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945© 2013 David G. Wild. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/B978-0-08-097037-0.00076-2

Although drug treatments are prescribed in terms of the dose to be administered, it is normally the subsequent con-centration of active drug in the patient’s blood (or plasma/serum) that more closely mirrors the concentration at the site of action and, therefore, its effects. The relationship between dose and plasma concentration can be highly vari-able between patients because of differences in absorption, distribution, excretion, and particularly metabolism. Mea-surement of the concentration of the drug in serum or plasma enables the dose of certain medicines to be adjusted to the optimum level.

Plasma drug concentrations are monitored for three main reasons:

� to ensure that the drug concentration is high enough to be therapeutically effective;

� to minimize dose-related (type A or toxic) side effects of the drug; and

� to check for patient adherence (sometimes also termed patient compliance or concordance) to the prescribed therapy.

Therapeutic drug monitoring (TDM) is of value only when the wanted or unwanted effects of the drug are related to the concentration in plasma. It is particularly useful when there is a wide variation in the rate of metabo-lism or excretion of the drug between individuals, resulting in marked differences in the plasma concentration at any given dose. Monitoring of serum or plasma concentrations is also particularly useful for those drugs in which the ther-apeutic concentration is close to the toxic concentration (low therapeutic ratio), such as digoxin and theophylline. Similarly, it is useful in monitoring those drugs (e.g., phe-nytoin [diphenylhydantoin]) where, due to saturation of metabolism, the drug concentrations can rise dispropor-tionately, with even small increases of dose, over the thera-peutic range of drug concentration so that the concentrations are not linearly related to dose.

TDM is used clinically to optimize the benefits and reduce the side effects of a range of medicines, including several important groups of agents not discussed in this chapter. The selection of the medicines included in this chapter is based in general on the availability of an immu-noassay technique.

All data shown refer to adults (unless specifically stated) and are for general guidance only; they are not intended for clinical use. The clinical applications listed for each medicine are based on those in the British National Formulary (BNF), and different indications may exist in other countries. The pharmacological effects are generally based on those cited in Drugbank. The pharmacokinetic data (unless otherwise stated) were taken from Dollery (1999). The therapeutic ranges and toxic concentrations quoted are from Valdes et al. (1998) for cardiac drugs, Patsalos et al. (2008) for antiepileptic drugs, White and

Wong (1998) for analgesics, and Linder and Keck (1998) for antidepressant drug monitoring. The lists of side effects are not comprehensive and authoritative sources should be consulted for fuller information. Information on the type of sample needed is predominantly taken from Hammett-Stabler and Dasgupta (2007).

Assay TechnologyThere are a number of methods for determining drug concentrations in serum or plasma. Excellent analytical methods are available that rely on extraction and chro-matographic separations, e.g., high-performance liquid chromatography (HPLC). Liquid chromatography– tandem mass spectrometry-based assays (LC-MS/MS) are widely used in clinical laboratories. They provide specific-ity and sensitivity and are capable of measuring several drugs on a single run. However, immunoassay techniques are also widely used for routine TDM because of simplic-ity, reliability, standardization across laboratories, and the wide availability of instrumentation (immunoassays are also used for a wide range of endogenous substances such as hormones). These issues have been recently discussed in detail (Brandhorst et al. 2012).

RADIOIMMUNOASSAYIn radioimmunoassay (RIA), a biological sample is mixed with a radiolabeled compound of interest and an immobi-lized antibody, specific to that agent. Measured radioactiv-ity bound to the immobilized antibody is inversely proportional to the agent of interest. The technique can be extremely sensitive and has historically been used widely for a number of agents found in very low concentrations (cannabis, lysergic acid diethylamide, digoxin, and para-quat). The disadvantages of the assay include the problems arising from the use of radiolabeled compounds (e.g., the need for expensive reagents and equipment, and the incon-veniences associated with the safe disposal of radioactive material) and the availability of those radiolabels. For these reasons, the technique is now rarely used for the purpose of TDM.

NONISOTOPIC IMMUNOASSAYNonisotopic immunoassay methods use inexpensive, sim-ple, and rapid measurements with similar specificity and sensitivity to RIA. Immunoassay methodology may be broadly separated into homogeneous and heterogeneous assays. Heterogeneous assays include the classical RIA tech-nique in that the labeled antigen competes with the unla-beled antigen for limited antibody-binding sites. After separation of the free antigen from the bound variety, the

Therapeutic Drug Monitoring (TDM)Philip A. Routledge ([email protected])

Alun D. Hutchings ([email protected])

C H A P T E R

9.22

946 The Immunoassay Handbook

measurement of enzyme activity in either fraction correlates with the concentration of free antigen. Homogeneous immunoassay, on the other hand, does not rely on this sepa-ration step, enabling simpler, faster protocols, so it is par-ticularly suited to random-access clinical chemistry analyzers. Although most homogeneous assays are less sen-sitive than heterogeneous assays, the majority of therapeutic drugs tend to circulate at concentrations well within the working ranges of these assays, making them ideally suited for TDM.

Enzyme-Multiplied Immunoassay TechniqueThe enzyme-multiplied immunoassay technique (EMIT®) is a simple, rapid homogeneous method now commonly employed to measure a wide range of substances (particu-larly drugs). The technique works on the basis that the drug present is proportional to the inhibition of an enzyme substrate reaction. A known quantity of drug is chemically labeled with an enzyme (e.g., glucose-6-phosphate dehy-drogenase), and antibodies specific to that drug bind that drug–enzyme complex so reducing enzyme activity. The introduction of a biological sample containing the same drug will release the enzyme-labeled drug from the anti-body complex, thereby increasing enzymatic activity. Enzyme activity correlates with drug concentration in the specimen as measured by absorbance changes resulting from the activity of the enzyme on a particular substrate. See HOMOGENEOUS IMMUNOASSAY.

Cloned Enzyme Donor ImmunoassayThis technique exploits antigen–antibody binding and spectophotometrically monitored enzyme activity in a similar fashion to EMIT. β-galactosidase is supplied as inactive fragments, the larger of which acts as enzyme acceptor while the smaller fragment behaves as enzyme donor. The two fragments combine to form the active enzyme. By conjugating hapten (analyte) to the donor fragment, antibodies to the hapten can prevent the for-mation of intact active enzyme. The analyte present in the test sample competes for binding sites on the anti-body, so an increase in analyte concentration will decrease binding of antibody to the donor fragment and increase enzyme activity. This activity is monitored spectrophoto-metrically through the production of chlorophenol red (CPR) from CPR-β-galactosidase. See HOMOGENEOUS IMMUNOASSAY.

Enzyme-Linked Immunosorbent AssayThe enzyme-linked immunosorbent assay (ELISA) tech-nique is a heterogeneous technique that depends on the antibody to a particular drug being bound to a microplate, and the antigen (analyte) is labeled with enzyme (typically horseradish peroxidase). For TDM applications, assays are usually in the competitive format. Analysis is relatively straightforward, and the process involves a small amount of antibody solution being added to each well, and the plate is then incubated for an appropriate length of time. This pro-cess binds the antibody to the wells after which the plate is washed with a blocking buffer and dried. Most commercial

kits are supplied with pre-coated plates in which case this initial step is unnecessary. Sample is added to the antibody-coated wells along with a buffer solution containing the enzyme-labeled analyte, and the plate is incubated at ambi-ent temperature to allow the antibody, analyte, and enzyme-labeled analyte to equilibrate. Finally, the plate is washed to remove any unbound enzyme, and the plate reacted with, e.g., 3,3′,5,5′-tetramethylbenzidine (TMB), if the enzyme is horseradish peroxidase. The oxidation of TMB results in a yellow color, which is monitored spectro-photometrically, and the intensity of that color is inversely proportional to the quantity of the analyte under test.

