Involvement of organic cation transporter 1 in the lactic acidosis caused by metformin

Use of biguanides to improve response to chemotherapy

Metformin is a commonly utilized antidiabetic agent, which has been associated with improved clinical outcomes in cancer patients. The precise mechanism of action remains unclear, but preclinical evidence suggests that metformin can sensitize tumor cells to the effects to conventional chemotherapeutic agents and ionizing radiation (IR). In this chapter we discuss the general background of an approach to evaluate the effects of metformin on conventional chemotherapeutic agent toxicity in a preclinical model.

Metformin in chronic kidney disease: time for a rethink

Metformin has traditionally been regarded as contraindicated in chronic kidney disease (CKD), though guidelines in recent years have been relaxed to permit therapy if the glomerular filtration rate (GFR) is > 30 mL/min. The main problem is the perceived risk of lactic acidosis (LA). Epidemiological evidence suggests that this fear is disproportionate. Lactic acidosis is a rare complication to type 2 diabetes mellitus (T2DM), with an incidence of 6/100,000 patient-years. The risk is not increased in metformin-treated patients. Metformin possesses a number of clinical effects independent of glucose reduction, including weight loss, which are beneficial to patients. The risk of death and cardiovascular disease is reduced by about a third in non-CKD patients. Since metformin intoxication undoubtedly causes LA, and metformin is renally excreted, inappropriate dosage of metformin will increase the risk of LA. It is suggested that introduction of metformin therapy to more advanced stages of CKD may bring therapeutic benefits that outweigh the possible risks.

Molecular pathways: BRAF induces bioenergetic adaptation by attenuating oxidative phosphorylation

Cancers acquire mutations in cooperating pathways that sustain their growth and survival. To support continued proliferation, tumor cells adapt their metabolism to balance energy production with their augmented biosynthetic needs. Although most normal differentiated cells use mitochondrial oxidative phosphorylation (OXPHOS) as the bioenergetic source, cancer cells have been proposed to rely principally on cytoplasmic glycolysis. The molecular basis for this shift, termed the Warburg effect, is the subject of intense investigation, because mechanistic understanding may lead to novel approaches to target the altered metabolism of cancer cells. Recently, mutations BRAF(V600E) have emerged as a major regulator of metabolic homeostasis. Melanoma cells may use a metabolic shift to circumvent BRAF(V600E)-induced senescence though limiting their reliance on OXPHOS and promote proliferation. Furthermore, BRAF(V600E) acts to suppress expression of the melanocyte master regulator microphthalmia-associated transcription factor (MITF) and the mitochondrial biogenesis coactivator PGC1α. Accordingly, therapeutic inhibition of BRAF(V600E) reverses metabolic reprogramming in melanoma cells and elevates OXPHOS through increased MITF-PGC1α levels. BRAF-targeted drugs modulate the metabolic state of malignant melanoma cells, and counteracting these adaptive responses using pharmacologic agents may prove useful in combinatorial therapeutic strategies. Clin Cancer Res; 20(9); 2257-63. ©2014 AACR.

Metformin and other antidiabetic agents in renal failure patients

This review mainly focuses on metformin, and considers oral antidiabetic therapy in kidney transplant patients and the potential benefits and risks of antidiabetic agents other than metformin in patients with chronic kidney disease (CKD). In view of the debate concerning lactic acidosis associated with metformin, this review tries to solve a paradox: metformin should be prescribed more widely because of its beneficial effects, but also less widely because of the increasing prevalence of contraindications to metformin, such as reduced renal function. Lactic acidosis appears either as part of a number of clinical syndromes (i.e., unrelated to metformin), induced by metformin (involving an analysis of the drug's pharmacokinetics and mechanisms of action), or associated with metformin (a more complex situation, as lactic acidosis in a metformin-treated patient is not necessarily accompanied by metformin accumulation, nor does metformin accumulation necessarily lead to lactic acidosis). A critical analysis of guidelines and literature data on metformin therapy in patients with CKD is presented. Following the present focus on metformin, new paradoxical issues can be drawn up, in particular: (i) metformin is rarely the sole cause of lactic acidosis; (ii) lactic acidosis in patients receiving metformin therapy is erroneously still considered a single medical entity, as several different scenarios can be defined, with contrasting prognoses. The prognosis for severe lactic acidosis seems even better in metformin-treated patients than in non-metformin users.

The synthesis and biodistribution of [(11)C]metformin as a PET probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1) in vivo

In order to develop a new positron emission tomography (PET) probe to study hepatobiliary transport mediated by the multi-drug and toxin extrusion transporter 1 (MATE1), (11)C-labelled metformin was synthesized and then evaluated as a PET probe. [(11)C]Metformin ([(11)C]4) was synthesized in three steps, from [(11)C]methyl iodide. Evaluation by small animal PET of [(11)C]4 showed that there was increased concentrations of [(11)C]4 in the livers of mice pre-treated with pyrimethamine, a potential inhibitor of MATEs, inhibiting the hepatobiliary excretion of metformin. Radiometabolite analysis showed that [(11)C]4 was not degraded in vivo during the PET scan. Biodistribution studies were undertaken and the organ distributions were extrapolated into a standard human model. In conclusion, [(11)C]4 may be useful as a PET probe to non-invasively study the in vivo function of hepatobiliary transport and drug-drug interactions, mediated by MATE1 in future clinical investigations.

Vitamin, mineral, and drug absorption following bariatric surgery

The prevalence of obesity continues to rise throughout the world. Increasingly, bariatric surgery is used for those with morbid obesity as a pivotal approach to achieve weight loss. Along with substantial weight loss, malabsorption of essential vitamins, minerals, and drugs also occurs. Therefore, more than ever, a better understanding of the physiology and mechanisms by which these deficiencies occur is essential. We review the normal physiology of vitamin, mineral, and drug absorption. This is followed by a description of currently performed bariatric surgeries in the United States. A detailed review of specific nutrient and mineral deficiency states is presented, based on the most significant studies published in the last two decades. Of note, screening and supplementation recommendations have been included. Drug absorption data after these procedures is presented and discussed. Studies were identified by searching the Cochrane Registry and MEDLINE using relevant search terms, as well as through review of the reference section of included manuscripts.

