First in vivo evaluation of liposome-encapsulated 223Ra as a potential alpha-particle-emitting cancer therapeutic agent

BACKGROUND

Liposomes carrying chemotherapeutics have had some success in cancer treatment and may also be suitable carriers for therapeutic radionuclides. This study was designed to evaluate the biodistribution and to estimate the radiation doses of the alpha emitter 223Ra loaded into pegylated liposomes in selected tissues.

MATERIALS AND METHODS

223Ra was encapsulated in pegylated liposomal doxorubicin (PLD) by ionophore-mediated loading. The biodistribution of liposomal 223Ra was compared to free cationic 223Ra in Balb/C mice.

RESULTS

Liposomal 223Ra circulated in the blood with an initial half-life in excess of 24 hours, which agreed well with that reported for PLD in rodents, while the blood half-life of cationic 223Ra was considerably less than an hour. When liposomal 223Ra was catabolized, the released 223Ra was either excreted or taken up in the skeleton. This skeletal uptake increased up to 14 days after treatment, but did not reach the level seen with free 223Ra. Pre-treatment with non-radioactive PLD 4 days in advance lessened the liver uptake of liposomal 223Ra. Dose estimates showed that the spleen, followed by bone surfaces, received the highest absorbed doses.

CONCLUSION

Liposomal 223Ra was relatively stable in vivo and may have potential for radionuclide therapy and combination therapy with chemotherapeutic agents.

Radiotoxicity of the alpha-emitting bone-seeker 223Ra injected intravenously into mice: histology, clinical chemistry and hematology

BACKGROUND

The alpha-emitter 223Ra, which localizes in osteoblastic active zones, including on skeletal surfaces and in osteoblastic metastases, has recently been introduced as a potential therapeutic agent against skeletal metastases. Here, the adverse effects of high dosages in animals were investigated.

MATERIALS AND METHODS

Balb/c mice received intravenously (i.v.) either 1250, 2500, or 3750 kBq/kg of dissolved 223RaCl2 and were followed in the initial toxicity phase. At the 4-week end-point, the animals were sacrificed and blood samples were collected to study the effects on clinical chemistry and hematological parameters. Selected organs were weighed and tissue samples examined by microscopy.

RESULTS

Treatment with 223Ra caused a dose-related minimal to moderate depletion of osteocytes and osteoblasts in the bones. Furthermore, a dose-related minimal to marked depletion of the hematopoietic cells in the bone marrow, and a minimal to slight extramedullary hematopoiesis in the spleen and in the mandibular and mesenteric lymph nodes were observed. The LD50 for acute toxicity, defined as death within 4 weeks of receiving the substance, was not reached.

CONCLUSION

This study demonstrated that high doses of the bone-seeker 223Ra did not completely inactivate the blood-producing cells. The relatively high tolerance to skeletal alpha doses was probably caused by the surviving pockets of red bone marrow cells beyond the range of alpha particles from the bone surfaces, and the recruitment of peripheral stems cells.

Initial evaluation of 227Th-p-benzyl-DOTA-rituximab for low-dose rate α-particle radioimmunotherapy

Radioimmunotherapy has proven clinically effective in patients with non-Hodgkin's lymphoma. Radioimmunotherapy trials have so far been performed with beta-emitting isotopes. In contrast to beta-emitters, the shorter range and high linear energy transfer (LET) of alpha particles allow for more efficient and selective killing of individually targeted tumor cells. However, there are several obstacles to the use of alpha-particle immunotherapy, including problems with chelation chemistry and nontarget tissue toxicity. The alpha-emitting radioimmunoconjugate (227)Th-DOTA-p-benzyl-rituximab is a new potential anti-lymphoma agent that might overcome some of these difficulties. The present study explores the immunoreactivity, in vivo stability and biodistribution, as well as the effect on in vitro cell growth, of this novel radioimmunoconjugate. To evaluate in vivo stability, uptake in balb/c mice of the alpha-particle-emitting nuclide (227)Th alone, the chelated form, (227)Th-p-nitrobenzyl-DOTA and the radioimmunoconjugate (227)Th-DOTA-p-benzyl-rituximab was compared in a range of organs at increasing time points after injection. The immunoreactive fraction of (227)Th-DOTA-p-benzyl-rituximab was 56-65%. During the 28 days after injection of radioimmunoconjugate only, very modest amounts of the (227)Th had detached from DOTA-p-benzyl-rituximab, indicating a relevant stability in vivo. The half-life of (227)Th-DOTA-p-benzyl-rituximab in blood was 7.4 days. Incubation of lymphoma cells with (227)Th-DOTA-p-benzyl-rituximab resulted in a significant antigen-dependent inhibition of cell growth. The data presented here warrant further studies of (227)Th-DOTA-p-benzyl-rituximab.

