Targeted Α-particle Therapy Research Articles

Effective Treatment of SSTR2-Positive Small Cell Lung Cancer Using 211At-Containing Targeted α-Particle Therapy Agent Which Promotes Endogenous Antitumor Immune Response.

Small cell lung cancer (SCLC) is a neuroendocrine tumor with a high degree of malignancy. Due to limited treatment options, patients with SCLC have a poor prognosis. We have found, however, that intravenously administered octreotide (Oct) armed with astatine-211 ([211At]SAB-Oct) is effective against a somatostatin receptor 2 (SSTR2)-positive SCLC tumor in SCLC tumor-bearing BALB/c nude mice. In biodistribution analysis, [211At]SAB-Oct achieved the highest concentration in the SCLC tumors up to 3 h after injection as time proceeded. A single intravenous injection of [211At]SAB-Oct (370 kBq) was sufficient to suppress SSTR2-positive SCLC tumor growth in treated mice by inducing DNA double-strand breaks. Additionally, a multitreatment course (370 kBq followed by twice doses of 370 kBq for a total of 1110 kBq) inhibited the growth of the tumor compared to the untreated control group without significant off-target toxicity. Surprisingly, we found that [211At]SAB-Oct could up-regulate the expressions of calreticulin and major histocompatibility complex I (MHC-I) on the tumor cell membrane surface, suggesting that α-particle internal irradiation may activate an endogenous antitumor immune response through the regulation of immune cells in the tumor microenvironment, which could synergically enhance the efficacy of immunotherapy. We conclude that [211At]SAB-Oct is a potential new therapeutic option for SSTR2-positive SCLC.

Evaluation of Candidate Theranostics for 227Th/89Zr Paired Radioimmunotherapy of Lymphoma.

227Th is a promising radioisotope for targeted α-particle therapy. It produces 5 α-particles through its decay, with the clinically approved 223Ra as its first daughter. There is an ample supply of 227Th, allowing for clinical use; however, the chemical challenges of chelating this large tetravalent f-block cation are considerable. Using the CD20-targeting antibody ofatumumab, we evaluated chelation of 227Th4+ for α-particle-emitting and radiotheranostic applications. Methods: We compared 4 bifunctional chelators for thorium radiopharmaceutical preparation: S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (p-SCN-Bn-DOTA), 2-(4-isothicyanatobenzyl)-1,2,7,10,13-hexaazacyclooctadecane-1,4,7,10,13,16-hexaacetic acid (p-SCN-Bn-HEHA), p-isothiacyanatophenyl-1-hydroxy-2-oxopiperidine-desferrioxamine (DFOcyclo*-p-Phe-NCS), and macrocyclic 1,2-HOPO N-hydroxysuccinimide (L804-NHS). Immunoconstructs were evaluated for yield, purity, and stability invitro and invivo. Tumor targeting of the lead 227Th-labeled compound invivo was performed in CD20-expressing models and compared with a companion 89Zr-labeled PET agent. Results: 227Th-labeled ofatumumab-chelator constructs were synthesized to a radiochemical purity of more than 95%, excepting HEHA. 227Th-HEHA-ofatumumab showed moderate invitro stability. 227Th-DFOcyclo*-ofatumumab presented excellent 227Th labeling efficiency; however, high liver and spleen uptake was revealed invivo, indicative of aggregation. 227Th-DOTA-ofatumumab labeled poorly, yielding no more than 5%, with low specific activity (0.08 GBq/g) and modest long-term invitro stability (<80%). 227Th-L804-ofatumumab coordinated 227Th rapidly and efficiently at high yields, purity, and specific activity (8 GBq/g) and demonstrated extended stability. In vivo tumor targeting confirmed the utility of this chelator, and the diagnostic analog, 89Zr-L804-ofatumumab, showed organ distribution matching that of 227Th to delineate SU-DHL-6 tumors. Conclusion: Commercially available and novel chelators for 227Th showed a range of performances. The L804 chelator can be used with potent radiotheranostic capabilities for 89Zr/227Th quantitative imaging and α-particle therapy.

Signaling Network Response to α-Particle-Targeted Therapy with the 225Ac-Labeled Minigastrin Analog 225Ac-PP-F11N Reveals the Radiosensitizing Potential of Histone Deacetylase Inhibitors.

