Kidney protection during peptide receptor radionuclide therapy with somatostatin analogues

Dimerization of a phage-display selected peptide for imaging of αvβ6- integrin: two approaches to the multivalent effect

The integrin α_vβ₆ is an emerging biomarker for non-small cell lung cancer (NSCLC). An α_vβ₆-binding peptide was previously selected from a phage-displayed peptide library. Here, we utilize a multivalent design to develop a peptidic probe for positron emission tomography (PET) imaging of α_vβ₆⁺ NSCLC tumors_. Multimeric presentation of this peptide, RGDLATLRQL, on a bifunctional copper chelator was achieved using two approaches: dimerization of the peptide followed by conjugation to the chelator (H₂-D10) and direct presentation of two copies of the peptide on the chelator scaffold (H₂-(M10)₂). Binding affinities of the divalent peptide conjugates are four-fold higher than their monovalent counterpart (H₂-M10), suggestive of multivalent binding. PET imaging using the bivalent ⁶⁴Cu-labeled conjugates showed rapid and persistent accumulation in α_vβ₆⁺ tumors. By contrast, no significant accumulation was observed in α_vβ₆^- tumors. Irrespective of the dimerization approach, all divalent probes showed three-fold higher tumor uptake than the monovalent probe, indicating the role of valency in signal enhancement. However, the divalent probes have elevated uptake in non-target organs, especially the kidneys. To abrogate nonspecific uptake, the peptide's N-terminus was acetylated. The resultant bivalent probe, ⁶⁴Cu- AcD10, showed drastic decrease of kidney accumulation while maintaining tumor uptake. In conclusion, we developed an α_vβ₆-integrin specific probe with optimized biodistribution for noninvasive PET imaging of NSCLC. Further, we have demonstrated that use of multivalent scaffolds is a plausible method to improve library selected peptides, which would be suboptimal or useless otherwise, for imaging probe development.

Novel 99mTc radiolabeled folate complexes with PEG linkers for FR-positive tumor imaging: synthesis and biological evaluation

The novel complex ^99mTc(FA-PEG2-HYNIC)(tricine/TPPTS) was clearly visualized at 120 min p.i. at the FR-positive tumor, highlighting its potential as an effective folate receptor tumor imaging agent.

Prospects in folate receptor-targeted radionuclide therapy

Targeted radionuclide therapy is based on systemic application of particle-emitting radiopharmaceuticals which are directed toward a specific tumor-associated target. Accumulation of the radiopharmaceutical in targeted cancer cells results in high doses of absorbed radiation energy whereas toxicity to non-targeted healthy tissue is limited. This strategy has found widespread application in the palliative treatment of neuroendocrine tumors using somatostatin-based radiopeptides. The folate receptor (FR) has been identified as a target associated with a variety of frequent tumor types (e.g., ovarian, lung, brain, renal, and colorectal cancer). In healthy organs and tissue FR-expression is restricted to only a few sites such as for instance the kidneys. This demonstrates why FR-targeting is an attractive strategy for the development of new therapy concepts. Due to its high FR-binding affinity (K_D < 10⁻⁹ M) the vitamin folic acid has emerged as an almost ideal targeting agent. Therefore, a variety of folic acid radioconjugates for nuclear imaging have been developed. However, in spite of the large number of cancer patients who could benefit of a folate-based radionuclide therapy, a therapeutic concept with folate radioconjugates has not yet been envisaged for clinical application. The reason is the generally high accumulation of folate radioconjugates in the kidneys where emission of particle-radiation may result in damage to the renal tissue. Therefore, the design of more sophisticated folate radioconjugates providing improved tissue distribution profiles are needed. This review article summarizes recent developments with regard to a therapeutic application of folate radioconjugates. A new construct of a folate radioconjugate and an application protocol which makes use of a pharmacological interaction allowed the first preclinical therapy experiments with radiofolates. These results raise hope for future application of such new concepts also in the clinic.

Rapid blood clearance and lack of long-term renal toxicity of 177Lu-DOTATATE enables shortening of renoprotective amino acid infusion

PURPOSE

The aim of the study was to investigate the feasibility of shortening the recommended 4-h renoprotective amino acid infusion in patients receiving peptide receptor chemoradionuclide therapy (PRCRT) using radiosensitizing 5-fluorouracil. We evaluated the clearance of radiopeptide from the blood, long-term nephrotoxicity in patients undergoing PRCRT with the conventional 4-h amino acid infusion and renal uptake in patients receiving an abbreviated infusion.

METHODS

The whole-blood clearance of (177)Lu-DOTA-octreotate (LuTate) was measured in 13 patients receiving PRCRT. A retrospective analysis of short-term and long-term changes in glomerular filtration rate (GFR) in 96 consecutive patients receiving a 4-h infusion was performed. Renal LuTate retention estimated using quantitative SPECT/CT in 22 cycles delivered with a 2.5-h amino acid infusion was compared with that in 72 cycles with the 4-h infusion.

RESULTS

LuTate demonstrated biexponential blood clearance with an initial clearance half-time of 21 min. Approximately 88 % of blood activity was cleared within 2 h. With the 4-h protocol, there was no significant change in GFR (1.2 ml/min mean increase from baseline; 95 % CI -6.9 to 4.4 ml/min) and no grade 3 or 4 nephrotoxicity at the end of induction PRCRT. The long-term decline in GFR after a median follow up of 22 months was 2.2 ml/min per year. There was no significant difference in the renal LuTate retention measured in patients receiving a 2.5-h amino acid infusion compared to those who had a 4-h infusion.

CONCLUSION

The greatest renal exposure to circulating radiopeptide occurs in the first 1 - 2 h after injection. This, combined with the safety of LuTate PRCRT, allows consideration of an abbreviated amino acid infusion, increasing patient convenience and reducing human resource allocation.

