Biodistribution of the radiolabeled 125I- and111In-DTPA- BsAb-1 and BsAb-2 in cynomolgus monkey liver
The biodistribution, including the tissue elimination and tissue accumulation kinetics of BsAb-1 and BsAb-2, were studied in cynomolgus monkeys to gain additional mechanistic insight into the differential peripheral clearance observations. Following a single IV injection of125I-labelled and 111In-labelled DTPA conjugated versions of the two BsAb molecules, blood, tissue (organ) and carcass concentrations were measured for each molecule over the course of 168 hours post administration (Figures 2 and 3). In addition, urine samples were also collected to calculate the total amount of radioactive recovery to monitor the elimination kinetics (Figure 3). The reason for using two different versions of each molecule was that the 125I-labelled and111In-labelled DTPA conjugated BsAbs have differential retention in tissues. In the case of the radiohalogen (125I) catabolic products are cleared from cells; hence, this labeling approach gives the rate of clearance of each BsAb from tissue and the amount of catabolic products recovered in urine. In contrast, the catabolism of the radiometal-chelate (111In-labelled DTPA) leads to the catabolite being trapped in cells because of polar nature of chelate; given this, the labeling approach gives the extent of accumulation of the BsAbs in tissues.
Radiometrically derived blood kinetics (with both labels) showed the clearance of BsAb-1 > BsAb-2 (Figure 2) consistent with the exposure profiles of the unlabeled counterpart of each construct suggesting labeling the molecules did not impact the disposition. Quantitative analyses of the rate and extent of accumulation of the111In-labeled BsAbs in the major highly vascularized tissues (liver, spleen, kidney, lung, skin, muscle and other tissue collected as per outlined in Materials and Methods section) showed no meaningful differences in how fast or how much BsAb-1 and BsAb-2 accumulated in these tissues (refer to combined organ data from111In-labeled BsAb-1 and BsAb-2 data in Figure 2). Liver had the highest percentage of the injected dose of each molecule relative to the other tissues (data not shown), however, consistent with the combined 111In-labeled organ data, no difference in the tissue accumulation kinetics (%ID/g from111In-labeled analyses) or amount (%ID/g from111In-labeled analyses) between BsAb-1 and BsAb-2 was observed (Figure 2). In contrast, quantitative analyses of the rate and extent of elimination of the 125I-labeled BsAbs in the major highly vascularized tissues (liver, spleen, kidney, lung, skin, muscle and other tissue collected as per outlined in Materials and Methods section) showed striking differences in how fast BsAb-1 and BsAb-2 were cleared from the tissues and catabolic products were measurable in urine (refer to data from 125In-labeled BsAb-1 and BsAb-2 data in Figure 2). BsAb-1 showed ~3- to 6- times greater concentrations of 125I in urine than BsAb-2 over time (Figure 3).
Comparison of the biodistribution data from the two labels for BsAb-2 over time shows that the findings are reasonably similar for125I-BsAb-2 and 111In-DTPA-BsAb-2 (Figures 2 and 3). The data for the two labels indicates BsAb-2 is largely in blood as there was ~2- to 3- fold increase (or retention) of the molecule in the organs with the111In-DTPA conjugation compared to125I-label over time (Figure 3). Additionally, a larger portion of the 125I-label percent recovery data was in the organs and blood relative to urine for BsAb-2 over the time course (Figure 3). The low urinary elimination and larger proportion of molecule in the blood over time in the case of both labels for BsAb-2 supports the contention that a large proportion of BsAb-2 taken into tissues is recycled back into blood. In contrast to the BsAb-2 results, for BsAb-1 which had shown more rapid peripheral clearance (Figures 1B and 3) there are striking differences in the findings between125I-BsAb-1 and 111In-DTPA-BsAb-1 (Figure 3). The data for the two labels indicates BsAb-1 is largely retained in the organs as there is ~7- to 18-fold increase (or retention) of the molecule in the organs with the111In-DTPA conjugation compared to125I-label over time (Figure 3). Additionally, a larger portion (65% to 92%) of the 125I-BsAb-1 percent recovery data was in the urine compared to the organs and blood over the time course (Figure 3). Taken together, these data indicate the two BsAbs are taken up into tissue to a comparable extent and rate but, there is more rapid catabolism of BsAb-1 than BsAb-2 in tissues and elimination of catabolite products in the urine.