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Current Radiopharmaceuticals, 2012, 5, 221-227 221 Bismuth-213 and Actinium-225 – Generator Performance and Evolving Therapeutic Applications of Two Generator-Derived Alpha-Emitting Radioisotopes Alfred Morgenstern*, Frank Bruchertseifer and Christos Apostolidis European Commission, Joint Research Centre, Institute for Transuranium Elements, PO Box 2340, 76125 Karlsruhe, Germany Abstract: The alpha emitters 225Ac and 213Bi are promising therapeutic radionuclides for application in targeted alpha therapy of cancer and infectious diseases. Both alpha emitters are available with high specific activity from established radionuclide generators. Their favourable chemical and physical properties have led to the conduction of a large number of preclinical studies and several clinical trials, demonstrating the feasibility, safety and therapeutic efficacy of targeted alpha therapy with 225Ac and 213Bi. This review describes methods for the production of 225Ac and 213Bi and gives an overview of 225Ac/213Bi radionuclide generator systems. Selected preclinical studies are highlighted and the current clinical experience with 225Ac and 213Bi is summarized. Keywords: Targeted alpha therapy, Alpha emitter, Actinium-225, Bismuth-213, Preclinical, Clinical studies. INTRODUCTION Alpha emitting radionuclides can kill targeted cells effectively and selectively through emission of highly energetic, short-range alpha particles. Due to the short range (<100 m) and high linear energy transfer (LET 100 keV/m) of alpha particles in human tissue, targeted alpha therapy (TAT) can deliver a highly cytotoxic dose to targeted cells while minimizing damage to surrounding healthy tissue. Cell death induced by alpha radiation is predominantly due to DNA double strand breaks occurring along the densely ionising particle trajectory and is largely independent of cell cycle phase and cell oxygenation status [1-4]. As alpha radiation is able to overcome resistance to chemotherapeutic drugs, betaand gamma radiation [5], targeted alpha therapy can offer an alternative therapy option for patients refractory to standard therapies. Among the few alpha emitters suitable for application in cancer therapy, the generator derived radionuclide pair 225Ac and 213Bi has emerged as particularly promising. The relatively long lived mother nuclide 225Ac (T1/2 = 10 days) can be applied as therapeutic nuclide or as source for production of its short-lived daughter nuclide 213Bi (T1/2 = 46 minutes). Both nuclides are available as no-carrier-added / high specific activity radionuclide, independently from local reactor or cyclotron production capabilities. While 225Ac is produced at centralised manufacturing sites, short-lived 213Bi is available in clinical settings from 225Ac / 213Bi radionuclide generator systems and is conveniently produced in house immediately before application. The favourable chemical properties of the trivalent metal ions Ac(III) and Bi(III) allow the stable linking to biomolecules using the established chelate molecules DOTA (1,4,7,10tetraazacyclododecane-1,4,7,10-tetraacetic acid) and DTPA *Address correspondence to this author at the European Commission, Joint Research Centre, Institute for Transuranium Elements, PO Box 2340, 76125 Karlsruhe, Germany; Tel: +49(0) 7247 95199618; Fax: +49(0) 7247 951248; E-mail: [email protected] 1874-47/12 $58.00+.00 (diethylene triamine pentaacetic acid). These advantageous characteristics of 225Ac and 213Bi have led to the generation of a wide body of preclinical data [6] that could be translated into several pioneering clinical trials. To date clinical experience with 213Bi has been established in clinical trials of leukemia [7,8], lymphoma [9], malignant melanoma [10-12], glioma [13,14] and neuroendocrine tumors [15], while clinical data on therapy with 225Ac are currently limited to a phase I study on leukemia [16]. DECAY CHARACTERISTICS OF 225Ac AND 213Bi 225 Ac is a pure alpha emitter with a half-life of 10 days. It decays via a cascade of