Radioiodinated biocytin derivatives are potentially useful for multi-s
tep tumour targeting using the avidin-biotin system. We synthesized a
radioiodinated biocytin derivative and evaluated its properties in viv
o. We labelled biocytin with I-125 by coupling biocytin to radioiodina
ted N-succinimidyl 3-(tri-n-butyl-stannyl) benzoate, and assessed its
binding to avidin and its biodistribution in normal and tumour-bearing
mite. When the synthesized biocytin was incubated with immobilized av
idin, more than 94% of the radioactivity was bound. However, after 2 h
incubation in serum, only 40% of the radioactivity was bound to the a
vidin. The iodinated biocytin derivative was characteristically taken
up by the liver and the kidneys when injected intravenously into mice.
In mice bearing an intraperitoneal tumour xenograft,I-125-biocytin an
d In-111-biotin were co-injected intraperitoneally 4 h after the intra
peritoneal administration of avidin, which accumulated in the intraper
itoneal tumours. At 2 and 24 h, the tumour uptake of I-125-biocytin wa
s 8.2 and 3.8% ID/g respectively, whereas that of In-111-biotin was 20
.0 and 18.7% ID/g respectively. When radioiodinated, biocytin retains
its binding capacity to avidin, and it localizes well with high tumour
-to-normal tissue ratios early post-injection using the two-step metho
d, but compared to In-111-biotin it is unstable. We conclude that the
stability of the product in serum needs to be improved prior to in-viv
o applications. ((C) 1998 Lippincott Williams & Wilkins).