Receptor binding of N-(methyl-11C) clozapine in the brain of rhesus monkey studied by positron emission tomography (PET)

18F-labeled radiotracers for in vivo imaging of DREADD with positron emission tomography

Designer Receptors Exclusively Activated by Designer Drugs (DREADD) are a preclinical chemogenetic approach with clinical potential for various disorders. In vivo visualization of DREADDs has been achieved with positron emission tomography (PET) using ¹¹C radiotracers. The objective of this study was to develop DREADD radiotracers labeled with ¹⁸F for a longer isotope half-life. A series of non-radioactive fluorinated analogs of clozapine with a wide range of in vitro binding affinities for the hM3Dq and hM4Di DREADD receptors has been synthesized for PET. Compound [¹⁸F]7b was radiolabeled via a modified ¹⁸F-deoxyfluorination protocol with a commercial ruthenium reagent. [¹⁸F]7b demonstrated encouraging PET imaging properties in a DREADD hM3Dq transgenic mouse model, whereas the radiotracer uptake in the wild type mouse brain was low. [¹⁸F]7b is a promising long-lived alternative to the DREADD radiotracers [¹¹C]clozapine ([¹¹C]CLZ) and [¹¹C]deschloroclozapine ([¹¹C]DCZ).

PET imaging-guided chemogenetic silencing reveals a critical role of primate rostromedial caudate in reward evaluation

The rostromedial caudate (rmCD) of primates is thought to contribute to reward value processing, but a causal relationship has not been established. Here we use an inhibitory DREADD (Designer Receptor Exclusively Activated by Designer Drug) to repeatedly and non-invasively inactivate rmCD of macaque monkeys. We inject an adeno-associated viral vector expressing the inhibitory DREADD, hM4Di, into the rmCD bilaterally. To visualize DREADD expression in vivo, we develop a non-invasive imaging method using positron emission tomography (PET). PET imaging provides information critical for successful chemogenetic silencing during experiments, in this case the location and level of hM4Di expression, and the relationship between agonist dose and hM4Di receptor occupancy. Here we demonstrate that inactivating bilateral rmCD through activation of hM4Di produces a significant and reproducible loss of sensitivity to reward value in monkeys. Thus, the rmCD is involved in making normal judgments about the value of reward.

Clozapine, atypical antipsychotics, and the benefits of fast-off D2 dopamine receptor antagonism

Drug-receptor interactions are traditionally quantified in terms of affinity and efficacy, but there is increasing awareness that the drug-on-receptor residence time also affects clinical performance. While most interest has hitherto been focused on slow-dissociating drugs, D(2) dopamine receptor antagonists show less extrapyramidal side effects but still have excellent antipsychotic activity when they dissociate swiftly. Fast dissociation of clozapine, the prototype of the "atypical antipsychotics", has been evidenced by distinct radioligand binding approaches both on cell membranes and intact cells. The surmountable nature of clozapine in functional assays with fast-emerging responses like calcium transients is confirmatory. Potential advantages and pitfalls of the hitherto used techniques are discussed, and recommendations are given to obtain more precise dissociation rates for such drugs. Surmountable antagonism is necessary to allow sufficient D(2) receptor stimulation by endogenous dopamine in the striatum. Simulations are presented to find out whether this can be achieved during sub-second bursts in dopamine concentration or rather during much slower, activity-related increases thereof. While the antagonist's dissociation rate is important to distinguish between both mechanisms, this becomes much less so when contemplating time intervals between successive drug intakes, i.e., when pharmacokinetic considerations prevail. Attention is also drawn to the divergent residence times of hydrophobic antagonists like haloperidol when comparing radioligand binding data on cell membranes with those on intact cells and clinical data.