six relatively short-lived radionuclide daughters to long-lived 209Bi (T1/2 = 1.9x1019 y) (Fig. (1)). The predominant decay path of 225Ac yields net 4 alpha particles with a large cumulative energy of 28 MeV and 2 beta disintegrations of 1.6 and 0.6 MeV maximum energy [17]. Gamma emissions useful for in vivo imaging are generated in the 225Ac decay path from disintegration of 221Fr (218 keV, 11.6% emission probability) and 213Bi (440 keV, 26.1% emission probability). Its relatively long half-life of 10 days and the multiple alpha particles generated in the rapid decay chain render 225Ac a particularly cytotoxic radionuclide. Its short-lived daughter nuclide 213Bi is a mixed alpha / beta emitter with a half-life of 46 min. It mainly decays via beta- emission to the ultra short-lived, pure alpha emitter 213 Po (T1/2 = 4.2 s, E = 8.4 MeV) with a branching ratio of 97.8% (Fig. (1)). The remaining 2.2 % of 213Bi decays lead to 209Tl via alpha particle emission (E = 5.5 MeV, 0.16%, E = 5.9 MeV, 2.01 %). Both 213Po and 209Tl finally decay via 209Pb (T1/2 = 3.25 h, beta-) into long-lived 209Bi. The 8.4 MeV alpha particle emitted by 213Po has a path length of 76 m in human tissue. It is contributing more than 98% of the alpha particle energy emitted per disintegration of 213Bi and can therefore be considered as mainly responsible for its cytotoxic effects. With 92.7 % the majority of the total parti© 2012 Bentham Science Publishers 222 Current Radiopharmaceuticals, 2012, Vol. 5, No. 3 Morgenstern et al. 233U : 1.6.105 y 229Th : 7340 y 225Ra : 14.9 d 225Ac : 10 d 5.8 MeV 221Fr : 4.9 m 6.3 MeV 217At : 46 min, 98% 1.4 MeV : 32 ms 7.1 MeV 213Bi 213Po : 4.2 s 8.4 MeV 209Pb : 2%, 46 min 5.9 MeV : 2.2 m 1.8 MeV 209Tl : 3.3 h 0.6 MeV 209Bi : 1.9x1019 y Fig. (1). Decay chain of 233Uranium (Decay data are taken from [17]). cle energy emitted per disintegration of 213Bi originates from alpha decay, while only 7.3 % of decay energy is contributed by beta particle emission, including the decay of 209Pb [1]. As mentioned above, the decay of 213Bi is accompanied by the emission of a 440 keV photon (emission probability of 26.1%) that allows to monitor 213Bi-biodistribution and to conduct pharmacokinetic and dosimetric studies using gamma cameras equipped with commercially available high energy collimators. PRODUCTION OF 225Ac Preclinical and clinical studies of targeted alpha therapy with 225Ac or 213Bi conducted to date have been using 225Ac / 213 Bi produced via radiochemical extraction from 229Th (T1/2 = 7340 y) sources available from the decay of 233U. 229 Th/225Ac generator sources of clinically relevant activities are currently available at the Institute for Transuranium Elements (ITU) in Karlsruhe, Germany [18,19], Oak Ridge National Laboratory (ORNL), USA [20] and at the Institute of Physics and Power Engineering (IPPE) in Obninsk [21]. These 229Th/225Ac sources have been obtained by separation from aged, fissile 233U originally produced by neutron irradiation of natural 232Th. The 225Ac product obtained from 229 Th is carrier-free with a specific activity of 2.1x1015 Bq/g (5.8x104 Ci/g). The methods for production of 225Ac from 229 Th/225Ac generator sources used at ITU and ORNL have been described in detail in [18-20], however, we are not aware of literature data describing the production process used at IPPE. The process developed at ITU for the separation of 225Ac from 229Th is a two-step process combining ion exchange and extraction chromatography [18,19]. At regular time intervals, typically every 8 weeks, 225Ra and 225Ac are separated from 229Th / 232Th using anion exchange in nitric acid media. In the second step of the separation process, 225Ac is separated from 225Ra and residual traces of 229Th / 232Th by extraction chromatography using a combination of UTEVA and TEHDGA resin. The 225Ra fraction is stored for subse- quent extractions of 225Ac in 2-3 week intervals. The resulting 225Ac product is supplied by ITU with a radiochemical purity of > 99.98% (< 2 x 10-5 of 225Ra and < 9 x 10-5 of 233U / 229Th relative to the activity of 225Ac) and is available as dried actinium nitrate, actinium chloride, or loaded on a 225 Ac / 213Bi radionuclide generator. The total annual production of 225Ac at ITU is approximately 13 GBq (350 mCi) with a maximum activity available in a single batch of 1.3 GBq (35 mCi). ORNL is using a four-step process combining anion and cation exchange chromatography [20]. 