A Novel Method for Real-Time Quantification of Radioligand Binding to Living Tumor Cells In Vitro

Background: Real-time quantification of radioligand binding to cells under in vivo-like conditions improves evaluation of clinical potential. Materials and Methods: SKOV-3 tumor cells were grown in a monolayer on a thin glass plate placed in a sealable shallow chamber with a continuous flow of 125I-trastuzumab solution. The time-dependent cell binding was measured using a NaI detector, and the binding parameters were derived by computational analysis. Results: The detection efficiency of 125I was 65 cps/kBq for radioligand bound to the cells. Experiments were analyzed to find the values of kon and koff. The resulting kon was 3.2-7.9 × 104 M-1 s-1 and koff was 0.11-4.2 × 10-5 s-1. Conclusions: Radioligands can be rapidly evaluated by binding to living cells for selection and optimization of radioconjugates for diagnostic and therapeutic purposes.

Pretargeted Alpha Therapy of Disseminated Cancer Combining Click Chemistry and Astatine-211

To enhance targeting efficacy in the radioimmunotherapy of disseminated cancer, several pretargeting strategies have been developed. In pretargeted radioimmunotherapy, the tumor is pretargeted with a modified monoclonal antibody that has an affinity for both tumor antigens and radiolabeled carriers. In this work, we aimed to synthesize and evaluate poly-L-lysine-based effector molecules for pretargeting applications based on the tetrazine and trans-cyclooctene reaction using ²¹¹At for targeted alpha therapy and ¹²⁵I as a surrogate for the imaging radionuclides ^{123, 124}I. Poly-L-lysine in two sizes was functionalized with a prosthetic group, for the attachment of both radiohalogens, and tetrazine, to allow binding to the trans-cyclooctene-modified pretargeting agent, maintaining the structural integrity of the polymer. Radiolabeling resulted in a radiochemical yield of over 80% for astatinated poly-L-lysines and a range of 66-91% for iodinated poly-L-lysines. High specific astatine activity was achieved without affecting the stability of the radiopharmaceutical or the binding between tetrazine and transcyclooctene. Two sizes of poly-L-lysine were evaluated, which displayed similar blood clearance profiles in a pilot in vivo study. This work is a first step toward creating a pretargeting system optimized for targeted alpha therapy with ²¹¹At.

Astatine-211 based radionuclide therapy: Current clinical trial landscape

Astatine-211 (²¹¹At) has physical properties that make it one of the top candidates for use as a radiation source for alpha particle-based radionuclide therapy, also referred to as targeted alpha therapy (TAT). Here, we summarize the main results of the completed clinical trials, further describe ongoing trials, and discuss future prospects.

Estimating the Risk for Secondary Cancer After Targeted α-Therapy with 211At Intraperitoneal Radioimmunotherapy

Intraperitoneal ²¹¹At-based targeted α-therapy (TAT) may hold great promise as an adjuvant therapy after surgery and chemotherapy in epithelial ovarian cancer to eradicate any remaining undetectable disease. This implies that it will also be delivered to patients possibly already cured by the primary treatment. An estimate of long-term risks is therefore sought to determine whether the treatment is justified. Methods: Baseline data for risk estimates of α-particle irradiation were collected from published studies on excess cancer induction and mortality for subjects exposed to either ²²⁴Ra treatments or Thorotrast contrast agent (25% ThO₂ colloid, containing ²³²Th). Organ dosimetry for ²²⁴Ra and Thorotrast irradiation were taken from the literature. These organ-specific risks were then applied to our previously reported dosimetry for intraperitoneal ²¹¹At-TAT patients. Results: Risk could be estimated for 10 different organ or organ groups. The calculated excess relative risk per gray (ERR/Gy) could be sorted into 2 groups. The lower-ERR/Gy group, ranging up to a value of approximately 5, included trachea, bronchus, and lung, at 0.52 (95% CI, 0.21-0.82); stomach, at 1.4 (95% CI, -5.0-7.9); lymphoid and hematopoietic system, at 2.17 (95% CI, 1.7-2.7); bone and articular cartilage, at 2.6 (95% CI, 2.0-3.3); breast, at 3.45 (95% CI, -10-17); and colon, at 4.5 (95% CI, -3.5-13). The higher-ERR/Gy group, ranging from approximately 10 to 15, included urinary bladder, at 10.1 (95% CI, 1.4-23); liver, at 14.2 (95% CI, 13-16); kidney, at 14.9 (95% CI, 3.9-26); and lip, oral cavity, and pharynx, at 15.20 (95% CI, 2.73-27.63). Applying a typical candidate patient (female, age 65 y) and correcting for the reference population mortality rate, the total estimated excess mortality for an intraperitoneal ²¹¹At-monoclonal antibody treatment amounted to 1.13 per 100 treated. More than half this excess originated from urinary bladder and kidney, 0.29 and 0.34, respectively. Depending on various adjustments in calculation and assumptions on competing risks, excess mortality could range from 0.11 to 1.84 per 100 treated. Conclusion: Published epidemiologic data on lifelong detriment after α-particle irradiation and its dosimetry allowed calculations to estimate the risk for secondary cancer after ²¹¹At-based intraperitoneal TAT. Measures to reduce dose to the urinary organs may further decrease the estimated relative low risk for secondary cancer from ²¹¹At-monoclonal antibody-based intraperitoneal TAT.

Covalent core-radiolabeling of polymeric micelles with 125I/211At for theranostic radiotherapy

Astatine-211 (²¹¹At) is one of the most promising α-emitters for targeted alpha therapy, especially of cancer metastases. However, the lack of a stable isotope, frequent in vivo deastatination, and limited radiochemical knowledge makes it challenging to apply. Here, we report a new strategy for radiolabeling the lipophilic core of polymeric micelles (PMs) with covalently bound ²¹¹At. The PMs were radiolabeled via either an indirect synthon-based method or directly on the amphipathic block copolymer. The radiochemistry was optimized with iodine-125 (¹²⁵I) and then adapted for ²¹¹At, enabling the use of both elements as a potential theranostic pair. PMs that were core-radiolabeled with both ¹²⁵I or ²¹¹At were prepared and characterized, based on a PEG(5k)-PLGA(10k) co-polymer. The stability of the radiolabeled PMs was evaluated in mouse serum for 21 h, showing radiochemical stability above 85%. After in vivo evaluation of the ²¹¹At- labeled PMs, 4-5 % ID/g of the ²¹¹At could still be detected in the blood, showing a promising in vivo stability of the PMs. Further, ²¹¹At-labeled PMs accumulated in the spleen (20-30 %ID/g) and the liver (2.5- 5.5 %ID/g), along with some detection of ²¹¹At in the thyroid (3.5-9 %ID/g). This led to the hypothesis that deastatination takes place in the liver, whereas good stability of the ²¹¹At core-radiolabel was observed in the blood.

Radium-223-induced Bystander Effects Cause DNA Damage and Apoptosis in Disseminated Tumor Cells in Bone Marrow

Radiation-induced bystander effects have been implicated in contributing to the growth delay of disseminated tumor cells (DTC) caused by ²²³RaCl₂, an alpha particle-emitting radiopharmaceutical. To understand how ²²³RaCl₂ affects the growth, we have quantified biological changes caused by direct effects of radiation and bystander effects caused by the emitted radiations on DTC and osteocytes. Characterizing these effects contribute to understanding the efficacy of alpha particle-emitting radiopharmaceuticals and guide expansion of their use clinically. MDA-MB-231 or MCF-7 human breast cancer cells were inoculated intratibially into nude mice that were previously injected intravenously with 50 or 600 kBq/kg ²²³RaCl₂. At 1-day and 3-days postinoculation, tibiae were harvested and examined for DNA damage (γ-H2AX foci) and apoptosis in osteocytes and cancer cells located within and beyond the range (70 μm) of alpha particles emitted from the bone surface. Irradiated and bystander MDA-MB-231 and MCF-7 cells harbored DNA damage. Bystander MDA-MB-231 cells expressed DNA damage at both treatment levels while bystander MCF-7 cells required the higher administered activity. Osteocytes also had DNA damage regardless of inoculated cancer cell line. The extent of DNA damage was quantified by increases in low (1-2 foci), medium (3-5 foci), and high (5+ foci) damage. MDA-MB-231 but not MCF-7 bystander cells showed increases in apoptosis in ²²³RaCl₂-treated animals, as did irradiated osteocytes. In summary, radiation-induced bystander effects contribute to DTC cytotoxicity caused by ²²³RaCl₂. IMPLICATIONS: This observation supports clinical investigation of the efficacy of ²²³RaCl₂ to prevent breast cancer DTC from progressing to oligometastases.

