Acute Seizures Due to a Probable Interaction between Valproic Acid and Meropenem

Effect of a Declined Plasma Concentration of Valproic Acid Induced by Meropenem on the Antiepileptic Efficacy of Valproic Acid

OBJECTIVE

This study aimed to indicate whether a declined plasma concentration of valproic acid (VPA) induced by co-administration of meropenem (MEPM) could affect the antiepileptic efficacy of VPA.

METHODS

We retrospectively reviewed data of hospitalized patients who were diagnosed with status epilepticus or epilepsy between 2010 and 2019. Patients co-administered VPA and MEPM during hospitalization were screened and assigned to the exposure group, while those co-administerd VPA and other broad-spectrum antibiotics were allocated to the control group.

RESULTS

The exposure group and control group included 50 and 11 patients, respectively. With a similar dosage of VPA, the plasma concentration of VPA significantly decreased during co-administration (24.6 ± 4.3 μg/mL) compared with that before co-administration (88.8 ± 13.6 μg/mL, p < 0.0001), and it was partly recovered with the termination of co-administration (39.8 ± 13.2 μg/mL, p = 0.163) in the exposure group. The inverse probability of treatment weighting estimated the treatment efficacy via changes in seizure frequency, seizure duration, and concomitant use of antiepileptic drugs, which were not significantly different between the exposure and control groups. In the exposure group, there was no significant differences in seizure frequency between the periods of before-during and before-after (p = 0.074 and 0.153, respectively). Seizure duration during VPA-MEPM co-administration was not significantly different from that before co-administration (p = 0.291).

CONCLUSIONS

In this study, the reduced plasma concentration of VPA induced by the co-administration of MEPM did not affect the antiepileptic efficacy of VPA. This conclusion should be interpreted with caution, and more research is warranted.

TRIAL REGISTRATION

Chinese Clinical Trial Registry: ChiCTR2000034567. Registered on 10 July 2020.

Clinical impact of carbapenems in critically ill patients with valproic acid therapy: A propensity-matched analysis

Background

Valproic acid (VPA) is one of the most widely used broad-spectrum antiepileptic drugs, and carbapenems (CBPs) remain the drug of choice for severe infection caused by multidrug-resistant bacteria in critically ill patients. The interaction between VPA and CBPs can lead to a rapid depletion of serum VPA level. This may then cause status epilepticus (SE), which is associated with significant mortality. However, the prognostic impact of drug interactions in critically ill patients remains an under-investigated issue.

Objective

The aim of this study was to compare the prognosis of critically ill patients treated with VPA and concomitant CBPs or other broad-spectrum antibiotics.

Methods

Adult patients admitted to a medical center intensive care unit between January 2007 and December 2017 who concomitantly received VPA and antibiotics were enrolled. The risk of reduced VPA serum concentration, seizures and SE, mortality rate, length of hospital stay (LOS), and healthcare expenditure after concomitant administration were analyzed after propensity score matching.

Results

A total of 1,277 patients were included in the study, of whom 264 (20.7%) concomitantly received VPA and CBPs. After matching, the patients who received CBPs were associated with lower VPA serum concentration (15.8 vs. 60.8 mg/L; p &lt; 0.0001), a higher risk of seizures (51.2 vs. 32.4%; adjusted odds ratio [aOR], 2.19; 95% CI, 1.48–3.24; p &lt; 0.0001), higher risk of SE (13.6 vs. 4.7%; aOR, 3.20; 95% CI, 1.51–6.74; p = 0.0014), higher in-hospital mortality rate (33.8 vs. 24.9%; aOR, 1.57; 95% CI, 1.03–2.20; p = 0.036), longer LOS after concomitant therapy (41 vs. 30 days; p &lt; 0.001), and increased healthcare expenditure (US$20,970 vs. US$12,848; p &lt; 0.0001) than those who received other broad-spectrum antibiotics.

Conclusion

The administration of CBPs in epileptic patients under VPA therapy was associated with lower VAP serum concentration, a higher risk of seizures and SE, mortality, longer LOS, and significant utilization of healthcare resources. Healthcare professionals should pay attention to the concomitant use of VPA and CBPs when treating patients with epilepsy. Further studies are warranted to investigate the reason for the poor outcomes and whether avoiding the co-administration of VPA and CBP can improve the outcomes of epileptic patients.

Clinical Evaluation of Meropenem-Vaborbactam Combination for the Treatment of Urinary Tract Infection: Evidence to Date

As antimicrobial resistance continues to grow, one of the biggest threats includes the members of the Enterobacterales order presenting with carbapenem resistance (CRE). Meropenem-vaborbactam, along with other beta-lactam/beta-lactamase agents, has been developed to help combat this growing concern and is currently approved to treat complicated urinary tract infections (cUTI), as well as acute pyelonephritis (AP), in the USA. Vaborbactam is a novel beta-lactamase inhibitor designed specifically to optimize and restore the activity of meropenem against resistant Enterobacterales. Vaborbactam inhibits a number of beta-lactamases, including in vitro activity against extended-spectrum beta-lactamases (ESBL) and the Klebsiella pneumoniae carbapenemase (KPC) group. KPC represents one of the most clinically relevant carbapenemase in the USA, accounting for the majority of carbapenemase-producing CRE. Meropenem-vaborbactam has been studied in the two Phase 3, noninferiority trials, TANGO I and TANGO II. TANGO I compared meropenem-vaborbactam against piperacillin-tazobactam in patients with cUTIs and was found to be noninferior for overall success and microbial eradication. TANGO II expanded to other disease states (bacteremia, hospital-acquired/ventilator-associated bacterial pneumonia [HAP/VAP], complicated intra-abdominal infection [cIAI], cUTI/AP) and was found to be noninferior against best available therapy (BAT) with respect to clinical cure at the end of treatment and the test of cure. Meropenem-vaborbactam maintained the established safety profile of meropenem alone, with headache as the most common adverse event in both phase 3 studies. Overall, clinical efficacy has been demonstrated and suggests the use of meropenem-vaborbactam for the treatment of cUTI is an option.

