Somatostatin receptor scintigraphy using [¹¹¹In-DTPA⁰]RC-160 in humans:A comparison with [¹¹¹In-DTPA₀]octreotide

Somatostatin receptor-positive lesions can be visualized by scintigraphy using [111In-DTPA0]octreotide. Recently, there have been reports of differences in receptor binding between somatostatin receptor subtypes and between somatostatin analogues, such as RC-160 and octreotide, as well as of differences in internalization between the somatostatin receptor subtypes. The possibility that certain somatostatin receptor-positive tissues and tumours which do not bind octreotide may bind and internalize RC-160 would open new scintigraphic or radiotherapeutic applications of radiolabelled RC-160. We investigated the metabolism and tissue distribution of [111In-DTPA0]RC-160 in comparison with [111InDTPA0]octreotide in four patients after injection of 250 MBq (10 μg) of these radiopharmaceuticals. Patient 1 had a metastatic follicular thyroid carcinoma, patient 2 a metastatic medullary thyroid carcinoma, patient 3 tuberculosis and patient 4 an insulinoma. The plasma clearance of the [111In-DTPA0]RC-160 was slower than that of [111In-DTPA0]octreotide, with 5% and 2%, respectively, of the initial plasma radioactivity remaining at 10 h p.i. The urinary excretion of [111In-DTPA0]RC-160 was initially also slower than that of [111]In-DTPA0]octreotide, but the cumulative excretion of radioactivity was not significantly different at 48 h p.i. Approximately 80% of injected radioactivity was cleared in the urine, while in one patient 20% of the injected dose was recovered in the faeces. The slower clearance of [111In-DTPA0]RC-160 resulted in a higher background in all organs studied i.e. liver, spleen, kidneys and lungs, at 24 h p.i. Although the target to background ratio with [111In-DTPA0]octreotide was higher, no differences were found between the two analogues with regard to their sensitivity in detecting lesions in these four patients. We conclude that although only four subjects were studied, [111In-DTPA0]RC-160 does not appear to have additional value for scintigraphy and is associated with higher background activity.</p

Somatostatin receptor scintigraphy using [¹¹¹In-DTPA⁰]RC-160 in humans:A comparison with [¹¹¹In-DTPA₀]octreotide

Somatostatin receptor-positive lesions can be visualized by scintigraphy using [111In-DTPA0]octreotide. Recently, there have been reports of differences in receptor binding between somatostatin receptor subtypes and between somatostatin analogues, such as RC-160 and octreotide, as well as of differences in internalization between the somatostatin receptor subtypes. The possibility that certain somatostatin receptor-positive tissues and tumours which do not bind octreotide may bind and internalize RC-160 would open new scintigraphic or radiotherapeutic applications of radiolabelled RC-160. We investigated the metabolism and tissue distribution of [111In-DTPA0]RC-160 in comparison with [111InDTPA0]octreotide in four patients after injection of 250 MBq (10 μg) of these radiopharmaceuticals. Patient 1 had a metastatic follicular thyroid carcinoma, patient 2 a metastatic medullary thyroid carcinoma, patient 3 tuberculosis and patient 4 an insulinoma. The plasma clearance of the [111In-DTPA0]RC-160 was slower than that of [111In-DTPA0]octreotide, with 5% and 2%, respectively, of the initial plasma radioactivity remaining at 10 h p.i. The urinary excretion of [111In-DTPA0]RC-160 was initially also slower than that of [111]In-DTPA0]octreotide, but the cumulative excretion of radioactivity was not significantly different at 48 h p.i. Approximately 80% of injected radioactivity was cleared in the urine, while in one patient 20% of the injected dose was recovered in the faeces. The slower clearance of [111In-DTPA0]RC-160 resulted in a higher background in all organs studied i.e. liver, spleen, kidneys and lungs, at 24 h p.i. Although the target to background ratio with [111In-DTPA0]octreotide was higher, no differences were found between the two analogues with regard to their sensitivity in detecting lesions in these four patients. We conclude that although only four subjects were studied, [111In-DTPA0]RC-160 does not appear to have additional value for scintigraphy and is associated with higher background activity.</p

Pre-clinical evaluation of [¹¹¹In-DTPA-Pro¹, Tyr⁴]bombesin, a new radioligand for bombesin-receptor scintigraphy

