Alpha-Emitters and Targeted Alpha Therapy in Oncology: from Basic Science to Clinical Investigations.

Alpha-emitters are radionuclides that decay through the emission of high linear energy transfer α-particles and possess favorable pharmacologic profiles for cancer treatment. When coupled with monoclonal antibodies, peptides, small molecules, or nanoparticles, the excellent cytotoxic capability of α-particle emissions has generated a strong interest in exploring targeted α-therapy in the pre-clinical setting and more recently in clinical trials in oncology. Multiple obstacles have been overcome by researchers and clinicians to accelerate the development of targeted α-therapies, especially with the recent improvement in isotope production and purification, but also with the development of innovative strategies for optimized targeting. Numerous studies have demonstrated the in vitro and in vivo efficacy of the targeted α-therapy. Radium-223 (223Ra) dichloride (Xofigo®) is the first α-emitter to have received FDA approval for the treatment of prostate cancer with metastatic bone lesions. There is a significant increase in the number of clinical trials in oncology using several radionuclides such as Actinium-225 (225Ac), Bismuth-213 (213Bi), Lead-212 (212Pb), Astatine (211At) or Radium-223 (223Ra) assessing their safety and preliminary activity. This review will cover their therapeutic application as well as summarize the investigations that provide the foundation for further clinical development.

Improved safety and efficacy of 213Bi-DOTATATE-targeted alpha therapy of somatostatin receptor-expressing neuroendocrine tumors in mice pre-treated with L-lysine.

BACKGROUND: Targeted alpha therapy (TAT) offers advantages over current ß-emitting conjugates for peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors. PRRT with 177Lu-DOTATATE or 90Y-DOTATOC has shown dose-limiting nephrotoxicity due to radiopeptide retention in the proximal tubules. Pharmacological protection can reduce renal uptake of radiopeptides, e.g., positively charged amino acids, to saturate in the proximal tubules, thereby enabling higher radioactivity to be safely administered. The aim of this preclinical study was to evaluate the therapeutic effect of 213Bi-DOTATATE with and without renal protection using L-lysine in mice. Tumor uptake and kinetics as a function of injected mass of peptide (range 0.03-3 nmol) were investigated using 111In-DOTATATE. These results allowed estimation of the mean radiation absorbed tumor dose for 213Bi-DOTATATE. Pharmacokinetics and dosimetry of 213Bi-DOTATATE was determined in mice, in combination with renal protection. A dose escalation study with 213Bi-DOTATATE was performed to determine the maximum tolerated dose (MTD) with and without pre-administration of L-lysine as for renal protection. Neutrophil gelatinase-associated lipocalin (NGAL) served as renal biomarker to determine kidney injury. RESULTS: The maximum mean radiation absorbed tumor dose occurred at 0.03 nmol and the minimum at 3 nmol. Similar mean radiation absorbed tumor doses were determined for 0.1 and 0.3 nmol with a mean radiation absorbed dose of approximately 0.5 Gy/MBq 213Bi-DOTATATE. The optimal mass of injected peptide was found to be 0.3 nmol. Tumor uptake was similar for 111In-DOTATATE and 213Bi-DOTATATE at 0.3 nmol peptide. Lysine reduced the renal uptake of 213Bi-DOTATATE by 50% with no effect on the tumor uptake. The MTD was <13.0 ± 1.6 MBq in absence of L-lysine and 21.7 ± 1.9 MBq with L-lysine renal protection, both imparting an LD50 mean renal radiation absorbed dose of 20 Gy. A correlation was found between the amount of injected radioactivity and NGAL levels. CONCLUSIONS: The therapeutic potential of 213Bi-DOTATATE was illustrated by significantly decreased tumor burden and improved overall survival. Renal protection with L-lysine immediately prior to TAT with 213Bi-DOTATATE prolonged survival providing substantial evidence for pharmacological nephron blockade to mitigate nephrotoxicity.

Influence of charge on cell permeability and tumor imaging of GPR30-targeted 111in-labeled nonsteroidal imaging agents.

Recent clinical studies implicate the role of G protein-coupled estrogen receptor, GPR30, in aggressive forms of breast, ovarian, and endometrial cancers. However, the functional role of GPR30 at cellular and molecular levels remains less clear and controversial, particularly its subcellular location. The primary objective of this study was to develop radiolabeled neutral and charged GPR30-targeted nonsteroidal analogues to understand the influence of ligand charge on cell binding, cellular permeability, and in vivo tumor imaging. Therefore, we developed a series of GPR30-targeted (111/113)In(III)-labeled analogues using macrocyclic and acyclic polyamino-polycarboxylate chelate designs that would render either a net negative or neutral charge. In vitro biological evaluations were performed to determine the role of negatively charged analogues on receptor binding and activation using calcium mobilization and phosphoinositide 3-kinase assays. In vivo evaluations were performed on GPR30-expressing human endometrial Hec50 tumor-bearing mice to characterize the biodistribution and potential application of GPR30-targeted imaging agents for translational research. In vitro functional assays revealed an effect of charge, such that only the neutral analogue activated GPR30-mediated rapid signaling pathways. These observations are consistent with expectations for initial rates of membrane permeability and suggest an intracellular rather than the cell surface location of functional receptor. In vivo studies revealed receptor-mediated uptake of the radiotracer in target organs and tumors; however, further structural modifications will be required for the development of future generations of GPR30-targeted imaging agents with enhanced metabolic properties and decreased nonspecific localization to the intestines.

