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

Overview of the Most Promising Radionuclides for Targeted Alpha Therapy: The "Hopeful Eight"

Among all existing radionuclides, only a few are of interest for therapeutic applications and more specifically for targeted alpha therapy (TAT). From this selection, actinium-225, astatine-211, bismuth-212, bismuth-213, lead-212, radium-223, terbium-149 and thorium-227 are considered as the most suitable. Despite common general features, they all have their own physical characteristics that make them singular and so promising for TAT. These radionuclides were largely studied over the last two decades, leading to a better knowledge of their production process and chemical behavior, allowing for an increasing number of biological evaluations. The aim of this review is to summarize the main properties of these eight chosen radionuclides. An overview from their availability to the resulting clinical studies, by way of chemical design and preclinical studies is discussed.

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.

Radioimmunotherapy of cancer with high linear energy transfer (LET) radiation delivered by radionuclides emitting α-particles or Auger electrons

Radioimmunotherapy (RIT) aims to selectively deliver radionuclides emitting α-particles, β-particles or Auger electrons to tumors by conjugation to monoclonal antibodies (mAbs) that recognize tumor-associated antigens/receptors. The approach has been most successful for treatment of non-Hodgkin's B-cell lymphoma but challenges have been encountered in extending these promising results to the treatment of solid malignancies. These challenges include the low potency of β-particle emitters such as ¹³¹I, ¹⁷⁷Lu or ⁹⁰Y which have been commonly conjugated to the mAbs, due to their low linear energy transfer (LET=0.1-1.0keV/μm). Furthermore, since the β-particles have a 2-10mm range, there has been dose-limiting non-specific toxicity to hematopoietic stem cells in the bone marrow (BM) due to the cross-fire effect. Conjugation of mAbs to α-particle-emitters (e.g. ²²⁵Ac, ²¹³Bi, ²¹²Pb or ²¹¹At) or Auger electron-emitters (e.g. ¹¹¹In, ⁶⁷Ga, ¹²³I or ¹²⁵I) would increase the potency of RIT due to their high LET (50-230keV/μm and 4 to 26keV/μm, respectively). In addition, α-particles have a range in tissues of 28-100μm and Auger electrons are nanometer in range which greatly reduces or eliminates the cross-fire effect compared to β-particles, potentially reducing their non-specific toxicity to the BM. In this review, we describe the results of preclinical and clinical studies of RIT of cancer using radioimmunoconjugates emitting α-particles or Auger electrons, and discuss the potential of these high LET forms of radiation to improve the outcome of cancer patients.

Application of 212Pb for Targeted α-particle Therapy (TAT): Pre-clinical and Mechanistic Understanding through to Clinical Translation

Targeted α-particle therapy (TAT), in which an α-particle emitting radionuclide is specifically directed to a biological target, is gaining more attention to treat cancers as new targets are validated. Bio-vectors such as monoclonal antibodies are able to selectively transport α-particles to destroy targeted cancer cells. TAT has the potential for an improved therapeutic ratio over β-particle targeted conjugate therapy. The short path length and the intense ionization path generated render α-emitters suitable for treatment and management of minimal disease such as micrometastases or residual tumor after surgical debulking. ²¹²Pb is the longer-lived parent radionuclide of ²¹²Bi and serves as an in vivo generator of ²¹²Bi. ²¹²Pb has demonstrated significant utility in both in vitro and in vivo models. Recent evaluation of ²¹²Pb-TCMC-trastuzumab in a Phase I clinical trial has demonstrated the feasibility of ²¹²Pb in TAT for the treatment of ovarian cancer patients. This review highlights progress in radionuclide production, radiolabeling chemistry, molecular mechanisms, and application of ²¹²Pb to targeted pre-clinical and clinical radiation therapy for the management and treatment of cancer.

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.