Peptide receptor radionuclide therapy for neuroendocrine tumors in Germany: first results of a multi-institutional cancer registry

Peptide receptor radionuclide therapy is an effective treatment option for patients with well-differentiated somatostatin receptor-expressing neuroendocrine tumors. However, published data result mainly from retrospective monocentric studies. We initiated a multi-institutional, prospective, board-reviewed registry for patients treated with peptide receptor radionuclide therapy in Germany in 2009. In five centers, 297 patients were registered. Primary tumors were mainly derived from pancreas (117/297) and small intestine (80/297), whereas 56 were of unknown primary. Most tumors were well differentiated with median Ki67 proliferation rate of 5% (range 0.9-70%). Peptide receptor radionuclide therapy was performed using mainly yttrium-90 and/or lutetium-177 as radionuclides in 1-8 cycles. Mean overall survival was estimated at 213 months with follow-up between 1 and 230 months after initial diagnosis, and 87 months with follow-up between 1 and 92 months after start of peptide receptor radionuclide therapy. Median overall survival was not yet reached. Subgroup analysis demonstrated that best results were obtained in neuroendocrine tumors with proliferation rate below 20%. Our results indicate that peptide receptor radionuclide therapy is an effective treatment for well- and moderately differentiated neuroendocrine tumors irrespective of previous therapies and should be regarded as one of the primary treatment options for patients with somatostatin receptor-expressing neuroendocrine tumors.

The joint IAEA, EANM, and SNMMI practical guidance on peptide receptor radionuclide therapy (PRRNT) in neuroendocrine tumours

Peptide receptor radionuclide therapy (PRRNT) is a molecularly targeted radiation therapy involving the systemic administration of a radiolabelled peptide designed to target with high affinity and specificity receptors overexpressed on tumours. PRRNT employing the radiotagged somatostatin receptor agonists (90)Y-DOTATOC ([(90)Y-DOTA(0),Tyr(3)]-octreotide) or (177)Lu-DOTATATE ([(177)Lu-DOTA(0),Tyr(3),Thr(8)]-octreotide or [(177)Lu-DOTA(0),Tyr(3)]-octreotate) have been successfully used for the past 15 years to target metastatic or inoperable neuroendocrine tumours expressing the somatostatin receptor subtype 2. Accumulated evidence from clinical experience indicates that these tumours can be subjected to a high absorbed dose which leads to partial or complete objective responses in up to 30 % of treated patients. Survival analyses indicate that patients presenting with high tumour receptor expression at study entry and receiving (177)Lu-DOTATATE or (90)Y-DOTATOC treatment show significantly higher objective responses, leading to longer survival and improved quality of life. Side effects of PRRNT are typically seen in the kidneys and bone marrow. These, however, are usually mild provided adequate protective measures are undertaken. Despite the large body of evidence regarding efficacy and clinical safety, PRRNT is still considered an investigational treatment and its implementation must comply with national legislation, and ethical guidelines concerning human therapeutic investigations. This guidance was formulated based on recent literature and leading experts' opinions. It covers the rationale, indications and contraindications for PRRNT, assessment of treatment response and patient follow-up. This document is aimed at guiding nuclear medicine specialists in selecting likely candidates to receive PRRNT and to deliver the treatment in a safe and effective manner. This document is largely based on the book published through a joint international effort under the auspices of the Nuclear Medicine Section of the International Atomic Energy Agency.

Systemic therapeutic options for carcinoid

"Carcinoids" are mostly slow-growing neuroendocrine neoplasms (NENs) with low proliferative activity. A wide range of therapeutic options with variable efficacy exist, including locoregional ablative strategies. Thereafter, some patients may not require medical therapy for years depending on the rate of progression or recurrence. However, the majority of patients require systemic treatment and therein lies the dilemma, since no antiproliferative agent is currently approved for carcinoids. Somatostatin analogs (SSAs), and to a lesser extent interferon-alpha, are standard therapy for carcinoids associated with the carcinoid syndrome. These drugs have some antiproliferative efficacy. SSAs rarely lead to tumor remission but may modestly prolong time to tumor progression. Chemotherapy is of limited value in carcinoids with low proliferation indices but may be useful in higher grade tumors. Peptide receptor-targeted radionuclide therapy may be of benefit and is mostly used after medical therapies fail. However, it is considered an investigational modality. More recently, targeted drugs such as mammalian target of rapamycin (mTOR) inhibitors and anti-angiogenics have been investigated. Objective remissions are rare. Their value remains to be rigorously elucidated. Increased efficacy requires a better understanding of the underlying tumor biology and identification of molecular pathological criteria to allow appropriate preselection of candidates for targeted therapies.

Differential uptake of (68)Ga-DOTATOC and (68)Ga-DOTATATE in PET/CT of gastroenteropancreatic neuroendocrine tumors

PURPOSE

Abundant expression of somatostatin receptors (sst) is a characteristic of neuroendocrine tumors (NET). Thus, radiolabeled somatostatin analogs have emerged as important tools for both in vivo diagnosis and therapy of NET. The two compounds most often used in functional imaging with positron emission tomography (PET) are (68)Ga-DOTATATE and (68)Ga-DOTATOC. Both analogs share a quite similar sst binding profile. However, the in vitro affinity of (68)Ga-DOTATATE in binding the sst subtype 2 (sst2) is approximately tenfold higher than that of (68)Ga-DOTATOC. This difference may affect their efficiency in detection of NET lesions, as sst2 is the predominant receptor subtype on gastroenteropancreatic NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs ((68)Ga-DOTATATE and (68)Ga-DOTATOC) in the same patients with gastroenteropancreatic NET.