Evaluation of therapeutic efficacy of 211At-labeled farletuzumab in an intraperitoneal mouse model of disseminated ovarian cancer

Introduction

Antibodies labeled with alpha-emitter astatine-211 have previously shown effective in intraperitoneal (i.p.) treatments of ovarian cancer. In the present work we explore the use of investigational farletuzumab, aimed at the folate receptor alpha. The aim was to evaluate the biodistribution and therapeutic effect of ²¹¹At-farletuzumab in in-vitro and in-vivo experiments and, using models for radiation dosimetry, to translate the findings to expected clinical result. The activity concentration used for therapy in mice (170 kBq/mL) was chosen to be in agreement with an activity concentration that is anticipated to be clinically relevant in patients (200 MBq/L).

Methods

For biodistribution, using intravenous injections and mice carrying subcutaneous (s.c.) tumors, the animals were administered either ²¹¹At-farletuzumab (n = 16); or with a combination of ¹²⁵I-farletuzumab and ²¹¹At-MX35 (n = 12). At 1, 3, 10 and 22 h, mice were euthanized and s.c.-tumors and organs weighted and measured for radioactivity. To evaluate therapeutic efficacy, mice were inoculated i.p. with 2 × 10⁶ NIH:OVCAR-3 cells. Twelve days later, the treatments were initiated by i.p.-administration. Specific treatment was given by ²¹¹At-labeled farletuzumab (group A; n = 22, 170 kBq/mL) which is specific for OVCAR-3 cells. Control treatments were given by either ²¹¹At-labeled rituximab which is unspecific for OVCAR-3 (group B; n = 22, 170 kBq/mL), non-radiolabeled farletuzumab (group C; n = 11) or PBS only (group D; n = 8).

Results

The biodistribution of ²¹¹At-farletuzumab was similar to that with ¹²⁵I as radiolabel, and also to that of ²¹¹At-labeled MX35 antibody. The tumor-free fraction (TFF) of the three control groups were all low (PBS 12%, unlabeled specific farletuzumab 9% and unspecific ²¹¹At-rituximab 14%). TFF following treatment with ²¹¹At-farletuzumab was 91%.

Conclusion

The current investigation of intraperitoneal therapy with ²¹¹At-farletuzumab, delivered at clinically relevant ²¹¹At-mAb radioactivity concentrations and specific activities, showed a 6 to 10-fold increase (treated versus controls) in antitumor efficacy. This observation warrants further clinical testing.

Realizing Clinical Trials with Astatine-211: The Chemistry Infrastructure

Despite the consensus around the clinical potential of the α-emitting radionuclide astatine-211 (211At), there are only a limited number of research facilities that work with this nuclide. There are three main reasons for this: (1) Scarce availability of the nuclide. Despite a relatively large number of globally existing cyclotrons capable of producing 211At, few cyclotron facilities produce the nuclide on a regular basis. (2) Lack of a chemical infrastructure, that is, isolation of 211At from irradiated targets and the subsequent synthesis of an astatinated product. At present, the research groups that work with 211At depend on custom systems for recovering 211At from the irradiated targets. Setting up and implementing such custom units require long lead times to provide a proper working system. (3) The chemistry of 211At. Compared with radiometals there are no well-established and generally accepted synthesis methods for forming sufficiently stable bonds between 211At and the tumor-specific vector to allow for systemic applications. Herein we present an overview of the infrastructure of producing 211At radiopharmaceuticals, from target to radiolabeled product including chemical strategies to overcome hurdles for advancement into clinical trials with 211At.

Isospin Properties of Nuclear Pair Correlations from the Level Structure of the Self-Conjugate Nucleus ^{88}Ru

The low-lying energy spectrum of the extremely neutron-deficient self-conjugate (N=Z) nuclide _{44}^{88}Ru_{44} has been measured using the combination of the Advanced Gamma Tracking Array (AGATA) spectrometer, the NEDA and Neutron Wall neutron detector arrays, and the DIAMANT charged particle detector array. Excited states in ^{88}Ru were populated via the ^{54}Fe(^{36}Ar,2nγ)^{88}Ru^{*} fusion-evaporation reaction at the Grand Accélérateur National d'Ions Lourds (GANIL) accelerator complex. The observed γ-ray cascade is assigned to ^{88}Ru using clean prompt γ-γ-2-neutron coincidences in anticoincidence with the detection of charged particles, confirming and extending the previously assigned sequence of low-lying excited states. It is consistent with a moderately deformed rotating system exhibiting a band crossing at a rotational frequency that is significantly higher than standard theoretical predictions with isovector pairing, as well as observations in neighboring N>Z nuclides. The direct observation of such a "delayed" rotational alignment in a deformed N=Z nucleus is in agreement with theoretical predictions related to the presence of strong isoscalar neutron-proton pair correlations.

Labeling of Anti-HER2 Nanobodies with Astatine-211: Optimization and the Effect of Different Coupling Reagents on Their in Vivo Behavior

The use of nanobodies (Nbs) as vehicles in targeted alpha therapy (TAT) has gained great interest because of their excellent properties. They combine high in vivo affinity and specificity of binding with fast kinetics. This research investigates a novel targeted therapy that combines the α-particle emitter astatine-211 (²¹¹At) and the anti-HER2 Nb 2Rs15d to selectively target HER2+ cancer cells. Two distinctive radiochemical methodologies are investigated using three different coupling reagents. The first method uses the coupling reagents, N-succinimidyl 4-(1,2-bis-tert-butoxycarbonyl)guanidinomethyl-3-(trimethylstannyl)benzoate (Boc₂-SGMTB) and N-succinimidyl-3-(trimethylstannyl)benzoate (m-MeATE), which are both directed to amino groups on the Nb, resulting in random conjugation. The second method aims at obtaining a homogeneous tracer population, via a site-specific conjugation of the N-[2-(maleimido)ethyl]-3-(trimethylstannyl)benzamide (MSB) reagent onto the carboxyl-terminal cysteine of the Nb. The resulting radioconjugates are evaluated in vitro and in vivo. 2Rs15d is labeled with ²¹¹At using Boc₂-SGMTB, m-MeATE, and MSB. After astatination and purification, the binding specificity of the radioconjugates is validated on HER2+ cells, followed by an in vivo biodistribution assessment in SKOV-3 xenografted mice. α-camera imaging is performed to determine uptake and activity distribution in kidneys/tumors. 2Rs15d astatination resulted in a high radiochemical purity >95% for all radioconjugates. The biodistribution studies of all radioconjugates revealed comparable tumor uptake (higher than 8% ID/g at 1 h). [²¹¹At]SAGMB-2Rs15d showed minor uptake in normal tissues. Only in the kidneys, a higher uptake was measured after 1 h, but decreased rapidly after 3 h. Astatinated Nbs consisting of m-MeATE or MSB reagents revealed elevated uptake in lungs and stomach, indicating the presence of released ²¹¹At. α-Camera imaging of tumors revealed a homogeneous activity distribution. The radioactivity in the kidneys was initially concentrated in the renal cortex, while after 3 h most radioactivity was measured in the medulla, confirming the fast washout into urine. Changing the reagents for Nb astatination resulted in different in vivo biodistribution profiles, while keeping the targeting moiety identical. Boc₂-SGMTB is the preferred reagent for Nb astatination because of its high tumor uptake, its low background signals, and its fast renal excretion. We envision [²¹¹At]SAGMB-2Rs15d to be a promising therapeutic agent for TAT and aim toward efficacy evaluation.

Intraperitoneal α-Emitting Radioimmunotherapy with 211At in Relapsed Ovarian Cancer: Long-Term Follow-up with Individual Absorbed Dose Estimations

Eliminating microscopic residual disease with α-particle radiation is theoretically appealing. After extensive preclinical work with α-particle-emitting ²¹¹At, we performed a phase I trial with intraperitoneal α-particle therapy in epithelial ovarian cancer using ²¹¹At conjugated to MX35, the antigen-binding fragments-F(ab')₂-of a mouse monoclonal antibody. We now present clinical outcome data and toxicity in a long-term follow-up with individual absorbed dose estimations. Methods: Twelve patients with relapsed epithelial ovarian cancer, achieving a second complete or nearly complete response with chemotherapy, received intraperitoneal treatment with escalating (20-215 MBq/L) activity concentrations of ²¹¹At-MX35 F(ab')_2. Results: The activity concentration was escalated to 215 MBq/L without any dose-limiting toxicities. Most toxicities were low-grade and likely related to the treatment procedure, not clearly linked to the α-particle irradiation, with no observed hematologic toxicity. One grade 3 fatigue and 1 grade 4 intestinal perforation during catheter implantation were observed. Four patients had a survival of more than 6 y, one of whom did not relapse. At progression, chemotherapy was given without signs of reduced tolerability. Overall median survival was 35 mo, with a 1-, 2-, 5-, and 10-y survival of 100%, 83%, 50%, and 25%, respectively. Calculations of the absorbed doses showed that a lower specific activity is associated with a lower single-cell dose, whereas a high specific activity may result in a lower central dose in microtumors. Individual differences in absorbed dose to possible microtumors were due to variations in administered activity and the specific activity. Conclusion: No apparent signs of radiation-induced toxicity or decreased tolerance to relapse therapy were observed. The dosimetric calculations show that further optimization is advisable to increase the efficacy and reduce possible long-term toxicity.