CONCLUSIONS

Bariatric surgery can be effectively used to achieve sustainable weight-loss in morbidly obese patients. It simultaneously brings forth important functional consequences on nutrient deficiencies and drug absorption that clinician's must be aware of. Further prospective, randomized research on specific procedures and deficiencies is required.

Involvement of carnitine/organic cation transporter OCTN1/SLC22A4 in gastrointestinal absorption of metformin

Metformin is a widely used oral anti-diabetic, but the molecular mechanism(s) of its gastrointestinal membrane permeation remains unclear. Here, we examined the role of carnitine/organic cation transporter OCTN1/SLC22A4, which is localized on apical membranes of small intestine in mice and humans, in metformin absorption. The maximum plasma concentration (Cmax ) after oral administration of metformin (50 mg/kg) in octn1 gene knockout mice (octn1 (-/-) ) was higher than that in wild-type mice, with only a minimal difference in terminal half-life, but Cmax in octn1(-/-) mice given a higher dose (175 mg/kg) was lower than that in wild-type mice. Systemic elimination of metformin after intravenous administration was similar in the two strains, suggesting the possible involvement of OCTN1 in the gastrointestinal absorption. OCTN1-mediated uptake of metformin was observed in human embryonic kidney 293 cells transfected with mouse OCTN1 gene, but much lower than the uptake of the typical substrate [(3) H]ergothioneine (ERGO). In particular, the distribution volume for OCTN1-mediated uptake increased markedly and then tended to decrease as the metformin concentration was increased. Efflux of metformin preloaded in intestinal epithelial cell line Caco-2 was inhibited by ERGO. Overall, the present findings suggest that OCTN1 transports metformin and may be involved in its oral absorption in small intestine.

Intracellular drug concentrations and transporters: measurement, modeling, and implications for the liver

Intracellular concentrations of drugs and metabolites are often important determinants of efficacy, toxicity, and drug interactions. Hepatic drug distribution can be affected by many factors, including physicochemical properties, uptake/efflux transporters, protein binding, organelle sequestration, and metabolism. This white paper highlights determinants of hepatocyte drug/metabolite concentrations and provides an update on model systems, methods, and modeling/simulation approaches used to quantitatively assess hepatocellular concentrations of molecules. The critical scientific gaps and future research directions in this field are discussed.

Transport of organic cationic drugs: effect of ion-pair formation with bile salts on the biliary excretion and pharmacokinetics

More than 40% of clinically used drugs are organic cations (OCs), which are positively charged at a physiologic pH, and recent reports have established that these drugs are substrates of membrane transporters. The transport of OCs via membrane transporters may play important roles in gastrointestinal absorption, distribution to target sites, and biliary and/or renal elimination of various OC drugs. Almost 40 years ago, a molecular weight (Mw) threshold of 200 was reported to exist in rats for monoquaternary ammonium (mono QA) compounds to be substantially (e.g., >10% of iv dose) excreted to bile. It is well known that some OCs interact with appropriate endogenous organic anions in the body (e.g., bile salts) to form lipophilic ion-pair complexes. The ion-pair formation may influence the affinity or binding of OCs to membrane transporters that are relevant to biliary excretion. In that sense, the association of the ion-pair formation with the existence of the Mw threshold appears to be worthy of examination. It assumes the ion-pair formation of high Mw mono QA compounds (i.e., >200) in the presence of bile salts in the liver, followed by accelerated transport of the ion-pair complexes via relevant bile canalicular transporter(s). In this article, therefore, the transport of OC drugs will be reviewed with a special focus on the ion-pair formation hypothesis. Such information will deepen the understanding of the pharmacokinetics of OC drugs as well as the physiological roles of endogenous bile salts in the detoxification or phase II metabolism of high Mw QA drugs.

Role of organic cation/carnitine transporter 1 in uptake of phenformin and inhibitory effect on complex I respiration in mitochondria

Phenformin causes lactic acidosis in clinical situations due to inhibition of mitochondrial respiratory chain complex I. It is reportedly taken up by hepatocytes and exhibits mitochondrial toxicity in the liver. In this study, uptake of phenformin and [(14)C]tetraethylammonium (TEA) and complex I inhibition by phenformin were examined in isolated liver and heart mitochondria. Uptake of phenformin into isolated rat liver mitochondria was higher than that into heart mitochondria. It was inhibited by several cat ionic compounds, which suggests the involvement of multispecific transport system(s). Similar characteristics were also observed for uptake of TEA; however, uptake of phenformin into mitochondria of organic cation/carnitine transporter 1 (OCTN1) knockout mice was lower than that in wild-type mice, whereas uptake of TEA was comparable between the two strains, suggesting the involvement of distinct transport mechanisms for these two cations in mitochondria. Inhibition by phenformin of oxygen consumption via complex I respiration in isolated rat liver mitochondria was greater than that in heart mitochondria, whereas inhibitory effect of phenformin on complex I respiration was similar in inside-out structured submitochondrial particles prepared from rat livers and hearts. Lactic acidosis provoked by iv infusion of phenformin was weaker in octn1(-/-) mice than that in wild-type mice. These observations suggest that uptake of phenformin into liver mitochondria is at least partly mediated by OCTN1 and functionally relevant to its inhibition potential of complex I respiration. This study was, thus, the first to demonstrate OCTN1-mediated mitochondrial transport and toxicity of biguanide in vivo in rodents.

