Abstract
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Objectives In pre-targeted radioimmunotherapy (PRIT), a bifunctional antibody is permitted to target tumor cells. Subsequently, a radioactive hapten is administered and captured by cell-bound antibody while unbound hapten clears rapidly from the body. We present a novel PRIT approach, with a bifunctional antibody specific to metal-loaded DOTA.
Methods Several 111In-DOTA-based haptens were compared in mice by measuring blood activity as a function of time and organ biodistribution at 4 and 24 hours post-injection. Directed evolution and yeast surface display were used to affinity mature the 2D12.5 antibody to DOTA, reformatted as a single chain variable fragment (scFv). A bispecific antibody construct was designed as an scFv fusion to the C-terminus of an IgG light chain.
Results While all haptens showed similar biodistribution, DOTA cleared significantly faster from the blood than DOTA-biotin, DOTA-fluorescein, and a di-DOTA peptide. Affinity maturation resulted in a 1000-fold affinity improvement to biotinylated DOTA-yttrium, yielding an 8.2 ± 1.9 picomolar binder. In addition to yttrium, the scFv binds DOTA complexes of lutetium and gadolinium with low picomolar affinity. The scFv retains its affinity when engineered into an IgG-like bispecific antibody.
Conclusions We have engineered an scFv with picomolar binding to several DOTA chelates that retains its affinity in a bispecific antibody format. Using simply DOTA as the hapten in PRIT, bone marrow dose is reduced several-fold due to faster blood clearance, while high tumor uptake is maintained via picomolar binding.
Research Support This work was supported by NIH CA101830 and a National Science Foundation Graduate Research Fellowship to KJD.
- © 2009 by Society of Nuclear Medicine