Half a century of antipsychotics and still a central role for dopamine D2 receptors

A review of the history of antipsychotics reveals that while the therapeutic effects of chlorpromazine and reserpine were discovered and actively researched almost concurrently, subsequent drug development has been restricted to drugs acting on postsynaptic receptors rather than modulation of dopamine release. The fundamental property of atypical antipsychotics is their ability to produce an antipsychotic effect in the absence of extrapyramidal side effects (EPS) or prolactin elevation. Modulation of the dopamine D2 receptor remains both necessary and sufficient for antipsychotic drug action, with affinity to the D2-receptor being the single most important discriminator between a typical and atypical drug profile. Most antipsychotics, including atypical antipsychotics, show a dose-dependent threshold of D2 receptor occupancy for their therapeutic effects, although the precise threshold is different for different drugs. Some atypical antipsychotics do not appear to reach the threshold for EPS and prolactin elevation, possibly accounting for their atypical nature. To link the biological theories of antipsychotics to their psychological effects, a hypothesis is proposed wherein psychosis is a state of aberrant salience of stimuli and ideas, and antipsychotics, via modulation of the mesolimbic dopamine system, dampen the salience of these symptoms. Thus, antipsychotics do not excise psychosis: they provide the neurochemical platform for the resolution of symptoms. Future generations of antipsychotics may need to move away from a "one-size-fits-all polypharmacy-in-a-pill" approach to treat all the different aspects of schizophrenia. At least in theory a preferred approach would be the development of specific treatments for the different dimensions of schizophrenia (e.g., positive, negative, cognitive, and affective) that can be flexibly used and titrated in the service of patients' presenting psychopathology.

Dopamine D(2) receptors and their role in atypical antipsychotic action: still necessary and may even be sufficient

"Atypical" antipsychotics are associated with a much lower propensity for extrapyramidal side effects and, with some exceptions, a lack of sustained prolactin elevation. The authors propose that a low-affinity and fast dissociation (in molecular terms) from the dopamine D(2) receptor, along with administration of the drug in doses that lead to appropriate levels of dopamine D(2) receptor blockade, are the most important requirements for atypicality. Actions at other receptors (5-HT(2), D(4), etc.) may not be necessary to achieve atypicality, and while action at these receptors may have benefits on symptoms such as mood and cognition, this is as yet to be conclusively proven. Why clozapine is effective in refractory patients is still elusive and efforts to make antipsychotics that are devoid of effects on the dopamine D(2) receptors so far have been unsuccessful. In light of this, the authors provide a heuristic model linking pathophysiology and therapeutics and suggest that the ideal treatment for schizophrenia is unlikely to be single-drug with multireceptor blockade (a sort of one-size-fits-all polypharmacy) but will require several specific and targeted treatment strategies that are titrated to match the variable expression of different dimensions of schizophrenia in each patient.

D(2) and 5HT(2A) receptor occupancy of different doses of quetiapine in schizophrenia: a PET study

OBJECTIVE

Quetiapine is a novel antipsychotic agent with many atypical features, including low D(2) and higher 5HT(2A) affinity in vitro, low propensity to induce extra-pyramidal side effects and minimal effects on prolactin levels. The purpose of this study was to investigate, using positron emission tomography (PET), the relationship between plasma concentrations of different doses of quetiapine and occupancy of D(2) and 5HT(2A) receptors in schizophrenic patients.

METHODS

Five patients were treated with quetiapine (titrated to 750 or 450 mg/day) for 28 days, subsequently reduced weekly in a descending-dose schedule. Dopamine D(2) and 5HT(2A) occupancies were determined using [(11)C] raclopride and [(11)C] N-methylspiperone as ligands, respectively, and PET imaging.

RESULTS

Mean D(2) receptor occupancies of 41 and 30% were observed at quetiapine doses of 750 and 450 mg/day. At lower dose levels no occupancy could be determined. Quetiapine induced a consistently higher degree of 5HT(2A) receptor occupancy, with mean occupancies of 74 and 57% at doses of 750 and 450 mg/day, respectively. No EPS emerged during the trial and most of the pre-trial EPS resolved during the study.

CONCLUSIONS

In clinically effective doses, quetiapine induced low occupancy at D(2) receptors, which is consistent with atypical antipsychotics such as clozapine, and probably explains the lack of EPS observed in this trial. Correlations between receptor occupancy and plasma concentrations of quetiapine could not be calculated, although receptor occupancy increased with higher plasma concentrations for the 450 and 750 mg doses.