Effect of drug interactions between carbapenems and valproate on serum valproate concentration: a systematic review and meta-analysis

Background: Concurrent use of valproate and carbapenem antibiotics may decrease serum valproate concentration (SVC). This study evaluated the effects of carbapenem-valproate drug interactions. Research design and methods: We screened PubMed, EMBASE, and Cochrane databases for eligible prospective or retrospective studies that evaluated the effect of concurrent use of carbapenem and valproate compared with valproate alone on SVC. Primary outcomes were the change in SVC from before the addition of the carbapenem to the SVC during the use of carbapenems and after carbapenem discontinuation, and seizure-related outcomes. Secondary outcomes were the influence of valproate or carbapenem dose on SVC and Drug Interaction Probability Scale scores. Results: Twelve studies (633 patients) were included. Compared with valproate alone, combination treatment with carbapenem substantially decreased mean SVC (mean difference, -43.98 mg/L; 95% confidence interval, -48.18 to -39.78). The onset of SVC decreases was within 1-3 days following carbapenem initiation. Seizure frequency increased by 26.3% during combination treatment. No difference was found in mean SVC between the different doses of valproate or carbapenem during combination treatment. Mean SVC increased to similar pre-carbapenem level within 1 to 2 weeks after carbapenem discontinuation. Conclusions: The drug interaction between valproate and carbapenem causes substantial SVC decreases, even to subtherapeutic levels, which may increase the risk of seizures.

Valproate Interaction With Carbapenems: Review and Recommendations

Introduction: Valproic acid is a commonly used antiepileptic drug. Combining valproate derivatives with carbapenem antibiotics is associated with a potential drug interaction that decreases serum concentration of valproate and may expose the patient to uncontrolled seizure risk from valproate subtherapeutic concentration. Raising awareness of this drug interaction among health care providers including emergency department physicians, neurologists, and pharmacists is highly needed. The aim of this article was to review the current literature about the potential drug interaction resulting from combining valproate derivatives with carbapenem antibiotics and to establish therapeutic recommendations regarding their use together. Methods: A review of the literature was conducted using Medline (through PubMed), Ovid, Embase, Cochrane library using the following keywords: valproate, valproic acid, carbapenem, ertapenem, doripenem, meropenem, imipenem, and valproate drug interaction. In addition, a manual search through major journals for articles referenced in PubMed was performed. Related publications from January 1998 till November 2018 were included in the initial search. Relevant publications were reviewed, and data regarding patients, type of carbapenem used, valproic acid dosing and level, interaction severity, and clinical outcome were summarized. Results and Discussion: Few clinical trials and multiple case reports have shown that carbapenem antibiotics including meropenem, ertapenem, imipenem, and doripenem can decrease the serum concentration of valproate derivatives leading to a subtherapeutic serum concentration and seizures in some patients. Valproic acid serum concentration may be significantly decreased with addition of a carbapenem antibiotic but generally return toward normal shortly after discontinuation of the carbapenem antibiotic. Conclusions: Generally, the concurrent use of carbapenem antibiotics with valproate derivatives should be avoided due to the potential of drug-drug interaction that results in subtherapeutic valproate serum concentration. Other antimicrobial agents should be considered as alternatives to carbapenems but if a concurrent carbapenem is necessary, using an additional antiepileptic agent is recommended. Therapeutic drug monitoring of valproate serum concentrations is warranted when a carbapenem-valproic acid combination therapy is unavoidable.

Pharmacokinetic evaluation of meropenem and vaborbactam for the treatment of urinary tract infection

INTRODUCTION

Meropenem/vaborbactam (M/V) represents the first carbapenem and β-lactamase inhibitor combination approved for treatment of complicated urinary tract infections (cUTIs), including pyelonephritis. Vaborbactam is a novel boronic acid, β-lactamase inhibitor with a high affinity for serine β-lactamases, including Klebsiella pneumoniae carbapenemase (KPC). This combination, Vabomere™, was approved in August 2017 by the United States Food and Drug Administration for the treatment of cUTIs in patients 18 years or older, including pyelonephritis, caused by the following susceptible microorganisms: Escherichia coli, K. pneumoniae, and Enterobacter cloacae species complex. Areas covered: Relevant literature regarding microbiology, pharmacokinetics, pharmacodynamics, and clinical trials evaluating efficacy, safety, and tolerability will be discussed. Expert opinion: Current treatment options for KPC-producing infections such as aminoglycosides, polymyxins, fosfomycin, and tigecycline are associated with concerns regarding efficacy, toxicities, optimal dosing, and/or development of resistance. Additionally, resistance to the new combination product of ceftazidime/avibactam has also emerged. Current clinical evidence supporting the use of M/V for KPC-producing infections is limited to an open-label, randomized, phase III study in a small number of patients with serious infections due to carbapenem-resistant Enterobacteriaceae. Although M/V is not approved for KPC-producing infections, we believe that M/V will become a preferred agent for KPC-producing Enterobacteriaceae infections.

Drug Interactions in Neurocritical Care

Drug-drug interactions (DDIs) are common and avoidable complications that are associated with poor patient outcomes. Neurocritical care patients may be at particular risk for DDIs due to alterations in pharmacokinetic profiles and exposure to medications with a high DDI risk. This review describes the principles of DDI pharmacology, common and severe DDIs in Neurocritical care, and recommendations to minimize adverse outcomes. A review of published literature was performed using PubMed by searching for 'Drug Interaction' and several high DDI risk and common neurocritical care medications. Key medication classes included anticoagulants, antimicrobials, antiepileptics, antihypertensives, sedatives, and selective serotonin reuptake inhibitors. Additional literature was also reviewed to determine the risk in neurocritical care and potential therapeutic alternatives. Clinicians should be aware of interactions in this setting, the long-term complications, and therapeutic alternatives.