α-particle emitters have recently been explored as valuable therapeutic radionuclides. Yet, toxicity to healthy organs and cancer radioresistance limit the efficacy of targeted α-particle therapy (TAT). Identification of the radiation-activated mechanisms that drive cancer cell survival provides opportunities to develop new points for therapeutic interference to improve the efficacy and safety of TAT. Methods: Quantitative phosphoproteomics and matching proteomics followed by the bioinformatics analysis were used to identify alterations in the signaling networks in response to TAT with the 225Ac-labeled minigastrin analog 225Ac-PP-F11N (DOTA-(dGlu)6-Ala-Tyr-Gly-Trp-Nle-Asp-Phe) in A431 cells, which overexpress cholecystokinin B receptor (CCKBR). Western blot analysis and microscopy verified the activation of the selected signaling pathways. Small-molecule inhibitors were used to validate the potential of the radiosensitizing combinatory treatments both invitro and in A431/CCKBR tumor-bearing nude mice. Results: TAT-induced alterations were involved in DNA damage response, cell cycle regulation, and signal transduction, as well as RNA transcription and processing, cell morphology, and transport. Western blot analysis and microscopy confirmed increased phosphorylations of the key proteins involved in DNA damage response and carcinogenesis, including p53, p53 binding protein 1 (p53BP1), histone deacetylases (HDACs), and H2AX. Inhibition of HDAC class II, ataxia-telangiectasia mutated (ATM), and p38 kinases by TMP269, AZD1390, and SB202190, respectively, sensitized A431/CCKBR cells to 225Ac-PP-F11N. As compared with the control and monotherapies, the combination of 225Ac-PP-F11N with the HDAC inhibitor vorinostat (suberoylanilide hydroxamic acid, SAHA) significantly reduced the viability and increased the DNA damage of A431/CCKBR cells, led to the most pronounced tumor growth inhibition, and extended the mean survival of A431/CCKBR xenografted nude mice. Conclusion: Our study revealed the cellular responses to TAT and demonstrated the radiosensitizing potential of HDAC inhibitors to 225Ac-PP-F11N in CCKBR-positive tumors. This proof-of-concept study recommends development of novel radiosensitizing strategies by targeting TAT-activated and survival-promoting signaling pathways.

Therapeutic efficacy of 211At-radiolabeled 2,6-diisopropylphenyl azide in mouse models of human lung cancer.

Targeted α-particle therapy (TAT) is an attractive alternative to conventional therapy for cancer treatment. Among the available radionuclides considered for TAT, astatine-211 (211At) attached to a cancer-targeting molecule appears very promising. Previously, we demonstrated that aryl azide derivatives could react selectively with the endogenous acrolein generated by cancer cells to give a diazo compound, which subsequently forms a covalent bond with the organelle of cancer cells in vivo. Herein, we synthesized 211At-radiolabeled 2,6-diisopropylphenyl azide (ADIPA), an α-emitting molecule that can selectively target the acrolein of cancer cells, and investigated its antitumor effect. Our results demonstrate that a single intratumor or intravenous administration of this simple α-emitting molecule to the A549 (human lung cancer) cell-bearing xenograft mouse model, at a low dose (70 kBq), could suppress tumor growth without inducing adverse effects. Furthermore, because acrolein is generally overproduced by most cancer cells, we believe ADIPA is a simple TAT compound that deserves further investigation for application in animal models and humans with various cancer types and stages.

Alpha-peptide receptor radionuclide therapy using actinium-225 labeled somatostatin receptor agonists and antagonists.

Peptide receptor radionuclide therapy (PRRT) has over the last two decades emerged as a very promising approach to treat neuroendocrine tumors (NETs) with rapidly expanding clinical applications. By chelating a radiometal to a somatostatin receptor (SSTR) ligand, radiation can be delivered to cancer cells with high precision. Unlike conventional external beam radiotherapy, PRRT utilizes primarily β or α radiation derived from nuclear decay, which causes damage to cancer cells in the immediate proximity by irreversible direct or indirect ionization of the cells' DNA, which induces apoptosis. In addition, to avoid damage to surrounding normal cells, PRRT privileges the use of radionuclides that have little penetrating and more energetic (and thus more ionizing) radiations. To date, the most frequently radioisotopes are β- emitters, particularly Yttrium-90 (90Y) and Lutetium-177 (177Lu), labeled SSTR agonists. Current development of SSTR-targeting is triggering the shift from using SSTR agonists to antagonists for PRRT. Furthermore, targeted α-particle therapy (TAT), has attracted special attention for the treatment of tumors and offers an improved therapeutic option for patients resistant to conventional treatments or even beta-irradiation treatment. Due to its short range and high linear energy transfer (LET), α-particles significantly damage the targeted cancer cells while causing minimal cytotoxicity toward surrounding normal tissue. Actinium-225 (225Ac) has been developed into potent targeting drug constructs including somatostatin-receptor-based radiopharmaceuticals and is in early clinical use against multiple neuroendocrine tumor types. In this article, we give a review of preclinical and clinical applications of 225Ac-PRRT in NETs, discuss the strengths and challenges of 225Ac complexes being used in PRRT; and envision the prospect of 225Ac-PRRT as a future alternative in the treatment of NETs.

225Ac-MACROPATATE: A Novel α-Particle Peptide Receptor Radionuclide Therapy for Neuroendocrine Tumors.

