90Y-DOTA-D-Phe1-Try3-octreotide in therapy of neuroendocrine malignancies

Rationale for the use of somatostatin analogs as antitumor agents

BACKGROUND

There is a need for novel antitumor agents that demonstrate efficacy in currently refractory tumors without adding to the toxicity of therapy. The somatostatin analogs, which have demonstrated antineoplastic activities in experimental tumor models, and good tolerability and safety profiles are attractive candidates.

MATERIALS AND METHODS

Data from preclinical studies provide evidence for direct and indirect mechanisms by which somatostatin analogs exert antitumor effects.

RESULTS

Direct antitumor activities, mediated through somatostatin receptors (sst(1)-sst(5)) expressed in tumor cells, include blockade of autocrine/paracrine growth-promoting hormone and growth factor production, inhibition of growth factor-mediated mitogenic signals and induction of apoptosis. Indirect antitumor effects include inhibition of growth-promoting hormone and growth factor secretion, and antiangiogenic actions. Many human tumors express more than one somatostatin receptor subtype, with sst(2) being predominant. Somatostatin analogs such as octreotide and lanreotide, which present a high affinity for sst(2), are in current clinical use to alleviate symptoms in patients with endocrine tumors, and radiolabeled somatostatin analogs have been developed for diagnosis and radiotherapy.

CONCLUSIONS

While the rationale exists for the use of somatostatin analogs as antitumor agents, studies are ongoing to identify analogs with activity across the range of receptor subtypes to maximize the potential of such treatment.

Radiolabeled peptides in oncology: role in diagnosis and treatment

There has been an exponential growth in the development of radiolabeled peptides for diagnostic and therapeutic applications in the last decade. The automated means of synthesizing these compounds in large quantities and the simplified methods of purifying, characterizing, and optimizing them have kindled attention to peptides as carrier molecules. These new techniques have accelerated the commercial development of radiolabelled peptides, which has provided additional radiopharmaceuticals for the nuclear medicine community. Peptides have many key properties including fast clearance, rapid tissue penetration, and low antigenicity, and can be produced easily and inexpensively. However, there may be problems with in vivo catabolism, unwanted physiologic effects, and chelate attachment. Radiolabeled peptides have made their greatest impact in the management of relatively rare neuroendocrine malignancies. Indeed, Indium-111 ((111)In)-pentetreotide ((111)In-DTPA-octreotide, Octreoscan), which binds to somatostatin receptors (SSTRs), has become the diagnostic 'gold standard' in these diseases. However, (111)In-pentetreotide has been less successful in the diagnosis of other more prevalent diseases in which SSTRs are upregulated. Technetium-99m (99mTc)-depreotide (NeoTect), a 99mTc-labeled SSTR-analog, could have wider impact since it has high sensitivity and specificity for lung cancer lesion detection. However, this impact may be minimized by the increased availability of positron emission tomography imaging with Fluorine-18 (18F)-flourodeoxyglucose, which has similar sensitivity and specificity for lesion identification in this disease, and is currently more widely used. The receptors for bombesin, alpha-melanocyte-stimulating hormone, neurotensin, and the integrin alpha(v)beta3, are under active investigation as targets for radiolabelled peptides, but are still in the pre-clinical stage. Compounds directed at the cholecystokinin-B/gastrin receptor have shown promising results in clinical trials in humans. Radiolabelled peptide therapy is usually indicated for patients with widespread disease that is not amenable to focused radiation therapy or is refractory to chemotherapy. Phase I/II studies using various radiolabelled peptides (including (111)In-pentetreotide, Yttrium-90 [90Y]-DOTA-Phe1-Tyr3-octreotide, 90Y-DOTA-lanreotide, and Lutetium-177 [177Lu]-DOTA-octreotate) for the treatment of patients with neuroendocrine malignancy are in progress. Over 400 patients have been treated, and the response rate has ranged from 60% to 75%, although few patients have had a complete response. Patients have been given individual doses ranging from 2 to 11 GBq with a slow infusion every 4-8 weeks (up to 12 times). The kidney is the dose-limiting organ and most patients experience a transient decline in blood cell counts. A concomitant infusion of an amino acid mixture can reduce kidney toxicity and increase the effective tumor dose. Other peptides currently under investigation, some of which have shown promising results, include Rhenium-188 (188Re)-P2045 and 90Y-alpha(v)beta3 antagonist.