225Ra and 225Ac are separated from thorium (228Th / 229Th / 232Th) by repeated anion exchange in 8 M HNO3, followed by separation of 225Ac from 225Ra via two sequential cation exchange steps. The radiochemical purity of the 225Ac product supplied by ORNL is > 99.99% with a 225Ra content of < 3 x 10-3 % and a 229Th content of < 2 x 10-3 %. Over an 8-week campaign, ORNL is producing 3.7 GBq (100 mCi) 225Ac in 5-6 batches ( 80 % of the theoretical yield), with the largest batch typically consisting of 1.85 GBq (50 mCi). IPPE is reporting production capacities of 2.2 GBq (60 mCi) per month [21], the radiochemical purity of the 225Ac product is > 99.9% (< 0.02 % 225Ra, < 0.02% 224Ra, < 0.007% 229Th), the sum of non-radioactive cations is stated as < 10 g/ml. Overall the current worldwide production of 225Ac from Th/225Ac generators amounts to approximately 63 GBq (1.7 Ci) per year. This level of production is sufficient to conduct preclinical studies and a limited number of clinical trials. However, for a widespread clinical application of 225 Ac and 213Bi their availability needs to be increased significantly. In this respect, a number of alternative production routes have been investigated, including the irradiation of 226 Ra targets using neutrons, protons, deuterons or gammarays [22-26] and the irradiation of 232Th targets with high energy protons [27-29]. Among these routes, the production of 225Ac by proton irradiation of 226Ra targets in a cyclotron through the reaction 226Ra(p,2n)225Ac seems most promising for cost-effective, large scale production [23]. The production can be performed in medium-sized cyclotrons at proton energies below 30 MeV with thick target yields of 18 229 Bismuth-213 and Actinium-225 – Generator Performance 100 Current Radiopharmaceuticals, 2012, Vol. 5, No. 3 223 225Ac Activity (%) . 80 60 213Bi 40 20 0 0 1 2 3 4 5 Time (h) Fig. (2). Ingrowth of 213Bi Activity (squares) from Decay of 225Ac (circles) Following Generator Elution. Two hours after Elution of the Generator 83% of the Maximum Activity of 213Bi have grown back in, while after three hours 93% are available. Polypropylene Fittings Perfluoroalkoxy Tubing AG-MP 50 Resin Inlet Outlet Silicone Tubing Frit Quartz Wool Frit Fig. (3). Schematics of the ITU Standard 225Ac/213Bi Radionuclide Generator. MBq/Ah at 25 MeV. In other words, a single irradiation of a thick target of 226Ra for 20 h at a moderate proton current of 50 A would result in the production of 18 GBq (486 mCi) 225Ac, corresponding to approximately one third of its current annual world-wide production from 229Th/225Ac generators. The 225Ac product obtained from proton irradiation of 226Ra is essentially carrier-free and suitable for synthesis of high specific targeting agents. Although the handling of cyclotron targets containing 226Ra is technically demanding, the production of 225Ac via cyclotron irradiation of 226Ra can be expected to provide the amounts of 225Ac required for widespread application in the mid-term future. PRODUCTION OF 213Bi FROM NUCLIDE GENERATORS 225 Ac / 213 Bi RADIO- Short-lived 213Bi can be obtained from radionuclide generators loaded with its longer lived mother nuclide 225Ac. Due to the 10 day half life of 225Ac the useful life time of 225 Ac/213Bi generators is several weeks. The generator derived 213Bi product is of high specific activity, containing variable, but low amounts of long-lived 209Bi (Fig. (1)). According to its 46 min half-life, a 213Bi activity corresponding to 93% of the activity of 225Ac loaded on the generator can be eluted every 3 hours, while after 2 hours 83% are available (Fig. (2)). In clinical settings generators are typically eluted every 2-3 hours, depending on the activity required per patient dose. Several types of 225Ac/213Bi radionuclide generators have been proposed, based on the use of cation and anion exchange or extraction chromatography [22, 3038]. 