Assessment of mouse-specific pharmacokinetics in kidneys based on 131I activity measurements using micro-SPECT

Background In order to acquire accurate drug pharmacokinetic information, which is required for tissue dosimetry, micro-SPECT must be quantitative to allow for an accurate assessment of radioligand activity in the relevant tissue. This study investigates the feasibility of deriving accurate mouse-specific time-integrated drug pharmacokinetic data in mouse kidneys from activity measurements using micro-SPECT. Methods An animal experiment was carried out to evaluate the accuracy of 131 I activity quantification in mouse kidneys (mean tissue volume of 0.140 mL) using a micro-SPECT system against conventional ex vivo gamma counting (GC) in a NaI(Tl) detector. The imaging setting investigated was that of the mouse biodistribution of a 131 I-labelled single-domain antibody fragment (sdAb), currently being investigated for targeted radionuclide therapy of HER2-expressing cancer. SPECT imaging of 131 I 365-keV photons was done with a VECTor/CT system (MILabs, Netherlands) using a high-energy mouse collimator with 1.6-mm-diameter pinholes. For both activity quantification techniques, the pharmacokinetic profile of the radioligand from approximately 1–73 h p.i. was derived and the time-integrated activity coefficient per gram of tissue ( ã / M ) was estimated. Additionally, SPECT activity recovery coefficients were determined in a phantom setting. Results SPECT activities underestimate the reference activities by an amount that is dependent on the 131 I activity concentration in the kidney, and thus on the time point of the pharmacokinetic profile. This underestimation is around − 12% at 1.5 h (2.89 MBq mL −1 mean reference activity concentration), − 13% at 6.6 h (149 kBq mL −1 ), − 40% at 24 h (17.6 kBq mL −1 ) and − 46% at 73 h (5.2 kBq mL −1 ) p.i. The ã / M value estimated from SPECT activities is, nevertheless, within − 14% from the reference (GC) ã / M value. Furthermore, better quantitative accuracy (within 2% from GC) in the SPECT ã / M value is achieved when SPECT activities are compensated for partial recovery with a phantom-based recovery coefficient of 0.85. Conclusion The SPECT imaging system used, together with a robust activity quantification methodology, allows an accurate estimation of time-integrated pharmacokinetic information of the 131 I-labelled sdAb in mouse kidneys. This opens the possibility to perform mouse-specific kidney-tissue dosimetry based on pharmacokinetic data acquired in vivo on the same mice used in nephrotoxicity studies.

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