PATIENTS AND METHODS

Twenty-seven patients with metastatic gastroenteropancreatic NET underwent (68)Ga-DOTATOC and (68)Ga-DOTATATE PET/CT as part of the workup before prospective peptide receptor radionuclide therapy (PRRT). The performance of both imaging methods was analyzed and compared for detection of individual lesions per patient and for eight defined body regions. A region was regarded as positive if at least one lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUV(max)) was compared for concordant lesions and renal parenchyma.

RESULTS

Fifty-one regions were found positive with both (68)Ga-DOTATATE and (68)Ga-DOTATOC. Overall, however, significantly fewer lesions were detected with (68)Ga-DOTATATE in comparison with (68)Ga-DOTATOC (174 versus 179, p < 0.05). Mean (68)Ga-DOTATATE SUV(max) across all lesions was significantly lower compared with (68)Ga-DOTATOC (16.9 ± 6.8 versus 22.1 ± 12.0, p < 0.01). Mean SUV(max) for renal parenchyma was not significantly different between (68)Ga-DOTATATE and (68)Ga-DOTATOC (12.6 ± 2.6 versus 12.6 ± 2.7).

CONCLUSIONS

(68)Ga-DOTATOC and (68)Ga-DOTATATE possess similar diagnostic accuracy for detection of gastroenteropancreatic NET lesions (with a potential advantage of (68)Ga-DOTATOC) despite their evident difference in affinity for sst2. Quite unexpectedly, maximal uptake of (68)Ga-DOTATOC tended to be higher than its (68)Ga-DOTATATE counterpart. However, tumor uptake shows high inter- and intraindividual variance with unpredictable preference of one radiopeptide. Thus, our data encourage the application of different sst ligands to enable personalized imaging and therapy of gastroenteropancreatic NET with optimal targeting of tumor receptors.

Radionuclide therapy via SSTR: future aspects from experimental animal studies

There is need for better therapeutic options for neuroendocrine tumours. The aim of this review was to summarize results of experimental animal studies and raise ideas for future radionuclide therapy based on high expression of somatostatin (SS) receptors by many neuroendocrine tumours. In summary, one of the major options is individualized treatment for each patient, including choice of SS analogues, radionuclides and treatment schedules. Other options are methods to increase the treatment effect on tumour tissue (increasing tumour uptake and retention by upregulation of receptor expression and avoiding saturation of receptor binding), methods to increase the tumour tissue response (by choice of radionuclides, SS analogues or combined therapies), and methods to reduce side effects (diminished uptake and retention in critical organs and reduced normal tissue response). Furthermore, combination therapy with other radiopharmaceuticals, cytotoxic drugs or radiosensitizers can be considered to enhance the effects of radiolabelled SS analogues.

Treatment of gastroenteropancreatic neuroendocrine tumors with peptide receptor radionuclide therapy

The primary treatment of gastroenteropancreatic neuroendocrine tumors (GEPNETs) is surgery with curative intent or debulking of the tumor mass. In case of metastatic disease, cytoreductive options are limited. A relatively new therapeutic modality, peptide receptor radionuclide therapy (PRRT) with radiolabeled somatostatin analogs, is currently available in a number of mostly European centers. Complete and partial responses obtained after treatment with [90Y-DOTA0,Tyr3]octreotide are in the same range as after treatment with [177Lu-DOTA0,Tyr3]octreotate (i.e. 10-30%). However, significant nephrotoxicity has been observed after treatment with [90Y-DOTA0,Tyr3]octreotide. Options to improve PRRT may include combinations of radioactive labeled somatostatin analogs, intra-arterial administration, and the use of radiosensitizing drugs combined with PRRT. Other therapeutic applications of PRRT may include additional therapy cycles in patients with progressive disease after benefit from initial therapy, PRRT in adjuvant or neoadjuvant setting, or PRRT combined with new targeted therapies, such as sunitinib or everolimus. Randomized clinical trials comparing PRRT with other treatment modalities, or comparing various radioactive labeled somatostatin analogs should be undertaken to determine the best treatment options and treatment sequelae for patients with GEPNETs.

Individualized dosimetry of kidney and bone marrow in patients undergoing 177Lu-DOTA-octreotate treatment

The organs at risk in radionuclide therapy with (177)Lu-octreotate are the bone marrow and the kidneys. The primary aim of this study was to develop an individualized dosimetry protocol for the bone marrow. The secondary aim was to identify those patients, undergoing fractionated therapy with 7.4 GBq/cycle, who first reached an accumulated dose of either 2 Gy to the bone marrow or 23 Gy to the kidneys.

METHODS

Two hundred patients with metastatic neuroendocrine tumors with high somatostatin receptor expression were included. After the administration of 7.4 GBq of (177)Lu-octreotate, blood samples were drawn 6 times within the first 24 h. In 50 patients, additional blood samples were obtained at 96 and 168 h. Moreover, urine was collected from 30 patients during the first 24 h. Planar whole-body and SPECT/CT images over the abdomen were acquired at 24, 96, and 168 h after the infusion. Calculation of the absorbed radiation dose to the bone marrow was based on blood and urinary activity curves combined with organ-based analysis of the whole-body images. The absorbed dose to the kidney was calculated from the pharmacokinetic data obtained from SPECT/CT.

RESULTS

For a single cycle of 7.4 GBq, the absorbed dose to the bone marrow and the kidney ranged from 0.05 to 0.4 Gy and from 2 to 10 Gy, respectively. In 197 of 200 patients, the kidneys accumulated an absorbed dose of 23 Gy before the bone marrow reached 2 Gy. Between 2 and 10 cycles of (177)Lu-octreotate could be administered before the upper dose limit for the individual patient was reached.