Criteria for the design and biological characterization of radiolabeled peptide-based pharmaceuticals

Radiolabeled peptide-based formulations are being evaluated for their application in oncological imaging and therapy using nuclear medicine techniques. A major breakthrough in the field was the discovery and identification of the G-protein coupled receptor superfamily that are overexpressed in a variety of human cancers. These receptors act as targets for endogenous compounds, often of peptidic nature, which can be radiolabeled and, therefore, could potentially be utilized as radiopharmaceuticals. This general strategy has proven successful for application in humans in only a few cases thus far. However, the use of more sophisticated structural methodology to enhance our understanding of the interactions between the receptor and the endogenous peptide or its analogs, and a more efficient preclinical evaluation process, may help to single out the most promising compounds for further development and eventual use in the clinical application of radiopharmaceuticals. This review analyzes current methods of approaching these key points. The rational process for developing peptide-based radiopharmaceuticals is presented, from the structural analysis of the peptide-receptor interaction for the identification and modeling of the peptide analogs to the synthesis, with an appropriate metal carrier, of compounds that mimic endogenous peptides. Finally, the in vitro and in vivo biological testing and evaluation in preclinical animal models is described. To render the entire process successful, expertise in different areas of drug development is indispensable.

Endocrine tumours of the gastrointestinal tract. Peptide receptor radionuclide therapy

Peptide receptor radionuclide therapy is a new treatment modality for patients with inoperable or metastasised neuroendocrine gastroenteropancreatic tumours. After the successful implementation of somatostatin receptor scintigraphy in daily clinical practice, the next logical step was to increase the radiation dose of the administered radiolabelled somatostatin analogue in an attempt to induce tumour shrinkage. Since then, an increasing number of patients has been successfully treated with this approach, resulting in a substantial numbers of patient with objective tumour shrinkage. Serious side-effects have been rare. This article reviews the effectiveness of the different radiolabelled somatostatin analogues used, the currently known side-effects and the survival data available. Furthermore, clinical issues, including indication and timing of therapy, are discussed. Finally, important directions for future research are briefly mentioned to illustrate that, although the currently available results already suggest a favourable outcome compared with other systemic therapies, new strategies are being developed to increase efficacy.

Nuclear medicine in the detection, staging and treatment of gastrointestinal carcinoid tumours

Carcinoid tumours belong to the family of neuroendocrine tumours with a capacity to take up and concentrate amines and precursors as well as peptides, and can thereby be detected by nuclear medicine techniques. These rare tumours are difficult to diagnose at earlier stages because of small size and multiplicity. Computed tomography (CT) and magnetic resonance imaging (MRI) are mostly of benefit for detection of larger primary tumours (1-3 cm) and liver and lymph-node metastases. A majority of carcinoid tumours express somatostatin receptors, particularly receptor type 2, and thus somatostatin receptor scintigraphy (SRS) can be used for detection and staging of carcinoid tumours. The detection rate of carcinoid tumours has been reported to be somewhere between 80 and 100% in different studies. The scintigraphy gives a good staging of the disease and detection of unexpected tumour sites, which were not determined by conventional imaging. This method also indicates content of somatostatin receptors, which might indicate efficacy of treatment with octreotide or other somatostatin analogues. Another new non-invasive technique for detection of carcinoid tumours is positron emission tomography (PET). The biological substance for study can be labelled for radioactive imaging with radionuclears, such as (11)C, (15)O and (18)F, with emission of positrons. More than 95% of patients studied displayed high tracer uptake from PET with (11)C-5HTP (5-hydroxytryptophan), which is significantly higher compared to both computer tomography and somatostatin receptor scintigraphy. MIBG has been used for decades to visualize carcinoid tumours, because MIBG is concentrated in the endocrine cells. It was initially developed to detect phaeochromocytomas of the adrenal with reported high sensitivity (87%) and specificity as high as 99%. The method can be used when other methods fail to localize carcinoid tumours and particularly when treatment with (131)I-MIBG is being considered. Tumour-targeted treatment for malignant carcinoid tumour is still investigational, but has become of significant interest with the use of radiolabelled somatostatin analogues. Since a majority of carcinoid tumours present somatostatin receptors and can therefore be visualized in vivo by using radiolabelled somatostatin analogues, it seems logical to try to target these tumours with radioactive substances, not only for visualization but also for treatment. (111)Indium-DTPA-octreotide has been used as the first tumour-targeted treatment, with rather low response rates (in the order of 10-20%) and no significant tumour shrinkage. The second radioactive analogue which has been applied in the clinic is (90)yttrium-DOTA-Tyr3-octreotide, which has given partial and complete remissions in 20-30% of patients. The most significant side-effects have been kidney dysfunction, thrombocytopenia and liver toxicity. The most recent compound is (177)lutetium-DOTA-Tyr3-octreotate, which has been applied by the Rotterdam group and has been reported to give partial remission in about 40% of the patients. In the near future, combined treatment with both (90)yttrium and (177)lutetium coupled to a somatostatin analogue might come into clinical trials. (177)Lutetium may be more effective for smaller tumours whereas (90)yttrium may be more effective for larger tumours.

