Margrete Larsen Burns

Long-term stability of morphine, codeine, and 6-acetylmorphine in real-life whole blood samples, stored at −20 °C

PURPOSE Stability of drugs during storage is important in forensic toxicology. For the analytes detected after intake of heroin (6-acetylmorphine (6-AM), morphine and codeine), long-time stability in real life whole blood samples are studied in only a small number of cases. METHODS Whole blood post mortem (n=37) and whole blood samples from living persons (n=22) containing morphine and codeine as well as 6-AM in blood or urine were selected. All cases represented intake of heroin. All samples contained fluoride and were initially analysed and stored in normal conditions (-20°C) for 4-9 years. All samples were then reanalysed using the same analytical methods and the results were compared. RESULTS For samples from living persons, the median change in concentration was -3.7% for morphine and -5.3% for codeine. For post mortem samples, the median change in concentration was -12% for morphine and -11% for codeine. Both for samples from living persons and post mortem samples, the decrease in the concentrations from the original analysis to reanalysis were statistically significant for morphine and codeine. Regarding 6-AM, all living samples were negative at reanalysis. For post mortem samples, four cases still tested positive for 6-AM at reanalysis with a median change in the concentrations of -81%. There was no significant change in the morphine to codeine concentration ratios neither for living nor post mortem samples. CONCLUSION This study showed that in real life whole blood samples, the concentrations of morphine and codeine are relatively stable during long-term storage at -20°C. 6-AM on the other hand, shows a considerable decrease in concentrations that is important to consider when interpreting results from reanalyses of forensic cases.

Experience from therapeutic drug monitoring and gender aspects of gabapentin and pregabalin in clinical practice

PURPOSE Gabapentin and pregabalin are antiepileptic drugs (AEDs) with epilepsy and neuropathic pain indications. The purpose of this study was to investigate pharmacokinetic variability of gabapentin and pregabalin and indications for therapeutic drug monitoring (TDM) in clinical practice with focus on gender aspects. METHOD Anonymous data from routine TDM-service at the National Center for Epilepsy regarding serum concentration measurements of gabapentin and pregabalin, 2009-2013, were utilised. All included samples were drug-fasting in the morning at steady-state. RESULTS In total, 356 patients were included; gabapentin 189 (66% women), average age 53 years and pregabalin 167 (56% women), average age 50 years. For gabapentin, mean serum concentration/dose(C/D)-ratio was similar across genders. Only 13% of the patients had concentrations above the lower limit of the reference range (70-120 μmol/L), which indicates a need for reevaluation of the reference range. For pregabalin, the C/D-ratio in women (0.08±0.06) was 42% higher than in men (0.056±0.05; p<0.05). The pharmacokinetic variability (C/D-ratio) was >100-fold for both gabapentin and pregabalin. An indication of use (epilepsy/pain/other) was stated in only 26% of the cases (n=94). Epilepsy was assumed as indication when other AEDs were also measured (50% of patients). This was similar for both genders and for both AEDs. Indications for TDM were stated in 155 cases (44%) and were similar for gabapentin and pregabalin. CONCLUSION Gabapentin and pregabalin are more used in women than in men, and routine use of TDM is most common in patients with epilepsy. Pharmacokinetic variability is extensive, highlighting a need for individualisation of therapy regardless of indication.

Long-term stability of GHB in post-mortem samples and samples from living persons, stored at −20 °C, using fluoride preservatives

PURPOSE Reanalyses are frequently requested in forensic toxicology, and knowledge of the stability of drugs in biological samples is of major importance for the interpretation of the toxicological findings. Currently, the literature on stability of gammahydroxybutyrate (GHB) in blood samples from living subjects and in post-mortem blood is limited. The purpose of this study was to evaluate the long-term stability of GHB in both blood samples from persons suspected of drug use and post-mortem blood samples. METHODS A total of 59 reanalyses were performed in whole blood samples, 27 samples from living subjects and 32 samples taken at autopsies. The samples were stored in the freezer between 0.4 and 7.2 years at -20°C in vials containing preservatives. Analyses were performed by GC-FID, and cut-off level was 10.3 mg/L. The concentrations in 22 of the samples were below cut-off. RESULTS The mean change in concentration between initial analysis and reanalysis was -0.8% for the positive samples from living persons and -7.1% for the positive post-mortem samples. Changes ranged from -32.4% to 21.0% for samples from living and from -30.4% to 34.4% for post-mortem samples. All negative samples were still negative at the time of reanalysis. CONCLUSION Reanalysis of these forensic whole blood samples stored several years at -20°C with fluoride preservation did not exhibit changes in GHB concentrations of practical significance for the interpretation of toxicological findings.