CONCLUSION

A method based on repeated whole-body imaging in combination with blood and urinary activity data over time was developed to determine the absorbed dose to the bone marrow. The dose-limiting organ was the kidney in 197 of 200 patients. In 50% of the patients, more than 4 cycles of 7.4 GBq of (177)Lu-octreotate could be administered, whereas 20% of the subjects were treated with fewer than 4 cycles. Individualized absorbed dose calculation is essential to optimize the therapy.

Differences in predicted and actually absorbed doses in peptide receptor radionuclide therapy

PURPOSE

An important assumption in dosimetry prior to radionuclide therapy is the equivalence of pretherapeutic and therapeutic biodistribution. In this study the authors investigate if this assumption is justified in sst2-receptor targeting peptide therapy, as unequal amounts of peptide and different peptides for pretherapeutic measurements and therapy are commonly used.

METHODS

Physiologically based pharmacokinetic models were developed. Gamma camera and serum measurements of ten patients with metastasizing neuroendocrine tumors were conducted using (111)In-DTPAOC. The most suitable model was selected using the corrected Akaike information criterion. Based on that model and the estimated individual parameters, predicted and measured (90)Y-DOTATATE excretions during therapy were compared. The residence times for the pretherapeutic (measured) and therapeutic scenarios (simulated) were calculated.

RESULTS

Predicted and measured therapeutic excretion differed in three patients by 10%, 31%, and 7%. The measured pretherapeutic and therapeutic excretion differed by 53%, 56%, and 52%. The simulated therapeutic residence times of kidney and tumor were 3.1 ± 0.6 and 2.5 ± 1.2 fold higher than the measured pretherapeutic ones.

CONCLUSIONS

To avoid the introduction of unnecessary inaccuracy in dosimetry, using the same substance along with the same amount for pretherapeutic measurements and therapy is recommended.

Estimation of absorbed dose to the kidneys in patients after treatment with 177Lu-octreotate: comparison between methods based on planar scintigraphy

BACKGROUND

Lu-[DOTA0, Tyr3]-octreotate (177Lu-octreotate) is used to treat neuroendocrine tumors with high somatostatin-receptor expression. 177Lu-octreotate is mainly excreted via the kidneys, but to some extent, accumulates in the kidney cortex due to, e.g., tubular reabsorption. Renal toxicity is one of the main limiting factors in 177Lu-octreotate treatment. Further knowledge of the biodistribution and dosimetry of 177Lu-octreotate in individual patients is needed. The aim of this study was to estimate the absorbed dose to the kidneys and compare the results obtained with planar imaging and different dosimetric methods: (1) conjugate-view (CV) method using patient-specific kidney sizes, (2) PA method, based on posterior images only, (3) CV method with reduced number of time points (CVreduced data), and (4) CV method using standard kidney sizes (CVstandard size).

METHODS

Totally, 33 patients each received 3.4 to 8.2 GBq of 177Lu-octreotate up to five times, with infusion of lysine and arginine to block the renal uptake. Whole-body planar gamma camera images were acquired on days 0, 1, 2, and 7. The 177Lu concentration in the kidneys was determined by the CV method, and the absorbed dose was estimated with patient-specific organ sizes. Comparison to the CV method was made using posterior images only, together with the influence of the number of time points and with standard organ sizes.

RESULTS

Large interindividual variations were found in the time-activity curve pattern and in the absorbed dose to the kidneys using the CV method: 0.33 to 2.4 Gy/GBq (mean =  0.80 Gy/GBq, SD = 0.30). In the individual patient, the mean deviation of all subsequent kidney doses compared to that of the first administration was 1% (SD = 19%) and 5% (SD = 23%) for the right and left kidneys, respectively. Excluding data for day 7 resulted in large variations in the absorbed dose.

CONCLUSION

Large interindividual variations in kidney dose were found, demonstrating the need for patient-specific dosimetry and treatment planning.

Lutetium-labelled peptides for therapy of neuroendocrine tumours

Treatment with radiolabelled somatostatin analogues is a promising new tool in the management of patients with inoperable or metastasized neuroendocrine tumours. Symptomatic improvement may occur with ¹⁷⁷Lu-labelled somatostatin analogues that have been used for peptide receptor radionuclide therapy (PRRT). The results obtained with ¹⁷⁷Lu-[DOTA⁰,Tyr³]octreotate (DOTATATE) are very encouraging in terms of tumour regression. Dosimetry studies with ¹⁷⁷Lu-DOTATATE as well as the limited side effects with additional cycles of ¹⁷⁷Lu-DOTATATE suggest that more cycles of ¹⁷⁷Lu-DOTATATE can be safely given. Also, if kidney-protective agents are used, the side effects of this therapy are few and mild and less than those from the use of ⁹⁰Y-[DOTA⁰,Tyr³]octreotide (DOTATOC). Besides objective tumour responses, the median progression-free survival is more than 40 months. The patients' self-assessed quality of life increases significantly after treatment with ¹⁷⁷Lu-DOTATATE. Lastly, compared to historical controls, there is a benefit in overall survival of several years from the time of diagnosis in patients treated with ¹⁷⁷Lu-DOTATATE. These findings compare favourably with the limited number of alternative therapeutic approaches. If more widespread use of PRRT can be guaranteed, such therapy may well become the therapy of first choice in patients with metastasized or inoperable neuroendocrine tumours.

New peptide receptor radionuclide therapy of invasive cancer cells: in vivo studies using 177Lu-DOTA-AE105 targeting uPAR in human colorectal cancer xenografts

The proposition of uPAR as a potential target in cancer therapy is advanced by its predominant expression at the invasive front of colorectal cancer (CRC) and its value as prognostic biomarker for poor survival in this disease. In this study, we provide the first in vivo proof-of-concept for a theranostic approach as treatment modality in a human xenograft colorectal cancer model.