Membrane transporters as mediators of Cisplatin effects and side effects

Transporters are important mediators of specific cellular uptake and thus, not only for effects, but also for side effects, metabolism, and excretion of many drugs such as cisplatin. Cisplatin is a potent cytostatic drug, whose use is limited by its severe acute and chronic nephro-, oto-, and peripheral neurotoxicity. For this reason, other platinum derivatives, such as carboplatin and oxaliplatin, with less toxicity but still with antitumoral action have been developed. Several transporters, which are expressed on the cell membranes, have been associated with cisplatin transport across the plasma membrane and across the cell: the copper transporter 1 (Ctr1), the copper transporter 2 (Ctr2), the P-type copper-transporting ATPases ATP7A and ATP7B, the organic cation transporter 2 (OCT2), and the multidrug extrusion transporter 1 (MATE1). Some of these transporters are also able to accept other platinum derivatives as substrate. Since membrane transporters display a specific tissue distribution, they can be important molecules that mediate the entry of platinum derivatives in target and also nontarget cells possibly mediating specific effects and side effects of the chemotherapeutic drug. This paper summarizes the literature on toxicities of cisplatin compared to that of carboplatin and oxaliplatin and the interaction of these platinum derivatives with membrane transporters.

Loss of multidrug and toxin extrusion 1 (MATE1) is associated with metformin-induced lactic acidosis

BACKGROUNDS AND PURPOSE

Lactic acidosis is a fatal adverse effect of metformin, but the risk factor remains unclear. Multidrug and toxin extrusion 1 (MATE1) is expressed in the luminal membrane of the kidney and liver. MATE1 was revealed to be responsible for the tubular and biliary secretion of metformin. Therefore, some MATE polymorphisms, that cause it to function abnormally, are hypothesized to induce lactic acidosis. The purpose of this study is to clarify the association between MATE dysfunction and metformin-induced lactic acidosis.

EXPERIMENTAL APPROACH

Blood lactate, pH and bicarbonate ion (HCO(3) (-) ) levels were evaluated during continuous administration of 3 mg·mL(-1) metformin in drinking water using Mate1 knockout (-/-), heterozygous (+/-) and wild-type (+/+) mice. To determine the tissue accumulation of metformin, mice were given 400 mg·kg(-1) metformin orally. Furthermore, blood lactate data were obtained from diabetic patients given metformin.

KEY RESULTS

Seven days after metformin administration in drinking water, significantly higher blood lactate, lower pH and HCO(3) (-) levels were observed in Mate1(-/-) mice, but not in Mate1(+/-) mice. The blood lactate levels were not affected in patients with the heterozygous MATE variant (MATE1-L125F, MATE1-G64D, MATE2-K-G211V). Sixty minutes after metformin administration (400 mg·kg(-1) , p.o.) the hepatic concentration of metformin was markedly higher in Mate1(-/-) mice than in Mate1(+/+) mice.

CONCLUSION AND IMPLICATIONS

MATE1 dysfunction caused a marked elevation in the metformin concentration in the liver and led to lactic acidosis, suggesting that the homozygous MATE1 variant could be one of the risk factors for metformin-induced lactic acidosis.

Metformin overdose causes platelet mitochondrial dysfunction in humans

INTRODUCTION

We have recently demonstrated that metformin intoxication causes mitochondrial dysfunction in several porcine tissues, including platelets. The aim of the present work was to clarify whether it also causes mitochondrial dysfunction (and secondary lactate overproduction) in human platelets, in vitro and ex vivo.

METHODS

Human platelets were incubated for 72 hours with saline or increasing doses of metformin (in vitro experiments). Lactate production, respiratory chain complex activities (spectrophotometry), mitochondrial membrane potential (flow-cytometry after staining with JC-1) and oxygen consumption (Clark-type electrode) were then measured. Platelets were also obtained from ten patients with lactic acidosis (arterial pH 6.97 ± 0.18 and lactate 16 ± 7 mmol/L) due to accidental metformin intoxication (serum drug level 32 ± 14 mg/L) and ten healthy volunteers of similar sex and age. Respiratory chain complex activities were measured as above (ex vivo experiments).

RESULTS

In vitro, metformin dose-dependently increased lactate production (P < 0.001), decreased respiratory chain complex I activity (P = 0.009), mitochondrial membrane potential (P = 0.003) and oxygen consumption (P < 0.001) of human platelets. Ex vivo, platelets taken from intoxicated patients had significantly lower complex I (P = 0.045) and complex IV (P < 0.001) activity compared to controls.

CONCLUSIONS

Depending on dose, metformin can cause mitochondrial dysfunction and lactate overproduction in human platelets in vitro and, possibly, in vivo.

TRIAL REGISTRATION

NCT 00942123.

Mitochondrial stress causes increased succination of proteins in adipocytes in response to glucotoxicity

2SC [S-(2-succino)-cysteine] is a chemical modification formed by a Michael addition reaction of fumarate with cysteine residues in proteins. Formation of 2SC, termed 'succination' of proteins, increases in adipocytes grown in high-glucose medium and in adipose tissues of Type 2 diabetic mice. However, the metabolic mechanisms leading to increased fumarate and succination of protein in the adipocyte are unknown. Treatment of 3T3 cells with high glucose (30 mM compared with 5 mM) caused a significant increase in cellular ATP/ADP, NADH/NAD+ and Δψm (mitochondrial membrane potential). There was also a significant increase in the cellular fumarate concentration and succination of proteins, which may be attributed to the increase in NADH/NAD+ and subsequent inhibition of tricarboxylic acid cycle NAD+-dependent dehydrogenases. Chemical uncouplers, which dissipated Δψm and reduced the NADH/NAD+ ratio, also decreased the fumarate concentration and protein succination. High glucose plus metformin, an inhibitor of complex I in the electron transport chain, caused an increase in fumarate and succination of protein. Thus excess fuel supply (glucotoxicity) appears to create a pseudohypoxic environment (high NADH/NAD+ without hypoxia), which drives the increase in succination of protein. We propose that increased succination of proteins is an early marker of glucotoxicity and mitochondrial stress in adipose tissue in diabetes.