Production of [(211)At]-astatinated radiopharmaceuticals and applications in targeted α-particle therapy

(211)At is a promising radionuclide for α-particle therapy of cancers. Its physical characteristics make this radionuclide particularly interesting to consider when bound to cancer-targeting biomolecules for the treatment of microscopic tumors. (211)At is produced by cyclotron irradiation of (209)Bi with α-particles accelerated at ~28 MeV and can be obtained in high radionuclidic purity after isolation from the target. Its chemistry resembles iodine, but there is also a tendency to behave as a metalloid. However, the chemical behavior of astatine has not yet been clearly established, primarily due to the lack of any stable isotopes of this element, which precludes the use of conventional analytical techniques for its characterization. There are also only a limited number of research centers that have been able to produce this element in sufficient amounts to carry out extensive investigations. Despite these difficulties, chemical reactions typically used with iodine can be performed, and a number of biomolecules of interest have been labeled with (211)At. However, most of these compounds exhibit unacceptable instability in vivo due to the weakness of the astatine-biomolecule bond. Nonetheless, several compounds have shown high potential for the treatment of cancers in vitro and in several animal models, thus providing a promising basis that has allowed initiation of the first two clinical studies.

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.

An overview of targeted alpha therapy

The effectiveness of targeted α-therapy (TAT) can be explained by the properties of α-particles. Alpha particles are helium nuclei and are ~8,000 times larger than β(-)-particles (electrons). When emitted from radionuclides that decay via an α-decay pathway, they release enormous amounts of energy over a very short distance. Typically, the range of α-particles in tissue is 50-100 μm and they have high linear energy transfer (LET) with a mean energy deposition of 100 keV/μm, providing a more specific tumor cell killing ability without damage to the surrounding normal tissues than β(-)-emitters. Due to these properties, the majority of pre-clinical and clinical trials have demonstrated that α-emitters such as (225)Ac, (211)At, (212)Bi, (213)Bi, (212)Pb, (223)Ra, and (227)Th are ideal for the treatment of smaller tumor burdens, micrometastatic disease, and disseminated disease. Even though these α-emitters have favorable properties, the development of TAT has been limited by high costs, unresolved chemistry, and limited availability of the radionuclides. To overcome these limitations, more potent isotopes, additional sources, and more efficient isotope production methods should be addressed. Furthermore, better chelation and labeling methods with the improvements of isotope delivery, targeting vehicles, molecular targets, and identification of appropriate clinical applications are still required.

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.

In vitro evaluation and biodistribution of HER2-targeted liposomes loaded with an (125)I-labelled DNA-intercalator

BACKGROUND

Increasing attention is currently focussed on the issue of finding strategies for the delivery of Auger-electron-emitting radionuclides into tumor cell nuclei.

PURPOSE

In this study, we investigated tumor-cell uptake and cell-killing ability in vitro as well as in vivo biodistribution of an (125)I-labelled anthracycline derivative administered by means of HER2-targeted liposomes.

METHODS

Anthracycline derivative Comp1 was radiolabelled with Auger-emitting (125)I and encapsulated in liposomes (DSPC:Chol:DSPE-PEG) using pH-gradient loading. Single-chain fragment F5 was anchored to the liposomes as targeting device for HER2. Uptake and specificity of (125)I-Comp1 delivered via targeting and non-targeting liposomes were analysed in cultured HER2-overexpressing cells. Cell-killing efficacy was evaluated in SKOV3 cells and biodistribution for up to 48 h was studied after intraperitoneal injection in tumor-bearing female BALB/c nu/nu mice.

RESULTS

(125)I-Comp1 was specifically taken up by the cultured cells when administered by means of HER2-targeted liposomes and a clear dose-effect correlation in survival of cells was seen with increasing specific activity. The biodistribution studies revealed that (125)I-Comp1 accumulated in tumors when distributed using HER2-targeted liposomes and that this effect was absent when using non-targeting liposomes.