Bombesin (BN) is a 14-amino-acid neuropeptide with a high affinity for the gastrin-releasing peptide receptor. This receptor has been found to be expressed in a variety of tumours, including lung, breast, prostate and pancreas. A newly synthesized BN analogue, [DTPA-Pro1, Tyr4]BN, was shown to be a high-affinity BN-receptor (BNR) agonist, stimulating prolactin secretion from 7315b cells with an IC50 of 8 nM. The 111In-labelled analogue was found to bind with high affinity to rat BNR in vitro and in vivo. The radioligand is internalized by BNR-expressing cells, in contrast to DTPA-conjugated BN antagonists. Therefore, we further studied the biodistribution of i.v. injected [111In-DTPA-Pro1, Tyr4]BN in rats. High and specific uptake was found in tissues of the gastrointestinal tract, notably pancreas. Uptake of radioactivity was blocked by pre- or co-injection of 100 μg [Tyr4]BN, but not when this was administered 30 min after the radioligand. This suggests BNR-mediated internalization of the radioligand within 30 min. The percentage injected dose (ID) taken up by BNR-positive tissues was a bell-shaped function of the amount (0.01-0.1 μg) of injected ligand. Next to the pancreas, highest uptake was observed in the kidneys, which was not blocked by excess [Tyr3]BN. Dynamic gamma camera studies showed rapid clearance of radioactivity from the blood compartment. Urinary excretion amounted to about 35% ID after 1 hr and to 70% ID after 24 hr, with a total body retention of 10% ID. Specific uptake was found in the BNR- positive CA20948 pancreas tumour and CC531 colon carcinoma in tumour-bearing rats. The CA20948 tumour, inoculated in the hindleg, was also visualized scintigraphically. [111In-DTPA-Pro1, Tyr4]BN appears to be a promising radioligand for scintigraphy of BNR-expressing tumours.</p

Pre-clinical evaluation of [¹¹¹In-DTPA-Pro¹, Tyr⁴]bombesin, a new radioligand for bombesin-receptor scintigraphy

Bombesin (BN) is a 14-amino-acid neuropeptide with a high affinity for the gastrin-releasing peptide receptor. This receptor has been found to be expressed in a variety of tumours, including lung, breast, prostate and pancreas. A newly synthesized BN analogue, [DTPA-Pro1, Tyr4]BN, was shown to be a high-affinity BN-receptor (BNR) agonist, stimulating prolactin secretion from 7315b cells with an IC50 of 8 nM. The 111In-labelled analogue was found to bind with high affinity to rat BNR in vitro and in vivo. The radioligand is internalized by BNR-expressing cells, in contrast to DTPA-conjugated BN antagonists. Therefore, we further studied the biodistribution of i.v. injected [111In-DTPA-Pro1, Tyr4]BN in rats. High and specific uptake was found in tissues of the gastrointestinal tract, notably pancreas. Uptake of radioactivity was blocked by pre- or co-injection of 100 μg [Tyr4]BN, but not when this was administered 30 min after the radioligand. This suggests BNR-mediated internalization of the radioligand within 30 min. The percentage injected dose (ID) taken up by BNR-positive tissues was a bell-shaped function of the amount (0.01-0.1 μg) of injected ligand. Next to the pancreas, highest uptake was observed in the kidneys, which was not blocked by excess [Tyr3]BN. Dynamic gamma camera studies showed rapid clearance of radioactivity from the blood compartment. Urinary excretion amounted to about 35% ID after 1 hr and to 70% ID after 24 hr, with a total body retention of 10% ID. Specific uptake was found in the BNR- positive CA20948 pancreas tumour and CC531 colon carcinoma in tumour-bearing rats. The CA20948 tumour, inoculated in the hindleg, was also visualized scintigraphically. [111In-DTPA-Pro1, Tyr4]BN appears to be a promising radioligand for scintigraphy of BNR-expressing tumours.</p

A standardised study to compare prostate cancer targeting efficacy of five radiolabelled bombesin analogues

