The use of 68Ga-EDTA PET allows detecting progressive decline of renal function in rats

Abstract

Introduction: Evaluation of glomerular filtration rate is very important in both preclinical and clinical setting, especially in the context of chronic kidney disease. It is typically performed using ⁵¹Cr-EDTA or by imaging with ¹²³I-Hippuran scintigraphy, which has a significantly lower resolution and sensitivity as compared to PET. ⁶⁸Ga-EDTA represents a valid alternative due to its quick availability using a ⁶⁸Ge/⁶⁸Ga generator, while PET/CT enables both imaging of renal function and accurate quantitation of clearance of activity from both plasma and urine. Therefore, we aimed at investigating the use of ⁶⁸Ga-EDTA as a preclinical tracer for determining renal function in a knock-in rat model known to present progressive decline of renal function.

Methods: ⁶⁸Ga-EDTA was injected in 23 rats, either wild type (n=10) or knock-in (n=13). By applying a unidirectional, two-compartment model and Rutland-Patlak Plot linear regression analysis, split renal function was determined from the age of 6 weeks to 12 months.

Results: Glomerular filtration ranged from 0.025±0.01 ml/min at 6 weeks to 0.049±0.05 ml/min at 6 months in wild type rats. Glomerular filtration was significantly lower in knock-in rats at 6 and 12 months (P<0.01). No significant difference was observed in renal volumes between knock-in and wild type animals, based on imaging-derived volume calculations.

Conclusions: ⁶⁸Ga-EDTA turned out to be a very promising PET/CT tracer for the evaluation of split renal function. This method allowed detection of progressive renal impairment in a knock-in rat model. Additional validation in a human cohort is warranted to further assess clinical utility in both, healthy individuals and patients with renal impairment.

Keywords: Glomerular filtration rate; gallium-68 EDTA; positron emission tomography; renal plasma function; rutland-patlak plot.

Figure 1

Micro-PET/CT image acquisition and pharmacokinetic modelling using an irreversible unidirectional, two-compartment model. A. A unidirectional, two-compartment model was used to model the kinetic of the cardiac, renal and bladder uptake of the radiotracer and Rutland-Patlak Plot was applied to calculate the filtration constant. In order to assure reduce variance in linear regression analysis, the first 120 seconds were taken into account during model fitting. B. Pharmacokinetic modelling of single-kidney split renal function and perfusion following ⁶⁸Ga-EDTA administration. The kidney is viewed as two separated compartments, with an external tubule-glomerular compartment and an inner papillary compartment. The filtration and elimination constants are shown. Legend: C(t)= concentration at time t, A _urine = total bladder activity after end of acquisition (20 minutes), K _gfr. = filtration constant, K _el = renal elimination constant. C. Region of interest (ROI) of the selected organs based on PET activity signal. The red, central area corresponding approximatively to the inner papillary compartment. D. PET and CT images were acquired separately and superimposed to show co-localization of anatomical and morphological images of both kidneys. E. Resulting time-activity pharmacokinetic curves for the different organs calculated using PMOD derived from the activity in the ROI over a time span of 20 minutes.

Figure 2

Estimation of pharmacokinetics parameters upon linear regression analysis. A. Glomerular filtration coefficient K _gfr was calculated at different time points, ranging from 6 weeks to 12 months, using the linear regression analysis and Patlak-Plot model. B. Corresponding glomerular filtration rate (GFR), which is proportional to the filtration coefficient and the renal volume, based on PET activity. C. Clearance of the radiotracer at different time points ranging from 6 weeks to 12 months, corresponding to the ratio of the total renal excreted activity (bladder VOI at the end of acquisition, set to 20 minutes) and the integral of the plasma curve (tubule-glomerular AUC). D. Renal volume over the lifespan of the animals, expressed as the average of both left and right kidneys. Volume calculation was based on distribution of tracer activity in the ROI and performed automatically by the PMOD software. Significant differences P<0.05 are indicated by (*).

Figure 3

Representative PET-CT images at the different time points investigated in the study and activity-based volume assessment in both wild-type and knock-in animals. A. PET signal activity in selected rats at different time points, ranging from 6 weeks to 12 months, showing both renal, bladder and cardiac activity. ROIs were then modelled around the activity regions and the different parameters determined using linear regression. B. Renal volume over the lifespan of the animals for wild-type and knock-in rats, calculated from PET signals. Values are averaged from both left and right kidneys.

Figure 4

Estimation of pharmacokinetics parameters upon linear regression analysis in knock-in animals compared to wild-type animals. A. Glomerular filtration coefficient K _gfr was calculated at different time points, ranging from 6 weeks to 12 months, using the linear regression analysis and Patlak-Plot model for both wild-type and knock-in rats. B. Corresponding glomerular filtration rate (GFR), which is proportional to the filtration coefficient and the renal volume, based on PET activity for both wild-type and knock-in rats. C. Clearance of the radiotracer at different time points ranging from 6 weeks to 12 months, corresponding to the ratio of the total renal excreted activity (bladder VOI at the end of acquisition, set to 20 minutes) and the integral of the plasma curve (tubule-glomerular AUC) for both wild-type and knock-in rats. D. Renal volume over the lifespan of the animals for both wild-type and knock-in rats, expressed as the average of both left and right kidneys. Significant differences are indicated by *P<0.05, ***P<0.01 and ****P<0.0001.