Angela K. Birnbaum

Pharmacological and clinical aspects of antiepileptic drug use in the elderly

In this article, epidemiological and clinical aspects related to the use of antiepileptic drugs (AEDs) in the elderly are highlighted. Studies have shown that people with epilepsy receiving AED treatment show important deficits in physical and social functioning compared with age-matched people without epilepsy. To what extent these deficits can be ascribed to epilepsy per se or to the consequences of AED treatment remains to be clarified. The importance of characterizing the effects of AEDs in an elderly population is highlighted by epidemiological surveys indicating that the prevalence of AED use is increased in elderly people, particularly in those living in nursing homes. Both the pharmacokinetics and the pharmacodynamics of AEDs may be altered in old age, which may contribute to the observation that AEDs are among the drug classes most commonly implicated as causing adverse drug reactions in an aged population. Age alone is one of several contributors to alterations in AED response in the elderly; other factors include physical frailty, co-morbidities, dietary influences, and drug interactions. Individualization of dosage, avoidance of unnecessary polypharmacy, and careful observation of clinical response are essential for an effective and safe utilization of AEDs in an elderly population.

High-speed simultaneous determination of nine antiepileptic drugs using liquid chromatography-mass spectrometry.

Therapeutic drug monitoring of antiepileptic drugs (AEDs) is important and widely practiced. However, simultaneous AED assays usually concentrate only on old or new AEDs. A new simultaneous assay was developed to monitor both older and newer AEDs in the same sample. This assay measures zonisamide (ZNS), lamotrigine (LTG), topiramate (TPM), phenobarbital (PB), phenytoin (PHT), carbamazepine (CBZ), carbamazepine-10,11-diol (CBZ-Diol), 10-hydroxycarbamazepine (MHD), and carbamazepine-10,11-epoxide (CBZ-E). Sample pretreatment consisted of a single solid-phase extraction (SPE) for all AEDs in a 100-μL plasma sample. HPLC separation was achieved on a Shimdazu Shimpack XR-ODS (4.6 id × 50 mm, 2.2-μm) column with a gradient mobile phase of acetate buffer, methanol, acetonitrile, and tetrahydrofuran. Four internal standards were used. Detection was achieved by atmospheric pressure chemical ionization mass spectrometry (APCI-MS) in selected-ion monitoring (SIM) mode with constant polarity switching. High recovery (88%-96%) was obtained for all compounds by SPE. Linearity was observed throughout an 80-fold concentration range, with correlation coefficient (r2) values higher than 0.99 for all AEDs. For the standards, the accuracy ranged from 89.3% to 111.8%. The within-run coefficient of variation (CVw) value was ≤9.7%, the between-run coefficient of variation (CVb) was ≤16.2%, and the total variability (CVt) was ≤16.8%. For the quality controls (QCs), accuracy ranged from 89.3% to 108.8%, CVw was ≤9.6%, CVb was ≤14.1%, and CVt was ≤15.1%. The correlation r2 values for comparison of this assay with existing validated assays in our laboratory (GC-MS or LC-MS) were 0.95, 0.91, 0.87, 0.95, and 0.95 for PHT, LTG, CBZ, CBZ-E, and CBZ-Diol, respectively.

Dosing equation for tacrolimus using genetic variants and clinical factors

AIM To develop a dosing equation for tacrolimus, using genetic and clinical factors from a large cohort of kidney transplant recipients. Clinical factors and six genetic variants were screened for importance towards tacrolimus clearance (CL/F). METHODS Clinical data, tacrolimus troughs and corresponding doses were collected from 681 kidney transplant recipients in a multicentre observational study in the USA and Canada for the first 6 months post transplant. The patients were genotyped for 2,724 single nucleotide polymorphisms using a customized Affymetrix SNP chip. Clinical factors and the most important SNPs (rs776746, rs12114000, rs3734354, rs4926, rs3135506 and rs2608555) were analysed for their influence on tacrolimus CL/F. RESULTS The CYP3A5*1 genotype, days post transplant, age, transplant at a steroid sparing centre and calcium channel blocker (CCB) use significantly influenced tacrolimus CL/F. The final model describing CL/F (l h(-1)) was: 38.4 ×[(0.86, if days 6-10) or (0.71, if days 11-180)]×[(1.69, if CYP3A5*1/*3 genotype) or (2.00, if CYP3A5*1/*1 genotype)]× (0.70, if receiving a transplant at a steroid sparing centre) × ([age in years/50](-0.4)) × (0.94, if CCB is present). The dose to achieve the desired trough is then prospectively determined using the individuals CL/F estimate. CONCLUSIONS The CYP3A5*1 genotype and four clinical factors were important for tacrolimus CL/F. An individualized dose is easily determined from the predicted CL/F. This study is important towards individualization of dosing in the clinical setting and may increase the number of patients achieving the target concentration. This equation requires validation in an independent cohort of kidney transplant recipients.

