Plasma and brain pharmacokinetics of amitriptyline and its demethylated and hydroxylated metabolites after one and six half-life repeated administrations to rats

Physiologically Based Pharmacokinetic (PBPK) Modeling for Predicting Brain Levels of Drug in Rat

One of the main obstacles in neurological disease treatment is the presence of the blood-brain barrier. New predictive high-throughput screening tools are essential to avoid costly failures in the advanced phases of development and to contribute to the 3 Rs policy. The objective of this work was to jointly develop a new in vitro system coupled with a physiological-based pharmacokinetic (PBPK) model able to predict brain concentration levels of different drugs in rats. Data from in vitro tests with three different cells lines (MDCK, MDCK-MDR1 and hCMEC/D3) were used together with PK parameters and three scaling factors for adjusting the model predictions to the brain and plasma profiles of six model drugs. Later, preliminary quantitative structure-property relationships (QSPRs) were constructed between the scaling factors and the lipophilicity of drugs. The predictability of the model was evaluated by internal validation. It was concluded that the PBPK model, incorporating the barrier resistance to transport, the disposition within the brain and the drug-brain binding combined with MDCK data, provided the best predictions for passive diffusion and carrier-mediated transported drugs, while in the other cell lines, active transport influence can bias predictions.

Dual action of amitriptyline on NMDA receptors: enhancement of Ca-dependent desensitization and trapping channel block

Although the tricyclic antidepressant amitriptyline (ATL) is widely used in the clinic, the mechanism underlying its high therapeutic efficacy against neuropathic pain remains unclear. NMDA receptors (NMDARs) represent a target for ATL and are involved in sensitization of neuropathic pain. Here we describe two actions of ATL on NMDARs: 1) enhancement of Ca²⁺-dependent desensitization and 2) trapping channel block. Inhibition of NMDARs by ATL was found to be dependent upon external Ca²⁺ concentration ([Ca²⁺]) in a voltage-independent manner, with an IC₅₀ of 0.72 μM in 4 mM [Ca²⁺]. The ATL IC₅₀ value increased exponentially with decreasing [Ca²⁺], with an e-fold change observed per 0.69 mM decrease in [Ca²⁺]. Loading neurons with BAPTA abolished Ca²⁺-dependent inhibition, suggesting that Ca²⁺ affects NMDARs from the cytosol. Since there is one known Ca²⁺-dependent process in gating of NMDARs, we conclude that ATL most likely promotes Ca²⁺-dependent desensitization. We also found ATL to be a trapping open-channel blocker of NMDARs with an IC₅₀ of 220 µM at 0 mV. An e-fold change in ATL IC₅₀ was observed to occur with a voltage shift of 50 mV in 0.25 mM [Ca²⁺]. Thus, we disclose here a robust dependence of ATL potency on extracellular [Ca²⁺], and demonstrate that ATL bound in the NMDAR pore can be trapped by closure of the channel.

Front and hind paw differential analgesic effects of amitriptyline, gabapentin, ibuprofen, and URB937 on mechanical and cold sensitivity in cisplatin-induced neuropathy

Cisplatin is a widely used platinum-derived antineoplastic agent that frequently results in peripheral neuropathy. Therapeutic strategies for neuropathic pain are limited and characterized by variable efficacy and severe adverse effects. Clinical translation of novel analgesics has proven difficult with many agents demonstrating preclinical efficacy failing in clinical trials. Preclinical studies frequently assess pain behaviors in the hind paws; however, the front paws have a greater degree of the fine sensorimotor functions characteristically damaged by chemotherapy-induced neuropathy. This is the first study to assess pain responses in the front paws. Here, we test the hypothesis that mouse front paws exhibit pain-related alterations in mechanical and thermal (cold) sensitivity in a murine model of cisplatin-induced neuropathy and that pharmacological treatment with amitriptyline, gabapentin, ibuprofen, and URB937 normalize pain behaviors in the front and hind paws. Cold (acetone withdrawal latencies) and mechanical (von Frey withdrawal thresholds) sensitivity were significantly increased and decreased respectively in both the front and the hind paws following initiation of weekly systemic (intraperitoneal) cisplatin injections (5 mg/kg). For the hind paws, systemic administration of amitriptyline (30 mg/kg), gabapentin (100 mg/kg), ibuprofen (0–10 mg/kg), or URB937 (0–10 mg/kg) resulted in a decrease in acetone withdrawal latencies and increase in von Frey withdrawal thresholds with return to normal values at the highest doses tested. For the front paws, return to baseline values for the highest doses was found for cold allodynia but not mechanical allodynia, where the highest doses failed to return to baseline values. These results indicate that mouse front paws exhibit pain-related changes in cisplatin-induced neuropathy and that drug effects can vary based on testing stimulus and location. This suggests that front paw responses across multiple modalities provide reliable and accurate information about pain-related drug effects. Future studies should be aimed at elucidating the mechanisms underlying these differential effects.

