0.66 Gy/min). Colonies were scored after a 12-day-incubation period. Also, the doubling time of this cell line was calculated. RESULTS: We observed a dose-dependent reduction in tumor-cell survival, which, at low doses, was similar for XRT and RT. For high-dose-rate XRT, the quadratic over linear component ratio (alpha/beta) for CA20948 was 8.3 Gy, whereas that ratio for low-dose-rate RT was calculated to be 86.5 Gy. The calculated doubling time of CA20948 cells was 22 hours. CONCLUSIONS: Despite the huge differences in dose rate, RT tumor cell-killing effects were approximately as effective as those of XRT at doses of 1 and 2 Gy, the latter being the common daily dose given in fractionated external-beam therapies. At higher doses, RT was less effective than XRT.

Reduction of skeletal accumulation of radioactivity by co-injection of DTPA in [90Y-DOTA0,Tyr3]octreotide solutions containing free 90Y3+.

Peptide receptor-targeted radionuclide therapy is nowadays being performed with radiolabeled DOTA-conjugated peptides, such as [90Y-DOTA0,Tyr3]octreotide (also known as OctreoTher or 90Y-DOTATOC). The incorporation of 90Y3+ is typically > or = 99%, however, since a total patient dose can be as high as 26 GBq or 700 mCi the amount of free 90Y3+ (= non-DOTA-incorporated) can be substantial. Free 90Y3+ accumulates in bone with undesired radiation of bone marrow as a consequence. 90Y-DTPA is excreted rapidly via the kidneys. Incorporation of free 90Y3+ into 90Y-DTPA might prevent this fraction from being accumulated into bone, therefore we have investigated: the biodistribution in rats of 90YCl3, [90Y-DOTA0,Tyr3]octreotide, and 90Y-DTPA; possibilities to complex 10% of free 90Y3+ in a [90Y-DOTA0,Tyr3]octreotide containing solution into 90Y-DTPA prior to intravenous injection; and effects of 10% free 90Y3+ in [90Y-DOTA0,Tyr3]octreotide solution, in the presence and in the absence of excess DTPA, on the biodistribution of in rats. The following results are presented: 90YCl3 showed high skeletal uptake (i.e., 1% ID (injected dose) per gram femur, with main localization in the epiphyseal plates) and a 24 h total body retention of 74% ID; 90Y-DTPA had rapid renal clearance, and 24 h total body retention of < 5% ID; added free 90Y3+ in [90Y-DOTA0,Tyr3]octreotide solution could rapidly be incorporated into 90Y-DTPA at room temperature; and accumulation of 90Y3+ in femur, blood, and liver was related to the amount of free 90Y3+, whereas these accumulations could be prevented by the addition of DTPA. In conclusion, the addition of excess DTPA to [90Y-DOTA0,Tyr3]octreotide with incomplete 90Y-incorporation is recommended.

Radiolabeled RGD-DTPA-Tyr3-octreotate for receptor-targeted radionuclide therapy.

The aim of this study was to develop and investigate a radiopeptide for the treatment of cancers which overexpress cell surface somatostatin receptors. The new radiopharmaceutical is composed of a somatostatin receptor-targeting peptide, a chelator (DTPA) to enable radiolabeling, and an apoptosis-inducing RGD (arginine-glycine-aspartate) peptide moiety. The receptor-targeting peptide portion of the molecule, Tyr3-octreotate, is specific for the somatostatin subtype-2 cell surface receptor (sst2), which is overexpressed on many tumor cells. Because of the rapid endocytosis of the somatostatin receptor, the entire molecule can thus be internalized, allowing the RGD portion to activate intracellular caspases, which in turn promotes apoptosis. In this paper, we present the synthesis and the in vitro and in vivo tumor binding and internalization characteristics of this hybrid peptide. In vitro internalization into sst2-positive tumor cells of the radiolabeled hybrid peptide appeared to be a rapid process and could be blocked by an excess of unlabeled octreotide, indicating an sst2-specific process. Tumor uptake in vivo in rats of radiolabeled RGD-DTPA-Tyr3-octreotate was in agreein vitro data and similar to that of radiolabeled DOTA-Tyr3-octreotate. The combined molecule is expected to significantly enhance the therapeutic efficacy of the somatostatin-based agent.

Uptake of [111In-DTPA0]octreotide in the rat kidney is inhibited by colchicine and not by fructose.

UNLABELLED: The high renal uptake of radiolabeled somatostatin analogs is dose limiting. Lowering this uptake permits higher radioactivity doses and, thus, tumor doses to be administered. We tested the effects of the microtubule drug colchicine on renal uptake of [(111)In-DTPA(0)]octreotide. Also, the effects of fructose were tested. METHODS: Organ radioactivity 24 h after injection of [(111)In-DTPA(0)]octreotide was determined in rats. RESULTS: Coinjection of 1 mg of colchicine per kilogram did not influence renal uptake of [(111)In-DTPA(0)]octreotide, whereas this dose administered 5 h before [(111)In-DTPA(0)]octreotide resulted in significant renal uptake reduction (63%). D-Lysine plus colchicine reduced the uptake by 76% (P < 0.01 vs. D-lysine alone). Liver and blood radioactivity levels were significantly elevated by colchicine. Fructose did not affect the biodistribution of [(111)In-DTPA(0)]octreotide. CONCLUSION: Renal uptake of [(111)In-DTPA(0)]octreotide is dependent on microtubule function in rats. The addition of colchicine to amino acid protocols may permit administration of higher doses, improving the therapeutic window of peptide receptor radionuclide therapy.