213 Bi-[ DOTA 0 , Tyr 3 ] Octreotide Peptide Receptor Radionuclide Therapy of Pancreatic Tumors in a Preclinical AnimalModel

Purpose:The somatostatin analogue [DOTA,Tyr]octreotide (DOTATOC) has previously been labeled with low linear energy transfer (LET) h-emitters, such as Lu or Y, for tumor therapy. In this study, DOTATOC labeled with the high-LET a-emitter, Bi, was evaluated. Experimental Design: The radiolabeling, stability, biodistribution, toxicity, safety, and therapeutic efficacy of Bi-DOTATOC (specific activity 7.4 MBq/Ag) were investigated. Biodistribution studies to determine somatostatin receptor specificity were done in Lewis rats at 1and 3 hours postinjection. Histopathology of various organs was used to evaluated toxicity and safety. Therapeutic efficacy of 4 to 22 MBq Bi-DOTATOC was determined in a rat pancreatic carcinomamodel. Results: Radiolabeling of the Bi-DOTATOC was achieved with radiochemical purity >95% and an incorporation yieldz99.9%. Biodistribution data showed specific binding to somatostatin receptor ^ expressing tissues. Administration of free Bi, compared with Bi-DOTATOC, resulted in higher radioactivity accumulation at 3 hours postinjection in the kidneys [34.47 F 1.40% injected dose/g (ID/g) tissue versus 11.15 F 0.46%, P < 0.0001] and bone marrow (0.31 F 0.01% ID/g versus 0.06 F 0.02%, P < 0.0324). A significant decrease in tumor growth rate was observed in rats treated with >11MBq of Bi-DOTATOC 10 days postinjection compared with controls (P < 0.025). Treatment with >20 MBq of Bi-DOTATOC showed significantly greater tumor reduction when compared with animals receiving <11MBq (P < 0.02). Conclusions: Bi-DOTATOC showed dose-related antitumor effects with minimal treatmentrelated organ toxicity. No acute or chronic hematologic toxicities were observed. Mild, acute nephrotoxicity was observedwithout evidence of chronic toxicity. Bi-DOTATOC is apromising therapeutic radiopharmaceutical for further evaluation. Somatostatin is a 14-amino-acid peptide hormone found on many cells of neuroendocrine origin that acts as a neurotransmitter in the central nervous system (1). Somatostatin receptors have been shown on the surface of human tumor cells, which includes the cells with amine precursor uptake and decarboxylation properties, such as pituitary tumors, endocrine pancreatic tumors, carcinoids, paragangliomas, small-cell lung cancers, medullary thyroid carcinomas, and pheochromocytomas (2, 3). Analogues of somatostatin were developed because human somatostatin has a very short half-life in circulation (2-3 minutes) and is easily broken down by endogenous peptidases (4). These analogues preserved two important molecular features of somatostatin (i.e., its cyclic form and the four amino acids involved in the binding to the receptor). One somatostatin analogue that has been extensively studied in vitro and in vivo is octreotide, which has been used as a hormonal treatment in patients with carcinoid syndrome (5–7). The presence of somatostatin receptors has been used to detect and localize carcinoid, islet cell tumors (8), and smallcell lung cancer (9). Despite good imaging and diagnostic results with In-labeled [DTPA]octreotide (Octreoscan) in the last few years, there have been several reports describing new somatostatin radioligands for studying somatostatin receptor expression. Some, like [DOTA, Tyr]octreotide (DOTATOC) labeled with I, Y, and Lu, are also being evaluated for use in the radionuclide therapy of tumors (10). The new peptide receptor radionuclide therapy (PRRT) using radiolabeled DOTATOC Cancer Therapy: Preclinical Authors’Affiliations: College of Pharmacy and Cancer ResearchTreatment Center, University of New Mexico, Albuquerque, New Mexico; Department of Nuclear Medicine, Erasmus Medical Center, Rotterdam, the Netherlands; and Radioimmune and Inorganic Chemistry Section, Radiation Oncology Branch, National Cancer Institute, Bethesda, Maryland Received 6/13/05; revised10/6/05; accepted11/4/05. Grant support: Somatostatin Peptide Imaging and Radioimmunotherapy funded by Mallinckrodt Medical, B.V., Petten, the Netherlands; the U.S. Department of Energy grant DE-FG01-001NE23554; and the University of New Mexico General Clinical Research Centers grant supported by Department of Health and Human Services/Public Health Service/NIH/National Center for Research Resources/ General Clinical Research Center, MO1RROO997. The costs of publication of this article were defrayed in part by the payment of page charges.This article must therefore be hereby marked advertisement in accordance with18 U.S.C. Section1734 solely to indicate this fact. Requests for reprints: Jeffrey P. Norenberg, College of Pharmacy, University of New Mexico, 2502 Marble Avenue, North East MSC09 5360, 1University of New Mexico, Albuquerque, NM 87131-0001. Phone: 505-272-4322; Fax: 505-2726749; E-mail: jpnoren@unm.edu. F2006 American Association for