Enhancement of somatostatin-receptor-targeted (177)Lu-[DOTA(0)-Tyr(3)]-octreotide therapy by gemcitabine pretreatment-mediated receptor uptake, up-regulation and cell cycle modulation.

INTRODUCTION: Clinical studies of patients treated with somatostatin-receptor (sstr)-targeted [DOTA(0)-Tyr(3)]-octreotide (DOTATOC) labeled with (177)Lu and (90)Y have shown overall response rates in the range of 9-33%. This study evaluates the potential for combination therapy with gemcitabine in an effort to improve clinical outcomes. METHODS: Human pancreatic adenocarcinoma Capan-2, rat pancreatic cancer AR42J and human small cell lung cancer NCI-H69 cells were each treated with 1 microg/ml gemcitabine for 4 days followed by replacement of the medium alone for four additional days. Cell cycle and direct receptor-uptake studies were performed with (177)Lu-DOTATOC after the total 8-day treatment as described. Cell viability and apoptosis experiments were performed to study the effects of gemcitabine pretreatment and (177)Lu-DOTATOC radionuclide therapy. Parallel control studies were performed with receptor-non-targeted (177)Lu-DOTA and DOTATOC. RESULTS: Cells treated with gemcitabine for 4 days showed a down-regulation of sstr expression as determined by (177)Lu-DOTATOC uptake. However, after 4 days of additional growth in absence of gemcitabine, the uptake of (177)Lu-DOTATOC was 1.5-3 times greater than that of the untreated control cells. In gemcitabine-pretreated Capan-2 cells, 84% of the cell population was in the G(2)M phase of the cell cycle. Due to sstr up-regulation and cell cycle modulations, synergistic effects of gemcitabine pretreatment were observed in cell viability and apoptosis assays. (177)Lu-DOTATOC resulted in two to three times greater apoptosis in gemcitabine-pretreated Capan-2 cells compared to the untreated cells. CONCLUSION: Gemcitabine pretreatment up-regulates sstr expression and acts as a radiosensitizer through cell cycle modulation. The rational combination of gemcitabine and sstr-targeted radiopharmaceuticals represents a promising chemoradiation therapeutic tool with great potential to improve clinical outcomes and, thus, merits further study.

Somatostatin-receptor-targeted alpha-emitting 213Bi is therapeutically more effective than beta(-)-emitting 177Lu in human pancreatic adenocarcinoma cells.

INTRODUCTION: Advance clinical cancer therapy studies of patients treated with somatostatin receptor (sstr)-targeted [DOTA(0)-Tyr(3)]octreotide (DOTATOC) labeled with low-linear-energy-transfer (LET) beta(-)-emitters have shown overall response rates in the range of 15-33%. In order to improve outcomes, we sought to compare the therapeutic effectiveness of sstr-targeted high-LET alpha-emitting (213)Bi to that of low-LET beta(-)-emitting (177)Lu by determining relative biological effectiveness (RBE) using the external gamma-beam of (137)Cs as reference radiation. METHODS: Sstr-expressing human pancreatic adenocarcinoma Capan-2 cells and A549 control cells were used for this study. The effects of different radiation doses of (213)Bi and (177)Lu labeled to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid and sstr-targeted DOTATOC were investigated with a clonogenic cell survival assay. Apoptosis was measured using the Cell Death Detection ELISA(PLUS) 10x kit. RESULTS: Using equimolar DOTATOC treatment with concurrent irradiation with a (137)Cs source as reference radiation, the calculated RBE of [(213)Bi]DOTATOC was 3.4, as compared to 1.0 for [(177)Lu]DOTATOC. As measured in terms of absorbance units, [(213)Bi]DOTATOC caused a 2.3-fold-greater release of apoptosis-specific mononucleosomes and oligonucleosomes than [(177)Lu]DOTATOC at the final treatment time of 96 h (P<.001) in sstr-expressing Capan-2 cells. CONCLUSIONS: In conclusion, at the same absorbed dose, [(213)Bi]DOTATOC is therapeutically more effective in decreasing survival than is [(177)Lu]DOTATOC in human pancreatic adenocarcinoma cells due to its comparatively higher RBE.