METHODS

A DOTA-conjugated 9-mer high affinity uPAR binding peptide (DOTA-AE105) was radiolabeled with (64)Cu and (177)Lu, for PET imaging and targeted radionuclide therapy study, respectively. Human uPAR-positive CRC HT-29 cells were inoculated in Nude mice and treated with (177)Lu-DOTA-AE105 once a visible tumor had formed. To evaluate the true effect of the targeted radiotherapy, two controls groups were included in this study, one receiving a (177)Lu-labeled non-binding control peptide and one receiving vehicle. All animals were treated day 0 and 7. A parallel (18)F-FLT PET/CT study was performed on day 0, 1, 3 and 6. Dosimetry calculations were based on a biodistribution study, where organs and tissue of interest were collected 0.5, 1.0, 2.0, 4.0 and 24h post injection of (177)Lu-DOTA-AE105. Toxicity was assessed by recording mouse weight and by H&E staining of kidneys in each treatment group.

RESULTS

uPAR-positive HT-29 xenograft was clearly visualized by PET/CT imaging using (64)Cu-DOTA-AE105. Subsequently, these xenograft transplants were locally irradiated using (177)Lu-DOTA-AE105, where a significant effect on tumor size and the number of uPAR-positive cells in the tumor was found (p<0.05). Evaluations of biodistribution and dosimetry revealed highest accumulation of radioactivity in kidneys and tumor tissue. (18)F-FLT PET/CT imaging study revealed a significant correlation between (18)F-FLT tumor uptake and efficacy of the radionuclide therapy. A histological examination of the kidneys from one animal in each treatment group did not reveal any gross abnormalities and the general performance of all treated animals also showed no indications of radioactivity-induced toxicity.

CONCLUSION

These findings document for the first time the in vivo efficacy of an uPAR-targeted radionuclide therapeutic intervention on both tumor size and its content of uPAR expressing cells thus setting the stage for future translation into clinical use.

Reduction of renal uptake of 111In-DOTA-labeled and A700-labeled RAFT-RGD during integrin αvβ3 targeting using single photon emission computed tomography and optical imaging

Integrin α(v)β(3) expression is upregulated during tumor growth and invasion in newly formed endothelial cells in tumor neovasculature and in some tumor cells. A tetrameric RGD-based peptide, regioselectively addressable functionalized template-(cyclo-[RGDfK])4 (RAFT-RGD), specifically targets integrin α(v)β(3) in vitro and in vivo. When labeled with indium-111, the RAFT-RGD is partially reabsorbed and trapped in the kidneys, limiting its use for further internal targeted radiotherapy and imaging investigations. We studied the effect of Gelofusine on RAFT-RGD renal retention in tumor-bearing mice. Mice were imaged using single photon emission computed tomography and optical imaging 1 and 24 h following tracer injection. Distribution of RAFT-RGD was further investigated by tissue removal and direct counting of the tracer. Kidney sections were analyzed by confocal microscopy. Gelofusine significantly induced a >50% reduction of the renal reabsorption of (111)In-DOTA-RAFT-RGD and A700-RAFT-RGD, without affecting tumor uptake. Injection of Gelofusine significantly reduced the renal retention of labeled RAFT-RGD, while increasing the tumor over healthy tissue ratio. These results will lead to the development of future therapeutic approaches.

Nephrotoxicity profiles and threshold dose values for [177Lu]-DOTATATE in nude mice

INTRODUCTION

In peptide receptor radionuclide therapy for neuroendocrine tumors the main dose-limiting tissue is found in the kidneys because of tubular reabsorption and retention of radioactivity. The aim of this study was to quantify late effects in renal cortex of nude mice exposed to high amounts of [(177)Lu]-DOTA-Tyr(3)-octreotate ([(177)Lu]-DOTATATE), and to determine whether a threshold dose value exists for these findings.

METHODS

Nude mice were exposed to 90, 120 or 150 MBq of [(177)Lu]-DOTATATE. Renal toxicity was evaluated up to 6 months after injection. Blood samples were collected to examine renal functional markers, and after sacrifice at 6 months changes in renal morphology were explored. Tissue damage was estimated by quantifying the relative area of the different subunits in the renal cortex using point counting. Additional morphological signs of radiation damage were also noted. The absorbed doses to the kidneys were estimated by previously determined kidney pharmacokinetics and Monte Carlo simulations for different assumptions regarding the activity distribution.

RESULTS

Increased serum creatinine and urea values indicated long-term renal toxicity. The tissue area occupied by proximal tubules decreased with increasing doses of [(177)Lu]-DOTATATE, whereas the other subunits in cortex slightly increased. The mean absorbed dose in the renal cortex for [(177)Lu]-DOTATATE was estimated to be 35-58 Gy for the different groups of animals. A dose-response relationship was observed for proximal tubular damage, and a threshold dose value of 24 Gy (BED 37 Gy) was determined.

CONCLUSIONS

Selective morphological changes in kidney cortex of nude mice were quantified and appeared in a dose dependent manner after injection of high amounts of [(177)Lu]-DOTATATE.

