Vascular-targeted radioimmunotherapy with the alpha-particle emitter 211At.

Astatine-211, an alpha-particle emitter, was employed in a model system for vascular-targeted radioimmunotherapy of small tumors in mouse lung to compare its performance relative to other radioisotopes in the same system. Astatine-211 was coupled to the lung blood vessel-targeting monoclonal antibody 201B with N-succinimidyl N-(4-[211At]astatophenethyl) succinamate linker. Biodistribution data showed that the conjugate delivered 211At to the lung (260-418% ID/g), where it remained with a biological half-time of about 30 h. BALB/c mice bearing about 100 lung tumor colonies of EMT-6 cells, each about 2000 cells in size, were treated with 211At-labeled monoclonal antibody 201B. The administered activity of 185 kBq per animal extended the life span of treated mice over untreated controls. Injections of 370 kBq, corresponding to an absorbed dose of 25-40 Gy, were necessary to eradicate all of the lung tumors. Mice receiving 740 kBq of 211At-labeled monoclonal antibody 201B developed pulmonary fibrosis 3-4 months after treatment, as did mice treated with 3700 kBq of the alpha-particle emitter 213Bi-labeled monoclonal antibody 201B in previous work. Animals that were injected with 211At bound to untargeted IgG or to glycine, as control agents, also demonstrated therapeutic effects relative to untreated controls. Control groups that received untargeted 211At required about twice as much administered activity for effective therapy as did groups with lung-targeted radioisotope. These results were not consistent with radioisotope biodistribution and dosimetry calculations that indicated that lung-targeted 211At should be at least 10-fold more efficient for lung colony therapy than 211At bound to nontargeting controls. The data showed that 211At is useful for vascular-targeted radioimmunotherapy because lung tumor colonies were eradicated in the mice. Work in this model system demonstrates that vascular targeting of alpha-particle emitters is an efficient therapy for small perivascular tumors and may be applicable to human disease when specific targeting agents are identified.

Preparation and in vivo evaluation of linkers for 211At labeling of humanized anti-Tac.

The syntheses, radiolabeling, antibody conjugation, and in vivo evaluation of new linkers for 211At labeling of humanized anti-Tac (Hu-anti-Tac), an antibody to the alpha-chain of the IL-2 receptor (IL-2Ralpha) shown to be a useful target for radioimmunotherapy are described. Synthesis of the organometallic linker precursors is accomplished by reaction of the corresponding bromo- or iodoaryl esters with bis(tributyltin) in the presence of a palladium catalyst. Subsequent conversion to the corresponding N-succinimidyl ester and labeling with 211At of two new linkers, N-succinimidyl 4-[211At]astato-3-methylbenzoate and N-succinimidyl N-(4-[211At]astatophenethyl)succinamate (SAPS), together with the previously reported N-succinimidyl 4-[211At]astatobenzoate and N-succinimidyl 3-[211At]astato-4-methylbenzoate, are each conjugated to Hu-anti-Tac. The plasma survival times of these conjugates are compared to those of directly iodinated (125I) Hu-anti-Tac. The N-succinimidyl N-(4-[211At]astatophenethyl)succinamate compound (SAPS) emerged from this assay as the most viable candidate for 211At-labeling of Hu-anti-Tac. SAPS, along with the directly analogous radio-iodinated reagent, N-succinimidyl N-(4-[125I]astatophenethyl)succinamate (SIPS), are evaluated in a biodistribution study along with directly iodinated (125I) Hu-anti-Tac. Blood clearance and biological accretion results indicate that SAPS is a viable candidate for further evaluation for radioimmunotherapy of cancer.

Multifunctional antioxidant activity of HBED iron chelator.

The use of N,N'-bis (2-hydroxybenzyl) ethylenediamine-N,N'-diacetic acid (HBED) for iron chelation therapy is currently being tested. Besides its affinity for iron, bioavailability, and efficacy in relieving iron overload, it is important to assess its anti- and/or pro-oxidant activity. To address these questions, the antioxidant/pro-oxidant effects of HBED in a cell-free solution and on cultured Chinese hamster V79 cells were studied using UV-VIS spectrophotometry, oximetry, spin trapping, and electron paramagnetic resonance (EPR) spectrometry. The results indicate that HBED facilitates Fe(II) oxidation but blocks O2(.-)-induced reduction of Fe(III) and consequently pre-empts production of .OH or hypervalent iron through the Haber-Weiss reaction cycle. The efficacy of HBED as a 1-electron donor (H-donation) was demonstrated by reduction of the 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)-derived nitrogen-centered radical cation (ABTS(.+)), accompanied by formation of a short-lived phenoxyl radical. HBED also provided cytoprotection against toxicity of H2O2 and t-BuOOH. Our results show that HBED can act both as a H-donating antioxidant and as an effective chelator lacking pro-oxidant capacity, thus substantiating its promising use in iron chelation therapy.