First clinical experience with alpha-emitting radium-223 in the treatment of skeletal metastases

PURPOSE

The main goals were to study the safety and tolerability of the alpha-emitter radium-223 (223Ra) in breast and prostate cancer patients with skeletal metastases. In addition, pain palliation was evaluated.

EXPERIMENTAL DESIGN

Fifteen prostate and 10 breast cancer patients enrolled in a phase I trial received a single i.v. injection of 223Ra. Five patients were included at each of the dosages: 46, 93, 163, 213, or 250 kBq/kg and followed for 8 weeks. Palliative response was evaluated according to the pain scale of the European Organization for Research and Treatment of Cancer QLQ C30 questionnaire at baseline and at 1, 4, and 8 weeks after injection.

RESULTS

Weekly blood sampling during follow-up revealed mild and reversible myelosuppression with nadir 2 to 4 weeks after the injection. Importantly, for thrombocytes only grade 1 toxicity was reported. Grade 3 neutropenia and leucopenia occurred in two and three patients, respectively. Mild, transient diarrhea was observed in 10 of the 25 patients. Nausea and vomiting was more frequently observed in the highest dosage group. Serum alkaline phosphatase decreased with nadir averages of 29.5% in females and 52.1% in males. Pain relief was reported by 52%, 60%, and 56% of the patients after 7 days, 4, and 8 weeks, respectively. 223Ra cleared rapidly from blood and was below 1% of initial level at 24 hours. Gamma camera images indicated, in accordance with pretreatment (99m)Tc-MDP scans, accumulation of 223Ra in skeletal lesions. Elimination was mainly intestinal. Median survival exceeded 20 months.

CONCLUSIONS

223Ra was well tolerated at therapeutically relevant dosages. Phase II studies have therefore been initiated.

Preparation and preclinical assessment of folate-conjugated, radiolabelled antibodies

BACKGROUND

The folate receptor (FR) is frequently over-expressed on human cancer cells and may be a suitable target for radiopharmaceuticals. Because of FR expression in the kidneys, the rapidly renal clearing folate is not well suited as a carrier for therapeutic radionuclides. As an alternative, folate-immunoglobulin conjugates were studied as potential carriers for radionuclides.

MATERIALS AND METHODS

Two types of conjugate were evaluated: (i) folate conjugated to osteosarcoma antigen directed murine monoclonal antibodies TP-1 and TP-3 or (ii) folate conjugated to non-specific polyclonal human IgG (HIg6). These constructs were labelled with 211At or 125I.

RESULTS

The folate-HIg6-radionuclide conjugate showed high affinity to immobilized folate binding protein and also to folate receptor-expressing cells. Folate conjugates of TP-1 and TP-3 had a selective binding in vitro to antigen-expressing tumor cells and also to cells expressing FR only, thus the folate antibody constructs possessed dual affinity binding. Comparisons between folate-conjugated and non-folated antibody in Balb/C mice showed that the folate did not markedly change the properties of the radiolabelled antibody.

CONCLUSION

It was demonstrated that folate-conjugated antibodies carrying therapeutic radionuclides have relevant properties for the targeting of tumor cells expressing FR.