225Ac / 213Bi generators based on AG MP-50 cation ex- change resin [32,33,38] are by far most widely used, have been applied for all patient studies with 213Bi to date and will be described in more detail below. The stable oxidation state for both actinium and bismuth under conditions relevant for generator operation is +3. In dilute acid both trivalent cations are efficiently sorbed to AG MP-50 cation exchange resin. As hard Lewis acid the Bi3+ cation has a strong affinity for oxygen and nitrogen donors and a strong affinity to form complexes with sulfur and halogens, in particular iodide. The strong affinity of Bi(III) for complexation with iodide is used for selective elution of 213 Bi from the cation exchange resin as anionic BiI4-/BiI52species [39]. A solution of 0.1 M HCl / 0.1 M NaI has been determined as most favorable as eluent, providing a high yield of 213Bi elution, low breakthrough of the parent nuclide and a medium suitable for subsequent labeling reactions [30,32]. The schematics of a typical 225Ac/213Bi radionuclide generator based on cation exchange are shown in Fig. (3). The standard generator produced at ITU shown here contains 0.3 ml of AG MP-50 resin in perflouroalkoxy tubing with polyproylene fittings and is equipped with silicone tubing. The distribution of 225Ac activity on AG MP-50 generator columns is determining the radiation dose absorbed by the organic cation exchange resin and has important consequences for the generator lifetime. Loading of firstgeneration generator systems with high levels of 225Ac in dilute hydrochloric acid directly onto pre-packed AGMP-50 columns resulted in the activity being deposited in a very small layer at the very top of the resin, leading to large local radiation doses delivered to the organic resin. This loading 224 Current Radiopharmaceuticals, 2012, Vol. 5, No. 3 method resulted in catastrophic failure of generators within 1-2 days of operation, due to radiolytic degradation and sintering of the resin and cracking of the plastic body of the column. The radiation dose (J/kg) to the resin can be decreased by spreading the activity over a larger mass of resin. McDevitt et al. at MSKCC have described a method to achieve a homogeneous distribution of activity over large parts of the resin by pre-loading a slurry of cation exchange resin outside the generator column. After sorption of 225Ac from dilute acid, the slurry is filled into a partially assembled generator column, containing a small aliquot of unloaded resin serving as catch plug at the outlet of the generator [32]. By spreading the activity over a larger resin mass this method significantly reduces the dose to the resin and has been shown to assure constant performance of the generators loaded with 740-1036 MBq for several weeks. Ma et al. [38] have further developed this generator principle and constructed tandem generators consisting of two serial columns loaded with 225Ac connected to a third, smaller, unloaded column acting as catch plug to trap parent breakthrough. This design allowed reliable operation of the generator with activities up to 2.6 GBq 225Ac and parent breakthrough of less than 5 ppm (activity). However, an obvious drawback of the generator loading method described by McDevitt and Ma is the need for manual and lengthy handling of large amounts of actinium activity when filling the pre-loaded slurry into the generator columns. This operation results in a significant radiation dose received by the operator and is associated with the risk of spilling, contamination and loss of activity. A much simplified method for generator loading that allows semi-automated, remote operation and greatly reduces dose to the operator has been developed at ITU [18]. 