Distribution of the methylpiperazinopyridobenzoxazepine derivative JL13, a potential antipsychotic, in rat brain

The brain uptake and distribution of the potential antipsychotic 5-(4-methylpiperazin-1-yl)-8-chloro-pyrido[2,3][1,5]benzoxazepine fumarate (JL13) was examined in rats after neuropharmacologically active doses. Plasma and brain concentrations of the compound were measured by reversed-phase HPLC with UV detection (210 nm). Clozapine was used as an internal standard. After an intraperitoneal dose of 10 mg kg(-1), the compound attained mean maximum plasma concentrations within 5 min of dosing, then declined with a mean elimination half-life of approximately 1 h. It rapidly crossed the blood-brain barrier and equilibrated with plasma, achieving mean maximum concentrations and area under the curve approximately 20-times those in plasma, with slight regional differences. Disappearance from whole brain almost paralleled its disappearance from plasma. There was a linear relationship between JL13 concentrations in plasma and brain regions, and in all tissues the concentrations of the compound increased almost linearly with the dose over the range of 5-20 mg kg(-1). It thus appears that JL13 brain pharmacokinetics parallels that in plasma, and that plasma concentrations accurately predict brain concentrations in rats.

Metabolites of the antipsychotic agent clozapine inhibit the replication of human immunodeficiency virus type 1

Schizophrenia is a serious and often debilitating neuropsychiatric disease of worldwide importance. Current therapy relies on the use of typical antipsychotic medications, which specifically inhibit binding of ligand at the D2 dopamine receptor, and atypical medications which display little activity for this receptor interaction. While atypical antipsychotic agents have been shown to variably inhibit other neuroreceptor-ligand interactions, the exact mechanisms for the therapeutic efficacy of these medications have not been completely defined. Clozapine, an atypical antipsychotic, and nine of its metabolites were studied in vitro for possible antiviral activity against a model of a human neurotropic virus, human immunodeficiency virus type 1 (HIV-1). In an assay for inhibition of virus-induced cytopathic effect (CPE) two metabolites demonstrated antiviral activity (ID50 = 37-85 micrograms/ml) (119-289 microM), while other atypical or novel antipsychotics as well as typical medications had no effect. Based on an ELISA, four chemically similar metabolites inhibited the production of p24, the major internal antigen of HIV (ID50 = 11.6-15.7 micrograms/ml) (38-51 microM). These data suggest that the therapeutic efficacy of some antipsychotics may be due in part to an ability to inhibit viral replication. Antiviral agents may prove to be effective adjuncts in the treatment of schizophrenia.

Serotonin-2 and dopamine-1 binding components of clozapine in frontal cortex and striatum in the human brain visualized by positron emission tomography

The specific binding of N-methyl-11C-clozapine in the human brain was studied in five healthy volunteers with positron emission tomography (PET). Four of the volunteers were reexamined after treatment with the dopamine D1 and D2 receptor antagonist flupenthixol, and all five volunteers were reexamined after pretreatment with the serotonin2 receptor antagonist ritanserin. The examinations after flupenthixol and ritanserin treatment were performed on different occasions. In the flupenthixol part of the study, two of the subjects were given an oral dose of 1 mg flupenthixol 2-3 h before the posttreatment study with PET. The other two subjects received 0.5 mg orally three times during the 24 h preceding the posttreatment PET study, with the last dose being administered < or = 4 h before the scan. All five ritanserin-treated subjects followed the same dosing regimen. During the 5 days preceding the posttreatment PET study, they were given a 10-mg tablet of ritanserin in the evening. The last dose was administered 2-1/2 hours before the study. Both flupenthixol and ritanserin pretreatment were associated with decreased binding of N-methyl-11C-clozapine in dorsolateral and medial frontal cortical regions. These results support previous findings that clozapine has affinity for both dopamine D1 and serotonin 5-HT2 receptors in the human frontal cortex. No consistent change of binding was observed in striatal regions following flupenthixol or ritanserin pretreatment. The clinical aspects of this feature are discussed, both with respect to efficacy and side effects.