Pharmacokinetics, microscale distribution, and dosimetry of alpha-emitter-labeled anti-PD-L1 antibodies in an immune competent transgenic breast cancer model

BACKGROUND

Studies combining immune checkpoint inhibitors with external beam radiation have shown a therapeutic advantage over each modality alone. The purpose of these works is to evaluate the potential of targeted delivery of high LET radiation to the tumor microenvironment via an immune checkpoint inhibitor.

METHODS

The impact of protein concentration on the distribution of 111In-DTPA-anti-PD-L1-BC, an 111In-antibody conjugate targeted to PD-L1, was evaluated in an immunocompetent mouse model of breast cancer. 225Ac-DOTA-anti-PD-L1-BC was evaluated by both macroscale (ex vivo biodistribution) and microscale (alpha-camera images at a protein concentration determined by the 111In data.

RESULTS

The evaluation of 111In-DTPA-anti-PD-L1-BC at 1, 3, and 10 mg/kg highlighted the impact of protein concentration on the distribution of the labeled antibody, particularly in the blood, spleen, thymus, and tumor. Alpha-camera images for the microscale distribution of 225Ac-DOTA-anti-PD-L1-BC showed a uniform distribution in the liver while highly non-uniform distributions were obtained in the thymus, spleen, kidney, and tumor. At an antibody dose of 3 mg/kg, the liver was dose-limiting with an absorbed dose of 738 mGy/kBq; based upon blood activity concentration measurements, the marrow absorbed dose was 29 mGy/kBq.

CONCLUSIONS

These studies demonstrate that 225Ac-DOTA-anti-PD-L1-BC is capable of delivering high LET radiation to PD-L1 tumors. The use of a surrogate SPECT agent, 111In-DTPA-anti-PD-L1-BC, is beneficial in optimizing the dose delivered to the tumor sites. Furthermore, an accounting of the microscale distribution of the antibody in preclinical studies was essential to the proper interpretation of organ absorbed doses and their likely relation to biologic effect.

Model of Intraperitoneal Targeted α-Particle Therapy Shows That Posttherapy Cold-Antibody Boost Enhances Microtumor Radiation Dose and Treatable Tumor Sizes

Intraperitoneally administered radiolabeled monoclonal antibodies (mAbs) have been tested in several clinical trials, often with promising results, but have never proven curative. Methods: We have previously presented simulations of clinically relevant amounts of intraperitoneal ⁹⁰Y-mAbs for treatment of minimal disease and shown that such treatments are unlikely to eradicate microtumors. Our previous model simulated the kinetics of intraperitoneally infused radiolabeled mAbs in humans and showed the benefit of instead using α-emitters such as ²¹¹At. In the current work, we introduce penetration of mAbs into microtumors with radii of up to 400 μm. Calculations were performed using dynamic simulation software. To determine the radiation dose distribution in nonvascularized microtumors of various sizes after intraperitoneal ²¹¹At-radioimmunotherapy, we used an in-house-developed Monte Carlo program for microdosimetry. Our aim was to find methods that optimize the therapy for as wide a tumor size range as possible. Results: Our results show that high-specific-activity radiolabeled mAbs that are bound to a tumor surface will penetrate slowly compared with the half-lives of ²¹¹At and shorter-lived radionuclides. The inner-core cells of tumors with radii exceeding 100 μm may therefore not be sufficiently irradiated. For lower specific activities, the penetration rate and dose distribution will be more favorable for such tumors, but the dose to smaller microtumors and single cells will be low. Conclusion: Our calculations show that the addition of a boost with unlabeled mAb 1-5 h after therapy results in sufficient absorbed doses both to single cells and throughout microtumors up to approximately 300 μm in radius. This finding should also hold for other high-affinity mAbs and short-lived α-emitters.

Abstract 834: Formation of DNA double-strand breaks in colon tumors after targeted alpha therapy with 211At-mAb

Introduction: Targeted alpha therapy has shown promising results in preclinical and clinical studies. Alpha particle irradiation gives a high fraction of DNA double-strand breaks (DSB), as shown in vitro, resulting in a high probability of cell death. We have previously examined the therapeutic effects of 211At on solid colon carcinoma tumors (diameter approximately 1 cm), with tolerable activities (5 MBq/animals) resulting in non-palpable tumors within one week p.i. The aim of the present study was to investigate the formation of DNA DSB during tumor regression after radioimmunotherapy with 211At-mAb in a syngeneic rat colon carcinoma model. Methods: 18 rats bearing solid colon tumors (1 cm in diameter) between peritoneum and the abdominal muscle were injected intravenously with 5 MBq/animal 211At-BR96. Tumors were excised and paraffin-embedded after 10 min, 2 h, 8 h, 18 h, 24 h, and 48 h p.i. (3 tumors per time point). 53BP1 was stained by immunohistochemistry and used as a marker for DNA DSB. Untreated tumors were used as controls (n=9). DNA DSB were counted in central and peripheral tumor areas selected at random. Results: A few DNA DSB were detected in untreated tumors. Already 10 min p.i., the number of DNA DSB had increased slightly in peripheral tumor tissue. The number peaked 8 h p.i., when the number of DNA DSB had increased 50 times in the tumor periphery and 24 times in the tumor center. The number of DNA DSB then declined, but the difference between center and periphery remained, as expected considering the intratumoral distribution of radioimmunoconjugate. This correlates with the 211At half-life of 7.2 h. Conclusion: DNA DSB are formed early after injection of 211At-mAb and follows the intratumoral distribution of mAbs.

Citation Format: Sophie E. Eriksson, Erika Elgström, Sture Lindegren, Tom Bäck. Formation of DNA double-strand breaks in colon tumors after targeted alpha therapy with 211At-mAb [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 834. doi:10.1158/1538-7445.AM2017-834

Therapeutic efficacy of α-radioimmunotherapy with different activity levels of the 213Bi-labeled monoclonal antibody MX35 in an ovarian cancer model

Background

The aim of this study was to compare the therapeutic efficacy of two different activity levels of the ²¹³Bi-labeled monoclonal antibody MX35 in an ovarian cancer model. Sixty female BALB/c (nu/nu) mice were inoculated intraperitoneally with human ovarian cancer cells (OVCAR-3). Two weeks later, 40 mice were injected intraperitoneal (i.p.) with 1 ml of ²¹³Bi-MX35, 3 MBq/mL (n = 20), or 9 MBq/mL (n = 20). An additional 20 mice received unlabeled MX35. Incidence of tumors and ascites was investigated 8 weeks after therapy. Body weight and white blood cell counts were monitored after treatment for possible signs of toxicity.

Results

The tumor-free fraction of the animals treated with 3 MBq/mL of ²¹³Bi-MX35 was 0.55, whereas that of animals treated with 9 MBq/mL of ²¹³Bi-MX35 was 0.78. The control group treated with unlabeled MX35 had a tumor-free fraction of 0.15. No significant reduction in white blood cell counts or weight loss was observed.

Conclusions

Tumor growth after i.p. treatment with ²¹³Bi-MX35 was significantly reduced compared to treatment with unlabeled MX35. Treatment with 9 MBq/mL of ²¹³Bi-MX35 resulted in higher tumor-free fraction compared with 3 MBq/mL of ²¹³Bi-MX35, but this difference was not statistically significant. No signs of toxicity were observed in the treated animals.