CONCLUSION

The HER2-targeted liposomes possess the properties needed to bring about tumor-specific delivery and therapeutic effect of (125)I-Comp1.

Intraperitoneal Alpha-Radioimmunotherapy of Advanced Ovarian Cancer in Nude Mice Using Different High Specific Activities

Background

The aim of this study was to investigate the therapeutic efficacy of advanced ovarian cancer in mice, using α-radioimmunotherapy with different high specific activities. The study was performed using the monoclonal antibody (mAb) MX35 F(ab′)2 labeled with the α-particle emitter ²¹¹At.

Methods

Animals were intraperitoneally inoculated with ≥1 × 10⁷ cells of the ovarian cancer cell line NIH:OVCAR-3. Four weeks later 9 groups of animals were given 25, 50, or 400 kBq ²¹¹At-MX35 F(ab′)2 with specific activities equal to 1/80, 1/500, or 1/1200 (²¹¹At atom/number of mAbs) for every activity level respectively (n = 10 in each group). As controls, animals were given PBS or unlabeled MX35 F(ab′)2 in PBS (n = 10 in each group). Eight weeks after treatment the animals were sacrificed and the presence of macroscopic tumors was determined by meticulous ocular examination of the abdominal cavity. Cumulated activity and absorbed dose calculations on tumor cells and tumors were performed using in house developed program. Specimens for scanning electron-microscopy analysis were collected from the peritoneum at the time of dissection.

Results

Summing over the different activity levels (25, 50, and 400 kBq ²¹¹At-MX35 F(ab′)2) the number of animals with macroscopic tumors was 13, 17, and 22 (n = 30 for each group) for the specific activities equal to 1/80, 1/500, or 1/1200, respectively. Logistic-regression analysis showed a significant trend that higher specific activity means less probability for macroscopic tumors (P = 0.02).

Conclusions

Increasing the specific activity indicates a way to enhance the therapeutic outcome of advanced ovarian cancer, regarding macroscopic tumors. Further studies of the role of the specific activity are therefore justified.

Intraperitoneal alpha-radioimmunotherapy in mice using different specific activities

PURPOSE

The aim of this study was to investigate the therapeutic efficacy of the alpha-radioimmunotherapy of ovarian cancer in mice, using different specific activities. This study was performed by using the monoclonal antibody, MX35 F(ab')(2), labeled with the alpha-particle-emitter, 211At.

METHODS

Animals were intraperitoneally inoculated with approximately 1 x 10(7) cells of the cell line, NIH:OVCAR-3. Four (4) weeks later, five groups of animals were given 400 kBq of 211At-MX35 F(ab')(2) with specific activities equal to 130, 65, 32, 16, or 4 kBq/microg, respectively (n = 18 in each group). As controls, animals were given unlabeled MX35 F(ab')(2) (n = 12). Eight (8) weeks after treatment, the animals were sacrificed and the presence of macro- and microscopic tumors and ascites was determined.

RESULTS

The tumor-free fractions (TFFs) of the animals, defined as the fraction of animals with no macro- and microtumors and no ascites, were 0.67, 0.73, 0.50, 0.50, and 0.17 when treated as above. Only the TFF of 0.17, for the specific activity of 4 kBq/microg, was significantly less, compared to that of the specific activity of 130 kBq/microg. The TFF for the specific activity of 4 kBq/microg showed a significant lowering, compared to the specific activity of 130 kBq/microg (p < 0.05). Treatment with unlabeled MX35 F(ab')(2) resulted in a TFF of zero.

CONCLUSIONS

A specific activity-dependent therapeutic outcome could not be shown in the interval of 130- to 16 kBq/mug. For lower specific activities (i.e., 4 kBq/microg), the therapeutic efficacy was significantly lowered.

