40 Therapeutic drug monitoring

• Serious toxicity in conjunction with poorly defined clinical endpoint for therapeutic efficacy
• Steep dose response curve
• Narrow therapeutic range
• Long term therapy
• Life threatening illnesses
• Significant inter individual differences in pharmacokinetics
• Non linear pharmacokinetics
• Frequent administration of the drug
• Reliable methods of determination

• P amikacin* (trough)

• P amikacin* (peak)

• P gentamicin* (trough)

• P gentamicin* (peak)

• P tobramycin* (trough)

• P tobramycin* (peak)

• P vancomycin (trough)

• P amitriptyline + nortriptyline

• P citalopram

• P clomipramine + norclomipramine

• P desipramine

• P doxepin + nordoxepin

• P duloxetine

• P escitalopram

• P fluoxetine + norfluoxetine

• P fluvoxamine

• P imipramine + desipramine

• P maprotiline

• P mirtazapine

• P nortriptyline

• P paroxetine

• P sertraline

• P trimipramine

• P venlafaxine + O-desmethylvenlafaxine

• P carbamazepine

• P 10-OH-carbazepine

• P ethosuximide

• P gabapentin

• P lamotrigine

• P levetiracetam

• P phenobarbital

• P phenytoin, total

• P phenytoin, free

• P primidone

• P topiramate

• P valproic acid

• P zonisamide

• P itraconazole

• P voriconazole

• P imatinib (trough)

• P methotrexate 24 h

• P methotrexate 48 h

• P methotrexate 72 h

• B cyclosporine (trough)

Kidney

Liver

Heart

Stem cells

• B tacrolimus (trough)

Kidney

Liver

Heart

Stem cells

• Concomitant cyclosporine

• Concomitant tacrolimus

• P MPA AUC (0–12 h)

• Kidney transplantation – Triple therapy with cyclosporine, corticosteroids, and sirolimus

• Dual therapy with sirolimus and corticosteroids

• Therapy with cyclosporine or tacrolimus, sirolimus, +/- corticosteroids

• Therapy with sirolimus +/- corticosteroids

• Triple therapy with cyclosporine, corticosteroids, and everolimus

• Organ transplantation (triple therapy with azathioprine)

• Chronic inflammatory bowel disease (azathioprine)

• Chemotherapy (6-mercaptopurine)

• E 6-methylmercaptopurine (6-MMP)

• P amiodarone

• P digitoxin

• P digoxin, in heart failure (sinus rhythm): 0.5–0.8 μg/L

• P lidocaine

• P aripiprazole

• P clozapine

• P haloperidol

• P olanzapine

• P quetiapine

• P risperidone + 9-OH risperidone

• S lithium

• P theophylline

• P theophylline, premature infants

Anticonvulsants – Generally

Indication for measuring concentration: therapy failure, e.g. due to unreliable patient compliance or pharmacokinetic reasons; suspected toxicity, e.g. cases with unclear neurological or psychiatric symptoms; drug therapy in patients with altered pharmacokinetics (e.g., in pregnant patients or patients on dialysis).

– Phenytoin

Therapeutic range: 10–20 mg/L adults, children; 6–14 mg/L premature and full-term newborns, infants (age 2–12 weeks). The therapeutic range for the free phenytoin concentration in the serum is 1–2 mg/L.

Elimination half-life: it does not make sense to specify a half-life since phenytoin has dose-dependent pharmacokinetics. The elimination of this drug is better described by the maximum rate of metabolism (V_max) and the concentration at which the half-maximum rate of metabolism (K_m) is present: V_max 100–1,000 mg/d; K_m 1–15 mg/L.

Recommendation for blood sampling: during the dose interval.

Elimination: metabolism in the liver; the metabolites have no significant anticonvulsive effect.

Clinical aspects: because phenytoin exhibits non-linear pharmacokinetics, no definite prediction can be made about the expected serum concentration after a specific dose. Doses should be increased in small increments only, otherwise toxicity can occur due to a disproportionate rise in the serum concentration. Toxic doses can have a pro convulsive effect. In the presence of chronic hepatic disease, the rate of phenytoin metabolism is reduced. During pregnancy, there is generally a decline in the serum phenytoin concentration. An increase in phenytoin dose seems to be necessary in patients with brain tumors or brain metastases undergoing chemotherapy (e.g., cisplatin) to maintain the phenytoin concentration within the therapeutic range. The concentration of free phenytoin not bound to serum proteins correlates generally with its concentration in the cerebrospinal fluid and saliva. In patients with altered serum protein binding or when an unexpected effect occurs, the serum free phenytoin concentration allows a better estimate of toxicity and efficacy than does the total phenytoin concentration /9/. Altered protein binding of phenytoin, for example, is observed with hypoalbuminemia, displacement from albumin binding (e.g., by valproic acid), hyperbilirubinemia, and uremia. Phenytoin concentrations > 20 mg/L are associated with side effects (nystagmus, ataxia). Concentrations > 35 mg/L are associated with increased tendency to seizures.

– Phenobarbital

Therapeutic range: 10–40 mg/L.

Elimination half-life: 50–120 h (adults), 40–70 h (children).

Recommendation for blood sampling: during the dose interval.

Elimination: metabolism in the liver; no known metabolites with anticonvulsive effect.

Clinical aspects: the interpretation of the serum phenobarbital concentration is complicated by the fact that tolerance to the effect on the central nervous system can develop. The upper limit of the therapeutic range can vary between patients. Therefore, the measurement of phenobarbital is of limited value. Because of its ability to induce hepatic microsomal enzymes, phenobarbital can influence the metabolism of other drugs.

– Primidone

Therapeutic range: 5–15 mg/L.

Elimination half-life: 4–22 h.

Recommendation for blood sampling: measure peak level 2–4 h after the last dose; measure trough level immediately prior to the next dose.

Elimination: primidone is metabolized in the liver into two metabolites with anticonvulsive effect (phenobarbital, phenyl ethyl malonamide) and one without anticonvulsive activity. Approximately 25% of the administered dose of primidone is metabolized to phenobarbital, which, due to its long half-life, accumulates and reaches serum concentrations more than double those of primidone.

Clinical aspects: phenobarbital resulting from the conversion of primidone is responsible for most of the anticonvulsive effect of primidone. To monitor therapy with primidone, multiple measurements of both primidone and phenobarbital are undertaken, with phenobarbital being of greater significance. Primidone concentrations > 15 mg/L are usually considered toxic.

– Carbamazepine

Therapeutic range: 4–10 mg/L.

Elimination half-life: 10–25 h (continuous therapy).

Recommendation for blood sampling: Measure peak level 6–18 h after the last dose; measure trough level immediately prior to the next dose.

