Prediction of interindividual variation in drug plasma levels in vivo from individual enzyme kinetic data and physiologically based pharmacokinetic modeling

CYP1A2 polymorphism may contribute to agomelatine-induced acute liver injury: Case report and review of the literature

RATIONALE

Liver function monitoring is recommended when agomelatine is prescribed, although liver enzymes are not considered predictive biomarkers. Most patients present with acute liver injury, with only a few presenting with levels of liver enzymes that are over 30 times the upper limit of normal. The patient-specific risk factors that are associated with liver injury remain unclear. Thus, this report provides new insights into the mechanism of agomelatine-induced acute hepatocellular injury based on cytochrome P450 family 1 subfamily A member 2 (CYP1A2) polymorphism.

PATIENT CONCERNS

We present a case of acute hepatocellular injury in a 75-year-old man who was taking agomelatine at a dose of 50 mg/qn. All hepatitis virus test results were negative. No history of liver disease was observed. As CYP1A2 is the main metabolic enzyme of agomelatine, CYP1A2 AA (rs762551) genetic polymorphism was analyzed.

DIAGNOSIS

The patient's transaminases level exceeded the critical value on day 72 after starting oral agomelatine.

INTERVENTIONS

The patient received intravenous magnesium isoglycyrrhizinate, a liver cell-protecting agent, followed by the withdrawal of agomelatine.

OUTCOMES

There was an improvement in the levels of the liver enzymes and no subsequent organ dysfunction was observed.

LESSONS

Here, we report a case of acute hepatocellular injury characterized by a very high aspartate aminotransferase level. Periodic liver function testing throughout the treatment period can help in the rapid and appropriate diagnosis of acute liver injury, particularly in the absence of typical clinical manifestations. Agomelatine hepatic toxicity might be related to an idiosyncratic metabolic reaction that depends on individual patient differences. As it is the main metabolic enzyme of agomelatine, CYP1A2 genetic polymorphism may contribute to liver injury by affecting its metabolites.

Use of Physiologically Based Kinetic Modeling-Based Reverse Dosimetry to Predict in Vivo Toxicity from in Vitro Data

The development of reliable nonanimal based testing strategies, such as in vitro bioassays, is the holy grail in current human safety testing of chemicals. However, the use of in vitro toxicity data in risk assessment is not straightforward. One of the main issues is that concentration-response curves from in vitro models need to be converted to in vivo dose-response curves. These dose-response curves are needed in toxicological risk assessment to obtain a point of departure to determine safe exposure levels for humans. Recent scientific developments enable this translation of in vitro concentration-response curves to in vivo dose-response curves using physiologically based kinetic (PBK) modeling-based reverse dosimetry. The present review provides an overview of the examples available in the literature on the prediction of in vivo toxicity using PBK modeling-based reverse dosimetry of in vitro toxicity data, showing that proofs-of-principle are available for toxicity end points ranging from developmental toxicity, nephrotoxicity, hepatotoxicity, and neurotoxicity to DNA adduct formation. This review also discusses the promises and pitfalls, and the future perspectives of the approach. Since proofs-of-principle available so far have been provided for the prediction of toxicity in experimental animals, future research should focus on the use of in vitro toxicity data obtained in human models to predict the human situation using human PBK models. This would facilitate human- instead of experimental animal-based approaches in risk assessment.

Characterizing novel metabolic pathways of melatonin receptor agonist agomelatine using metabolomic approaches

Agomelatine (AGM), an analog of melatonin, is a potential agonist at melatonin receptors 1/2 and a selective antagonist at 5-hydroxytryptamine 2C receptors. AGM is widely used for the treatment of major depressive episodes in adults. However, multiple adverse effects associated with AGM have been reported in clinical practice. It is little known about AGM metabolism in vitro and in vivo, although metabolism plays a pivotal role in its efficacy and safety. To elucidate metabolic pathways of AGM, we systemically investigated AGM metabolism and its bioactivation in human liver microsomes (HLM) and mice using metabolomic approaches. We identified thirty-eight AGM metabolites and adducts, among which thirty-two are novel. In HLM, we uncovered five GSH-trapped adducts and two semicarbazide-trapped aldehydes. Moreover, we characterized three N-acetyl cysteine conjugated-AGM adducts in mouse urine and feces, which were formed from the degradation of AGM_GSH adducts. Using recombinant CYP450 isoenzymes and chemical inhibitors, we demonstrated that CYP1A2 and CYP3A4 are primary enzymes contributing to the formation of AGM_GSH adducts and AGM_hydrazones. This study provided a global view of AGM metabolism and identified the novel pathways of AGM bioactivation, which could be utilized for further understanding the mechanism of adverse effects related to AGM and possible drug-drug interactions.