Metformin-associated lactic acidosis in a patient with normal kidney function

The existence of metformin-induced lactic acidosis has been questioned, in particular in the absence of specific risk factors such as impaired renal function. This report describes the presence of lactic acidosis in a patient with normal kidney function and normal doses of metformin. Subsequent positive rechallenge with metformin confirms causality.

Metformin overdose, but not lactic acidosis per se, inhibits oxygen consumption in pigs

INTRODUCTION

Hepatic mitochondrial dysfunction may play a critical role in the pathogenesis of metformin-induced lactic acidosis. However, patients with severe metformin intoxication may have a 30 to 60% decrease in their global oxygen consumption, as for generalized inhibition of mitochondrial respiration. We developed a pig model of severe metformin intoxication to validate this clinical finding and assess mitochondrial function in liver and other tissues.

METHODS

Twenty healthy pigs were sedated and mechanically ventilated. Ten were infused with a large dose of metformin (4 to 8 g) and five were not (sham controls). Five others were infused with lactic acid to clarify whether lactic acidosis per se diminishes global oxygen use. Arterial pH, lactatemia, global oxygen consumption (VO2) (metabolic module) and delivery (DO2) (cardiac output by thermodilution) were monitored for nine hours. Oxygen extraction was computed as VO2/DO2. Activities of the main components of the mitochondrial respiratory chain (complex I, II and III, and IV) were measured with spectrophotometry (and expressed relative to citrate synthase activity) in heart, kidney, liver, skeletal muscle and platelets taken at the end of the study.

RESULTS

Pigs infused with metformin (6 ± 2 g; final serum drug level 77 ± 45 mg/L) progressively developed lactic acidosis (final arterial pH 6.93 ± 0.24 and lactate 18 ± 7 mmol/L, P < 0.001 for both). Their VO2 declined over time (from 115 ± 34 to 71 ± 30 ml/min, P < 0.001) despite grossly preserved DO2 (from 269 ± 68 to 239 ± 51 ml/min, P = 0.58). Oxygen extraction accordingly fell from 43 ± 10 to 30 ± 10% (P = 0.008). None of these changes occurred in either sham controls or pigs infused with lactic acid (final arterial pH 6.86 ± 0.16 and lactate 22 ± 3 mmol/L). Metformin intoxication was associated with inhibition of complex I in the liver (P < 0.001), heart (P < 0.001), kidney (P = 0.003), skeletal muscle (P = 0.012) and platelets (P = 0.053). The activity of complex II and III diminished in the liver (P < 0.001), heart (P < 0.001) and kidney (P < 0.005) while that of complex IV declined in the heart (P < 0.001).

CONCLUSIONS

Metformin intoxication induces lactic acidosis, inhibits global oxygen consumption and causes mitochondrial dysfunction in liver and other tissues. Lactic acidosis per se does not decrease whole-body respiration.

Disposition of metformin: variability due to polymorphisms of organic cation transporters

Considerable interindividual variability in clinical efficacy is recognized in the treatment of type 2 diabetes mellitus with the biguanide metformin. Metformin is a substrate of organic cation transporters, which play important roles in gastrointestinal absorption, renal and biliary elimination, and distribution to target sites of substrate drugs. This raises the question of whether genetic variations in these transporters affect efficacy and risk of adverse events associated with metformin use. In this review, the pharmacogenetics of metformin is discussed in the light of the most recent literature. Overall, results from healthy volunteers support the notion that metformin pharmacokinetics can be affected by polymorphisms in genes encoding organic cation transporters. When considering the glycemic response to metformin in patients, however, the likely multifactorial nature of metformin response masks the effects of transporter polymorphisms observed in some clinical studies.

Drug transporters in drug efficacy and toxicity

Drug transporters are now widely acknowledged as important determinants governing drug absorption, excretion, and, in many cases, extent of drug entry into target organs. There is also a greater appreciation that altered drug transporter function, whether due to genetic polymorphisms, drug-drug interactions, or environmental factors such as dietary constituents, can result in unexpected toxicity. Such effects are in part due to the interplay between various uptake and efflux transporters with overlapping functional capabilities that can manifest as marked interindividual variability in drug disposition in vivo. Here we review transporters of the solute carrier (SLC) and ATP-binding cassette (ABC) superfamilies considered to be of major importance in drug therapy and outline how understanding the expression, function, and genetic variation in such drug transporters will result in better strategies for optimal drug design and tissue targeting as well as reduce the risk for drug-drug interactions and adverse drug responses.