Elimination: carbamazepine is metabolized in the liver. Only small amounts (approx. 1%) of the parent drug are excreted in the urine. Multiple metabolites are created, of which carbamazepine-10, 11-epoxide has shown anticonvulsive efficacy in animal studies. How much this metabolite contributes to the therapeutic effect of carbamazepine is unclear. The serum concentration of carbamazepine-10, 11-epoxide is 5–81% (average: 30%) of that of carbamazepine. The metabolism of carbamazepine can be induced by the substance itself or by other anticonvulsants (e.g., phenytoin or phenobarbital). The clearance of carbamazepine is reduced in the presence of underlying liver diseases. During pregnancy, a decrease in the carbamazepine serum concentration is observed.

Clinical aspects: the serum concentration of carbamazepine is difficult to estimate due to slow and incomplete intestinal absorption, low renal excretion, and large interindividual differences in the half-life. Therefore, it is important to monitor it. In general, the measurement of carbamazepine-10, 11-epoxide appears to be unnecessary for routine clinical practice. Toxic doses of carbamazepine can have a pro convulsive effect. Carbamazepine concentrations > 15 mg/L are associated with side effects (visual disturbances, ataxia, nystagmus).

– Oxcarbazepine

Therapeutic range: oxcarbazepine is a prodrug that is metabolized rapidly and almost completely to the active metabolite 10-hydroxycarbazepine. A therapeutic range for 10-hydroxycarbazepine has not yet been established. In seizure-free adults, plasma concentrations of 13–35 mg/L have been reported /10/. Concentrations of 15–55 mg/L have been reported in children. Side effects are more frequent at plasma concentrations of 35–40 mg/L.

Elimination half-life: 8–10 h for 10-hydroxycarbazepine.

Recommendation for blood sampling: measure trough concentration immediately prior to the next dose.

Elimination: 10-hydroxycarbazepine is eliminated mainly by conjugation to glucuronic acid, which is excreted by the kidneys. In contrast to carbamazepine, repeated doses of oxcarbazepine do not affect its own metabolism. Induction and/or inhibition of the cytochrome P450 system has little effect on the pharmacokinetics of oxcarbazepine and its active metabolites.

Clinical aspects: since there appears to be a linear relationship between the oxcarbazepine dose and the 10-hydroxycarbazepine plasma concentration, monitoring the plasma concentration of the active metabolite is of limited usefulness. An advantage of oxcarbazepine is that 10-hydroxycarbazepine does not undergo epoxidation, thereby preventing the production of carbamazepine epoxide, which is primarily responsible for the toxic side effects of carbamazepine. HPLC methods are available for determining 10-hydroxycarbazepine.

– Lamotrigine

Therapeutic range: 3–14 mg/L. These values are tentative, however, since a therapeutic range has not yet been firmly established /10/.

Elimination half-life: approximately 25 h (15–35 h).

Recommendation for blood sampling: measure trough concentration immediately prior to the next dose.

Elimination: mainly renal, as glucuronide.

HPLC and LC-MS/MS methods are available for determining plasma concentrations.

Clinical aspects: therapeutic drug monitoring is recommended for lamotrigine because of significant inter individual differences in its pharmacokinetics. The clearance of lamotrigine is age-dependent. It is higher in children than in adults and declines even further in elderly patients. It also appears to be significantly increased in the later stages of pregnancy. Lamotrigine is associated with significant drug interactions. Phenytoin, carbamazepine, and barbiturates induce the metabolism of lamotrigine, reducing its half-life to 15 h (8–20 h). Conversely, valproic acid inhibits the metabolism of lamotrigine, increasing its half-life to 60 h (30–90 h). HPLC methods are available for determining plasma concentrations of lamotrigine /10/.

– Topiramate

Therapeutic range: 5–20 mg/L /10/. These values are tentative.

Elimination half-life: 20–30 h.

Recommendation for blood sampling: Measure trough concentration immediately prior to the next dose.

Elimination: topiramate is mainly eliminated unchanged in the urine; only a certain proportion is metabolized by oxidation.

Clinical aspects: because the correlation between concentration and effect has not been studied systematically to date, the available therapeutic range is only tentative. In clinical studies, serum topiramate concentrations between 3.4 and 5.2 mg/L have been associated with an adequate anticonvulsive effect. In refractory patients, concentrations > 4 mg/L were necessary for an adequate response, and there are indications that concentrations > 10 mg/L may be required. Further studies are required to define the therapeutic window more clearly. Phenytoin and carbamazepine induce the metabolism of topiramate, leading to a significantly reduced serum concentration. Valproic acid can also lower the concentration of topiramate, but to a lesser extent. Due to these pharmacokinetic interactions, therapeutic drug monitoring for topiramate would appear to be useful in selected patients /10/.

– Ethosuximide

Therapeutic range: 40–100 mg/L.

Elimination half-life: 30–60 h (adults), 20–55 h (children).

Recommendation for blood sampling: during the dose interval.

Elimination: pharmacologically active metabolites do not occur; approximately 20% of the daily dose is excreted unchanged in the urine.

Clinical aspects: ethosuximide is used to treat petit mal epilepsy. Serum concentrations of greater than 150 mg/L are usually considered toxic. The frequency of absences can increase with toxic doses.

– Valproic acid

Therapeutic range: 50–100 mg/L.

Elimination half-life: 10–16 h.

Recommendation for blood sampling: measure peak level 1–4 (–8) h after the last dose; measure trough level immediately prior to the next dose.

Elimination: metabolism in the liver; 1–3% of the dose is excreted unchanged in the urine.

Clinical aspects: there does not seem to be a direct correlation between the serum concentration of valproic acid and its efficacy /11/. Routine serum measurements of valproic acid are therefore of limited value for optimizing the dose. Measurement of the valproic acid concentration is, however, clinically useful for checking patient compliance.

Cardiac glycosides – Generally

Indication for measuring concentration: suspected toxicity, e.g. arrhythmias; therapy failure, e.g. due to unreliable patient compliance or pharmacokinetic reasons. Therapeutic use in patients with altered pharmacokinetics, e.g. in renal failure, severe decompensated heart failure, thyroid dysfunction, or drug interactions. Lack of information about prior therapeutic use of cardiac glycosides (e.g., in the case of unconscious patients). In the case of a non-visible digitalis effect due to preexisting ECG abnormalities (e.g., in patients with a cardiac pacemaker or in ventricular bundle branch block).

– Digoxin

Therapeutic range: 0.8–2.0 μg/L; 0.5–0.8 μg/L (heart failure in sinus rhythm) /12/.

Elimination half-life: 1–2 d.

Recommendation for blood sampling: 8–24 h after the last dose.

Elimination: only a small percentage is converted in the liver to metabolites with cardiac effects and the significantly less active metabolite dihydrodigoxin. Elimination is predominantly renal.

