Introduction Aim of TDM Pharmacokinetics/Pharmacodynamics Therapeutic range Therapeutic index TDM of established drugs Laboratory detection

2. Case scenario 1
• A 28-year-old male with a history of bipolar disorder is brought in by
ambulance for altered mental status.
• His mother discovered him lying on the ground at home with an
empty bottle of immediate-release valproic acid near him; he was last
seen well 6 hours prior.
• He had started the valproic acid for bipolar disorder a couple days
ago, and the bottle had contained 60 tabs of 500 mg.
• Vitals are T 37.2C, HR 88, BP 118/72, RR 16, SpO2 99%.
• Exam reveals a young adult male who appears somnolent, lethargic
and confusion. He mentions feeling nauseated and abdominal pain.

3. OVERVIEW
 Introduction
 Aim of TDM
 Pharmacokinetics/Pharmacodynamics
 Therapeutic range
 Therapeutic index
 TDM of established drugs
 Laboratory detection

4. INTRODUCTION
• Therapeutic drug monitoring (TDM) is the clinical practice of
measuring specific drugs at designated intervals to maintain a
constant concentration in a patient’s bloodstream, thereby
optimizing individual dosage regimens.
• In other words, TDM refers to the individualization of drug dosage
by maintaining plasma or blood drug concentrations within a
targeted therapeutic range or window.
• The goal of this process is to individualize therapeutic regimens for
optimal patient benefit.
• By combining knowledge of pharmaceutics, pharmacokinetics and
pharmacodynamics, TDM enables the assessment of the efficacy and
safety of a particular medication in a variety of clinical settings.

5. Aim of TDM
• TDM is based on the principle that for some drugs there is a close
relationship between the plasma level of the drug and its clinical
effect.
• Another assumption is that drug metabolism varies from patient to
patient.
• When a precise therapeutic end point is difficult to define,
monitoring of drug levels may be of considerable therapeutic
assistance
• Therapeutic drug monitoring aims to promote optimum drug
treatment by maintaining serum drug concentration within a
‘Therapeutic Range’.
• Major aim to aid the clinician in the choice of drug dosage in order to
provide the optimum treatment for the patient and, in particular, to
avoid iatrogenic toxicity

6. Pharmacokinetics
• what the body does to drugs
• the processes of:
 absorption,
 Distribution
 metabolism and
 excretion
Pharmacodynamics
• what drugs do to the body
• the interaction of
pharmacologically active
substances with target sites
(receptors) and the biochemical
and physiological consequences
of these interactions
• Few drugs act by their physical
and chemical properties.
• Most of them act by effecting
other functional proteins in the
body such as enzymes, ion
channels, transporters and
receptors.

7. Conceptual relationship between pharmacodynamics and

8. Therapeutic Range/ Therapeutic Window
• The therapeutic range/ therapeutic window is the
concentration range of drug in plasma where the drug has been
shown to be efficacious without causing toxic effects in most people.

9. • Therapeutic window is a range of doses that produces therapeutic
response without causing any significant adverse effect in patients.
• It is a ratio between minimum effective concentrations (MEC) to the
minimum toxic concentration (MTC).
• The levels of drug should always be between MEC and MTC in order
to provide risk free therapeutic effects.
• If any drug crosses MTC then it will surely elicit toxic effects
• if drug is unable to surpass MEC then it will cause therapeutic failure
• Therapeutic window is also termed as safety window and can be
quantified by therapeutic index.

11. Therapeutic Index (TI)
• TI (therapeutic ratio) is a comparison of the amount of a therapeutic agent
that causes the therapeutic effect to the amount that causes toxicity.
• In animals;
• For humans;
• LD50 : lethal dose of a drug for 50% of the population
• TD50 : the dose of drug that causes adverse effects at an
incidence/severity not compatible with the targeted indication i.e toxic
dose in 50% of subjects
• ED50 : dose that leads to the desired pharmacological effect in 50% of
subjects i.e minimum effective dose for 50% of the population

12. Calculation of TI:
Certain safety factor :LD1/ED99

14. USE OF THERAPEUTIC DRUG MONITORING
• Once the narrow range of drugs for which TDM can provide useful
information has been defined, it should not be assumed that TDM is
necessary for every patient on these drugs at every visit.