Comparison of cerebral Open Flow Microperfusion and Microdialysis when sampling small lipophilic and small hydrophilic substances

BACKGROUND

Assessment of drug concentration in the brain interstitial fluid (ISF) is crucial for development of brain active drugs, which are mainly small, lipophilic substances able to cross the blood-brain barrier (BBB). We aimed to compare the applicability of cerebral Open Flow Microperfusion (cOFM) and Microdialysis (MD) to sample the lipophilic substance amitriptyline (AMI), its metabolites Hydroxyamitriptyline (HYA), Nortriptyline (NOR), Amitriptyline-N-Oxide (ANO), deuterated water (D₂O) and the hydrophilic substance sodium fluorescein (Naf) in brain ISF. NEW METHOD: cOFM has been refined to yield increased spatial resolution and performance.

COMPARISON OF COFM AND MD AND RESULTS

Performance of cOFM and MD was assessed by in vivo AUC ratios of probe samples (AUC_COFM/AUC_MD) and the in vivo relative recovery of D₂O (RR_vv,D2O). Adsorption of AMI and Naf to MD and cOFM was assessed by the in vitro relative recovery (RR_vt) prior to the in vivo experiments. The in vivo AUC ratio of AMI and RR_vv,D2O was about two times higher for cOFM than for MD (AUC_OFM/AUC_MD = 2.0, RR_vv,D2O(cOFM)/RR_vv,D2O(MD) = 2.1). cOFM detected all investigated AMI metabolites except NOR. MD did not detect HYA, NOR, ANO and Naf. In vitro adsorption of AMI and Naf to the MD membrane was strong (RR_vt,AMI = 4.4%, RR_vt,Naf = 1.5%) but unspecific adsorption to cOFM was negligibly small (RR_vt,AMI = 98% and RR_vt,Naf = 98%).

CONCLUSIONS

cOFM showed better performance when sampling AMI and its metabolites, Naf and D₂O, and had an about two times higher RR_vv,D2O than MD. MD did not detect HYA, NOR, ANO and Naf, most likely due to membrane adsorption.

Nortriptyline inhibits aggregation and neurotoxicity of alpha-synuclein by enhancing reconfiguration of the monomeric form

The pathology of Parkinson's disease and other synucleinopathies is characterized by the formation of intracellular inclusions comprised primarily of misfolded, fibrillar α-synuclein (α-syn). One strategy to slow disease progression is to prevent the misfolding and aggregation of its native monomeric form. Here we present findings that support the contention that the tricyclic antidepressant compound nortriptyline (NOR) has disease-modifying potential for synucleinopathies. Findings from in vitro aggregation and kinetics assays support the view that NOR inhibits aggregation of α-syn by directly binding to the soluble, monomeric form, and by enhancing reconfiguration of the monomer, inhibits formation of toxic conformations of the protein. We go on to demonstrate that NOR inhibits the accumulation, aggregation and neurotoxicity of α-syn in multiple cell and animal models. These findings suggest that NOR, a compound with established safety and efficacy for treatment of depression, may slow progression of α-syn pathology by directly binding to soluble, native, α-syn, thereby inhibiting pathological aggregation and preserving its normal functions.