Clinical aspects: because the effects of digitalis are dependent on numerous factors, there is an overlap between the therapeutic and toxic ranges for digoxin /13/. The interpretation of serum values is therefore only meaningful in conjunction with the assessment of the overall clinical situation. Factors that influence the serum concentration or efficacy of cardiac glycosides at therapeutic doses are:

Disturbances of electrolyte status and acid-base balance. A serum digoxin concentration usually considered therapeutic can exert toxic effects due to the presence of hypokalemia, e.g. as found with the combination of digoxin plus diuretics, hypercalcemia, and hypomagnesemia. Tolerance to digoxin is also reduced in acid-base disorders, tissue hypoxia, acute myocardial infarction, cardiomyopathies, and valvular heart diseases.

Concomitant diseases or conditions that result in a change in the pharmacokinetics of digoxin. These include decreased glomerular filtration in patients with advanced renal failure and in elderly patients and impaired intestinal absorption in patients with malabsorption syndrome. Patients with hypothyroidism generally seem to have higher, and patients with hyperthyroidism on average lower, digoxin concentrations than would be expected from the dose.

Drug interactions. A significant increase in the serum digoxin concentration can result from concomitant use of quinidine at a customary dose (750–1500 mg/d). Digoxin concentrations that do not correlate with the administered dose can occur with concomitant use of drugs that impair the intestinal absorption of digoxin (e.g., cholestyramine, neomycin, antacids, or kaolin/pectin).

High digoxin concentrations are frequently measured in patients with cardiac pacemakers because glycoside-induced ECG changes cannot be discerned.

– Digitoxin

Therapeutic range: 10–25 μg/L.

Elimination half-life: 6–8 d.

Recommendation for blood sampling: 8–24 h after the last dose. Digitoxin is much more strongly bound to protein (90–97%) than digoxin.

Elimination: digitoxin is metabolized in the liver. During this process, approximately 10% of the administered dose is converted into digoxin and 30% is eliminated renally.

Clinical aspects: see under digoxin.

Antiarrhythmic agents – Lidocaine

Therapeutic range: 1.5–5.0 mg/L.

Elimination half-life: 70–140 min.

Recommendation for blood sampling: during the infusion.

Elimination: metabolized in the liver. The clearance of lidocaine is reduced in the presence of liver diseases (e.g., cirrhosis) and reduced hepatic perfusion due to heart failure.

Clinical aspects: parenteral therapy of ventricular arrhythmias. See under disopyramide. Lidocaine can also be used as a test substance for the purpose of assessing liver function /14/. In this context, the cytochrome-P450-mediated formation of the lidocaine metabolite monoethylglycinexylidide (MEGX) serves as an indicator of liver function. Decreased cytochrome P450 3 A4 activity in the hepatocyte as well as disturbances of hepatic perfusion lower the rate of formation of this metabolite. The MEGX test was developed based on this concept. This test is particularly useful in the field of liver transplantation, where it is used to assess the quality of donor organs, to estimate the prognosis of transplant candidates, and to evaluate the function of the graft during the early postoperative course. In addition, the formation of MEGX is an early predictor of posttraumatic multi organ failure.

MEGX test protocol: blood sampling before, and 15 or 30 min. after, an i.v. bolus injection (administered over 2 min.) of 1 mg/kg lidocaine hydrochloride. Measurement of MEGX in the serum by HPLC, LC-MS/ MS /15/. If the MEGX concentration measured before the test dose is administered is above the detection limit (3 mg/L), this concentration is to be subtracted from the MEGX values at 15 and 30 min respectively. This test must not be performed in patients with known lidocaine allergy or cardiac injury.

Interpretation of test results /14/:

• MEGX 15 or 30 min. levels > 50 mg/L in liver donors are consistent with a functionally intact donor organ.
• MEGX 30 min. levels < 10 mg/L in transplant candidates with chronic liver disease indicate a high mortality risk.
• MEGX 15 min. levels < 20 mg/L within the first 3 days after liver transplantation are consistent with hepatic dysfunction and reduced survival of the transplanted organ.
• MEGX 15 min. levels < 30 mg/L on the 3rd day post injury in a patient with multiple trauma are predictive of multiple organ failure.

Based on current data, MEGX test results < 20 mg/L appear to be associated with hepatic dysfunction in adults. When interpreting the MEGX test results, it is important to consider the overall clinical picture, because extrahepatic factors, e.g. heart failure or therapeutic drugs, can affect hepatic perfusion or the activity of the cytochrome P450 system.

– Phenytoin

Therapeutic range: 10–18 mg/L.

Blood sampling, elimination: see under anticonvulsants.

Clinical aspects: oral and intravenous therapy for ventricular arrhythmias.

– Procainamide, N-acetylprocainamide

Therapeutic range: procainamide 4–10 mg/L, N-acetylprocainamide 6–20 mg/L. For the total serum concentration of procainamide and its active metabolite N-acetylprocainamide, a therapeutic range of 10–30 mg/L has been proposed. The clinical significance of this range, however, has not been adequately clarified.

Elimination half-life: procainamide 3–5 h, N-acetylprocainamide 6–10 h.

Recommendation for blood sampling: measure trough level immediately prior to the next dose; measure peak level 1–5 h after the last oral dose.

Elimination: metabolized in the liver to the active metabolite N-acetylprocainamide (20–50%). Due to N-acetyltransferase polymorphism, significant interindividual differences in the metabolism of this substance occur. Approximately 50% of the dose is excreted unchanged in the urine /16/. An increase in the serum concentration is seen in heart failure and renal failure.

Clinical aspects: prophylaxis and therapy of atrial and ventricular arrhythmias.

– Quinidine

Therapeutic range: 2–5 mg/L.

Elimination half-life: approximately 6 h.

 Recommendation for blood sampling: measure peak level 1–3 h after dose (for sustained release preparations, approximately 8 h); measure trough level immediately prior to the next dose.

Elimination: metabolized in the liver to metabolites that are likely to be active; 10–20% is excreted unchanged in the urine. Reduced elimination can result from chronic liver disease, heart failure, or urinary alkalinization.

Clinical aspects: continuous oral therapy and prophylaxis of atrial and ventricular cardiac arrhythmias.

– Disopyramide

Therapeutic range: 2–5 mg/L.

Elimination half-life: 7 h.

Recommendation for blood sampling: measure peak level 2–3 h after dose; measure trough level immediately prior to the next dose.

Elimination: N-desisopropyldisopyramide, a metabolite formed in the liver, exerts a lesser antiarrhythmic effect in animals. Approximatey 40–60% of the administered dose is excreted unchanged in the urine.

Clinical aspects: continuous oral therapy and prophylaxis of atrial and ventricular cardiac arrhythmias.