Neurological Adverse Effects Attributable to β-Lactam Antibiotics: A Literature Review

β-lactam antibiotics are commonly prescribed antibiotic drugs. To describe the clinical characteristics, risk markers and outcomes of β-lactam antibiotic-induced neurological adverse effects, we performed a general literature review to provide updated clinical data about the most used β-lactam antibiotics. For selected drugs in each class available in France (ticarcillin, piperacillin, temocillin, ceftazidime, cefepime, cefpirome, ceftaroline, ceftobiprole, ceftolozane, ertapenem and aztreonam), a systematic literature review was performed up to April 2016 via an electronic search on PubMed. Articles that reported original data, written in French, Spanish, Portuguese or English, with available individual data for patients with neurological symptoms (such as seizure, disturbed vigilance, confusional state, myoclonia, localising signs, and/or hallucinations) after the introduction of a β-lactam antibiotic were included. The neurological adverse effects of piperacillin and ertapenem are often described as seizures and hallucinations (>50 and 25% of cases, respectively). Antibiotic treatment is often adapted to renal function (>70%), and underlying brain abnormalities are seen in one in four to one in three cases. By contrast, the neurological adverse drug reactions of ceftazidime and cefepime often include abnormal movements but few hallucinations and seizures. These reactions are associated with renal insufficiency (>80%) and doses are rarely adapted to renal function. Otherwise, it appears that monobactams do not have serious neurological adverse drug reactions and that valproic acid and carbapenem combinations should be avoided. The onset of disturbed vigilance, myoclonus, and/or seizure in a patient taking β-lactam antibiotics, especially if associated with renal insufficiency or underlying brain abnormalities, should lead physicians to suspect adverse drug reactions and to consider changes in antibacterial therapy.

Effect of Multi Drug Resistance Protein 4 (MRP4) Inhibition on the Pharmacokinetics and Pharmacodynamics of Ciprofloxacin in Normal and Rats with LPS-Induced Inflammation

BACKGROUND AND OBJECTIVES

Infection and inflammation are known to cause wide variability in disposition of drugs through modulation of drug transporters. However, the effects of inhibition of multidrug resistance protein 4 (MRP4) on pharmacokinetics and pharmacodynamics are poorly understood in normal and inflamed conditions. We hypothesized that inflammation alters the pharmacokinetic parameters of ciprofloxacin; and Pharmacokinetic/Pharmacodynamic indices, such as ratio of peak plasma concentration to minimum inhibitory concentration (C _max/MIC) and ratio of area under the plasma drug concentration-time curve to minimum inhibitory concentration (AUC/MIC) of ciprofloxacin will be improved with the co-administration of a MRP4 inhibitor, dipyridamole, in inflammatory conditions.

METHODS

In this study, the role of MRP4 on the pharmacokinetic and pharmacodynamic parameters of ciprofloxacin was investigated by the co-administration of dipyridamole in rats with or without lipopolysaccharide (LPS)-induced inflammation. The pharmacokinetic parameters for ciprofloxacin were calculated by non-compartmental approach. MIC of ciprofloxacin was determined using broth microdilution technique.

RESULTS

Induction of inflammation in rats resulted in marked reduction in C _max and AUC; and an increase in the volume of distribution (V _d/F) and clearance (Cl/F) of ciprofloxacin, compared to normal rats. Co-administration of dipyridamole with ciprofloxacin in inflamed rats resulted in a threefold increase in AUC, a twofold decrease in V _d/F and a threefold decrease in Cl/F of ciprofloxacin with significantly prolonged half-life compared to inflamed rats who received ciprofloxacin alone. Co-administration of dipyridamole enhanced AUC/MIC values of ciprofloxacin in both normal and inflamed rats.

CONCLUSIONS

The results suggest that MRP4 inhibition increases the systemic exposure of ciprofloxacin in both normal and inflammatory conditions.

The Effect of Different Carbapenem Antibiotics (Ertapenem, Imipenem/Cilastatin, and Meropenem) on Serum Valproic Acid Concentrations

BACKGROUND

Carbapenem antibiotics (CBPMs) may significantly reduce the serum concentration of valproic acid (VPA), but the extent of this effect among various CBPMs is unknown. This study compared the extent and onset of the interactions among ertapenem, imipenem/cilastatin, and meropenem.

METHODS

A 5-year retrospective study was performed. Hospitalized patients over 18 years old who received VPA and a CBPM concurrently were enrolled via the pharmacy computer system. Patients who lacked VPA serum concentration measurements before or during CBPMs' use, had concurrent medication(s) that might interfere with VPA metabolism, or had a history of liver cirrhosis were excluded. Total VPA serum concentrations before and during CBPMs' use and after its discontinuation were recorded, and differences among various CBPMs were analyzed.

RESULTS

Fifty-two patients were included in this analysis. Irrespective of the route of administration, VPA serum concentrations were subtherapeutic in 90% of the subjects during CBPMs' use. There was a significant decrease (P < 0.001) in VPA serum concentrations during the use of CBPMs: 72% ± 17%, 42% ± 22%, and 67% ± 19% in the ertapenem (N = 9), imipenem/cilastatin (N = 17), and meropenem (N = 26) groups, respectively. The effect of ertapenem and meropenem on VPA was significantly more expressed than that of imipenem/cilastatin (P < 0.005). The onset of this drug interaction occurred within 24 hours of CBPMs' administration, and VPA serum concentrations returned to 90% of baseline within 7 days of CBPMs' discontinuation along with a 20% increase in VPA dose. Increasing VPA dose during the use of ertapenem or meropenem did not result in elevating VPA serum concentrations to therapeutic levels during the combined therapy period.

CONCLUSIONS

CBPMs reduced VPA serum concentration within 24 hours of administration by approximately 60%. Ertapenem and meropenem had a greater effect on VPA serum concentration than imipenem/cilastatin. Because of the dramatic reduction of VPA serum concentration during CBPMs' use, concomitant use of VPA and CBPMs should be avoided.

Carbapenems and valproate: A consumptive relationship

A clinical case series is presented to characterize the interaction between carbapenem antibiotics and sodium valproate. Six illustrative cases are presented in which carbapenem therapy led to the rapid depletion of serum valproate levels, and one case is presented to demonstrate the difficulty of initiating valproate therapy in patients already on meropenem. The speed of valproate depletion after the initiation of carbapenem therapy, the effect of treatment duration, clinical manifestations, delay in valproate level normalization after carbapenem therapy, the efficacy of supplemental valproate doses, and the usefulness of valproate dose escalation are evaluated. Five out of the 7 patients became acutely symptomatic owing to their subtherapeutic valproate levels. The presented cases also highlight the relatively slow normalization of valproate levels after discontinuation of the antibiotic therapy. Our cases suggest that the interaction is not absorption‐mediated because all of our patients received intravenous valproate. We observed that the introduction of alternative antiepileptic drugs (AEDs) may be preferable to valproate dose escalation, which is ineffective in the presence of concomitant meropenem therapy. The characterization and recognition of this interaction have implications for the management of a particularly vulnerable patient cohort.