Neuroendocrine tumors (NETs) express somatostatin receptors (SSTRs) 2 and 5. Modified variants of somatostatin, the cognate ligand for SSTR2 and SSTR5, are used in treatment for metastatic and locoregional disease. Peptide receptor radionuclide therapy with 177Lu-DOTATATE (DOTA-octreotate), a β-particle-emitting somatostatin derivative, has demonstrated survival benefit in patients with SSTR-positive NETs. Despite excellent results, a subset of patients has tumors that are resistant to treatment, and alternative agents are needed. Targeted α-particle therapy has been shown to kill tumors that are resistant to targeted β-particle therapy, suggesting that targeted α-particle therapy may offer a promising treatment option for patients with 177Lu-DOTATATE-resistant disease. Although DOTATATE can chelate the clinically relevant α-particle-emitting radionuclide 225Ac, the labeling reaction requires high temperatures, and the resulting radioconjugate has suboptimal stability. Methods: We designed and synthesized MACROPATATE (MACROPA-octreotate), a novel radioconjugate capable of chelating 225Ac at room temperature, and assessed its invitro and invivo performance. Results: MACROPATATE demonstrated comparable affinity to DOTATATE (dissociation constant, 21 nM) in U2-OS-SSTR2, a SSTR2-positive transfected cell line. 225Ac-MACROPATATE demonstrated superior serum stability at 37°C over time compared with 225Ac-DOTATATE. Biodistribution studies demonstrated higher tumor uptake of 225Ac-MACROPATATE than of 225Ac-DOTATATE in mice engrafted with subcutaneous H69 NETs. Therapy studies showed that 225Ac-MACROPATATE exhibits significant antitumor and survival benefit compared with saline control in mice engrafted with SSTR-positive tumors. However, the increased accumulation of 225Ac-MACROPATATE in liver and kidneys and subsequent toxicity to these organs decreased its therapeutic index compared with 225Ac-DOTATATE. Conclusion: 225Ac-MACROPATATE and 225Ac-DOTATATE exhibit favorable therapeutic efficacy in animal models. Because of elevated liver and kidney accumulation and lower administered activity for dose-limiting toxicity of 225Ac-MACROPATATE, 225Ac-DOTATATE was deemed the superior agent for targeted α-particle peptide receptor radionuclide therapy.

Effective Treatment of Human Breast Carcinoma Xenografts with Single-Dose 211At-Labeled Anti-HER2 Single-Domain Antibody Fragment.

Single-domain antibody fragments (sdAbs) are attractive for targeted α-particle therapy, particularly with 211At, because of their rapid accumulation in tumor and clearance from normal tissues. Here, we evaluate the therapeutic potential of this strategy with 5F7 and VHH_1028-2 sdAbs that bind with high affinity to domain IV of human epidermal growth factor receptor type 2 (HER2). Methods: The HER2-specific sdAbs and HER2-irrelevant VHH_2001 were labeled using N-succinimidyl-3-211At-astato-5-guanidinomethyl benzoate (iso-211At-SAGMB). The cytotoxicity of iso- 211At-SAGMB-5F7 and iso- 211At-SAGMB-VHH_2001 were compared on HER2-expressing BT474 breast carcinoma cells. Three experiments in mice with subcutaneous BT474 xenografts were performed to evaluate the therapeutic effectiveness of single doses of iso- 211At-SAGMB-5F7 (0.7-3.0 MBq), iso- 211At-SAGMB-VHH_1028 (1.0-3.0 MBq), and iso- 211At-SAGMB-VHH_1028 and iso- 211At-SAGMB-VHH_2001 (∼1.0 MBq). Results: Clonogenic survival of BT474 cells was reduced after exposure to iso- 211At-SAGMB-5F7 (D0 = 1.313 kBq/mL) whereas iso- 211At-SAGMB-VHH_2001 was ineffective. Dose-dependent tumor growth inhibition was observed with 211At-labeled HER2-specific 5F7 and VHH_1028 but not with HER2-irrelevant VHH_2001. At the 3.0-MBq dose, complete tumor regression was seen in 3 of 4 mice treated with iso- 211At-SAGMB-5F7 and 8 of 11 mice treated with iso- 211At-SAGMB-VHH_1028; prolongation in median survival was 495% and 414%, respectively. Conclusion: Combining rapidly internalizing, high-affinity HER2-targeted sdAbs with the iso- 211At-SAGMB residualizing prosthetic agent is a promising strategy for targeted α-particle therapy of HER2-expressing cancers.

Basis for the ICRP’s updated biokinetic model for systemic astatine

The International Commission on Radiological Protection (ICRP) recently updated its biokinetic models for workers in a series of reports called the OIR (occupational intakes of radionuclides) series. A new biokinetic model for astatine (At), the heaviest member of the halogen family, was adopted in OIR Part 5 (ICRP in press). Occupational intakes of radionuclides: Part 5). This paper provides an overview of available biokinetic data for At; describes the basis for the ICRP’s updated model for At; and tabulates dose coefficients for intravenous injection of each of the two longest lived and most important At isotopes, 211At and 210At. At-211 (T 1/2 = 7.214 h) is a promising radionuclide for use in targeted α-particle therapy due to several favourable properties including its half-life and the absence of progeny that could deliver significant radiation doses outside the region of α-particle therapy. At-210 (T 1/2 = 8.1 h) is an impurity generated in the production of 211At in a cyclotron and represents a potential radiation hazard via its long-lived progeny 210Po (T 1/2 = 138 days). Tissue dose coefficients for injected 210At and 211At based on the updated model are shown to differ considerably from values based on the ICRP’s previous model for At, particularly for the thyroid, stomach wall, salivary glands, lungs, spleen, and kidneys.