Variability of total phenytoin serum concentrations within elderly nursing home residents.

Background: Approximately 6% of all elderly nursing home residents receive phenytoin. Phenytoin concentrations are often measured to guide therapy. Objective: To evaluate the intraresident variability among multiple measurements of total phenytoin serum concentrations in nursing home residents. Methods: This was an observational study of 56 elderly (≥65 years) nursing home residents from 32 nursing homes who had at least 3 phenytoin concentrations measured while on the same dose of phenytoin for at least 4 weeks and who were not taking any interfering concomitant medications. These were a subset of 387 elderly nursing home residents from 112 nursing homes across the United States who had total phenytoin concentration measurements between June 1998 and December 2000. Results: The mean age was 80.1 years (range, 65 to 100 years) and 58.9% were women. The mean daily dose of phenytoin per resident was 4.9 ± 1.5 mg/kg. Total phenytoin concentrations within an elderly nursing home resident varied as much as two- to threefold, even though there was no change in dose. The person with the smallest variability had a minimum concentration of 10.0 μg/mL and a maximum of 10.4 μg/mL. The person with the largest variability had a minimum concentration of 9.7 μg/mL and a maximum of 28.8 μg/mL. Conclusions: There is considerable variability in the total phenytoin concentrations in the elderly nursing home resident and measurement of a single total phenytoin concentration should not be used to guide treatment.

Epilepsy in the elderly

The elderly, often defined as those 65 years or older, are the most rapidly growing segment of the population, and onset of epilepsy is higher in this age group than in any other. This paper reviews recent developments, including a new proposed definition of epilepsy, a transgenic mouse model of Alzheimers disease that exhibits complex partial seizures, evidence that the highest incidence of epilepsy may occur after admission to a nursing home, a challenge to the vitamin D hypothesis of osteoporosis associated with antiepileptic drugs (AEDs), evidence that the genetic complement of hepatic isoenzymes is more predictive of metabolic rate than age, and data showing that there is considerable variability in serum levels of AEDs in many nursing home residents during constant dosing conditions.

