Labeling of monoclonal antibodies with radionuclides

Antibodies, specifically monoclonal antibodies, are potentially very useful and powerful carriers of therapeutic agents to target tissues and diagnostic agents. The loading or charging of antibodies with agents, especially radiotracers, is reviewed here. The choice of radioisotope for immunodetection and/or immunotherapy is based on its availability, half-life, nature of the radiation emitted, and the metabolic pathways of the radionuclide in the body. Most important of all are the derivatization techniques available for labeling the antibody with the given radionuclide. Isotopes of iodine and divalent metal ions are the most commonly used radionuclides. Antibodies labeled with iodine at tyrosine residues are metabolized rapidly in vivo. This leads to the incorporation of metabolized radioactive iodine into various tissues, mainly the thyroid gland and stomach, and to the accumulation of high levels of circulating iodine in the blood, which masks tumor uptake considerably. To overcome these limitations, the use of iodohippurate as an iodine-anchoring molecule to the protein should be considered. When divalent or multivalent metal ions are used as the preferred radionuclide, bifunctional chelating reagents such as ethylenediaminepentaacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) are first coupled to the protein or antibody. These chelating molecules are attached to the protein by formation of an isopeptide linkage between the carboxylate of the chelating reagent and the amino group of the protein. Several procedures are available to generate the isopeptide linkage. When the anchoring of the chelating agent through isopeptide linkage results in the inactivation of the antibody, periodate oxidation of the carbohydrate moiety of the antibody, followed by reductive coupling of chelator, could be considered as an alternative. There is still a need for better, simpler, and more direct methods for labeling antibodies with radionuclides.     

166Ho and 90Y labeled 6D2 monoclonal antibody for targeted radiotherapy of melanoma: Comparison with 188Re radiolabel

IntroductionAn approach to radioimmunotherapy (RIT) of metastatic melanoma is the targeting of melanin pigment with monoclonal antibodies (mAbs) to melanin radiolabeled with therapeutic radionuclides. The proof of principle experiments were performed using a melanin-binding antibody 6D2 of IgM isotype radiolabeled with a β emitter ¹⁸⁸Re and demonstrated the inhibition of tumor growth. In this study we investigated the efficacy of 6D2 antibody radiolabeled with two other longer lived β emitters ⁹⁰Y and ¹⁶⁶Ho in treatment of experimental melanoma, with the objective to find a possible correlation between the efficacy and half-life of the radioisotopes which possess high energy β (E_max > 1.5 MeV) emission properties.Methods6D2 was radiolabeled with longer lived β emitters ⁹⁰Y and ¹⁶⁶Ho in treatment of experimental melanoma in A2058 melanoma tumor-bearing nude mice. The immunoreactivity of the radiolabeled 6D2 mAb, its in vitro binding to the MNT1 human melanoma cells, the biodistribution and therapy in A2058 human melanoma bearing nude mice as well as dosimetry calculations were performed.ResultsWhen labeled with the longer lived ⁹⁰Y radionuclide, the 6D2 mAb did not produce any therapeutic effect in tumor bearing mice while the reduction of the tumor growth by ¹⁶⁶Ho-6D2 was very similar to the previously reported therapy results for ¹⁸⁸Re-6D2. In addition, ¹⁶⁶Ho-labeled mAb produced the therapeutic effect on the tumor without any toxic effects while the administration of the ⁹⁰Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect.Conclusions¹⁶⁶Ho-labeled mAb to melanin produced some therapeutic effect on the tumor without any toxic effects while the administration of the ⁹⁰Y-labeled radioconjugate was toxic to mice with no appreciable anti-tumor effect. We concluded that the serum half-life of the 6D2 carrier antibody matched well the physical half-life of ¹⁶⁶Ho to deliver the tumoricidal absorbed dose to the tumor. Further investigation of this radionuclide for RIT of melanoma is warranted.     

Radiolabeling of monoclonal antibody against carcinoembryonic antigen with 88Y and biodistribution studies

The biodistribution of the 202 monoclonal antibody against CEA labeled with 88Y by the bicyclic DTPA anhydride method was studied in normal Balb/c mice. The in vitro binding to 1 X 10(7) CO112, LS174T and WiDR colon cancer cells was 21.0, 27.3 and 18.8%, respectively. The binding to an equal number of KM-3 leukemia cells and normal human lymphocytes was 8.9 and 3.2%, respectively. Liver, spleen, kidney and blood were the tissues that showed the highest uptake of radiolabeled antibody in vivo.     