Seizures as adverse events of antibiotic drugs: A systematic review

OBJECTIVE

Antibiotic drugs are commonly associated with seizures. Tailoring antibiotics to the individual risk for seizures is challenged as avoidance of certain antibiotic classes may no longer be possible due to the emergence of resistant bacteria. We performed a systematic review regarding the current evidence for seizures associated with all antibiotic classes, their underlying mechanisms, and predisposing factors.

METHODS

The medical search engine PubMed was systematically screened to identify articles in English published between 1960 and 2013. All study designs were considered and evidence was assessed.

RESULTS

We included 143 articles involving 25,712 patients and 25 different antibiotics. Evidence for antibiotic-related symptomatic seizures is low to very low, mainly deriving from studies regarding β-lactams, especially unsubstituted penicillins and fourth-generation cephalosporins, as well as carbapenems, mainly imipenem, all administered in high doses or in patients with renal dysfunction, brain lesions, or known epilepsy. Evidence regarding symptomatic seizures from fluoroquinolones only relies on case reports and case series with most reports for ciprofloxacin in patients with renal dysfunction, mental disorders, prior seizures, or coadministered theophylline.

CONCLUSIONS

Evidence for an association between antibiotic drugs and symptomatic seizures is low to very low (evidence Class III-IV). Despite this, numerous reports point to an increased risk for symptomatic seizures especially of unsubstituted penicillins, fourth-generation cephalosporins, imipenem, and ciprofloxacin in combination with renal dysfunction, brain lesions, and epilepsy. During administration of such antibiotics in patients with particular predispositions, close monitoring of serum levels is advocated. As most seizures associated with cephalosporins are nonconvulsive, continuous EEG should be considered in patients with altered levels of consciousness.

Pharmacological interaction between valproic acid and carbapenem: what about levels in pediatrics?

Valproic acid (VPA) is the most commonly used antiepileptic drug in pediatric patients, but its major drawback is its multiple pharmacological interactions.

OBJECTIVE

To study children who had been simultaneously treated with carbapenems and valproic acid, considering drug levels, pharmacological interactions and clinical follow-up.

MATERIAL AND METHODS

Retrospective study of children who simultaneously received treatment with VPA and carbapenems between January 2003 and December 2011. Demographic variables, indication of treatment, dose, VPA plasma levels, interactions, clinical manifestations and medical management were analyzed.

RESULTS

28 children with concomitant treatment with both drugs were included in the study. 64.3% were males. 78.6% of the interactions were observed in the Intensive Care Unit. 60.7% of children had been previously treated VPA and its major indication were generalized seizures. Basal plasma levels of VPA were recorded in 53% and at 24 h after admittance in 60%. "40% of basal VPA levels were below therapeutic range prior to the administration of carbapenem. After the introduction of carbapenem 88% of level determinations were below therapeutic range". 54.5% of the patients that were chronically receiving VPA and had good control of epilepsy before admission had seizures during the coadministration. One patient that was on VPA before admission but with bad control of epilepsy worsened, and one patient that acutely received VPA did not achieve seizure freedom. In these cases it was necessary to either increase VPA dose or change to a different antiepileptic drug.

CONCLUSIONS

Little is known about the mechanism of pharmacologic interactions between carbapenems and VPA, but it leads to a reduction in plasma levels that may cause a loss of seizure control, so simultaneous use of both drugs should be avoided when possible. If not, VPA levels should be monitored.

Reduced valproic acid serum concentrations due to drug interactions with carbapenem antibiotics: overview of 6 cases

BACKGROUND

The plasma concentrations of valproic acid (VPA) are known to decrease during the concomitant administration of carbapenem antibiotics, such as meropenem, imipenem, and ertapenem. This study summarizes 6 cases of drug-drug interactions between VPA and carbapenem antibiotics.

METHODS

To investigate the onset and severity of the reductions in the concentration of VPA in patients with or without the coadministration of carbapenem antibiotics, the authors performed a retrospective evaluation of therapeutic drug monitoring (TDM) reports that described a decrease in the serum concentrations of VPA during the concomitant use of carbapenem antibiotics from January 2008 to December 2010 in the Seoul National University Hospital. The evaluated TDM reports included 6 cases. The decrement ratio of the VPA serum concentration was calculated from the TDM reports, and the change in the half-life of the VPA was also estimated.

RESULTS

Six cases presented with changes in the VPA serum concentration before and after the administration of carbapenem antibiotics. (Three cases were treated with meropenem, 2 were treated with ertapenem, and 1 was treated with imipenem.) The VPA concentrations reduced by (mean ± SD) 88.7 ± 5.3% (3 cases of meropenem), 74.0 ± 9.8% (2 cases of ertapenem), and 73.3% (1 case of imipenem), respectively, and the half-life of VPA reduced by 80.1 ± 9.0%, 64.4 ± 24.2%, and 50.6%, respectively.

CONCLUSION

The interaction between VPA and carbapenem antibiotics caused decreases in the VPA serum concentrations; the extent of this decrease was greater in the meropenem-treated patients than in the imipenem-treated or ertapenem-treated cases. Because the therapeutic effect of VPA depends on its serum concentration, it should be recognized that there may be a loss of seizure control in patients using VPA with carbapenem antibiotics.

Population-Based Assessments of Clinical Drug-Drug Interactions: Qualitative Indices or Quantitative Measures?

Population-based assessments of drug-drug interactions have become more common since the introduction and acceptance of the population pharmacokinetic approach. Unlike traditional methods, population-based studies provide clinically relevant results that can be applied directly to a target patient population. Furthermore, population-based studies do not demand the traditional requirements of intensive pharmacokinetic sampling, rigorous inpatient stays, or stringent assessment schedules. As such, the population-based approach can effectively be used to confirm known drug-drug interactions and further characterize anticipated interactions. A prospectively designed analysis can also reveal drug-drug interactions that might otherwise have gone undetected with traditional methods. Ultimately, these results could help to alleviate clinicians' concerns about using widely marketed drugs in combination therapies and also reduce patients' risk of experiencing unacceptable side effects. This article intends to provide a balanced overview of the population-based approach and its merits, drawbacks, and potential utility in the assessment of drug-drug interactions during clinical drug development.

