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Isothiocyanate group for conjugation of porphyrins and dipyrromethenes to antibodies

The synthesis of porphyrins bearing single isothiocyanate (NCS) group enables their bioconjugation to antibodies under mild conditions [1]. This methodology can be employed in the synthesis of porphyrins as new third generation photosensitisers that would target tumour via bioconjugation to monoclonal antibodies (Mabs). We have synthesised four cationic isothiocyanato-5, 15-diphenyl porphyrins and one tetraphenyl analogue. Diphenyl porphyrins were prepared from dipyrromethanes in [2+2] condensation, where one half of the molecule 'provides' amino group, which is easily converted to NCS group. This group reacts with amino groups contained on lysine residues on the antibody. To achieve better solubility under those conditions and to minimise non-covalent binding, the other half of the molecule contains quaternised nitrogen. Applying a known route, tetraphenyl analogue was also synthesised. Their bioconjugation to several monoclonal antibodies was successfully carried out without compromising the binding of the antibodies. The potential of these compounds as targeted photosensitisers for use in photodynamic therapy (PDT) has been demonstrated [2], and recently, bioconjugation to single chain Fv fragment was also successful proving even greater potential of this methodology for PDT applications [3]. We have also synthesised two BODIPY analogues (boron complexes of 5-phenyldipyrromethenes) bearing NCS group. The molecule with hindered rotation of the phenyl ring shows good photophysical properties common for this type of fluorophore. Both molecules were successfully conjugated to two monoclonal antibodies that bind to antigens over-expressed on cancer cells. This demonstrates their applicability for fluorescence labelling of proteins bearing primary amino groups and great potential advantages in the areas of fluorescence microscopy [4].

porphyrin; dipyrromethene; bioconjugation; monoclonal antibodies; isothiocyanate

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