The Impact of Pharmacokinetic Interactions with Eslicarbazepine Acetate Versus Oxcarbazepine and Carbamazepine in Clinical Practice

Background: Eslicarbazepine acetate (ESL) is a new anti-epileptic drug (AED) chemically related to oxcarbazepine (OXC) and carbamazepine (CBZ) and is increasingly used in clinical practice. The purpose of the study was to investigate 2-way pharmacokinetic interactions between ESL and other AEDs as compared to OXC and CBZ. Methods: Anonymous data regarding age, gender, use of AEDs, daily doses and serum concentration measurements of ESL, OXC, CBZ and lamotrigine (LTG) and other AEDs were retrieved from 2 therapeutic drug monitoring (TDM) databases in Norway. Drugs were categorized according to their known potential for interactions. Concentration/dose (C/D) ratios were calculated. Results: Data from 1100 patients were available. The C/D ratios of ESL and OXC were unchanged in combination with enzyme-inducing AEDs or valproate (VPA). The C/D ratio of CBZ decreased by 40% and 22% in combination with other enzyme-inducing AEDs or VPA, respectively, pointing to an increased clearance. ESL demonstrated no significant enzyme-inducing effect on LTG metabolism although there was a 20% and 34% decrease in the C/D ratio of LTG in combination with OXC and CBZ, respectively. Conclusions: Possible pharmacokinetic interactions have been studied for ESL as compared to OXC and CBZ. The pharmacokinetics of ESL is not affected by enzyme-inducing AEDs or VPA and does not affect the metabolism of LTG in contrast to OXC and CBZ. The study demonstrates the value of using TDM databases to explore the potential for pharmacokinetic interactions of new AEDs.

Therapeutic Drug Monitoring of Clobazam and Its Metabolite-Impact of Age and Comedication on Pharmacokinetic Variability.

Background: Clobazam (CLB) has been used as an antiepileptic drug for several decades. There is still insufficient data regarding its pharmacokinetic variability in clinical practice. The purpose of this study was to investigate pharmacokinetic variability of CLB with emphasis on the impact of age and comedication in patients with epilepsy. Methods: Serum concentration measurements of CLB and its metabolite N-desmethylclobazam (NCLB), as well as demographic and clinical data were retrieved from the routine therapeutic drug monitoring service at the National Center for Epilepsy, Norway, 2009–2013. NCLB/CLB and total (CLB + NCLB), CLB and NCLB concentration/dose (C/D) ratios were calculated. Results: 550 patients (296 women/254 men), average age 27 years (range 1–86), were included. The interindividual pharmacokinetic variability was extensive, as illustrated by a 100-fold variability in serum concentration compared with dose (total C/D ratio 0.03–3.29 µmol·L−1·mg−1). The CLB C/D ratio was 36% lower in young children (2–9 years) than in adults (18–64 years), reflecting a higher clearance. In patients receiving phenytoin, felbamate, stiripentol, oxcarbazepine or eslicarbazepine acetate, valproate, phenobarbital, zonisamide or carbamazepine one or more of the calculated ratios were significantly different from that in patients receiving no or neutral comedications. The mean values for the different groups were in the order of 20%–230% of C/D ratios in the neutral group and 200%–950% of the NCLB/CLB ratio. Conclusions: The pharmacokinetic variability of CLB and its metabolite NCLB in clinical practice is extensive, and is influenced by drug–drug interactions, age, and pharmacogenetics. Therapeutic drug monitoring of CLB and NCLB is therefore valuable in patient management.

Pharmacokinetic variability of valproate in women of childbearing age

The purpose was to investigate pharmacokinetic variability of valproic acid (VPA) in women of childbearing age by therapeutic drug monitoring (TDM) data to elucidate the variable relationship between dose and serum concentrations with the ultimate aim of facilitating safer use of VPA. Anonymized retrospective data from the TDM database (2006–2015) at the National Center for Epilepsy in Norway were used. Trough total concentrations of VPA at assumed steady state in women aged 14–46 years were analyzed. Data from 643 nonpregnant women of childbearing age (mean age = 27 years) were included. Mean dose and serum concentration of VPA were 968 (standard deviation [SD] = 453) mg/day and 411 (SD = 138) μmol/L, respectively, and 59% used polytherapy. The pharmacokinetic variability in serum concentration/dose (C/D) ratios between women was extensive. For doses <700 mg/day (n = 202; 32%; 150–625 mg/day), mean serum concentration was 336 μmol/L and variability in C/D ratio was 10‐fold. The variability decreased with increasing dose to eightfold (≥700 to <1,500 mg/day, n = 358) and fourfold (≥1,500 mg/day, n = 96). This study demonstrates the extensive pharmacokinetic variability of VPA among women of childbearing age, which is most pronounced at low doses. In future studies, serum concentrations of VPA, rather than dosage, should be used as a guide for exposure of VPA and possible risks of teratogenicity to evaluate safety aspects of VPA in women.