Radiopeptide imaging and therapy in the United States

Radiolabeled peptides targeted against receptors on the cell surface have been shown to be remarkably specific and effective in the diagnosis and therapy of malignant disease. Much of the early work in this field took place outside the United States, but in recent years the research effort within the United States has accelerated. Most of the initial studies in the United States focused on somatostatin receptor ligands. (111)In-pentetreotide was approved in 1994 and has been used extensively in the diagnosis and management of a wide variety of neuroendocrine tumors, particularly carcinoid. This work was extended to (99m)Tc-labeled analogs, and the most successful, (99m)Tc-depreotide, was approved in 1999. This agent was found to be accurate in the diagnosis of lung cancer, but it was not particularly successful because it was supplanted by (18)F-FDG imaging with positron tomography. More recently, studies with (68)Ga-labeled somatostatin analogs were initiated in the United States. This effort was delayed relative to that in other parts of the world because of difficulty in obtaining the necessary generators and regulatory uncertainty, both of which are less of a problem currently. Several ligands are being developed to image melanoma through targeting of the melanocyte-stimulating hormone receptor. Other ligands are being developed to use the arginine-glycine-aspartate oligopeptide to target angiogenesis and to use bombesin analogs to target the gastrin-releasing peptide receptor for the diagnosis and potential therapy of prostate cancer. Peptide dimers that target 2 receptors simultaneously are also being constructed, potentially increasing the selectivity of the approach significantly. Radiopeptide therapy has been explored with these ligands, initially with high-dose (111)In-pentetreotide. This step has been followed by U.S. participation in several trials with (90)Y-, (177)Lu-, and (188)Re-labeled analogs. Some of these agents are now available clinically outside the United States, and it is important to design and conduct the appropriate trials so that this therapy can be offered within the United States.

Minor changes in effective half-life during fractionated 177Lu-octreotate therapy

AIMS

Fractionated (177)Lu-DOTA-octreotate therapy has been reported to be an effective treatment option for patients with generalized neuroendocrine tumors. In our clinic, full individual dosimetry is performed during the first therapy cycle, while dosimetry at later cycles is based on the 24 h uptake measurement assuming an unchanged effective half-life. Our aim was to evaluate this assumption and the variation in the 24 h uptake during therapy.

PATIENTS

Thirty patients, 13 women and 17 men, were included in the study.

METHODS

During the first therapy cycle the (177)Lu-concentration was measured with SPECT/CT over the abdomen at 24 h, 96 h and 168 h after infusion. The effective half-life was determined for the kidneys, liver and spleen. The procedure was repeated at cycle 4 or 5.

RESULTS

The median ratio between the effective half-lives of the latter and the first cycle was 0.97 and 1.01 for the right and left kidney, with a range of 0.89-1.01 (1st-3rd quartile) and 0.93-1.05, respectively.

DISCUSSION

The mean value of the ratios was slightly lower than one, indicating a tendency towards increased activity elimination during therapy. In individual patients, significant changes were found for all organs, often when a large tumor burden reduction occurred during treatment. Possible contributing factors appeared to be larger amounts of non-tumor bound tracer, improved organ function (kidneys), decrease of vessel obstruction (spleen), less scatter from large tumors and reduction of small metastases (liver and spleen).

CONCLUSION

With most patients it is safe to estimate absorbed doses to kidneys, liver and spleen from 24 h activity concentration assuming an unchanged effective half-life during therapy. Patients with risk factors for kidney dysfunction need to be monitored in more detail. Simplified dosimetry based on the assumption of unchanged effective half-life can function as guidance to the number of therapy cycles an individual patient can tolerate.

68Ga-DOTATOC versus 68Ga-DOTATATE PET/CT in functional imaging of neuroendocrine tumors

Radiolabeled somatostatin analogs represent valuable tools for both in vivo diagnosis and therapy of neuroendocrine tumors (NETs) because of the frequent tumoral overexpression of somatostatin receptors (sst). The 2 compounds most often used in functional imaging with PET are (68)Ga-DOTATATE and (68)Ga-DOTATOC. Both ligands share a quite similar sst binding profile. However, the in vitro affinity of (68)Ga-DOTATATE in binding the sst subtype 2 (sst2) is approximately 10-fold higher than that of (68)Ga-DOTATOC. This difference may affect their efficiency in the detection of NET lesions because it is the sst2 that is predominantly overexpressed in NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs ((68)Ga-DOTATATE and (68)Ga-DOTATOC) in the same NET patients.

METHODS

Forty patients with metastatic NETs underwent (68)Ga-DOTATOC and (68)Ga-DOTATATE PET/CT as part of the work-up before prospective peptide receptor radionuclide therapy. The performance of both imaging methods was analyzed and compared for the detection of individual lesions per patient and for 8 defined body regions. A region was regarded positive if at least 1 lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUVmax) was compared for concordant lesions and renal parenchyma.

RESULTS

Seventy-eight regions were found positive with (68)Ga-DOTATATE versus 79 regions with (68)Ga-DOTATOC (not significant). Overall, however, significantly fewer lesions were detected with (68)Ga-DOTATATE than with (68)Ga-DOTATOC (254 vs. 262, P < 0.05). Mean (68)Ga-DOTATATE SUVmax across all lesions was significantly lower than (68)Ga-DOTATOC (16.0 ± 10.8 vs. 20.4 ± 14.7, P < 0.01). Mean SUVmax for renal parenchyma was not significantly different between (68)Ga-DOTATATE and (68)Ga-DOTATOC (12.7 ± 3.0 vs. 13.2 ± 3.3).

CONCLUSION

(68)Ga-DOTATOC and (68)Ga-DOTATATE possess a comparable diagnostic accuracy for the detection of NET lesions, with (68)Ga-DOTATOC having a potential advantage. The approximately 10-fold higher affinity for the sst2 of (68)Ga-DOTATATE does not prove to be clinically relevant. Quite unexpectedly, SUVmax of (68)Ga-DOTATOC scans tended to be higher than their (68)Ga-DOTATATE counterparts.

Repeated cycles of peptide receptor radionuclide therapy (PRRT)--results and side-effects of the radioisotope 90Y-DOTA TATE, 177Lu-DOTA TATE or 90Y/177Lu-DOTA TATE therapy in patients with disseminated NET

PURPOSE

PRRT is a known tool in the management of patients with disseminated and inoperable NETs. The aim of study was to assess the effectiveness of the repeated cycles of PRRT in patients with disseminated and inoperable NETs.