Clinical aspects of antiarrhythmic agents: monitoring the serum concentration of antiarrhythmic agents is clinically relevant for detecting the therapeutic endpoint. A clear relationship exists between the serum concentrations of some antiarrhythmic agents and their therapeutic efficacy. For instance, in patients with stable therapeutic serum concentrations of quinidine or procainamide, no new episodes of ventricular fibrillation were observed /17/. High serum concentrations of quinidine, procainamide, and N-acetylprocainamide can induce ventricular arrhythmias.

Theophylline

Therapeutic range: 8–20 mg/L (adults, children), 6–11 mg/L (premature infants).

Elimination half-life: 3–12 h adults; 2–6 h children (1–17 years) and smokers; approximately 30 h premature infants and adults with liver cirrhosis.

Recommendation for blood sampling: during continuous infusion until steady state is reached 4–8, 12, 24, 48 h after the start of the infusion and during further therapy for dose adjustments. Measure trough level immediately prior to the next maintenance dose; measure peak level approximately 1 h (for sustained release preparations, approximately 4 h) after the last dose.

Elimination: theophylline is largely converted to relatively inactive metabolites in the liver. In newborns, theophylline is also metabolized to caffeine. Only approximately 10% of the dose is excreted unchanged in the urine.

Clinical aspects: theophylline is used as a bronchodilator and for the treatment of apnea in premature infants. The clearance of theophylline is reduced in the presence of heart failure, liver cirrhosis, acute viral respiratory infections, and during the concomitant use of certain drugs (e.g., cimetidine, erythromycin, or allopurinol). Due to significant interindividual differences in pharmacokinetics, theophylline dose regimens based on body weight alone are unreliable and sometimes even dangerous for the patient.

Indication for measuring theophylline concentration: suspected toxicity, therapy failure (e.g., due to unreliable patient compliance or pharmacokinetic reasons), or administration of theophylline as a continuous i.v. infusion (e.g., in severe respiratory obstruction); lack of information about any prior therapeutic use of theophylline; or therapeutic use in patients with altered pharmacokinetics (e.g. in cases of concomitant disease, changes in patterns of tobacco use, or drug interactions). Pharmacokinetic methods can be used for individual dose adjustments /8/. Based on the theophylline concentration in the saliva, it is possible to estimate whether the correct dose was chosen and adequate patient compliance is present. The ratio of the saliva/serum concentration of theophylline is on average 0.68 (ranging from 0.5 to 0.85). In cases of suspected toxicity and for optimizing the dose, however, it is necessary to measure the theophylline concentration in the serum.

Methotrexate

Therapeutic range: a therapeutic range does not exist. To avoid serious toxic side effects, the serum methotrexate concentrations during high-dose methotrexate therapy (infusion over a period of 4–6 h) should be kept below the following values:

• ≤ 10 μmol/L 24 h after the start of the infusion
• ≤ 1.0 μmol/L 48 h after the start of the infusion
• ≤ 0.1 μmol/L 72 h after the start of the infusion.

Recommendation for blood sampling: 24, 48, and frequently also 72 h after the start of the methotrexate infusion. In cases of delayed elimination, further blood sampling is required until the methotrexate concentration is < 0.05–0.10 μmol/L.

Elimination half-life: biexponential decline in serum concentration with half-life of 2–4 h and 10–20 h respectively /18/.

Elimination: methotrexate is largely (80%) excreted unchanged in the urine. A small percentage of the dose reaches the intestine in the bile and undergoes enterohepatic circulation. It would appear that methotrexate can be metabolized by the intestinal bacterial flora to 4-amino-4-deoxy-N10-methylpteroic acid. 7-hydroxymethotrexate is another important metabolite, which is considered to be potentially nephrotoxic. Both metabolites exhibit significantly less inhibition of dihydrofolate reductase than methotrexate.

Clinical aspects: methotrexate is an inhibitor of dihydrofolate reductase. It is used in chemotherapy for malignant diseases (e.g., osteogenic sarcoma). Methotrexate clearance is reduced in the presence of renal and hepatic diseases as well as in aciduria. In the case of ascites or pleural effusions, the half-life of methotrexate is prolonged. Toxic side effects occur when the concentration exceeds the specified threshold values. The severity of toxicity is determined more by the length of time by which the time limit is exceeded than by the extent of the rise in the methotrexate concentration above the concentration limit. Monitoring the methotrexate concentration allows any possible risk to the patient to be detected at an early stage. If necessary, life-threatening side effects can then be prevented by administering the antidote leucovorin (citrovorum factor) and implementing measures to increase the renal excretion of methotrexate. Administration of leucovorin should be continued until the serum methotrexate concentration has decreased below 0.10 μmol/L), or better still, 0.01 μmol/L.

Lithium

Therapeutic range: 0.6–1.2 mmol/L /19/.

Elimination half-life: 14–33 h.

Recommendation for blood sampling: 12 h after the last dose.

Elimination: renal excretion, which is increased by a high intake of sodium and water.

Clinical aspects: even if renal function is normal and a standard maintenance dose is administered, significant variations in the serum lithium concentration can occur. Toxic side effects such as muscle twitching, ataxia, and drowsiness occur at concentrations > 1.5 mmol/L; seizures, dehydration, and coma occur at concentrations > 3.0 mmol/L; concentrations > 4.0 mmol/L are potentially lethal. It is therefore necessary to monitor the serum lithium concentration during continuous lithium therapy for depressive and manic-depressive disorders.

Amino glycosides

Therapeutic ranges:

• Amikacin: 20–30 mg/L (peak), < 5 (8) mg/L (trough)
• Gentamicin: 5–10 mg/L (peak), < 2 mg/L (trough)
• Netilmicin: 5–12 mg/L (peak), < 3 mg/L (trough)
• Tobramycin: 5–10 mg/L (peak), < 2 mg/L (trough)

Elimination half-life: 0.5–3.0 h in the presence of normal renal function.

Recommendation for blood sampling: measure peak level 30 min. after completion of an i.v. infusion over 30 min. or 1 h after i.m. administration; measure trough level immediately prior to the next dose.

Elimination: excreted unchanged in the urine. No known metabolites.

Clinical aspects: amino glycosides are used to treat serious infections involving gram-negative bacteria. Impaired renal function results in reduced clearance of amino glycosides. In dehydration, the distribution volume decreases. High concentrations of certain penicillins, e.g. carbenicillin, can inactivate amino glycosides in vitro and in vivo. Accumulation of amino glycosides in the tissues can result in ototoxicity and nephrotoxicity. The trough serum concentration (prior to the administration of the next dose) is an important indicator for the accumulation of these substances. A suitable dosing regimen has to be set up such that, on the one hand, an adequate peak serum concentration of the amino glycoside above the minimum inhibitory concentration for the pathogen in question can be achieved, while on the other hand, the trough serum concentration remains below the threshold above which toxicity can be expected. The use of pharmacokinetic methods for individual dosage adjustment is highly recommended /7/. For once-daily dosing of amino glycosides, no particular recommendations exist with regard to drug monitoring /20/. Once-daily dosing with 6.6 mg netilmicin/kg, for instance, resulted in an average peak serum concentration of 21.3 mg/L /21/.