Purpose: Prostate-specific antigen (PSA)-based screening for prostate cancer (PC) has dramatically increased early diagnosis. Current imaging techniques are not optimal to stage early PC adequately. A promising alternative to PC imaging is peptide-based scintigraphy using radiolabelled bombesin (BN) analogues that bind to gastrin-releasing peptide receptors (GRPR) being overexpressed in PC. When labelled to appropriate radionuclides BN targeting of GRPRs may also provide applications for peptide radionuclide receptor therapy (PRRT). Assessment studies under identical experimental conditions allowing a reliable comparison of the potential of such analogues are lacking. This study was performed to evaluate and directly compare five promising radiolabelled BN analogues for their targeting efficacy for PC under standardised conditions. Methods: The BN agonists [111In]DOTA-PESIN, [111In]AMBA, [111In]MP2346 and [111In]MP2653 and one antagonist [99mTc]Demobesin-1 were evaluated in GRPR-overexpressing human PC-3 tumour-bearing mice to determine peptide stability in vivo, biodistribution and GRPR targeting potential by animal SPECT/CT imaging and ex vivo autoradiography. Results: HPLC analysis of blood showed intact Demobesin-1 at 5 and 15 min after injection (64.1 ± 1.6% and 41.0 ± 01%, respectively) being much less for the other compounds. AMBA, the second most stable analogue, showed 36.1 ± 2.7% and 9.8 ± 1.1% intact peptide after 5 and 15 min. PC-3 tumour uptake at 1 h was comparable for Demobesin-1, AMBA, PESIN and MP2346 (3.0 ± 0.4, 2.7 ± 0.5, 2.3 ± 0.5 and 2.1 ± 0.9%ID/g, respectively), but very low for MP2653 (0.9 ± 0.2%ID/g). In addition, MP2346 showed undesirably high uptake in the kidneys (7.9 ± 1.9%ID/g) being significantly less for the other analogues. AMBA, MP2346 and PESIN revealed favourable increases in tumour to blood ratios over time while changes in tumour to kidney and pancreas ratios for Demobesin-1 from 1 to 24 h after injection were significantly better than for the other analogues. All analogues visualised PC-3 tumours by SPECT/CT and autoradiography. Conclusion: In the present study the BN antagonist Demobesin-1 was the best performing analogue showing superior in vivo stability, highest tumour uptake and retention while pancreatic and renal clearance were rapid. PESIN and AMBA were the best GRP agonists with sufficient in vivo stabilities as well as high tumour uptake and retention. Based on these results all three analogues deserve further evaluation for clinical use in PC patients.ISSN:1619-7070ISSN:1619-708

Crucial role for somatostatin receptor subtype 2 in determining the uptake of [¹¹¹In-DTPA-D-Phe¹]octreotide in somatostatin receptor-positive organs

Human somatostatin (SS) receptor (sst)-positive tumors can be visualized by gamma camera scintigraphy after the injection of [111In- diethylenetriaminepentaacetic acid (DTPA)-D-Phe1] octreotide. Uptake of [111In-DTPA-D-Phe1]octreotide is dependent on sst-mediated internalization of the radioligand by the tumor cells. Human sst-positive tumors frequently express multiple sst subtypes. In vitro studies have demonstrated that the 5 sst subtypes (ssti1-5) differentially internalize sst-bound ligand. The present study was performed to evaluate the role of sst2 in vivo in determining the uptake of [ 111In-DTPA-D-Phe1]octreotide, as well as of the more "universal" ligand [111In-DTPA]SS-14, by sst-positive organs expressing multiple sst subtypes. Methods: Wild-type and sst2 knockout mice (n = 4 per treatment group) were injected intravenously with 1 MBq (0.1 μg) [111In-DTPA-D-Phe1]octreotide or [ 111In-DTPA]SS-14. After 24 h, the animals were sacrificed and radioactivity in the organs under investigation was determined. In addition, the sst subtype messenger RNA (mRNA) expression pattern in these organs was determined by reverse transcriptase polymerase chain reaction (RT-PCR) analysis. Results: RT-PCR analysis demonstrated the presence of all 5 sst subtype mRNAs in the adrenals and pituitary of wild-type mice but no sst2 in the knockout mice. The thymus expressed mRNA for sst2 and sst4 mRNA in wild-type mice, whereas no sst2 was detected in knockout mice. In wildtype mice, the in vivo uptake values (in percentage injected dose per gram of tissue) of [111In-DTPA-D-Phe1]octreotide for the pituitary, adrenals, pancreas, and thymus amounted to 1.2 ± 0.2, 0.26 ± 0.03, 0.18 ± 0.03, and 0.30 ± 0.05, respectively, in wild-type mice. Compared with wild-type mice, sst2 knockout mice had dramatically lower uptake values in these organs - lower by 97%, 83%, 96%, and 94%, respectively (P &lt; 0.01 vs. wild type). Comparable differences in the uptake of radioactivity between wild-type and knockout mice were found using [111In-DTPA]SS-14 as the radiotracer. Interestingly, in some organs expressing sst2 mRNA (liver, muscle, and peripheral blood mononuclear cells), no specific binding of [111In-DTPA-D-Phe1] octreotide or [111In-DTPA]SS-14 to sst in vivo was found, suggesting that the sst2 protein expression level was very low in these tissues. Conclusion: The uptake of [111In-DTPA-D-Phe1]octreotide and [111In-DTPA]SS-14 in sst-positive organs is determined predominantly by sst2.</p