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

Pharmacokinetic drug interactions of antimicrobial drugs: a systematic review on oxazolidinones, rifamycines, macrolides, fluoroquinolones, and Beta-lactams

Like any other drug, antimicrobial drugs are prone to pharmacokinetic drug interactions. These drug interactions are a major concern in clinical practice as they may have an effect on efficacy and toxicity. This article provides an overview of all published pharmacokinetic studies on drug interactions of the commonly prescribed antimicrobial drugs oxazolidinones, rifamycines, macrolides, fluoroquinolones, and beta-lactams, focusing on systematic research. We describe drug-food and drug-drug interaction studies in humans, affecting antimicrobial drugs as well as concomitantly administered drugs. Since knowledge about mechanisms is of paramount importance for adequate management of drug interactions, the most plausible underlying mechanism of the drug interaction is provided when available. This overview can be used in daily practice to support the management of pharmacokinetic drug interactions of antimicrobial drugs.

[Analysis of the valproic acid-meropenem interaction in hospitalised patients]

INTRODUCTION

Published data demonstrate a serious interaction between valproic acid and meropenem. However, recommendations about the management of concomitant treatment are contradictory; some experts recommend closer monitoring of valproic acid serum concentrations and others recommend avoiding concurrent therapy. The purpose of this study is to critically analyse the interaction and to evaluate the impact of pharmaceutical intervention in the use of these drugs in hospitalised patients.

MATERIAL AND METHODS

Study of the concomitant prescription of valproic acid and meropenem in a general hospital of 1,080 beds divided in to two periods; the first period was retrospective and observational and it was followed by a prospective period involving pharmaceutical intervention. The prescription habits between both periods were compared.

RESULTS

A total of 26 patients received concurrent treatment with valproic acid and meropenem (13 per period) and none of them maintained therapeutic serum levels of the antiepileptic drug. Pharmaceutical intervention modified prescription habits, reducing by half the number of days of concomitant treatment, changing the antibiotherapy and/or monitoring serum concentrations more often.

CONCLUSIONS

The interaction between valproic acid and meropenem is serious, especially because of the dramatic decrease in the antiepileptic serum concentrations. The concomitant use of both drugs should be avoided, replacing the antibiotherapy empirically, or according to the resistance profiles of the microorganism and maintaining the same the anti-epileptic treatment.

Characterization of inhibitory effect of carbapenem antibiotics on the deconjugation of valproic acid glucuronide

Serum concentrations of valproic acid (VPA) are markedly decreased by coadministration of carbapenem antibiotics (CBPMs). Although inhibition of deconjugation of VPA-glucuronide (VPA-G) to VPA by CBPMs has been proposed as one of the mechanisms to account for this drug-drug interaction, little information is available on the mode of inhibition. In the present study, we characterized the enzyme involved in the deconjugation of VPA-G by using human and rat liver cytosol. It is suggested that 1) deconjugation activity inhibited by CBPMs may be selective for VPA-G, 2) deconjugation of VPA-G may be mediated by enzyme(s) other than β-glucuronidase, and 3) the irreversible inactivation may be responsible for the inhibition of deconjugation of VPA-G by CBPMs. Finally, the kinetic parameters for inactivation (K'(app) and k(inact)) were determined for four CBPMs of diverse structure from in vitro experiments. Based on the results of simulation analyses with these parameters and the degradation rate constant of the putative VPA-G deconjugation enzyme obtained from experiments using rats, it is probable that the deconjugation enzyme for VPA-G in the liver is rapidly and mostly inactivated by these CBPMs under clinical situations.

Drug interaction between carbapenems and extended-release divalproex sodium in a patient with schizoaffective disorder

BACKGROUND

Clinicians prescribing divalproex sodium (DVX) are well aware of its potential to cause a drug-drug interaction. One specific interaction occurs between the carbapenem antibiotics and DVX resulting in decreased valproic acid (VPA) levels immediately following the initiation of this antibiotic class.

OBJECTIVE/METHOD

We describe a case of a 46 year-old Caucasian male who had an undetectable VPA level following treatment with carbapenems.

RESULTS

On admission the patient's VPA level was 115 μg/ml; however, a routine VPA level on day 19 of his hospitalization returned a value of 16 μg/ml. At this point, he had received a total of 15 days of carbapenem antibiotics for treatment of lower leg cellulitis. His DVX dose was increased to a maximum of 6g daily, twice his home dose, but it did not produce a therapeutic VPA concentration. The patient was lost to follow-up before an outpatient VPA level was drawn.

CONCLUSION

Our case report is the first to document this drug-drug interaction in a patient diagnosed with schizoaffective disorder, bipolar type.

The Effect of Carbapenem Antibiotics on Plasma Concentrations of Valproic Acid

Objective:

To critically evaluate the mechanisms of the interaction between valproic acid and carbapenem antibiotics.

Data Sources:

A PubMed search (January 1971–June 2009) was performed to identify literature on the interaction between valproic acid and carbapenem antibiotics. Additional references were identified through review of bibliographies of identified articles.

Study Selection and Data Extraction:

Data on the mechanisms of the interaction between valproic acid and carbapenem antibiotics were extracted from identified references that were published in English.

Data Synthesis:

Valproic acid plasma concentrations fall markedly during concomitant administration with carbapenem antibiotics due to a combination of absorption, distribution, and metabolism mechanisms. Carbapenems appear to inhibit the intestinal transporter of valproic acid, thereby reducing absorption of orally administered valproic acid. In vivo experiments in rats demonstrate a 57% reduction in absorption of orally administered valproic acid in the presence of imipenem. Followup studies in Caco-2 cells suggest that the inhibition probably occurs at the basolateral membrane. In addition, enterohepatic recycling of valproic acid may be diminished due to carbapenem activity against gut flora producing β-glucuronidase. When rabbits and rats were given intravenous valproic acid-glucuronide, the glucuronide metabolite of valproic acid, 50–90% of the conversion back into valproic acid was inhibited in the presence of a carbapenem. An increase in erythrocyte distribution of valproic acid has also been observed in the presence of carbapenems. After intravenous administration of a carbapenem and valproic acid, valproic acid plasma concentrations fell in the presence of a carbapenem, yet whole blood concentrations of valproic acid did not change significantly. Follow-up studies suggest that the mechanism of this distribution shift is that multidrug resistance proteins on adenosine triphosphate–binding cassette transporters on erythrocyte membranes are inhibited by carbapenems. Thus, valproic acid is not effluxed out of the erythrocytes. Finally, carbapenems may enhance glucuronidation of valproic acid by increasing UDP-glucuronic acid levels. In rats, UDP-glucuronic acid levels increased by 1,7-fold in the presence of panipenem, which was proportionate to the increase in valproic acid-glucuronide formation.