Melatonergic drugs in development

Melatonin (N-acetyl-5-methoxytryptamine) is widely known as "the darkness hormone". It is a major chronobiological regulator involved in circadian phasing and sleep-wake cycle in humans. Numerous other functions, including cyto/neuroprotection, immune modulation, and energy metabolism have been ascribed to melatonin. A variety of studies have revealed a role for melatonin and its receptors in different pathophysiological conditions. However, the suitability of melatonin as a drug is limited because of its short half-life, poor oral bioavailability, and ubiquitous action. Due to the therapeutic potential of melatonin in a wide variety of clinical conditions, the development of new agents able to interact selectively with melatonin receptors has become an area of great interest during the last decade. Therefore, the field of melatonergic receptor agonists comprises a great number of structurally different chemical entities, which range from indolic to nonindolic compounds. Melatonergic agonists are suitable for sleep disturbances, neuropsychiatric disorders related to circadian dysphasing, and metabolic diseases associated with insulin resistance. The results of preclinical studies on animal models show that melatonin receptor agonists can be considered promising agents for the treatment of central nervous system-related pathologies. An overview of recent advances in the field of investigational melatonergic drugs will be presented in this review.

Synthetic melatoninergic ligands: achievements and prospects

Pineal hormone melatonin is widely used in the treatment of disorders of circadian rhythms. The presence of melatonin receptors in various animal tissues motivates the use of this hormone in some other diseases. For this reason, in recent years investigators continued the search for synthetic analogues of melatonin which are metabolically stable and selective to receptors. This review includes recent information about the most famous melatonin analogues, their structure, properties, and physiological features of the interaction with melatonin receptors.

Could agomelatine be the ideal antidepressant?

Depressive disorders are a common cause of chronic and recurrent psychiatric dysfunction, constituting the fourth leading cause of global diseases. Depression is associated with a high rate of morbidity and mortality, and is a leading cause of global disability. Despite the effectiveness of most currently available antidepressants, many of them have a number of undesirable side effects. Agomelatine is the first melatonin (MT)(1)/MT(2) agonist having 5-hydroxytryptamine (5-HT)(2C) and 5-HT(2B) antagonist properties and antidepressant activity. Agomelatine is effective in several animal models of depression and anxiety. In addition, three large, multicenter, multinational, placebo-controlled studies and several double-blind, placebo-controlled trials of agomelatine have demonstrated that it is a clinically effective and well-tolerated antidepressant in acute trials. Since currently available antidepressants are not always adequate to cause complete remission of symptoms in severely depressed patients, the superior rate of response achieved with agomelatine in this group of patients underlines its future for clinical use in depressive disorders. In summary, the clinical advantage of agomelatine is attributed to its novel mechanism of action, which helps not only to exert antidepressant action, but also to regulate the sleep-wake rhythm.

Synthesis, chiral resolution, absolute configuration assignment and pharmacological evaluation of a series of melatoninergic ligands

(−)-R-Enantiomers of agomelatine analogues were more potent at serotonin 5-HT_2C receptors than (+)-S-enantiomers, and showed antidepressant-like properties in a tail suspension test.

Characterization of the human kinetic adjustment factor for the health risk assessment of environmental contaminants

A default uncertainty factor of 3.16 (√10) is applied to account for interindividual variability in toxicokinetics when performing non-cancer risk assessments. Using relevant human data for specific chemicals, as WHO/IPCS suggests, it is possible to evaluate, and replace when appropriate, this default factor by quantifying chemical-specific adjustment factors for interindividual variability in toxicokinetics (also referred to as the human kinetic adjustment factor, HKAF). The HKAF has been determined based on the distributions of pharmacokinetic parameters (e.g., half-life, area under the curve, maximum blood concentration) in relevant populations. This article focuses on the current state of knowledge of the use of physiologically based algorithms and models in characterizing the HKAF for environmental contaminants. The recent modeling efforts on the computation of HKAF as a function of the characteristics of the population, chemical and its mode of action (dose metrics), as well as exposure scenario of relevance to the assessment are reviewed here. The results of these studies, taken together, suggest the HKAF varies as a function of the sensitive subpopulation and dose metrics of interest, exposure conditions considered (route, duration, and intensity), metabolic pathways involved and theoretical model underlying its computation. The HKAF seldom exceeded the default value of 3.16, except in very young children (i.e., <≈ 3 months) and when the parent compound is the toxic moiety. Overall, from a public health perspective, the current state of knowledge generally suggest that the default uncertainty factor is sufficient to account for human variability in non-cancer risk assessments of environmental contaminants.