FluoroimmunoassayFluoroimmunoassay offers an alternative signal genera-tion and detection system to RIA with potentially greater sensitivity. Fluoroimmunoassays may also be categorized as homogeneous and heterogeneous. Heterogeneous methods require a separation step since the activity of the fluorophor is unaffected by its antibody binding. In contrast, the fluorophor in the homogenous assay is affected by its attachment to the antibody and, therefore, does not require a separation step, making it particularly useful for clinical chemistry systems and enabling faster assay times. See SIGNAL GENERATION AND DETECTION SYSTEMS.

Fluorescence enhancement and quenching assays are based on the principle that the fluorescence properties of a fluorescent-labeled antigen are altered once bound to an antibody. These assays do not require a separation step and, unlike many immunoassay techniques, do not rely on specialist equipment, using conventional fluorimeters for their measurements. See HOMOGENEOUS IMMUNOASSAYS.

Polarization fluoroimmunoassay is based on the behavior of a fluorophor-labeled antigen when subjected to plane-polarized light. Preferential excitement occurs when the axes of the molecules lie parallel to the light plane, and rotation of the molecule results in depolariza-tion of that light. Since the degree of depolarization is dependent on the size of the molecule (large molecules rotate more slowly), the binding of the complex to a large antibody will significantly reduce this rotation, resulting in a reduction of depolarization. In an immunoassay of this type, unlabeled antigen competes with the fluorescent-labeled antigen for binding sites on the antibody, and the depolarization of the emitted light is proportional to the concentration of drug in the sample. See HOMOGENEOUS IMMUNOASSAYS.

Nonisotopic immunoassays have largely replaced RIA approaches due to their simplicity. As with RIA, no sample pretreatment (e.g., solvent extraction) is required, so the technique is ideally suited to the emergency situation. Like RIA, however, the technique often suffers from a lack of specificity, and cross-reactivity with other substances, such as metabolites, is common. All immunoassays depend on the availability of suitable antibodies, which may not always be available. For example, it has not been possible to pro-duce an antibody for amiodarone that does not cross-react with triiodothyronine and thyroxine. Nevertheless, the ease with which such assays may be performed has in many cases reduced the need for specialist laboratory services and

947CHAPTER 9.22 Therapeutic Drug Monitoring (TDM)

placed such assays within the remit of the general hospital biochemistry laboratory, thereby making such measure-ments more widely available.

Microparticle Enzyme ImmunoassayThis technique is similar to ELISA except that the analyte is captured by antibody-coated beads rather than by a microwell plate. Separation is facilitated by a suitable method such as fiberglass mat or magnet and the beads incubated with an anti-analyte antibody, labeled with an enzyme and a suitable substrate. This is not a competitive assay, and the fluorescent or chemiluminescent product is proportional to the amount of analyte under test.

Measurement of Free Drug ConcentrationA basic principle of TDM is that the circulating drug con-centration in blood or plasma is related to the concentra-tion of the drug at its site of action and, therefore, correlates with the magnitude of drug effect. Most drug assays mea-sure the total concentration (bound plus free) of the sub-stance in blood or plasma/serum, and in most cases, this is an adequate reflection of drug concentration at the site of action. Certain drugs are highly bound in plasma to albu-min, alpha-1-acid glycoprotein (AAG), lipoproteins, and other proteins, and it is the free (unbound) drug concen-tration that reflects the concentration at the site of action (Routledge, 1986). Provided there is a reasonable correla-tion between the free and total concentration, measure-ment of the latter is a useful surrogate measure for the former. Any marked variability in the plasma protein bind-ing in a population will reduce the validity of total concen-tration monitoring. Such variability is generally relatively low if the drug is predominantly bound to albumin and the patient group is healthy (Ebden et al., 1984; Rimmer et al., 1984). However, conditions causing low albumin concen-trations (e.g., renal or liver disease or malnutrition), or dis-placement of the drug from binding sites (e.g., by other drugs or nonesterified fatty acids, or in uremia) will mean that the total drug concentration may underestimate the free (active) drug concentration in plasma. Measurement of free drug concentration may, therefore, be useful in such circumstances.

AAG is an acute phase protein that may vary markedly in its plasma concentration, both within and between indi-viduals. Concentrations rise after burns, trauma, inflam-mation, and acute myocardial infarction. Lower than normal concentrations may occur in neonates, chronic liver disease, and nephrotic syndrome, and during preg-nancy. A number of basic drugs (e.g., lidocaine and quini-dine) are bound to a significant degree to this protein, and the total drug concentration may not adequately reflect the free drug concentration in these circumstances (Rout-ledge et al., 1986). Even in healthy individuals, the plasma AAG concentration may vary with time, possibly because of intercurrent viral or other infections, resulting in changes in the plasma protein binding of some (often basic) drugs. Free drug concentrations can be estimated if

the total drug concentration and the concentration of the major binding protein are known, and methods have been developed for drugs binding significantly to AAG, such as lidocaine (Routledge et al. 1985), and for drugs binding predominantly to albumin, such as phenytoin (Krasowski & Penrod, 2012).

Despite these considerations, total drug concentrations generally correlate well with free drug concentrations of commonly measured drugs (e.g., phenytoin or theophylline) in most general patient populations so that routine measure-ment of free drug concentrations has not become common. Nevertheless, the availability of rapid, reliable, and inexpen-sive techniques to measure free drug concentration would be useful in circumstances where variability in protein binding of certain drugs might be greater than normal.

Practical Aspects of TDMSince drugs are normally given at fixed intervals, the plasma concentration varies between doses during the pro-cesses of absorption, distribution metabolism, and excre-tion. The greatest variability is in rate of absorption, so samples taken when absorption is relatively complete are therefore more reflective of the average (steady-state) con-centration between doses. Thus, samples should be taken at least 8 h after digoxin administration and around 12 h after lithium. In certain cases, peak levels are more impor-tant in assessing efficacy (e.g., with aminoglycoside antibi-otics), and since they occur around 30–60 min after an intramuscular injection or immediately at the end of an intravenous infusion, samples should ideally be taken at those times. Peak levels of orally-administered drugs are achieved at 30–180 min after conventional formulations and later after modified-release preparations. For many drugs, a sample taken just before the next dose is due (pre-dose or “trough” concentration) will correlate best with the average (steady-state) concentration, although it will of course be lower.

It takes around five half-lives of a drug before the plasma concentrations reach their maximum, steady-state level. Sampling before this time has elapsed is therefore most usually considered only if toxicity of the drug due to exces-sive accumulation is suspected or anticipated. In other cir-cumstances, five half-lives should be allowed to elapse before the steady-state plasma concentrations are mea-sured. This may be a considerable time in certain cases (e.g., 7–8 months in the case of amiodarone, which has a half-life of around 45 days). Details of sampling time rela-tive to dose, time of last dosage change, and present daily dose schedule should always be stated on the assay request form to aid in the clinical interpretation of the plasma con-centration. Several immunosuppressants (e.g., ciclosporin) are measured in whole blood, with EDTA as anticoagu-lant. Not only are they distributed in erythrocytes as well as plasma, but redistribution during processing of speci-mens (e.g., due to ambient temperature changes) means that anticoagulated whole blood is preferred to plasma.