MATERIAL AND METHODS

Eighty nine patients were included in the PRRT. Among them 16 patients (18%) were qualified for a repeated PRRT cycle due to progression of the disease. In one of the patients qualified for the repeated cycle, PRRT was used as neoadjuvant therapy. The results and side-effects of the repeated cycles of PRRT were analyzed.

RESULTS

Disease stabilization was observed in 10 patients 6 months after the repeated PRRT cycle and in 5 patients after 12 and 18 months. Ten of the patients who had received repeated PRRT cycles died. In the case of neoadjuvant therapy, further reduction of the tumor size was observed, enabling qualification for surgery. Clinically significant reduction in the mean values of morphological parameters was not observed. Only after 12 and 18 months the mean values of creatinine levels were higher than the normal range (only in 2 patients).

CONCLUSIONS

The repeated cycles of PRRT did not cause a clinically significant increase of the toxicity of PRRT. The changes in kidney and blood morphology parameters were transient. The repeated cycles of PRRT enabled stabilization of the disease.

Effect of amino acid infusion on potassium serum levels in neuroendocrine tumour patients treated with targeted radiopeptide therapy

PURPOSE

Administration of positively charged amino acids has been introduced to reduce the nephrotoxicity of targeted radiopeptide therapy (TRT). However, the amino acid solution may have side effects, including hyperkalaemia. The aim of this study was to evaluate the frequency and the magnitude of hyperkalaemia in neuroendocrine tumour (NET) patients undergoing TRT.

METHODS

Enrolled in the study were 31 patients with NET eligible for TRT with [(90)Y-DOTA(0),Tyr(3)]octreotide ((90)Y-DOTATOC). Their mean age was 54 ± 14 years. Of these 31 patients, 21 (67%) were referred for the first treatment cycle, while 10 (33%) were referred for a subsequent therapy cycle. Patients were treated with therapeutic doses of (90)Y-DOTATOC ranging from 7,030 to 35,520 MBq. To inhibit tubular reabsorption of (90)Y-DOTATOC, 1 l of physiological saline solution containing 25 g of arginine hydrochloride and 25 g of lysine hydrochloride was given over 4 h starting 1 h before (90)Y-DOTATOC injection. All patients underwent a standard biochemical blood analysis at baseline, and 4 h and 24 h after the beginning of the amino acid infusion.

RESULTS

ANOVA repeated measures showed a significant overall effect on K(+) levels over time (F = 118.2, df = 2, P < 0.0001). Mean serum levels of K(+) were 4.00 ± 0.33 mmol/l at baseline, 5.47 ± 0.57 mmol/l at 4 h, and 4.38 ± 0.63 mmol/l at 24 h after the beginning of the infusion. Post-hoc analysis showed that K(+) levels at 4 h were significantly (P < 0.05) higher than at baseline. K(+) levels at 24 h were significantly (P < 0.05) lower than at 4 h but they were still significantly (P < 0.05) higher than K(+) levels at baseline. On a subject-by-subject basis, none of the 31 patients had increased K(+) levels at baseline. At 4 h, 24 of the 31 patients (77%) had K(+) levels above the normal range, and 6 patients (19%) experienced severe hyperkalaemia (K(+) ≥ 6 mmol/l). All patients with increased K(+) levels were clinically asymptomatic. At 24 h, only 4 patients (13%) had increased K(+) serum levels. The magnitude of the increase in K(+) levels between baseline and 4 h was relatively homogeneous over the whole group (1.41 ± 0.50 mmol/l) and it was not related (linear regression, P>0.05) to baseline K(+) levels. Intravenous administration of 40 mg furosemide 1 h after the beginning of the amino acid infusion did not have a significant effect on K(+) levels (P>0.05). No clinical characteristic was predictive for the increase in K(+) levels (chi-squared test, P > 0.05).

CONCLUSION

Hyperkalaemia is a frequent, potentially life-threatening side effect of basic amino acid infusion during TRT. K(+) levels 4 h after the beginning of the infusion should be monitored in patients at risk of complications, such as those with heart disease and those with risk factors for nephrotoxicity.

Renal uptake of different radiolabelled peptides is mediated by megalin: SPECT and biodistribution studies in megalin-deficient mice

PURPOSE

Radiolabelled peptides used for peptide receptor radionuclide therapy are excreted mainly via the kidneys and are partly reabsorbed and retained in the proximal tubular cells. The resulting high renal radiation dose can cause nephrotoxicity, limiting the maximum activity dose and the effectiveness of peptide receptor radionuclide therapy. The mechanisms of kidney reabsorption of these peptides are incompletely understood, but the scavenger receptor megalin has been shown to play a role in the reabsorption of (111)In-octreotide. In this study, the role of megalin in the renal reabsorption of various relevant radiolabelled peptides was investigated.

METHODS

Groups of kidney-specific megalin-deficient mice and wild-type mice were injected with (111)In-labelled somatostatin, exendin, neurotensin or minigastrin analogues. Single photon emission computed tomographic (SPECT) images of the kidneys were acquired and analysed quantitatively, or the animals were killed 3 h after injection and the activity concentration in the kidneys was measured.

RESULTS

Megalin-deficient mice showed significantly lower uptake of all studied radiolabelled peptides in the kidneys, ranging from 22% ((111)In-octreotide) to 65% ((111)In-exendin) of uptake in wild-type kidneys. Quantitative analysis of renal uptake by SPECT and ex vivo measurements showed a very good correlation.