Cure of Human Ovarian Carcinoma Solid Xenografts by Fractionated α-Radioimmunotherapy with 211At-MX35-F(ab')2: Influence of Absorbed Tumor Dose and Effect on Long-Term Survival

The goal of this study was to investigate whether targeted α-therapy can be used to successfully treat macrotumors, in addition to its established role for treating micrometastatic and minimal disease. We used an intravenous fractionated regimen of α-radioimmunotherapy in a subcutaneous tumor model in mice. We aimed to evaluate the absorbed dose levels required for tumor eradication and growth monitoring, as well as to evaluate long-term survival after treatment. Methods: Mice bearing subcutaneous tumors (50 mm³, NIH:OVCAR-3) were injected repeatedly (1-3 intravenous injections 7-10 d apart, allowing bone marrow recovery) with ²¹¹At-MX35-F(ab')₂ at different activities (close to acute myelotoxicity). Mean absorbed doses to tumors and organs were estimated from biodistribution data and summed for the fractions. Tumor growth was monitored for 100 d and survival for 1 y after treatment. Toxicity analysis included body weight, white blood cell count, and hematocrit. Results: Effects on tumor growth after fractionated α-radioimmunotherapy with ²¹¹At-MX35-F(ab')₂ was strong and dose-dependent. Complete remission (tumor-free fraction, 100%) was found for tumor doses of 12.4 and 16.4 Gy. The administered activities were high, and long-term toxicity effects (≤60 wk) were clear. Above 1 MBq, the median survival decreased linearly with injected activity, from 44 to 11 wk. Toxicity was also seen by reduced body weight. White blood cell count analysis after α-radioimmunotherapy indicated bone marrow recovery for the low-activity groups, whereas for high-activity groups the reduction was close to acute myelotoxicity. A decrease in hematocrit was seen at a late interval (34-59 wk after therapy). The main external indication of poor health was dehydration. Conclusion: Having observed complete eradication of solid tumor xenografts, we conclude that targeted α-therapy regimens may stretch beyond the realm of micrometastatic disease and be eradicative also for macrotumors. Our observations indicate that at least 10 Gy are required. This agrees well with the calculated tumor control probability. Considering a relative biological effectiveness of 5, this dose level seems reasonable. However, complete remission was achieved first at activity levels close to lethal and was accompanied by biologic effects that reduced long-term survival.

Synthesis and Evaluation of Astatinated N-[2-(Maleimido)ethyl]-3-(trimethylstannyl)benzamide Immunoconjugates

Effective treatment of metastasis is a great challenge in the treatment of different types of cancers. Targeted alpha therapy utilizes the short tissue range (50-100 μm) of α particles, making the method suitable for treatment of disseminated occult cancers in the form of microtumors or even single cancer cells. A promising radioactive nuclide for this type of therapy is astatine-211. Astatine-211 attached to tumor-specific antibodies as carrier molecules is a system currently under investigation for use in targeted alpha therapy. In the common radiolabeling procedure, astatine is coupled to the antibody arbitrarily on lysine residues. By instead coupling astatine to disulfide bridges in the antibody structure, the immunoreactivity of the antibody conjugates could possibly be increased. Here, the disulfide-based conjugation was performed using a new coupling reagent, maleimidoethyl 3-(trimethylstannyl)benzamide (MSB), and evaluated for chemical stability in vitro. The immunoconjugates were subsequently astatinated, resulting in both high radiochemical yield and high specific activity. The MSB-conjugate was shown to be stable with a long shelf life prior to the astatination. In a comparison of the in vivo distribution of the new immunoconjugate with other tin-based immunoconjugates in tumor-bearing mice, the MSB conjugation method was found to be a viable option for successful astatine labeling of different monoclonal antibodies.

Biokinetic Modeling and Dosimetry for Optimizing Intraperitoneal Radioimmunotherapy of Ovarian Cancer Microtumors

A biokinetic model was constructed to evaluate and optimize various intraperitoneal radioimmunotherapies for micrometastatic tumors. The model was used to calculate the absorbed dose to both anticipated microtumors and critical healthy organs and demonstrated how intraperitoneal targeted radiotherapy can be optimized to maximize the ratio between them.

METHODS

The various transport mechanisms responsible for the biokinetics of intraperitoneally infused radiolabeled monoclonal antibodies (mAbs) were modeled using a software package. Data from the literature were complemented by pharmacokinetic data derived from our clinical phase I study to set parameter values. Results using the β-emitters (188)Re, (177)Lu, and (90)Y and the α-emitters (211)At, (213)Bi, and (212)Pb were compared. The effects of improving the specific activity, prolonging residence time by introducing an osmotic agent, and varying the activity concentration of the infused agent were investigated.

RESULTS

According to the model, a 1.7-L infused saline volume will decrease by 0.3 mL/min because of lymphatic drainage and by 0.7 mL/min because of the transcapillary convective component. The addition of an osmotic agent serves to lower the radiation dose to the bone marrow. Clinically relevant radioactivity concentrations of α- and β-emitters bound to mAbs were compared. For α-emitters, microtumors receive high doses (>20 Gy or 100 Sv [relative biological effect = 5]). Since most of the tumor dose originates from cell-bound radionuclides, an increase in the specific activity would further increase the tumor dose without affecting the dose to peritoneal fluid or bone marrow. For β-emitters, tumors will receive almost entirely nonspecific irradiation. The dose from cell-bound radiolabeled mAbs will be negligible by comparison. For the long-lived (90)Y, tumor doses are expected to be low at the maximum activity concentration delivered in clinical studies.

CONCLUSION

According to the presented model, α-emitters are needed to achieve radiation doses high enough to eradicate microscopic tumors.

Automated astatination of biomolecules--a stepping stone towards multicenter clinical trials

To facilitate multicentre clinical studies on targeted alpha therapy, it is necessary to develop an automated, on-site procedure for conjugating rare, short-lived, alpha-emitting radionuclides to biomolecules. Astatine-211 is one of the few alpha-emitting nuclides with appropriate chemical and physical properties for use in targeted therapies for cancer. Due to the very short range of the emitted α-particles, this therapy is particularly suited to treating occult, disseminated cancers. Astatine is not intrinsically tumour-specific; therefore, it requires an appropriate tumour-specific targeting vector, which can guide the radiation to the cancer cells. Consequently, an appropriate method is required for coupling the nuclide to the vector. To increase the availability of astatine-211 radiopharmaceuticals for targeted alpha therapy, their production should be automated. Here, we present a method that combines dry distillation of astatine-211 and a synthesis module for producing radiopharmaceuticals into a process platform. This platform will standardize production of astatinated radiopharmaceuticals, and hence, it will facilitate large clinical studies focused on this promising, but chemically challenging, alpha-emitting radionuclide. In this work, we describe the process platform, and we demonstrate the production of both astaine-211, for preclinical use, and astatine-211 labelled antibodies.

Absorbed Doses and Risk Estimates of (211)At-MX35 F(ab')2 in Intraperitoneal Therapy of Ovarian Cancer Patients

PURPOSE

Ovarian cancer is often diagnosed at an advanced stage with dissemination in the peritoneal cavity. Most patients achieve clinical remission after surgery and chemotherapy, but approximately 70% eventually experience recurrence, usually in the peritoneal cavity. To prevent recurrence, intraperitoneal (i.p.) targeted α therapy has been proposed as an adjuvant treatment for minimal residual disease after successful primary treatment. In the present study, we calculated absorbed and relative biological effect (RBE)-weighted (equivalent) doses in relevant normal tissues and estimated the effective dose associated with i.p. administration of (211)At-MX35 F(ab')2.

METHODS AND MATERIALS

Patients in clinical remission after salvage chemotherapy for peritoneal recurrence of ovarian cancer underwent i.p. infusion of (211)At-MX35 F(ab')2. Potassium perchlorate was given to block unwanted accumulation of (211)At in thyroid and other NIS-containing tissues. Mean absorbed doses to normal tissues were calculated from clinical data, including blood and i.p. fluid samples, urine, γ-camera images, and single-photon emission computed tomography/computed tomography images. Extrapolation of preclinical biodistribution data combined with clinical blood activity data allowed us to estimate absorbed doses in additional tissues. The equivalent dose was calculated using an RBE of 5 and the effective dose using the recommended weight factor of 20. All doses were normalized to the initial activity concentration of the infused therapy solution.

RESULTS

The urinary bladder, thyroid, and kidneys (1.9, 1.8, and 1.7 mGy per MBq/L) received the 3 highest estimated absorbed doses. When the tissue-weighting factors were applied, the largest contributors to the effective dose were the lungs, stomach, and urinary bladder. Using 100 MBq/L, organ equivalent doses were less than 10% of the estimated tolerance dose.

CONCLUSION

Intraperitoneal (211)At-MX35 F(ab')2 treatment is potentially a well-tolerated therapy for locally confined microscopic ovarian cancer. Absorbed doses to normal organs are low, but because the effective dose potentially corresponds to a risk of treatment-induced carcinogenesis, optimization may still be valuable.