Finally, the concentration–response relationship is a con-tinuous one, and adequate efficacy may be seen in some patients when the plasma drug concentration is below the accepted “therapeutic range” for the population. Therapeutic


ranges should, therefore, be considered as guidelines based on the average patient group, and the best dose of drug for an individual patient is the lowest one consistent with adequate efficacy and minimum toxicity.

INTERFERENCES WITH IMMUNOASSAYSBilirubin, lipids, hemolysis, paraproteins, drug metabolites, and endogenous substances can all cause interferences in immunoassays. This may result in positive or negative bias. This area is discussed in detail elsewhere (Dasgupta, 2007, 2012). See INTERFERENCES IN IMMUNOASSAYS.

Some endogenous substances are detected by many digoxin immunoassays (Dasgupta, 2012). They are known collectively as digoxin-like factors (DLFs), digoxin-like immunoreactive factors (DLIFs), or digoxin-like immuno-reactive substances (DLISs). The extent of the interfer-ence varies from assay to assay primarily due to the specificity of the antisera. As a result of DLF, “digoxin” concentrations have been found in patients who have never received the drug, primarily neonates, pregnant women, and patients with renal or hepatic failure. DLF may be the natural ligands for digoxin receptors in the body, but their exact identity is as yet unknown. In some affected assays, extending the assay incubation time can reduce the inter-ference. Pre-ultrafiltration is an effective method for reducing DLF in samples.

Certain exogenous substances, including various drugs and, particularly, the drug “Digibind” (made from Fab frag-ments of digoxin antibodies and used to treat life-threatening digoxin overdoses), can cause interference in the digoxin immunoassay. In the case of “Digibind,” the antibody frag-ments mop up the free digoxin, rendering it inactive in the patient. However, the extra digoxin antibody present in the sample can cause misleading results in immunoassays. For example, in a solid-phase competitive assay, the labeled digoxin may be bound by the antibody in the sample, lead-ing to a low signal and apparently high concentration. In a polyethylene glycol precipitation assay, this labeled digoxin-Fab complex is precipitated, and the apparent concentration of digoxin may be zero. Using an antibody with a relatively higher affinity for digoxin than the Digibind antibodies may reduce the interference. Alternatively, a pretreatment stage that removes proteins may be effective.

There may be significant cross-reactivity of the major metabolite of carbamazepine, carbamazepine 10,11-epox-ide, of up to 94% with the parent compound (Dasgupta, 2012). Cross-reactivity of tricyclic antidepressant metabo-lites with their parent compounds is discussed later in this chapter. Finally, cross-reactivity between the metabolites of several immunosuppressants and their parent com-pounds can be significant and can vary according to the particular immunoassay used.

Antiarrhythmic AgentsThese agents are generally basic compounds, some of which (e.g., lidocaine) are used predominantly for the control of ventricular arrhythmias, whereas others (e.g., disopyramide) are used to treat arrhythmias arising either from ventricular or supraventricular sites.

ACECAINIDE (N-ACETYLPROCAINAMIDE)See Fig. 1.

Clinical ApplicationsAcecainide (N-acetylprocainamide) is used (relatively rarely) in some countries in the treatment of ventricular arrhythmias, in particular life-threatening arrhythmias in patients with procainamide-induced lupus erythematosus. It is the major metabolite of procainamide.

Mode of AdministrationOrally in divided doses.

Pharmacological EffectsThe drug increases the effective refractory period with a selective lengthening of the cardiac potential by length-ening repolarization. Since this is accomplished without an effect on depolarization, the drug is classified as a Class III antiarrhythmic agent (procainamide is a Class 1A agent).

PharmacokineticsAcecainide is 60–90% excreted unchanged in the urine in patients with normal renal function (Connolly & Kates, 1982) and declines in plasma with an elimination half-life (t½,elim) of 6–9 h.

Therapeutic Range10–20 mg/L.10–20 µg/mL.

Potentially Toxic Concentration>40 mg/L.>40 µg/mL.

Side EffectsCommon adverse effects include light headedness, insom-nia, nausea, stomach upset, and blurred vision. It is thought to be unlikely to be associated with the production of drug-induced lupus.

Type of SampleSerum or plasma.

DIGOXINDigoxin is one of the cardiac glycosides that are character-ized by a complex multi-ring structure (Fig. 2).

NH

O

O

NH

N

FIGURE 1 Acecainide (N-acetylprocainamide).


Clinical ApplicationsDigoxin is used in the treatment of heart failure and in certain supraventricular arrhythmias, particularly atrial fibrillation and atrial flutter.

Mode of AdministrationDigoxin is most often administered orally. It can also be given as a slow intravenous infusion in an emergency, although this is rarely necessary.

Pharmacological EffectsDigoxin increases the force of contraction of the heart and slows the resting heart rate. It also slows conduction through the atrioventricular node and works in this way to terminate or reduce the risk of recurrence of supraventric-ular tachycardia.

PharmacokineticsDigoxin is well absorbed after oral administration and eliminated predominantly by renal glomerular filtration (90% is normally excreted unchanged in urine) with a t½,elim of 20–50 h, which is prolonged in renal dysfunction (REDUCE DOSE IN RENAL DYSFUNCTION).

Therapeutic Range0.0005–0.002 mg/L.0.5–2.0 µg/L.0.5–2.0 ng/mL.Sample must be taken at least 6 h after dose.

Potentially Toxic Concentration>0.003 mg/L.>3 µg/L.>3 ng/mL.

Lower if predisposing factors are present (e.g., low blood potassium [hypokalemia]).

Side EffectsLoss of appetite, nausea and vomiting, diarrhea, abdomi-nal pain, slowing of the heart, and cardiac arrhythmias (both abnormally fast and abnormally slow) have been reported in association with digoxin. Effects on the central

nervous system (CNS) include visual disturbances, head-ache, fatigue, drowsiness, confusion, delirium, and hallucinations.

Type of SampleSerum or plasma (EDTA or heparin).

DIGITOXINDigitoxin has a similar structure to digoxin.

Clinical ApplicationsSee DIGOXIN.

Mode of AdministrationOral.

Pharmacological EffectsSee DIGOXIN. However, digitoxin has a longer half-life and is primarily metabolized in the liver.

PharmacokineticsDigitoxin is almost completely absorbed after oral admin-istration and largely metabolized in the liver, with an aver-age t½,elim in plasma of 7.5 days.

Therapeutic Range (Adult)0.01–0.03 mg/L.10–30 µg/L.10–30 ng/mL.

Potentially Toxic Concentration>0.045 mg/L.>45 µg/L.>40 ng/mL.

Lower, if predisposing factors are present (e.g., low blood potassium [hypokalemia]).

Side EffectsSee DIGOXIN. Adverse effects may be more prolonged than after digoxin, due to its much longer elimination half-life.

FIGURE 2 Digoxin.


Type of SampleSerum or plasma (heparin, fluoride, or oxalate).

PROCAINAMIDESee Fig. 3.

Clinical ApplicationsProcainamide is used in the treatment of ventricular arrhythmias (especially after myocardial infarction) and atrial tachycardia.