Synthesis of n.c.a. carbon-11 labelled clozapine and its major metabolite clozapine-N-oxide and comparison of their biodistribution in mice

N.c.a. [11C]clozapine, [8-chloro-11-(4-[methyl-11C]-methyl-1-piperazinyl)-5H-dibenzo[b,e]-1, 4-diazepine], 1, an atypical neuroleptic was synthesized by N-methylation of the desmethyl compound norclozapine, 3, using [11C]methyl iodide or [11C]methyl triflate for comparison. Subsequent oxidation of 1 with m-chloroperoxybenzoic acid yielded clozapine-N-oxide, 2, the major metabolite of 1. Purification of both radiolabelled products was carried out using a combined semi-preparative HPLC/solid phase extraction procedure. In preparative scale runs overall radiochemical yields for 1 and 2 were 70 and 65%, respectively. The radiochemical purities of both compounds exceeded 98% and the specific activities were in the range of 92-130 GBq/mumol (2.5-3.5 Ci/mumol). Biodistribution of 1 and 2 has been studied in NMRI mice. 10 min p.i. clozapine shows a 24-fold higher brain uptake than its major metabolite. At 60 min p.i., however, the cerebral uptake of both compounds is almost identical.

Treatment of schizophrenia: a clinical and preclinical evaluation of neuroleptic drugs

Forty years after the first clinical report on the effectiveness of chlorpromazine in psychiatric patients, neuroleptic drugs are still the most widely used drugs in the treatment of schizophrenia. Indeed, there are no other drugs which have proven to be as effective in the treatment of this severe psychiatric disorder. Yet, there are still many unresolved problems relating to neuroleptic drugs. The present review gives a comprehensive overview of our knowledge (and our lack of knowledge) with respect to the clinical and preclinical effects of neuroleptic drugs and tries to integrate this knowledge in order to identify the neuronal mechanisms underlying the therapeutic and side effects of neuroleptic drugs.

PET radioligands for dopamine receptors and re-uptake sites: chemistry and biochemistry

This report, based on the past experience of European centres, offers practical guidance on the chemistry and biochemistry of PET radioligands used for the in vivo imaging of dopamine receptors and re-uptake sites. It mainly summarizes methods for the preparation of D1 and D2 receptor ligands labelled with positron-emitting radioisotopes. Some of these ligands (11C-labelled SCH23390, raclopride and nomifensine, 18F-labelled butyrophenones, [76Br]bromolisuride), which have been found useful in PET clinical investigations, have been emphasized. This report is intended as an introduction and guideline for new PET-groups who want to start research in the dopaminergic neurotransmission imaging field.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Positron emission tomography studies of brain receptors

Probing the regional distribution and affinity of receptors in the brain, in vivo, in human and non human primates has become possible with the use of selective ligands labelled with positron emitting radionuclides and positron emission tomography (PET). After describing the techniques used in positron emission tomography to characterize a ligand receptor binding and discussing the choice of the label and the limitations and complexities of the in vivo approach, the results obtained in the PET studies of various neurotransmission systems: dopaminergic, opiate, benzodiazepine, serotonin and cholinergic systems are reviewed.

Combination of positron emission tomography with liquid chromatography in neuropharmacologic research

Positron emission tomography (PET) is an in vivo autoradiographic technique that determines the radioactive distribution and kinetics of a radiolabelled tracer in a tissue. By choice of tracer, it is possible to study physiological processes in living animals and man non-invasively. PET has certain disadvantages such as limited spatial resolution and simultaneous measurement of radiolabelled tracer with the metabolites formed. For an adequate interpretation of the data obtained, complementary techniques such as column liquid chromatography of radioactive composition in blood, plasma, urine and tissue samples have to be used. The prerequisites for any chromatographic technique used for the radioanalysis of substances are speed, high selectivity and high separation efficiency. Examples from PET studies in combination with chromatographic analysis will be given. The utilization of L-dopa in the brain constitutes several steps. Analysis by column liquid chromatography of metabolites in plasma and in monkey brain tissue will make it possible to elucidate different utilization processes of the tracer. Kinetic studies of 11C-labelled neuropeptides such as methionine-enkephalin and substance P revealed high radioactivities in the brain of monkeys. However, simultaneous determination plasma and urine radioactivities using liquid chromatography with radiochemical and photometric detection both indicated that the brain radioactivities emanated to a large extent from 11C-labelled metabolites formed in vivo. Studies with PET using radiotracers having a rapid and extensive metabolism require complementary techniques in the evaluation. High detection selectivity, by combination of photometric and radiochemical detection and rapid and efficient separation, will make liquid chromatography a most important complement in the analysis.