Sterically stabilized liposomes as a carrier for alpha-emitting radium and actinium radionuclides

The alpha-particle emitting radionuclides (223)Ra (t(1/2) = 11.4 d), (224)Ra (t(1/2) = 3.6 d), and (225)Ac(t(1/2) = 10.0 d) may have a broad application in targeted radiotherapy provided that they could be linked to vehicles with tumor affinity. The potential usefulness of liposomes as carriers was studied in the present work. Radium and actinium radionuclides could be loaded in good yields into sterically stabilized liposomes. Subsequent coating of the liposomes with a folate-F(ab')(2) construct yielded a product with affinity towards tumor cells expressing folate receptors. Radionuclide loaded liposomes showed excellent stability in serum in vitro.

Thorium and actinium polyphosphonate compounds as bone-seeking alpha particle-emitting agents

The present study explores the use of alpha-particle-emitting, bone-seeking agents as candidates for targeted radiotherapy. Actinium and thorium 1,4,7,10 tetraazacyclododecane N,N',N'',N''' 1,4,7,10-tetra(methylene) phosphonic acid (DOTMP) and thorium-diethylene triamine N,N',N'' penta(methylene) phosphonic acid (DTMP) were prepared and their biodistribution evaluated in conventional Balb/C mice at four hours after injection. All three bone-seeking agents showed a high uptake in bone and a low uptake in soft tissues. Among the soft tissue organs, only kidney had a relatively high uptake. The femur/kidney ratios for 227Th-DTMP, 228-Ac-DOTMP and 227Th-DOTMP were 14.2, 7.6 and 6.0, respectively. A higher liver uptake of 228Ac-DOTMP was seen than for 227Th-DTMP and 227Th-DOTMP. This suggests that some demetallation of the 228Ac-DOTMP complex had occurred. The results indicate that 225Ac-DOTMP, 227Th-DOTMP and 227Th-DTMP have promising properties as potential therapeutic bone-seeking agents.

Targeting of osseous sites with alpha-emitting 223Ra: comparison with the beta-emitter 89Sr in mice

The bone-seeking property and the potential exposure of red marrow by the alpha-particle emitter (223)Ra (half-life, 11.43 d) were compared with those of the beta-emitter (89)Sr (half-life, 50.53 d).

METHODS

The biodistributions of (223)Ra and (89)Sr were studied in mice. Tissue uptake was determined at 1 h, 6 h, 1 d, 3 d, and 14 d after intravenous administration. Radiation absorbed doses were calculated for soft tissues and for bone. Multicellular-level doses were estimated for bone marrow cavities.

RESULTS

Both (89)Sr and (223)Ra selectively concentrated on bone surfaces relative to soft tissues. The measured bone uptake of (223)Ra was slightly higher than that of (89)Sr. At 24 h, the femur uptake of (223)Ra was 40.1% +/- 7.7% of the administered activity per gram of tissue. The uptake in spleen and most other soft tissues was higher for (223)Ra than for (89)Sr. Although predominant clearance of (223)Ra was observed from the soft tissues within the first 24 h, the bone uptake of (223)Ra, which was not significantly different from maximum after only 1 h, was not significantly reduced during the 14 d. Furthermore, little redistribution of (223)Ra daughter products away from bone was found (2% at 6 h and less than 1% at 3 d). Estimates of dose to marrow cavities showed that the (223)Ra alpha-emitter might have a marrow-sparing advantage compared with beta-emitters for targeting osteoid surfaces because the short-range alpha-particles irradiate a significantly lower fraction of the marrow volumes. At the same time, the bone surfaces will receive a therapeutically effective radiation dose.

CONCLUSION

The results of this study indicate that (223)Ra is a promising candidate for high-linear-energy transfer alpha-particle irradiation of cancer cells on bone surfaces. (223)Ra can, together with its daughter radionuclides, deliver an intense and highly localized radiation dose to the bone surfaces with substantially less irradiation of healthy bone marrow compared with standard bone-seeking beta-emitters.

