Physical and chemical properties of radionuclide therapy

Theoretical aspects on the use of single-time-point dosimetry for radionuclide therapy

Objective: This study considers the error distributions for time-integrated activity (TIA) of single-time-point (STP) methods for patient-specific dosimetry in radionuclide therapy.Approach: The general case with the same pharmaceutical labelled with different radionuclides for imaging and therapy are considered for a mono-exponential time-activity curve. Two methods for STP dosimetry, both based on the combination of one activity estimate with the population-mean effective decay constant, are investigated. The cumulative distribution functions (CDFs) and the probability density functions for the two methods are analytically derived for arbitrary distributions of the biological decay constant. The CDFs are used for determining 95 % coverage intervals of the relative errors for different combinations of imaging time points, physical decay constants, and relative standard deviations of the biological decay constant. Two examples, in the form of kidney dosimetry in [¹⁷⁷Lu]Lu-DOTA-TATE therapy and tumour dosimetry for Na[¹³¹I]I therapy for thyroid cancer with dosimetry based on imaging of Na[¹²⁴I]I, are also studied in more detail with analysis of the sensitivity with respect to errors in the mean biological decay constant and to higher moments of the distribution.Main results: The distributions of the relative errors are negatively skewed, potentially leading to the situation that some TIA estimates are highly underestimated even if the majority of estimates are close to the true value.Significance: The main limitation of the studied STP dosimetry methods is thereby the risk of large underestimations of the TIA.

State of the Art and Recent Developments of Radiopharmaceuticals for Pancreatic Neuroendocrine Tumors Imaging

Background:

Neuroendocrine Tumors (NETs) are relatively rare tumors, mainly originating from the digestive system, that tend to grow slowly and are often diagnosed when metastasised. Surgery is the sole curative option but is feasible only in a minority of patients. Among them, pancreatic neuroendocrine tumors (pancreatic NETs or pNETs) account for less than 5% of all pancreatic tumors. Viable therapeutic options include medical treatments such as biotherapies and more recently Peptide Receptor Radionuclide Therapies (PRRT) with radiolabeled somatostatin analogues. Molecular imaging, with main reference to PET/CT, has a major role in patients with pNETs.

Objective:

The overexpression of specific membrane receptors, as well as the ability of cells to take up amine precursors in NET, have been exploited for the development of specific targeting imaging agents.

Methods:

SPECT/CT and PET/CT with specific isotopes such as [68Ga]-1,4,7,10-tetra-azacyclododecane- N,N’,N’’,N’’’-tetra-acetic acid (DOTA)-somatostatin analogs, [18F]-FDG and [18F]-fluorodopa have been clinically explored.

Results:

To overcome the limitations of SSTR imaging, interesting improvements are connected with the availability of new radiotracers, activating with different mechanisms compared to somatostatin analogues, such as glucagon-like peptide 1 receptor (GLP-1 R) agonists or antagonists.

Conclusion:

This paper shows an overview of the RPs used so far in the imaging of pNETs with insight on potential new radiopharmaceuticals currently under clinical evaluation.

Radioimmunotherapy for Prostate Cancer--Current Status and Future Possibilities

Prostate cancer (PCa) is one of the most common cancers in men and is the second leading cause of cancer-related deaths in the USA. In the United States, it is the second most frequently diagnosed cancer after skin cancer, and in Europe it is number one. According to the American Cancer Society, approximately 221,000 men in the United States would be diagnosed with PCa during 2015, and approximately 28,000 would die of the disease. According to the International Agency for Research on Cancer, approximately 345,000 men were diagnosed with PCa in Europe during 2012, and despite more emphasis placed on early detection through routine screening, 72,000 men died of the disease. Hence, the need for improved therapy modalities is of utmost importance. And targeted therapies based on radiolabeled specific antibodies or peptides are a very interesting and promising alternative to increase the therapeutic efficacy and overall chance of survival of these patients. There are currently several preclinical and some clinical studies that have been conducted, or are ongoing, to investigate the therapeutic efficacy and toxicity of radioimmunotherapy (RIT) against PCa. One thing that is lacking in a lot of these published studies is the dosimetry data, which are needed to compare results between the studies and the study locations. Given the complicated tumor microenvironment and overall complexity of RIT to PCa, old and new targets and targeting strategies like combination RIT and pretargeting RIT are being improved and assessed along with various therapeutic radionuclides candidates. Given alone or in combination with other therapies, these new and improved strategies and RIT tools further enhance the clinical response to RIT drugs in PCa, making RIT for PCa an increasingly practical clinical tool.