Crucial role for somatostatin receptor subtype 2 in determining the uptake of [¹¹¹In-DTPA-D-Phe¹]octreotide in somatostatin receptor-positive organs

Human somatostatin (SS) receptor (sst)-positive tumors can be visualized by gamma camera scintigraphy after the injection of [111In- diethylenetriaminepentaacetic acid (DTPA)-D-Phe1] octreotide. Uptake of [111In-DTPA-D-Phe1]octreotide is dependent on sst-mediated internalization of the radioligand by the tumor cells. Human sst-positive tumors frequently express multiple sst subtypes. In vitro studies have demonstrated that the 5 sst subtypes (ssti1-5) differentially internalize sst-bound ligand. The present study was performed to evaluate the role of sst2 in vivo in determining the uptake of [ 111In-DTPA-D-Phe1]octreotide, as well as of the more "universal" ligand [111In-DTPA]SS-14, by sst-positive organs expressing multiple sst subtypes. Methods: Wild-type and sst2 knockout mice (n = 4 per treatment group) were injected intravenously with 1 MBq (0.1 μg) [111In-DTPA-D-Phe1]octreotide or [ 111In-DTPA]SS-14. After 24 h, the animals were sacrificed and radioactivity in the organs under investigation was determined. In addition, the sst subtype messenger RNA (mRNA) expression pattern in these organs was determined by reverse transcriptase polymerase chain reaction (RT-PCR) analysis. Results: RT-PCR analysis demonstrated the presence of all 5 sst subtype mRNAs in the adrenals and pituitary of wild-type mice but no sst2 in the knockout mice. The thymus expressed mRNA for sst2 and sst4 mRNA in wild-type mice, whereas no sst2 was detected in knockout mice. In wildtype mice, the in vivo uptake values (in percentage injected dose per gram of tissue) of [111In-DTPA-D-Phe1]octreotide for the pituitary, adrenals, pancreas, and thymus amounted to 1.2 ± 0.2, 0.26 ± 0.03, 0.18 ± 0.03, and 0.30 ± 0.05, respectively, in wild-type mice. Compared with wild-type mice, sst2 knockout mice had dramatically lower uptake values in these organs - lower by 97%, 83%, 96%, and 94%, respectively (P &lt; 0.01 vs. wild type). Comparable differences in the uptake of radioactivity between wild-type and knockout mice were found using [111In-DTPA]SS-14 as the radiotracer. Interestingly, in some organs expressing sst2 mRNA (liver, muscle, and peripheral blood mononuclear cells), no specific binding of [111In-DTPA-D-Phe1] octreotide or [111In-DTPA]SS-14 to sst in vivo was found, suggesting that the sst2 protein expression level was very low in these tissues. Conclusion: The uptake of [111In-DTPA-D-Phe1]octreotide and [111In-DTPA]SS-14 in sst-positive organs is determined predominantly by sst2.</p

Pre-clinical comparison of [DTPA⁰] octreotide, [DTPA⁰,Tyr³] octreotide and [DOTA⁰,Tyr³] octreotide as carriers for somatostatin receptor-targeted scintigraphy and radionuclide therapy