Significant antitumor effect from bone-seeking, alpha-particle-emitting (223)Ra demonstrated in an experimental skeletal metastases model

The therapeutic efficacy of the alpha-particle-emitting radionuclide (223)Ra (t(1/2) = 11.4 days) in the treatment against experimental skeletal metastases in rats was addressed. Biodistribution studies, involving measurement of (223)Ra in bone marrow samples, were performed in rats after i.v. injection. To study the therapeutic effect of (223)Ra, an experimental skeletal metastases model in nude rats was used. Animals that had received 10(6) MT-1 human breast cancer cells were treated with (223)Ra doses in the range of 6-30 kBq after 7 days. The biodistribution experiment demonstrated that (223)Ra was selectively concentrated in bone as compared with soft tissues. The femur content of (223)Ra was 800 +/- 56% of injected dose per gram tissue times gram body weight (b.w.; mean +/- SD) 1 day after the injection and 413 +/- 23% of injected dose per gram tissue times gram b.w. at 14 days. The femur:kidney ratio increased from (5.9 +/- 2.0).10(2) at 1 day to (7.2 +/- 3.0).10(2) at 14 days, whereas the femur:liver ratio increased from (6.2 +/- 0.2).10(2) to (9.1 +/- 6.6).10(2). Femur:spleen ratio increased from (8.1 +/- 0.3).10(2) at 1 day to (6.4 2.2).10(3) at 14 days. The femoral bone:marrow ratio was 6.5 +/- 2.1 after day 1 and larger than 15 at day 14. All of the tumor-bearing control animals had to be sacrificed because of tumor-induced paralysis 20-30 days after injection with tumor cells, whereas the rats treated with > or =10 kBq of (223)Ra had a significantly increased symptom-free survival (P < 0.05). Also 36% (5 of 14) of rats treated with 11 kBq and 40% (2 of 5) of rats treated with 10 kBq were alive beyond the 67-day follow-up period. No signs of bone marrow toxicity or b.w. loss were observed in the groups of treated animals. The significant antitumor effect of (223)Ra at doses that are tolerated by the bone marrow is most likely linked to the intense and highly localized radiation dose from alpha-particles at the bone surfaces. The results of this study indicate that (223)Ra should be additionally studied as a potential bone marrow-sparing treatment of cancers involving the skeleton.

Evaluation of potential chelating agents for radium

The alpha-particle-emitting radionuclide 223Ra (t(1/2) = 11.4 d) is of interest for use in targeted radionuclide therapy. In order to provide radium-labeled monoclonal antibodies, the development of a chelator binding radium in a stable fashion is required. As a part of the search for potentially useful radium chelators, the relative stability of 223Ra-chelates with linear and cyclic chelating agents was evaluated by means of competition extraction experiments.

Radiation doses to non-Hodgkin's lymphoma cells and normal bone marrow exposed in vitro. Comparison of an alpha-emitting radioimmunoconjugate and external gamma-irradiation

PURPOSES

The alpha-emitting radionuclide 211At conjugated to the CD20 targeting chimeric monoclonal antibody rituximab was studied to: (a) Estimate radiation dose components to lymphoma and bone marrow (BM) cells exposed in vitro. (b) Calculate the mean absorbed radiation doses in various normal tissues of mice following intravenous injection.

MATERIALS AND METHODS

B-lymphoma cells (RAEL) and normal human BM cells were incubated with increasing concentrations of the radioimmunoconjugate. Based on binding kinetics and on measured cellular and nuclear diameters, the radiation doses were calculated using microdosimetric methods.

RESULTS

Targeting of 211At-rituximab to RAEL cells was extensive and stable compared with the binding to BM cells. The absorbed radiation doses from cell-bound activity at an initial activity concentration of 10 kBq ml(-1) were 0.645 and 0.021 Gy to RAEL and BM cells, respectively. In comparison, the contribution from unbound conjugate in the medium during 1h exposure was 0.042 and 0.043 Gy. The D(0) value for RAEL cells was 0.55 Gy, but only 0.34 Gy for BM cells, whereas corresponding D(0) values were 0.72 and 1.21 Gy after a single exposure to external 60Co gamma-rays. Mean absorbed doses of 1.31, 0.48 and 0.36 Gy for blood, lungs and heart were calculated in mice injected with 5.4kBq g(-1) of 211At-rituximab.

