New strategies are needed to protect normal organs from radiation in
cancer radioimmunotherapy (RIT). This can be achieved by rapid clearance of radiometal in the circulation after accumulation of
radioimmunoconjugates (RIC) in the
tumor. Our strategy is to place highly efficient and specific cleavable linkers between radiometal chelates and the
tumor targeting agents. Such linkers must be resistant to cleavage by
enzymes present in the plasma and the
tumor. After radiotargeting agents have accumulated in the
tumor, a cleaving agent can be administered "on demand" to cleave a specific linker, resulting in the release of radiometal from the circulating RIC in a form that will have rapid renal clearance. We have selected
TNKase, a
thrombolytic agent approved for patient use, as our model on-demand cleaving agent. To identify
TNKase-specific linkers, we screened fluorescent-quenched random "one-bead-one-compound" (OBOC) combinatorial
peptide libraries. d-
Amino acid containing
peptides that were specific for
TNKase but were resistant to cleavage by plasma and
tumor-associated
proteases were identified. One of these
peptide substrates (rqYKYkf) was used to link the
DOTA chelate to
ChL6, a
monoclonal antibody known to target
breast cancer. This antibody conjugate was stable in plasma for 7 days while preserving the immunoreactivity to intact
tumor cells. The addition of
TNKase at clinical achievable plasma level (10 mug/mL) resulted in the release of 28% of the radiometal from the
radioimmunoconjugate within 72 h. This lead linker, after further optimization to increase its response to
TNKase, may be useful in the development of more effective radioimmunotherapeutic and radioimaging agents.