Labeling of phosphorothioate antisense oligonucleotides with yttrium-90

Novel yttrium-90 (90Y)-labeled phosphorothioate antisense oligonucleotides were designed as a potential targeted radionuclide therapeutic agent for malignant tumors. A 15-mer phosphorothioate antisense oligonucleotide, which was complementary to the translation start region of the N-myc oncogene mRNA, was conjugated with isothiocyanobenzyl ethylenediamine tetraacetic acid (SCN-Bn-EDTA), via a C-5-substituted deoxyuridine that had replaced a thymine in the oligonucleotide, and was then labeled with 90Y-acetate. Following purification, the radiochemical purity of the 90-Y-Bn-EDTA-phosphorothioate antisense oligonucleotides was estimated by 2.0% agarose gel electrophoresis, and the specific hybridization of 90Y-Bn-EDTA-phosphorothioate antisense oligonucleotide to a phosphorodiester sense oligonucleotide was investigated by 20% polyacrylamide gel electrophoresis in a cell-free system. Radiochemical purity was 98.7 +/- 0.4% at 72 h after labeling and 90.3 +/- 0.9% after 72-h incubation with human normal serum. The 90Y-Bn-EDTA-phosphorothioate antisense oligonucleotide hybridized specifically to a complementary phosphorodiester sense oligonucleotide. In conclusion, phosphorothioate antisense oligonucleotides can be labeled stably with 90Y using SCN-Bn-EDTA without loss of hybridization properties.     

Labelling monoclonal antibodies with yttrium-90

Using the cyclic DTPA derivatisation procedure developed by Hnatowich, conditions have been optimised for labelling three tumour-associated monoclonal antibodies with 90Y. High labelling efficiencies (greater than 95%) at modest specific activities (1 mCi/mg) could be routinely obtained. Radiochemical stability of the radiolabelled preparations in vitro was good, but radiolysis resulted in early losses of antibody immunoreactivity.     

Labelling monoclonal antibodies with Yttrium-90

Using the cyclic DTPA derivatisation procedure developed by Hnatowich, conditions have been optimised for labelling three tumour-associated monoclonal antibodies with ⁹⁰Y. High labelling efficiencies (>95%) at modest specific activities (1 mCi/mg) could be routinely obtained. Radiochemical stability of the radiolabelled preparations in vitro was good, but radiolysis resulted in early losses of antibody immunoreactivity.Abbreviations PBS Phosphate buffered saline - BSA Bovine Serum Albumin - HSA Human Serum Albumin - ITLC Instant thin layer chromatography - HPLC High pressure liquid chromatography - CAE Cellulose acetate electrophoresis - ELISA Enzyme linked immunosorbent assay - RIA Radioimmune assay - DTPA Diethylenetriaminepentaacetic acid     

Synovectomy of the knee with 90Y

In 33 patients with chronic arthritis of the knee, 48 knees were treated with an intra-articular injection of 5 mCi yttrium silicate (90Y). There were 27 patients with rheumatoid arthritis (RA) and 6 with osteoarthrosis (OA); the mean follow-up period was 33 months. At clinical investigation after 1 year, no signs of pain or swelling were found in 15 knees. In most cases, pain and swelling improved subjectively, with a mean duration of 11 months; in 20 knees, the improvement lasted more than 22 months. When radiographs showed severe destruction, 90Y treatment was unsuccessful, but an important new finding was that most patients with mild or moderate radiological abnormalities appeared to have a long-lasting improvement. The result did not correlate with erythrocyte sedimentation rate (ESR), haemoglobin or Rose titre at the time of injection or at follow up, suggesting that the result of the treatment is more dependent on local factors than on the disease activity. The results of 90Y treatment in 6 OA knees with persistent swelling were promising regarding swelling, even in patients with moderate radiological abnormalities. The main side-effect was a sometimes painful swelling of the knee, which was always successfully treated with an intraarticular corticosteroid injection. In 90Y-treated knees, the incidence of unstable joints was not significantly higher than in non-treated knees. In conclusion, 90Y synovectomy may be a successful treatment for patients older than 50 years with chronic arthritis of the knee due to RA and probably also OA, even when moderate radiological abnormalities are present.     