CONCLUSION

Despite the higher inherent sensitivity of the BM cells to the alpha-irradiation, there was, related to the radioactivity concentrations of 211At-rituximab, several logs greater cell kill in RAEL cells, illustrating the tumour-specific nature of the targeting.

Optimisation of cyclotron production parameters for the 209Bi(alpha, 2n) 211At reaction related to biomedical use of 211At

The cyclotron alpha beam production of 211At and of the contaminant 210At related to beam energy were studied. Radiochemical purification of 211At from the other main contaminant, 210Po, by an extraction procedure was also evaluated. To avoid impurities 28MeV has previously been considered as a maximum beam energy, but by using instead 29.1 MeV as a limit a large increase in EOB yield and sufficient radiochemical purity of extracted 211At were obtained. More cyclotrons could thereby deliver quantities useful for clinical cancer trials.

Demonstration of highly specific toxicity of the alpha-emitting radioimmunoconjugate(211)At-rituximab against non-Hodgkin's lymphoma cells

The ability of an alpha-emitter conjugated to a chimaeric anti-CD20 monoclonal antibody to kill selectively human B-lymphoma cells in vitro is reported. Two B-lymphoma cell lines RAEL and K422, and normal haematopoietic progenitor cells from human bone marrow aspirates were incubated with(211)At-rituximab (Rituxan(R) or MabTheratrade mark) and plated in clonogenic assays for survival analyses. Following 1 h incubation with(211)At-rituximab, in concentrations which gave an initial activity of 50 kBq ml(-1), a high tumour cell to normal bone marrow cell toxicity ratio was obtained; 4.1 to 1.0 log cell kill. Biodistribution studies of(211)At-rituximab in Balb/c mice showed similar stability as that of the iodinated analogue. The data indicate that testing of(211)At-rituximab in human patients is warranted.

Exposure of human osteosarcoma and bone marrow cells to tumour-targeted alpha-particles and gamma-irradiation: analysis of cell survival and microdosimetry

PURPOSE

This study was designed to compare the cytotoxic effects of an alpha-emitting radioimmunoconjugate, which binds to osteosarcoma but not to bone marrow cells, with those of external gamma-irradiation.

MATERIALS AND METHODS

The human osteosarcoma cell line, OHS-s1, and mononuclear cells from bone marrow (BM) harvested from healthy donors, were used for these experiments. Cells in suspension were added to various activity concentrations of the anti-osteosarcoma monoclonal antibody TP-3 radiolabelled with 211At. Following incubation for 1 h, unbound radioactivity was washed off and cell survival was determined from clonogenic assays. Microdosimetry was calculated based on binding and retention kinetics of 211At to the cells, as well as cellular and nuclear diameters. For comparison, cell suspensions were irradiated with a single dose of 60Co gamma-rays.

RESULTS

211At-labelled TP-3 showed heterogeneous binding to OHS-s1 cells, with a considerable variation among experiments. About 78% of the initially bound 211At decayed while associated with the OHS-s1 cells. D0 values estimated by microdosimetry were 0.33 (0.22-0.48, range) Gy and 1.18 (0.89-1.89) Gy for OHS-s1 and BM cells, respectively, whereas D0 values after external beam irradiation were 0.86+/-0.07Gy and 1.71+/-0.22Gy. The relative biological effectiveness (RBE) of 211At-labelled TP-3 at 37% survival was 3.43 for OHS-s1 and 1.55 for BM.

CONCLUSIONS

High-LET targeted alpha-particle exposure killed osteosarcoma cells more effectively than bone marrow cells, although heterogeneous antigen expression among these tumour cells limited the magnitude of this effect.