Pharmacokinetic variability of valproate during pregnancy – Implications for the use of therapeutic drug monitoring

BACKGROUND AND PURPOSE Use of valproate (VPA) in women of childbearing age is restricted due to dose-dependent risk of teratogenicity. The purpose of this study was to characterise pharmacokinetic variability of VPA in pregnancy, and discuss use of therapeutic drug monitoring (TDM) as guidance to exposure in women. METHOD Measurements of trough total and unbound VPA concentrations before, during and after pregnancy, at assumed steady-state were collected from the TDM-database (2006-2016) at the National Center for Epilepsy in Norway. Additional clinical data were obtained from the Oppland county Perinatal Database (1994-2011). RESULTS Data from 51 pregnancies in 33 women aged 19-40 years were included. Each woman underwent 1-4 pregnancies, and 1-7 measurements per pregnancy were performed. The variability in total concentration/dose (C/D)-ratios between women was 13-fold, and intra-patient variability extensive. Total C/D-ratios were reduced by 46% from before pregnancy to third trimester (0.48-0.29 μmol/L/mg). Unbound concentrations of VPA were only requested in 10% of the pregnancies. Repeated measurements from two pregnancies in one women revealed increased unbound concentration of VPA during pregnancy. There were 19 with idiopathic generalized epilepsy and two focal based on clinical data from 21 women and 38 pregnancies; 1 major congenital malformation was noted. CONCLUSION There is pronounced pharmacokinetic variability of VPA during pregnancy. Unbound concentrations are rarely requested. TDM should be used by measurements of both total and unbound concentrations since total concentrations may be misleading for efficacy and fetal exposure of VPA.

Utilisation and polypharmacy aspects of antiepileptic drugs in elderly versus younger patients with epilepsy: A pharmacoepidemiological study of CNS-active drugs in Norway, 2004-2015

BACKGROUND AND PURPOSE Many patients with epilepsy use antiepileptic drugs (AEDs) in combination. The elderly is a vulnerable group regarding polypharmacy. The purpose of this study was to investigate changes in utilisation of AEDs, and the extent of polypharmacy with other CNS-active drugs in elderly versus younger patients in Norway. METHODS This pharmacoepidemiological study included all prescriptions of antiepileptic, antidepressant and antipsychotic drugs from Norwegian pharmacies in the Norwegian Prescription Database (NorPD) (2004-2015). Variables included number of patients, utilisation in defined daily doses, age, gender, and diagnosis specific reimbursement codes for AEDs. RESULTS The use of AEDs has increased in all age groups in this population-based study in Norway. In the elderly, AEDs used in neuropathic pain (mainly gabapentin and pregabalin) have increased more than 10-fold (from 0.7 to 9.6 DDDs/1000 elderly/day, 2004-2015), while the prevalence of users is four times more than in younger patients. Polypharmacy between antiepileptic, antidepressant and antipsychotic drugs occurred in 35% of elderly and 38% of younger patients with epilepsy. The use of enzyme-inducers was common, and occurred more often in elderly patients. A total of 42 different interactions that may have clinical implications were identified among these drugs. CONCLUSION The use of AEDs in elderly compared to younger patients is increasing, especially in neuropathic pain. Polypharmacy with antiepileptic, antidepressant and/or antipsychotic drugs was documented in more than one third of the patients. Awareness of increased drug utilisation, polypharmacy with potential drug interactions, and focus on elderly patients are important for increased patient safety.

Reference ranges for antiepileptic drugs revisited: a practical approach to establish national guidelines

Background and objective Laboratories sometimes use different reference ranges for the same antiepileptic drug (AED), particularly for new and poorly investigated drugs. This may contribute to misunderstandings, concerns or inappropriate dose changes, which in turn may affect therapeutic effect, drug safety or treatment adherence. Therefore, the Norwegian Association of Clinical Pharmacology wished to update and harmonize the reference ranges for AEDs and establish national guidelines for Norway. Methods A working group collected information on the reference ranges used by Norwegian laboratories for all commonly used AEDs. These reference ranges were compared to recent recommendations by the International League Against Epilepsy, current literature, applicable clinical studies, reference ranges used by leading Northern European epilepsy centers outside of Norway, and routine data derived from Norwegian laboratory databases. Results Reference ranges varied between laboratories for four of 23 available AEDs (lamotrigine, valproate, eslicarbazepine and oxcarbazepine). For four AEDs (brivaracetam, perampanel, stiripentol and sulthiame), reference ranges had not previously been established. In total, 13 reference ranges were either harmonized, updated or newly established. No changes were applied to the remaining 10 AEDs. Conclusion Updated and harmonized reference ranges are now available for 22 of the 23 AEDs available in Norway. The exception is vigabatrin (reference range not applicable). Revision of reference ranges is an important part of pharmacovigilance of AEDs and must be a continuous process based on current literature and clinical experience.

Felles referanseområder for antiepileptika

ARNE REIMERS E-mail: arne.reimers@stolav.no Arne Reimers (born 1965), Dr. Philos., is a specialist in clinical pharmacology and senior consultant at the Department of Clinical Pharmacology, St. Olavs University Hospital, and associate professor at the Department of Laboratory Medicine, Children’s and Women’s Health, Norwegian University of Science and Technology. The author has completed the ICMJE form and reports no conflicts of interest.