Melperone and clozapine: neuroendocrine effects of atypical neuroleptic drugs

The effects of atypical neuroleptics within the neuroendocrine axis of rodents can be distinguished from those of typical neuroleptics by the production of: 1) a shortlived increase in serum PRL concentrations, 2) an acute increase in the activity of TIDA neurons, and 3) a marked increase in serum corticosterone concentrations. It is of interest to speculate that the pharmacological properties of atypical neuroleptics which mediate the unique neuroendocrine responses are of relevance to an understanding of the mechanisms which underlie the clinical profile of these antipsychotic agents.

Dopamine receptors in human brain: autoradiographic distribution of D2 sites

We have studied the detailed anatomical distribution of D2 receptors in human post mortem brain tissue using quantitative autoradiographic techniques. D2 receptors were labeled using the specific D2 agonist [3H]CV 205-502 and the antagonist [3H]spiroperidol. The pattern of D2 receptor distribution observed with the two ligands was very similar. The highest densities were found in the nucleus caudatus, putamen, nucleus accumbens and olfactory tubercle followed by the substantia nigra, where D2 receptors were mainly concentrated in the pars compacta. Lower but still significant densities were associated with the lateral part of the globus pallidus and CA1 and CA3 fields of the hippocampus. The medial part of the globus pallidus, the dentate gyrus and the amygdala showed low to very low densities of D2 receptors. Almost negligible amounts of binding were observed in the olfactory bulb, diencephalon, brainstem, cerebellum and most parts of the neocortex. Our results are comparable with previously reported localizations of D2 receptors in the human and rat brain. We also report the lack of the so-called spirodecanone binding sites in the human brain. The localization of D2 receptors is compared with the distribution of D1 receptors.

Striatal and frontal cortex binding of 11-C-labelled clozapine visualized by positron emission tomography (PET) in drug-free schizophrenics and healthy volunteers

The binding of 11C-labelled clozapine in the brain was studied in three drug-free schizophrenic patients and in three healthy volunteers. High radioactivities were found in the striatum and in the frontal cortex. The rate constant k3, which is proportional to receptor association rate and the number of receptors, was lower in the frontal cortex compared to the striatum. No obvious difference between the two brain areas was seen for the dissociation rate constant from the receptors (k4). Two schizophrenic patients were reexamined after pretreatment with haloperidol, one after 6 weeks of treatment with a low oral dose, the other one after an IV injection 1 h before 11C-clozapine was given. After haloperidol pretreatment, the binding of 11C-clozapine in striatum and frontal cortex was reduced, more pronounced in the striatum, indicating competition for D-2 dopamine binding sites. Our finding indicates that clozapine has an affinity for a receptor population in the frontal cortex that is predominantly not of the dopamine-D2 type. This feature might be of importance for the unique clinical profile of the drug.

D1 and D2 dopamine receptors in caudate-putamen of nonhuman primates (Macaca fascicularis)

D1 and D2 dopamine receptors were characterized in the caudate-putamen region of nonhuman primate brains (Macaca fascicularis). D1 dopamine receptors were identified with [3H]SCH 23390 and D2 receptors with [3H]-spiperone. Scatchard analysis of [3H]SCH 23390 saturation data using washed membranes revealed a single high-affinity binding site (KD, 0.352 +/- 0.027 nM) with a density (Bmax) of 35.7 +/- 2.68 pmol/g original wet tissue weight (n = 10). The affinity of [3H]spiperone for the D2 site was 0.039 +/- 0.007 nM and the density was 25.7 +/- 1.97 pmol/g original wet tissue weight (n = 10). D1 and D2 receptors in nonhuman primates may be differentiated on the basis of drug affinities and stereoselectivity. In competition experiments, RS-SKF 38393 was the most selective D1 agonist, whereas (+)-4-propyl-9-hydroxynaphthoxazine [(+)-PHNO] was the most selective D2 agonist. Apomorphine was essentially nonselective for D1 or D2 binding sites. Of the antagonists, R-SKF 83566 and SCH 23390 were the most selective for the D1 site, whereas YM-09151-2 was the most selective for the D2 site. cis-Flupentixol and (S)-butaclamol were the least selective dopamine antagonists. D1 receptors bound benzazepine antagonists (SCH 23390/SCH 23388, R-SKF 83692/RS-SKF 83692) stereoselectively whereas D2 receptors did not. Conversely D2 receptors bound (S)-sulpiride and (+)-PHNO more potently than their enantiomers whereas D1 receptors showed little stereoselectively for each of these isomeric pairs. These binding characteristics may be utilized for evaluation of individual receptor function in vivo.

