Abstract 3576: Pixantrone is chemically and mechanistically related to mitoxantrone (Mito) and doxorubicin (Dox) but lacks their iron binding properties and myocyte toxicity in vitro

The anthracyclines (Dox), anthracenediones (Mito) and anthrapyrazoles (losoxantrone) are all intercalating topoisomerase II inhibitors that have significant anti-tumor activity but have the potential to cause severe, treatment limiting cardiotoxicity. Each of these drugs contains at least one hydroxyquinone element capable of binding iron. Given the putative role of iron in free radical generation and drug induced cardiotoxicity, we synthesized Pixantrone (BBR 2778; Pix), an aza-anthracenedione heterocyclic compound devoid of hydroxyquinone moieties. Pix has recently successfully completed a Phase 3 clinical trial showing excellent anti-tumor activity. Results obtained in animal models demonstrated that Pix caused minimal cardiotoxicity in mice, even those pretreated with Dox. To determine whether Pix binds iron, we used a well established spectrophotometric method to assess drug: iron interactions. Adding increasing amounts of iron to a drug solution we noted the previously reported hypochromic and bathychromic shifts in drug absorption patterns for both Dox and Mito in a 3:1 and 2:1 drug: iron ratios, respectively. In contrast, no iron binding was noted when iron was added to Pix solution at iron: drug ratios ranging from 0.1 to 10.0. To assess the effect of pH on drug binding Pix was incubated with iron at pH of 5, 6, 7 and 8 and at no pH was iron binding detected. Further studies assessing the effect of incubation temperature (4 0 C - 50 0 C) and time of incubation (1 to120 min) demonstrated the lack of iron binding by Pix. Studies performed with the rat myocardial cell line H9C2 indicated that Pix (ID 50 >50 μg/ml) was far less toxic to confluent myocardial cells than doxorubicin (ID 50 ∼1μg/ml). Further, Pix did not induce a significant increase in reactive oxygen species (ROS) compared to vehicle treated controls. In summary, our results demonstrate that the lack of iron binding and concomitant lack of SOD production may provide the mechanism for lack of cardiotoxicity in animals and an apparent decrease in cardiotoxic potential in humans for Pix. Our results also substantiate the idea that ROS are not necessary for tumor cell cytotoxicity. 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 3576.