Introduction
Beta particles (electrons) emitted by radioisotopes are known to efficiently kill cancer cells. This finding has already been clinically exploited by using 131I to treat thyroid cancer [1], a strategy still employed successfully in more than 50% of such patients in the United States, with over a 90% cure rate. Similarly, beta particle-emitting radiolabeled antibodies directed against CD20, including 131I-Bexxar (tositumomab) and 90Y-Zevalin (ibritumomab tiuxetan), have been used against non-Hodgkin’s lymphoma [2,3]. Moreover, 90Y-labeled somatostatin receptor ligand is utilized to treat neuroendocrine tumors [4]. Electrons emitted by 32P have an energy level intermediate between those of 131I and the more powerful 90Y, resulting in a path length of up to 5 mm in human tissues [5]. Electrons emitted from radioisotopes can strike thousands of cells. The resulting bystander effect amplifies the lethal potential of each beta particle emitted in or near a tumor. However, as we shall show below, we have discovered that among all available beta-emitting isotopes, 32P possesses a unique chemically and radiologically-based double-strand breakage mechanism, which confers greater anti-tumor efficacy than other beta-emitters of comparable power.
Human cancer-derived cell lines established in immunocompromised mice are a valuable tool for testing the effectiveness of candidate anti-cancer agents [6-9]. We previously found that a single, low-dose intravenous injection of [32P]ATP significantly inhibits tumor growth for several weeks in murine xenograft models [10,11]. Because ATP is a small naturally-occurring molecule, its radiolabeled form poses some advantages over larger synthetic compounds as a potential anti-cancer therapeutic, including lower immunogenicity, greater tumor penetration, and superior pharmacokinetics [12]. Inorganic [32P]PO4, a simple aqueous ion, has been used for decades as a therapeutic agent for polycythemia vera and essential thrombocythemia [13]. This ion was also previously used for palliation of bone pain due to metastases, where it was thought to be incorporated into the extracellular matrix [14]. However, aqueous 32P use has never been established as a primary anti-cancer strategy per se.
The clinical application of 32P was first attempted in the 1930’s [15-18]. Since that time, 32P usage has generally been restricted to a colloidal suspension form, wherein 32P forms a component of a complex, insoluble particle [19-22]. This form of 32P is typically injected directly into the tumor, with the colloidal suspension preventing the radioisotope from leaving the intended target and disseminating throughout the body. The administration of aqueous 32P as a primary anti-cancer agent has not been studied, aside from its palliative use for relief of pain due to bone metastases.
Recent experimental findings have led to the development and use of the alpha- and beta-emitter 223Ra to selectively target bone metastases in patients with castration-resistant prostate cancer [23,24]. Originally developed by a Norwegian company Algeta, Alpharadin was approved for use in the United States in 2013, and is now marketed by Bayer under the name Xofigo [25]. Thus, 223Ra is the latest simple radioactive element to become an effective anti-cancer drug.
We now report that a single, low-dose intravenous injection of aqueous 32P results in rapid, significant growth inhibition of pre-established tumor growth in an immunocompetent (syngeneic) murine model. We also show that 32P is more efficient than equivalent doses of higher-energy extracellular electrons, such as those emitted by 90Y, a beta-emitting radioisotope in common use today. We provide evidence that this higher efficiency results from the direct incorporation of 32P into nascent DNA, causing double-strand DNA breakage via a combined chemical-radiological mechanism that cannot duplicated by other beta-emitting radioisotopes, such as 131I and 90Y. This finding has immediate ramifications for the expanded treatment of primary human cancers.