Binding Affinity, Specificity and Comparative Biodistribution of the Parental Murine Monoclonal Antibody MX35 (Anti-NaPi2b) and Its Humanized Version Rebmab200

The aim of this preclinical study was to evaluate the characteristics of the monoclonal antibody Rebmab200, which is a humanized version of the ovarian-specific murine antibody MX35. This investigation contributes to the foundation for future clinical α-radioimmunotherapy of minimal residual ovarian cancer with 211At-Rebmab200. Here, the biodistribution of 211At-Rebmab200 was evaluated, as was the utility of 99mTc-Rebmab200 for bioimaging. Rebmab200 was directly compared with its murine counterpart MX35 in terms of its in-vitro capacity for binding the immobilized NaPi2B epitope and live cells; we also assessed its biodistribution in nude mice carrying subcutaneous OVCAR-3 tumors. Tumor antigen and cell binding were similar between Rebmab200 and murine MX35, as was biodistribution, including normal tissue uptake and in-vivo tumor binding. We also demonstrated that 99mTc-Rebmab200 can be used for single-photon emission computed tomography of subcutaneous ovarian carcinomas in tumor-bearing mice. Taken together, our data support the further development of Rebmab200 for radioimmunotherapy and diagnostics.

Alpha particle induced DNA damage and repair in normal cultured thyrocytes of different proliferation status

Childhood exposure to ionizing radiation increases the risk of developing thyroid cancer later in life and this is suggested to be due to higher proliferation of the young thyroid. The interest of using high-LET alpha particles from Astatine-211 ((211)At), concentrated in the thyroid by the same mechanism as (131)I [1], in cancer treatment has increased during recent years because of its high efficiency in inducing biological damage and beneficial dose distribution when compared to low-LET radiation. Most knowledge of the DNA damage response in thyroid is from studies using low-LET irradiation and much less is known of high-LET irradiation. In this paper we investigated the DNA damage response and biological consequences to photons from Cobolt-60 ((60)Co) and alpha particles from (211)At in normal primary thyrocytes of different cell cycle status. For both radiation qualities the intensity levels of γH2AX decreased during the first 24h in both cycling and stationary cultures and complete repair was seen in all cultures but cycling cells exposed to (211)At. Compared to stationary cells alpha particles were more harmful for cycling cultures, an effect also seen at the pChk2 levels. Increasing ratios of micronuclei per cell nuclei were seen up to 1Gy (211)At. We found that primary thyrocytes were much more sensitive to alpha particle exposure compared with low-LET photons. Calculations of the relative biological effectiveness yielded higher RBE for cycling cells compared with stationary cultures at a modest level of damage, clearly demonstrating that cell cycle status influences the relative effectiveness of alpha particles.

Ex vivo activity quantification in micrometastases at the cellular scale using the α-camera technique

Targeted α-therapy (TAT) appears to be an ideal therapeutic technique for eliminating malignant circulating, minimal residual, or micrometastatic cells. These types of malignancies are typically infraclinical, complicating the evaluation of potential treatments. This study presents a method of ex vivo activity quantification with an α-camera device, allowing measurement of the activity taken up by tumor cells in biologic structures a few tens of microns.

METHODS

We examined micrometastases from a murine model of ovarian carcinoma after injection of a radioimmunoconjugate labeled with (211)At for TAT. At different time points, biologic samples were excised and cryosectioned. The activity level and the number of tumor cells were determined by combined information from 2 adjacent sections: one exposed to the α-camera and the other stained with hematoxylin and eosin. The time-activity curves for tumor cell clusters, comprising fewer than 10 cells, were derived for 2 different injected activities (6 and 1 MBq).

RESULTS

High uptake and good retention of the radioimmunoconjugate were observed at the surface of tumor cells. Dosimetric calculations based on the measured time-integrated activity indicated that for an injected activity of 1 MBq, isolated tumor cells received at least 12 Gy. In larger micrometastases (≤ 100 μm in diameter), the activity uptake per cell was lower, possibly because of hindered penetration of radiolabeled antibodies; however, the mean absorbed dose delivered to tumor cells was above 30 Gy, due to cross-fire irradiation.

CONCLUSION

Using the α-camera, we developed a method of ex vivo activity quantification at the cellular scale, which was further applied to characterize the behavior of a radiolabeled antibody administered in vivo against ovarian carcinoma. This study demonstrated a reliable measurement of activity. This method of activity quantification, based on experimentally measured data, is expected to improve the relevance of small-scale dosimetry studies and thus to accelerate the optimization of TAT.

Successful radioimmunotherapy of established syngeneic rat colon carcinoma with 211At-mAb

Background

Most carcinomas are prone to metastasize despite successful treatment of the primary tumor. One way to address this clinical challenge may be targeted therapy with α-emitting radionuclides such as astatine-211 (²¹¹At). Radioimmunotherapy utilizing α-particle emitting radionuclides is considered especially suitable for the treatment of small cell clusters and single cells, although lesions of different sizes may also be present in the patient. The aim of this study was primarily to evaluate the toxicity and secondarily in vivo efficacy of a ²¹¹At-labeled monoclonal antibody (mAb) directed against colon carcinoma with tumor diameters of approximately 10 mm.

Methods

Eighteen rats with subperitoneal syngeneic colon carcinoma were allocated to three groups of six animals together with three healthy rats in each group. The groups were injected intravenously with either 150 μg of unlabeled mAbs (controls) or 2.5 or 5 MBq ²¹¹At-mAbs directed towards the Lewis Y antigen expressed on the cell membrane of several carcinomas. Tumor volume, body weight, and blood cell counts were monitored for 100 days after treatment.

Results

Local tumors were non-palpable in five out of six rats after treatment with both activities of ²¹¹At-mAbs, compared to one out of six in the control group. At the study end, half of the animals in each group given ²¹¹At-BR96 and one animal in the control group were free from disease. Radioimmunotherapy resulted in dose-dependent, transient weight loss and myelotoxicity. Survival was significantly better in the groups receiving targeted alpha therapy than in those receiving unlabeled mAbs.

Conclusions

This study demonstrates the possibility of treating small, solid colon carcinoma tumors with α-emitting radionuclides such as ²¹¹At bound to mAbs, with tolerable toxicity.

Comparison of 211At-PRIT and 211At-RIT of ovarian microtumors in a nude mouse model

Abstract Purpose: Pretargeted radioimmunotherapy (PRIT) against intraperitoneal (i.p.) ovarian microtumors using avidin-conjugated monoclonal antibody MX35 (avidin-MX35) and (211)At-labeled, biotinylated, succinylated poly-l-lysine ((211)At-B-PLsuc) was compared with conventional radioimmunotherapy (RIT) using (211)At-labeled MX35 in a nude mouse model.

METHODS

Mice were inoculated i.p. with 1×10(7) NIH:OVCAR-3 cells. After 3 weeks, they received PRIT (1.0 or 1.5 MBq), RIT (0.9 MBq), or no treatment. Concurrently, 10 additional animals were sacrificed and examined to determine disease progression at the start of therapy. Treated animals were analyzed with regard to presence of tumors and ascites (tumor-free fraction; TFF), 8 weeks after therapy.

RESULTS

Tumor status at baseline was advanced: 70% of sacrificed animals exhibited ascites. The TFFs were 0.35 (PRIT 1.0 MBq), 0.45 (PRIT 1.5 MBq), and 0.45 (RIT). The 1.5-MBq PRIT group exhibited lower incidence of ascites and fewer tumors >1 mm than RIT-treated animals.

CONCLUSIONS

PRIT was as effective as RIT with regard to TFF; however, the size distribution of tumors and presence of ascites indicated that 1.5-MBq PRIT was more efficient. Despite advanced disease in many animals at the time of treatment, PRIT demonstrated good potential to treat disseminated ovarian cancer.

Quantification of activity by alpha-camera imaging and small-scale dosimetry within ovarian carcinoma micrometastases treated with targeted alpha therapy

Targeted alpha therapy (TAT) a promising treatment for small, residual, and micrometastatic diseases has questionable efficacy against malignant lesions larger than the α-particle range, and likely requires favorable intratumoral activity distribution. Here, we characterized and quantified the activity distribution of an alpha-particle emitter radiolabelled antibody within >100-µm micrometastases in a murine ovarian carcinoma model. Nude mice bearing ovarian micrometastases were injected intra-peritoneally with 211At-MX35 (total injected activity 6 MBq, specific activity 650 MBq/mg). Animals were sacrificed at several time points, and peritoneal samples were excised and prepared for alpha-camera imaging. Spatial and temporal activity distributions within micrometastases were derived and used for small-scale dosimetry. We observed two activity distribution patterns: uniform distribution and high stable uptake (>100% IA/g at all time points) in micrometastases with no visible stromal compartment, and radial distribution (high activity on the edge and poor uptake in the core) in tumor cell lobules surrounded by fibroblasts. Activity distributions over time were characterized by a peak (140% IA/g at 4 h) in the outer tumor layer and a sharp drop beyond a depth of 50 µm. Small-scale dosimetry was performed on a multi-cellular micrometastasis model, using time-integrated activities derived from the experimental data. With injected activity of 400 kBq, tumors exhibiting uniform activity distribution received <25 Gy (EUD=13 Gy), whereas tumors presenting radial activity distribution received mean absorbed doses of <8 Gy (EUD=5 Gy). These results provide new insight into important aspects of TAT, and may explain why micrometastases >100 µm might not be effectively treated by the examined regimen.

