BIOLOGICAL PROPERTIES OF BIOTIN-CHELATE CONJUGATES FOR PRETARGETED DIAGNOSIS AND THERAPY WITH THE AVIDIN BIOTIN SYSTEM/

Three-step pretargeting increases target-to-background ratios in radio immunodetection and can potentially decrease harmful radiation to norm al tissues in radioimmunotherapy. We studied four biotin-chelate conju gates (BCCs) for use in the avidin/biotin pretargeting system. Methods : Pharmacokinetics and biodistribution were studied in normal BALB/c ( IA(k)-negative), normal C3H (IA(k)-positive) and LS174T tumor-bearing BALB/c severe combined immunodeficient mice, Streptavidin alone and an tibody-streptavidin conjugates [monoclonal antibody (MAb) 10-3.6 anti- IA(k) IgG2a] were used, Indium-111- or Y-88-BCCs were given alone intr avenously; they were mixed with streptavidin or MAb-streptavidin conju gate and given intravenously; or streptavidin and MAb-streptavidin con jugate were pretargeted, and 2-3, 5 and 21 hr later, BCCs were injecte d intravenously. Samples were taken 2-3 hr after intravenous injection of labeled BCCs, Results: Three of the four BCCs were rapidly excrete d by the kidneys, with <2.5%/g in any organ or tumor at 2-3 hr. Gut ex cretion eliminated inyl-(S)-1-p-aminobenzylethylenediaminetetraacetic acid (EDTA) for use in pretargeting, Ninety percent of BCCs were bound to circulating pretargeted streptavidin at 1-6 hr, and similar to 15% were bound to pretargeted streptavidin at 24 hr. Kidney uptakes were: preformed streptavidin-BCC given intravenously, similar to 80%/g (24 hr); streptavidin pretargeted for 23 hr, similar to 60%/g; and strepta vidin pretargeted for 5-21 hr, similar to 10%-20%/g, Kidney uptake was dose-dependent: 0.2, 0.67 and 1.0 nmol of streptavidin pretargeted fo r 21 hr showed increasing concentrations (24 hr). Uptake of monoclonal anti-IA(k)-streptavidin-CC complex into spleen (70% +/- 10%/g; p < 0. 05) and lymph nodes (10% +/- 3.5%/g; p < 0.01) was higher in IA(k)-pos itive C3H mice than it was in IA(k)-negative control BALB/c mice, and it was much higher than that in streptavidin controls. No significant target uptake was seen with anti-IA(k) MAb-streptavidin pretargeted fo r 3 or 20 hr. Kidney uptake similar to 20%/g, which was lower than tha t of streptavidin alone. Conclusion: Three biotinyl chelates bind the diagnostic and therapeutic radiometals In-111 and Y-88 land, by analog y, Y-90) with the required in vivo stability and physiological propert ies for pretargeted diagnosis and therapy. Kidney uptake of streptavid in was decreased by conjugation to MAb. Failure of anti IA(k) MAb-stre ptavidin conjugate to bind BCC after pretargeting may be due to rapid internalization of MAb-streptavidin-IA(k) complex by the lymphocyte or to endogenous biotin. Either or both of these would make streptavidin unavailable to subsequent BCCs.