Current Radiopharmaceuticals - Volume 9, Issue 1, 2016

Peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin analogs has become an established procedure for the treatment of patients suffering from inoperable neuroendocrine cancers over-expressing somatostatin receptors. Success of PRRT depends on the availability of the radiolabeled peptide with adequately high specific activity, so that required therapeutic efficacy can be achieved without saturating the limited number of receptors available on the target lesions. Specific activity of the radionuclide and the radiolabeled somatostatin analog are therefore an important parameters. Although these analogs have been investigated and improved, and successfully applied for PRRT for more than 15 years, there are still many possibilities for further improvements that fully exploit PRRT with 177Lu-DOTA-TATE. The here summarized data presented herein on increased knowledge of the components of 177Lu-DOTA-TATE (especially the purity of 177Lu and specific activity of 177Lu) and the reaction kinetics during labeling 177Lu-DOTA-TATE clearly show that the peptide dose and dose in GBq can be varied. Here we present an overview of the development, formulation and optimisation of 177Lu-DOTA-TATE, mainly addressing radiochemical parameters

Peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin analogues has shown encouraging results in various somatostatin receptor positive tumors. Partial remission rates up to 30% have been documented as well as significant improvements in quality of life and survival. This treatment takes advantage of the high specific binding of the radiolabeled peptide to somatostatin receptors overexpressed by the tumors thus being more effective on the tumor cells with less systemic side-effects. The development of macrocyclic chelators conjugated to peptides made possible the stable binding with various radionuclides. In particular 177Lu features favourable physical characteristics with a half-life of 6.7 days, emission of β- with energy of 0.5 MeV for treatment and γ-emissions suitable for imaging. The present contribution describes the learning process achieved at the European Institute of Oncology (IEO) since the first application of 90 labeled peptides to the therapy of neuroendocrine tumors back in 1997. Continuous improvements led to the preparation of a safe 177Lu labeled peptide for human use. Our learning curve began with the identification of the optimal characteristics of the isotope paying attention to its chemical purity and specific activity along with the optimization of the parameters involved in the radiolabeling procedure. Also the radiation protection issues have been improved along the years and recently more and more attention has been devoted to the pharmaceutical aspects involved in the preparation. The overall issue of the quality has now been completed by drafting an extensive documentation with the goal to deliver a safe and reliable product to our patients.

The rare-earth radionuclides that decay by beta particle (β-&) emission are considered to be ideal in the context of targeted radiotherapy. The rare-earth isotopes exist primarily in the 3+ oxidation state and are considered to be hard metal centers, requiring multidentate, hard donor ligands such as the poly(aminocarboxylates) for in vivo kinetic inertness. 177Lu is a rare-earth radionuclide that is produced in moderate specific activity (740 GBq/mg) by direct neutron capture of enriched 176Lu via the 176Lu(n,γ)177Lu nuclear reaction. 177Lu has a half-life of 6.71 d, decays by beta emission (Ebmax = 0.497 MeV), and emits two imagable photons (113keV, 3% and 208kev, 11%). High specific activity, no-carrier-added 177Lu can also be prepared by an indirect neutron capture nuclear reaction on a 176 target. Herein, we report upon bombesin (BBN) peptides radiolabeled with 177Lu. The impetus driving many of the research studies that we have described in this review is that the high-affinity gastrin releasing peptide receptor (GRPR, BBN receptor subtype 2, BB2) has been identified in tissue biopsy samples and immortalized cell lines of many human cancers and is an ideal biomarker for targeting early-stage disease. Early on, the ability of GRPR agonists to be rapidly internalized coupled with a high incidence of GRPR expression on various neoplasias was a driving force for the design and development of new diagnostic and therapeutic agents targeting GRP receptor-positive tumors. Recent reports, however, show compelling evidence that radiopharmaceutical design and development based upon antagonist-type ligand frameworks clearly bears reexamination. Last of all, the ability to target multiple biomarkers simultaneously via a heterodimeric targeting ligand has also provided a new avenue to investigate the dual targeting capacity of bivalent radioligands for improved in vivo molecular imaging and treatment of specific human cancers. In this report, we describe recent advances in 177Lu-labeled bombesin peptides for targeted radiotherapy that includes agonist, antagonist, and multivalent cell-targeting agents. In vitro, in vivo translational, and in vivo human clinical investigations are described.