225Ac loading of prepacked AG-MP 50 generator columns from 4 M HNO3 solution has been found to result in distribution of the activity over the first two thirds of the resin and can be performed using a peristaltic pump without manual intervention. ITU standard 225Ac/213Bi radionuclide generators loaded in this manner afford 76±3 % elution yield of 213Bi in 0.6 ml of 0.1 M NaI / 0.1 M HCl (two column bed volumes) with low parent breakthrough of < 0.2 ppm [33]. To date the standard ITU generator has been used for preparation of patient doses in clinical studies of neuroendocrine tumors, glioma, malignant melanoma and lymphoma with > 90 patients. Quality Control of 213Bi Preparations 213 Bi produced from 225Ac/213Bi radionuclide generators can be used for labeling of monoclonal antibodies or peptides following established labeling protocols [14, 32]. Owing to the short half live of 213Bi, quality control of 213Bi preparations is preferably performed within few minutes to avoid significant decay losses. The radiochemical purity of 213 Bi labeled antibodies is typically assessed by ITLC within less than 5 minutes, while 213Bi labeled peptides can be rapidly analyzed by ITLC and/or Radio-HPLC. However, direct quantification of 225Ac parent breakthrough from the generator and the content of 225Ac in the final preparation is not feasible within an acceptable time frame. Due to the low ratio of 225Ac over 213Bi activity of less < 10-6 and the low emission probability of gamma rays associated with 225Ac decay of < 2%, analysis of 225Ac activity is typically performed via gamma spectrometry after quantitative decay of Morgenstern et al. eluted 213Bi, typically after 24 hours. An acceptable level of Ac activity in 213Bi preparations used for clinical application is therefore typically assured in an indirect manner, deduced from previous testing of generators of identical type. 225 Ac/213Bi generators based on AG MP-50 resin that have been used in humans studies to date allow the synthesis of 213 Bi labelled antibodies and peptides with a content of 225Ac of less than 1 ppm (activity of 225Ac over activity of 213Bi at time of injection). To date 225Ac/213Bi radionuclide generators are not available in sterile form, sterility of the 213Bi preparation is typically assured via aseptic handling, microbial monitoring and sterile filtration of the final formulation. 225 SELECTED PRECLINICAL STUDIES WITH AND 213Bi 225 Ac In the last decades a wide body of preclinical studies on targeted alpha therapy using 225Ac and 213Bi has been generated and has been summarized in e.g. [6, 40, 41]. A variety of targeting strategies has been investigated, using full antibodies, antibody fragments, low molecular weight peptides or liposomes as vehicles. The considerable difference in half life between 225Ac and 213Bi has important implications on the choice of successful targeting strategies. When using short half lived 213Bi, rapid targeting is essential and can be achieved e.g with locoregional application [42], pretargeting [43] or with fast-diffusible peptides as carrier molecules [44]. In contrast, the 10 day half life of 225Ac also allows targeting with full antibodies that may require up to several days for maximum tumor uptake. To give a detailed overview of all preclinical studies with 225Ac and 213Bi is beyond the scope of this review, therefore we have attempted to summarize a selection of more recent reports. In a series of studies Hong et al. have investigated targeted alpha therapy of breast cancer metastases in a neu-N transgenic mouse model that closely resembles the pattern of metastatic spread in breast cancer patients [45-48]. Treatment with a 213Bi-labeled anti–HER-2/neu monoclonal antibody at the maximum tolerated dose of 4.44 MBq was found to be effective in treating early-stage HER-2/neu–expressing micrometastases, however, due to the relatively slow tumor uptake of the labeled antibody, radiation doses delivered to metastases with 213Bi were limited [46]. When labeled with longer-lived 225Ac, the anti–HER-2/neu antibody radioconjugate delivered a nearly 5 fold higher dose to metastases