Modification of antibody isoelectric point affects biodistribution of 111-indium-labeled antibody

We evaluated the influence of changes in charge on the biodistribution of In-111-labeled purified rabbit antihuman serum albumin (R-HSA) IgG conjugated to diethylenetriaminepentaacetic dianhydride (DTPA). Optimization of isoelectric point (pi) may influence the biodistribution profile, especially retention in vital organs, which ultimately affects radioimmunoimaging, Experiments were designed to modify the pI of R-HSA by conjugating various molar ratios of DTPA (DTPA:R HSA ratios 5:1 to 100:1). The pI of the conjugates was determined by isoelectricfocusing (IEF). In-111-labeled DTPA:R-HSA with known pi range was injected intraperitoneally into BALB/c mice to evaluate biodistribution. There was a proportional relationship between the molar ratio of DTPA to R-HSA IgG and the number of DTPA substituted. Molar ratios of 5, 10, 20, 50 and 100 gave, on average, 2.0, 3.6, 5.1, 9.5 and 16.0 DTPA per R-HSA IgG, respectively. An anodal shift in the pf of In-111-labeled R-HSA IgG was noted with increased number of DTPA conjugation. Biodistribution studies at both 4 and 24 h showed sequential increase in the liver activity with increasing number of DTPA per antibody, whereas colon and small intestine showed a decrease in the activity at 4 h. The organ-specific increase (e.g., liver) or decrease (e,g., colon and small intestine) in the activity may depend on a critical balance of charge of a particular organ and its interaction with the amount of negative charge carried by the antibody conjugate. The results suggest that pI optimized In-111-labeled antibody could be used to increase or decrease colon and hepatic retention for more efficient radioimmunoimaging of colon tumors and their hepatic metastasis.