Abstract
353
Objectives: Drug pharmacokinetics and target organ availability are defined by the drug’s retention in the blood pool. Since effective therapy usually requires a sustained minimum blood concentration, drugs with short blood half-life require higher and/or more frequent doses to maintain therapeutic levels. Such dosing levels may increase the likelihood of undesirable side effects. In order extend blood half-life of drugs using albumin as a carrier molecule, we developed an “add-on” molecule featuring (i) truncated Evans blue (EB) dye molecule (ii) a metal chelate and (iii) a maleimide. The “add-on” provides moderate albumin binding, due to the EB, that prolongs half-life in the blood, allows radiolabeling for imaging and radiotherapy, and can be easily conjugated to targeting molecules containing a free thiol group. Materials and Methods: The truncated EB was conjugated with or 1,4,7,10-tetraazacyclododecane-tetraacetic acid (DOTA) chelator. As a proof-of-concept, we coupled DOTA conjugated EB to an octreotate analogy (TATE), a somatostatin receptor-binding peptide (EB-TATE). EB-TATE were radiolabeled with Lu-177 for SPECT imaging and targeted radiotherapy, respectively, and compared to the peptides without the “add-on” (DOTA-TATE) in patient-derived xenograft model of pancreatic neuroendocrine tumors (PDX-PNET).
Results: After approval from the internal review board, samples of metastatic PNET (Grade 3) are obtained from patient undergoing percutaneous biopsy, and the PENT-PDX models were subsequently established. EB-TATE was radiolabeled at a specific activity of 35-40 GBq/μmol, with a radiochemical yield of more than 90%, and a radiochemical purity of more than 95%. The resulting radiolabeled conjugates showed prolonged circulation half-life and enhanced tumor accumulation in somatostatin receptor-expressing PDX-PNETs (Figure 1). In 177Lu-EB-TATE SPECT imaging, the tumor to background ratio (T/B) reached 13.3±2.9 at 24 h, and remained 13.9±3.4 at 72 h post injection. The ex vivo biodistribution result showed that tumor uptake of EB-TATE was markedly increased compared to the peptides without the Evans blue conjugation (13.2 ±2.1 %ID/g vs. 0.2 ± 0.1 %ID/g, Figure 2). Regarding targeted radiotherapy, 177Lu-EB-TATE revealed a great inhibition of tumor growth in the 500 μCi treated group (Day 10, 401.6 ± 140.6 mm3; Day 20, 472.2 ± 149.6 mm3), compared with TATE monomer treated groups (Day 10, 755.6 ± 237.5 mm3; Day 20, 1280.8 ± 426.3 mm3) (P <0.01) and untreated controls (Day 10, 853.7 ± 358.6 mm3; Day 20, 1395.1 ± 197.6 mm3) (P <0.01) (Figure 3). No systemic toxicity due to radiotherapy was observed by monitoring animal body weight.
Conclusions: Conjugation of our novel “add-on” molecules to TATE peptides significantly improved both imaging and radiotherapy with these agents. These results show that our “add-on” improves blood half-life and tumor uptake, and can transform drugs into theranostic entities. Furthermore, our PDX-PNET model is the available, validated PDX model for PNET, and preclinical data from the use of this model suggests that 177Lu-EB-TATE may be an effective new treatment option for patients with PNET.