and was consequently therapeutically more effective [47]. In a related study Lingappa et al. attempted to increase the number of 213Bi-atoms delivered to the target sites by engineering liposomal vesicles that carry a greater number of 213 Bi atoms than radiolabeled anti–HER-2/neu antibodies. It could be demonstrated that these high–specific activity 213Biconjugated immunoliposome constructs were stable in vitro and in vivo, significantly increasing the median survival over untreated mice and merit further development [48]. Pretargeting with antibody-streptavidin constructs and Bi-labeled DOTA-biotin has been investigated by Park et al. [43]. In a model of athymic mice bearing Ramos lymphoma xenografts pretargeting resulted in rapid and specific targeting of tumor sites, displayed a favorable biodistribution profile with excellent therapeutic efficacy and clearly merits further investigation. 213 Bismuth-213 and Actinium-225 – Generator Performance Current Radiopharmaceuticals, 2012, Vol. 5, No. 3 225 Table 1. Overview of Clinical Studies with 225Ac and 213Bi Conducted to Date Cancer Type Leukemia Lymphoma Melanoma Radioconjugate Phase No. of Patients Reference 213 Phase I 18 [7] 213 Bi-HuM195mAb Phase I/II 31 [8] 225 Ac-HuM195mAb Phase I 15 [16] 213 Bi-rituximab Phase I 12 [9] 213 Bi-9.2.27mAb Phase I (intralesional) 16 [10] 213 Bi-9.2.27mAb Phase I (systemic) 38 [11,12] Bi-Substance P Pilot Phase I 2 5 [13,14] Pilot 12 [15] Bi-HuM195mAb Glioma 213 Neuroendocrine Tumors 213 Bi-DOTATOC Rapid targeting of tumor sites can often also be achieved by use of low molecular weight peptides as carrier molecules. Potential advantages of small radiopeptides over monoclonal antibodies include their fast diffusion, lack of immunogenicity, and fast blood clearance. The somatostatin analogue [DOTA0,Tyr3]octreotide (DOTATOC) labeled with low linear energy transfer (LET) -emitters, such as 177Lu or 90 Y is increasingly used for peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors and has yielded impressive results on tumor response, overall survival, and quality of life in a large number of patients [49]. Norenberg et al. have evaluated DOTATOC labeled with 213Bi in an animal model of rat pancreatic carcinoma [50] and have compared its therapeutic effectiveness with 177LuDOTATOC in vitro [51]. In the animal study 213BiDOTATOC showed dose-related antitumor effects with minimal treatment related organ toxicity, no acute or chronic hematologic toxicities were observed. In vitro 213BiDOTATOC was found to be more effective in decreasing clonogenic survival at the same absorbed dose than 177LuDOTATOC in human pancreatic adenocarcinoma cells due to its comparatively higher RBE. Based on these promising findings peptide receptor alpha therapy with 213BiDOTATOC is now in clinical testing [15]. In a recent study of peptide receptor radiotherapy Wild et al. [44] compared 213Bi- vs. 177Lu-radiopeptide therapy of prostate carcinoma in vivo. Two novel 213Bi-labeled peptides, DOTA-PEG4-bombesin (DOTA-PESIN) and DO3ACH2CO-8-aminooctanoyl-Q-W-A-V-G-H-L-M-NH2 (AMBA) were evaluated in a human androgen-independent prostate carcinoma xenograft model (PC-3 tumor) in comparison with 177Lu-DOTA-PESIN. At the maximum tolerated dose, 213Bi-DOTA-PESIN and 213Bi-AMBA prolonged median survival significantly longer than 177Lu-DOTA-PESIN. The authors conclude that -therapy with 213Bi-DOTAPESIN or 213Bi-AMBA is more effective than -therapy and represents a possible new approach for treating recurrent prostate cancer. Drecoll et al. [52] investigated a 213Bi-labeled dimer of the peptide F3 (213Bi-DTPA-[F3]2) in an animal model of intraperitoneally growing xenograft tumors of peritoneal carcinomatosis. F3 is a 32 amino acid peptide that is internalized into the nucleus of tumor cells upon binding to nu- cleolin on the cell surface. 