Vancomycin

Therapeutic range: 5–20 mg/l (trough) /22/.

Elimination half-life: 4–10 h (adults), 2–3 h (children), 6–10 h (newborns).

Recommendation for blood sampling: measure peak level 1 h after completion of an i.v. infusion; measure trough level immediately prior to the next dose.

Elimination: more than 90% of the administered dose is excreted unchanged in the urine.

Clinical aspects: vancomycin is effective against gram-positive bacteria. A decline in renal function can cause the serum vancomycin concentration to increase to a toxic level. In patients with reduced liver function, elimination of vancomycin is delayed. At serum concentrations above the therapeutic range, vancomycin exerts ototoxic and nephrotoxic effects. The degree of ototoxicity is increased by concomitant use of amino glycosides or furosemide.

Cyclosporine

Therapeutic range: this is difficult to define since there are no easy-to-measure parameters for assessing the immunosuppressive effect. The trough serum concentration of cyclosporine prior to the next dose is generally used to guide cyclosporine therapy after organ transplantation /23/. The following tentative range for the cyclosporine trough concentration in whole blood is recommended for specific analytic methods such as LC-MS/MS: 100–300 μg/L. Therapeutic ranges for trough concentrations of cyclosporine are shown in Tab. 40-2 – Therapeutic ranges. During the early postoperative phase, cyclosporine concentrations should be maintained within the upper half of the therapeutic range. Usually, the dose of cyclosporine is reduced in a stepwise manner within 3–6 months after transplantation. The cyclosporine concentrations during maintenance therapy are adjusted to lie within the lower part of the therapeutic range. Cyclosporine concentrations > 400 μg/L are associated with a high risk of toxicity /23/. It is still unclear whether lower therapeutic ranges are adequate when cyclosporine is used in combination with other immunosuppressive drugs such as mycophenolate mofetil or sirolimus. An alternative approach is C2 monitoring. The cyclosporine concentration (C2) determined 2 h after the administration of the micro emulsion formulation Sandimmun^® Optoral allows a better estimate of cyclosporine exposure and the risk of acute rejection or toxicity than an estimate based on the trough level. The most significant inhibition of calcineurin activity and maximum reduction in IL-2 production by cyclosporine occur in the first two hours after cyclosporine administration. The particular importance of adequate cyclosporine exposure within the first 3–5 days after transplantation to prevent acute rejection has been documented in prospective studies of patients who have received kidney or liver transplants. However, prospective studies have been unable to provide evidence to support the theoretical benefit of C2 monitoring /24/.

Elimination half-life: terminal half-life in healthy individuals 6.3 (4.7–9.5) h, in kidney transplant patients 10.7 (4.3–53.4) h, and in patients with liver cirrhosis 20.4 (10.8–48.0) h /25/.

Recommendation for blood sampling: measure trough level immediately prior to the next dose, e.g. 12 h after each dose with twice-daily dosing. C₂ concentration: 2 h ± 15 min. after the last dose. Additional subsequent blood sampling, e.g. 6 h post-dose, is necessary in patients with reduced or delayed absorption. EDTA is the anticoagulant of choice.

Elimination: 99% of cyclosporine is metabolized in the liver and exhibits a first-pass effect. Multiple metabolites are produced /25/. Less than 1% of the dose is excreted unchanged in the urine or bile.

Clinical aspects: Cyclosporine metabolites are excreted mainly in the bile. Other immunosuppressive drugs such as mycophenolate mofetil or sirolimus. An alternative approach is C2 monitoring. The cyclosporine concentration (C2) determined 2 h after the administration of the microemulsion formulation Sandimmun^® Optoral allows a better estimate of cyclosporine exposure and the risk of acute rejection or toxicity than an estimate based on the trough level. The most significant inhibition of calcineurin activity and maximum reduction in IL-2 production by cyclosporine occur in the first two hours after cyclosporine administration. The particular importance of adequate cyclosporine exposure within the first 3–5 days after transplantation to prevent acute rejection has been documented in prospective studies of patients who have received kidney or liver transplants. However, prospective studies have been unable to provide evidence to support the theoretical benefit of C2 monitoring /24/.

Elimination half life: terminal half-life in healthy individuals 6.3 (4.7–9.5) h, in kidney transplant patients 10.7 (4.3–53.4) h, and in patients with liver cirrhosis 20.4 (10.8–48.0) h /25/.

Recommendation for blood sampling: measure trough level immediately prior to the next dose (e.g., 12 h after each dose with twice-daily dosing). C₂ concentration: 2 h ± 15 min. after the last dose. Additional subsequent blood sampling (e.g., 6 h post dose) is necessary in patients with reduced or delayed absorption. EDTA is the anticoagulant of choice.

Elimination: 99% of cyclosporine is metabolized in the liver and exhibits a first-pass effect. Multiple metabolites are produced /25/. Less than 1% of the dose is excreted unchanged in the urine or bile. Cyclosporine metabolites are excreted mainly in the bile.

Clinical aspects: cyclosporine is a highly effective immunosuppressive substance used to prevent graft rejection (kidney, liver, heart, lung, pancreas, bone marrow). It inhibits calcineurin and suppresses the synthesis of cytokines (e.g., IL-2). As a result of immunosuppression, there is a risk of infection, which is significantly higher in the presence of toxic cyclosporine concentrations. Cyclosporine can also cause serious nephrotoxic, hepatotoxic, and neurotoxic effects. These side effects are usually dose-dependent. In the event of deteriorating renal function, e.g. serum creatinine concentration > 2 mg/dL (177 μmol/L) 3–6 months after transplantation, cyclosporine can be reduced or discontinued and the use of other immunosuppressive drugs such as mycophenolate mofetil and sirolimus can be increased. The pharmacokinetics of cyclosporine show significant intraindividual and inter individual differences. After oral administration of the micro emulsion preparation Sandimmun^® Optoral in comparison to Sandimmun^®, less variability in the cyclosporine kinetics is observed as well as less dependence on bile acids and no interaction with food intake. The time required to reach the peak blood concentration after oral intake of Sandimmun^® Optoral is 2–3 h. Multiple drug interactions have been observed. Certain drugs (e.g., ketoconazole, phenytoin, phenobarbital, and steroids) affect the metabolism of cyclosporine whereas other substances (e.g., amino glycosides and cephalosporins) increase its nephrotoxicity.

