An important limitation in attaining significant tumour/non-tumour ratio during radio-immuno therapy is the nonspecific targeting of antibodies to the liver, spleen and other parts of reticulo-endothelial system (R.E.S.). We have earlier described methods of reducing this by blocking the R.E.S., using large molecular weight substance such as nonspecific gammaglobulin. Since pretargeting is another approach to enhance tumour targeting and because radiolabeled biotin is the effector therapy molecule in many such strategies, we investigated the biodistribution of radiolabeled biotin in experimental mice.
To 900 gm biotin in 1 ml equi-volume solution of EtoH/H2O (pH-7.5), we added 500, l sodium-potassium tartrate, (160mg/10 ml d.w.), 10, l SnF2 (0.1%), 40, l (0.1%) sodium-oxalate & finally 5 mCi Tc-generator eluate. The reaction mixture was injected into the tail vein of normal rats. The animal were imaged at 1 hrs & 4 hrs after this.
There was a high concentration of radio-labeled biotin in the liver reminiscent of the liver imaging with radio-labeled gamma-globulin. This was quite distinct from the distribution of free Tc.
Radiolabeled biotin, like radiolabeled gammaglobulin does have a large nonspecific targeting to the liver. Methods of reducing this need to be studied for successful use of radio-labeled biotin for therapy.
Radio Immuno therapy, Pretargeting, Radio-labeled biotin, biodistribution