Radiopharmaceuticals for bone metastasis therapy and beyond: a voyage from the past to the present and a look to the future

Bone cancer can be divided into primary and secondary (metastatic) bone cancer. Osteosarcoma is the most common type of primary bone cancer, but still is a rare cancer. The development of bone metastases is a common event for the cancer patient and the main cause of treatment failure and death, being chronic pain syndrome the most important complication. There are currently several therapeutic modalities for the treatment of metastatic bone disease, including radiation therapy. Treatment with radionuclides (β- and α-particle emitters and Auger electron cascades) is a safe and effective tool of medicine. There is a great deal of interest in diphosphonic acids in nuclear medicine as ligands for radiometals in bone-seeking diagnostic and therapeutic agents. Several radiopharmaceuticals have been designed with the phosphonates as ligands. A recent approach to develop an effective radiopharmaceutical for therapy of bone cancer was the design of a water-soluble polymer that would exploit the disrupted vasculature in tumors according to the enhanced permeability and retention effect. To enhance the effect of radionuclide therapy on the cancer cells, new strategies have recently been investigated, such as the combined radionuclide and chemotherapy, high-dose radionuclide therapy, and repeated radionuclide therapy.

Dosimetric effectiveness of targeted radionuclide therapy based on a pharmacokinetic landscape

Assessment of targeted radionuclide therapy (TRT) agent effectiveness based on its pharmacokinetic (PK) properties could provide means to expedited agent development or its rejection. A broad PK model that predicts the relative effectiveness of TRT agents based on the relationship between their normal body (k(12), k(21)) and tumor (k(34), k(43)) PK parameters has been developed. A classic two-compartment open model decoupled from a tumor was used to represent the body. Analytically solved differential equations were used to develop a relationship that predicts TRT effectiveness. Various PK scenarios were created by pairing normal body PK parameters of 38 pharmaceuticals found in the literature with estimated tumor PK parameters. Each PK scenario resulted in a maximum permissible injected activity that limited the whole-body dose to 2 Gy and yielded a maximum delivered tumor dose. The model suggests that a k(34):k(43) ratio greater than 5 and a k(12):k(21) ratio less than 1 is effective at delivering doses that ensure sufficient solid tumor control. It was also shown that there is no direct relationship between tumor dose and acid dissociation constant (pK(a)), lipophilicity (log P), and fraction unbound (fu), which are important physicochemical properties. This study suggests that although effective TRT may be difficult to achieve for solid tumors, good TRT agents must have extremely desirable normal body PKs in conjunction with very high tumor retention. The developed PK TRT model could serve as a tool to compare the relative dosimetric effectiveness of existing TRT agents and novel TRT agents early in the developmental phase to potentially reject those that possess unfavorable PKs.

The biological effectiveness of targeted radionuclide therapy based on a whole-body pharmacokinetic model

Biologically effective dose (BED) may be more of a relevant quantity than absorbed dose for establishing tumour response relationships. By taking into account the dose rate and tissue-specific parameters such as repair and radiosensitivity, it is possible to compare the relative biological effects of different targeted radionuclide therapy (TRT) agents. The aim of this work was to develop an analytical tumour BED calculation for TRT that could predict a relative biological effect based on normal body and tumour pharmacokinetics. This work represents a step in the direction of establishing relative pharmacokinetic criteria of when the BED formalism is more applicable than absorbed dose for TRT. A previously established pharmacokinetic (PK) model for TRT was used and adapted into the BED formalism. An analytical equation for the protraction factor, which incorporates dose rate and repair rate, was derived. Dose rates within the normal body and tumour were related to the slopes of their time-activity curves which were determined by the ratios of their respective PK parameters. The relationships between the tumour influx-to-efflux ratio (k(34):k(43)), central compartment efflux-to-influx ratio (k(12):k(21)), central elimination (k(el)), and tumour repair rate (μ), and tumour BED were investigated. As the k(34):k(43) ratio increases and the k(12):k(21) ratio decreases, the difference between tumour BED and D increases. In contrast, as the k(34):k(43) ratios decrease and the k(12):k(21) ratios increase, the tumour BED approaches D. At large k(34):k(43) ratios, the difference between tumour BED and D increases to a maximum as k(el) increases. At small k(34):k(43) ratios, the tumour BED approaches D at very small k(el). At small μ and small k(34):k(43) ratios, the tumour BED approaches D. For large k(34):k(43) ratios, large μ values cause tumour BED to approach D. This work represents a step in the direction of establishing relative PK criteria of when the BED formalism is more applicable than absorbed dose for TRT. It also provides a framework by which the biological effects of different TRT agents can be compared in order to predict efficacy.

Cross sections for production of the therapeutic radioisotopes 198Au and 199Au in proton and deuteron induced reactions on 198Pt

The possible production of the therapeutic radioisotopes 198Au and 199Au using charged particle reactions at cyclotrons was investigated. For the first time, excitation functions for the 198Pt(p,n)198gAu, 198Pt(d,2n)198m,gAu and 198Pt(d,x) 199Au nuclear reactions were measured up to 40 MeV proton and 20 MeV deuteron energies. The results were compared with calculations performed with the computer code ALICE-IPPE. Acceptable overall agreement was obtained. Production and product quality parameters are discussed on the basis of the measured cross sections and from the deduced integral yields.