Gold nanorod-mediated hyperthermia enhances the efficacy of HPMA copolymer-90Y conjugates in treatment of prostate tumors

IntroductionThe treatment of prostate cancer using a radiotherapeutic ⁹⁰Y labeled N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer can be enhanced with localized tumor hyperthermia. An ¹¹¹In labeled HPMA copolymer system for single photon emission computerized tomography (SPECT) was developed to observe the biodistribution changes associated with hyperthermia. Efficacy studies were conducted in prostate tumor bearing mice using the ⁹⁰Y HPMA copolymer with hyperthermia.MethodsHPMA copolymers containing 1, 4, 7, 10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) were synthesized by reversible addition-fragmentation transfer (RAFT) copolymerization and subsequently labeled with either ¹¹¹In for imaging or ⁹⁰Y for efficacy studies. Radiolabel stability was characterized in vitro with mouse serum. Imaging and efficacy studies were conducted in DU145 prostate tumor bearing mice. Imaging was performed using single photon emission computerized tomography (SPECT). Localized mild tumor hyperthermia was achieved by plasmonic photothermal therapy using gold nanorods.ResultsHPMA copolymer-DOTA conjugates demonstrated efficient labeling and stability for both radionuclides. Imaging analysis showed a marked increase of radiolabeled copolymer within the hyperthermia treated prostate tumors, with no significant accumulation in non-targeted tissues. The greatest reduction in tumor growth was observed in the hyperthermia treated tumors with ⁹⁰Y HPMA copolymer conjugates. Histological analysis confirmed treatment efficacy and safety.ConclusionHPMA copolymer-DOTA conjugates radiolabeled with both the imaging and treatment radioisotopes, when combined with hyperthermia can serve as an image guided approach for efficacious treatment of prostate tumors.     

Administration guidelines for radioimmunotherapy of non-Hodgkin's lymphoma with (90)Y-labeled anti-CD20 monoclonal antibody.

90Y-ibritumomab tiuxetan is a novel radioimmunotherapeutic agent recently approved for the treatment of relapsed or refractory low-grade, follicular, or CD20+ transformed non-Hodgkin's lymphoma (NHL). (90)Y-ibritumomab tiuxetan consists of a murine monoclonal antibody covalently attached to a metal chelator, which stably chelates (111)In for imaging and (90)Y for therapy. Both health care workers and patients receiving this therapy need to become familiar with how it differs from conventional chemotherapy and what, if any, safety precautions are necessary. Because (90)Y is a pure beta-emitter, the requisite safety precautions are not overly burdensome for health care workers or for patients and their families. (90)Y-ibritumomab tiuxetan is dosed on the basis of the patient's body weight and baseline platelet count; dosimetry is not required for determining the therapeutic dose in patients meeting eligibility criteria similar to those used in clinical trials, such as <25% lymphomatous involvement of the bone marrow. (111)In- and (90)Y-ibritumomab tiuxetan are labeled at commercial radiopharmacies and delivered for on-site dose preparation and administration. Plastic and acrylic materials are appropriate for shielding during dose preparation and administration; primary lead shielding should be avoided because of the potential exposure risk from bremsstrahlung. Because there are no penetrating gamma-emissions associated with the therapy, (90)Y-ibritumomab tiuxetan is routinely administered on an outpatient basis. Furthermore, the risk of radiation exposure to patients' family members has been shown to be in the range of background radiation, even without restrictions on contact. There is therefore no need to determine activity limits or dose rate limits before patients who have been treated with (90)Y radioimmunotherapy are released, as is necessary with patients who have been treated with radiopharmaceuticals that contain (131)I. Standard universal precautions for handling body fluids are recommended for health care workers and patients and their family members after (90)Y-ibritumomab tiuxetan administration. In summary, (90)Y-ibritumomab tiuxetan introduces (90)Y into clinical practice and expands the role nuclear medicine plays in the care of patients with cancer. Understanding the unique properties of this novel radioimmunoconjugate will facilitate its safe and effective use.     