Conclusions:

Published data demonstrate a serious and complex interaction between valproic acid and carbapenem antibiotics. Coadministration should be avoided, but if no other antibiotic therapies exist, it is imperative to monitor valproic acid concentrations more frequently. Clinicians should anticipate higher doses of valproic acid to maintain therapeutic serum concentrations during coadministration and subsequent dose reductions upon discontinuation of the carbapenem antibiotic.

The use of carbapenems in the treatment of serious infections

Inadequate initial antimicrobial treatment in serious infections leads to increased mortality. Achieving adequate treatment is increasingly difficult because of the increasing prevalence of multidrug-resistant (MDR) pathogens. The carbapenems are potent, broad-spectrum antibiotics that have been shown to be safe and efficacious therapies in the treatment of serious infections. This review is intended to compare the 4 major members of the carbapenem class, which include imipenem, meropenem, ertapenem, and doripenem, with other widely used antimicrobial agents in the intensive care unit (ICU). The carbapenems are potent, broad-spectrum antibiotics that have been shown to be safe and efficacious therapies in the treatment of serious infections. They provide better gram-negative coverage than other beta-lactams and are stable against extended-spectrum beta-lactamases and AmpC beta-lactamases, making them effective in the treatment of many MDR bacteria. The newly approved carbapenem, doripenem, may help preserve the utility of the carbapenem class.

Effect of concomitant administration of meropenem and valproic acid in an elderly Chinese patient

BACKGROUND

Meropenem is a carbapenem with a broad spectrum of activity against beta-lactamase-producing organisms. Valproic acid is widely used in the treatment of generalized tonic-clonic and partial seizures. Concomitant administration of meropenem and valproic acid reportedly leads to a rapid decline in serum concentrations of valproic acid, which is sometimes associated with seizures.

CASE SUMMARY

This report describes an 85-year-old Chinese male inpatient who twice received concomitant administration of meropenem and valproic acid for the treatment of pneumonia and poststroke epilepsy, respectively. Rapid declines in valproic acid concentrations were observed both times after meropenem administration. No seizures occurred in the first treatment period; however, when the patient suffered pneumonia again 3 months later, the same concomitant therapy was prescribed, and seizures occurred. It is difficult to identify a single etiology of the seizures. Based on a score of 7 on the Naranjo adverse drug reaction probability scale, the seizures were considered to be probably related to the concomitant administration of meropenem and valproic acid.

CONCLUSIONS

Various factors make the effect of concomitant administration of meropenem and valproic acid unpredictable, even in the same patient. Caution should be used when administering meropenem and valproic acid concomitantly, especially in elderly patients with central nervous system disorders, even if the patient has had a successful prior experience with these 2 drugs. If concomitant administration is essential, very close serum concentration monitoring and clinical observation are necessary.

Clinical review: balancing the therapeutic, safety, and economic issues underlying effective antipseudomonal carbapenem use

Antipseudomonal carbapenems have played a useful role in our antimicrobial armamentarium for 20 years. However, a review of their use during that period creates concern that their clinical effectiveness is critically dependent on attainment of an appropriate dosing range. Unfortunately, adequate carbapenem dosing is missed for many reasons, including benefit/risk misconceptions, a narrow therapeutic window for imipenem and meropenem (due to an increased rate of seizures at higher doses), increasingly resistant pathogens requiring higher doses than are typically given, and cost containment issues that may limit their use. To improve the use of carbapenems, several initiatives should be considered: increase awareness about appropriate treatment with carbapenems across hospital departments; determine optimal dosing regimens for settings where multidrug resistant organisms are more likely encountered; use of, or combination with, an alternative antimicrobial agent having more favorable pharmacokinetic, pharmacodynamic, or adverse event profile; and administer a newer carbapenem with lower propensity for resistance development (for example, reduced expression of efflux pumps or greater stability against carbapenemases).

Interaction between valproic acid and carbapenem antibiotics

The serum concentration of valproic acid (VPA) in epilepsy patients decreased by the administration of carbapenem antibiotics, such as meropenem, panipenem or imipenem, to a sub-therapeutic level. This review summarized several case reports of this interaction between VPA (1-4 g dose) and carbapenem antibiotics to elucidate the possible mechanisms decreasing VPA concentration by carbapenem antibiotics. Studies to explain the decrease were carried out using rats by the following sites: absorption of VPA in the intestine, glucuronidation in the liver, disposition in blood and renal excretion. In the intestinal absorption site, there are two possible mechanisms: inhibition of the intestinal transporter for VPA absorption by carbapenem antibiotics, and the decrease of beta-glucuronidase supplied from enteric bacteria, which were killed by antibiotics. This is consistent with a view that the decrease of VPA originated from VPA-Glu, relating to entero-hepatic circulation. The second key site is in the liver, because of no decreased in VPA level by carbapenem antibiotics in hepatectomized rats. There are three possible mechanisms in the liver to explain the decreased phenomenon: first, decrease of the UDPGA level by carbapenem antibiotics. UDPGA is a co-factor for UDP-glucuronosyltransferase (UGT)-mediated glucuronidation of VPA. Second, the direct activation of UGT by carbapenem antibiotics. This activation was observed after pre-incubation of human liver microsomes with carbapenem antibiotics. Third, the inhibition of beta-glucuronidase in liver by carbapenem antibiotics and the decreased VPA amount liberated from VPA-Glu. The third site is the distribution of VPA in blood (erythrocytes and plasma). Plasma VPA distributed to erythrocytes by the inhibition of transporters (Mrp4), which efflux VPA from erythrocytes to plasma, by carbapenem antibiotics. The increase of renal excretion of VPA as VPA-Glu depends on the increase of VPA-Glu level by UGT. One or a combination of some factors in these mechanisms might relate to the carbapenem-mediated decrease of the plasma VPA level.