Valproic acid pathway: pharmacokinetics and pharmacodynamics

Valproic acid (VPA) is a branched short-chain fatty acid derived from naturally occurring valeric acid. VPA is used primarily in the treatment of epilepsy and seizures, but is also used in migraine, bipolar, mood, anxiety, and psychiatric disorders [1]. Recent work has explored its use as an adjuvant agent in cancer, HIV therapy, and neurodegenerative disease because of its action as histone deacetylase (HDAC) inhibitor [2]. VPA is widely used in pediatric epilepsy because of its multiple mechanisms of action and acceptable safety profile [3]. The dose requirements for VPA are highly variable (10-fold differences in mean dose in adults) [4] and interactions with other drugs are common (discussed below). Therapeutic drug monitoring is commonly used, although both the clinical and toxic effects of the drug are considered to be poorly correlated with total serum concentrations [3]. The drug label carries a black box warning for life-threatening adverse drug reactions (ADR) including hepatoxocity, teratogenicity, and pancreatitis [1]. Compared with adults, children appear to be at an increased risk for severe hepatotoxic reactions to VPA, in particular those younger than 2 years of age undergoing polytherapy, with existing developmental delays and coincident metabolic disorders [1]. Hyperammonemia is also a documented ADR of VPA treatment, although this is usually successfully resolved by cessation of VPA and treatment with carnitine [5]. The Food and Drug Administration has issued recommendations for patient testing and advises that VPA is contraindicated in patients with known urea cycle disorders (see Clinical PGx tab at http://www.pharmgkb.org/drug/PA451846). However, specific genetic tests for the determination of at-risk patients are not mentioned. The aim of this study was to introduce candidate genes in the pharmacokinetics (PK) (Fig. 1), pharmacodynamics (PD) of VPA (Fig. 2), and discuss results from pharmacogenomic studies so far. Fig. 1 Graphic representation of the candidate genes involved in valproic acid (VPA) pharmacokinetics. A fully interactive version of this pathway is available online at PharmGKB at http://www.pharmgkb.org/pathway/PA165964265. CYP, cytochrome P450. Fig. 2 Graphic representation of the candidate genes involved in valproic acid (VPA) pharmacodynamics. A fully interactive version of this pathway is available online at PharmGKB at http://www.pharmgkb.org/pathway/PA165959313. Pharmacokinetics VPA is highly protein bound (87–95%) resulting in low clearance (6–20 ml/h/kg) [6]. There are at least three routes of VPA metabolism in humans: glucuronidation, β oxidation in the mitochondria (both considered major routes accounting for 50 and 40% of dose, respectively), and cytochrome P450 (CYP)-mediated oxidation (considered a minor route, ~10%) [7-9]. Valproate glucuronide is the major urinary metabolite of VPA (~30–50%) [8]. In-vitro studies of human liver microsomes and purified recombinant proteins have reported glucuronidation of VPA by UGT1A3, UGT1A4, UGT1A6, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B15 [8,10,11]. Other studies have disputed the role of UGT2B15, suggesting that VPA inhibits UGT2B15, but is not glucuronidated by it [12]. UGT1A1 does not have activity against VPA in vitro [8,12]. VPA is a fatty acid and can be metabolized through endogenous pathways in the mitochondria (Fig. 1). It has been observed that some of the mitochondrial metabolites of VPA generated by this pathway are hepatotoxic. The current understanding of VPA bioactivation involves the entry of 4-ene-VPA into the mitochondria, formation of a 4-ene-VPA-CoA ester with the help of ACADSB, and subsequent β-oxidation to form the reactive 2,4-diene-VPA-CoA ester [13,14]. Studies have demonstrated that the β-oxidation is blocked in fluorinated derivatives of 4-ene-VPA [15], and that the fluoro derivative of 4-ene-VPA cannot form a CoA ester [16], indicating a specific role of β oxidation of 4-ene-VPA in the formation of the 2,4-diene metabolite. This putative cytotoxic metabolite (2,4-diene-VPA-S-CoA) further gets conjugated with glutathione to form thiol conjugates. These chemically reactive metabolites generated from 4-ene-VPA have the potential to deplete mitochondrial glutathione pools [13] and form conjugates with CoA [17], in turn inhibiting enzymes in the β-oxidation pathway [18,19]. Identification of N-acetylcysteine conjugates of (E)-2,4-diene-VPA in human urine demonstrated that the reactive thiol conjugates of VPA arise primarily from the biotransformation of (E)-2,4-diene VPA in humans [20]. The key CYP-mediated branch of the VPA pathway is the generation of the metabolite 4-ene-VPA by CYP2C9, CYP2A6, and to a lesser extent by CYP2B6 [21,22]. In addition, these metabolizing enzymes also mediate the metabolism of VPA to the inactive 4-OH-VPA and 5-OH-VPA [23]. CYP2A6 also contributes partially to the formation of 3-OH-VPA [22].

Association of carbamazepine major metabolism and transport pathway gene polymorphisms and pharmacokinetics in patients with epilepsy

AIM The aim of this study was to evaluate the association of genetic variants in the major genes involved in carbamazepine (CBZ) metabolism and transport with its pharmacokinetics in epilepsy patients. MATERIALS & METHODS Twenty-five SNPs within seven CBZ pathway genes, namely CYP3A4, CYP3A5, EPHX1, NR1I2, UGT2B7, ABCB1 and ABCC2, were analyzed for association with CBZ pharmacokinetics in 90 epilepsy patients. RESULTS The CYP3A4*1B SNP was significantly associated with CBZ clearance. Significant association of EPHX1 SNPs was observed with greater carbamazepine-10,11-trans dihydrodiol:carbamazepine 10-11 epoxide ratios. Among drug transporters, ABCB1 and ABCC2 SNPs were significantly associated with altered CBZ clearance. CONCLUSION SNPs within CBZ pathway genes contribute to interpatient variation in CBZ pharmacokinetics and might contribute to pharmacoresistant epilepsy. Although our results need further clinical validation in a larger patient cohort, they indicate that genetic variation in CBZ pathway genes could influence its pharmacokinetics, and hence would have clinical significance.