Evaluation of effects on the peritoneum after intraperitoneal α-radioimmunotherapy with (211)At

The introduction of the short-lived α-emitter (211)At to intraperitoneal radioimmunotherapy has raised the issue of the tolerance dose of the peritoneum. The short range of the α-particles (70 μm) and the short half-life (7.21 h) of the nuclide yield a dose distribution in which the peritoneum is highly irradiated compared with other normal tissues. To address this issue, mice were injected with (211)At-trastuzumab to irradiate the peritoneum to absorbed doses ranging between 0 and 50 Gy and followed for up to 34 weeks. The peritoneum-to-plasma clearance of a small tracer, (51)Cr-ethylenediamine tetraacetic acid, was measured for evaluation of the small solute transport capacity of the peritoneal membrane. The macroscopic status of the peritoneum and the mesenteric windows was documented when the mice were sacrificed. Biopsies of the peritoneum were taken for morphology and immunohistochemical staining against plasminogen activator inhibitor-1 and calprotectin. Peritoneum-to-plasma clearance measurements indicated a dose-dependent decrease in peritoneal transport capacity in irradiated mice. However, macroscopic and microscopic evaluations of the peritoneal membrane showed no difference between irradiated mice versus controls. The results imply that the peritoneal membrane tolerates absorbed doses as high as 30-50 Gy from α-particle irradiation with limited response.

State estimation for anaerobic digesters using the ADM1

The optimization of full-scale biogas plant operation is of great importance to make biomass a competitive source of renewable energy. The implementation of innovative control and optimization algorithms, such as Nonlinear Model Predictive Control, requires an online estimation of operating states of biogas plants. This state estimation allows for optimal control and operating decisions according to the actual state of a plant. In this paper such a state estimator is developed using a calibrated simulation model of a full-scale biogas plant, which is based on the Anaerobic Digestion Model No.1. The use of advanced pattern recognition methods shows that model states can be predicted from basic online measurements such as biogas production, CH4 and CO2 content in the biogas, pH value and substrate feed volume of known substrates. The machine learning methods used are trained and evaluated using synthetic data created with the biogas plant model simulating over a wide range of possible plant operating regions. Results show that the operating state vector of the modelled anaerobic digestion process can be predicted with an overall accuracy of about 90%. This facilitates the application of state-based optimization and control algorithms on full-scale biogas plants and therefore fosters the production of eco-friendly energy from biomass.

In vivo distribution of avidin-conjugated MX35 and (211)At-labeled, biotinylated poly-L-lysine for pretargeted intraperitoneal α-radioimmunotherapy

PURPOSE

Avidin-coupled monoclonal antibody MX35 (avidin-MX35) and astatine-211-labeled, biotinylated, succinylated poly-l-lysine ((211)At-B-PL(suc)) were administered in mice to assess potential efficacy as an intraperitoneal (i.p.) therapy for microscopic tumors. We aimed to establish a timeline for pretargeted radioimmunotherapy using these substances, and estimate the maximum tolerable activity.

METHODS

(125)I-avidin-MX35 and (211)At-B-PL(suc) were administered i.p. in nude mice. Tissue distributions were studied at various time points and mean absorbed doses were estimated from organ uptake of (211)At-B-PL(suc). Studies of myelotoxicity were performed after administration of different activities of (211)At-B-PL(suc).

RESULTS

We observed low blood content of both (125)I-avidin-MX35 and (211)At-B-PL(suc), indicating fast clearance. After sodium perchlorate blocking, the highest (211)At uptake was found in kidneys. Red bone marrow (RBM) accumulated some (211)At activity. Mean absorbed doses of special interest were 2.3 Gy/MBq for kidneys, 0.4 Gy/MBq for blood, and 0.9 Gy/MBq for RBM. An absorbed dose of 0.9 Gy to the RBM was found to be safe. These values suggested that RBM would be the key dose-limiting organ in the proposed pretargeting scheme, and that blood data alone was not sufficient for predicting its absorbed dose.

CONCLUSIONS

To attain a favorable distribution of activity and avoid major toxicities, at least 1.0 MBq of (211)At-B-PL(suc) can be administered 24 hours after an i.p. injection of avidin-MX35. These results provide a basis for future i.p. therapy studies in mice of microscopic ovarian cancer.

Measurement of the correlation between electron spin and photon linear polarization in atomic-field bremsstrahlung

Atomic-field bremsstrahlung has been studied with a longitudinally polarized electron beam. The correlation between the initial orientation of the electron spin and the angle of photon polarization has been measured at the photon high energy tip region. In the time reversal this corresponds to a so-far unobserved phenomenon of production of longitudinally polarized electrons by photoionization of unpolarized atoms with linearly polarized photons. The results confirm the fully relativistic calculations for radiative recombination and suggest a new method for electron beam polarimetry.

Evidence for a spin-aligned neutron-proton paired phase from the level structure of (92)Pd

Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing, in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus (92)Pd. Gamma rays emitted following the (58)Ni((36)Ar,2n)(92)Pd fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution γ-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction. We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.

The alpha-camera: a quantitative digital autoradiography technique using a charge-coupled device for ex vivo high-resolution bioimaging of alpha-particles

Bioconjugates used in internal radiotherapy exhibit heterogeneous distributions in organs and tumors, implying a risk of nonuniform dose distribution in therapeutic applications using α-particle emitters. Tools are required that provide data on the activity distribution for estimation of absorbed dose on a suborgan level. The α-camera is a quantitative imaging technique developed to detect α-particles in tissues ex vivo. The aim of this study was to evaluate the characteristics of this imaging system and to exemplify its potential use in the development of α-radioimmunotherapy.

METHODS

The α-camera combines autoradiography with a scintillating technique and optical registration by a charge-coupled device (CCD). The imaging system characteristics were evaluated by measurements of linearity, uniformity, and spatial resolution. The technique was applied for quantitative imaging of (211)At activity distribution in cryosections of tumors, kidney, and whole body. Intratumoral activity distributions of tumor-specific (211)At-MX35-F(ab')(2) were studied at various times after injection. The postinjection activity distributions in the renal cortex and whole kidneys were compared for (211)At-F(ab')(2) and (211)At-IgG trastuzumab.

RESULTS

Quantitative analysis of α-camera images demonstrated that the pixel intensity increased linearly with activity in the imaged specimen. The spatial resolution was 35 ± 11 μm (mean ± SD) and the uniformity better than 2%. Kidney cryosections revealed a higher cortex-to-whole kidney ratio for (211)At-F(ab')(2) than for (211)At-IgG (1.38 ± 0.03 and 0.77 ± 0.04, respectively) at 2 h after injection. Nonuniform intratumoral activity distributions were found for tumor-specific (211)At-MX35-F(ab')(2) at 10 min and 7 h after injection; after 21 h, the distribution was more uniform.

CONCLUSION

The characteristics of the α-camera are promising, suggesting that this bioimaging system can assist the development, evaluation, and refinement of future targeted radiotherapy approaches using α-emitters. The α-camera provides quantitative data on the activity distribution in tissues on a near-cellular scale and can therefore be used for small-scale dosimetry, improving the prediction of biologic outcomes with α-particles with short path length and high linear energy transfer.

Intraperitoneal alpha-particle radioimmunotherapy of ovarian cancer patients: pharmacokinetics and dosimetry of (211)At-MX35 F(ab')2--a phase I study

The alpha-emitter (211)At labeled to a monoclonal antibody has proven safe and effective in treating microscopic ovarian cancer in the abdominal cavity of mice. Women in complete clinical remission after second-line chemotherapy for recurrent ovarian carcinoma were enrolled in a phase I study. The aim was to determine the pharmacokinetics for assessing absorbed dose to normal tissues and investigating toxicity.

METHODS

Nine patients underwent laparoscopy 2-5 d before the therapy; a peritoneal catheter was inserted, and the abdominal cavity was inspected to exclude the presence of macroscopic tumor growth or major adhesions. (211)At was labeled to MX35 F(ab')(2) using the reagent N-succinimidyl-3-(trimethylstannyl)-benzoate. Patients were infused with (211)At-MX35 F(ab')(2) (22.4-101 MBq/L) in dialysis solution via the peritoneal catheter. gamma-Camera scans were acquired on 3-5 occasions after infusion, and a SPECT scan was acquired at 6 h. Samples of blood, urine, and peritoneal fluid were collected at 1-48 h. Hematology and renal and thyroid function were followed for a median of 23 mo.