A Whole-Body Physiologically Based Pharmacokinetic Model for Alpha Particle Emitting Bismuth in Rats

Background: α particle emitting bismuth (212Bi) as decay product of 212Pb-labeled pharmaceuticals has been effective in targeted α particle therapy (TAT). Estimating the contribution of 212Bi released from its chelator to the absorbed doses in nontarget tissues is challenging in TAT. Physiologically based pharmacokinetic (PBPK) modeling can help overcome this limitation. Therefore, a whole-body 212Bi-PBPK model was developed to describe the pharmacokinetics (PKs) of 212Bi in rats. Materials and Methods: The rat 212Bi-PBPK model was implemented using the modeling software SAAM II with data and parameter values from the literature. Besides other mechanisms, 212Bi interactions with red blood cells, high molecular weight plasma protein, and intracellular biological thiols are described. Important PK parameters were fitted to time-activity data. Absorbed dose coefficients (ADCs) were calculated for injecting 0.774 fmol of 212Bi. Results: 212Bi uptake rates of liver, bone, small intestine, bone marrow, skin, and muscle were (0.86 ± 0.13), (3.85 ± 0.63), (0.27 ± 0.05), (1.44 ± 0.29), (0.04 ± 0.01), and (0.007 ± 0.007) per min with corresponding ADCs of 0.09, 0.03, 0.03, 0.07, 0.01, and 0.003 mGy/kBq, respectively. An ADC of 0.70 mGy/kBq was determined for kidneys. Conclusions: Kidneys are the dose-limiting organs in 212Bi-based TAT. The 212Bi-PBPK model is an effective tool to investigate the 212Bi biodistribution in murine models. Integrating the 212Bi-PBPK model into other murine and human PBPK models of α particle generators can help study the efficacy and safety of TAT.

Biodistribution and Multicompartment Pharmacokinetic Analysis of a Targeted α Particle Therapy.

Targeted α particle therapy (TAT) is ideal for treating disease while minimizing damage to surrounding nontargeted tissues due to short path length and high linear energy transfer (LET). We developed a TAT for metastatic uveal melanoma, targeting the melanocortin-1 receptor (MC1R), which is expressed in 94% of uveal melanomas. Two versions of the therapy are being investigated: 225Ac-DOTA-Ahx-MC1RL (225Ac-Ahx) and 225Ac-DOTA-di-d-Glu-MC1RL (225Ac-di-d-Glu). The biodistribution (BD) from each was studied and a multicompartment pharmacokinetic (PK) model was developed to describe drug distribution rates. Two groups of 16 severe combined immunodeficient (SCID) mice bearing high MC1R expressing tumors were intravenously injected with 225Ac-Ahx or 225Ac-di-d-Glu. After injection, four groups (n = 4) were euthanized at 24, 96, 144, and 288 h time points for each cohort. Tumors and 13 other organs were harvested at each time point. Isomeric γ spectra were measured in tissue samples using a scintillation γ detector and converted to α activity using factors for γ ray abundance per α decay. Time activity curves were calculated for each organ. A five-compartment PK model was built with the following compartments: blood, tumor, normal tissue, kidney, and liver. This model is characterized by a system of five ordinary differential equations using mass action kinetics, which describe uptake, intercompartmental transitions, and clearance rates. The ordinary differential equations were simultaneously solved and fit to experimental data using a genetic algorithm for optimization. The BD data show that both compounds have minimal distribution to organs at risk other than the kidney and liver. The PK parameter estimates had less than 5% error. From these data, 225Ac-Ahx showed larger and faster uptake in the liver. Both compounds had comparable uptake and clearance rates for other compartments. The BD and PK behavior for two targeted radiopharmaceuticals were investigated. The PK model fit the experimental data and provided insight into the kinetics of the compounds systematically.