Agomelatine: Its Role in the Management of Major Depressive Disorder

Circadian rhythm abnormalities, as shown by sleep/wake cycle disturbances, constitute one the most prevalent signs of depressive illness; advances or delays in the circadian phase are documented in patients with major depressive disorder (MDD), bipolar disorder, and seasonal affective disorder (SAD). The disturbances in the amplitude and phase of rhythm in melatonin secretion that occur in patients with depression resemble those seen in chronobiological disorders, thus suggesting a link between disturbed melatonin secretion and depressed mood. Based on this, agomelatine, the first MT1/MT2 melatonergic agonist displaying also 5-HT2C serotonergic antagonism, has been introduced as an antidepressant. Agomelatine has been shown to be effective in several animal models of depression and anxiety and it has beneficial effects in patients with MDD, bipolar disorder, or SAD. Among agomelatine's characteristics are a rapid onset of action and a pronounced effectiveness for correcting circadian rhythm abnormalities and improving the sleep/wake cycle. Agomelatine also improves the 3 functional dimensions of depression—emotional, cognitive, and social—thus aiding in the full recovery of patients to a normal life.

Therapeutic potential of melatonin agonists

Melatonin, a hormone secreted by the pineal gland, has been successfully employed to improve sleep in both normal patients and insomniacs, and for the treatment of circadian rhythm sleep disorders. Melatonergic MT₁ and MT₂ receptors exist in high concentrations in the suprachiasmatic nucleus of the hypothalamus and have been shown to be instrumental for the sleep-promoting and circadian rhythm-regulating effects of melatonin. A lack of consistency among reports on the therapeutic efficacy of melatonin has been attributed to differences in melatonin's bioavailability and the short half-life of the hormone. In view of the need for longer acting melatonergic agonists that improve sleep efficiency without causing drug abuse or dependency, ramelteon (Rozerem™, Takeda) was developed. Ramelteon, which acts via MT₁/MT₂ melatonergic agonism, has been found clinically effective for improving total sleep time and sleep efficiency in insomniacs. Agomelatine (Valdoxan™, Servier) is another MT₁/MT₂ melatonergic agonist that also displays antagonist activity at 5-HT_2C serotonin receptors. Agomelatine has been found effective in treating depression and sleep disorders in patients with major depressive disorder. A slow-release preparation of melatonin (Circadin™, Neurim) has been shown to be effective in treating sleep disorders in the elderly population.

Agomelatine: an antidepressant with a novel mechanism of action

Despite advances in understanding potential disease mechanisms in depression and in therapeutic approaches to the management of major depressive disorder, the disease continues to carry an enormous personal, social and economic burden. Agomelatine is a melatonergic antidepressant with melatonin (MT1 and MT2) agonistic and serotonin (5-HT2C) antagonistic properties. It represents a promising approach to treating depression through a unique synergistic mechanism. There is also preliminary evidence to suggest that agomelatine has anxiolytic properties in depressed patients. Clinical data confirm that agomelatine, in a dose range of 25–50 mg daily, is an effective antidepressant with a very favorable side-effect profile. In particular, sleep restorative effects in the absence of daytime sedation and a favorable effect on sexual function suggest that agomelatine will offer a high efficacy–tolerability index for patients with major depressive disorder. Such results have been anticipated based on preclinical studies using various animal models of both depression and anxiety.

Drug treatment of depression in the 2000s: An overview of achievements in the last 10 years and future possibilities

During the past 10 years our knowledge about the pharmacotherapy of depression has been consolidated, and a variety of very interesting new compounds launched onto the market. The pipeline of the pharmaceutical industry is still filled with an assortment of new developments and very promising new approaches towards the pharmacotherapy of depressive disorders. Future pharmacological treatments of depression will not only enhance serotonergic and noradrenergic neurotransmission: other systems, such as the melatonergic receptor system and the hypothalamus-pituitary-adrenal axis, are also the targets of newly developed and upcoming substances with putative antidepressant effects. The main advantages of the currently available newer pharmacotherapeutic options are the broadening of the spectrum of possible antidepressant treatments, which is of particular importance for the growing number of patients suffering from difficult-to-treat depression, and a far better tolerability profile in comparison to older compounds such as tricyclic antidepressants. Unresolved issues are the unacceptably high rate of non-responsiveness during antidepressant treatment, a latency of sometimes several weeks until clinical improvement and remission can be achieved, and a variety of possible side effects also present during treatment with modern compounds. This review mainly presents the development of antidepressant pharmacotherapies during the past 10 years, together with pharmacokinetic and pharmacodynamic information and a comparison of different pharmacological treatment principles evaluated in randomized controlled clinical trials. In addition, new pharmacological strategies that are not yet available on the market and strategies currently under development are reviewed in detail. The study of new treatment options is of major importance to provide better strategies for the clinical management of depression in the future, and is thus also of great socio-economic importance.

