RADIOIMMUNOTHERAPY & EXTRACORPOREAL ADSORPTION Preclinical Studies of the Pharmacokinetics & Biotinylation of Radioimmunoconjugates in Immunocompetent Rats

Extracorporeal adsorption (ECAT) is a novel strategy to reduce activity in whole body (WB), blood and radiosensitive organs by removing the excess of radiolabeled and biotinylated monoclonal antibody (MAb) from blood so that tumor/normal tissue ratio (T/N) could be improved. By increasing T/N, higher amounts of radiolabeled MAb might be administered, making it possible to treat disseminated carcinomas. Paper 1. To determine whether ECAT also could be used to reduce activity after intraperitoneal administration (111In-HMFG1-biotin) in rats and to compare the pharmacokinetics +/- attached biotin. When ECAT was employed, the WB and blood radioactivity were reduced by 35-40% (P < 0.05) and 75-86% (P < 0.01), respectively. After the completion of ECAT, the activity uptake in organs was significantly decreased. Paper 2. When N-hydroxysuccinimido (NHS) biotin reagent was used for biotinylation of 111In-hMN14-DOTA, accelerated blood clearance and increased liver uptake of the radiolabeled MAb were observed, probably due to altered hydrophobicity after MAb biotinylation. An alternative, water-soluble biotin reagent sulfo-NHS-biotin (S-NHS) was therefore chosen and evaluated. No fragmentation or aggregation was observed using sulfo-NHS-biotin. When ECAT started at 6 h post injection, the WB and blood activity were reduced by 64% and 98%, respectively. The uptake in organs sensitive to radiation was also reduced, varying between 39% for the liver and 84% for the lungs and bone marrow. Paper 3. To investigate if pharmacokinetics from one radiolabeled MAb in rats can be generalized to other MAbs labeled with the same chelator (1033) and radionuclide (111In) to avoid time-consuming pharmacokinetic studies for each individual MAb used. The pharmacokinetics of three chimeric/humanized MAbs (rituximab, trastuzumab, BR96) were evaluated. No statistical difference was detected between blood pharmacokinetics. The WB activity clearance and the organ uptake for three groups were very similar. A further MAb (hMN14,) which was labeled to another chelator (DOTA-biotin), was found to have a faster blood pharmacokinetics, WB activity clearance and higher liver activity uptake. The pharmacokinetics could be predicted irrespective of the IgG1-MAb chosen prior to ECAT when labeled with the same tri-functional chelator 1033 and 111In. A small tumor burden did not change the biokinetics. Paper 4. To determine and compare the maximal tolerable dose (MTD) of the MAb BR96 labeled with 90Y or 177 Lu by using immunocompetent rat models, which better reflects the clinical situation. This model showed a good reproducibility and dose-dependent toxicity for both the radionuclides indicating that the MTD of 177Lu-BR96 is 1.7 times the MTD of 90Y-BR96. The longer T1/2 of 177Lu vs. 90Y implies that it will be possible to delay the onset of the ECAT procedure allowing a greater accumulation of the radioimmunoconjugate in the tumor without unacceptable exposure to normal tissue.