Enhanced efficacy of 90Y-radiolabeled anti-Lewis Y humanized monoclonal antibody hu3S193 and paclitaxel combined-modality radioimmunotherapy in a breast cancer model.

Radioimmunotherapy (RIT) of solid tumor is often limited in efficacy because of restrictions in achieved tumor dose. In an effort to overcome this, the combination of RIT with other therapeutic modalities was investigated in an animal model of breast carcinoma. The rationale for this combined-modality RIT (CMRIT) was to increase the therapeutic efficacy of RIT through the use of paclitaxel to arrest cells in the radiosensitive G(2)/M phase of the cell cycle. METHODS: In this study, the biodistribution and therapeutic efficacy of (90)Y-radiolabeled humanized anti-Lewis Y hu3S193 monoclonal antibody ((90)Y-hu3S193) RIT in combination with paclitaxel chemotherapy was explored in a Lewis Y-expressing MCF-7 tumor xenografted BALB/c nude mouse model of breast cancer. RESULTS: Biodistribution studies demonstrated excellent tumor targeting and limited normal tissue uptake by (90)Y-hu3S193. A therapeutic study with established tumors assessed (90)Y-hu3S193 as a single agent and demonstrated significant antitumor effects in all animals receiving a single intravenous 1.85 or 3.70 MBq dose of this treatment compared with phosphate-buffered saline placebo controls (P = 0.0008 vs. P < 0.0001). Complete responses were observed in all animals in the 3.70 MBq study arm for the duration of the study. Single-dose (90)Y-hu3S193 plus paclitaxel (600 microg) CMRIT displayed improved efficacy over single-modality therapies, with a significant difference (P < 0.0001) between the mean percentage change in tumor volume in mice receiving 0.46 MBq (90)Y-hu3S193 alone and when combined with 600 mug paclitaxel. CONCLUSION: The significant efficacy of (90)Y-hu3S193 and paclitaxel CMRIT at low radiation doses in this model of breast carcinoma indicates its therapeutic potential and warrants further investigation into this promising therapeutic approach.     

Optimization of radioimmunotherapy of renal cell carcinoma: labeling of monoclonal antibody cG250 with 131I, 90Y, 177Lu, or 186Re.

Radioimmunotherapy (RIT) can be performed with various radionuclides. We tested the stability, biodistribution, and therapeutic efficacy of various radioimmunoconjugates ((131)I, (88/90)Y, (177)Lu, and (186)Re) of chimeric antirenal cell cancer monoclonal antibody G250 (mAb cG250) in nude mice with subcutaneous renal cell cancer (RCC) tumors. METHODS: The (88/90)Y and (177)Lu labeling procedures of cG250 conjugated with cyclic diethylenetriaminepentaacetic acid anhydride (cDTPA), isothiocyanatobenzyl-DTPA (SCN-Bz-DTPA), or 1,4,7,10-tetraazacyclododecanetetraacetic acid (DOTA) were characterized. Stability of the labeled conjugates in plasma at 37 degrees C was assessed. Biodistribution and therapeutic efficacy of labeled cG250 were compared in nude mice with SK-RC-52 human RCC xenografts. RESULTS: Both SCN-Bz-DTPA and DOTA were stable in vitro (<5% release of the radiolabel during 14 and 21 d of incubation) and in vivo (uptake in bone 相似文献   

Cytogenetic damage in marrow cells of mice after injections of Yttrium-90-labeled monoclonal antibody

Chromosome aberrations, micronuclei and sister-chromatid exchanges were quantified in marrow cells of athymic nude and B6C3F1 mice at various times up to 14 days after injection of ⁹⁰Y-labeled monoclonal antibody CO17-1A. Aberrations, predominantly of the chromatid type, were sharply elevated at 24 h post-injection then declined in a curvilinear fashion over the 14 days. Micronucleus numbers among polychromatic erythrocytes peaked 3–4 days after treatment, then declined exponentially but remained at higher than expected levels. Sister-chromatid exchanges were roughly double the control rate with no apparent relation to post-treatment time.     