We have evaluated the potential usefulness of radiolabelled [DTPA0,Tyr3]octreotide and [DOTA0,Tyr3]octreotide as radiopharmaceuticals for somatostatin receptor-targeted scintigraphy and radiotherapy. In vitro somatostatin receptor binding and in vivo metabolism in rats of the compounds were investigated in comparison with [111In-DTPA0] octreotide. Comparing different peptide-chelator constructs, [DTPA0,Tyr3]octreotide and [DOTA0, Tyr3]octreotide were found to have a higher affinity than [DTPA0]octreotide for subtype 2 somatostatin receptors (sst2) in mouse AtT20 pituitary tumour cell membranes (all IC50 values obtained were in the low nanomolar range). In vivo studies in CA20948 tumor-bearing Lewis rats revealed a significantly higher uptake of both 111In-labelled [DOTA0,Tyr3]octreotide and [DTPA0,Tyr3]octreotide in sst2-expressing tissues than after injection of [111In-DTPA0]octreotide, showing that substitution of Tyr for Phe at position 3 in octreotide results in an increased affinity for its receptor and in a higher target tissue uptake. Uptake of 111In-labelled [DTPA0]octreotide, [DTPA0,Tyr3]octreotide and [DOTA0,Tyr3]octreotide in pituitary, pancreas, adrenals and tumour was decreased to less than 7% of control by pre-treatment with 0.5 mg unlabelled octreotide/rat, indicating specific binding to sst2. Comparing different radionuclides, [90Y- DOTA0,Tyr3]octreotide had the highest uptake in sst2-positive organs, followed by the [111In-DOTA0,Tyr3]octreotide, whereas [DOTA0,125I- Try3]octreotide uptake was low compared to that of the other radiopharmaceuticals, when measured 24 hr after injection. Renal uptake of 111In-labelled [DTPA0]octreotide, [DTPA0, Tyr3]octreotide and [DOTA0,Tyr3]octreotide was reduced over 50% by an i.v. injection of 400 mg/kg D-lysine, whereas radioactivity in blood, pancreas and adrenals was not affected.</p

Pre-clinical comparison of [DTPA⁰] octreotide, [DTPA⁰,Tyr³] octreotide and [DOTA⁰,Tyr³] octreotide as carriers for somatostatin receptor-targeted scintigraphy and radionuclide therapy

We have evaluated the potential usefulness of radiolabelled [DTPA0,Tyr3]octreotide and [DOTA0,Tyr3]octreotide as radiopharmaceuticals for somatostatin receptor-targeted scintigraphy and radiotherapy. In vitro somatostatin receptor binding and in vivo metabolism in rats of the compounds were investigated in comparison with [111In-DTPA0] octreotide. Comparing different peptide-chelator constructs, [DTPA0,Tyr3]octreotide and [DOTA0, Tyr3]octreotide were found to have a higher affinity than [DTPA0]octreotide for subtype 2 somatostatin receptors (sst2) in mouse AtT20 pituitary tumour cell membranes (all IC50 values obtained were in the low nanomolar range). In vivo studies in CA20948 tumor-bearing Lewis rats revealed a significantly higher uptake of both 111In-labelled [DOTA0,Tyr3]octreotide and [DTPA0,Tyr3]octreotide in sst2-expressing tissues than after injection of [111In-DTPA0]octreotide, showing that substitution of Tyr for Phe at position 3 in octreotide results in an increased affinity for its receptor and in a higher target tissue uptake. Uptake of 111In-labelled [DTPA0]octreotide, [DTPA0,Tyr3]octreotide and [DOTA0,Tyr3]octreotide in pituitary, pancreas, adrenals and tumour was decreased to less than 7% of control by pre-treatment with 0.5 mg unlabelled octreotide/rat, indicating specific binding to sst2. Comparing different radionuclides, [90Y- DOTA0,Tyr3]octreotide had the highest uptake in sst2-positive organs, followed by the [111In-DOTA0,Tyr3]octreotide, whereas [DOTA0,125I- Try3]octreotide uptake was low compared to that of the other radiopharmaceuticals, when measured 24 hr after injection. Renal uptake of 111In-labelled [DTPA0]octreotide, [DTPA0, Tyr3]octreotide and [DOTA0,Tyr3]octreotide was reduced over 50% by an i.v. injection of 400 mg/kg D-lysine, whereas radioactivity in blood, pancreas and adrenals was not affected.</p