D. Marshall et al., CLEARANCE OF CIRCULATING RADIO-ANTIBODIES USING STREPTAVIDIN OR 2ND ANTIBODIES IN A XENOGRAFT MODEL, British Journal of Cancer, 69(3), 1994, pp. 502-507
The improved tumour to non-tumour ratios needed for effective tumour t
argeting with antibodies requires that blood background radioactivity
is reduced. We investigated the effect of streptavidin as a clearing a
gent for I-125-labelled biotinylated anti-CEA antibodies in a human co
lon carcinoma xenograft model. By comparing the biodistribution of the
monoclonal antibody A5B7 with four, nine or 22 biotins per antibody m
olecule, we investigated how the degree of biotinylation of the primar
y radiolabelled antibody affects its clearance with streptavidin. Limi
ting the degree of biotinylation limited blood clearance, whereas nine
or 22 biotins per antibody molecule resulted in a 13- to 14-fold redu
ction in blood radioactivity, the streptavidin-biotinylated antibody c
omplexes clearing rapidly via the liver and spleen. Although a reducti
on in tumour activity was also seen, a 6.6-fold improvement in the tum
our to blood ratio was achieved. A comparative study of streptavidin v
ersus second antibody clearance was carried out using the polyclonal a
ntibody PK4S biotinylated with 12 biotins per antibody molecule. This
study indicated that second antibody was superior for clearance of the
polyclonal antibody, resulting in a larger and faster reduction in bl
ood radioactivity and improved tumour to blood ratios. In this case th
e primary antibody was polyclonal, and therefore nonuniformity of biot
inylation may affect complexation with streptavidin. Therefore, the de
gree of biotinylation and type of antibody must be carefully considere
d before the use of streptavidin clearance.