Determination of specific radioactivity for (76)Br-labeled compounds measuring the ratio between (76)Br and (79)Br using packed capillary liquid chromatography mass spectrometry

Evaluation of ((4-hydroxyphenyl)ethyl)maleimide for site-specific radiobromination of anti-HER2 affibody

Affibody molecules are a new class of small phage-display selected proteins using a scaffold domain of the bacterial receptor protein A. They can be selected for specific binding to a large variety of protein targets. An affibody molecule binding with high affinity to a tumor antigen HER2 was recently developed for radionuclide diagnostics and therapy in vivo. The use of the positron-emitting nuclide (76)Br (T(1/2) = 16.2 h) could improve the sensitivity of detection of HER2-expressing tumors. A site-specific radiobromination of a cysteine-containing variant of the anti-HER2 affibody, (Z(HER2:4))(2)-Cys, using ((4-hydroxyphenyl)ethyl)maleimide (HPEM), was evaluated in this study. It was found that HPEM can be radiobrominated with an efficiency of 83 +/- 0.4% and thereafter coupled to freshly reduced affibody with a yield of 65.3 +/- 3.9%. A "one-pot" labeling enabled the radiochemical purity of the conjugate to exceed 97%. The label was stable against challenge with large excess of nonlabeled bromide and in a high molar strength solution. In vitro cell tests demonstrated that radiobrominated affibody binds specifically to the HER2-expressing cell-line, SK-OV-3. Biodistribution studies in nude mice bearing SK-OV-3 xenografts have shown tumor accumulation of 4.8 +/- 2.2% IA/g and good tumor-to-normal tissue ratios.

Radiobromination of humanized anti-HER2 monoclonal antibody trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate, a potential label for immunoPET

Combining the specificity of radioimmunoscintigraphy and the high sensitivity of PET in an in vivo detection technique could improve the quality of nuclear diagnostics. Positron-emitting nuclide (76)Br (T(1/2)=16.2 h) might be a possible candidate for labeling monoclonal antibodies (mAbs) and their fragments, provided that the appropriate labeling chemistry has been established. For internalizing antibodies, such as the humanized anti-HER2 monoclonal antibody, trastuzumab, radiobromine label should be residualizing, i.e., ensuring that radiocatabolites are trapped intracellularly after the proteolytic degradation of antibody. This study evaluated the chemistry of indirect radiobromination of trastuzumab using N-succinimidyl 5-(tributylstannyl)-3-pyridinecarboxylate. Literature data indicated that the use of this method provided residualizing properties for iodine and astatine labels on some antibodies. An optimized "one-pot" procedure produced an overall labeling efficiency of 45.5+/-1.2% over 15 min. The bromine label was stable under physiological and denaturing conditions. The labeled trastuzumab retained its capacity to bind specifically to HER2-expressing SKOV-3 ovarian carcinoma cells in vitro (immunoreactivity more than 75%). However, in vitro cell test did not demonstrate that the radiobromination of trastuzumab using N-succinimidyl 5-bromo-3-pyridinecarboxylate improves cellular retention of radioactivity in comparison with the use of N-succinimidyl 4-bromobenzoate.

Hybridisation of [76Br]-labelled antisense oligonucleotides to Chromogranin A mRNA verified by RT-PCR

Methods have been developed to label oligonucleotides (ODNs) in the 5'-position with (76)Br via a prosthetic group on a hexylamino-linker. The purpose of the study was to explore whether the labelling procedure would prevent specific hybridisation by using reverse transcription-polymerase chain reaction (RT-PCR) followed by sequencing of the PCR product. Antisense ODNs (30 mer, specific for rat Chromogranin A [CgA] mRNA) with phosphodiester (O-ODN) or phosphothioate (S-ODN) backbone, either unlabelled or labelled with (76)Br, served as one of the primers in individual PCR reactions. Using O-ODN as a primer, irrespective of being labelled or not, a selected 225-bp PCR fragment was successfully amplified. However, no amplification was obtained using S-ODN as a primer. The proper PCR products were only detected in the sample prepared from the adrenal gland, but not in that from the heart, liver or kidney. Autoradiographic recording of the gel, after gel electrophoresis, revealed radioactive signals corresponding to the amplified PCR products. The sequence of the PCR product matched the rat CgA mRNA sequence obtained from the EMBL database. RT-PCR is an attractive method to identify the selective binding of modified ODNs to target mRNA. This method confirmed that the labelling with (76)Br did not change the hybridisation ability of antisense O-ODN.