Prostate specific membrane antigen (PSMA) is the single most well-validated prostate cancer (PCa)-specific cell membrane antigen known. It is present in high levels in 95% of PCa, and is an ideal target to develop radiopharmaceuticals for imaging studies and radionuclide therapy. Humanized J591 monoclonal antibody (mAb) binds specifically with nanomolar affinity to the extracellular domain of PSMA. After binding, the PSMA–antibody complex is rapidly internalized, increasing the potential utility of PSMA as a target for the delivery of mAb-conjugated radionuclides or cytotoxins. J591 mAb was labeled with 177 at a high specific activity (10-30 mCi/mg) using DOTA as the bifunctional chelate. The preclinical data in PSMA positive xenografts, strongly suggested that 177 mAb is an ideal radiopharmaceutical for RIT of metastatic PCa. Since October 2000, five clinical studies (phase I and II) were performed in subjects with metastatic castration-resistant prostate cancer (CRPC) using 177 The methodology and the results of these clinical studies are briefly reviewed in this article. The maximum tolerated dose (MTD) as a single dose was 70 mCi/m2&. Based on dose fractionation (DF), MTD was 90 mCi/m2& (2 doses of 45 mCi/m2&, 2 wks apart). Phase II study in patients with progressive metastatic CRPC, at a dose of 65- 70 mCi/m2& resulted in significant PSA declines in 60% of the patients. While myelosuppression was the dose limiting toxicity, DF alone or in combination with docetaxel also resulted in significant PSA declines with much less toxicity. 177 imaging studies demonstrated accurate targeting of known metastatic sites in >90% of patients and those with stronger PSMA expression by semi-quantitative imaging had more PSA declines. These clinical studies clearly documented the potential therapeutic value of radioimmunotherapy (RIT) in metastatic PCa.

The optimization of DOTA-NHS-ester conjugation to Rituximab using different Ab:DOTA molar ratios (1:10, 1:20, 1:50 and 1:100) was studied. High radiochemical yield, in vitro stability and immunoreactive fraction were obtained for the Rituximab conjugated at 1:50 molar ratio, resulting in the incorporation of an average number of 4.9 ± 1.1 DOTA per Rituximab molecule. Labeling with 177 was performed in high specific activity with great in vitro stability. Biodistribution in healthy and xenographed mice showed tumor uptake and high in vivo stability as evidenced by low uptake in bone. The properties of 177 prepared from DOTA-NHS-ester suggest the potential for the application of the 177 antibody in preliminary clinical studies.

177 is currently being investigated as a potential agent for providing palliative care to the patients suffering from bone pain due to metastatic skeletal carcinoma. The present article describes the evaluation of 177Lu-EDTMP complex in four different canine patients with different types of primary and metastatic skeletal lesions with respect to its pharmacokinetic properties, dosimetry and therapeutic efficacy. The dogs were treated with a dose of ~44.4 MBq (1.2 mCi) per kg body weight of 177, synthesized in-house with high radiochemical purity (98.8 ± 0.4 %) and excellent in vitro stability. The radiopharmaceutical showed favourable pharmacokinetic properties, such as, preferential accumulation at skeletal lesion sites and fast clearance from blood and other non-target organs through urinary route. The administered dose of the radiopharmacutical showed excellent therapeutic efficacy in case of a dog suffering from skeletal metastasis originating from primary tumor elsewhere. On the other hand, two of the remaining three patients with primary bone cancer showed stable disease intially with palliative effect. The fourth patient having metal implant induced osteosarcoma with severe limb oedema did not show any response to the treatment.

