Relative proportions of serum carbamazepine and its pharmacologically active 10,11-epoxy derivative: effect of polytherapy and renal insufficiency

Quality, origins and limitations of common therapeutic drug reference intervals

Therapeutic drug monitoring (TDM) is used to manage drugs with a narrow window between effective and toxic concentrations. TDM involves measuring blood concentrations of drugs to ensure effective therapy, avoid toxicity and monitor compliance. Common drugs for which TDM is used include aminoglycosides for infections, anticonvulsants to treat seizures, immunosuppressants for transplant patients and cardiac glycosides to regulate cardiac output and heart rate. An essential element of TDM is the provision of accurate and clinically relevant reference intervals. Unlike most laboratory reference intervals, which are derived from a healthy population, TDM reference intervals need to relate to clinical outcomes in the form of efficacy and toxicity. This makes TDM inherently more difficult to develop as healthy individuals are not on therapy, so there is no "normal value". In addition, many of the aforementioned drugs are old and much of the information regarding reference intervals is based on small trials using methods that have changed. Furthermore, individuals have different pharmacokinetics and drug responses, particularly in the context of combined therapies, which exacerbates the challenge of universal TDM targets. This focused review examines the origins and limitations of existing TDM reference intervals for common drugs, providing targets where possible based on available guidelines.

Markedly Elevated Carbamazepine-10,11-epoxide/Carbamazepine Ratio in a Fatal Carbamazepine Ingestion

Carbamazepine is a widely used anticonvulsant. Its metabolite, carbamazepine-10,11-epoxide, has been found to display similar anticonvulsant and neurotoxic properties. While the ratio of parent to metabolite concentration varies significantly, at therapeutic doses the epoxide concentration is generally about 20% of the parent. We report a case of fatal carbamazepine overdose in which the epoxide metabolite concentration was found to be 450% higher than the parent compound, suggesting a potential role for metabolite quantification in severe toxicity.

Impact of interferences including metabolite crossreactivity on therapeutic drug monitoring results

Therapeutic drug monitoring is an integral part of services offered by toxicology laboratories because certain drugs require routine monitoring for dosage adjustment to achieve optimal therapeutic response and avoid adverse drug reactions. Immunoassays are widely used for therapeutic drug monitoring. However, immunoassays suffer from interferences from both exogenous and endogenous compounds including metabolites of the parent drug. Digoxin immunoassays are affected more commonly than any other immunoassays used for therapeutic drug monitoring. Digoxin immunoassays are affected by endogenous digoxin-like immunoreactive substances and exogenous compounds such as various drugs, certain herbal supplements, and Digibind. Carbamazepine is metabolized to carbamazepine 10, 11-epoxide, and the crossreactivity of this metabolite with carbamazepine immunoassay may vary from 0% to 94%. Immunoassays used for measuring concentrations of tricyclic antidepressants are affected by tricyclic antidepressant metabolites and by a number of other drugs. Immunoassays for immunosuppressants are also subjected to significant interferences from metabolites, and liquid chromatography combined with mass spectrometry or tandem mass spectrometry is recommended for therapeutic drug monitoring of immunosuppressants. However, liquid chromatography combined with mass spectrometry may also suffer from interferences, for example, due to ion suppression or from isobaric ions.

Miscellaneous central nervous system intoxicants

The topic of central nervous system intoxicants encompasses a multitude of agents. This article focuses on three classes of therapeutic drugs, with specific examples in which overdoses require admission to the intensive care unit. Included are some of the newer antidepressants, the atypical neuroleptic agents, and selected anticonvulsant drugs. The importance of understanding pertinent physiology and applicable supportive care is emphasized.

Discordant carbamazepine values between two immunoassays: carbamazepine values determined by ADVIA Centaur correlate better with those determined by LC-MS/MS than PETINIA assay

Discordant carbamazepine values as determined by two different immunoassays may be due to different cross-reactivities with the active metabolite carbamazepine 10, 11-epoxide and may cause confusion in interpreting carbamazepine serum levels. In this study, we compared carbamazepine values in samples containing carbamazepine and the epoxide metabolite, as determined by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) and by two commercial carbamazepine immunoassays: the PETINIA and the ADVIA Centaur carbamazepine. Clinical specimens were used for the comparative studies wherein we determined carbamazepine concentrations using the PETINIA, ADVIA Centaur, and LC-MS/MS assays. We observed an excellent correlation between carbamazepine concentrations determined by the ADVIA Centaur and LC-MS/MS methods while carbamazepine values were overestimated using the PETINIA assay. When aliquots of drug-free serum were supplemented with clinically relevant concentrations of the carbamazepine epoxide metabolite, we observed negligible cross-reactivity of epoxide with the ADVIA Centaur assay but over 90% cross-reactivity with the PETINIA assay. We conclude that the ADVIA centaur assay accurately measures carbamazepine concentrations in plasma or serum and that the PETINIA assay significantly overestimates true carbamazepine concentration. Such discordance may cause confusion in interpreting serum carbamazepine levels.

Estimation of carbamazepine and carbamazepine-10,11-epoxide concentrations in plasma using mathematical equations generated with two carbamazepine immunoassays

Carbamazepine is metabolized to an active metabolite known as carbamazepine-10,11-epoxide, or simply the "epoxide" metabolite. The presence of this metabolite can have clinically significant implications in therapeutic drug monitoring of carbamazepine, but accurate quantification of the epoxide metabolite is currently limited to chromatographic techniques. In this study, mathematical equations are proposed for the estimation of carbamazepine and epoxide metabolite concentrations based on values generated by common carbamazepine immunoassays. Three immunoassays were studied: particle-enhanced turbidimetric inhibition immunoassay (PETINIA, Siemens Healthcare Diagnostics, Deerfield, IL), ADVIA Centaur (Siemens Healthcare Diagnostics), and a cloned enzyme donor immunoassay (CEDIA; Roche, Indianapolis, IN). Equations were based on observed cross-reactivity of epoxide with the PETINIA (average, 96.2%; range, 86.6%-105.7%) and epoxide cross-reactivity with the ADVIA Centaur assay (average, 6.5%; range, 5.3%-7.7%). In addition, equations were developed using average cross-reactivity of epoxide with the PETINIA and with the CEDIA. Values determined by calculation correlated well with carbamazepine and epoxide concentrations in supplemented and patient samples, for which values of carbamazepine (2.2-12.0 microg/mL [9-51 micromol/L]) and the epoxide metabolite (0.6-2.4 microg/mL) were also verified by liquid chromatography-tandem mass spectrometry.