Clinical development of radioimmunotherapy for B-cell non-Hodgkin's lymphoma

Over the past several decades, several biomolecules have been investigated for their ability to deliver radiation to cancer cells, but antibodies have been the carriers of choice in systemic targeted radionuclide therapy (STaRT). Two radioimmunotherapy agents that target the CD20 antigen, (131)I-tositumomab and (90)Y-ibritumomab tiuxetan, have been approved by the U.S. Food and Drug Administration for the treatment of patients with relapsed or refractory B-cell non-Hodgkin's lymphoma (NHL), and clinical trials have shown that they are effective as monotherapies in the salvage setting, producing response rates that are often higher and durations of response that are often longer than those with chemotherapy. Escalated doses of these agents can be supported with stem cell transplantation and can produce high rates of complete response and greater survival in patients with relapsed NHL. The quality and duration of responses are greater with radioimmunotherapy when it is used earlier in the course of treatment.

Overview of dosimetry for systemic targeted radionuclide therapy (STaRT)

The purposes of systemic targeted radionuclide therapy dosimetry include compiling a database of normal organ radiation-absorbed doses that are carrier- and radionuclide-specific, and assuring that the normal organ radiation doses are within a safe range before therapy. Also of importance is quantitation of radiation delivery to tumors vs. normal tissues to correlate absorbed dose with tumor control. For agents with significant and variable excretion, estimates of individual patient distribution/clearance may be needed to optimize the dose-response relationship.

153Sm and 166Ho complexes with tetraaza macrocycles containing pyridine and methylcarboxylate or methylphosphonate pendant arms

A set of tetraaza macrocycles containing pyridine and methylcarboxylate (ac(3)py14) or methylphosphonate (MeP(2)py14 and P(3)py14) pendant arms were prepared and their stability constants with La(3+), Sm(3+), Gd(3+) and Ho(3+) determined by potentiometry at 25 degrees C and 0.10 M ionic strength in NMe(4)NO(3). The metal:ligand ratio for (153)Sm and (166)Ho and for ac(3)py14, MeP(2)py14 and P(3)py14, as well as the pH of the reaction mixtures, were optimized to achieve a chelation efficiency higher than 98%. These radiocomplexes are hydrophilic and have a significant plasmatic protein binding. In vitro stability was studied in physiological solutions and in human serum. All complexes are stable in saline and PBS, but 20% of radiochemical impurities were detected after 24 h of incubation in serum. Biodistribution studies in mice indicated a slow rate of clearance from blood and muscle, a high and rapid liver uptake and a very slow rate of total radioactivity excretion. Some bone uptake was observed for complexes with MeP(2)py14 and P(3)py14, which was enhanced with time and the number of methylphosphonate groups. This biological profile supports the in vitro instability found in serum and is consistent with the thermodynamic stability constants found for these complexes.

Radionuclide production for therapeutic radiopharmaceuticals

A fundamental task within the framework of a project searching for new radiopharmaceuticals for systemic therapy was the evaluation of the capabilities of the Portuguese Research Reactor (RPI) for the production of several important radionuclides. The feasibility of producing 64Cu, 77As, 153Sm, 165Dy, 166Ho, 170Tm, 177Lu, 186Re, 199Au and 111Ag in useful quantities was evaluated for the present RPI operation schedule (12 h cycles) and for continuous operation. The main evaluation criteria are expressed in terms of specific activity for continuous irradiation and/or 12 h cycle and the use of natural or enriched targets if necessary. Selected samples were irradiated and a comparison between measured activities and values calculated according to the irradiation schedule and using the same software was performed.

Electrophilic chelating agents for irreversible binding of metal chelates to engineered antibodies

Radiolabeled monoclonal antibodies are widely used in the detection and treatment of cancer. However, several problems still prevent full clinical exploitation of these reagents. Low tumor/background ratios in radioimmunoscintigraphy and high background radioactivity in therapy are the foremost among these. The strategy of pretargeting which separates the tumor-targeting step from radiolocalization step may overcome these limitations. One pretargeting approach, based on the streptavidin-biotin system, has been demonstrated to successfully treat cancer in preclinical models (Proc. Natl. Acad. Sci. 97 (2000) 1802). In this report we describe the synthesis of several electrophilic chelates, designed for use in vivo. In this new pretargeting approach, we have used protein engineering to prepare an antibody that can bind selectively and irreversibly to certain of these metal chelates. This improves upon approaches based on the immunogenic protein streptavidin and the endogenous ligand biotin.