Simulation and prediction of in vivo drug metabolism in human populations from in vitro data

The perceived failure of new drug development has been blamed on deficiencies in in vivo studies of drug efficacy and safety. Prior simulation of the potential exposure of different individuals to a given dose might help to improve the design of such studies. This should also help researchers to focus on the characteristics of individuals who present with extreme reactions to therapy. An effort to build virtual populations using extensive demographic, physiological, genomic and in vitro biochemical data to simulate and predict drug disposition from routinely collected in vitro data is outlined.

Whole body pharmacokinetic models

The aim of the current review is to summarise the present status of physiologically based pharmacokinetic (PBPK) modelling and its applications in drug research, and thus serve as a reference point to people interested in the methodology. The review is structured into three major sections. The first discusses the existing methodologies and techniques of PBPK model development. The second describes some of the most interesting PBPK model implementations published. The final section is devoted to a discussion of the current limitations and the possible future developments of the PBPK modelling approach. The current review is focused on papers dealing with the pharmacokinetics and/or toxicokinetics of medicinal compounds; references discussing PBPK models of environmental compounds are mentioned only if they represent considerable methodological developments or reveal interesting interpretations and/or applications.The major conclusion of the review is that, despite its significant potential, PBPK modelling has not seen the development and implementation it deserves, especially in the drug discovery, research and development processes. The main reason for this is that the successful development and implementation of a PBPK model is seen to require the investment of significant experience, effort, time and resources. Yet, a substantial body of PBPK-related research has been accumulated that can facilitate the PBPK modelling and implementation process. What is probably lagging behind is the expertise component, where the demand for appropriately qualified staff far outreaches availability.

The use of in vitro metabolic parameters and physiologically based pharmacokinetic (PBPK) modeling to explore the risk assessment of trichloroethylene

A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,1,2-trichloroethene, TRI) for rat and humans, based on in vitro metabolic parameters. These were obtained using individual cytochrome P450 and glutathione S-transferase enzymes. The main enzymes involved both for rats and humans are CYP2E1 and the μ- and π-class glutathione S-transferases. Validation experiments were performed in order to test the predictive value of the enzyme kinetic parameters to describe 'whole-body' disposition. Male Wistar rats were dosed orally or intravenously with different doses of trichloroethylene. Obtained exhaled radioactivity, excreted radioactivity in urine, and obtained blood concentration-time curves of trichloroethylene for all dosing groups were compared to predictions from the PBPK model. Subsequently, using the scaling factor derived from the rat experiments predictions were made for the extreme cases to be expected in humans, based on interindividual variations of the key enzymes involved. On comparing these predictions with literature data a very close match was found. This illustrates the potential application of in vitro metabolic parameters in risk assessment, through the use of PBPK modeling as a tool to understand and predict in vivo data. From a hypothetical 8 h exposure scenario to 35 ppm trichloroethylene in rats and humans, and assuming that the glutathione S-transferase pathway is responsible for the toxicity of trichloroethylene, it was concluded that humans are less sensitive for trichloroethylene toxicity than rats.

Prediction of isoprene diepoxide levels in vivo in mouse, rat and man using enzyme kinetic data in vitro and physiologically-based pharmacokinetic modelling

The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters K(m) and V(max) were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice.

Predicting the impact of physiological and biochemical processes on oral drug bioavailability

Recent advances in computational methods applied to the fields of drug delivery and biopharmaceutics will be reviewed with a focus on prediction of the impact of physiological and biochemical factors on simulation of gastrointestinal absorption and bioavailability. Our application of a gastrointestinal simulation for the prediction of oral drug absorption and bioavailability will be described. First, we collected literature data or we estimated biopharmaceutical properties by application of statistical methods to a set of 2D and 3D molecular descriptors. Second, we integrated the differential equations for an advanced compartmental absorption and transit (ACAT) model in order to determine the rate, extent, and approximate gastrointestinal location of drug liberation (for controlled release), dissolution, passive and carrier-mediated absorption, and saturable metabolism and efflux. We predict fraction absorbed, bioavailability, and C(p) vs. time profiles for common drugs and compare those estimates to literature data. We illustrate the simulated impact of physiological and biochemical processes on oral drug bioavailability.