15. TDM-Indications
1. Unpredictable dose and plasma conc. Eg phenytoin
2. Narrow therapeutic window. Eg Lithium, phenytoin, and digoxin
3. Drugs with steep dose response curve: e.g. theophylline.
4. Difficulty interpreting clinical evidence. Eg digitalis
5. Difficult to clinically predict the response. Eg immunosuppressant drugs
6. Renal disease
7. Drug interactions:
8. For diagnosis of suspected toxicity & determining drug abuse
9. To evaluate compliance of patient
10.Guiding withdrawal of therapy: Antiepileptics, Cyclosporine.

17. Limitations of TDM
• Scientific accuracy of the drug assay
• Laboratory variability in reporting
• Limited accessibility and infrastructure facilities in the rural areas
• Validity of suggested target ranges
• Lack of training and skills
• Cost involved

18. TDM of established drugs
1. Antiepileptic drugs : Phenytoin, phenobarbitone, benzodiazepines,
carbamazepine, valproic acid and ethosuximide
2. Antimicrobial agents: Aminoglycoside
3. Antineoplastics: Methotrexate, Busulfan, 6-Mercaptopurine
4. Cardioactive Drugs : Amiodarone, digoxin, digitoxin, disopyramide,
lignocaine,procainamide,propranolol and quinidine
5. Immunosuppressants: Cyclosporine, Tacrolimus, Sirolimus
6. Antidepressants : Lithium and tricyclic antidepressants
7. Bronchodilators : Theophylline, Caffeine

20. • Many drugs are available for treating seizures; prevent or minimize
seizures by augmenting inhibitory processes
• MOA by enhancing γ-aminobutyric acid (GABA)-mediated
neurotransmission or inhibiting excitatory processes (e.g., voltage- or
ligandgated ion channels, glutamate-mediated neurotransmission) in
the brain.
• These drugs are also used as sedatives; to treat neuropathic pain,
migraine headaches, and psychiatric conditions; and to manage
addictions.
Antiepileptics Drugs

21. Antiepileptics Drugs

22. Valproic Acid
• Valproic acid : brand names Depakene and Depakote, but also
available under other names and in generic form
• Used for the treatment of absence seizures and also useful against
tonic-clonic and partial seizures when used in conjunction with other
antiepileptic agents, such as phenobarbital or phenytoin.
• The drug inhibits the enzyme GABA transaminase, resulting in an
increase in the concentration of GABA in the brain.
• GABA is a potent inhibitor of presynaptic and postsynaptic discharges
in the central nervous system.

23. MECHANISM
• Valproic acid also modulates the synaptic sodium channel by prolonging
inactivation, which reduces the ability of the neuron to respond at high
frequency.
• This action gives it some activity against tonic-clonic seizures.
• Valproic acid is rapidly and almost completely absorbed after oral
administration and has a very low Vd (0.1 to 0.5 L/kg).
• Peak concentrations occur 1 to 4 hours after an oral dose of conventional
tablets and solutions, but they are extended for enteric-coated and
sustained-release formulations.
• Valproic acid is highly protein bound (>90%), but the extent of protein
binding decreases with increasing concentration, leading to nonlinear
kinetics in some patients.
• The metabolism of valproic acid is extensive, involving β-oxidation (≈30%
of dose) and production of several glucuronide conjugates (≈40% of dose).