Chronic amitriptyline treatment attenuates nigrostriatal degeneration and significantly alters trophic support in a rat model of parkinsonism

In addition to alleviating depression, long-term adaptive changes induced by antidepressants may regulate neural plasticity in the diseased brain, providing symptomatic and disease-modifying effects in Parkinson's disease. The present study investigated whether chronic treatment with a frequently prescribed tricyclic antidepressant was neuroprotective in a 6-hydroxydopamine (6-OHDA) rat model of parkinsonism. In lesioned animals, chronic amitriptyline (AMI; 5 mg/kg) treatment resulted in a significant sparing of tyrosine hydroxylase-immunoreactive (THir) neurons in the substantia nigra pars compacta (SNpc) compared with saline treatment. Additionally, striatal fibers were preserved and functional motor deficits were attenuated. Although 6-OHDA lesions did not induce anhedonia in our model, the dose of AMI utilized had antidepressant activity as demonstrated by reduced immobility. Recent in vitro and in vivo data provide evidence that trophic factors such as brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) may be key mediators of the therapeutic response to antidepressants. Therefore, we investigated whether AMI mediates changes in these specific trophic factors in the intact and degenerating nigrostriatal system. Chronic AMI treatment mediates an increase in nigral BDNF both before and during ongoing degeneration, suggesting it may contribute to neuroprotection observed in vivo. However, over time, AMI reduced BDNF levels in the striatum, indicating tricyclic therapy differentially regulates trophic factors within the nigrostriatal system. Combined, these results suggest that AMI treatment attenuates dopamine neuron loss and elicits significant trophic changes relevant to dopamine neuron survival.

Tricyclic antidepressant treatment evokes regional changes in neurotrophic factors over time within the intact and degenerating nigrostriatal system

In addition to alleviating depression, trophic responses produced by antidepressants may regulate neural plasticity in the diseased brain, which not only provides symptomatic benefit but also potentially slows the rate of disease progression in Parkinson's disease (PD). Recent in vitro and in vivo data provide evidence that neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) may be key mediators of the therapeutic response to antidepressants. As such, we conducted a cross-sectional time-course study to determine whether antidepressant-mediated changes in neurotrophic factors occur in relevant brain regions in response to amitriptyline (AMI) treatment before and after intrastriatal 6-hydroxydopamine (6OHDA). Adult male Wistar rats were divided into seven cohorts and given daily injections (i.p.) of AMI (5mg/kg) or saline throughout the duration of the study. In parallel, various cohorts of intact or parkinsonian animals were sacrificed at specific time points to determine the impact of AMI treatment on trophic factor levels in the intact and degenerating nigrostriatal system. The left and right hemispheres of the substantia nigra, striatum, frontal cortex, piriform cortex, hippocampus, and anterior cingulate cortex were dissected, and BDNF and GDNF levels were measured with ELISA. Results show that chronic AMI treatment elicits effects in multiple brain regions and differentially regulates levels of BDNF and GDNF depending on the region. Additionally, AMI halts the progressive degeneration of dopamine (DA) neurons elicited by an intrastriatal 6-OHDA lesion. Taken together, these results suggest that AMI treatment elicits significant trophic changes important to DA neuron survival within both the intact and degenerating nigrostriatal system.

Segmental spinal root avulsion in the adult rat: a model to study avulsion injury pain

Road traffic accidents are the most common cause of avulsion injury, in which spinal roots are torn from the spinal cord. Patients suffer from a loss of sensorimotor function, intractable spontaneous pain, and border-zone hypersensitivity. The neuropathic pains are particularly difficult to treat because the lack of a well-established animal model of avulsion injury prevents identifying the underlying mechanisms and hinders the development of efficacious drugs. This article describes a hindlimb model of avulsion injury in adult rats where the L5 dorsal and ventral spinal root are unilaterally avulsed (spinal root avulsion [SRA]), leaving the adjacent L4 spinal root intact. SRA produced a significant ipsilateral hypersensitivity to mechanical and thermal stimulation by 5 days compared with sham-operated or naïve rats. This hypersensitivity is maintained for up to 60 days. No autotomy was observed and locomotor deficits were minimal. The hypersensitivity to peripheral stimuli could be temporarily ameliorated by administration of amitriptyline and carbamazepine, drugs that are currently prescribed to avulsion patients. Histological assessment of the L4 ganglion cells revealed no significant alterations in calcitonin gene-related peptide (CGRP), IB4, transient receptor potential cation channel subfamily V member 1 (TrpV1), or N52 staining across groups. Immunohistochemistry of the spinal cord revealed a localized glial response, phagocyte infiltration, and neuronal loss within the ipsilateral avulsed segment. A comparable response from glia and phagocytes was also found in the intact L4 spinal cord, supporting the role for central mechanisms within the L4-5 spinal cord in contributing to the generation of the pain-related behavior. The SRA model provides a platform to investigate possible new pharmacological treatments for avulsion injuries.

