Carbamazepine and its metabolites in neuralgias: concentration-effect relations

Therapeutic drug monitoring of gabapentin in various indications

OBJECTIVES

Gabapentin has been increasingly used in various indications in recent years. Despite variable pharmacokinetics, therapeutic drug monitoring (TDM) is scarcely described in other indications than epilepsy. The aim of the study was to investigate the use and pharmacokinetic variability of gabapentin in epilepsy and non-epilepsy indications and to further evaluate the use of TDM in patients with restless legs syndrome (RLS).

MATERIALS & METHODS

Population-based data from the Norwegian Prescription Database, retrospective TDM data from the section for Clinical Pharmacology, the National Center for Epilepsy, Norway, and prospective observational data on patients with RLS were used.

RESULTS

The use of gabapentin increased by 30% from 2014 to 2017 (32 181 to 42 675 users). TDM data from 120 patients showed a 22-fold pharmacokinetic variability in concentration/dose ratios, and this ratio was elevated in elderly patients (≥65 years). The majority of elderly used gabapentin for non-epilepsy indications. In patients with RLS, intake in the evening/night only was common due to nocturnal symptoms, in contrast to regular dosing regimens in epilepsy. Thus, drug fasting concentrations do not reflect concentrations at the time of required therapeutic effect. TDM was still found useful in most patients to support dosage increase or evaluate adverse effects.

CONCLUSION

Due to extensive pharmacokinetic variability, TDM can benefit patients using gabapentin. Challenges with applying TDM in new indications such as RLS include different dosage regimens and consequently different interpretation of serum concentrations. Thus, TDM should be requested on clear clinical grounds and the service tailored according to the therapeutic indication.

Pharmacology update: tapentadol for neuropathic pain

Neuropathic pain in a common problem encountered in palliative care. When neuropathic pain is diagnosed, appropriate treatment is important in limiting the severe psychosocial impairment that can ensue with undertreated pain. Proper evaluation of the patient to clarify the type of pain experienced is the first step to determine appropriate management. Tapentadol is an oral mu-opioid receptor agonist and a noradrenaline reuptake inhibitor developed by Ortho-McNeil Janssen Pharmaceuticals and approved by the Food and Drug Administration in November 2008 for the treatment of moderate-to-severe acute pain in adult patients and for chronic pain in August 2011 in an extended release form. Tapentadol has been studied for use in nociceptive pain but few studies have yet been done to assess its efficacy in the treatment of neuropathic pain.

Memantine (Namenda) for Neuropathic Pain

Neuropathic pain is common in the palliative care population; unless adequately treated, the pain can lead to chronic anxiety, depression, and social impairment. Many treatments have been proposed for neuropathic pain; however, it remains underdiagnosed, under-treated, and often requires long-term therapy with risk of adverse effects. Memantine (Namenda), an N-Methyl, D-aspartate receptor inhibitor currently marketed for the treatment of dementia, has been proposed as a medication for the treatment of neuropathic pain for its mechanism, safety, lack of serious adverse effects, and relatively rapid onset of action. However, clinical trials have not been promising so far and its routine use in neuropathic pain is not currently recommended.

Pain and psychiatry: a critical analysis and pharmacological review

Pain is one of the most difficult medical problems to diagnose and treat and can be a common symptom of several psychiatric disorders. Pain-related issues are heterogeneous and often underestimated or misinterpreted, with the result that psychiatric interventions, which might have been beneficial from the outset, are often delayed or requested only as a last measure. Several problems arise from the definition, classification and assessment of pain, when documented according to the different scales which are commonly used, since these attempt to cover a multitude of analytical requirements, without really succeeding. An area of constant debate regards the connection between pain and various psychiatric disorders, and the difficulty in the classification of pain disorders within the currently existing framework. The pharmacological treatment of pain is complex and implies a variety of different compounds, from opioids to psychotropic medications like antidepressants and anticonvulsivants.

This paper explores the mutual and reciprocal influence between pain and psychiatric disorders reviewing the latest developments in the definition, assessment and treatment of pain, with special emphasis on the impact of pain on psychiatric disorders (and vice versa), and on the use of psychotropic drugs in the treatment of pain syndromes.

The response of neuropathic pain and pain in complex regional pain syndrome I to carbamazepine and sustained-release morphine in patients pretreated with spinal cord stimulation: a double-blinded randomized study

Forty-three patients with peripheral neuropathic pain, exclusively pain reduced by spinal cord stimulation (SCS), were switched into a painful state after SCS inactivation. This mode was used to assess the pain-relieving effect of carbamazepine (CMZ) and opioids in a double-blinded, placebo-controlled trial. In Phase 1, the patients were randomly allocated to receive either CMZ (600 mg/d) or placebo during an SCS-free period of 8 days. In Phase 2, after a CMZ elimination interval of 7 days, 38 patients received either sustained-release morphine (90 mg/d) or placebo for 8 days. In cases of intolerable pain, the patients were authorized to reactivate their SCS. The pain intensity was rated on a numeric analog scale. In 38 patients who completed Phase 1, significant delay in pain increase was observed in the CMZ group as compared with placebo (P = 0.038). In Phase 2, the trend observed with morphine was insignificant (P = 0.41). Two CMZ patients and one morphine patient showed complete pain relief and preferred to continue the medication. Thirty-five patients returned to SCS. We conclude that CMZ is effective in peripheral neuropathic pain. Morphine obviously requires larger individually titrated dosages than those used in this study for results to be adequately interpreted.

