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

UNLABELLED: 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)C-labeled metformin would be a suitable PET tracer for quantification of renal function. METHODS: (11)C-metformin was prepared by (11)C-methylation of 1-methylbiguanide. In vitro cell uptake of (11)C-metformin was studied in LLC-PK1 cells in the presence of increasing doses of unlabeled metformin. In vivo small-animal PET studies in Sprague-Dawley 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= 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-PK1 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)C-metformin 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.

Imaging tumour physiology and vasculature to predict and assess response to heat.

The vascular supply of tumours and the tumour microenvironment both play an important role when tumours are treated with hyperthermia. Blood flow is one of the major vehicles by which heat is dissipated thus the vascular supply will influence the ability to heat the tumour. It also influences the type of microenvironment that exists within tumours, and it is now well-established that cells existing in areas of oxygen deficiency, nutrient deprivation and acidic conditions are more sensitive to the effect of hyperthermia. The vascular supply and microenvironment are also affected by hyperthermia. In general, mild heat temperatures transiently improve blood flow and oxygenation, while higher hyperthermia temperatures cause vascular collapse and so increase the adverse microenvironmental conditions. Being able to image these vascular and microenvironmental parameters both before and after heating will help in our ability to predict and assess response. Here we review the various techniques that can be applied to supply this information, especially using non-invasive imaging approaches.