Wholebody Clearance Of Three ⁸⁹Zr Labeled Antibodies:Differences between PET based and Whole Body Probe Based Clearance

Objectives For dosimetry,it is important to obtain whole body(WB) clearance data for the radiolabeled antibody that is being tested.We have been using both WB probe based and PET based clearance data.The aim of this study was to compare WB clearance data for 3 novel radiolabeled antibodies using probe based vs PET based measurements.

Methods We compared 10 patients injected with ⁸⁹Zr-DFO-trastuzumab,10 with ⁸⁹Zr-DFO-huJ591, and 6 with ⁸⁹Zr-DFO-MSTP2109A(~5mCi). WB clearance was calculated based on background subtracted geometric mean counts from NaI(Tl) probe.PET based clearance was calculated by integrating total activity in all serial PET images.Probe measurements were made pre and post-void and on 3 additional days over an 8 day period.Data were fit to a single exponential and T1/2 effective was obtained.Probe measurements were compared to PET measurements using Bland-Altman analysis.

Results The coefficient of variation for the effective half-live (Teff) as measured by PET and WB probe was less than 10% for all antibodies. PET and WB probe based estimates of Teff were not significantly different for ⁸⁹Zr-DFO-trastuzumab and ⁸⁹Zr-DFO-MSTP2109A (p=0.25 and p=0.17), but were 7% higher for PET based measurements for ⁸⁹Zr-DFO-huJ591 (p<0.002). This was probably due to concentration of activity in the liver over time.

Conclusions Estimates of WB clearance for ⁸⁹Zr-antibodies based on either probe or PET images generally show only small differences.Systematic differences may occur if activity is over time concentrated in the field of view of the PET camera as observed in the present study for ⁸⁹Zr-DFO-huJ591.