Modes of AdministrationProcainamide can be given orally, by slow intravenous injection, or by intravenous infusion.

Pharmacological EffectsProcainamide is a Class 1A antiarrhythmic that prolongs the action potential duration and the effective refractory period. It can also reduce the force of contraction of the heart.

PharmacokineticsProcainamide is 75%–95% absorbed after oral administra-tion. Approximately 50%–60% of the drug is excreted unchanged in the urine with a variable proportion as the active metabolite N-acetylprocainamide (see ACECAINIDE), since N-acetylation (which occurs in the liver) rate is sub-ject to genetic polymorphism. The t½,elim of procainamide in plasma is around 2.5–4.7 h.



Side EffectsProcainamide can reduce blood pressure by causing myo-cardial depression as well as nausea, vomiting, diarrhea, and on occasion, psychosis. It may be associated with drug-induced lupus erythematosus in as many as 30% of those patients taking procainamide for six months or longer. This condition does not appear to be caused by N-acetyl-procainamide, the major metabolite of procainamide, which has pharmacological activity in its own right (see ACECAINIDE).


QUINIDINESee Fig. 4.

Clinical ApplicationsQuinidine is used for the treatment and prevention of both supraventricular and ventricular arrhythmias.


Pharmacological EffectsQuinidine is a Class 1A agent that increases the action potential duration and the effective refractory period. It also reduces the force of contraction of the heart and pos-sesses anticholinergic activity.

PharmacokineticsQuinidine is almost completely absorbed after oral admin-istration and extensively metabolized in the liver. The average t½,elim in plasma is 7 h (range 4–12 h).

Therapeutic Range2.0–5 mg/L.2.0–5 µg/mL.


Side EffectsQuinidine may cause nausea and vomiting and the syn-drome of “cinchonism” (tinnitus, dizziness), and it may also cause hypotension or serious ventricular arrhythmias. In addition, quinidine may produce various reactions related to hypersensitivity.

Assay LimitationsSome immunoassays suffer from cross-reactivity from dihydroquinidine (drug impurity) and quinidine metabo-lites (e.g., O-desmethylquinidine).


FIGURE 3 Procainamide hydrochloride. FIGURE 4 Quinidine.


Drugs Acting on the Respiratory SystemTHEOPHYLLINESee Fig. 5.

Clinical ApplicationsTheophylline is used in the treatment of patients with reversible airways obstruction (asthma and chronic obstructive lung disease).

Modes of AdministrationTheophylline is normally administered orally as a modi-fied-release preparation. Theophylline can also be given in the form of an ethylenediamine salt (aminophylline) orally or rarely intravenously (must be given slowly). Theophyl-line (80 mg) is equivalent to aminophylline (100 mg).

Pharmacological EffectsIt may act in part by blocking adenosine receptors.

PharmacokineticsTheophylline is well absorbed after oral administration and is extensively metabolized, largely in the liver. The t½,elim of conventional formulations is short (mean 8.2, range 3.6–12.8 h), so it is normally administered orally in a variety of modified formulations with different release characteristics. In such circumstances, it is advised that samples for TDM should normally be taken 4–6 h after a dose and at least 5 days after starting treatment (BNF, 2012a).

Therapeutic Range10–20 mg/L (BNF, 2012a).10–20 µg/mL.


There is a narrow margin between therapeutic and toxic dose (i.e., a low “therapeutic index”).

Side EffectsNausea, vomiting, tachycardia, palpitations, arrhythmias, headache, insomnia, and convulsions have been reported in association with theophylline use.

Type of SampleSerum or plasma (heparin, EDTA).

AntibioticsMany of the antibiotics (e.g., penicillins and cephalosporins) have a wide safety margin, and plasma monitoring is there-fore not required. TDM is of great value with the aminogly-coside antibiotics (e.g., amikacin, gentamicin, and tobramycin) and vancomycin, which in excessive dose may cause kidney damage or damage to the eighth cranial nerve (serving hear-ing and balance). Therefore, serum or plasma concentrations should be measured in all patients receiving these drugs by injection or infusion. Those at particular risk are patients with renal impairment, the elderly, obese individuals, patients with cystic fibrosis and, particularly, if high doses are being administered. The role of TDM in antimicrobial use in gen-eral is discussed in detail elsewhere (Roberts et al., 2012).

AMIKACINSee Fig. 6.

Clinical ApplicationsAmikacin is a semisynthetic aminoglycoside antibiotic used in the treatment of Gram-negative infections resis-tant to gentamicin.

Mode of AdministrationAmikacin is not absorbed orally and is given by either intramuscular or slow intravenous injection either once or twice daily or, in severe infections, three times daily.

Pharmacological EffectsLike all aminoglycoside antibiotics, amikacin blocks the production of protein by binding to the 30S ribosome and thus inhibiting messenger RNA in the bacterial cell. A conventional “therapeutic range” is not applicable to anti-biotics, in which the aim is to achieve a concentration above the minimum inhibitory concentration for the bac-teria while ensuring that trough levels are sufficiently low to reduce the risk of damage to susceptible organs.

FIGURE 5 Theophylline. FIGURE 6 Amikacin.


PharmacokineticsAmikacin is excreted by the kidney with an average t½,elim in plasma of 2.3 h (range 2.2–2.5 h) in patients with normal renal function. REDUCE DOSE IN RENAL IMPAIRMENT.

Target Peak ConcentrationFor the multiple-daily dose regimen, the peak concentra-tion (at 1 h) should not exceed 30 mg/L (BNF, 2012a).

Target Trough ConcentrationFor the multiple-daily dose regimen, the target trough concentration (before the next dose) should be less than 10 mg/L (BNF, 2012a). For the once-daily dose regimen, the pre-dose (trough) concentration should be less than 5 mg/L (BNF, 2012a).

Potentially Toxic ConcentrationPeak concentration >32 mg/L (>32 µg/mL) (Dasgupta et al., 2007).

Side EffectsAbove the target peak or trough concentrations, there is an increased risk of damage to the eighth cranial nerve, par-ticularly the vestibular branch, which mediates balance (but also the auditory branch, which subserves hearing). This may be irreversible. Above the target peak or trough con-centrations, there is also an increased risk of nephrotoxic-ity. More rarely, aminoglycosides may be associated with nausea and vomiting and stomatitis, antibiotic-associated colitis, peripheral neuropathy, and electrolyte disturbances (e.g., hypomagnesemia, and sometimes hypocalcemia or hypokalemia). Given in large doses during surgery, they have been associated with a transient myasthenic syndrome (muscle weakness and fatigability) in patients with normal neuromuscular function.


GENTAMICINSee Fig. 7.

Clinical ApplicationsGentamicin is an aminoglycoside antibiotic used in the treat-ment of severe infections (often in combination with other antibiotics) including endocarditis (also in combination with other antibiotics). It is used to treat pneumonia in hospital patients and as adjunctive therapy in listerial meningitis.

Mode of AdministrationGentamicin is not absorbed orally and so is given by intra-muscular injection or by slow intravenous injection or infusion, normally three times daily. It is sometimes also given as a once-daily dose.

Pharmacological EffectsSee AMIKACIN.

PharmacokineticsGentamicin is largely excreted unchanged by the kidney (predominantly by glomerular filtration) with an average t½,elim in plasma of 1–4 h in patients with normal renal function. REDUCE DOSE IN RENAL IMPAIRMENT.

