Improved safety and efficacy of 213Bi-DOTATATE-targeted alpha therapy of somatostatin receptor-expressing neuroendocrine tumors in mice pre-treated with L-lysine

Background: Targeted alpha therapy (TAT) offers advantages over current beta-emitting conjugates for peptide receptor radionuclide therapy (PRRT) of neuroendocrine tumors. PRRT with Lu-177-DOTATATE or Y-90-DOTATOC has shown dose-limiting nephrotoxicity due to radiopeptide retention in the proximal tubules. Pharmacological protection can reduce renal uptake of radiopeptides, e.g., positively charged amino acids, to saturate in the proximal tubules, thereby enabling higher radioactivity to be safely administered. The aim of this preclinical study was to evaluate the therapeutic effect of Bi-213-DOTATATE with and without renal protection using L-lysine in mice. Tumor uptake and kinetics as a function of injected mass of peptide (range 0.03-3 nmol) were investigated using In-111-DOTATATE. These results allowed estimation of the mean radiation absorbed tumor dose for Bi-213-DOTATATE. Pharmacokinetics and dosimetry of Bi-213-DOTATATE was determined in mice, in combination with renal protection. A dose escalation study with Bi-213-DOTATATE was performed to determine the maximum tolerated dose (MTD) with and without pre-administration of L-lysine as for renal protection. Neutrophil gelatinase-associated lipocalin (NGAL) served as renal biomarker to determine kidney injury. Results: The maximum mean radiation absorbed tumor dose occurred at 0.03 nmol and the minimum at 3 nmol. Similar mean radiation absorbed tumor doses were determined for 0.1 and 0.3 nmol with a mean radiation absorbed dose of approximately 0.5 Gy/MBq Bi-213-DOTATATE. The optimal mass of injected peptide was found to be 0.3 nmol. Tumor uptake was similar for In-111-DOTATATE and Bi-213-DOTATATE at 0.3 nmol peptide. Lysine reduced the renal uptake of Bi-213-DOTATATE by 50% with no effect on the tumor uptake. The MTD was < 13.0 +/- 1.6 MBq in absence of L-lysine and 21.7 +/- 1.9 MBq with L-lysine renal protection, both imparting an LD50 mean renal radiation absorbed dose of 20 Gy. A correlation was found between the amount of injected radioactivity and NGAL levels. Conclusions: The therapeutic potential of Bi-213-DOTATATE was illustrated by significantly decreased tumor burden and improved overall survival. Renal protection with L-lysine immediately prior to TAT with Bi-213-DOTATATE prolonged survival providing substantial evidence for pharmacological nephron blockade to mitigate nephrotoxicity.