TC-99M LABELING OF THE ANTITUMOR ANTIBODY PR1A3 BY PHOTOACTIVATION

Irradiation of antibody with ultraviolet light leads to reduction of d isulphide bonds, Thus irradiation can be used to generate free thiols prior to direct labelling of antibody with technetium-99m, and has a p otential advantage over methods using chemical reducing agents such as mercaptoethanol or tin, in that no purification step is needed to rem ove excess reducing agent. We have used the photoactivation method dev eloped by Sykes et al. to label the anti-tumour antibody PR1A3 with Tc -99m. The antibody was irradiated at 300 nm using a Rayonet photochemi cal reactor with eight RMR3000 lamps. In a typical experiment, the ant ibody solution was injected into a nitrogen-filled borosilicate glass vial and purged with nitrogen. A degassed solution containing stannous fluoride and methylene diphosphonate was then added to the antibody a nd the vial was irradiated, Following the irradiation, [Tc-99m]pertech netate was injected into the vial and the reaction mixture was incubat ed for 30 min at room temperature before being analysed by size-exclus ion high-pressure liquid chromatography and instant thin-layer chromat ography, Labelling yields greater than 95% were obtained using antibod y concentrations ranging from 0.5 mg/ml to 5 mg/ml. Irradiation times as short as 5 min and tin to antibody ratios in the range between 11 a nd 32 mu g tin per mg antibody gave high labelling yields, Labelling y ields greater than 95% were obtained after storage of the photoactivat ed antibody at -70 degrees C for several weeks. The stability of the T c-99m-labelled photoactivated PR1A3 was similar to that of Tc-99m-labe lled mercaptoethanol-reduced PR1A3, The mean immunoreactive fraction w as 77% for the photoactivation-labelled PR1A3, compared to 93% for PR1 A3 labelled by mercaptoethanol reduction, Biodistribution studies were carried out using Tc-99m-photoactivation-labelled PR1A3 or PR1A3 labe lled by mercaptoethanol reduction in Balb/c mice and in nude mice with MKN45 human tumour xenografts. There was no significant difference in tumour uptake between the mice that received photoactivated PR1A3 and those that received mercaptoethanol-reduced PR1A3, There was also no significant difference in uptake in most organs in Balb/c mice; howeve r, the photoactivated antibody cleared more rapidly from the blood, an d whole-body clearance was also faster for the photoactivated PR1A3, I n conclusion, the photoactivation technique provides a very convenient ''one-pot'' method for labelling antibodies with Tc-99m.