Clinical trials of alpha-particle emitters have demonstrated the expected hallmarks of targeted alpha-particle emitter therapy — anti-tumor efficacy with minimal toxicity. The 213Bi Phase I/II trials against acute myeloid leukemia (AML) demonstrated complete responses in patients whose tumor burden had been previously reduced by cytarabine. The responses in this very high risk population lasted up to 12 months. Myelosuppression was tolerable and no significant extramedullary toxicity was observed [33] and [34]. In addition to this, there are also on-going trials in Europe and Australia. These trials are investigating targeted 213Bi against lymphoma, progressive glioma, and melanoma [35], [36], [37] and [38]. Median survival in recurrent malignant brain cancer patients following administration of 211At-labeled anti-tenascin antibody into the surgically created tumor resection cavity was increased from the historically expected 25 to 30 weeks to 54 weeks [39]. As of the last review of these data in 2004, two patients with recurrent glioblastoma were alive 151 and 153 weeks after 211At-labeled chimeric 81C6 therapy [40]. Clinical investigations in humans, using Ra-223 for therapy of painful skeletal metastases in prostate and breast cancer patients, showed a strong and consistent reduction in alkaline phosphatase levels [41] and [42]. In a large fraction of prostate cancer patients, this was accompanied by reduced prostate-specific antigen relative to baseline. Myelosuppression was minimal and thrombocytopenia was not dose-limiting.
The alpha-emitting radionuclide, 225Ac has a decay scheme that includes 3 alpha-particle emitting daughters. The last alpha-emitting daughter in the series is 213Bi. The cytotoxicity of this in vivo isotope generator or “nanogenerator” is 1000 times more potent than 213Bi, in vitro, and has demonstrated remarkable efficacy in pre-clinical studies [43]. In a first-in-human phase I dose escalation study of this nanogenerator, AML patients treated with a single infusion of 23 to 170 µCi (0.5 to 2 µCi/kg) have demonstrated dose-related reduction in peripheral blood and bone marrow blasts with no acute or delayed toxicity at 10 month follow-up [44]. Accrual to this trial continues.