Safety profile of meropenem: an updated review of over 6,000 patients treated with meropenem

Meropenem is a broad-spectrum carbapenem antibacterial with potent antimicrobial activity against a broad range of Gram-negative, Gram-positive and anaerobic bacteria. The second parenteral carbapenem to be introduced worldwide, meropenem has been in clinical use since 1994. Two previous safety reviews have established that meropenem has a favourable and acceptable safety profile. This new review was conducted after the approval of meropenem in the US in 2005 for the treatment of patients with complicated skin and skin-structure infections, in addition to the previously approved indications of intra-abdominal infections and paediatric bacterial meningitis. The analysis includes the clinical trial data from the previous safety reviews, updated with expanded experience across a number of serious bacterial infections, including a large international study in patients with skin or skin-structure infections and further experience in patients with intra-abdominal infections and bacterial meningitis. A total of 6154 patients with 6308 meropenem exposures were compared with 4483 patients treated with comparator agents (4593 exposures), and the paediatric population base for which safety data are available has doubled to over 1000 patients. The data presented reinforce the favourable safety profile of meropenem. In general, the incidence and pattern of adverse events occurring with meropenem were similar to those of the first carbapenem, imipenem/cilastatin, and to those of the cephalosporin- and clindamycin-based regimens to which it had been compared. The most common adverse events reported for meropenem were diarrhoea (2.5%), rash (1.4%) and nausea/vomiting (1.2%). No adverse event occurred in more than 3% of patient exposures to meropenem, indicating a low overall frequency of adverse events as well as excellent gastrointestinal tolerability. Furthermore, no unexpected adverse events were identified, and the very low incidence of seizures in patients with meningitis was not considered to be drug related. In infections other than meningitis, the incidence of seizures considered by investigators to be related to meropenem treatment was 0.07%. In the new studies that updated the earlier safety data, no new cases of drug-related seizure were reported for any treatment or patient group (meningitis/non-meningitis infections). In conclusion, meropenem is well tolerated and has good CNS and gastrointestinal tolerability when used for the treatment of serious bacterial infections in a wide range of adult and paediatric patient populations.

Acute Seizures in a Patient Receiving Divalproex Sodium After Starting Ertapenem Therapy

Divalproex sodium is an anticonvulsant widely prescribed to treat several types of seizure disorders, including tonic-clonic and simple or complex partial seizures. We describe a 41-year-old man who experienced recurring tonic-clonic seizures after a drug interaction between divalproex sodium and ertapenem, a carbapenem antibiotic. The patient's valproic acid serum concentration was 130 mug/ml approximately 3 months before he started ertapenem 2000 mg/day (20.6 mg/kg/day). On day 7 of ertapenem therapy, the patient was brought to the emergency department with tonic-clonic seizures; his valproic acid serum concentration was 70 microg/ml. His divalproex sodium dosage was increased, and he was released from the emergency department only to return 4 days later with recurring seizures. This time his valproic acid serum concentration was 10.7 microg/ml. Ertapenem was discontinued, and his divalproex sodium dosage was increased further. The patient's valproic acid level rapidly returned to a therapeutic level 2 days after ertapenem discontinuation, and he had no further seizures. Using the Naranjo adverse drug reaction probability scale to determine the probability of the drug interaction, we found that the likelihood of the interaction was probable (score of 7). Similar interactions have been reported between other carbapenem antibiotics and valproic acid. Clinicians should be aware of this potential interaction between divalproex sodium and ertapenem; concurrent administration of these two drugs should be approached with caution. In patients prescribed this combination, the valproic acid serum concentration should be carefully monitored to prevent recurring seizures.

Adverse events caused by drug interactions involving glucuronoconjugates of zidovudine, valproic acid and lamotrigine, and analysis of how such potential events are discussed in package inserts of Japan, UK and USA

BACKGROUND AND OBJECTIVE

As pharmacokinetic drug interactions frequently cause adverse events, it is important that the relevant information is given in package inserts (PIs). We previously analysed the provision of PIs for HMG-CoA reductase inhibitors and Ca antagonists, for which metabolism by cytochrome P450 could be a major interaction mechanism. In this article, we focus on interactions involving glucuronoconjugates because many drugs and their metabolites undergo this conjugation.

METHODS

We reviewed clinical drug interactions related to glucuronoconjugates, focusing on reports of adverse events. Then, we picked out three important drugs (zidovudine, valproic acid and lamotrigine), and examined how the literature information is reflected in the relevant PIs in Japan, UK and USA.

RESULTS AND DISCUSSION

Pharmacokinetic interactions related to glucuronoconjugates were found with 33 drug combinations. Of these, five combinations induced clear adverse events: (i) severe anaemia due to zidovudine and caused by interaction with valproic acid, (ii) recurrence/increased frequency of seizure or increased manic states from a reduction in therapeutic effects of valproic acid caused by panipenem, (iii) meropenem or (iv) ritonavir and (v) of lamotrigine caused by oral contraceptives. Analysis of PIs showed a lack of description of the interaction of zidovudine with valproic acid in the Japanese PI. The UK PI mentioned this interaction without quantitative data, whereas full information was given in the US PI. A lack of description was also present on the interaction between valproic acid with ritonavir, reported in 2006, in the PIs of all three countries. For the interactions involving valproic acid and panipenem or meropenem, even though marked reduction of blood valproic acid level has been reported, no quantitative data were provided in any of the PIs.

CONCLUSION

Five combinations were identified to cause severe adverse events because of interactions related to glucuronoconjugates. This information, including quantitative data, is not always properly provided in the relevant PIs in Japan, UK or USA. PIs should be improved to better inform healthcare providers and thereby help them and their patients.

Interaction between valproate and meropenem: a retrospective study

BACKGROUND

Valproate and meropenem are frequently used in the intensive care unit to treat seizures and serious infections, respectively. Several case reports have described a remarkable interaction between the drugs when administered concurrently. The interaction leads to a significant drop in plasma concentrations of valproate within 24 hours and relapse of seizures in some patients.

OBJECTIVE

To evaluate a consecutive population of hospitalized patients who were simultaneously treated with meropenem and valproate and assess the effect on epileptic activity.

METHODS

A retrospective study of an 18 month period was performed to assess the extent and clinical impact of this interaction. To assess the relevance of the interaction, the time-relationship between the drop in plasma concentrations and relapse in seizure activity and/or deterioration of electroencephalogram recordings was determined. We investigated other contributing proconvulsive cofactors and concomitant antiepileptic treatment. Drug interaction probability scale (DIPS) scores were calculated.