Receptor binding and selectivity of three 11C-labelled dopamine receptor antagonists in the brain of rhesus monkeys studied with positron emission tomography

The regional distribution of 3 11C-labelled dopamine receptor antagonists, N-methyl spiperone, raclopride and clozapine, in the brain of Rhesus monkeys was studied by positron emission tomography (PET). The measured radioactivities in the striatal area were similar for the 3 antagonists, although the highest selectivity as compared to cerebellum was found for 11C-raclopride 60 min after administration. The selectivity of the radiotracers for the serotonin and D2-dopamine receptors was evaluated after pretreatment of the monkeys with serotonin and dopamine receptor antagonists. 11C-N-methylspiperone and 11C-clozapine both bound to serotonin receptors in the frontal cortex and to D2-dopamine receptors in the striatal area. Raclopride was selectively bound to the D2-dopamine receptors. The radioactivities measured in the striatal area with cerebellum as reference were fitted to a 3-compartment model which made possible evaluation of receptor binding characteristics. The rate proportional to the association rate constant for the receptor, kon and number of receptors, Bmax, varied from 0.02-0.07 min-1 between the studied radiolabelled drugs, whereas the apparent dissociation rate was highest for clozapine. This means that clozapine had the lowest affinity for the receptors in the striatum, assuming that the Bmax values are identical. The observed difference in selective receptor binding and binding characteristics of the 3 tracers may have an influence both on the clinical efficacy and side effects of the studied dopamine receptor antagonists.

Future treatment of schizophrenia

In spite of 35 years of experience with antipsychotic drugs, the psychiatrists are still faced with the limitations of these drugs: no or minimal therapeutic effect in hallucinations and delusions in about 25% of schizophrenic patients; persisting anergia and emotional withdrawal in otherwise successfully treated patients; a great spectrum of side effects, some irreversible. Quo vadis? An incidental discovery of a completely new drug, a new "chlorpromazine" would be the ideal solution, but for the present, one has to continue with the small pragmatic steps, especially within the following areas: (a) the selective antidopaminergic drugs, especially the substituted benzamides, may be further developed in the direction of antipsychotic selectivity with fewer and fewer extrapyramidal side effects; (b) the atypical clozapine ought soon to have successors, hopefully without the risk of bone marrow depression and cardiovascular side effects; (c) the D1 antagonists as well as the D1 agonists may imply therapeutically valuable effects; (d) the dopamine autoreceptor has long been in focus, but until now, no pure agonist has been found, and the drugs available, including (-)3-PPP, appear to have many side effects; (e) serotonin antagonists may be an interesting possibility; and (f) when it may be possible to influence brain peptides more efficiently than up to now, this area will probably provide us with several psychotropic drugs. Furthermore, during the search for new antipsychotic drugs, one must not forget to improve the practical use of available neuroleptics and of nonpharmacological, psychosocial treatment modalities.

Clinical pharmacokinetics of clozapine in chronic schizophrenic patients

The clinical pharmacokinetics of clozapine, an atypical neuroleptic, was evaluated in 10 chronic schizophrenic male patients after intravenous and oral administration. The mean equilibrium-state concentration ratio between blood and plasma was experimentally determined to be 0.87. The average values for blood clearance, hepatic extraction ratio and oral bioavailability were 250 ml/min, 0.2 and 0.27, respectively. Plasma concentration peaked on average at 3 h. The mean volume of distribution at steady-state and the terminal half-life was 1.6 l/kg and 10.3 h, respectively. A large fraction of the dose is most probably metabolized by some extrahepatic presystemic routes. The large inter-individual variability in the bioavailability and clearance is probably the main reason for large variation in the steady-state plasma level in patients receiving the same oral dosage regimen.