The response of neuropathic pain and pain in complex regional pain syndrome I to carbamazepine and sustained-release morphine in patients pretreated with spinal cord stimulation: a double-blinded randomized study

Forty-three patients with peripheral neuropathic pain, exclusively pain reduced by spinal cord stimulation (SCS), were switched into a painful state after SCS inactivation. This mode was used to assess the pain-relieving effect of carbamazepine (CMZ) and opioids in a double-blinded, placebo-controlled trial. In Phase 1, the patients were randomly allocated to receive either CMZ (600 mg/d) or placebo during an SCS-free period of 8 days. In Phase 2, after a CMZ elimination interval of 7 days, 38 patients received either sustained-release morphine (90 mg/d) or placebo for 8 days. In cases of intolerable pain, the patients were authorized to reactivate their SCS. The pain intensity was rated on a numeric analog scale. In 38 patients who completed Phase 1, significant delay in pain increase was observed in the CMZ group as compared with placebo (P = 0.038). In Phase 2, the trend observed with morphine was insignificant (P = 0.41). Two CMZ patients and one morphine patient showed complete pain relief and preferred to continue the medication. Thirty-five patients returned to SCS. We conclude that CMZ is effective in peripheral neuropathic pain. Morphine obviously requires larger individually titrated dosages than those used in this study for results to be adequately interpreted.

Carbamazepine-10,11-epoxide in therapeutic drug monitoring

Carbamazepine (CBZ) is widely used in the treatment of epilepsy, frequently in combination with other anticonvulsants. Its metabolite, carbamazepine-10,11-epoxide, is pharmacologically active and is increased with concurrent use of valproate and other anticonvulsants. This pharmacokinetic interaction may be particularly important because CBZ, its epoxide, phenytoin, and lamotrigine all act on fast voltage-dependent sodium channels. Over a 2-month period, routine serum requests for CBZ (n=47) (excluding known cases of overdose) were also analyzed for CBZ epoxide, phenytoin, and lamotrigine using a simultaneous high performance liquid chromatographic (HPLC) method. Valproate was measured using fluorescence polarization immunoassay (FPIA). With concurrent phenytoin and lamotrigine administration, there was a relative increase in CBZ epoxide and a significant decrease in the ratio of CBZ to epoxide (from more than 5 to 3). If valproate was also present, the concentration of parent and metabolite increased significantly, causing potential toxicity. Two patients in this latter group had significant clinical toxicity, with parent CBZ concentrations in the reference range; a third patient suffered from poor control of seizures. This study illustrates the importance of awareness of the contribution of active metabolites in therapeutic drug monitoring and raises questions about the role of the routine monitoring of such metabolites.

A double-blind study of the sedative effects of the thalidomide enantiomers in humans

This study investigated the sedative effects of the enantiomers of thalidomide in humans. Since the enantiomers undergo chiral inversion in vivo this entailed application of suitable statistical methodology to distinguish the effects of each enantiomer in the presence of the other one. Six healthy male volunteers received single oral doses of (+)-(R)-thalidomide, (-)-(S)-thalidomide or racemic thalidomide in a double-blind study, and the results of this study were compared with those of a similar study where three subjects received (+)-(R)-thalidomide, (-)-(S)-thalidomide or placebo in a double-blind fashion. Blood samples for analysis of (+)-(R)-thalidomide and (-)-(S)-thalidomide were obtained. Prior to sampling it was noted whether the subject was awake or asleep, and his feelings of tiredness and heaviness were estimated using Borg scales. After blood sampling continuous reaction time was measured by a 10-min series of auditory signals. The concentration-effect relationships were analyzed by logistic regression techniques and Cox regression for the reaction time data. The (+)-(R)-thalidomide concentrations, but not the (-)-(S)-thalidomide concentrations, exhibited significant positive influences on all sedative effects studied (sleep, tiredness, and reaction times). In some of the analyses of reaction times the (-)-(S)-thalidomide concentrations had a significant effect in the opposite direction. The time course and intensity of sedative effect is thus influenced by which enantiomer is administered and by the kinetics of in vivo chiral inversion.

Advances in comprehensive pain management

Chronic pain is a significant public health problem and frustrating for everyone affected by it. Psychiatrists should take an active role in the care of patients with chronic pain. They no longer should wait to make a psychiatric diagnosis by exclusion in the patient who has failed to respond to multiple treatments over a period of years. Recent advances in the treatment of chronic pain include the diagnosis and treatment of psychiatric comorbidity, the application of primary psychiatric treatments to chronic pain, and the development of interdisciplinary efforts to provide comprehensive health care to the patient suffering with chronic pain.