Previous studies have shown that pretargeting allows rapid visualization of renal cell carcinomas (RCC) with an (111)In-labeled bivalent peptide. For radioimmunotherapy, a beta-emitting radionuclide labeled to a bivalent peptide is required. Therapeutic efficacy of these radionuclides depends on the E(max), physical half-life, and residence time of the radiolabel in the tumor. The (131)I radiolabel generally clears rapidly from the tumor after internalization and subsequent degradation of the bivalent l-amino acid peptide (l-a.a. peptide) in the tumor cells. To improve the residence time of the iodine label in the tumor, a new bivalent peptide was synthesized that is peptidase resistant and consists of 4 d-amino acids (d-a.a. peptide). Here we investigated the characteristics of the residualizing iodine label in SK-RC-52 RCC tumors.
Methods: The d-a.a. peptide was manually synthesized according to standard solid-phase Fmoc/HBTU (2-[1H-benzotriazole-1-yl]-1,1,3,3-tetramethyluronium hexafluorophosphate) chemistry. The uptake and retention in the tumor of (111)In-/(125)I-labeled bivalent peptides (l-a.a. peptide and d-a.a. peptide) were studied in female BALB/c athymic mice with subcutaneous SK-RC-52 RCC tumors. Tumors were pretargeted with the bispecific monoclonal antibody (bs-mAb) G250xDTIn-1 and, 72 h later, mice were injected intravenously with one of both radiolabeled peptides. The effect of bs-mAb-diDTPA-bs-mAb (DTPA is diethylenetriaminepentaacetic acid) bridging at the tumor cell surface on the internalization of the bs-mAb-diDTPA complex was investigated in SK-RC-52 tumor-bearing mice.
Results: The maximum uptake and retention of (125)I-labeled l-a.a. peptide in the tumor were significantly lower compared with that of the (111)In-labeled l-a.a. peptide. In contrast, the tumor uptake and retention of the (125)I-labeled d-a.a. peptide) were similar to that of the (111)In-labeled l-a.a. peptide but were superior at later time points. The biodistribution of the radioiodinated d-a.a. peptide was highly similar to that of the (111)In-labeled d-a.a. peptide, and both radiolabeled peptides were retained significantly better in the tumor than the (111)In-labeled l-a.a. peptide. bs-mAb-diDTPA-bs-mAb bridge formation did not affect internalization of the bs-mAb-diDTPA complex.
Conclusion: Uptake and retention in the tumor of the iodinated peptide after pretargeting with a bs-mAb can be significantly improved using d-a.a. peptides. Accordingly, the radiation dose to the tumor, correlating with the therapeutic efficacy of pretargeted RCC, can be enhanced substantially.