Development of an electrochemical 90 Sr-90 Y generator for separation of 90 Y suitable for targeted therapy

90 Y of high specific activity and very high radionuclidic purity (>99.998%) is essential for targeted therapy. Since the current methods used for the preparation of 90 Y from 90 Sr are not adaptable for use in a central/hospital radiopharmacy, a simple 90 Sr-90 Y generator system based on electrochemical separation technique was developed. METHODS: Two-cycle electrolysis procedure was developed for separation of 90 Y from 90 Sr in nitrate solution. The first electrolysis was performed for 90 min in 90 Sr(NO3)2 feed solution at pH 2-3 at a potential of -2.5 V with 100-200 mA current using platinum electrodes. The second electrolysis was performed for 45 min in 3 mM HNO3 at a potential of -2.5 V with 100 mA current. In this step, the cathode from the first electrolysis containing 90 Y was used as anode along with a fresh circular platinum electrode as cathode. The 90 Y deposited on the circular cathode after the second electrolysis was dissolved in acetate buffer to obtain 90 Y acetate, suitable for radiolabeling. RESULTS: >96% recovery of 90 Y could be achieved in each cycle, with an overall decay corrected yield of >90%. The recovered 90 Y had high radionuclidic purity with barely 30.2+/-15.2 kBq (817+/-411 nCi) of 90 Sr per 37 GBq (1 Ci) of 90 Y (0.817+/-0.411 ppm). Consistent and repeated separation could be demonstrated using up to 1.85 GBq (50 mCi) of 90 Sr. The generator was named "Kamadhenu," the eternally milk-yielding Indian mythological cow. CONCLUSIONS: A technique suitable for adaptation at central radiopharmacies for obtaining therapeutic quantities of pure 90 Y has been developed.     

Labeling of monoclonal antibodies with a 67Ga-phenolic aminocarboxylic acid chelate

As a chelating agent for labeling antibodies (Abs) with metallic radionuclides, a propionic acid substituted ethylenediamine N,N-di-[(o-hydroxyphenyl) acetic acid] (P-EDDHA), which tighly complexes ⁶⁷Ga, was synthesized. The ⁶⁷Ga-P-EDDHA chelate was coupled in aqueous solution to IgG at a molar ratio of 1:1 via carbodiimide. The average coupling yield was 15%. A specific activity of 4 mCi/mg IgG could be obtained with commercially supplied ⁶⁷Ga. In vitro stability was evaluated in human serum at 37° C and showed a half-life of about 120 h for the release of ⁶⁷Ga from the labeled Ab during the initial phase of incubation. This in vitro halflife is similar to that measured for ¹¹¹In-DTPA labeled Abs. Because of the high stability of the ⁶⁷Ga-P-EDDHA chelate, the in vivo formation of radioactive labeled transferrin by transchelation, as described for ¹¹¹In-DTPA labeled Abs, should, however, be reduced by this labeling technique.     

Treatment of keloids by 90Sr-90Y beta-rays

The use of radiations for the treatment of keloids was the topic of debate for years. Because of the benign nature of the keloids, surgery (keloidectomy) was treatment of choice. However, the use of surgery alone for arresting the keloids growth does not give satisfactory results due to the high frequency of recurrences. In this study 110 symptomatic cases were treated with 90Sr-90Y beta-radiation either alone for flat keloids or in combination with surgery for thick keloids. The results obtained with this method were found to be quite satisfactory. Patients were given four fractions of 5 Gy per fraction either as weekly or twice weekly schedules. Radiation dose of 2000 cGy given twice weekly in four fractions showed response in 86% of the cases as compared to 73% in those receiving four fractions of 5 Gy weekly. Further observations on different time dose fractionation schedules would open up newer dimensions in the radiotherapy of keloids.     

90锶/90钇敷贴治疗增生性疤痕疙瘩疗效观察

我们对增生性疤痕疙瘩采用90锶/90钇(90Sr/90Y)敷贴和手术切除联合90Sr/90Y敷贴综合治疗,根据疤痕疙瘩生长的部位、病变大小以及病程长短等因素,选择不同照射剂量和疗效程。1资料与方法1.1临床资料增生性疤痕疙瘩患者166例,男102例,女64例,年龄9~56岁。166例中采用单纯90Sr/90Y敷贴治疗组98例,综合治疗(手术切除联合90Sr/90Y治疗)组68例。1.2方法90Sr/90Y敷贴器的表面剂量率为0.015Gy/s,采用小剂量分次治疗每次敷贴剂量4~6 GY,每周1次,3次为一疗程。综合治疗组选择增生性疤痕厚度超过0.5 cm的患者先进行手术切除,术后3~5 d即行90Sr/90…     