Influence of pretreatment with 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (APB) on organ uptake of 211At and 125I-labeled amidobisphosphonates in mice

To minimize nontarget organ uptake in animals receiving radiolabeled amidobisphosphonates, the influence of pretreatment with cold 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (APB, pamidronate) was studied. Three groups of animals were given pure 3-[125I]iodobenzamide-N-3-hydroxypropylidene-3,3-bisphosphonate (IBPB) and 3-[211At]astatobenzamide-N-3-hydroxypropylidene-3,3-bisphosphonate (ABPB) (control); co-injection of APB and IBPB/ABPB; and 1 h preinjection of APB followed by IBPB/ABPB, respectively. A significant reduction of uptake in normal tissue was observed, whereas the bone uptake remained constant at 35-50%ID/g tissue. This study suggests that co- or preinjection of pamidronate may reduce the normal organ radiation doses when using these radiohalogenated bisphosphonates for endoradiotherapeutic procedures.

Radiotoxicity of systemically administered 211At-labeled human/mouse chimeric monoclonal antibody: a long-term survival study with histologic analysis

PURPOSE

The antitenascin human/mouse chimeric monoclonal antibody labeled with the alpha-particle-emitting radionuclide 211At is of interest as an endoradiotherapeutic agent for the treatment of brain tumors. To facilitate the investigation of 211At-labeled chimeric 81C6 in patients, the long-term radiotoxicity of this radiopharmaceutical has been evaluated.

METHODS AND MATERIALS

Antibody labeling was performed using N-succinimidyl 3-[211At]astato-benzoate. After an initial dose-finding experiment, a second toxicity study was carried out at 4 dose levels in groups of 30 nonthyroid blocked B6C3F1 mice per group (15 males, 15 females). Male mice received either saline or 15-81 kBq/g and females received either saline or 16-83 kBq/g of 211At-labeled antibody. Ten animals (5 males, 5 females) were followed for 6 months and the remainder for 1 year.

RESULTS

The lethal dose in 10% of animals (LD10) for 211At-labeled chimeric 81C6 was 46 kBq/g in females and 102 kBq/g in males. Toxic effects--perivascular fibrosis of the intraventricular septum of the heart, bone marrow suppression, splenic white pulp atrophy, and spermatic maturational delay--generally were confined to a few animals receiving the highest doses of labeled antibody.

CONCLUSIONS

The LD10 of 211At-labeled chimeric 81C6 in this mouse strain was about half that of [211At]astatide. These results establish the preclinical maximum tolerated dose of 211At-labeled chimeric 81C6 and define in the mouse the target organs for toxicity. These studies will be useful for determining starting doses for clinical studies with 211At-labeled chimeric 81C6.

211At- and 131I-labeled bisphosphonates with high in vivo stability and bone accumulation

Bisphosphonates were synthesized for use as carriers for astatine and iodine radioisotopes to target bone neoplasms.

METHODS

Radiohalogenated activated esters were coupled to the amino group in the side chain of the bisphosphonate. The bisphosphonate 3-amino-1-hydroxypropylidene bisphosphonate was combined with four different acylation agents: N-succinimidyl 3-[211At]astatobenzoate, N-succinimidyl 3-[131I]iodobenzoate, N-succinimidyl-5-[211At]astato-3-pyridinecarboxylate and N-succinimidyl-5-[131I]iodo-5-pyridinecarboxylate. The products, 3-[131I]iodobenzamide-N-3-hydroxypropylidene-3,3-bisphosphonate (IBPB), 3-[211At]astato-benzamide-N-3-hydroxypropylidene-3,3-bisphosphonat e (ABPB), 5-[131I]iodopyridine-3-amide-N-3-hydroxypropylidene-3,3-bisphospho nate (IPPB) and 5-[211At]astatopyridine-3-amide-N-3-hydroxypropylidene-3,3-bisphos phonate (APPB), were injected intravenously into Balb/c mice. MIRD and Monte Carlo methods were used on the basis of cumulated activity calculated from biodistribution data to estimate dose to organs and bone segments.