Genetic signature of prostate cancer mouse models resistant to optimized hK2 targeted α-particle therapy

Hu11B6 is a monoclonal antibody that internalizes in cells expressing androgen receptor (AR)-regulated prostate-specific enzyme human kallikrein-related peptidase 2 (hK2; KLK2). In multiple rodent models, Actinium-225-labeled hu11B6-IgG1 ([225Ac]hu11B6-IgG1) has shown promising treatment efficacy. In the present study, we investigated options to enhance and optimize [225Ac]hu11B6 treatment. First, we evaluated the possibility of exploiting IgG3, the IgG subclass with superior activation of complement and ability to mediate FC-γ-receptor binding, for immunotherapeutically enhanced hK2 targeted α-radioimmunotherapy. Second, we compared the therapeutic efficacy of a single high activity vs. fractionated activity. Finally, we used RNA sequencing to analyze the genomic signatures of prostate cancer that progressed after targeted α-therapy. [225Ac]hu11B6-IgG3 was a functionally enhanced alternative to [225Ac]hu11B6-IgG1 but offered no improvement of therapeutic efficacy. Progression-free survival was slightly increased with a single high activity compared to fractionated activity. Tumor-free animals succumbing after treatment revealed no evidence of treatment-associated toxicity. In addition to up-regulation of canonical aggressive prostate cancer genes, such as MMP7, ETV1, NTS, and SCHLAP1, we also noted a significant decrease in both KLK3 (prostate-specific antigen ) and FOLH1 (prostate-specific membrane antigen) but not in AR and KLK2, demonstrating efficacy of sequential [225Ac]hu11B6 in a mouse model.

Separation of Ac-225 from Lanthanide Fission Products using a Reverse Phase Chromatographic Process Incorporating a Solvent Impregnated Resin

Targeted α-particle therapy (TAT) has been shown in recent years to be a promising route for the treatment of various forms of cancer, and may serve as an alternative to more traditional cancer treatment options. TAT takes advantage of an α-emitting radionuclide’s ability to deliver cytotoxic doses of radiation to tumor cells while causing minimal damage to surrounding healthy tissue. This is possible because of the short path length and high linear energy transfer associated with α -particles, which are on the order of 100 μm or less and around 100 keV/μm, respectively. To this end, a number of α-emitting radionuclides have explored for therapeutic applications including Ac-225, Bi-213, Bi-212, At-211, and Ra-223 with others currently in development. Of these, Ac-225 in particular has proven itself to be an especially appealing isotope for the treatment of some cancers. The value of Ac-225 is derived in part from its four alpha decay chain, which includes Bi-213 (t1/2 = ∼46 minutes), which is in itself a desirable isotope for TAT. The targeted delivery of Ac-225 to tumor sites has been shown in a number of instances to be highly effective for the treatment of several forms of cancer when a conjugate with an antibody, peptide, or protein is produced which can carry the isotope to the target site. One of the more notable instances of the successful treatment of human cancer with Ac-225 has been in the treatment of prostate cancer, for which Ac-225 may be conjugated with Prostate Specific Membrane Antigen for targeted doses to prostate cancer tumor cells. Because of the proven efficacy of Ac-225-containing conjugate species in anticancer treatment applications, demand for this isotope is expected to rise above current production capabilities. The work presented herein describes a new method for the separation of Ac-225 from radiolanthanide impurities resulting from fission of proton irradiated Th-232 target foils. The process incorporates a solvent impregnated resin to achieve the separation, and is the first in a series of resins to be tested as alternatives to current technologies incorporating branched DGA resin. The current resin is comprised of an Amberlite XAD7HP resin that has been loaded with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide ([Bmim][NTf2]) and the extractant N,N-dioctyldiglycolamic acid (DODGAA). Going forward, the resin will be tuned to achieve optimum results by loading the Amberlite scaffold with varying combinations of ionic liquids and extractants. Data presented will include batch extraction experiments including the solvent impregnated resin with complex matrices of metals (La-Lu, Sc, Y, U, Th, Ac) in HCl, HNO3, and citrate-phosphate buffer media over a range of pH. Additionally, results of dynamic column experiments will be presented, and comparisons will be made to separations technologies currently in use.