Serotoninergic and non-serotoninergic effects of two tricyclic antidepressants on visceral nociception in a rat model

OBJECTIVE

Tricyclic antidepressants (TCAs) are well established in the treatment of patients with irritable bowel syndrome (IBS). The effects are believed to be linked to serotoninergic antinociceptive properties, but data on the antinociceptive effects of various TCAs with variable serotoninergic and non-serotoninergic properties have not been investigated. The aim of this study was to compare the antinociceptive effects of different TCAs.

MATERIAL AND METHODS

Colorectal distension (CRD) using a barostat device was carried out in rats and the visceromotor response (VMR) to CRD was quantified by abdominal wall electromyography. Prior to CRD, saline (control), amitriptyline (AM), desipramine (DES), reserpine (RES) or a combination of TCAs and RES (AM + RES or DES + RES) was applied intraperitoneally. Serum 5-HT levels were determined using high-performance liquid chromatography (HPLC). RES was used to antagonize the serotoninergic actions of TCAs in order to discriminate between these effects and others.

RESULTS

Both TCAs decreased the VMR compared to placebo. After RES application without TCAs, the VMR was increased compared to controls (6403 microV+/-1772 microV). Co-administration of AM and RES resulted in a modest decrease in VMR (5774 microV+/-1953 microV), while in rats treated with RES and DES the VMR again was significantly lower (3446 microV (+/-1347 microV; p <0.05)). 5-HT levels were higher in TCA pretreated rats than those in controls and significantly lower 5-HT levels were found in all rats pretreated with RES.

CONCLUSIONS

AM and DES have antinociceptive properties while RES is pro-nociceptive. The antinociceptive effects of DES are not abolished by RES pretreatment, while AM only attenuates the pro-nociceptive effects of RES. The non-serotoninergic properties of TCAs substantially contribute to the differences in the antinococeptive effects of various TCAs.

Serotonin transporter density and anxiolytic-like effects of antidepressants in mice

BACKGROUND

Chronic treatment with the dual serotonin/noradrenaline reuptake inhibitor (SNRI) duloxetine reduces the density of serotonin transporter sites in cortex and engenders an anxiolytic-like response. To determine the reproducibility of these effects and their generality to other antidepressants we compared the effects of chronic duloxetine treatment with another SNRI, venlafaxine, and two selective serotonin reuptake inhibitors, paroxetine and fluoxetine.

METHODS

Separate groups of mice were administered vehicle, fluoxetine (15 mg/kg), paroxetine, duloxetine or venlafaxine (10 mg/kg) perorally twice daily for 28 days and tested in the mouse zero-maze and in motility cages on days 21 and 22, respectively, to determine effects on anxiety and motor activity. On day 28 brains were analysed for serotonin transporter (SERT) density in cortex and noradrenaline transporter (NET) density in cortex and hippocampus.

RESULTS

Duloxetine and fluoxetine both reduced SERT density in cortex and induced anxiolytic-like effects. Paroxetine had an identical profile, but it is unclear if this drug down-regulated the SERT since extensive washing of cortical tissue did not remove all drug. Venlafaxine had no effect on behavioural or biochemical parameters. Only duloxetine reduced NET density in cortex, although not hippocampus.

CONCLUSIONS

The reduction in SERT density and anxiolytic-like effects with duloxetine, fluoxetine and, potentially, paroxetine suggest that down-regulation of the SERT may be a relevant mechanism in therapeutic response to these antidepressants.