Target Peak ConcentrationFor the multiple-daily dose regimen, the peak concentra-tion (at 1 h) should be 5–10 mg/L (3–5 mg/L for endocar-ditis). For once-daily dose regimen, consult local guidelines (BNF, 2012a).

Target Trough ConcentrationFor multiple-daily dose regimen, the target trough con-centration (before the next dose) should be less than 2 mg/L (<1 mg/L for endocarditis). For once-daily dose regimen, consult local guidelines (BNF, 2012a).

Potentially Toxic ConcentrationAbove target peak or trough concentrations.

Side EffectsSee AMIKACIN.


TOBRAMYCINSee Fig. 8.

Clinical ApplicationsTobramycin is very similar to gentamicin, see GENTAMICIN. It has slightly better efficacy against Pseudomonas aeruginosa but less activity against some other Gram-negative bacteria.

Modes of AdministrationIt is normally given by intramuscular injection. It is also given by nebulizer or by inhalation of powder to treat chronic pulmonary P. aeruginosa infection in cystic fibrosis.

FIGURE 7 Gentamicin: C1, R1 = R2 = CH3; C2, R1 = CH3, R2 = H; C1A, R1 = R2 = H.


Pharmacological EffectsSee AMIKACIN.

PharmacokineticsTobramycin is largely excreted unchanged by the kidney with an average t½,elim in plasma of 2–3 h in individuals with normal renal function. REDUCE DOSE IN RENAL IMPAIRMENT.

Target Peak ConcentrationFor the multiple-daily dose regimen, the peak concentra-tion (at 1 h) should not exceed 10 mg/L (BNF, 2012a).

Target Trough ConcentrationFor the multiple-daily dose regimen, the target trough concentration (i.e., before the next dose) should be less than 2 mg/L (BNF, 2012a).


Side EffectsSee AMIKACIN. It may cause less ototoxicity than other ami-noglycosides when given for more than 10 days.


VANCOMYCIN

Clinical ApplicationsThe glycopeptide antibiotic vancomycin is used systemi-cally in the prophylaxis and treatment of endocarditis and other serious infections caused by Gram-positive cocci, including methicillin-resistant Staphylococcus aureus (MRSA). Vancomycin (added to dialysis fluid) is also used in the treatment of peritoneal dialysis-associated peritoni-tis (an unlicensed indication).

Mode of AdministrationBy intravenous infusion, normally twice daily. Given orally (when it is not significantly absorbed), it is effective in the treatment of antibiotic-associated (pseudomembranous) colitis.

Pharmacological EffectsVancomycin inhibits Gram-positive bacterial cell wall syn-thesis at a different site from the beta-lactam antibiotics (cross-resistance therefore does not occur) and is normally bactericidal. It cannot penetrate the outer membrane of most Gram-negative organisms and so has limited activity against these organisms.

PharmacokineticsVancomycin is largely excreted unchanged by the kidney (predominantly by glomerular filtration) with average t½,elim in plasma of 5–11 h in normal renal function. REDUCE DOSE IN RENAL IMPAIRMENT.

Target Peak ConcentrationUncertain. The target trough concentration has been rec-ommended as the most accurate and practical method for monitoring efficacy (Martin et al., 2010).

Target Trough ConcentrationThe target trough concentration (before the next dose) should be l0–15 mg/L (15–20 mg/L for less sensitive strains of methicillin-resistant S. aureus) (BNF, 2012a).


Side EffectsAfter parenteral administration, vancomycin may be associ-ated with renal damage and ototoxicity (discontinue the drug if the patient experiences tinnitus). If the infusion is given rapidly, severe hypotension (including shock and car-diac arrest), wheezing, dyspnea, urticaria, pruritus, flushing of the upper body (“red man” syndrome), pain, and muscle spasm in the back and chest have been reported. All patients require measurement of serum/plasma vancomycin con-centration. This should be measured after three or four doses, if the patient’s renal function is normal prior to treatment, or earlier, if there is preexisting renal impairment.


AnticonvulsantsAnticonvulsant monitoring is used both to reduce the risk of drug toxicity and to indicate likely therapeutic concen-trations, because the clinical end point (absence of sei-zures) can only be assessed prospectively (i.e., if the patient remains seizure free). Many patients require more than one agent for optimal therapy, and since several

FIGURE 8 Tobramycin.


anticonvulsant drugs can induce the metabolism of others, e.g., by increasing activity of hepatic cytochrome P450 enzymes, and some can inhibit the metabolism of others, TDM is widely used to ensure that serum/plasma concen-trations of these agents are optimal. The topic of combina-tion and drug interactions is comprehensively addressed elsewhere (Majkowski et al., 2005). The t½,elim times, in plasma quoted below, are for monotherapy, but because of drug interactions caused by enzyme induction, these t½,elim times may be shorter during combination therapy. Pat-salos and colleagues have provided estimates of t½,elim times for combination therapy and have comprehensively reviewed the role of TDM in epilepsy (Patsalos et al., 2008).

CARBAMAZEPINESee Fig. 9.

Clinical ApplicationsCarbamazepine is an anticonvulsant used in the treatment of partial and secondary generalized tonic–clonic seizures, but not in primary generalized seizures. It is also used to treat trigeminal neuralgia and in the prevention of bipolar affective disorder (manic-depressive psychosis) unrespon-sive to lithium. It is used as a part of some approaches to alcohol withdrawal and in diabetic neuropathy (although these indications are both unlicensed in the UK).

Mode of AdministrationCarbamazepine is normally given orally but can also be administered by suppository.

Pharmacological EffectsCarbamazepine reduces the spread of impulses from epi-leptic foci. It also increases the activity of cytochrome P450 in the liver (enzyme induction) and may thus reduce the plasma concentration of drugs metabolized by this route (e.g., estrogens in oral contraceptives), resulting in clinically significant drug–drug interactions. The principal metabolite carbamazepine 10,11-epoxide has around one third of the anticonvulsant activity of the parent drug.

PharmacokineticsCarbamazepine is extensively metabolized in the liver. After a single dose, t½,elim in plasma is around 30–40 h, but because of “autoinduction” (carbamazepine induces its own metabolism), t½,elim may fall to around 12 h after long-term monotherapy. Carbamazepine, as a potent enzyme inducer, is associated with interactions with other drugs by increasing their clearance.


Potentially Toxic Concentration>15 mg/L (Broussard, 2007).>15 µg/mL.

Side EffectsUnsteadiness, drowsiness, headache, confusion, agita-tion (particularly in the elderly) double vision, drowsi-ness, nausea, vomiting, loss of appetite, constipation, and diarrhea have been reported. Hyponatremia (low plasma sodium concentration) is also reported. Hyper-sensitivity reactions (e.g., blood, liver, or skin disorders) may occur rarely. There is a risk of Stevens–Johnson syndrome in presence of HLA-B*1502 allele, which is more prevalent in individuals of Han Chinese or Thai origin.


GABAPENTINSee Fig. 10.

Clinical ApplicationsGabapentin is used alone or in combination with other medicines for the treatment of partial seizures with or without secondary generalization. It is also licensed for the treatment of neuropathic pain and is sometimes used in the prevention of episodes of migraine (an unlicensed indication).


Pharmacological EffectsIt is thought to act on T-type calcium channel function, inhibiting the release of various neurotransmitters and modulators.

