A PET Tracer for Renal Organic Cation Transporters, ¹¹C-Metformin: Radiosynthesis and Preclinical Proof-of-Concept Studies

Abstract

Organic cation transporters (OCTs) in the kidney proximal tubule (PT) participate in renal excretion of drugs and endogenous compounds. PT function is commonly impaired in kidney diseases, and consequently quantitative measurement of OCT function may provide an important estimate of kidney function. Metformin is a widely used drug and targets OCT type 2 located in the PT. Thus, we hypothesized that 11 Clabeled metformin would be a suitable PET tracer for quantification of renal function. Methods: 11 C-metformin was prepared by 11 Cmethylation of 1-methylbiguanide. In vitro cell uptake of 11 C-metformin was studied in LLC-PK 1 cells in the presence of increasing doses of unlabeled metformin. In vivo small-animal PET studies in SpragueDawley rats were performed at baseline and after treatment with OCT inhibitors to evaluate renal uptake of 11 C-metformin. Kidney and liver pharmacokinetics of 11 C-metformin was investigated in vivo by dynamic 11 C-metformin PET/CT in 6 anesthetized pigs, and renal clearance of 11 C-metformin was compared with renal clearance of 51 Cr-ethylenediaminetetraacetic acid (EDTA). Formation of 11 C metabolites was investigated by analysis of blood and urine samples. Results: The radiochemical yield of 11 C-metformin was 15% ± 3% (n 5 40, decay-corrected), and up to 1.5 GBq of tracer were produced with a radiochemical purity greater than 95% in less than 30 min. Dose-dependent uptake of 11 C-metformin in LLC-PK 1 cells was rapid. Rat small-animal PET images showed 11 C-metformin uptake in the kidney and liver, the kinetics of which were changed after challenging animals with OCT inhibitors. In pigs, 80% of the injected metformin dose was rapidly present in the kidney, and a high dose of metformin caused a delayed renal uptake and clearance compared with baseline consistent with transporter-mediated competition. Renal clearance of 11 C-metformin was approximately 3 times the renal clearance of 51 Cr-EDTA. Conclusion: We successfully synthesized an 11 Cmetformin tracer, and PET studies in rats and pigs showed a rapid kidney uptake from the blood and excretion into the bladder similar to other radiopharmaceuticals developed for γ-camera renography.

In Vivo Imaging of Human 11C-Metformin in Peripheral Organs: Dosimetry, Biodistribution, and Kinetic Analyses

“…As recently demonstrated by our group using a porcine model (8), 80% of intravenously administered 11 C-metformin is rapidly taken up by the kidneys and excreted into the urine. Renal metformin uptake and extrusion involves OCT2 (13,27,28), MATE1, MATE2k (19), and possibly OCT1 (29).…”

“…Preparation of 11 C-Metformin 11 C-metformin (isotope half-life, 20.4 min) was prepared as recently described (8) and contained 0.02-0.4 mg/mL metformin dissolved in aqueous (NH 4 ) 2 HPO 4 (100 mM, pH 5).…”

“…We recently reported that metformin can be labeled with 11 C using a novel 1-step synthesis and have shown in cell and largeanimal studies that the radiotracer may be a potential candidate to accurately quantify kidney function (8). The aim of this paper was to extend these preclinical findings to humans.…”

Renoprotective Effects of Metformin are Independent of Organic Cation Transporters 1 & 2 and AMP-activated Protein Kinase in the Kidney

“…[ 11 C]-metformin was prepared as described earlier56 Briefly, [ 11 C]-Methyl iodide was reacted with 1-methylbiguanide dissolved in acetone and the crude product was purified by reverse phase HPLC. The final product (0.2–1.0 GBq) contained 0.1–0.5 microgram/ml metformin in aqueous (NH 4 ) 2 HPO 4 (100 mM, pH 7).…”

[11C]-Labeled Metformin Distribution in the Liver and Small Intestine Using Dynamic Positron Emission Tomography in Mice Demonstrates Tissue-Specific Transporter Dependency

“…11 C]-labeled metformin ([ 11 C]-metformin) was recently demonstrated to be a powerful tool to noninvasively determine distribution of metformin in specific tissues (14)(15)(16). Therefore, the primary aim of the present study was to use dynamic positron emission tomography (PET) with [ 11 C]-metformin in mice to investigate the importance of OCT and MATE in dynamic hepatic and small intestinal distribution of metformin in vivo.…”