P-glycoprotein reduces the ability of amitriptyline metabolites to cross the blood brain barrier in mice after a 10-day administration of amitriptyline

P-glycoprotein (P-gp) is a 170-kDa membrane protein and the gene product of the multiple drug resistance (MDR1 or ABCB1) gene. It constitutes an important part of the blood-brain barrier and actively exports a number of molecules across the blood-brain barrier back into the vascular space, subsequently reducing central nervous system (CNS) bioavailability of these substances. The aim of the present study was to investigate the pharmacokinetics of amitriptyline and its metabolites in P-gp (also called mdr1ab or abcb1ab) knockout mice and controls after a long-term adminstration for 10 days. Knockout mice and controls received s.c. injections of amitriptyline (10 microg/g bodyweight) twice daily for 10 days. After 10 days, the animals were sacrificed and the concentrations of amitriptyline and nortriptyline and both their E-10-OH and Z-10-OH metabolites were measured with high-performance liquid chromatography in the cerebrum, plasma, spleen, kidney, testes, lung, liver, muscle and fat. Except for amitriptyline, the brain concentrations of all other examined substances were significantly higher in the P-gp knockout mice. Compared to controls, concentrations of nortriptyline were 2.6-fold higher, E-10-OH-nortriptyline 10-fold higher, Z-10-OH-nortriptyline seven-fold higher, E-10-OH-amitriptyline two-fold higher and Z-10-OH-amitriptyline five-fold higher. The present study confirms that P-gp plays an important role in the interaction between CNS drugs and the blood-brain barrier. Without P-gp at the blood-brain barrier, the brain concentrations of the substances were up to 10-fold higher, showing that P-gp plays an active role in exporting CNS drugs out of the brain. Recent clinical studies showing different side-effects in patients with P-gp polymorphisms confirm the clinical importance of these findings.

The use of gabapentin for the treatment of postherpetic neuralgia

BACKGROUND

Varicella-zoster virus causes chickenpox and can reemerge later in life to cause herpes zoster or shingles. One of the most common and disabling complications of herpes zoster is postherpetic neuralgia (PHN).

OBJECTIVES

This article reviews the current primary literature about the efficacy and tolerability of gabapentin for the treatment of PHN. Gabapentin pharmacokinetics and drug interactions are also reviewed.

METHODS

A literature search in the English language was conducted using OVID Web, which contained the following databases: MEDLINE (1966-present), EMBASE (1980-2002), Current Contents/Clinical Medicine (1999-2002), Cochrane Controlled Trials Register (1898-present), Cochrane Database of Systemic Reviews (fourth quarter, 2002), and International Pharmaceutical Abstracts (1970-2002). Search terms used were postherpetic neuralgia; zoster; gabapentin; neuropathic pain; pain; pharmacoeconomic; cost; controlled clinical trial; randomized, controlled trial; postherpetic neuralgia and gabapentin; gabapentin and pain; treatment and postherpetic neuralgia; gabapentin and age; gabapentin and gender; gabapentin and ethnicity; and gabapentin and pharmacokinetics.

RESULTS

Gabapentin displays nonlinear absorption kinetics, is minimally protein bound (< 3%), has a high mean (SD) volume of distribution (50.4 [8.0] L), and is excreted via the kidneys as unchanged drug. Two randomized, placebo-controlled, parallel-group, multicenter clinical trials demonstrated the effectiveness of gabapentin at doses of up to 3600 mg/d to significantly reduce pain (P < 0.01 and P < 0.001), improve sleep (P < 0.01), and improve some parameters on the Short Form-McGill Pain Questionnaire (P < 0.05). Dizziness and somnolence were the most common side effects leading to withdrawal from the trials. The recommended dosage in adults is 300 mg at bedtime on day 1,300 mg BID on day 2, and 300 mg TID on day 3, titrating up as needed to 2400 to 3600 mg/d. To reduce adverse events in patients with renal impairment, the dose should be adjusted based on the patient's creatinine clearance.