PharmacokineticsOral absorption is around 60% and is dose dependent (absorption reduces with increasing dose). Gabapentin is predominantly excreted unchanged by the kidney (pre-dominantly by glomerular filtration) with an average t½,elim in plasma of 5–7 h in patients with normal renal function. REDUCE DOSE IN RENAL IMPAIRMENT.

FIGURE 9 Carbamazepine.

O

OHNH2

FIGURE 10 Gabapentin.


Therapeutic RangeUncertain, but effective concentrations may be between 2–20 mg/L (Patsalos et al., 2008) and 10–20 mg/L (Brous-sard, 2007).

Potentially Toxic ConcentrationUncertain. One source quotes >85 mg/L (Broussard, 2007).

Side EffectsA wide variety of side effects, particularly involving the gastrointestinal (GI) system and CNS, have been described. Hypersensitivity reactions have been reported in associa-tion with gabapentin.


LAMOTRIGINESee Fig. 11.

Clinical ApplicationsLamotrigine is used alone or in combination, for the treat-ment of focal seizures and generalized seizures, including tonic–clonic seizures. It is also used in seizures associated with Lennox–Gastaut syndrome. It is used as monother-apy in typical absence seizures in children. Finally, it is used for the prevention of depressive episodes associated with bipolar disorder.


Pharmacological EffectsUnknown. It may suppress the release of the excitatory amino acid, glutamate.

PharmacokineticsLamotrigine is completely absorbed after oral administra-tion and eliminated largely by hepatic metabolism. The mean t½,elim in plasma is 29 h.

Therapeutic RangeUncertain. 2.5–15 mg/L.

Potentially Toxic ConcentrationUncertain. One source quotes >20 mg/L (Broussard, 2007).

Side EffectsA wide variety of side effects, particularly involving the GI system and CNS, have been described. Serious skin reac-tions including Stevens–Johnson syndrome and toxic epi-dermal necrolysis have also been described in association with lamotrigine use.


LEVETIRACETAMSee Fig. 12.

Clinical ApplicationsLevetiracetam is licensed for use alone or in combination for the treatment of focal seizures, with or without second-ary generalization, and for the adjunctive therapy of myo-clonic seizures in patients with juvenile myoclonic epilepsy and primary generalized tonic–clonic seizures.

Mode of AdministrationIt is normally given orally but can also be given by intrave-nous infusion.

Pharmacological EffectsLevetiracetam is thought to prevent hypersynchronization of epileptiform burst firing and propagation of seizure activity.

PharmacokineticsIt is virtually completely absorbed after oral administra-tion and more than half is excreted unchanged in the urine. The t½,elim in plasma is 6–8 h (Drugbank).

Therapeutic RangeUncertain but one source quotes a reference range of 12–46 mg/L (Patsalos et al., 2008).

Potentially Toxic ConcentrationUncertain.

Side EffectsA wide range of side effects, particularly involving the GI system and CNS, have been described. Serious skin reac-tions, including Stevens–Johnson syndrome and toxic epi-dermal necrolysis, have also been described in association with levetiracetam.

N

CI

CI

NN

NH2H2N

FIGURE 11 Lamotrigine.

OH

NO

NH2

FIGURE 12 Levetiracetam.



PHENOBARBITALSee Fig. 13.

Clinical ApplicationsPhenobarbital is used in the treatment of all forms of epi-lepsy (except typical absence seizures) and in the treatment of status epilepticus.

Mode of AdministrationOral. It can also be given by slow intravenous injection (e.g., in status epilepticus).

Pharmacological EffectsPhenobarbital increases the seizure threshold and reduces the spread of discharge from an epileptic focus.

PharmacokineticsPhenobarbital is completely absorbed after oral adminis-tration and extensively metabolized by the liver. The aver-age t½,elim is 100 h (range 50–150 h). Phenobarbital is a potent cytochrome P450 enzyme inducer, leading to inter-actions with other drugs by increasing their clearance.



Side EffectsSedation, lethargy, depression, unsteadiness, stupor, and coma have been reported. Paradoxical excitement, restlessness, and confusion may occur in the elderly, and hyperkinesia may be a problem in children. Megaloblas-tic anemia (sometimes responsive to folic acid) and osteomalacia may occur after prolonged use. Very rarely, Stevens–Johnson syndrome and toxic epidermal necrolysis have been associated with phenobarbital therapy.


PHENYTOIN (DIPHENYLHYDANTOIN)See Fig. 14.

Clinical ApplicationsPhenytoin (diphenylhydantoin) is used in the treatment of status epilepticus and in acute symptomatic seizures asso-ciated with neurosurgery or trauma to the head.

Mode of AdministrationBy mouth or (in emergency) by slow intravenous injection or infusion. Phenytoin (diphenylhydantoin) is poorly and variably absorbed after intramuscular administration and should not be given by this route.

Pharmacological EffectsThe mode of action is not fully understood, but phenytoin (diphenylhydantoin) appears to reduce the spread of epileptic discharges, thus reducing seizure propagation.

PharmacokineticsPhenytoin (diphenylhydantoin) is almost completely absorbed after oral administration and is extensively metabolized by the liver. Hepatic metabolism is character-ized by Michaelis–Menten (dose-dependent “saturable”) pharmacokinetics so that small increases in dose can result in disproportionately greater increases in plasma concen-tration, even over the therapeutic range of plasma concen-trations. The t½,elim is therefore highly variable (range 7–60 h) and also dose dependent. Phenytoin is a potent enzyme inducer, leading to interactions with other drugs by increasing their clearance.



Side EffectsPhenytoin (diphenylhydantoin) can cause nystagmus (abnormal jerky movements of the eyes), nausea, vomiting, confusion, tremor, insomnia, nervousness, drowsiness and coma. Long-term use of the drug may be associated with GI symptoms, acne, gingival hyperplasia, and hirsutism. Hypersensitivity (non-dose-related) reactions (e.g., leuco-penia, severe skin rashes, and more rarely, hepatotoxicity) are rare but may occur in association with phenytoin.

FIGURE 13 Phenobarbital. FIGURE 14 Phenytoin.


There is a risk of Stevens–Johnson syndrome in presence of HLA-B*1502 allele, which is more prevalent in indi-viduals of Han Chinese or Thai origin.


PRIMIDONESee Fig. 15.

Clinical ApplicationsPrimidone is used in the treatment of all forms of epilepsy except absence seizures. It is also used to treat essential tremor.


Pharmacological EffectsSee PHENOBARBITAL.

PharmacokineticsPrimidone is well absorbed (c. 75%) after oral administra-tion. The mean t½,elim in plasma is 10 h (range 4–22 h). Primidone is partly metabolized in the liver to two major active metabolites, phenylethylmalonamide and pheno-barbital (phenobarbitone), both of which have longer t½,elim values than the parent compound. Primidone is a cytochrome 450 enzyme inducer, leading to interactions with other drugs by increasing their hepatic clearance.


Phenobarbital concentrations should also be measured (see PHENOBARBITAL).

Potentially Toxic Concentration>15 mg/L.>15 mg/mL.

Phenobarbital concentrations should also be measured.

Side EffectsSee PHENOBARBITAL. Visual disturbances have also been reported. Very rarely psychosis, lupus erythematosus and arthralgia have been associated with primidone use.


TOPIRAMATESee Fig. 16.