Cyclosporine significantly decreases exposure to mycophenolic acid, an immunosuppressive agent frequently used in combination with cyclosporine. In patients with abnormal liver function, reduced absorption and delayed elimination of cyclosporine mean that lower doses are generally required. Children have a higher cyclosporine clearance than adults.

Monitoring of the blood concentration of cyclosporine is necessary to optimize the dose regimen. Recent studies have shown that acute rejection in the first year following kidney transplantation negatively impacts long-term graft survival.

Because numerous factors (e.g., temperature, lipoprotein concentration, and hematocrit) influence the blood/plasma concentration ratio, cyclosporine should be measured in hemolyzed blood. During the early postoperative phase in patients with kidney, liver, and heart transplants, cyclosporine blood monitoring is carried out 4–7 times per week. Following this period of close monitoring, the frequency of measurement is reduced in a stepwise manner. For instance, in kidney transplant patients with an uncomplicated course, the cyclosporine concentration should be monitored once a month during the first year and every 1–3 months thereafter. Additional measurements are necessary if the clinical situation requires a change in dose. This is also the case during concomitant treatment with drugs known to interact with the metabolism of cyclosporine. Changes in dose should be monitored the following week by measuring the cyclosporine level.

Specific HPLC and LC-MS/MS methods or immunoassays are available for measuring cyclosporine in the blood. For individual dose adjustment, especially in patients post liver transplantation or with abnormal liver function, specific methods such as LC-MS/MS /26/ or a specific immunoassay should ideally be used to determine the trough level. With regard to C₂ monitoring, differences in the specificities of the immunoassays are less relevant (more favorable ratio of parent substance to cross-reactive metabolites), which means that assay-specific reference values are unnecessary. Impaired liver function leads to the accumulation of cyclosporine metabolites in the blood. Routine measurements of metabolite concentrations, however, do not appear to be justified.

Tacrolimus

Therapeutic range: as in the case of cyclosporine, it is difficult to define a specific range. The trough serum concentration of tacrolimus prior to the next dose is used to guide tacrolimus therapy following organ transplantation. If commercial tacrolimus assays are used, a tentative trough concentration range of 5–18 μg/L should be used while, based on our experiences, the corresponding range for LC-MS/MS is 4–15 μg/L.

During the early postoperative phase, tacrolimus concentrations should be maintained within the upper half of the therapeutic range. The dose of tacrolimus is usually reduced in a stepwise manner within 3–6 months after transplantation (Tab. 40-2 – Therapeutic ranges). Concentrations > 20 μg/L are associated with an increased incidence of toxic side effects.

Elimination half-life: terminal half life approximately 16 h in patients with kidney transplant and approximately 12 h in patients with liver transplant; if hepatic function is severely impaired, the elimination half-life may be > 50 h.

Recommendation for blood sampling: immediately prior to the next dose, e.g. 12 h after each dose with twice-daily dosing. EDTA is the anticoagulant of choice.

Elimination: tacrolimus is mainly metabolized by cytochrome P450 3A in the liver. Multiple metabolites are produced. Less than 5% of the dose is excreted unchanged in the bile. Tacrolimus metabolites are excreted mainly in the bile.

Clinical aspects: tacrolimus is a highly effective immunosuppressive substance used to prevent graft rejection (e.g., liver, kidney, heart). Like cyclosporine, it inhibits calcineurin. As a result of immunosuppression, there is a risk of infection, which is significantly higher in the presence of toxic tacrolimus concentrations. Like cyclosporine, tacrolimus can cause dose-dependent nephrotoxic and neurotoxic side effects (such as tremor). Another side effect is hypertension. Tacrolimus also has a diabetogenic effect. However, it does not appear to have a significant effect on the plasma cholesterol concentration. The peak concentration of tacrolimus is reached after approx. 2 h (0.7–6 h). Taking tacrolimus with food delays the increase in the blood concentration. Multiple drug interactions have been observed. Certain therapeutic drugs (e.g., ketoconazole, erythromycin, fluconazole, diltiazem, cimetidine) and high-dose steroids increase the tacrolimus concentration while other substances (e.g., rifampicin, barbiturates, and carbamazepine) taken at the same time reduce the concentration. The pharmacokinetics of tacrolimus show significant intraindividual and inter individual differences. Monitoring of the blood concentration of tacrolimus is necessary to optimize the dose regimen. During the first two weeks after organ transplantation, tacrolimus levels should be measured four to seven times per week. The frequency of measurement can then be reduced in a step wise manner (as with cyclosporine). A sustained release formulation is available for once daily dosing. In concentration a formulation licensed for twice daily dosing, the trough concentrations achieved with once daily formulation are on average 12.7% lower at the same dose /27/. The two formulations are considered to be bioequivalent. The same TDM approach to the therapeutic range of tacrolimus applies for both formulations but the differences in the trough levels must be taken into account when interpreting the results /28/.

Immunoassays and highly specific LC-MS/MS /26/ methods are available for measuring tacrolimus in the blood. Cross reactivity with tacrolimus metabolites (M-II, M-III, M-V) is observed when immunoassays are used. Accumulation of tacrolimus metabolites (e.g. due to cholestasis) can therefore lead to falsely high tacrolimus concentrations if certain immunoassays are used.

Azathioprine

Therapeutic range: azathioprine itself is a prodrug and does not have immunosuppressive effects. It is converted into the active metabolite 6-thioguanine nucleotide (6-TGN). The tentative therapeutic range for the erythrocyte 6-TGN concentration in transplant recipients undergoing triple therapy with cyclosporine and prednisone is 100–450 pmol/0.8 × 10⁹ erythrocytes and in patients receiving azathioprine therapy for chronic inflammatory bowel disease is 250–450 pmol/0.8 × 10⁹ erythrocytes /29/. For chemotherapy with 6-mercaptopurine, a 6-TGN concentration of 500–1,000 pmol/0.8 × 10⁹ erythrocytes has been proposed. An increased risk of azathioprine intolerance is observed in patients with thiopurine methyl transferase (TPMT) deficiency /29/. Erythrocyte TPMT activity < 2.8 nmol/(mL Ery × h) indicates homozygous TPMT deficiency while TPMT activity of 2.8–10.0 nmol/(mL Ery × h) indicates heterozygous TPMT deficiency. During therapy with thiopurines (e.g., azathioprine, 6-mercaptopurine), induction of TPMT activity occurs, so that even patients with constitutively intermediate activity levels can have values > 10 nmol/(mL Ery × h).

Recommendation for blood sampling: during the dose interval. The measurement is performed in erythrocytes from heparinized or EDTA whole blood.

Elimination: in vivo, azathioprine is metabolized via 6-mercaptopurine to 6-TGN, by xanthine oxidase to 6-thiouric acid, and by TPMT to 6-methyl mercaptopurine. In TPMT deficiency, relatively more 6-TGN is produced.