Alpha-Particle Therapy for Acute Myeloid Leukemia

Early studies using the humanized anti-CD33 monoclonal antibody lintuzumab labeled with β-emitters showed significant activity against acute myeloid leukemia (AML) but produced prolonged myelosuppression necessitating hematopoietic cell transplantation (HCT). Targeted α-particle therapy may produce more efficient tumor killing while sparing normal cells. An initial phase I trial of bismuth-213 (213Bi)-lintuzumab in relapsed and refractory (R/R) AML provided proof-of-principle for systemically administered α-particle therapy (Jurcic, Blood 2002). 213Bi-lintuzumab demonstrated rapid targeting of disease sites without significant extramedullary toxicity. Target-to-whole body dose ratios were greatly enhanced compared to β-emitting immunoconjugates. Anti-leukemic effects were seen across all dose levels with a decrease in marrow blasts in 78% of patients. In a subsequent phase I/II trial, 213Bilintuzumab was administered following partial cytoreduction with cytarabine (Rosenblat, Clin Cancer Res 2010). Among patients receiving the maximum tolerated dose of 37 MBq/kg or higher, responses were seen in 24% of patients. The 46-minute half-life of 213Bi, however, remained an obstacle to its widespread use. Therefore, a more potent second-generation construct containing actinium-225 (225Ac) (t½ = 10 days), which generates four α-particle emissions, was developed. A phase I study demonstrated that a single dose of 225Aclintuzuamb was safe at doses of 111 kBq/kg or less and produced marrow blast reductions in 67% of evaluable patients with R/R AML (Jurcic, ASH 2011). Dose-limiting toxicity was prolonged myelosuppression, and no evidence of radiation-induced nephrotoxicity was seen. Based on these findings, 225Ac-lintuzumab was investigated in a multicenter phase I/II trial in combination with low-dose cytarabine for older patients with untreated AML (Jurcic, SNMMI 2017). During the first cycle of therapy, two fractions of 225Ac-lintuzumab (18.5-74 kBq/kg/fraction) were administered one week apart after completion of chemotherapy. Five of 18 patients (28%) had objective responses. All responses occurred after the first cycle. The baseline peripheral blood blast count was a strong predictor of outcome, as responses were seen only in patients with peripheral blast counts < 200/μL. This is most likely explained by preferential antibody binding to peripheral sites in patients with higher circulating blast counts, leading to decreased marrow targeting. Because of this observation, a multicenter phase II trial of 225Ac-lintuzumab monotherapy using hydroxyurea to lower peripheral blast counts if needed was undertaken in older AML patients (Finn, ASH 2017). Objective responses were seen in nine of 13 patients (69%) after two doses of 225Ac-lintuzumab (74 kBq/kg) administered one week apart. Myelosuppression, however, was considered to be longer than acceptable in this population, and additional patients were treated with two fractions of 55.5 kBq/kg each. 225Ac-lintuzumab has shown significant activity in AML both alone and in combination with chemotherapy. Future development of 225Ac-lintuzumab includes combinations with standard chemotherapy and novel targeted agents for R/R AML, treatment for measurable residual disease in AML, and conditioning before HCT in patients with high-risk myelodysplastic syndromes.

Separation of Ac-225 from Lanthanide Fission Products using a Reverse Phase Chromatographic Process Incorporating a Solvent Impregnated Resin

Targeted α-particle therapy (TAT) has been shown in recent years to be a promising route for the treatment of various forms of cancer, and may serve as an alternative to more traditional cancer treatment options. TAT takes advantage of an α-emitting radionuclide’s ability to deliver cytotoxic doses of radiation to tumor cells while causing minimal damage to surrounding healthy tissue. This is possible because of the short path length and high linear energy transfer associated with α -particles, which are on the order of 100 μm or less and around 100 keV/μm, respectively. To this end, a number of α-emitting radionuclides have explored for therapeutic applications including Ac-225, Bi-213, Bi-212, At-211, and Ra-223 with others currently in development. Of these, Ac-225 in particular has proven itself to be an especially appealing isotope for the treatment of some cancers. The value of Ac-225 is derived in part from its four alpha decay chain, which includes Bi-213 (t1/2 = ∼46 minutes), which is in itself a desirable isotope for TAT. The targeted delivery of Ac-225 to tumor sites has been shown in a number of instances to be highly effective for the treatment of several forms of cancer when a conjugate with an antibody, peptide, or protein is produced which can carry the isotope to the target site. One of the more notable instances of the successful treatment of human cancer with Ac-225 has been in the treatment of prostate cancer, for which Ac-225 may be conjugated with Prostate Specific Membrane Antigen for targeted doses to prostate cancer tumor cells. Because of the proven efficacy of Ac-225-containing conjugate species in anticancer treatment applications, demand for this isotope is expected to rise above current production capabilities. The work presented herein describes a new method for the separation of Ac-225 from radiolanthanide impurities resulting from fission of proton irradiated Th-232 target foils. The process incorporates a solvent impregnated resin to achieve the separation, and is the first in a series of resins to be tested as alternatives to current technologies incorporating branched DGA resin. The current resin is comprised of an Amberlite XAD7HP resin that has been loaded with the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide ([Bmim][NTf2]) and the extractant N,N-dioctyldiglycolamic acid (DODGAA). Going forward, the resin will be tuned to achieve optimum results by loading the Amberlite scaffold with varying combinations of ionic liquids and extractants. Data presented will include batch extraction experiments including the solvent impregnated resin with complex matrices of metals (La-Lu, Sc, Y, U, Th, Ac) in HCl, HNO3, and citrate-phosphate buffer media over a range of pH. Additionally, results of dynamic column experiments will be presented, and comparisons will be made to separations technologies currently in use.