CONCLUSION

Megalin is involved in the renal reabsorption of radiolabelled octreotide, octreotate, exendin, neurotensin and minigastrin. This knowledge may help in the design of strategies to reduce this reabsorption and the resulting nephrotoxicity in peptide receptor radionuclide therapy, enabling more effective therapy. Small-animal SPECT is an accurate tool, allowing in vivo quantification of renal uptake and serial measurements in individual mice.

Identification of a new peptide for fibrosarcoma tumor targeting and imaging in vivo

A 12-mer amino acid peptide SATTHYRLQAAN, denominated TK4, was isolated from a phage-display library with fibrosarcoma tumor-binding activity. In vivo biodistribution analysis of TK4-displaying phage showed a significant increased phage titer in implanted tumor up to 10-fold in comparison with normal tissues after systemic administration in mouse. Competition assay confirmed that the binding of TK4-phage to tumor cells depends on the TK4 peptide. Intravenous injection of ¹³¹I-labeled synthetic TK4 peptide in mice showed a tumor retention of 3.3% and 2.7% ID/g at 1- and 4-hour postinjection, respectively. Tumor-to-muscle ratio was 1.1, 5.7, and 3.2 at 1-, 4-, and 24-hour, respectively, and tumors were imaged on a digital γ-camera at 4-hour postinjection. The present data suggest that TK4 holds promise as a lead structure for tumor targeting, and it could be further applied in the development of diagnostic or therapeutic agent.

Dynamic and static small-animal SPECT in rats for monitoring renal function after 177Lu-labeled Tyr3-octreotate radionuclide therapy

High kidney radiation doses during clinical peptide receptor radionuclide therapy (PRRT) with β-particle-emitting radiolabeled somatostatin analogs will lead to renal failure several months after treatment, urging the coinfusion of the cationic amino acids lysine and arginine to reduce the renal radiation dose. In rat PRRT studies, renal protection by the coadministration of lysine was confirmed by histologic examination of kidney specimens indicating nephrotoxicity. In the current study, we investigated dedicated small-animal SPECT/CT renal imaging in rats to monitor renal function in vivo during follow-up of PRRT, with and without lysine.

METHODS

The following 3 groups of rats were imaged using a multipinhole SPECT/CT camera: controls (group 1) and rats at more than 90 d after therapy with 460 MBq (15 μg) of (177)Lu-DOTA-Tyr(3)-octreotate without (group 2) or with (group 3) a 400-mg/kg lysine coinjection as kidney protection (n ≥ 6 per group). At 90 and 140 d after therapy, static kidney scintigraphy was performed at 2 h after injection of 25 MBq of (99m)Tc-dimercaptosuccinic acid ((99m)Tc-DMSA). In addition, dynamic dual-isotope renography was performed using 50 MBq of (111)In-diethylenetriaminepentaacetic acid ((111)In-DTPA) and 50 MBq of (99m)Tc-mercaptoacetyltriglycine ((99m)Tc-MAG3) at 100-120 d after therapy.

RESULTS

(111)In-DTPA and (99m)Tc-MAG3 studies revealed a time-activity pattern comparable to those in patients, with a peak at 2-6 min followed by a decline of renal radioactivity. Reduced (111)In-DTPA, (99m)Tc-MAG3, and (99m)Tc-DMSA uptake indicated renal damage in group 2, whereas group 3 showed only a decrease of (99m)Tc-MAG3 peak activity. These results indicating nephrotoxicity in group 2 and renal protection in group 3 correlated with levels of urinary protein and serum creatinine and urea and were confirmed by renal histology.

CONCLUSION

Quantitative dynamic dual-isotope imaging using both (111)In-DTPA and (99m)Tc-MAG3 and static (99m)Tc-DMSA imaging in rats is feasible using small-animal SPECT, enabling longitudinal monitoring of renal function. (99m)Tc-MAG3 renography, especially, appears to be a more sensitive marker of tubular function after PRRT than serum chemistry or (99m)Tc-DMSA scintigraphy.

Preclinical and clinical studies of peptide receptor radionuclide therapy

In the 1980s, the (111)In-labeled somatostatin analog OctreoScan (Covidien, Hazelwood, MO) was developed for imaging of somatostatin receptor subtype 2 (sst(2)) overexpressing tumors. On the basis of this success, peptide receptor radionuclide therapy (PRRT) was developed using similar somatostatin analogs with different therapeutic radionuclides. Clinical application of PRRT demonstrated impressive results on tumor response, overall survival, and quality of life in patients with gastroenteropancreatic neuroendocrine tumors. The peptides 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), Tyr(3)-octreotate (DOTATATE) and DOTA, Tyr(3)-octreotide (DOTATOC) (brand name Onalta), predominantly targeting sst(2), have been granted Orphan Drug status by the European Medicines Agency and the US Food and Drug Administration for application in PRRT. Besides somatostatin receptor-targeting peptides, multiple other radiopeptide analogs were developed targeting several other receptors overexpressed on various tumors. Some of these peptide analogs, including cholecystokinin, gastrin, gastrin-releasing peptide, arginine-glycine-aspartate (RGD)-peptides, and glucagon-like peptide 1 analogs appeared very promising in preclinical and clinical imaging and PRRT studies. Although the success of PRRT with radiolabeled somatostatin analogs has been established, there is still room for improvement. The therapeutic window of PRRT could be enlarged by the use of new and improved targeting compounds, of which new antagonists with excellent tumor to background ratios are very promising. Furthermore, locoregional administration, improved healthy tissue protection, and combination treatment can be applied to increase the effectiveness of PRRT. Combination treatment might include cocktails of different peptide analogs of different therapeutic radionuclides and of radiolabeled peptides with chemotherapeutic or radiosensitizing agents. This review summarizes results of PRRT and describes clinical and preclinical studies regarding PRRT optimizing strategies.