Cancer Research. doi:10.1158/1078-0432.CCR-05-1264 www.aacrjournals.org Clin Cancer Res 2006;12(3) February1, 2006 897 Cancer Research. on October 31, 2017. © 2006 American Association for clincancerres.aacrjournals.org Downloaded from has led to tumor responses in the majority of patients but has also posed problems with renal and hematologic toxicities (10). In previous studies, kidney failures have been reported after treatment with DOTATOC labeled to the h-particle emitter Y (11–13). In previous clinical studies, it was observed that 10% to 34% patients had complete remission following Y-DOTATOC treatment (14). The results of these studies illustrate the partial treatment potentials of this agent and the possible higher relapse rates that may occur in the future (15). The primary challenges that Yor Lu-labeled DOTATOC faces are renal toxicities and incomplete treatments, especially in radioresistant tumors. One solution is to use a high linear energy transfer (LET) a-emitter. Several a-emitters have been considered and proposed for this purpose, including At that was recently evaluated for targeting somatostatin receptor– expressing D341 Med human medulloblastoma s.c. xenografts in a murine model (16). At is attractive due to its short halflife (7.2 hours), but has notable limitations due to a daughter, Bi, which has a long a half-life (38 years) and a h-emitting decay product, Pb. Further, At requires onsite cyclotron production and target processing facilities. Some years ago, Bi was proposed for a-immunotherapy (17, 18). It can be readily obtained from a Ac/Bi radionuclide generator system (19). Bi decays mainly by h-emission (98%), with a 440 keV g-emission and a half life (t1/2) of 45.6 minutes to the ultra-short-lived high-energy a-emitter Po (8.375 MeV, t1/2 = 4.2 As). Bi also has a direct decay pathway by a-emission (2%, 5.87 MeV) to the h-particle emitter Tl (3.98 MeV; ref. 20). More detailed information of the decay scheme is shown in Fig. 1. Several studies have shown the successful use of high-LET a-emitters for targeted radionuclide therapy, suggesting their superiority over low-LET h-emitters in the treatment of solid tumors (21, 22). In this study, we aim to evaluate the quantitative radiolabeling methods, stability, biodistribution, safety, and therapeutic efficacy of Bi labeled to DOTATOC in the treatment of somatostatin receptor–expressing pancreatic tumors. Materials andMethods Radionuclide The Bi used in these procedures was obtained from a Ac/Bi radionuclide generator system (NIH, National Cancer Institute, Bethesda, MD; ref. 23). Before each elution, the Ac generator column was first rinsed with distilled water and then flushed with air to remove the water. To selectively elute the Bi daughter, the column was eluted with 10 mL of 0.1 mol/L hydrochloric acid. The eluate was diluted with water at 5.6 times the eluate volume of water (56 mL). This dilution was loaded onto a MP-50 cation-exchange column. This column was then reverse eluted with an additional 0.4 mL of freshly prepared 0.1 mol/L hydroiodic acid that contained the desired Bi (23). Radiolabeling and serum stability Freshly eluted Bi (4 MBq) was added to 0.5 Ag of DOTATOC solution and incubated for 5 minutes at 100jC in a hot block. Before heating, the pH of the final solution was adjusted to 6 to 7 using 3 mol/L NH4OAc solution. The specific activity of Bi-DOTATOC was 7.4 MBq/AG for all experiments. Incorporation yield was assessed using Silica Gel instant TLC (ITLC) with 0.9% sodium chloride as the mobile phase. The radiolabeled samples were diluted with 4 mmol/L diethylenetriaminepentaacetic acid (DTPA) at pH 4.1. Five microliters of the diluted sample were spotted on an ITLC silica gel strip and allowed to develop in a chromatography chamber. Upon completion of the migration to the solvent front, the ITLC sample strips were allowed to dry, cut in half, and counted on a Wallac Wizard g-counter (Perkin-Elmer, Boston, MA) to determine the incorporation yield. Radiochemical purity was assessed via high-performance liquid chromatography (HPLC) analysis. The liquid chromatography system (Thermo Separation Products, San Jose, CA) consisted of a multisolvent delivery pump, an autosampler; a radiometric detector (g-RAM, IN/US Systems, Inc., Tampa, FL); and a C18, 5 Am, 4.6 250 mm, reverse-phase HPLC column. The mobile phase consisted of buffer A [0.5 mol/L ammonium acetate in HPLC grade water (pH 5.5)] and buffer B (100% HPLC grade methanol). The HPLC samples were analyzed with a 1:10 dilution in 4 mmol/L DTPA. The flow rate was 1.0 mL/min and the retention time for the radiolabeled product was 14.0 to 14.5 minutes. The radiolabeled product, Bi-DOTATOC, was incubated at 37jC in a CO2 incubator for 24 hours in rat serum obtained from a male Lewis rat to study in vitro stability. After incubation, the product was analyzed by the ITLC and HPLC methods previously described.