Clinical ApplicationsTopiramate can be given alone or in combination with other agents in generalized tonic–clonic seizures or focal seizures with or without secondary generalization. It can be used in combination with other agents for the treat-ment of seizures associated with Lennox–Gastaut syn-drome. Finally, it is also licensed for the prevention of migraine.


Pharmacological EffectsThe mechanism of action is unknown, although topiramate enhances gamma-aminobutyric acid (GABA)-activated chloride channels and may have other inhibitory effects on excitatory neurotransmission (Drugbank).

PharmacokineticsTopiramate is well (>80%) absorbed after oral adminis-tration and primarily eliminated, unchanged in the urine (approximately 70% of an administered dose, Drug-bank). The t½,elim in plasma is 20–30 h (Patsalos et al., 2008).

Therapeutic RangeUncertain, but one source quotes a reference range of 5–20 mg/L (Patsalos et al., 2008).


Side EffectsA wide range of side effects, particularly involving the GI system and CNS, has been described. Topiramate has also been associated with acute myopia with secondary angle-closure glaucoma.


VALPROIC ACIDSee Fig. 17.

FIGURE 15 Primidone.

O

O O

O

O O

NH2OS

H

H H

O

FIGURE 16 Topiramate.


Clinical ApplicationsValproic acid is effective against a wide variety of seizure types including generalized (tonic–clonic) seizures and partial seizures. It is sometimes used in the treatment of migraine (an unlicensed indication).

Mode of AdministrationOral and intravenous administration.

Pharmacological EffectsValproic acid may act by binding to, and inhibiting, GABA transaminase. This increases the CNS concentrations of GABA (an inhibitory neurotransmitter at the GABA–ben-zodiazepine complex).

PharmacokineticsValproate is completely absorbed after oral administration and almost completely metabolized in the liver. The aver-age t½,elim in plasma is 12 h (range 9–21 h).



Side EffectsNausea and vomiting, drowsiness, confusion, ataxia and tremor, transient alopecia (with subsequent growth of curly hair), increased appetite, weight gain, and edema have been reported. Blood disorders, liver dysfunction (including fatal hepatic failure) and very rarely, pancre-atitis, toxic epidermal necrolysis, and Stevens–Johnson syndrome have occurred in association with valproate use.


ZONISAMIDESee Fig. 18.

Clinical ApplicationsZonisamide is used (in combination with other treatments) for the management of refractory focal seizures with or without secondary generalization.


Pharmacological EffectsIt is reported that zonisamide (a sulfonamide) binds to sodium channels and voltage-sensitive calcium channels, thus suppressing neuronal depolarization and hypersyn-chronization (Drugbank).

PharmacokineticsZonisamide is rapidly but variably absorbed after oral administration. It is extensively metabolized in the liver. The t½,elim in plasma is 50–70 h (Patsalos et al., 2008).

Therapeutic RangeUncertain, but one source quotes a reference range of 10–40 mg/L (Patsalos et al., 2008).


Side EffectsA wide range of side effects, particularly involving the GI system and CNS, have been described in association with zonisamide. Very rarely, Stevens–Johnson syndrome, and toxic epidermal necrolysis have been reported in associa-tion with zonisamide therapy.


Drugs for Malignancy and ImmunosuppressionCICLOSPORIN (CYCLOSPORIN)Ciclosporin is the internationally accepted name and the British approved name (previously Cyclosporin). Cyclosporin is a US adopted name.

Clinical ApplicationsCiclosporin is an immunosuppressant used in the field of organ and tissue (bone marrow, kidney, liver, pancreas, heart, lung, and heart–lung) transplantation to prevent graft rejection and for the prophylaxis of graft-versus-host disease. It is also used in severe acute ulcerative colitis refractory to corticosteroid treatment (an unlicensed indi-cation), nephrotic syndrome, rheumatoid arthritis, atopic dermatitis, and psoriasis.

Mode of AdministrationThe drug can be given orally or by intravenous infusion.

FIGURE 17 Valproic acid.

N

S

O O

ONH2

FIGURE 18 Zonisamide.


Pharmacological EffectsBy binding to cyclophilin, ciclosporin acts as a calcineurin inhibitor (the latter is normally responsible for activating transcription of interleukin) Thus, ciclosporin inhibits activation of T lymphocytes, particularly T (helper) cells, and reduces production of lymphokines, especially inter-leukin-2 (IL-2).

PharmacokineticsCiclosporin is variably absorbed after oral administration. It is extensively metabolized in the liver, with little renal excretion of unchanged drug. The drug is also excreted in the bile. The t½,elim in blood is 6–27 h (Butch, 2007).

Therapeutic RangeVariable, in part dependent on the specificity of the tech-nique used (some methods also measure a number of metabolites).

Potentially Toxic ConcentrationVariable, in part dependent on the specificity of the tech-nique used (some methods also measure a number of metabolites).

Toxic Side EffectsCiclosporin is relatively nontoxic to blood cells (non-myelotoxic) but is very nephrotoxic. Tremor, hyperten-sion, hepatic dysfunction, nausea and vomiting, increased hair growth, and overgrowth of the gums (gingival hyper-plasia) may also occur. Rare side effects reported in asso-ciation with ciclosporin include visual disturbances secondary to benign intracranial hypertension. Anaphy-laxis has been reported in association with intravenous administration.

Assay limitationsMetabolite cross-reactivity may result in differing results with different immunoassays. This is discussed in detail by Butch (2007).

Type of SampleWhole blood (EDTA anticoagulant).

METHOTREXATESee Fig. 19.

Clinical ApplicationsMethotrexate is used for the maintenance therapy of childhood acute lymphoblastic leukemia and other malig-nancies, including choriocarcinoma, non-Hodgkin’s lym-phoma, and some solid tumors. Intrathecal methotrexate is used in the CNS prophylaxis of childhood acute lym-phoblastic leukemia and as a treatment for established meningeal cancer or lymphoma. It is also used in the treatment of severe psoriasis and rheumatoid arthritis. It has also been used in Crohn’s disease (an unlicensed indication).

Mode of AdministrationMethotrexate can be given orally, intravenously, intramus-cularly, or intrathecally.

Pharmacological EffectsMethotrexate is an antimetabolite, which, by inhibiting the enzyme dihydrofolate reductase, inhibits the synthesis of the purines and pyrimidines that are necessary for nucleic acid synthesis.

PharmacokineticsMethotrexate is well absorbed after oral administration. A significant proportion (44–100%) of the dose is excreted unchanged by the kidney (by glomerular filtration and active tubular secretion). The t½,elim is 3–10 h at low doses and 8–15 h at high doses (Drugbank).

Therapeutic RangeEffective concentrations are dependent upon the indica-tion (Dasgupta et al., 2007).

Potentially Toxic ConcentrationToxicity is dependent on duration of exposure. Metho-trexate concentrations are used to guide the use of folinic acid to speed recovery from methotrexate-induced muco-sitis or myelosuppression (folinic acid rescue). For chil-dren in the UK, folinic acid rescue therapy is normally continued until the plasma methotrexate concentration falls to 45–90 µg/L (45–90 ng/mL) (BNF, 2012b).

Side EffectsMethotrexate is associated with a range of side effects. These include bone marrow suppression (myelosuppres-sion), ulceration of the mucus membranes (mucositis) and more rarely pulmonary toxicity (particularly in rheuma-toid arthritis), and renal or liver damage. Stevens–Johnson syndrome and toxic epidermal necrolysis have been reported in association with methotrexate.