RESULTS

Thirty-nine patients were treated simultaneously with valproate and meropenem. The pharmacokinetic interaction was observed in all 39 patients, with an average drop in valproate plasma concentrations of 66%. The decrease occurred within 24 hours, as shown in 19 patients who had daily plasma concentration monitoring. The clinical impact of the interaction could not be assessed in 19 (49%) patients due to death (n = 13) or incomplete charts (n = 6). In the remaining 20 (51%) patients, DIPS scores were calculated and clinical relevance was assessed. The interaction was considered to be probable in 16 patients and possible in 4, as calculated by the DIPS. The interaction contributed to electroclinical deterioration in 11 patients.

CONCLUSIONS

The pharmacokinetic interaction between valproate and meropenem was present in all patients and led to a drop of valproate concentrations with an average of 66% within 24 hours. This interaction was clinically relevant with electroclinical deterioration in 55% of patients. To avoid patients' possible neurologic deterioration, meropenem and valproate should not be administered together.

Comparative Review of the Carbapenems

The carbapenems are beta-lactam antimicrobial agents with an exceptionally broad spectrum of activity. Older carbapenems, such as imipenem, were often susceptible to degradation by the enzyme dehydropeptidase-1 (DHP-1) located in renal tubules and required co-administration with a DHP-1 inhibitor such as cilastatin. Later additions to the class such as meropenem, ertapenem and doripenem demonstrated increased stability to DHP-1 and are administered without a DHP-1 inhibitor. Like all beta-lactam antimicrobial agents, carbapenems act by inhibiting bacterial cell wall synthesis by binding to and inactivating penicillin-binding proteins (PBPs). Carbapenems are stable to most beta-lactamases including AmpC beta-lactamases and extended-spectrum beta-lactamases. Resistance to carbapenems develops when bacteria acquire or develop structural changes within their PBPs, when they acquire metallo-beta-lactamases that are capable of rapidly degrading carbapenems, or when changes in membrane permeability arise as a result of loss of specific outer membrane porins. Carbapenems (imipenem, meropenem, doripenem) possess broad-spectrum in vitro activity, which includes activity against many Gram-positive, Gram-negative and anaerobic bacteria; carbapenems lack activity against Enterococcus faecium, methicillin-resistant Staphylococcus aureus and Stenotrophomonas maltophilia. Compared with imipenem, meropenem and doripenem, the spectrum of activity of ertapenem is more limited primarily because it lacks activity against Pseudomonas aeruginosa and Enterococcus spp. Imipenem, meropenem and doripenem have in vivo half lives of approximately 1 hour, while ertapenem has a half-life of approximately 4 hours making it suitable for once-daily administration. As with other beta-lactam antimicrobial agents, the most important pharmacodynamic parameter predicting in vivo efficacy is the time that the plasma drug concentration is maintained above the minimum inhibitory concentration (T>MIC). Imipenem/cilastatin and meropenem have been studied in comparative clinical trials establishing their efficacy in the treatment of a variety of infections including complicated intra-abdominal infections, skin and skin structure infections, community-acquired pneumonia, nosocomial pneumonia, complicated urinary tract infections, meningitis (meropenem only) and febrile neutropenia. The current role for imipenem/cilastatin and meropenem in therapy remains for use in moderate to severe nosocomial and polymicrobial infections. The unique antimicrobial spectrum and pharmacokinetic properties of ertapenem make it more suited to treatment of community-acquired infections and outpatient intravenous antimicrobial therapy than for the treatment of nosocomial infections. Doripenem is a promising new carbapenem with similar properties to those of meropenem, although it appears to have more potent in vitro activity against P. aeruginosa than meropenem. Clinical trials are required to establish the efficacy and safety of doripenem in moderate to severe infections, including nosocomial infections.

Meropenem –valproic acid interaction in patients with cefepime-associated status epilepticus

PURPOSE

Two case reports of rapid decreases in valproic acid levels after initiation of meropenem in patients who developed new-onset seizure activity during treatment with cefepime are presented.

SUMMARY

A 60-year-old Caucasian woman with myelodysplasia was transferred to the medical intensive care unit (MICU) on day 11 of her hospitalization. Cefepime was given as empiric therapy for febrile neutropenia. Pulmonary invasive aspergillosis was diagnosed. On day 16 of hospitalization, epileptic activity was confirmed. Valproic acid was initiated. Cefepime was discontinued and meropenem was initiated for treatment of cefepime-resistant pneumonia. Serum valproic acid levels decreased to subtherapeutic levels within 24 hours. Meropenem was discontinued and ceftazidime was started on day 22; serum valproic acid levels gradually increased but never reached therapeutic levels again. The patient died of intractable invasive aspergillosis on day 33. A 54-year-old Caucasian man with myelodysplasia was admitted to the MICU for nonconvulsive status epilepticus. Ten days before admission, cefepime had been started empirically for the treatment of neutropenic fever. One day before MICU admission, valproic acid was initiated as treatment for status epilepticus. The next day, serum valproic acid levels were therapeutic; cefepime was switched to meropenem. Serum valproic acid levels decreased within 24 hours and phenytoin was added. On day 4, the patient's serum valproic acid levels decreased further and meropenem was discontinued. Although the valproic acid dosage was increased, valproic acid levels did not return to the therapeutic range. The patient died on day 11.

CONCLUSION

Following cefepime therapy, a clinically important interaction between meropenem and valproic acid occurred in two critically ill patients with new-onset status epilepticus.

Epileptic seizures caused by low valproic acid levels from an interaction with meropenem

A 55-year-old woman was diagnosed with pneumonia and was treated with meropenem; 5 days later she developed epileptic seizures. She had been treated with valproic acid for 16 years to control her epileptic seizures. Her serum valproic acid concentration was low during treatment with meropenem than previously recorded despite an increase of valproic dose. As soon as administration of meropenem was withdrawn, valproic acid concentration increased to previous levels and her seizures stopped. Meropenem decreases valproic acid concentration, and may promote the development of epileptic seizures in previously controlled epileptic patients. The acute lowering of serum valproate produced by meropenem probably precludes their concomitant use.