Alpha-Particle Therapy for Acute Myeloid Leukemia

Early studies using the humanized anti-CD33 monoclonal antibody lintuzumab labeled with β-emitters showed significant activity against acute myeloid leukemia (AML) but produced prolonged myelosuppression necessitating hematopoietic cell transplantation (HCT). Targeted α-particle therapy may produce more efficient tumor killing while sparing normal cells. An initial phase I trial of bismuth-213 (213Bi)-lintuzumab in relapsed and refractory (R/R) AML provided proof-of-principle for systemically administered α-particle therapy (Jurcic, Blood 2002). 213Bi-lintuzumab demonstrated rapid targeting of disease sites without significant extramedullary toxicity. Target-to-whole body dose ratios were greatly enhanced compared to β-emitting immunoconjugates. Anti-leukemic effects were seen across all dose levels with a decrease in marrow blasts in 78% of patients. In a subsequent phase I/II trial, 213Bilintuzumab was administered following partial cytoreduction with cytarabine (Rosenblat, Clin Cancer Res 2010). Among patients receiving the maximum tolerated dose of 37 MBq/kg or higher, responses were seen in 24% of patients. The 46-minute half-life of 213Bi, however, remained an obstacle to its widespread use. Therefore, a more potent second-generation construct containing actinium-225 (225Ac) (t½ = 10 days), which generates four α-particle emissions, was developed. A phase I study demonstrated that a single dose of 225Aclintuzuamb was safe at doses of 111 kBq/kg or less and produced marrow blast reductions in 67% of evaluable patients with R/R AML (Jurcic, ASH 2011). Dose-limiting toxicity was prolonged myelosuppression, and no evidence of radiation-induced nephrotoxicity was seen. Based on these findings, 225Ac-lintuzumab was investigated in a multicenter phase I/II trial in combination with low-dose cytarabine for older patients with untreated AML (Jurcic, SNMMI 2017). During the first cycle of therapy, two fractions of 225Ac-lintuzumab (18.5-74 kBq/kg/fraction) were administered one week apart after completion of chemotherapy. Five of 18 patients (28%) had objective responses. All responses occurred after the first cycle. The baseline peripheral blood blast count was a strong predictor of outcome, as responses were seen only in patients with peripheral blast counts < 200/μL. This is most likely explained by preferential antibody binding to peripheral sites in patients with higher circulating blast counts, leading to decreased marrow targeting. Because of this observation, a multicenter phase II trial of 225Ac-lintuzumab monotherapy using hydroxyurea to lower peripheral blast counts if needed was undertaken in older AML patients (Finn, ASH 2017). Objective responses were seen in nine of 13 patients (69%) after two doses of 225Ac-lintuzumab (74 kBq/kg) administered one week apart. Myelosuppression, however, was considered to be longer than acceptable in this population, and additional patients were treated with two fractions of 55.5 kBq/kg each. 225Ac-lintuzumab has shown significant activity in AML both alone and in combination with chemotherapy. Future development of 225Ac-lintuzumab includes combinations with standard chemotherapy and novel targeted agents for R/R AML, treatment for measurable residual disease in AML, and conditioning before HCT in patients with high-risk myelodysplastic syndromes.

Melanocortin 1 Receptor-Targeted α-Particle Therapy for Metastatic Uveal Melanoma.

New effective therapies are greatly needed for metastatic uveal melanoma, which has a very poor prognosis with a median survival of less than 1 y. The melanocortin 1 receptor (MC1R) is expressed in 94% of uveal melanoma metastases, and a MC1R-specific ligand (MC1RL) with high affinity and selectivity for MC1R was previously developed. Methods: The 225Ac-DOTA-MC1RL conjugate was synthesized in high radiochemical yield and purity and was tested in vitro for biostability and for MC1R-specific cytotoxicity in uveal melanoma cells, and the lanthanum-DOTA-MC1RL analog was tested for binding affinity. Non-tumor-bearing BALB/c mice were tested for maximum tolerated dose and biodistribution. Severe combined immunodeficient mice bearing uveal melanoma tumors or engineered MC1R-positive and -negative tumors were studied for biodistribution and efficacy. Radiation dosimetry was calculated using mouse biodistribution data and blood clearance kinetics from Sprague-Dawley rat data. Results: High biostability, MC1R-specific cytotoxicity, and high binding affinity were observed. Limiting toxicities were not observed at even the highest administered activities. Pharmacokinetics and biodistribution studies revealed rapid blood clearance (<15 min), renal and hepatobillary excretion, MC1R-specific tumor uptake, and minimal retention in other normal tissues. Radiation dosimetry calculations determined pharmacokinetics parameters and absorbed α-emission dosages from 225Ac and its daughters. Efficacy studies demonstrated significantly prolonged survival and decreased metastasis burden after a single administration of 225Ac-DOTA-MC1RL in treated mice relative to controls. Conclusion: These results suggest significant potential for the clinical translation of 225Ac-DOTA-MC1RL as a novel therapy for metastatic uveal melanoma.

Anti-tumor effects and potential therapeutic response biomarkers in α-emitting meta-211At-astato-benzylguanidine therapy for malignant pheochromocytoma explored by RNA-sequencing.

