Renal PET-imaging with (11)C-metformin in a transgenic mouse model for chronic kidney disease

Hepatic exposure of metformin in patients with non-alcoholic fatty liver disease

AIMS

Metformin is first-line treatment of type 2 diabetes mellitus and reduces cardiovascular events in patients with insulin resistance and type 2 diabetes. Target tissue for metformin action is thought to be the liver, where metformin distribution depends on facilitated transport by polyspecific transmembrane organic cation transporters (OCTs). Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the western world with strong associations to insulin resistance and the metabolic syndrome, but whether NAFLD affects metformin biodistribution to the liver is not known. In this study, the primary aim was to investigate in vivo hepatic uptake of metformin dynamically in humans with variable degrees of liver affection. As a secondary aim, we wished to correlate hepatic metformin distribution with OCT gene transcription determined in diagnostic liver biopsies.

METHODS

Eighteen patients with biopsy-proven NAFLD were investigated using 11C-metformin PET/CT technique. Gene transcripts of OCTs were determined by real-time polymerase chain reaction (PCR).

RESULTS

We observed similar hepatic volume of distribution of metformin between patients with simple steatosis and non-alcoholic steatohepatitis (NASH) (Vd 2.38 ± 0.56 vs. 2.10 ± 0.39, P = 0.3). There was no association between hepatic exposure to metformin and the degree of inflammation or fibrosis, and no clear correlation between metformin distribution and OCT gene transcription.

CONCLUSION

Metformin is distributed to the liver in patients with NAFLD and the distribution is not impaired by inflammation or fibrosis. The findings imply that metformin action in liver in patients with NAFLD may be preserved.

Imaging techniques to study drug transporter function in vivo

Transporter systems involved in the permeation of drugs and solutes across biological membranes are recognized as key determinants of pharmacokinetics. Typically, the action of membrane transporters on drug exposure to tissues in living organisms is inferred from invasive procedures, which cannot be applied in humans. In recent years, imaging methods have greatly progressed in terms of instruments, synthesis of novel imaging probes as well as tools for data analysis. Imaging allows pharmacokinetic parameters in different tissues and organs to be obtained in a non-invasive or minimally invasive way. The aim of this overview is to summarize the current status in the field of molecular imaging of drug transporters. The overview is focused on human studies, both for the characterization of transport systems for imaging agents as well as for the determination of drug pharmacokinetics, and makes reference to animal studies where necessary. We conclude that despite certain methodological limitations, imaging has a great potential to study transporters at work in humans and that imaging will become an important tool, not only in drug development but also in medicine. Imaging allows the mechanistic aspects of transport proteins to be studied, as well as elucidating the influence of genetic background, pathophysiological states and drug-drug interactions on the function of transporters involved in the disposition of drugs.

Imaging in Pharmacogenetics

An increasing collection of imaging technologies makes it possible to differentiate treatment responders from nonresponders based on genetic variation. This chapter will review some of the imaging technologies currently available in nuclear medicine to visualize drug absorption, distribution, metabolism, and elimination. Some of the commonly used techniques to detect radiation-emitting compounds are the two-dimensional scintigraphy and the three-dimensional single-photon emission computed tomography (SPECT) which both detect photons using a gamma camera, and the three-dimensional positron emission tomography (PET), which detect the decay of positron-emitting radionuclides. Current examples include visualization of functional effects of genetic variants, and these provide proof of concept for imaging in pharmacogenetics as a tool to improve efficacy and safety of drugs.

Prediction of the Effects of Renal Impairment on Clearance for Organic Cation Drugs that Undergo Renal Secretion: A Simulation-Based Study

Renal impairment (RI) is a major health concern with a growing prevalence. RI leads to various physiologic changes, in addition to a decrease in glomerular filtration rate, that impact the pharmacokinetics (PK) and, specifically, the renal clearance (CL_R) of compounds, including alterations of drug transporter (DT)/drug-metabolizing enzyme expression and activity, as well as protein binding. The objectives of this study were to use a physiologically based pharmacokinetic modeling platform to 1) assess the impact of alterations in DT expression, toxin-drug interactions (TDIs), and free fraction (f_u) on PK predictions for the organic cation transporter 2/multidrug and toxin extrusion protein 1 substrate metformin in RI populations; and 2) use available in vitro data to improve predictions of CL_R for two actively secreted substrates, metformin and ranitidine. The goal was to identify changes in parameters other than glomerular filtration rate-namely, f_u and DT expression/activity-that are consistent with in vitro and clinical data in RI, and predict the importance of these parameters in the PK of metformin and ranitidine in RI patients. Our results demonstrated that including alterations in DT expression and f_u, and including TDIs affecting DT activity, as indicated by in vitro data, improved the simulated predictions of CL_R and other PK parameters for both metformin and ranitidine in RI. Our simulations suggest that modifications of DT expression/activity and f_u are necessary for improved predictions of CL_R in RI for compounds that are actively secreted, and that improvement of PK predictions in RI populations for metformin and ranitidine can be obtained by incorporating in vitro data.

The Effect of Uremic Solutes on the Organic Cation Transporter 2

Chronic kidney disease (CKD) is characterized by the accumulation of uremic solutes; however, little is known about how these solutes affect drug absorption and disposition. The goal of this study is to evaluate the effect of uremic solutes on the organic cation transporter, OCT2, which plays a key role in the renal secretion of many basic drugs. As a second goal, we reviewed the literature to determine whether there was evidence for the effect of CKD on the renal secretion of basic drugs. We first screened 72 uremic solutes as inhibitors of [¹⁴C]-labeled metformin uptake by OCT2. Seven were identified as inhibitors and 3 of them were determined to be clinically relevant. Of the 7 solutes, dimethylamine, malondialdehyde, trimethylamine, homocysteine, indoxyl-β-d-glucuronide, and glutathione disulfide were novel OCT2 inhibitors. For 6 drugs that are known OCT2 substrates, both secretory clearance and glomerular filtration rate declined in parallel with progression of CKD from stage 2 to 4, suggesting that selective effects of uremic solutes on net tubular secretion of organic cations do not occur. Further clinical studies are warranted with a broader range of OCT2 substrates to determine whether CKD may differentially affect tubular secretion of drugs especially in patients with advanced CKD.