24. • Mostly metabolized by the liver, with a half-life of 5-20 hours (longer
at toxic levels)
• The half-life is shorter still in children than in adults, with the
exception of neonates with hepatic disease, in whom the half-life
becomes prolonged.
• Other drugs that induce hepatic oxidative enzymes result in increased
valproic acid clearance, requiring a higher dose to maintain effective
therapeutic concentrations.
• The minimum effective therapeutic concentration of valproic acid is
50 mg/L.
• Concentrations greater than 100 mg/L have been associated with
hepatic toxicity and acute toxic encephalopathy.
• Nausea, lethargy and weight gain; adverse effect

25. Antimicrobial Agents
AMINOGLYCOSIDES (AMIKACIN, GENTAMICIN, TOBRAMYCIN)
• Aminoglycoside antibiotics inhibit protein synthesis to kill aerobic,
gram-negative bacteria.
• Because oral absorption is poor, aminoglycosides are administered
intravenously or by intramuscular injection.
• Elimination is largely renal, almost entirely as the unchanged parent
drug.
• Plasma half-life is 2–3 hr & the drug may accumulate in tissues.
• If therapy is continued for longer than a week, tissue sites become
saturated and plasma concentrations may rise.

26. • Narrow Therapeutic index
• efficacy of these drugs correlates well with the ratio of the maximum
concentration in plasma (Cmax) relative to the MIC of the organism; this
characteristic is termed concentration dependent killing.
• Ratios of Cmax / MIC ≥ 10 are preferred for optimal effect against
sensitive bacteria.

27. • Toxicity:
• These drugs exhibit significant systemic toxicity at plasma
concentrations just above those necessary for bactericidal activity.
• Aminoglycosides are very toxic & can accumulation in kidney,
middle ear, etc
• Aminoglycosides are associated with the risk of serious toxicity like
nephrotoxicity (renal tubular necrosis) and potentially irreversible
ototoxicity (auditory nerve degeneration) leading to hearing loss.

28. Antineoplastic drugs

29. Methotrexate
Mechanism:
• Methotrexate inhibits DNA synthesis by decreasing the availability
of pyrimidine nucleotides.
• It competitively inhibits the enzyme dihydrofolate reductase, thus
decreasing the concentrations of the tetrahydrofolate essential to
the methylation of the pyrimidine nucleotides and consequently
the rate of pyrimidine nucleotide synthesis
• Methotrexate is metabolized to 7-hydroxymethotrexate and
• both the parent drug and metabolite are excreted via the kidneys.

30. • Usually given in high doses by the intravenous route over periods of 6–12 h.
• It is also used orally at low dose (7.5–25 mg weekly) in Crohn disease,
rheumatoid arthritis and as an anti-psoriatic agent.
• Serum concentrations of methotrexate are commonly monitored during high-
dose therapy (>50 mg/m2) to identify the time at which active intervention by
leucovorin rescue should be initiated.
• Criteria for serum concentrations indicative of a potential for toxicity after
single-bolus, high-dose therapy are as follows:
1. Methotrexate concentration greater than 10 μmol/L 24 hours after dose.
2. Methotrexate concentration greater than 1 μmol/L 48 hours after dose.
3. Methotrexate concentration greater than 0.1 μmol/L 72 hours after dose.

31. • TDM indication:
• Measurement of methotrexate concentrations may be required in
two circumstances :-
• to allow adjustment of infusion to maintain a steady plasma
concentration at 10–4 M and
• to ensure that the concentrations at 24 and 48 h after the start
of the infusion have fallen to <5–10 μmol/L and <0.5–1.0
μmol/L, respectively, depending on the protocol.
• Methotrexate concentration monitoring is not usually
necessary in patients on low-dose regimens.
• serum concentrations are monitored at 24, 48, and 72 hours after the
single dose, and leucovorin is administered when methotrexate
concentrations are inappropriately high for a post-dose phase.

32. • Toxicity:
• They have low solubility so are nephrotoxic
• Thus prehydration, alkalinization and good renal function are
essential for the safe use of this drug.
• Hepatotoxicity – also seen in some patients.