Targeted α-particle therapy is a promising option for patients with malignant pheochromocytoma. Recent observations regarding meta-211At-astato-benzylguanidine (211At-MABG) in a pheochromocytoma mouse model showed a strong anti-tumor effect, though the molecular mechanism remains elusive. Here, we present the first comprehensive RNA-sequencing (RNA-seq) data for pheochromocytoma cells based on in vitro 211At-MABG administration experiments. Key genes and pathways in the tumor α-particle radiation response are also examined to obtain potential response biomarkers.Methods: We evaluated genome-wide transcriptional alterations in the rat pheochromocytoma cell line PC12 at 3, 6, and 12 h after 211At-MABG treatment; a control experiment using 60Co γ-ray irradiation was carried out to highlight 211At-MABG-specific gene expression. For comparisons, 10% and 80% iso-survival doses (0.8 and 0.1 kBq/mL for 211At-MABG and 10 and 1 Gy for 60Co γ-rays) were used.Results: Enrichment analysis of differentially expressed genes (DEGs) and analysis of the gene expression profiles of cell cycle checkpoints revealed similar modes of cell death via the p53-p21 signaling pathway after 211At-MABG treatment and γ-ray irradiation. The top list of ranked DEGs demonstrated the expression of key genes on the decrease in the survival following 211At-MABG exposure, and four potential genes (Mien1, Otub1, Vdac1 and Vegfa genes) of 211At-MABG therapy. Western blot analysis indicated increased expression of TSPO in 211At-MABG-treated cells, suggesting its potential as a PET imaging probe.Conclusion: Comprehensive RNA-seq revealed contrasting cellular responses to γ-ray and α-particle therapy, leading to the identification of four potential candidate genes that may serve as molecular imaging and 211At-MABG therapy targets.

Targeted cancer cell ablation in mice by an α-particle-emitting astatine-211-labeled antibody against major histocompatibility complex class I chain-related protein A and B

Major histocompatibility complex class I chain-related protein A and B (MICA/B) are ligands of the immune receptor, natural-killer group 2 member D. MICA/B expression is often found in several types of cancer but is restricted in normal tissues. Here, we show that an α-particle emitting astatine-211 (211At)-labeled antibody targeting MICA/B (211At-anti MICA/B Ab) efficiently ablates cancer cells in vitro and in vivo. We generated 211At-anti MICA/B Ab, an anti-MICA/B antibody conjugated with a highly cytotoxic α-particle emitting radionuclide 211At. 211At-anti MICA/B Ab binds to human osteosarcoma SaOS2 and U2OS cells that exhibit high levels of MICA/B expression and efficiently kills those cells in vitro. Biodistribution analysis using xenograft mouse models of HCT116 p53−/− positive for MICA/B expression, showed increased 211At in the xenografts for up to 22 h after injection as time proceeded. A single dose of 211At-anti MICA/B Ab (1 MBq) showed significant reduction in the tumor growth rate of HCT116 p53−/− xenografts compared to 211At-labeled mouse IgG (1 MBq) at 21 days after injection. No body weight loss and erythrocytopenia was evident in mice that received 211At-anti MICA/B. Leukocytopenia and thrombocytopenia were observed within a week after 211At-anti MICA/B injection, but counts of red blood cells and platelets were recovered to control levels at about 3–4 weeks after injection. Taken together, these data strongly demonstrate that targeted α-particle therapy using 211At-anti-MICA/B Ab emitting highly cytotoxic α-particles is a potential new therapeutic option for several types of cancer.

Clinical Studies with Bismuth-213 and Actinium-225 for Hematologic Malignancies.

Due to the shorter range and higher linear energy transfer of α-particles compared to β-particles, targeted α-particle therapy may produce more efficient tumor killing while sparing neighboring healthy cells. We will review the clinical studies using α-particle therapy for Acute Myeloid Leukemia (AML). A series of clinical trials were conducted to assess the safety, feasibility, and anti-leukemic effects of lintuzumab, an anti-CD33 humanized monoclonal antibody, labeled with the α-emitters bismuth- 213 (213Bi) and actinium-225 (225Ac). An initial phase I study conducted in 18 patients with relapsed or refractory AML demonstrated the safety and antitumor effects of 213Bi-lintuzumab. Subsequently, 213Bi-lintuzumab produced remissions in AML patients after partial cytoreduction with cytarabine in phase I/II trial. The 46- minute half-life of 213Bi and need for an onsite generator has limited its utility. Therefore, a secondgeneration construct was developed using 225Ac, a radiometal that yields four α-particle emissions. A phase I trial demonstrated that a single infusion of 225Ac-lintuzumab could be given safely at doses up to 111 kBq/kg with anti-leukemic activity across all dose levels studied. In a second phase I study, 28% of older patients with untreated AML had objective responses after receiving fractionated-dose 225Aclintuzumab and low-dose cytarabine. Based upon the encouraging results seen in phase I trials of 225Ac-lintuzumab, a phase II study of 225Ac-lintuzumab monotherapy for older patients with untreated AML is now in progress and is also being studied in a subset of patients with CD33-positive multiple myeloma.

Radio-pharmaceuticals for cancer treatment: are they ready for prime time yet?

Radio-pharmaceuticals for cancer treatment: are they ready for prime time yet?