Abstract 5552: Lack of protective effect by hypoxia on alpha particle radiation-induced cytotoxicity from radium-223, Alpharadin

Abstract

Abstract A central theme in radiation therapy of cancer is the protective effect induced by reduced oxygen tension to radiation-induced cytotoxicity. However, cytotoxicity induced by alpha particle radiation has been described as not responding to classical oxygen enhancement. Here we show that the new alpha particle cancer therapeutic drug Alpharadin (radium-223 chloride) induces similar levels of cytotoxicity in vitro under reduced oxygen conditions (4% O2) as in an oxygenated (20% O2) atmosphere. Alpharadin, radium-223, is in clinical phase 3 testing for activity against bone metastases from prostate cancer. The drug was provided by Algeta ASA (Oslo, Norway) at a radioactive concentration of 1000 kBq/ml. NHIK 3025 cells, a cervical carcinoma cell line, were grown as monolayers in a standard CO2 incubator holding 5% CO2 and atmospheric oxygen (approximately 20% O2). Additionally, cells were also grown for >6 months under continuous hypoxic conditions in an HA 35 hypoxia workstation at 4% O2 and 5% CO2. Oxygen concentrations are given in the gas phase. Treatment with Alpharadin (radium-223) was carried out by diluting the drug in cell culture medium to the desired radioactive concentration (kBq/ml). Treatment of single cells at various dose-rates was carried out either in 20% or 4% oxygen concentrations. All experiments were performed at least three separate times. For treatment under hypoxic conditions, all procedures including preparation of radium-containing medium, cell trypsinization, and exposure to radium-223 were carried out under 4% O2. Following treatment, cytotoxicity was determined by counting colonies of over 40 cells formed from surviving cells cultured under 20% O2. The radioactive dose to medium was recalculated to give the cellular dose. The data show that the cell survival curve of cells treated at atmospheric oxygen (20% O2) had a form described by the Linear-Quadratic Model (S = eαD + ßD2) with and α-value of 3.8 and a ß-value of 0.4, essentially describing a linear relationship between cellular dose of radioactivity and cell survival when survival is plotted on a log scale. When cells that were established and continually cultured under 4% O2 (i.e. a chronic state of hypoxia) were treated with radium-223, the number of surviving cells was similar to that found when cells were treated under atmospheric oxygen. There is apparently no protective effect of reduced oxygen concentration on radium-223-induced cytotoxicity. These data can be interpreted to indicate that Alpharadin (radium-223) induces similar cytotoxic effects irrespective of oxygen concentration. Combined with the bone-targeting potential of Alpharadin, these data support the use of radium-223 in clinical settings where tumors may have areas of reduced oxygen concentration. This work has been funded with the support from METOXIA project no.222741 under the 7th Research Framework Programme of the European Union. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5552.