Labeling internalizing anti-epidermal growth factor receptor variant III monoclonal antibody with 177Lu: in vitro comparison of acyclic and macrocyclic ligands

IntroductionThe monoclonal antibody (mAb) L8A4, reactive with the epidermal growth factor receptor variant III (EGFRvIII), internalizes rapidly in glioma cells after receptor binding. Combining this tumor-specific mAb with the low-energy β-emitter ¹⁷⁷Lu would be an attractive approach for brain tumor radioimmunotherapy, provided that trapping of the radionuclide in tumor cells after mAb intracellular processing could be maximized.Materials and MethodsL8A4 mAb was labeled with ¹⁷⁷Lu using the acyclic ligands [(R)-2-amino-3-(4-isothiocyanatophenyl)propyl]-trans-(S,S)-cyclohexane-1,2-diamine-pentaacetic acid (CHX-A″-DTPA), 2-(4-isothiocyanatobenzyl)-diethylenetriaminepenta-acetic acid (pSCN-Bz-DTPA) and 2-(4-isothiocyanatobenzyl)-6-methyldiethylenetriaminepentaacetic acid (1B4M-DTPA), and the macrocyclic ligands S-2-(4-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-tetraacetic acid (C-DOTA) and α-(5-isothiocyanato-2-methoxyphenyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (MeO-DOTA). Paired-label internalization and cellular processing assays were performed on EGFRvIII-expressing U87.ΔEGFR glioma cells over 24 h to directly compare ¹⁷⁷Lu-labeled L8A4 to L8A4 labeled with ¹²⁵I using either iodogen or N-succinimidyl 4-guanidinomethyl-3-[¹²⁵I]iodobenzoate ([¹²⁵I]SGMIB). In order to facilitate comparison of labeling methods, the primary parameter evaluated was the ratio of ¹⁷⁷Lu to ¹²⁵I activity retained in U87.ΔEGFR cells.ResultsAll chelates demonstrated higher retention of internalized activity compared with mAb labeled using iodogen, with ¹⁷⁷Lu/¹²⁵I ratios of >20 observed for the three DTPA chelates at 24 h. When compared to L8A4 labeled using SGMIB, except for MeO-DOTA, internalized activity for ¹²⁵I was higher than ¹⁷⁷Lu from 1-8 h with the opposite behavior observed thereafter. At 24 h, ¹⁷⁷Lu/¹²⁵I ratios were between 1.5 and 3, with higher values observed for the three DTPA chelates.ConclusionsThe nature of the chelate used to label this internalizing mAb with ¹⁷⁷Lu influenced intracellular retention in vitro, although at early time points, only MeO-DOTA provided more favorable results than radioiodination of the mAb via SGMIB.     

Labeling of monoclonal antibodies with samarium-153 for combined radioimmunoscintigraphy and radioimmunotherapy

The labeling of a monoclonal antibody K-1-21 with 153Sm has been investigated using the bifunctional chelate cyclic diethylenetriaminepentaacetic acid (DTPA) anhydride. Labeling efficiencies greater than 60% were obtained using high specific activity [153Sm]chloride and a cDTPAa:MAb conjugation ratio of 20:1. The resultant labeled antibody had a s.a. greater than 150 MBq.mg-1 and a % retained immunoreactivity greater than 90%. Imaging and biodistribution studies in a rat model demonstrated that specific uptake of 153Sm-K-1-21 into s.c. implants of the target antigen could be clearly detected in scintigrams at 6 days p.i. The specific uptake (1.90 +/- 0.45% ID/g, 19.95 +/- 2.20 Implant:Blood ratio) compared favorably to 131I- and 111In-labeled K-1-21 (2.52 +/- 0.20 and 3.33 +/- 0.20% ID/g, 7.69 +/- 0.45 and 10.10 +/- 0.60 I:B, respectively). Labeling of MAbs with 153Sm for combined scintigraphy/therapy is feasible at clinically appropriate specific activities using cDTPAa, with the resultant conjugates retaining immunoreactivity and in vivo antigen localization.