177 has been proposed as a potent bone pain palliation agent owing to theoretical advantage of reduced bone marrow suppression resulting from the low β-& energy and a suitably long half-life facilitating its wider distribution with less loss from radioactive decay. Herein, we report the pharmacokinetics, dosimetry and toxicity analysis of 177 in patients (phase-0/I study). In a phase-0 study, the biokinetics of skeletal and non-skeletal uptake of 177 was assessed in 6 patients with metastatic prostate cancer using tracer doses (172.7–206.9MBq). Data of whole skeletal uptake, blood and fractionated urine samples were obtained and dosimetric calculations were performed using the OLINDA/EXM 1.0 software. Prolonged bone retention was observed in all patients. Excretion was mainly via the renal route and blood clearance was rapid and biphasic. Mean estimated red marrow dose was 0.80±0.15mGy/MBq while mean total-body dose was 0.16±0.04mGy/MBq. A maximum tolerated dose (MTD) of 2000-3250MBqfor 177 was calculated. For the phase-I study, 21 patients with metastatic prostate cancer were given a therapeutic dose of 177 EDTMP (692-5550MBq). Toxiciy (WHO), evaluated by assessment of hemoglobin levels, platelet and leukocyte counts over 12 weeks, was mainly limited to anemia or thrombocytopenia. Only transient toxicity was observed in 14/21 patients, of which 6 had baseline toxicity. Beyond the MTD, a significantly higher number of patients displayed grade 3-4 toxicity. Pain relief, assessed by VAS pain score, was observed in 86% patients with median relief duration of 7 weeks. The results demonstrate that 177 has excellent pharmacokinetic and dosimetric properties, besides being safe and effective. Along with estimating radiation dose values to certain critical organs, we have also proposed an MTD for 177 that correlated well with toxicity data. The encouraging dosimetry and toxicity data of 177 reported provide the basis for subsequent phases of the studies to establish complete effectiveness and safety of 177 as an attractive alternative to other radioactive bone pain palliation agents.

Lutetium-177 is an emerging radionuclide due its convenient chemical and nuclear properties. In this paper we describe the development and evaluation in Uruguay of the targeted 177 labelled radiopharmaceuticals EDTMP (for bone pain palliation) and DOTA-TATE (neuroendocrine tumors). We optimized the preparation of these 177 radiopharmaceuticals including radiolabelling, quality control methods, in vitro and in vivo stability and their therapeutic application in patients. Radiation dosimetry aspects of 177 are also included. Nine male patients with prostate cancer and four female patients with breast carcinoma with multiple bone metastatic lesions were treated with 177 Four patients with gastroentheropancreatic neuroendocrine tumors (GEP-NET) and one patient with bronchial NET were treated with 1- 3 cycles with a cumulative dose of 4.44-22.2 GBq of 177 Scintigraphic images of the patients treated with 177 evidenced high and rapid uptake in bone metastasis, remaining after 7 days post administration. Images allow skeletal visualization with high definition and demonstrate increased uptake in bone metastases. For 177, partial remissions were obtained in 4 patients and the remaining patient did not show significant progression 3 months after the second cycle. No serious adverse effects were registered, even in two patients with confirmed renal disease and high risk for renal disease Dosimetry assessments confirm the predictive value of the personalized therapy with radiolabelled peptides. We found it is possible to accumulate high therapeutic doses in tumours in sequential administrations of 177, increasing the probability of biological response without significant impairment ofthe renal function in patients with risk factors. These results demonstrate the attractive therapeutic properties of these two 177 labelled agents and the feasibility of this metabolic therapy in regions far away from 177 producing countries

Lutetium-177 has been widely discussed as a radioisotope of choice for targeted radionuclide therapy. The simultaneous emission of imageable gamma photons [208 keV (11%) and 113 keV (6.4%)] along with particulate β-& emission [β(max) = 497 keV] makes it a theranostically desirable radioisotope. In the present article, the possibility of using two 177 agents viz. 177 and 177 for theranostic applications in metastatic bone pain palliation (MBPP) and peptide receptor radionuclide therapy (PRRT), have been explored. In the case of 177, the whole-body images obtained are compared with those recorded using 99m in the same patient. On the other hand, pre-therapy images acquired with 177 are compared with similar images obtained with standard agents, such as 99m (SPECT) and 68 (PET) in the same patient. The advantage of the long physical half-life (T1/2) of 177 has been utilized in mapping the pharmacokinetics of two additional agents, 177 hydroxyapatite (HA) in radiation synovectomy of knee joints and 177 for therapy of hepatocellular carcinoma. Results of these multiple studies conclusively document the potential of 177 as a theranostic radioisotope.