33. Antiarrhythmics and cardiac glycosides

34. Digoxin
• Mechanism:
• It act through inhibition of the Na+K+-ATPase that regulates cation
flux in myocardial cells
• Its direct and indirect activities coordinate to slow heart rate,
increase the strength and velocity of cardiac contraction, and
regulate nervous (sympathetic) and endocrine (renin-angiotensin)
systems, affecting cardiovascular function.
• Half life: Long half life (20-60 hours)

35. • TDM:
• Sample: drawn at least 8 hours after the last dose- as Digoxin
distributes extensively into tissue & this process requires several hours
after a dose is administered.
• Effective plasma concentrations are 0.5–2.0 μg/L (0.6–2.6 nmol/L), is
recommended for the treatment of heart failure.
• Toxicity: plasma concentrations  3.0 nmol/L
• Toxicity inversely proportional to plasma K+ concentrations.
• Significant numbers of patients die with digoxin concentrations >2.3
μg/L
• Sometimes in neonates digoxin-like interfering Substances (DLIS) can
give false high values of digoxin levels. This is overcome by
measuring digoxin in protein free ultrafiltrates.
• Toxicity: Vomiting, Insomnia, Hypokalemia

36. Immunosuppressant drugs
Used to prevent or treat graft rejection in kidney, liver and heart transplant
patients.

37. Tacrolimus
• also a calcineurin phosphatase inhibitor; inhibits T-cell activation and
proliferation, humoral immuity
• metabolized in the liver by CYP3A and is concentrated in RBC
• TDM is essential, particularly in hepatic impairment and during or following
treatment with drugs that induce or inhibit metabolism.
• Target concentration ranges 5–15 μg/L
But vary with transplant type,other drug therapy and analytical method
Toxicity:
• not intrinsically nephrotoxic, but does cause renal vasoconstriction,
which leads to chronic graft dysfunction
• Adverse effects are minimized by using the lowest dose consistent with
efficacy and by the use of combined therapy with other drugs such as
mycophenolic acid.

38. Antidepressants

39. MECHANISM
• Inhibits postsynaptic D2 receptor supersensitivity.
• Alters cation transport in nerve and muscle cells and influences reuptake of serotonin
or norepinephrine
• It is rapidly and completely absorbed, exhibits moderate distribution (1 L/kg)
without protein binding that is excreted renally.
• Half life:
• Lithium has a half-life of 20–40 h depending on the duration of treatment
• steady-state concentrations are obtained approximately seven days after the start
of therapy.
• The usual therapeutic range is 0.4–1.0 mmol/L in samples taken 12 h post-dose,
with an upper limit of 0.8 mmol/L in the elderly.
• Concentrations up to 1.2 mmol/L may be required for the acute treatment of
mania in younger patients
Lithium:

40. Toxicity:
• extremely toxic, with a low therapeutic index.
• It is nephrotoxic and Plasma concentrations >1.5 mmol/L
• Early signs of toxicity include lethargy, muscle weakness and
tremors, and speech difficulties
• concentrations greater than 2.5 mmol/L can produce signs of
severe intoxication, including muscle rigidity and life-threatening
seizures
• Concentrations >3.0 mmol/L on chronic therapy are potentially
fatal.
• side-effects of lithium therapy includes; Thyroid dysfunction (most
often hypothyroidism), hyperparathyroidism and nephrogenic
diabetes insipidus

41. METHOD OF ANALYSIS
Sample collection:
• Timing of specimen collection is the single most important factor in TDM. In
general, trough concentrations for most drugs are drawn right before the next
dose; peak concentrations are drawn 1 hour after an orally administered
dose.
• Heparinized plasma is suitable for most drug analysis.
• Certain drugs have a tendency to be absorbed into the gel of certain serum
separator collection tubes.
• ethylenediaminetetracetic acid (EDTA), citrated and oxalated plasma are not
usually acceptable specimens.
Methods:
• LC-MS
• HPLC
• IMMUNOASSAY
• RADIOIMMUNOASSAY
• TLC

42. Drug Monitoring and toxicology lab TUTH
2075
1. Cholinesterase
2. Paracetamol
3. Phenytoin
4. Carbamazepine
5. Phenobarbitone