Repeated Intraperitoneal alpha-Radioimmunotherapy of Ovarian Cancer in Mice

The aim of this study was to investigate the therapeutic efficacy of α-radioimmunotherapy of ovarian cancer in mice using different fractionated treatment regimens. The study was performed using the monoclonal antibody MX35 F(ab′)₂ labeled with the α-particle emitter ²¹¹At. Methods. Nude mice were intraperitoneally inoculated with ~1 × 10⁷ cells of the cell line NIH:OVCAR-3. Four weeks later 6 groups of animals were given 400 kBq ²¹¹At-MX35 F(ab′)₂ as a single or as a repeated treatment of up to 6 times (n = 18 in each group). The fractionated treatments were given every seventh day. Control animals were treated with unlabeled MX35 F(ab′)₂ (n = 12). Eight weeks posttreatment the animals were sacrificed and the presence of macro- and microscopic tumors and ascites was determined. Results. The tumor-free fractions (TFFs) of the animals, defined as the fraction of animals with no macro- and microtumors and no ascites, were 0.17, 0.11, 0.39, 0.44, 0.44, and 0.67 when treated with 400 kBq ²¹¹At-MX35 F(ab′)₂ once or 2, 3, 4, 5, or 6 times, respectively. Repeated treatment 3 times or more resulted in a significantly higher (P < .05) TFF than compared to treatment once or twice. The presence of ascites decreased from 15 out of 18 animals in the group given only one treatment to zero for the 2 groups given 5 or 6 fractions. Treatment with unlabeled MX35 F(ab′)₂ resulted in a TFF of zero. Conclusion. Weekly repeated intraperitoneal injections of tolerable amounts of activity of ²¹¹At-MX35 F(ab′)₂ of up to 6 times produced increased therapeutic efficacy without observed toxicity, indicating a potential increase of the therapeutic index.

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.

Astatine Radiopharmaceuticals: Prospects and Problems

For the treatment of minimum residual diseases such micrometastases and residual tumor margins that remain after debulking of the primary tumor, targeted radiotherapy using radiopharmaceuticals tagged with alpha-particle-emitting radionuclides is very attractive. In addition to the their short range in tissue, which helps minimize harmful effects on adjacent normal tissues, alpha-particles, being high LET radiation, have several radiobiological advantages. The heavy halogen, astatine-211 is one of the prominent alpha-particle-emitting radionuclides in practice. Being a halogen, it can often be incorporated into biomolecules of interest by adapting radioiodination chemistry. A wide spectrum of compounds from the simple [(211)At]astatide ion to small organic molecules, peptides, and large proteins labeled with (211)At have been investigated with at least two reaching the stage of clinical evaluation. The chemistry, cytotoxic advantages, biodistribution studies, and microdosimetry/pharmacokinetic modeling of some of these agents will be reviewed. In addition, potential problems such as the harmful effect of radiolysis on the synthesis, lack of sufficient in vivo stability of astatinated compounds, and possible adverse effects when they are systemically administered will be discussed.

Complexation study on no-carrier-added astatine with insulin: a candidate radiopharmaceutical

No-carrier-added astatine radionuclides produced in the (7)Li-irradiated lead matrix were separated from bulk lead nitrate target by complexing At with insulin, followed by dialysis. The method offers simultaneous separation of At from lead as well as its complexation with insulin. The At-insulin complex might be a potential radiopharmaceutical in the treatment of hepatocellular carcinoma. The stability of At-insulin complex was checked by dialysis against deionized water and Ringer lactate (RL) solution. It has been found that the half-life of At-insulin complex is about approximately 12h, when dialyzed against deionized water and is only 6h, when dialyzed against RL solution having the same composition as blood serum. The 6h half-life of this Insulin-At complex is perfect for killing cancer cells from external cell surfaces as the half-life of internalization of insulin molecule inside the cell is 7-12h.