Radiolabeled somatostatin analog [177Lu-DOTA0,Tyr3]octreotate in patients with endocrine gastroenteropancreatic tumors

PURPOSE

There are few treatment options for patients with metastasized or inoperable endocrine gastroenteropancreatic (GEP) tumors. Chemotherapy can be effective, but the response is usually less than 1 year. Here, we present the results of treatment with a radiolabeled somatostatin analog, [177Lu-DOTA0,Tyr3]octreotate (177Lu-octreotate).

PATIENTS AND METHODS

One hundred thirty-one patients with somatostatin receptor-positive tumors were treated with up to a cumulative dose of 600 to 800 mCi (22.2 to 29.6 GBq) of 177Lu-octreotate.

RESULTS

One patient developed renal insufficiency, and another patient developed hepatorenal syndrome. Creatinine clearance did not change significantly in the other patients. WHO hematologic toxicity grade 3 or 4 occurred after less than 2% of the administrations. We observed complete remission in three patients (2%), partial remission in 32 patients (26%), minor response (tumor diameter decrease of 25% to 50%) in 24 patients (19%), stable disease (SD) in 44 patients (35%), and progressive disease (PD) in 22 patients (18%). Higher remission rates were positively correlated with high uptake on pretherapy somatostatin receptor imaging and a limited number of liver metastases, whereas PD was significantly more frequent in patients with a low performance score and extensive disease. Median time to progression in 103 patients who either had SD or tumor regression was more than 36 months.

CONCLUSION

Treatment with 177Lu-octreotate results in tumor remission in a high percentage of patients with GEP tumors. Serious side effects are rare. The median time to progression compares favorably with chemotherapy. Results are better in patients with a limited tumor load. Therefore, early treatment, even in patients who have no PD, may be better.

Somatostatin analogues in the treatment of gastroenteropancreatic neuroendocrine tumors

Gastroenteropancreatic neuroendocrine tumors constitute a heterogeneous group of neoplasms that are often associated with typical symptoms due to excessive and uncontrolled release of diverse hormones. Because these tumors are usually slow growing, surgery is the cornerstone of treatment. However, these rare tumors can present with rapid progression that requires aggressive systemic therapy or diffuse metastatic disease not amenable to surgical palliation. For most patients, medical approaches are necessary at some point in the course of their disease, especially since most tumors are at an advanced stage at the time of diagnosis. Most gastroenteropancreatic neuroendocrine tumors express high levels of somatostatin receptors, which are bound by somatostatin or its synthetic analogues. These agents, alone or combined with other therapies, such as Interferon or radioisotopes, are therefore used frequently to control hormone-related symptoms and, for some patients, the growth of the disease Itself. This article reviews the evidence for the use of somatostatin analogues in the treatment of gastroenteropancreatic neuroendocrine tumors based on a MEDLINE search of literature published from January 1970 to July 2003.