Clinical pharmacokinetics of metformin

Metformin is widely used for the treatment of type 2 diabetes mellitus. It is a biguanide developed from galegine, a guanidine derivative found in Galega officinalis (French lilac). Chemically, it is a hydrophilic base which exists at physiological pH as the cationic species (>99.9%). Consequently, its passive diffusion through cell membranes should be very limited. The mean ± SD fractional oral bioavailability (F) of metformin is 55 ± 16%. It is absorbed predominately from the small intestine. Metformin is excreted unchanged in urine. The elimination half-life (t(&frac12;)) of metformin during multiple dosages in patients with good renal function is approximately 5 hours. From published data on the pharmacokinetics of metformin, the population mean of its clearances were calculated. The population mean renal clearance (CL(R)) and apparent total clearance after oral administration (CL/F) of metformin were estimated to be 510 ± 130 mL/min and 1140 ± 330 mL/min, respectively, in healthy subjects and diabetic patients with good renal function. Over a range of renal function, the population mean values of CL(R) and CL/F of metformin are 4.3 ± 1.5 and 10.7 ± 3.5 times as great, respectively, as the clearance of creatinine (CL(CR)). As the CL(R) and CL/F decrease approximately in proportion to CL(CR), the dosage of metformin should be reduced in patients with renal impairment in proportion to the reduced CL(CR). The oral absorption, hepatic uptake and renal excretion of metformin are mediated very largely by organic cation transporters (OCTs). An intron variant of OCT1 (single nucleotide polymorphism [SNP] rs622342) has been associated with a decreased effect on blood glucose in heterozygotes and a lack of effect of metformin on plasma glucose in homozygotes. An intron variant of multidrug and toxin extrusion transporter [MATE1] (G>A, SNP rs2289669) has also been associated with a small increase in antihyperglycaemic effect of metformin. Overall, the effect of structural variants of OCTs and other cation transporters on the pharmacokinetics of metformin appears small and the subsequent effects on clinical response are also limited. However, intersubject differences in the levels of expression of OCT1 and OCT3 in the liver are very large and may contribute more to the variations in the hepatic uptake and clinical effect of metformin. Lactic acidosis is the feared adverse effect of the biguanide drugs but its incidence is very low in patients treated with metformin. We suggest that the mean plasma concentrations of metformin over a dosage interval be maintained below 2.5 mg/L in order to minimize the development of this adverse effect.

Effects of a MATE protein inhibitor, pyrimethamine, on the renal elimination of metformin at oral microdose and at therapeutic dose in healthy subjects

A microdose study of metformin was conducted to investigate the predictability of drug-drug interactions at the therapeutic dose (ThD). Healthy subjects received a microdose (100 µg) or ThD (250 mg) of metformin orally, with or without a potent and competitive multidrug and toxin extrusion (MATE) inhibitor, pyrimethamine (50 mg, p.o.), in a crossover fashion. Pyrimethamine significantly reduced the renal clearance of metformin by 23 and 35% at the microdose and ThD, respectively. At ThD, but not at microdose, it caused significant increases in the maximum concentration (C(max)) and area under the plasma concentration-time curve (AUC) of metformin (142 and 139% of control values, respectively). Human canalicular membrane vesicles showed pyrimethamine-inhibitable metformin uptake. Pyrimethamine did not affect plasma lactate/pyruvate after ThD of metformin but significantly reduced the renal clearance of creatinine, thereby causing elevation of plasma creatinine level. This microdose study quantitatively predicted a drug-drug interaction involving the renal clearance of metformin at ThD by pyrimethamine. Pyrimethamine is a useful in vivo inhibitor of MATE proteins.

Organic cation transporters (OCTs, MATEs), in vitro and in vivo evidence for the importance in drug therapy

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 and multidrug and toxin extrusion (MATE) transporters of the SLC47 family have been identified as uptake and efflux transporters, respectively, for xenobiotics including several clinically used drugs such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin. Expression of human OCT1 (SLC22A1) and OCT2 (SLC22A2) is highly restricted to the liver and kidney, respectively. By contrast, OCT3 (SLC22A3) is more widely distributed. MATEs (SLC47A1, SLC47A2) are predominantly expressed in human kidney. Data on in vitro studies reporting a large number of substrates and inhibitors of OCTs and MATEs are systematically summarized. Several genetic variants of human OCTs and in part of MATE1 have been reported, and some of them result in reduced in vitro transport activity corroborating data from studies with knockout mice. A comprehensive overview is given on currently known genotype-phenotype correlations for variants in OCTs and MATE1 related to protein expression, pharmacokinetics/-dynamics of transporter substrates, treatment outcome, and disease susceptibility.

Current concepts in drug-induced mitochondrial toxicity

Mitochondria generate most of the cell's ATP and play key roles in fatty acid oxidation, steroid synthesis, heme synthesis, thermogenesis, calcium homeostasis, and apoptosis. With the development of new methods to study mitochondrial function, it is becoming clear that drug-induced mitochondrial dysfunction is one of the causes of drug toxicity. Mitochondria can be impaired by drugs in a variety of ways. These include inhibition of oxidative phosphorylation, uncoupling of electron transport from ATP synthesis, irreversible opening of the mitochondrial permeability transition pore, inhibition of transporters within the mitochondrial inner membrane, increased oxidative stress, inhibition of the citric acid cycle, inhibition of fatty acid oxidation, and impairment of either mtDNA replication or mtDNA-encoded protein synthesis. This unit provides an overview on the physiological roles of mitochondria and the mechanisms by which they can be adversely affected by drugs.

Role of organic cation transporters in drug-induced toxicity

INTRODUCTION

Membrane transporters are important determinants of in vivo drug disposition, therapeutic efficacy and adverse drug reactions. Many commonly used drugs are organic cations and substrates of organic cation transporters (OCTs). These transporters have a large binding site containing partially overlapping interaction domains for different substrates and are specifically distributed around the body. Consequently, drug interactions with these transporters can result in specific toxicity.

AREAS COVERED

This review describes the general properties of OCT and illustrates their importance for the development of important drug toxicities using the examples of metformin and cisplatin. Additionally, this review discusses the role of OCT polymorphisms in the modulation of these toxic effects.

EXPERT OPINION

Understanding how drugs interact with membrane transporters is pivotally important in explaining the mechanisms of specific toxicities and also in designing new drugs or new therapeutic protective protocols by specific competition at the transporter. Defining the pharmacogenomics of these transporters will be essential to personalized medicine, enabling physicians to choose drugs for patients based on efficacy, availability, cost, safety, tolerability and convenience.

