Pharmacokinetics and toxicity of 213Bi-labeled PAI2 in preclinical targeted alpha therapy for cancer

Objectives: The plasminogen activator inhibitor type 2 (PAI2) when labelled with 213Bi forms the 213Bi-PAI2 alpha conjugate (AC). This AC has been shown to be efficacious in preclinical studies with breast, ovarian, prostate and pancreatic cancers. The objectives of this study were to investigate the pharmacokinetics and in vivo stability of 213Bi-PAI2 in mice, its toxicity in mice and rabbits; and to determine whether a prior injection of a metal chelation (Ca-DTPA) or lysine can reduce toxicity by decreasing renal uptake. Methods: Two chelators (CHX-A″-DTPA and cDTPA) were used for preparation of the 213Bi-PAI2 conjugate, for intraperitoneal administration in mice and ear vein injection in rabbits. The mice were sacrificed at different time points for pharmacokinetic studies. Blood and organs were collected for toxicity studies for all groups. Results: Both chelators were found to have similar %ID/g in the kidneys over 4 hours. Mice and rabbits did not show any short term toxicity over 13 weeks at 1420 MBq/kg and 120 MBq/kg 213Bi-PAI2 respectively. Kidney uptake was decreased three fold by lysine. Radiation nephropathy was observed at 20-30 weeks in mice, leading to severe weight loss, whereas severe and widespread renal tubular necrosis was observed at 13 weeks in rabbits. Conclusions: Radiation nephropathy is the dose limiting toxicity observed in mice and rabbits. Lysine can reduce kidney uptake by three fold. Based on long-term monitoring, the maximum tolerance doses (MTD) are 350 and 120 MBq/kg for mice and rabbits respectively.