RESULTS

Pharmacokinetics and dosimetric results were related to the initial activity concentration (IC) of the infused solution. The decay-corrected activity concentration decreased with time in the peritoneal fluid to 50% IC at 24 h, increased in serum to 6% IC at 45 h, and increased in the thyroid to 127% +/- 63% IC at 20 h without blocking and less than 20% IC with blocking. No other organ uptakes could be detected. The cumulative urinary excretion was 40 kBq/(MBq/L) at 24 h. The estimated absorbed dose to the peritoneum was 15.6 +/- 1.0 mGy/(MBq/L), to red bone marrow it was 0.14 +/- 0.04 mGy/(MBq/L), to the urinary bladder wall it was 0.77 +/- 0.19 mGy/(MBq/L), to the unblocked thyroid it was 24.7 +/- 11.1 mGy/(MBq/L), and to the blocked thyroid it was 1.4 +/- 1.6 mGy/(MBq/L) (mean +/- SD). No adverse effects were observed either subjectively or in laboratory parameters.

CONCLUSION

This study indicates that by intraperitoneal administration of (211)At-MX35 F(ab')(2) it is possible to achieve therapeutic absorbed doses in microscopic tumor clusters without significant toxicity.

Direct procedure for the production of 211At-labeled antibodies with an epsilon-lysyl-3-(trimethylstannyl)benzamide immunoconjugate

(211)At-labeled tumor-specific antibodies have long been considered for the treatment of disseminated cancer. However, the limited availability of the nuclide and the poor efficacy of labeling procedures at clinical activity levels present major obstacles to their use. This study evaluated a procedure for the direct astatination of antibodies for the production of clinical activity levels.

METHODS

The monoclonal antibody trastuzumab was conjugated with the reagent N-succinimidyl-3-(trimethylstannyl)benzoate, and the immunoconjugate was labeled with astatine. Before astatination of the conjugated antibody, the nuclide was activated with N-iodosuccinimide. The labeling reaction was evaluated in terms of reaction time, volume of reaction solvent, immunoconjugate concentration, and applied activity. The quality of the astatinated antibodies was determined by in vitro analysis and biodistribution studies in nude mice.

RESULTS

The reaction proceeded almost instantaneously, and the results indicated a low dependence on immunoconjugate concentration and applied activity. Radiochemical labeling yields were in the range of 68%-81%, and a specific radioactivity of up to 1 GBq/mg could be achieved. Stability and radiochemical purity were equal to or better than those attained with a conventional 2-step procedure. Dissociation constants for directly astatinated, conventionally astatinated, and radioiodinated trastuzumab were 1.0+/-0.06 (mean+/-SD), 0.44+/-0.06, and 0.29+/-0.02 nM, respectively. The tissue distribution in non-tumor-bearing nude mice revealed only minor differences in organ uptake relative to that obtained with the conventional method.

CONCLUSION

The direct astatination procedure enables the high-yield production of astatinated antibodies with radioactivity in the amounts required for clinical applications.

Therapeutic efficacy of astatine-211-labeled trastuzumab on radioresistant SKOV-3 tumors in nude mice

PURPOSE

To investigate the potential use of astatine-211 (211At)-labeled trastuzumab for the treatment of HER-2-positive, radioresistant ovarian carcinoma.

METHODS AND MATERIALS

Four-week-old nude mice were inoculated intraperitoneally with 5 . 10(6) SKOV-3 cells in 0.4 mL saline on Day 0. The endpoint was the total tumor weight in each mouse on Day 63. Three experiments were performed in which the response to single-dose and fractionated treatment with unlabeled and 211At-labeled antibody was evaluated.

RESULTS

Experiment 1 showed, for the same total amount of trastuzumab, a dose-response relationship between 211At activity (0-400 kBq on Day 7) and therapeutic efficacy (p = 0.001). The effect of varying the amount of unlabeled trastuzumab was studied in Experiment 2. All mice, except for the controls, received 400 kBq 211At-trastuzumab, and different groups received 5, 50, or 500 microg trastuzumab on Day 7. The increase from 5 to 50 microg trastuzumab reduced the tumors by 78% in weight. No tumors were present in mice given 500 microg trastuzumab. In Experiment 3, the effect of a fractionated treatment regimen was studied. Mice that received 100 kBq 211At-trastuzumab on Days 7 and 8 had a 42% smaller tumor burden than did controls. Groups of mice injected with 200 + 100 kBq on Days 7 and 21 and mice injected with 100 kBq on Days 7, 8, and 21 both had 24% less tumor weight than the corresponding controls.

CONCLUSION

The combination of 500 microg trastuzumab and 400 kBq 211At-trastuzumab had the greatest effect, with complete eradication of the tumors in this nude mouse model.

Administered activity and metastatic cure probability during radioimmunotherapy of ovarian cancer in nude mice with 211At-MX35 F(ab')2

PURPOSE

To elucidate the therapeutic efficacy of alpha-radioimmunotherapy of ovarian cancer in mice. This study: (i) estimated the minimum required activity (MRA), giving a reasonable high therapeutic efficacy; and (ii) calculated the specific energy to tumor cell nuclei and the metastatic cure probability (MCP) using various assumptions regarding monoclonal-antibody (mAb) distribution in measured tumors. The study was performed using the alpha-particle emitter Astatine-211 (211At) labeled to the mAb MX35 F(ab')2.

METHODS AND MATERIALS

Animals were inoculated intraperitoneally with approximately 1 x 10(7) cells of the cell line NIH:OVCAR-3. Four weeks later animals were treated with 25, 50, 100, or 200 kBq 211At-MX35 F(ab')2 (n = 74). Another group of animals was treated with a nonspecific mAb: 100 kBq 211At-Rituximab F(ab')2 (n = 18). Eight weeks after treatment the animals were sacrificed and presence of macro- and microscopic tumors and ascites was determined. An MCP model was developed and compared with the experimentally determined tumor-free fraction (TFF).

RESULTS

When treatment was given 4 weeks after cell inoculation, the TFFs were 25%, 22%, 50%, and 61% after treatment with 25, 50, 100, or 200 kBq (211)At-MX35 F(ab')2, respectively, the specific energy to irradiated cell nuclei varying between approximately 2 and approximately 400 Gy.

CONCLUSION

As a significant increase in the therapeutic efficacy was observed between the activity levels of 50 and 100 kBq (TFF increase from 22% to 50%), the conclusion was that the MRA is approximately 100 kBq (211)At-MX35 F(ab')2. MCP was most consistent with the TFF when assuming a diffusion depth of 30 mum of the mAbs in the tumors.

Alpha-radioimmunotherapy of intraperitoneally growing OVCAR-3 tumors of variable dimensions: Outcome related to measured tumor size and mean absorbed dose

The purpose of this work was to (a) investigate the efficacy of radioimmunotherapy using 211At-MX35 F(ab')2 or 211At-Rituximab F(ab')2 (nonspecific antibody) against differently advanced ovarian cancer in mice; (b) image the tumor growth on the peritoneum; and (c) calculate the specific energy and mean absorbed dose to tumors and critical organs.

METHODS

Two experiments with 5-wk-old nude mice (n = 100 + 93), intraperitoneally inoculated with approximately 1 x 10(7) NIH:OVCAR-3 cells, were done. At either 1, 3, 4, 5, or 7 wk after inoculation animals were intraperitoneally treated with approximately 400 kBq 211At-MX35 F(ab')2 (n = 50 + 45), approximately 400 kBq 211At-Rituximab F(ab')2 (n = 25 + 24), or unlabeled Rituximab F(ab')2 (n = 25 + 24). At the time of treatment 29 animals were sacrificed and biopsies were taken for determination of tumor sizes using scanning electron microscopy (SEM). Eight weeks after each treatment the animals were sacrificed and the presence of macro- and microscopic tumors and ascites was determined. The specific energy and mean absorbed dose to tumors were calculated. The activity concentration was measured in critical organs and abdominal fluid.

RESULTS

When given treatment 1, 3, 4, 5, or 7 wk after cell inoculation the tumor-free fraction (TFF) was 95%, 68%, 58%, 47%, 26%, and 100%, 80%, 20%, 20%, and 0% when treated with 211At-MX35 F(ab')2 or 211At-Rituximab F(ab')2, respectively. The SEM images revealed maximum tumor radius of approximately 30 mum 1 wk after cell inoculation, increasing to approximately 340 mum at 7 wk. Specific energy to cell nuclei varied between 0 and approximately 540 Gy, depending on assumptions regarding activity distribution and tumor size. The mean absorbed dose to thyroid, kidneys, and bone marrow was approximately 35, approximately 4, and approximately 0.3 Gy, respectively.