Lactic acidosis induced by metformin: incidence, management and prevention

Lactic acidosis associated with metformin treatment is a rare but important adverse event, and unravelling the problem is critical. First, this potential event still influences treatment strategies in type 2 diabetes mellitus, particularly in the many patients at risk of kidney failure, in those presenting contraindications to metformin and in the elderly. Second, the relationship between metformin and lactic acidosis is complex, since use of the drug may be causal, co-responsible or coincidental. The present review is divided into three parts, dealing with the incidence, management and prevention of lactic acidosis occurring during metformin treatment. In terms of incidence, the objective of this article is to counter the conventional view of the link between metformin and lactic acidosis, according to which metformin-associated lactic acidosis is rare but is still associated with a high rate of mortality. In fact, the direct metformin-related mortality is close to zero and metformin may even be protective in cases of very severe lactic acidosis unrelated to the drug. Metformin has also inherited a negative class effect, since the early biguanide, phenformin, was associated with more frequent and sometimes fatal lactic acidosis. In the second part of this review, the objective is to identify the most efficient patient management methods based on our knowledge of how metformin acts on glucose/lactate metabolism and how lactic acidosis may occur (at the organ and cellular levels) during metformin treatment. The liver appears to be a key organ for both the antidiabetic effect of metformin and the development of lactic acidosis; the latter is attributed to mitochondrial impairment and subsequent adenosine triphosphate depletion, acceleration of the glycolytic flux, increased glucose uptake and the generation of lactate, which effluxes into the circulation rather than being oxidized further. Haemodialysis should systematically be performed in severe forms of lactic acidosis, since it provides both symptomatic and aetiological treatment (by eliminating lactate and metformin). In the third part of the review (prevention), the objective is to examine the list of contraindications to metformin (primarily related to renal and cardiovascular function). Diabetes is above all a vascular disease and metformin is a vascular drug with antidiabetic properties. Given the importance of the liver in lactate clearance, we suggest focusing on the severity of and prognosis for liver disease; renal dysfunction is only a prerequisite for metformin accumulation, which may only be dangerous per se when associated with liver failure. Lastly, in view of metformin's impressive overall effectiveness profile, it would be paradoxical to deny the majority of patients with long-established diabetes access to metformin because of the high prevalence of contraindications. The implications of these contraindications are discussed.

Metformin action on AMP-activated protein kinase: a translational research approach to understanding a potential new therapeutic target

Clinical studies in Type 2 diabetes mellitus have shown that the effects of metformin go beyond improving HbA(1c) and include reductions in cardiovascular endpoints. Metformin therapy has been widely used in the treatment of Type 2 diabetes for many years, yet the precise mode of action remains uncertain. It has recently been proposed that metformin-mediated stimulation of hepatic AMP-activated protein kinase (AMPK) underlies the hypoglycaemic effects of metformin. AMPK is a heterotrimeric enzyme that is expressed in many tissues and plays a central role in the regulation of energy homoeostasis. Furthermore, there is increasing evidence that AMPK is implicated in the pathophysiology of cardiovascular and metabolic diseases. The generation of more specific and potent activators of AMPK, however, could have additional metabolic and vascular benefits for patients with Type 2 diabetes.

Increased plasma d-lactic acid associated with impaired memory in rats

AIM

d-Lactic acidosis is associated with memory impairment in humans. Recent research indicates that d-lactic acid may inhibit the supply of energy from astrocytes to neurons involved with memory formation. However, little is known about the effects of increased hind-gut fermentation due to changes in diet on circulating lactic acid concentrations and memory.

METHOD

Thirty-six male Wistar rats were fed three dietary treatments: a commercial rat and mouse chow, a soluble carbohydrate based diet or a fermentable carbohydrate based diet. The parameters estimating memory were examined by employing the object recognition test. Physical parameters of fermentation including hind-gut and plasma lactic acid concentrations were examined after sacrifice, either 3 or 21h after feeding.

RESULTS

Increased fermentation in the hind-gut of rats, indicated by lower caecum pH, was associated with increased plasma l-lactic acid (r=-0.41, p=0.020) and d-lactic acid (r=-0.33, p=0.087). Memory, being able to discriminate between a familiar and a novel object during the object recognition test, was reduced with increasing plasma d-lactic acid (r=-0.51, p=0.021).

CONCLUSIONS

Memory impairment was associated with alterations in plasma d-lactic acid following the fermentation of carbohydrate in the hind-gut. Further work is still required to determine whether these effects are mediated centrally or via direct connections through the enteric nervous system.

Sitagliptin attenuates metformin-mediated AMPK phosphorylation through inhibition of organic cation transporters

To assess potential interactions between sitagliptin and metformin, we sought to characterize the in vitro inhibitory potency of sitagliptin on the uptake of MPP(+) and metformin, representative substrates for OCTs, and to evaluate the pharmacological pathways that may be affected by the combination of metformin and sitagliptin. Among the OATs and OCTs screened, OAT3-mediated salicylate uptake and OCT1- and OCT2-mediated MPP(+) uptake were inhibited by sitagliptin. The K(i) values of sitagliptin for OCT1- and OCT2-mediated metformin uptake were 34.9 and 40.8 μM, respectively. As OCT1 is the gate protein for metformin action in the liver, we investigated whether sitagliptin-mediated OCT1 inhibition affected metformin-induced activation of AMPK signalling. Treatment with sitagliptin in MDCK-OCT1 and HepG2 cells resulted in a reduced level of phosphorylated AMPK, with K(i) values of 38.8 and 43.3 μM, respectively. These results suggest that the inhibitory potential of sitagliptin on OCT1 may attenuate the first step of metformin action, that is, the phosphorylation of AMPK. Nevertheless, the likelihood of a drug-drug interaction between sitagliptin and metformin is believed to be remote in usual clinical setting.