CONCLUSIONS

Gabapentin appears to be effective and well tolerated for the short-term treatment of PHN. However, future controlled studies are needed to determine whether the effectiveness of gabapentin for PHN is maintained for > 2 months, to establish the optimal dose of gabapentin for PHN, and to compare the efficacy of gabapentin with that of other pharmacologic agents used for the treatment of PHN.

Different effects of amitriptyline and imipramine on the pharmacokinetics and metabolism of perazine in rats

The aim of this study was to search for possible effects of imipramine and amitriptyline on the pharmacokinetics and metabolism of perazine at steady state in rats. Perazine (10 mg kg(-1), i.p.) was administered to rats twice daily for two weeks, alone or jointly with imipramine or amitriptyline (10 mg kg(-1) i.p.). Concentrations of perazine and its two main metabolites (5-sulphoxide and N-desmethylperazine) in the plasma and brain were measured at 30 min (Cmax), 6h and 12h (slow disposition phase) after the last dose of the drugs. Liver microsomes were prepared 24 h after withdrawal of the drugs. Amitriptyline increased the plasma and brain concentrations of perazine (up to 300% of the control) and N-desmethylperazine, while not affecting those of 5-sulphoxide. Imipramine only tended to increase the neuroleptic concentration in the plasma and brain. Studies with control liver microsomes showed that amitriptyline and imipramine added to the incubation mixture in-vitro, competitively inhibited N-demethylation (Ki (inhibition constant) = 16 microM and 164 microM, respectively) and 5-sulphoxidation (Ki = 57 microM and 86 microM, respectively) of perazine, amitriptyline being a more potent inhibitor of perazine metabolism, especially with respect to N-demethylation. Studies with microsomes of rats treated chronically with perazine or tricyclic antidepressants, or both, did not show significant differences in the rate of perazine metabolism between perazine- and perazine+antidepressant-treated rats. The data obtained were compared with the results of analogous experiments with promazine and thioridazine. It was concluded that elevations of perazine concentration were caused by direct inhibition of the neuroleptic metabolism by the antidepressants. Similar interactions, possibly leading to exacerbation of the pharmacological action of perazine, may be expected in man. Since the interactions between phenothiazines and tricyclic antidepressants may proceed in two directions, reduced doses of both the neuroleptic and the antidepressant are recommended when the drugs are administered jointly.

Pharmacokinetics and metabolism of thioridazine during co-administration of tricyclic antidepressants

1. Because of serious side-effects of thioridazine and tricyclic antidepressants (cardiotoxicity), a possible influence of imipramine and amitriptyline on the pharmacokinetics and metabolism of thioridazine was investigated in a steady state (2-week treatment) in rats. 2. Imipramine and amitriptyline (5 and 10 mg kg(-1) i.p., respectively) elevated 30 and 20 fold, respectively, the concentration of thioridazine (10 mg kg(-1) i.p.) and its metabolites (N-desmethylthioridazine, 2-sulphoxide, 2-sulphone, 5-sulphoxide) in blood plasma. Similar, yet weaker increases in the thioridazine concentration were found in the brain. Moreover, an elevation of thioridazine/metabolite ratios was observed. 3. Imipramine and amitriptyline added to control liver microsomes in vitro inhibited the metabolism of thioridazine via N-demethylation (an increase in K(m)), mono-2-sulphoxidation (an increase in K(m) and a decrease in V(max)) and 5-sulphoxidation (mainly a decrease in V(max)). Amitriptyline was a more potent inhibitor than imipramine of the thioridazine metabolism. 4. The varying concentration ratios of antidepressant/thioridazine in vivo appear to be more important to the final result of the pharmacokinetic interactions than are relative direct inhibitory effects of the antidepressants on thioridazine metabolism observed in vitro. 5. Besides direct inhibition of the thioridazine metabolism, the decreased activity of cytochrome P-450 towards 5-sulphoxidation, produced by chronic joint administration of thioridazine and the antidepressants, seems to be relevant to the observed in vivo interaction. 6. The obtained results may also point to inhibition of another, not yet investigated, metabolic pathway of thioridazine, which may be inferred from the simultaneous elevation of concentrations of both thioridazine and the measured metabolites.