Clinical aspects: azathioprine is used to treat chronic inflammatory bowel disease (e.g. Crohn’s disease) and following organ transplantation. Patients with a deficiency of the enzyme TPMT are at risk of azathioprine intolerance. In patients with homozygous TPMT deficiency, which is observed in 0.3% of Caucasians, normal doses of azathioprine lead to severe bone marrow suppression. Determination of TPMT activity can therefore be used to screen patients for homozygous TPMT deficiency before starting azathioprine therapy. In these patients, the possibility of using an alternative form of immunosuppressive therapy must be examined. If azathioprine is still to be used, it is essential to monitor the 6-TGN concentration. In these cases, approximately one-tenth of the normal dose must be used. The risks associated with TPMT deficiency are also relevant for the use of 6-mercaptopurine or thioguanine in chemotherapy, for acute juvenile leukemia in particular /30/. If the TPMT activity is consistent with a ”wild type” genotype [(10–20) nmol/mL Ery × h], therapy with standard doses of thiopurine drugs (e.g., azathioprine, 6-mercaptopurine) can be started immediately in accordance with the usual level of monitoring recommended by specialists (blood count, including platelet count). If intermediate TPMT activity [(2.8–9.9) nmol/(mL Ery × h] is demonstrated, typically due to a heterozygous TPMT gene mutation, therapy should be commenced at 30–50% of the standard dose and, if tolerated (WBC count, 6-TGN concentration), increased weekly until the target dose is reached. Note that in vitro assays cannot clearly identify inhibition of TPMT activity by concomitantly administered drugs (e.g., amino salicylate, sulfasalazine). In these cases, measurement of the 6-TGN concentration should be used to guide therapy. In patients with increased TPMT activity (> 20 nmol/mL Ery × h), there is evidence to suggest that using standard doses results in lower concentrations of active 6-TGN, which can lead to therapy failure. In these patients, increasing the dose can result in increased production of hepatotoxic metabolites (6-MMP, 6-methyl mercaptopurine; 6-MMPR, 6-methyl mercaptopurine ribonucleotide). The measured 6-TGN concentration in conjunction with monitoring of aminotransferases should be used to guide therapy.

An HPLC method is used to determine the 6-TGN concentration in erythrocytes. The activity of TPMT in the erythrocytes is also measured using suitable HPLC methods. However, it is not possible to determine the TPMT phenotype reliably based on the TPMT activity for 30–60 days after a blood transfusion. In such cases, and in patients with diseases associated with over-aged erythrocytes, genotyping must be performed. Twenty allelic variants for the TPMT gene are currently known /31/. Due to a lack of standardization, the ranges specified for the 6-TGN concentration and TPMT activity depend on the method used.

Sirolimus

Therapeutic range:

• 4–12 μg/L (trough) for triple therapy with cyclosporine, corticosteroids, and sirolimus in kidney transplant patients
• 12–20 μg/L (trough) for dual therapy with corticosteroids and sirolimus in kidney transplant patients
• 3–6 μg/L (trough) for liver transplant patients with secondary immunosuppression (e.g., due to calcineurin inhibitor nephrotoxicity) with a 50% reduction in the calcineurin trough concentration
• 5–8 μg/L (trough) for liver transplant patients without calcineurin.

Elimination half-life: 59 ± 19 h (adults), 10–23 h (children aged 5–11 yrs.), approx. 110 h (patients with abnormal liver function) /32/.

Recommendation for blood sampling: immediately prior to the next dose. EDTA is the anticoagulant of choice.

Elimination: sirolimus is mainly metabolized by CYP3A4 in the liver into multiple relatively inactive metabolites. Approximately 2.2% of the dose is excreted by the kidneys.

Clinical aspects: sirolimus (rapamycin) is a highly effective immunosuppressive substance used to prevent graft rejection (e.g. kidney, heart, liver). It inhibits T-cell activation and proliferation by inhibiting mTOR (mammalian target of rapamycin). The pharmacokinetics of sirolimus show significant intraindividual and inter individual differences. It has a bioavailability of approximately 14%. The low bioavailability of sirolimus following oral administration is due to its extensive metabolism by CYP3A4 in the intestine and liver and P-glycoprotein counter transport in the intestine. Peak blood concentrations are achieved after 1–2 h and the plasma free fraction is only about 8%. Multiple drug interactions have been observed. Ketoconazole, itraconazole, cyclosporine, erythromycin, nelfinavir, and diltiazem increase sirolimus exposure while rifampicin decreases it. Conversely, sirolimus increases cyclosporine exposure approximately 2-fold /32/. Sirolimus does not affect mycophenolic acid exposure. Although sirolimus is nephrotoxic only in the presence of cyclosporine, it has dose-dependent side effects such as hyperlipidemia and thrombocytopenia. According to current tentative drug monitoring recommendations, the sirolimus blood concentration should be determined as follows: following administration of the loading dose, following the establishment of a new steady state after a change in dose (approx. 7–10 days after the dose change), and after starting or discontinuing inhibitors or inducers of CYP3A4 and the P-glycoprotein transporter. The sirolimus trough level should be measured weekly for the first month after transplantation and every 2 months thereafter. Monitoring is also advisable in the event of a change in the dose or steady state concentration of cyclosporine; a change in the relative dosing intervals of sirolimus and cyclosporine; hyperlipidemia, leukopenia, thrombocytopenia, or liver disease; and to check compliance. Close monitoring is recommended for pediatric patients and patients with abnormal liver function.

Highly specific LC-MS/MS methods /26/ or LC-UV methods and a less specific immunoassay are available for measuring blood sirolimus levels. The specified therapeutic ranges only apply to specific methods (e.g., LC-MS/MS).

Everolimus

Therapeutic range: 3–8 μg/L (trough) for triple therapy with cyclosporine, corticosteroids, and everolimus in kidney transplant patients /33/.

Elimination half-life: 28 (24–35) h.

Recommendation for blood sampling: immediately prior to the next dose. EDTA is the anticoagulant of choice.

Elimination: everolimus is mainly metabolized by CYP3A4 in the liver.

Clinical aspects: everolimus is a highly effective immunosuppressive substance used to prevent graft rejection (e.g., kidney, heart, liver). It is also used to treat renal cell carcinoma and sub ependymal giant cell astrocytoma [SEGA]. In this setting, a tentative therapeutic range of 5–15 μg/L is used. Everolimus works by inhibiting mTOR. The pharmacokinetics of everolimus show significant intraindividual and inter individual differences. It has a bioavailability of 5–26%. The low bioavailability of everolimus following oral administration is due to its extensive metabolism by CYP3A4 in the intestine and liver and P-glycoprotein counter transport in the intestine. Peak blood concentrations are achieved after 1–2.2 h and the plasma free fraction is about 26%. As with sirolimus, multiple drug interactions have been observed. The everolimus trough level should be measured weekly for the first month after transplantation and every 2 months thereafter. The indications for drug monitoring are analogous to those for sirolimus.