Glutamine addiction: a new therapeutic target in cancer

Most cancers depend on a high rate of aerobic glycolysis for their continued growth and survival. Paradoxically, some cancer cell lines also display addiction to glutamine despite the fact that glutamine is a nonessential amino acid that can be synthesized from glucose. The high rate of glutamine uptake exhibited by glutamine-dependent cells does not appear to result solely from its role as a nitrogen donor in nucleotide and amino acid biosynthesis. Instead, glutamine plays a required role in the uptake of essential amino acids and in maintaining activation of TOR (target of rapamycin) kinase. Moreover, in many cancer cells, glutamine is the primary mitochondrial substrate and is required for maintenance of mitochondrial membrane potential and integrity and for support of the NADPH production needed for redox control and macromolecular synthesis.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Oxygen consumption is depressed in patients with lactic acidosis due to biguanide intoxication

INTRODUCTION

Lactic acidosis can develop during biguanide (metformin and phenformin) intoxication, possibly as a consequence of mitochondrial dysfunction. To verify this hypothesis, we investigated whether body oxygen consumption (VO2), that primarily depends on mitochondrial respiration, is depressed in patients with biguanide intoxication.

METHODS

Multicentre retrospective analysis of data collected from 24 patients with lactic acidosis (pH 6.93 +/- 0.20; lactate 18 +/- 6 mM at hospital admission) due to metformin (n = 23) or phenformin (n = 1) intoxication. In 11 patients, VO2 was computed as the product of simultaneously recorded arterio-venous difference in O2 content [C(a-v)O2] and cardiac index (CI). In 13 additional cases, C(a-v)O2, but not CI, was available.

RESULTS

On day 1, VO2 was markedly depressed (67 +/- 28 ml/min/m2) despite a normal CI (3.4 +/- 1.2 L/min/m2). C(a-v)O2 was abnormally low in both patients either with (2.0 +/- 1.0 ml O2/100 ml) or without (2.5 +/- 1.1 ml O2/100 ml) CI (and VO2) monitoring. Clearance of the accumulated drug was associated with the resolution of lactic acidosis and a parallel increase in VO2 (P < 0.001) and C(a-v)O2 (P < 0.05). Plasma lactate and VO2 were inversely correlated (R2 0.43; P < 0.001, n = 32).

CONCLUSIONS

VO2 is abnormally low in patients with lactic acidosis due to biguanide intoxication. This finding is in line with the hypothesis of inhibited mitochondrial respiration and consequent hyperlactatemia.

SLC22A2 gene 808 G/T variant is related to plasma lactate concentration in Chinese type 2 diabetics treated with metformin

AIM

To investigate the potential relationship between the SLC22A2 gene polymorphism and blood lactate concentration in Shanghai Hans suffering from type 2 diabetes mellitus (T2DM).

METHODS

The SLC22A2 single nucleotide polymorphism (SNP) 808G/T was genotyped in 400 T2DM patients, including a metformin-treated group (n=200) and a non-metformin-treated group (n=200). Fasting plasma lactic acid levels were measured with an enzyme-electrode assay. Biochemical indexes, including plasma alanine aminotransferase (ALT), creatinine (Cr), and glycolated hemoglobin (HbA1c), were also measured.

RESULTS

The fasting plasma lactate concentration in the metformin-treated group was significantly higher than that in the non-metformin-treated group (1.29+/-0.45 mmol/L vs 1.18+/-0.44 mmol/L, P=0.015). Additionally, the ratio of patients with hyperlactacidemia was 8% (16/200) for the metformin-treated group and 5.5% (11/200) for the non-metformin-treated group, with no lactic acidosis found in either group. The frequency of the SLC22A2 808G/T T allele was 12.9%. Patients with the mutant genotype (TT) had a higher blood lactate concentration in the metformin-treated group than those in the non-metformin-treated group (t=2.492, P=0.013). This trend was not observed in the GG and GT genotypes when compared with metformin-treated and non-metformin-treated groups. Patients with the mutant genotype (TT) in the metformin-treated group also had a higher incidence of hyperlactacidemia compared with the GG genotype (40.0% vs 6.9%, P=0.050) in the metformin-treated group and the GG (6.0%, P=0.042) or GT (4.3%, P=0.043) genotypes in the non-metformin-treated group. In the metformin-treated group, there were significant gender differences in lactate concentrations in the TT (2.18+/-0.15 vs 1.04+/-0.27 mmol/L, P=0.008) and GG genotypes (1.40+/-0.51 vs 1.19+/-0.35 mmol/L, P=0.004). The lactate levels of women with the TT genotype were the highest in the metformin-treated group, but differences in lactate levels among the genotypes were not observed in the non-metformin-treated group.

CONCLUSION

There is an 808G/T polymorphism in the SLC22A2 gene in Chinese Hans with T2DM. The 808G>T variance in the SLC22A2 gene can affect the plasma lactate level and the incidence of hyperlactacidemia in T2DM patients undergoing metformin therapy. Additionally, the female patients carrying the TT genotype are prone to lactatemia.

Can acute overdose of metformin lead to lactic acidosis?

INTRODUCTION

Metformin-associated lactic acidosis (MALA) is well described in patients taking therapeutic metformin who develop renal failure or other serious comorbid conditions. Metformin-associated lactic acidosis from acute overdose has also been described in case series but is debated by some clinicians, arguing that metformin overdose does not cause lactic acidosis. Our aim was to perform a multicenter poison control database review to determine if MALA can occur in mono-overdose patients with no comorbid conditions.

METHODS

This was a retrospective chart review of the Illinois and Washington Poison Centers between the 2001-2006 and 1999-2006 periods, respectively. Metformin overdoses that were referred to health care facilities were categorized into mono-overdose with or with out MALA and polypharmacy overdose with or without MALA.

RESULTS

The overall prevalence of MALA was 14 (3.5%) of 398 cases referred to a health care facility. Metformin-associated lactic acidosis occurred in 9.1% of mono-overdose and in 0.7% of polypharmacy overdose patients referred to health care facilities and was 16% for intentional mono-overdoses. There was one death of 132 mono-overdoses referred to health care facilities.