RESULTS

All 131I- and 211At-labeled analogs were strongly incorporated into osseous tissue and retained there at stable levels, while a rapid clearance from blood was observed. The bone uptake was found to be similar for 211At- and 131I-labeled bisphosphonate when compared in paired label experiments. Bone uptake and bone-to-tissue ratios were better for IBPB compared with IPPB, and ABPB compared with APPB. All four compounds appeared to be highly resistant to in vivo dehalogenation as indicated by low uptake of 131I/211At in the thyroid gland and stomach. According to dosimetric estimates, the bone surface-to-bone marrow ratio was three times higher with 211At than with 131I.

CONCLUSION

Both the beta-particle- and alpha-particle-emitting compounds showed high in vivo stability and excellent affinity for osseous tissue. Further preclinical evaluation is therefore warranted.

Synthesis, purification, and in vitro stability of 211At- and 125I-labeled amidobisphosphonates

A method is described for preparing 211At- and radioiodinated amidobisphosphonates. The active esters N-succinimidyl 3-(tri-methylstannyl) benzoate (ATE) and N-succinimidyl 5-(tri-methylstannyl)-3-pyridinecarboxylate (SPC) were used as precursors. The isolated and purified radiolabeled intermediates were coupled to 3-amino-1-hydroxypropylidene-1,1-bisphosphonate (APB) in high yields ranging from 60% to 97%. The lipophilicity of the compounds was found to depend on the nature of the labeled template and the halogen. High in vitro stability in mouse, fetal calf, and human serum was documented by high performance liquid chromatography.

Blocking [211At]astatide accumulation in normal tissues: preliminary evaluation of seven potential compounds

Normal tissue accumulation of 211At must be minimized during targeted radiotherapy with 211At-labeled compounds. Therefore, we investigated the ability of seven compounds to block normal organ uptake of [211At]astatide in mice: potassium iodide, sodium thiocyanate, sodium perchlorate, sodium periodate, cysteine, 2,3-dimercapto-1-propanesulfonic acid, and meso-2,3-dimercaptosuccinic acid. The monovalent anions I-, SCN-, and ClO4- reduced 211At uptake in stomach and thyroid, while thiocyanate and cysteine were the only compounds to significantly reduce activity levels in lungs and spleen. This study suggests that blocking agents may help reduce normal organ radiation doses in endoradiotherapeutic procedures with 211At-labeled radiopharmaceuticals.

Intratumour injection of immunoglobulins labelled with the alpha-particle emitter 211At: analyses of tumour retention, microdistribution and growth delay

To determine the effects of 211At-labelled antibodies in solid tumour tissue, nude mice carrying OHS human osteosarcoma xenografts received intratumour injections at dosages of 1, 2 or 4 MBq (-1) tumour. The radioisotope was conjugated to either the osteosarcoma-specific monoclonal antibody TP-3 or the non-specific polyclonal antibody hlgGkappa. Tumour retention of injected radioimmunoconjugate (RIC), measured as the percentage of injected activity dosage per gram, was significantly higher for the [211At]TP-3 (203 +/- 93 at 24.1 h post injection) compared with the [211At]hlgGkappa (57 +/- 22 at 23.2 h post injection). The radioactive count rates in body (measured at neck and abdomen) were significantly lower with the TP-3 than with the hlgGkappa. Microautoradiography of the tumour radionuclide distribution was different for the two RICs, i.e. the [211At]TP-3 was to a larger extent concentrated near the injection site, whereas the [211At]hlgGkappa was more evenly distributed all over the tumour. The tumour growth was significantly delayed as a function of the injected activity dosage but without significant difference between the specific and the non-specific RIC. According to this study, it is possible to deliver highly selective radiation doses to solid tumours using intratumour injection of alpha-particle-emitting RICs. Improved tumour retention caused by antigen binding indicates that reduced normal tissue exposure can be obtained with antigen-specific antibodies. The heterogeneous tumour dose distribution observed is, however, a major impediment to the use of alpha-particle emitters against solid tumours.