MYCOPHENOLIC ACIDSee Fig. 20.

Clinical ApplicationsMycophenolic acid is used in the prophylaxis of acute renal, cardiac, or hepatic transplant rejection (in combina-tion with ciclosporin and corticosteroids).

FIGURE 19 Methotrexate.


Mode of AdministrationOrally or intravenously.

Pharmacological EffectsMycophenolic acid is a selective reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH), thus blocking the de novo pathway of guanosine nucleotide syn-thesis and producing cytostatic effects in T and B lympho-cytes (Drugbank).

PharmacokineticsMycophenolate can be given as mycophenolic acid or as a pro-drug, mycophenolate mofetil (the doses of the two forms are not equal), which is metabolized to mycopheno-lic. However, unnecessary switching between the two should be avoided because of pharmacokinetic differences between the two forms. The t½,elim of mycophenolic acid (the active agent) is 8–16 h (Drugbank).

Therapeutic RangeUncertain. One source quotes effective concentrations of 1.0–3.5 mg/L (trough, Butch, 2007).


Side EffectsA range of adverse effects affecting the GI tract has been reported. Myelosuppression (including leucopenia, ane-mia, thrombocytopenia, pancytopenia, and red cell aplasia) has also been reported. It may be associated with an increased risk of opportunistic infections (e.g., cytomegalovirus).


SIROLIMUS

Clinical ApplicationsSirolimus is used for the prevention of organ rejection in kidney allograft recipients (initially in combination with ciclosporin and a corticosteroid and then with a corticoste-roid only).


Pharmacological EffectsSirolimus binds to the immunophilin, FK-binding pro-tein-12 (FKBP-12), to generate an immunosuppressive complex that then binds to and inhibits the “mammalian target of rapamycin” (mTOR). Sirolimus is therefore an mTOR inhibitor (not a calcineurin inhibitor like ciclospo-rin), which selectively inhibits cytokine production, thus inhibiting T-lymphocyte activation and proliferation (Drugbank).

PharmacokineticsIt is relatively poorly absorbed and metabolized in the intestine and liver, with a t½,elim in blood of 57–63 h (Drugbank).

Therapeutic RangeUncertain. Depends on the assay used. The whole blood sirolimus (trough) concentration is normally measured.

Potentially Toxic ConcentrationVariable.

Side EffectsA range of side effects affecting particularly the GI tract, blood and skin disorders, and metabolic effects have been described in association with sirolimus.

Assay LimitationsImmunoassays may have significant cross-reactivity with metabolites of sirolimus. Different immunoassays can, therefore, all give clinically significant differences in results compared with each other or compared with HPLC. Switching between the assays is, therefore, not recom-mended, since it could result in inappropriate dose adjust-ments (BNF, 2012a). The therapeutic range should, therefore, be interpreted with knowledge of the particular assay used.


TACROLIMUS

Clinical ApplicationsTacrolimus is used in the prevention of organ rejection in heart, kidney and liver allograft recipients, and also in allograft rejection resistant to conventional immunosup-pressive regimens. It is also used in moderate to severe atopic eczema.

Mode of AdministrationTacrolimus is given orally or by intravenous infusion.

Pharmacological EffectsTacrolimus is (like ciclosporin) a calcineurin inhibitor that inhibits both T-lymphocyte signal transduction and IL-2 transcription (Drugbank).

O

OOH

HO

O

O

FIGURE 20 Mycophenolic acid.


PharmacokineticsIt is absorbed after oral administration but bioavailability is low (20% or less). It is metabolized largely in the liver with mean t½,elim in blood of 11.3 h (range 3.5–40.6 h) (Drugbank).

Therapeutic RangeUncertain. Depends on the assay used. The whole blood tacrolimus trough concentration is normally measured.

Potentially Toxic ConcentrationVariable.

Side EffectsSimilar to ciclosporin, but the incidence of neurotoxicity may be greater in association with tacrolimus and distur-bances of glucose homeostasis may also occur more fre-quently. Cardiomyopathy has also been reported in children.

Assay LimitationsImmunoassays may have significant cross-reactivity with metabolites of tacrolimus. Different immunoassays can, therefore, all give clinically significant differences in results compared with each other or compared with HPLC. The therapeutic range should, therefore, be interpreted with knowledge of the particular assay used.


MiscellaneousACETAMINOPHEN (PARACETAMOL)See Fig. 21.

Clinical ApplicationsAcetaminophen is used in the treatment of mild to moder-ate pain and in the relief of pyrexia.

Dose and Mode of AdministrationOral.

Pharmacological EffectsThe mode of action of acetaminophen is unknown. It may involve central inhibition of cyclo-oxygenase-3 (COX-3).

PharmacokineticsAcetaminophen is virtually completely absorbed after oral administration and almost completely metabolized, largely

in the liver, with a mean t½,elim in plasma of 1–3 h (mean 2.3 h) h. Around 10% is metabolized to a highly reactive intermediate compound (N-acetyl-p-benzoquinoneimine (NABQI)), which is then conjugated with glutathione and excreted in the urine as a mercapturate conjugate. In over-dose, glutathione stores may be insufficient to conjugate (and thus detoxify) the increased amount of NABQI pro-duced. This can result in liver damage (which may be severe) and also sometimes renal damage.


Potentially Toxic ConcentrationsPlasma or serum acetaminophen concentrations are used as a guide in the management of acetaminophen poison-ing. The risk of toxicity appears to be related to the plasma concentration at any given time, as well as to the nutri-tional state, chronic alcohol misuse, and concomitant drug therapy. Above the appropriate recommended treatment line(s), antidotal treatment (e.g., N-acetylcysteine) should be used. Such treatment reduces the risk or severity of liver damage after overdose. The recommended treatment pro-tocols differ in different countries.

Toxic Side EffectsIn overdose, liver damage and more rarely renal damage.


TRICYCLIC ANTIDEPRESSANTS

Clinical ApplicationsTricyclic antidepressants are most effective in treating moderate to severe endogenous depression but there may be an interval of 2–4 weeks before beneficial effects are seen. They are also effective in the management of panic disorder. Although many related compounds are available, amitriptyline, imipramine, and desipramine are well-established compounds for which TDM may occasionally be helpful. Further details concerning TDM of antide-pressants are provided by Linder and Keck (1998).


Pharmacological EffectsTricyclic antidepressants inhibit the uptake of norepi-nephrine and serotonin in adrenergic and serotonergic neurons. Most tricyclic antidepressants also have periph-eral and central anticholinergic properties.

Therapeutic RangeAmitriptyline: 120–250 µg/L (120–250 ng/mL).Imipramine: 180–350 µg/L (180–300 ng/mL).Desipramine: 115–250 µg/L (115–250 mg/mL).

FIGURE 21 Acetaminophen.


Potentially Toxic ConcentrationAmitriptyline: >500 µg/L (>500 ng/mL).Imipramine: >500 µg/L (>500 ng/mL).Desipramine: >500 µg/L (>500 ng/mL).

Side EffectsDry mouth, sedation (especially with amitriptyline), blurred vision, constipation, difficulty in passing urine. Tachycardia and postural fall in blood pressure also occur.


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the immuassay handbook parte93

Health & Medicine

plasma drug concentrations

plasma concentrations

drug treatments

antidepressant drug

plasma serum

toxic concentrations

centration of active

monitoring of serum