Co-administration of morphine and oxycodone vaccines reduces the distribution of 6-monoacetylmorphine and oxycodone to brain in rats

Opioid conjugate vaccines have shown promise in animal models as a potential treatment for opioid addiction. Individual vaccines are quite specific and each targets only a limited number of structurally similar opioids. Since opioid users can switch or transition between opioids, we studied a bivalent immunization strategy of combining 2 vaccines that could target several of the most commonly abused opioids; heroin, oxycodone and their active metabolites. Morphine (M) and oxycodone (OXY) haptens were conjugated to keyhole limpet hemocyanin (KLH) through tetraglycine (Gly)(4) linkers at the C6 position. Immunization of rats with M-KLH alone produced high titers of antibodies directed against heroin, 6-monoacetylmorphine (6-MAM) and morphine. Immunization with OXY-KLH produced high titers of antibodies against oxycodone and oxymorphone. Immunization with the bivalent vaccine produced consistently high antibody titers against both immunogens. Bivalent vaccine antibody titers against the individual immunogens were higher than with the monovalent vaccines alone owing, at least in part, to cross-reactivity of the antibodies. Administration of a single concurrent intravenous dose of 6-MAM and oxycodone to rats immunized with the bivalent vaccine increased 6-MAM, morphine and oxycodone retention in serum and reduced the distribution of 6-MAM and oxycodone to brain. Vaccine efficacy correlated with serum antibody titers for both monovalent vaccines, alone or in combination. Efficacy of the individual vaccines was not compromised by their combined use. Consistent with the enhanced titers in the bivalent group, a trend toward enhanced pharmacokinetic efficacy with the bivalent vaccine was observed. These data support the possibility of co-administering two or more opioid vaccines concurrently to target multiple abusable opioids without compromising the immunogenicity or efficacy of the individual components.

Zonisamide discontinuation due to psychiatric and cognitive adverse events A case-control study

Objectives: Zonisamide (ZNS) is an antiepileptic drug (AED) that has been associated with psychiatric adverse events (PAE) and cognitive adverse events (CAE); controlled studies evaluating these adverse events are limited. Our objectives were to 1) determine the incidence of PAE and CAE leading to the discontinuation of ZNS and 2) identify risk factors for PAE and CAE associated with the discontinuation of ZNS. Methods: All patients exposed to ZNS at MINCEP Epilepsy Care between March 2000 and September 2008 were identified. Reasons for discontinuing ZNS were documented. Separate case-control studies were performed to identify risk factors associated with the discontinuation of ZNS due to PAE or CAE via multivariate binary logistic regression. Results: A total of 544 patients were exposed to ZNS during the study period. PAE and CAE were the most frequently identified reasons for terminating ZNS therapy. The incidence of PAE severe enough to be associated with the discontinuation of ZNS was 6.9%; the incidence of CAE was 5.8%. Factors associated with termination of ZNS therapy due to PAE were past psychiatric history (p = 0.005), symptomatic generalized epilepsy (p = 0.027), and lower maximum ZNS serum concentration (mean = 17.9 mg/L vs 34.7 mg/L, p < 0.001). Independent variables associated with discontinuing ZNS due to CAE were greater number of concomitant AEDs (p = 0.011) and lower maximum ZNS serum concentration (mean = 16.6 mg/L vs 30.6 mg/L, p = 0.002). Conclusions: We have identified clinically relevant risk factors associated with the discontinuation of ZNS. Our findings support the concept that selected patients are relatively more vulnerable to CNS adverse events when exposed to ZNS.

Cytochrome P450 isozymes 3A4 and 2B6 are involved in the in vitro human metabolism of thiotepa to TEPA

Purpose. To establish the cytochrome P450 (CYP) isozymes involved in the metabolism of the alkylating agent, thiotepa, to the pharmacologically active metabolite, TEPA.Methods. In vitro chemical inhibition studies were conducted by incubating thiotepa and pooled human hepatic microsomes in the presence of known inhibitors to CYP1A2, CYP2A6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. Studies were also performed with cloned, expressed CYP3A4, CYP2A6, CYP2E1 and CYP2B6 microsomes, and anti-CYP2B6 monoclonal antibody.Results. Known CYP3A4 inhibitors reduced TEPA production. Inhibition with CYP2E1 inhibitors was inconsistent. All other inhibitors produced little or no change in TEPA formation. Cloned, expressed CYP2B6 and CYP3A4 microsomes catalyzed TEPA formation, whereas CYP2A6 and CYP2E1 did not. Incubation of thiotepa with anti-CYP2B6 antibody and cloned, expressed CYP2B6 microsomes resulted in reductions in the formation of TEPA, but no change in TEPA formation occurred in human liver microsomes.Conclusions. Thiotepa is metabolized in human liver microsomes by CYP3A4 (major) and CYP2B6 (minor). There is a potential for CYP-mediated drug interactions with thiotepa. Pharmacokinetic variability of thiotepa may be related to expression of hepatic CYP isozymes.