Cytotoxicity of alpha-particle-emitting astatine-211-labelled antibody in tumour spheroids: no effect of hyperthermia

The high linear energy transfer, alpha-particle-emitting radionuclide astatine-211 (211At) is of interest for certain therapeutic applications; however, because of the 55- to 70-microm path length of its alpha-particles, achieving homogeneous tracer distribution is critical. Hyperthermia may enhance the therapeutic efficacy of alpha-particle endoradiotherapy if it can improve tracer distribution. In this study, we have investigated whether hyperthermia increased the cytotoxicity of an 211At-labelled monoclonal antibody (MAb) in tumour spheroids with a radius (approximately 100 microm) greater than the range of 211At alpha-particles. Hyperthermia for 1 h at 42 degrees C was used because this treatment itself resulted in no regrowth delay. Radiolabelled chimeric MAb 81C6 reactive with the extracellular matrix antigen tenascin was added to spheroids grown from the D-247 MG human glioma cell line at activity concentrations ranging from 0.125 to 250 kBq ml(-1). A significant regrowth delay was observed at 125 and 250 kBq ml(-1) in both hyperthermia-treated and untreated spheroids. For groups receiving hyperthermia, no increase in cytotoxicity was seen compared with normothermic controls at any activity concentration. These results and those from autoradiographs indicate that hyperthermia at 42 degrees C for 1 h had no significant effect on the uptake or distribution of this antitenascin MAb in D-247 MG spheroids.

The cytotoxicity and microdosimetry of astatine-211-labeled chimeric monoclonal antibodies in human glioma and melanoma cells in vitro

The cytotoxicity of alpha-particle-emitting endoradiotherapeutic compounds is of increasing interest because clinical evaluation of these potential therapeutic agents is commencing. Astatine-211 is a radionuclide with a 7.2-h half-life that emits 5.87 and 7.45 MeV alpha particles. In the present work, we have investigated the in vitro cytotoxicity of 211At-labeled chimeric monoclonal antibodies (mAbs) in monolayers of D-247 MG human glioma cells and SK-MEL-28 human melanoma cells. The mAbs studied were 81C6, reactive with the extracellular matrix antigen tenascin, Mel-14, directed against the cell membrane antigen proteoglycan chondroitin sulfate, and a nonspecific control mAb, TPS3.2. Cell uptake increased as a function of activity concentration after a 1-h exposure to the 211At-labeled mAbs. The retention of activity was also measured to calculate cumulative activity associated with the cells and the medium. The clonogenic survival as a function of activity concentration was linear in all cases with no detectable shoulder. Microdosimetric analyses were performed based on measured cell geometry, cumulative activity and Monte Carlo transport of alpha particles. Using 18 kBq/ml activity concentration and 1 h of incubation, a two to five times higher activity bound to the microcolonies was found for the specific mAbs compared to the nonspecific mAb. These calculations indicated that a survival fraction of 0.37 was achieved with 0.24-0.28 Gy for D-247 MG cells and 0.27-0.29 Gy for SK-MEL-28 cells. The microdosimetric cell sensitivity, z0, for D-247 MG cells was significantly lower than for SK-MEL-28 cells (0.08 compared to 0.15 Gy). For both cell lines, reduction in survival to 0.37 required an average of only 1-2 alpha-particle hits to the cell nucleus.

Serological assays for the identification of Oesophagostomum dentatum infections in pigs

Oesophagostomum dentatum antigen preparations of third (L3) or fourth (L4) stage larvae were characterised by Western blotting. Panels of sera obtained from pigs infected with O dentatum and Ascaris suum, respectively, reacted with the same bands of L3 antigen. In contrast high specificity against a characteristic band, was observed when L4 extract was employed as antigen. To establish an enzyme-linked immunosorbent assay (ELISA), a panel of homologous and heterologous sera was tested against O dentatum L4 extract. The best combined specificity and sensitivity was obtained when horseradish peroxidase (HRP) goat anti swine IgG conjugate was used rather than HRP rabbit anti swine Ig conjugate. Testing series of sera from pigs infected with single doses of either 2000, 20,000 or 200,000 infective larvae by the ELISA, a significant dose dependency in the antibody response was observed between the low and high dosage groups. This assay may be useful in future studies of the immune-mechanisms against nodular worm infections in pigs.