CONCLUSIONS

Apparent metformin mono-overdose is associated with MALA. Dosages that place patients at risk for MALA will require additional study.

A comparison of uptake of metformin and phenformin mediated by hOCT1 in human hepatocytes

Metformin, a biguanide that has been used to treat type 2 diabetes mellitus, is reportedly transported into human hepatocytes by human organic cation transporter 1 (hOCT1). The objective of this study was to investigate differences in the hepatic uptake of metformin and phenformin, a biguanide derivative similar to metformin. Special focus was on the role of active transport into cells. Experiments were therefore performed using human cryopreserved hepatocytes and hOCT1 expressing oocytes. Both biguanides proved to be good substrates for hOCT1. However, phenformin exhibited a much higher affinity and transport activity, with a marked difference in uptake kinetics compared with metformin. Both biguanides were transported actively by hOCT1, with the active transport components much greater than passive transport components in both cases, suggesting that functional changes in hOCT1 might affect the transport of both compounds to the same degree. This study for the first time produced detailed comparative findings for uptake profiles of metformin and phenformin in human hepatocytes and hOCT1 expressing oocytes. It is considered that hOCT1 may not be the only key factor that determines the frequency of metformin and phenformin toxicity, considering the major contribution of this transporter to the total hepatic uptake and comparable width of their therapeutic concentrations.

Potent and specific inhibition of mMate1-mediated efflux of type I organic cations in the liver and kidney by pyrimethamine

This report describes a potent and selective inhibitor of multidrug and toxin extrusion (MATE) protein, pyrimethamine (PYR), and examines its effect on the urinary and biliary excretion of typical Mate1 substrates in mice. In vitro inhibition studies demonstrated that PYR is a potent inhibitor of mouse (m)Mate1 (K(i) = 145 nM) among renal organic cation transporters mOctn1 and mOctn2 (K(i) > 30 microM), mOct1 (K(i) = 3.6 microM), and mOct2 (K(i) = 6.0 microM). PYR inhibited the uptake of metformin by kidney brush-border membrane vesicles (BBMVs) (K(i) = 41 nM) and canalicular membrane vesicles in the presence of outward gradient of H+. PYR treatment significantly increased the kidney-to-plasma ratio of tetraethylammonium, and both the liver- and kidney-to-plasma ratios of metformin in mice, whereas it did not affect their plasma concentrations and urinary excretion rates. Furthermore, the plasma lactate concentration, a biomarker for inhibition of gluconeogenesis by metformin, was significantly higher in the PYR-treated group than in the control group. These results not only suggest the importance of mMate1 in the efflux of organic cations into the urine and bile in mice but also the importance of canalicular efflux mediated by MATE proteins for the therapeutic efficacy of metformin. PYR is a potent inhibitor of human (h)MATE1 and hMATE2-K (K(i) = 77 and 46 nM, respectively) and H+ and organic cation exchanger in human kidney BBMVs (K(i) = 31 nM) in the presence of outward gradient of H+. Taken together, PYR can be used as a potent probe inhibitor of human MATE transporters.

Interaction of beta-blockers with the renal uptake transporter OCT2

AIM

The uptake of drugs from the blood into the renal tubular cells is a key determinant for renal secretion and may influence their systemic plasma concentrations and extrarenal effects. Metformin, used for treatment of type 2 diabetes, is taken up into renal tubular cells by the organic cation transporter 2 (OCT2). Because many patients with type 2 diabetes receiving metformin are concomitantly treated with beta-blockers, we tested whether beta-blockers can inhibit OCT2-mediated drug transport.

METHOD

Using Madin-Darby canine kidney II cells stably expressing the uptake transporter OCT2, we analysed whether the beta-blockers bisoprolol, carvedilol, metoprolol and propranolol inhibit the transport of OCT2 substrates 1-methyl-4-phenylpyridinium (MPP(+)) and metformin.

RESULTS

Neither bisoprolol nor metoprolol significantly inhibited the uptake of MPP(+), whereas a significant inhibition was observed for carvedilol und propranolol (half maximal inhibitory concentration IC(50): 26.3 and 67.5 microM) respectively. Moreover, all beta-blockers significantly inhibited OCT2-mediated metformin uptake (IC(50) for bisoprolol: 2.4 microM, IC(50) for carvedilol: 2.3 microM, IC(50) for metoprolol: 50.2 microM and IC(50) for propranolol: 8.3 microM).

CONCLUSION

These in vitro results demonstrate that alterations of uptake transporter function by beta-blockers have to be considered as potential mechanisms underlying drug-drug interactions in the kidney.

In vitro-in vivo extrapolation of transporter-mediated clearance in the liver and kidney

Transporters govern drug movement into and out of tissues, thereby playing an important role in drug disposition in plasma and to the site of action. The molecular cloning of such transporters has clarified the importance of members of the solute carrier family, such as OATP/SLCO, OCT/SLC22, OAT/SLC22, and MATE/SLC47, and the ATP-binding cassette transporters, such as P-glycoprotein/ABCB1, MRPs/ABCC, and BCRP/ABCG2. Elucidation of molecular characteristics of transporters has allowed the identification of transporters as mechanisms for drug-drug interactions, and of interindividual differences in drug dispositions and responses. Cumulative studies have highlighted the cooperative roles of uptake transporters and metabolic enzymes/efflux transporters. In this way, the concept of a rate-limiting process in hepatic/renal elimination across epithelial cells has developed. This review illustrates the concept of the rate-limiting step in the hepatic elimination mediated by transporters, and describes the prediction of the in vivo pharmacokinetics of drugs whose disposition is determined by transporters, based on in vitro experiments using pravastatin as an example. This review also illustrates the transporters regulating the peripheral drug concentrations.