Receptor-Mediated Tumor Targeting with Radiopeptides. Part 1. General Concepts and Methods: Applications to Somatostatin Receptor-Expressing Tumors

Radiolabeled peptides have become important tools in nuclear oncology, both as diagnostics and more recently also as therapeutics. They represent a distinct sector of the molecular targeting approach, which in many areas of therapy will implement the old "magic bullet" concept by specifically directing the therapeutic agent to the site of action. In this three-part review, we present a comprehensive overview of the literature on receptor-mediated tumor targeting with the different radiopeptides currently studied. Part I summarizes the general concepts and methods of targeting, the selection of radioisotopes, chelators, and the criteria of peptide ligand development. Then, the >400 studies on the application to somatostatin/somatostatin-release inhibiting factor receptor-mediated tumor localization and treatment will be reviewed, demonstrating that peptide radiopharmaceuticals have gained an important position in clinical medicine.

A comparison of (111)In-DOTATOC and (111)In-DOTATATE: biodistribution and dosimetry in the same patients with metastatic neuroendocrine tumours

[Yttrium-90-DOTA-Tyr(3)]-octreotide (DOTATOC) and [(177)Lu-DOTA-Tyr(3)-Thr(8)]-octreotide (DOTATATE) are used for peptide receptor-mediated radionuclide therapy (PRMRT) in neuroendocrine tumours. No human data comparing these two compounds are available so far. We used (111)In as a surrogate for (90)Y and (177)Lu and examined whether one of the (111)In-labelled peptides had a more favourable biodistribution in patients with neuroendocrine tumours. Special emphasis was given to kidney uptake and tumour-to-kidney ratio since kidney toxicity is usually the dose-limiting factor. Five patients with metastatic neuroendocrine tumours were injected with 222 MBq (111)In-DOTATOC and (111)In-DOTATATE within 2 weeks. Up to 48 h after injection, whole-body scans were performed and blood and urine samples were collected. The mean absorbed dose was calculated for tumours, kidney, liver, spleen and bone marrow. In all cases (111)In-DOTATATE showed a higher uptake (%IA) in kidney and liver. The amount of (111)In-DOTATOC excreted into the urine was significantly higher than for (111)In-DOTATATE. The mean absorbed dose to the red marrow was nearly identical. (111)In-DOTATOC showed a higher tumour-to-kidney absorbed dose ratio in seven of nine evaluated tumours. The variability of the tumour-to-kidney ratio was high and the significance level in favour of (111)In-DOTATOC was P=0.065. In five patients the pharmacokinetics of (111)In-DOTATOC and (111)In-DOTATATE was found to be comparable. The two peptides appear to be nearly equivalent for PRMRT in neuroendocrine tumours, with minor advantages for (111)In/(90)Y-DOTATOC; on this basis, we shall continue to use (90)Y-DOTATOC for PRMRT in patients with metastatic neuroendocrine tumours.

[177Lu]Bz-DTPA-EGF: Preclinical Characterization of a Potential Radionuclide Targeting Agent Against Glioma

Patients with glioblastoma multiforme have a poor prognosis due to recurrences originating from spread cells. The use of radionuclide targeting might increase the chance of inactivating single tumor cells with minimal damage to surrounding healthy tissue. As a target, overexpressed epidermal growth factor receptors (EGFR) may be used. A natural ligand to EGFR, the epidermal growth factor (EGF) is an attractive targeting agent due to its low molecular weight (6 kDa) and high affinity for EGFR. 177Lu (T(1/2) = 6.7 days) is a radionuclide well suited for treatment of small tumor cell clusters, since it emits relatively low-energy beta particles. The goal of this study was to prepare and preclinically evaluate both in vitro and in vivo the [177Lu]Bz-DTPA-EGF conjugate. The conjugate was characterized in vitro for its cell-binding properties, and in vivo for its pharmacokinetics and ability to target EGFR. [177Lu]Bz-DTPA-EGF bound to cultured U343 glioblastoma cells with an affinity of 1.9 nM. Interaction with EGFR led to rapid internalization, and more than 70% of the cell-associated radioactivity was internalized after 30 minutes of incubation. The retention of radioactivity was good, with more than 65% of the 177Lu still cell-associated after 2 days. Biodistribution studies of i.v. injected [177Lu]Bz-DTPA-EGF in NMRI mice demonstrated a rapid blood clearance. Most of the radioactivity was found in the liver and kidneys. The liver uptake was receptor-mediated, since it could be significantly reduced by preinjection of unlabeled EGF. In conclusion, [177Lu]Bz-DTPA-EGF seems to be a promising candidate for locoregional treatment of glioblastoma due to its high binding affinity, low molecular weight, and ability to target EGFR in vivo.