CONCLUSION

Treatment with 211At-MX35 F(ab')2 or 211At-Rituximab F(ab')2 resulted in a TFF of 95%-100% when the tumor radius was < or =30 microm. The TFF was decreased (TFF < or = 20%) for 211At-Rituximab F(ab')2 when the tumor radius exceeded the range of the alpha-particles. The specific antibody gave for these tumor sizes a significantly better TFF, explained by a high mean absorbed dose (>22 Gy) from the activity bound to the tumor surface and probably some contribution from penetrating activity.

211At radioimmunotherapy of subcutaneous human ovarian cancer xenografts: evaluation of relative biologic effectiveness of an alpha-emitter in vivo

The use of alpha-particle emitters in radioimmunotherapy (RIT) appears to be promising. We previously obtained convincing results in the treatment of microscopic intraperitoneal ovarian cancer in nude mice by using the alpha-emitter 211At. This study was performed to evaluate the relative biological effectiveness (RBE) of 211At compared with that of 60Co gamma-irradiation in an RIT model. Our endpoint was growth inhibition (GI) of subcutaneous xenografts.

METHODS

GI after irradiation was studied with subcutaneous xenografts of the human ovarian cancer cell line NIH:OVCAR-3 implanted in nude mice. The animals received an intravenous injection of 211At-labeled monoclonal antibody MX35 F(ab')2 at different levels of radioactivity (0.33, 0.65, and 0.90 MBq). Control mice received unlabeled MX35 F(ab')2 only. To calculate the mean absorbed dose to tumor, a separate biodistribution study established the uptake of 211At in tumors and organs at different times after injection. External irradiation of the tumors was performed with 60Co. Tumor growth was monitored, and the normalized tumor volume (NTV) was calculated for each tumor. GI was defined by dividing the NTV values by the fitted NTV curve obtained from the corresponding control mice. To compare the biologic effects of the 2 radiation qualities, the mean value for GI (from day 8 to day 23) was plotted for each tumor as a function of its corresponding absorbed dose. From exponential fits of these curves, the doses required for a GI of 0.37 (D37) were derived, and the RBE of 211At was calculated.

RESULTS

The biodistribution study showed the uptake of the immunoconjugate by the tumor (amount of injected radioactivity per gram) to be 14% after 7 h. At 40 h, the ratio of uptake in tumors to uptake in blood reached a maximum value of 6.2. The administered activities of 211At corresponded to doses absorbed by tumors of 1.35, 2.65, and 3.70 Gy. The value (mean+/-SEM) for D37 was 1.59+/-0.08 Gy. Tumor growth after 60Co external irradiation showed a value for D37 of 7.65+/-1.0 Gy. The corresponding RBE of 211At irradiation was 4.8+/-0.7.

CONCLUSION

Using a tumor GI model in nude mice, we were able to derive an RBE of alpha-particle RIT with 211At. The RBE was found to be 4.8+/-0.7.

Therapeutic efficacy and tumor dose estimations in radioimmunotherapy of intraperitoneally growing OVCAR-3 cells in nude mice with (211)At-labeled monoclonal antibody MX35

The purpose of this study was to investigate the therapeutic efficacy of-and to estimate the absorbed dose to-tumor cells from radioimmunotherapy (RIT) in an ovarian cancer model using the alpha-particle-emitting nuclide (211)At labeled to monoclonal antibody (mAb) MX35. Previous studies on mAb MOv18 did not allow for dosimetry because of antigen shedding in vitro.

METHODS

Five-week-old female nude BALB/c nu/nu mice were inoculated intraperitoneally with 1 x 10(7) cells of the human tumor cell line OVCAR-3. Three weeks later, the animals were given approximately 400, 800, or 1,200 kBq of (211)At-labeled mAb MX35 intraperitoneally. As controls, one group of animals was injected with unlabeled mAb and another group was injected with phosphate-buffered saline (PBS). Another group was given approximately 400 kBq of (211)At labeled to the previously investigated mAb MOv18 for efficacy comparison. Two months after treatment, the animals were sacrificed and the presence of macroscopic and microscopic tumors, as well as ascites, was determined. The absorbed dose to tumor cells on the peritoneal surface was estimated in terms of the sum of a specific and a nonspecific contribution. The specific contribution, arising from mAbs binding to the antigenic sites on the cell membrane, was calculated using a dynamic compartment model developed in-house and Monte Carlo software. The model used as input values the number of mAbs injected into the abdominal cavity, N(mAb), the specific activity, A(sp), the association rate constant, k(on), and the maximal number of mAbs bound per cell, B(max)-all determined by in vitro experiments. This specific component of the absorbed dose was calculated for assumed cell cluster sizes with radii of 25, 50, and 100 microm. The nonspecific contribution to the absorbed dose was derived from unbound mAbs freely circulating in the abdominal cavity, also using the Monte Carlo software.

RESULTS

In the control groups given unlabeled MX35 or PBS, all 18 animals had ascites, 6 of 9 animals in each group had macroscopic tumors, and all animals had microscopic growth. In the 3 groups given different amounts of (211)At-MX35, only 3 of 25 animals developed ascites. None of these animals had any sign of macroscopic tumors, but 8 had microscopic growth. In the group given (211)At-MOv18, no animals had ascites or macroscopic tumors, but 3 of 10 animals had microscopic tumors. After injecting 400 kBq of (211)At-MX35, the absorbed dose due to specific binding, for a cell cluster with a radius of 50 microm, ranged from 413 to 223 Gy between 0- and 45-microm distance from the cluster center, assuming a homogeneous distribution of (211)At-MX35 in the cluster. The contribution from unbound (211)At-MX35 and (211)At-MX35 only distributed on the cluster surface, for this cluster size, ranged from 7 to 14 Gy and from 29 to 94 Gy, between 0- and 45-microm distance from the cluster center, respectively. The calculated total absorbed doses are in a clinically relevant range and were effective as verified in the nude mice with subclinical intraperitoneal growth of OVCAR-3 cells.

CONCLUSION

(211)At-MX35 injected intraperitoneally exhibits a high efficacy when treating micrometastatic growth of the ovarian cancer cell line OVCAR-3 on the peritoneum of nude mice.

Single-cell irradiation from [211At] astatine-labeled C215 monoclonal antibody: improved estimates of radiosensitivity from measurements on cellular uptake and retention

New data on the biological effect of 211At-C215 monoclonal antibody in a slowly rotating, widely dispersed single-cell suspension of the human cancer cell line Colo-205 is presented. Cell growth curves of each experiment were used to calculate an apparent cell survival after irradiation. Uptake measurements provided the data needed to calculate the average number of 211At decays per cell in the cell suspension. The results from each experiment were then fit to a mono-exponential function. From the exponential fit, an average of 35 +/- 2 (SD) astatine-211 decays per cell are required for 37% apparent cell survival (D0).

Synthesis and biodistribution of 211At-labeled, biotinylated, and charge-modified poly-L-lysine: evaluation for use as an effector molecule in pretargeted intraperitoneal tumor therapy

Poly-L-lysine (7, 21, and 204 kDa) has been evaluated as an effector carrier for use in pretargeted intraperitoneal tumor therapy. For the synthesis, the epsilon-amino groups on the poly-L-lysine were modified in three steps utilizing conjugate biotinylation with biotin amidocaproate N-hydroxysuccinimide ester (BANHS), conjugate radiolabeling with (211)At using the intermediate reagent N-succinimidyl 3-(trimethylstannyl)benzoate (m-MeATE), and charge modification using succinic anhydride, resulting in an increase in the molecular weight of approximately 80% of the final product. The labeling of the m-MeATE reagent and subsequent conjugation of the polymer were highly efficient with overall radiochemical yields in the range of 60-70%. The in vitro avidin binding ability of the modified polymer was almost complete (90-95%), as determined by binding to avidin beads using a convenient filter tube assay. Following intraperitoneal (ip) injection in athymic mice, the 13 kDa polymer product was cleared mainly via the kidneys with fast kinetics (biological half-live T(b) approximately 2 h) and with low whole-body retention. The clearance of the 38 kDa polymer was distributed between kidneys and liver, and the 363 kDa polymer was mainly sequestered by the liver with a T(b) of 8 h. Increased tissue uptake in the thyroid, lungs, stomach, and spleen following the distribution of the large effector molecules (38 and 363 kDa) suggests that degradation of the polymers by the liver may release some of the label as free astatine/astatide.