Production and separation of Astatine Radionuclides: some new addition to Astatine Chemistry

For the first time no-carrier-added (nca) (209,210)At radionuclides were produced by heavy ion ((7)Li) activation from 4 mg/cm(2) lead nitrate target. Astatine was separated from the bulk target by three different approaches. Nca astatine radionuclide was selectively partitioned in the (i) polymer rich phase of an aqueous biphasic system consisting of polyethylene glycol (PEG) 4000 (50% w/w) and 2M Na(2)SO(4) solution (ii) aqueous phase of a liquid liquid extraction system being comprised of a liquid cation exchanger, HDEHP (di-2-ethylhexyl phosphoric acid) (0.5%) and liquor ammonia (iii) liquid phase of a solid liquid extraction system of cation exchange resin Dowex-50 and 10(-6)M HCl. Very high separation factors have been achieved in all the three methods.

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.

Fractionated radioimmunotherapy of intraperitoneally growing ovarian cancer in nude mice with 211At-MX35 F(ab')2: therapeutic efficacy and myelotoxicity

OBJECTIVE

The aim of this study was to investigate the therapeutic efficacy and myelotoxicity during fractionated radioimmunotherapy of ovarian cancer in mice. The study was performed using the monoclonal antibody MX35 F(ab')(2) labeled with the alpha-particle emitter (211)At.

METHODS

Animals were intraperitoneally inoculated with approximately 1x10(7) cells of the cell line NIH:OVCAR-3. Four weeks later, the mice were given the first treatment. Six groups of animals were intraperitoneally injected with approximately 800, 3x approximately 267, approximately 400, 3x approximately 133, approximately 50 or 3x approximately 17 kBq (211)At-MX35 F(ab')(2) (n=18 in each group). The second and third injections for Groups 2, 4 and 6 were given 4 and 8 days after the first injection, respectively. As controls, animals were treated with unlabeled MX35 F(ab')(2) (n=12). Eight weeks after the last injection, the animals were sacrificed and the presence of macro- and microscopic tumors and ascites was determined. Blood counts were determined for each mouse in Groups 1 and 2 before the first injection and 3, 7, 11, 15 and 23 days after the first injection. The calculation of the mean absorbed dose to the bone marrow was based on the ratio between the (211)At-activity concentration in bone and blood [i.e., the bone-to-blood ratio (BBLR)] as well as that between the (211)At-activity concentration in bone marrow and blood [i.e., the bone-marrow-to-blood ratio (BMBLR)] and the cumulated activity and absorbed fraction of the alpha-particles emitted by (211)At in the bone marrow.

RESULTS

The tumor-free fractions of animals were 56% and 41% when treated with approximately 800 kBq and 3x approximately 267 kBq (211)At-MX35 F(ab')(2), respectively; 39% and 28% when treated with approximately 400 kBq and 3x approximately 133 kBq (211)At-MX35 F(ab')(2), respectively; and 17% and 22% when treated with approximately 50 kBq or 3x approximately 17 kBq (211)At-MX35 F(ab')(2), respectively. The nadir of the white blood cell (WBC) counts was decreased (from 46% to 19%, compared with the baseline WBC counts) and delayed (from Day 4 to Day 11 after the first injection) during the fractionated treatment compared with the single-dose treatment. The percentage of injected activity per gram (%IA/g) for blood, bone and bone marrow all peaked 6 h after injection at 13.80+/-1.34%IA/g, 4.00+/-0.69%IA/g and 8.28+/-1.38%IA/g, respectively. The BBLR and BMBLR were 0.20+/-0.04 and 0.58+/-0.01, respectively. The mean absorbed dose to bone marrow was approximately 0.4 Gy after intraperitoneally injecting approximately 800 kBq (211)At-MX35 F(ab')(2).

CONCLUSION

No advantage was observed in the therapeutic efficacy of using a fractionated regimen compared with a single administration, with the same total amount of administered activity. Alleviation of the myelotoxicity was observed during the fractionated regimen in terms of decreased suppression and delayed nadir of the WBC counts. No thrombocytopenia was observed during either regimen.