213Bi-DTPA-[F3]2 significantly increased survival time of the animals when injected 4-14 days or 16-26 days after tumor cell inoculation in a fractionated regime without signs of toxicity. A follow up study comparing the therapeutic efficacy of 213Bi-DTPA-F3 and 225 Ac-labeled DOTA-F3 in the same tumor model has demonstrated similar therapeutic efficacy of the 225Ac labelled peptide at 1000 fold lower activities with comparable mild renal toxicity [53]. CLINICAL EXPERIENCE WITH 225Ac AND 213Bi To date targeted therapy with 225Ac and 213Bi has been clinically investigated for the treatment of leukemia, lymphoma, malignant melanoma, glioma and neuroendocrine tumors. An overview of the current status of clinical trials is given in Table 1. Although the numbers of patients that have been treated with 225Ac or 213Bi are still moderate, these studies have provided evidence that targeted therapy with both alpha emitters is feasible and safe and can have anti-tumor efficacy with minimal toxicity. We have summarized 213Bi studies on leukemia, lymphoma, malignant melanoma and glioma in [6]. More recently, a pilot study of therapy of neuroendocrine tumors using 213Bi-DOTATOC has been started in collaboration between University Hospital Heidelberg, Germany, and ITU [15]. To date, twelve patients with unresectable neuroendocrine primary tumor or liver metastases, refractory to a previous treatment with 90Y-/177Lu-DOTATOC, have been treated with 213Bi-DOTATOC. The cumulative activity administered was up to 10 GBq 213Bi per patient and the activity administered during a single treatment cycle has been escalated from 1- 7.5 GBq. The activity was injected in fractions of 0.5 -1.5 GBq into an arterial catheter placed in the tumor feeding vessel. While no acute kidney, endocrine or hematologic toxicity higher than grade 0/I were observed, short term follow up demonstrated reduced tumor-perfusion (as assessed by contrast enhanced sonography) and tumor shrinkage (as assessed by MRI) in several patients. Clinical experience with 225Ac therapy is currently still limited to an ongoing phase I study on therapy of relapsed/refractory AML using the 225Ac labeled anti-CD33 antibody lintuzumab [16]. To date, 15 patients (median age, 226 Current Radiopharmaceuticals, 2012, Vol. 5, No. 3 65 yrs; range, 45-80 yrs) have been treated with total injected activities of 225Ac ranging from 851 to 14,430 kBq with antibody doses of 0.7 to 2.6 mg. While no acute toxicities were seen, dose-limiting toxicities (myelosuppression lasting > 35 days; death due to sepsis) were seen in one patient treated with 111 kBq/kg and in both patients receiving 148 kBq/kg. Extramedullary toxicities were limited to transient grade 2/3 liver function abnormalities. With a median follow-up of 5 months (range, 1-24 months), no evidence of radiation nephritis was seen. Peripheral blood blasts were eliminated in 7 of 11 evaluable patients. Bone marrow blast reductions of > 33% were seen in 6 of 10 evaluable patients at 4 weeks, including 2 patients with < 5% blasts. This first in human study shows that therapy with 225Ac-lintuzumab is feasible, tolerable at doses < 148 kBq/kg and has antileukemic activity. Morgenstern et al. [5] [6] [7] [8] [9] CONCLUSIONS AND OUTLOOK The alpha emitters 225Ac and 213Bi can be reliably produced from established radionuclide generator systems with high specific activity and high radionuclidic purity. Their availability in clinical settings is independent from local reactor or cyclotron facilities, their favourable chemical properties allow the synthesis of stable radioconjugates using established chelate molecules. Consequently, a large number of preclinical studies and several pioneering clinical trials have been conducted, showing that targeted alpha therapy with 225Ac and 213Bi is feasible, safe and may provide new therapeutic options to patients refractory to standard therapies. In particular peptide receptor alpha therapy with 225Ac and 213Bi using fast diffusible, low molecular weight peptides such as DOTATOC and Substance P as carrier molecules is promising and can be expected to play an increasing role in future clinical application. 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Revised: March 28, 2012 Accepted: April 03, 2012