Manipulation of blood clearance to optimize delivery of residualizing label-antibody conjugates to tumor cells in vivo.

UNLABELLED: We have attempted to improve the therapeutic index of radioimmunotherapy by manipulating the blood clearance rate and the catabolism of the radiolabel. The general strategy is to allow the antibody (Ab) to circulate in the blood for 2-3 days, then to clear it rapidly by a method that delivers the Ab to hepatocytes. In addition, the radiolabel selected has two key properties: it is a residualizing label (which is lysosomally trapped after catabolism), so it is retained well by tumor cells, but is excreted rapidly by hepatocytes into bile.
METHODS: In initial experiments, three residualizing radiolabels were tested for their rate of excretion after specific delivery in vivo to either hepatocytes, via galactosylated Ab, or Kupffer cells, via immune complexes. A label showing rapid biliary excretion only after delivery to hepatocytes, (111)In-benzyl-diethylenetriamine tetraacetic acid, was then used for radioimmunodetection in a protocol of delayed rapid blood clearance in which clearance was by hepatocytes. This was achieved by using galactosylated Ab, combined with temporary inhibition of the asialo-glycoprotein receptor on hepatocytes. Ab RS11 and the lung adenocarcinoma Calu-3 xenograft in nude mice were used. Control experiments were performed with a conventional 125I label and with 125I-dilactitol-tyramine.
RESULTS: Indium-benzyl-diethylenetriamine tetraacetic acid was identified as a label that was excreted more rapidly from hepatocytes than from Kupffer cells, by biliary excretion. Using this radiolabel with delayed rapid blood clearance, very high tumor/blood ratios were obtained, 166:1 at day 3, but tumor/normal tissue ratios for other tissues were not as high. There appeared to be some uptake of the radiolabel by all normal tissues tested, including the lungs and muscle. Dosimetry calculations suggested that the therapeutic index was no better than with a simple Ab injection.
CONCLUSION: Antibody catabolism can be directed towards either hepatocytes or Kupffer cells, and this difference can strongly affect the excretion rate of radiolabels, since only hepatocytes can excrete degradation products into bile. Processing will also depend on the particular radiolabel. These factors are particularly important for protocols involving delayed rapid blood clearance, since liver uptake is so rapid. The methods described should stimulate other approaches of manipulating Ab blood clearance and radiolabel catabolism to achieve improved therapeutic results.