LC-MS/MS methods /26/ and an immunoassay are available for measuring everolimus levels. The specified therapeutic range applies if LC-MS/MS is used.

Mycophenolic acid

Therapeutic range:

• 1.3–3.5 mg/L (plasma concentration prior to the administration of the next dose, C0) for triple therapy with cyclosporine, corticosteroids, and mycophenolate mofetil (MMF) in kidney transplant patients in the early post-transplantation phase (≤ 3 months)
• 1.9–4.0 mg/L with concomitant use of tacrolimus
• (30–60) mg × h/L MPA AUC012for triple therapy with cyclosporine or tacrolimus, corticosteroids, and MMF /33, 34/.

Algorithms have been developed to determine the MPA AUC using three measurement points /34/. The MPA AUC can also be estimated using Bayesian forecasting (https://pharmaco.chu-limoges.fr). The ranges specified apply if HPLC or LC-MS/MS is used. According to provisional results, the therapeutic ranges for liver and heart transplantation are similar. Plasma MPA concentrations below the specified therapeutic ranges are associated with an increased risk of rejection. Studies performed to date have shown that MPA concentrations above the therapeutic ranges do not appear to be associated with increased efficacy. Significant correlations between toxicity and the height of the MPA AUC or the C0 concentration have not been established to date. It appears that the free MPA concentration is a better indicator of leukopenia as a side effect of MPA. For EC-MPS, the MPA concentrations of C0 are higher than those for MMF at an equimolar dose, which means that the therapeutic range cannot be used for EC-MPS.

Elimination half-life: 17 h (healthy adults).

Recommendation for blood sampling: immediately prior to the next dose (C₀) or AUC estimation using three measurement points. MPA is determined in the plasma and EDTA is the anticoagulant of choice. Storing samples at room temperature can lead to an increase in the plasma MPA concentration.

Elimination: mycophenolic acid is largely metabolized, mainly into inactive mycophenolic acid glucuronide (MPAG), which is excreted primarily by the kidneys but also in the bile. In the intestine, MPAG undergoes cleavage by bacterial glucuronidases to MPA, which is subsequently reabsorbed. MPA is also metabolized to the active metabolite mycophenolic acid glucuronide (AcMPAG), an inactive 7-O-glucoside, and a cytochrome P450 oxidation product.

Clinical aspects: mycophenolic acid is a highly effective immunosuppressive substance that is available as mycophenolate mofetil and EC-MPS and is used to prevent graft rejection (e.g., kidney, heart, liver). MMF is hydrolyzed in vivo to mycophenolic acid (MPA), which is a selective, reversible inhibitor of inosine mono phosphate dehydrogenase (IMPDH) in de novo purine synthesis. By inhibiting this enzyme, it blocks the conversion of inosine mono phosphate to xanthine mono phosphate and subsequently to guanosine mono phosphate. In contrast to other cells, T and B lymphocytes do not have a salvage pathway. Therefore, inhibition of IMPDH leads to depletion of intracellular guanosine triphosphate and reduced proliferation. The pharmacokinetics of mycophenolic acid show significant inter individual differences. There are various reasons for this. Hypoalbuminemia leads to increased MPA clearance as a result of reduced protein binding. Dis continuation of corticosteroids leads to an increase in MPA exposure due to a reduction in UDP glucuronyltransferase (UGT) activity. Cyclosporine reduces the enterohepatic recirculation of MPA by inhibiting the MRP2 transporter. Decreased creatinine clearance leads to increased MPA exposure. Low UGT1A9 activity increases MPA exposure, while UGT1A9 promoter polymorphism increases MPA clearance, which results in decreased MPA exposure. In addition, numerous drug interactions have been observed.

Antacids, cholestyramine, metronidazole, and norfloxacin all decrease MPA exposure. Rifampicin also reduces MPA exposure by inducing UGT. Mycophenolic acid has no nephrotoxic or neurotoxic side effects. Furthermore, it does not influence lipoprotein metabolism. It is not diabetogenic and does not cause hypertension. Elevated free MPA levels can lead to infections and leukopenia.

Another side effect is anemia. Gastrointestinal side effects (e.g., diarrhea) can also occur; these are primarily dose-dependent. The active MPA metabolite AcMPAG is involved in the development of diarrhea.

A drug monitoring approach for MPA has not yet been established /35/. Three prospective studies have investigated the therapeutic benefits of TDM for mycophenolic acid. In the FDCC study /36/, a significant correlation was observed between the MPA AUC in the early post transplant phase and the incidence of acute rejection during the first year after kidney transplantation. However, a clear overall benefit for drug monitoring was not established. In contrast to the FDCC study, systematic dose adjustment was carried out in the APOMYGRE study, based on the MPA AUC estimated using the Bayesian method. A significantly lower rejection rate was observed in patients who underwent concentration-controlled dose adjustment /37/. The Opticept study /38/ showed a potential benefit for MPA C0 monitoring when reducing the dose of calcineurin inhibitors.

The following tentative recommendations for measuring MPA plasma levels have been proposed: it would seem reasonable to aim for therapeutic MPA concentrations in the early post transplantation phase (day 3–5). Drug monitoring for MPA is recommended for heart transplant patients due to their minimal tolerance for rejection episodes and in liver transplant patients due to significant inter individual differences in pharmacokinetics. In kidney transplant patients, MPA monitoring is useful in certain circumstances, e.g. reduced CNI exposure, delayed graft function, poor HLA matching, compliance problems in young patients, and suspected drug interactions /34/.

With respect to the frequency of MPA monitoring in kidney transplant patients, an initial evaluation (preferably an abbreviated AUC estimation) is recommended once during the first week after transplantation (day 3–5). In addition, either measurement of the pre-dose concentration or, preferably, an abbreviated MPA AUC estimation should be carried out once between days 10 and 14 and again after 3–4 weeks. If the MPA AUC is in the range (30–60) mg × h/L, a dose adjustment is generally not necessary. If the MPA AUC is < 30 mg × h/L, a dose adjustment is recommended. The same applies if the MPA AUC is > 60 mg × h/L and side effects are present. A confirmatory analysis is then required after 3–5 days, by which time a new steady state plasma concentration has been reached. A therapeutic range for long-term therapy has not yetbeen established.

HPLC or LC-MS/MS can be used to measure MPA. It can also be measured using EMIT, PETINIA, or an IMPDH-based enzyme inhibition assay. The IMPDH-based enzyme inhibition assay displays a lack of significant AcMPAG cross reactivity /39/.