Therapeutic drug Monitoring
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Therapeutic drug Monitoring. What is therapeutic drug monitoring (TDM)? Individualization of drug doses by maintaining plasma/blood drug concentrations within a target range---- therapeutic range therapeutic window.

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Therapeutic drug Monitoring

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Therapeutic drug monitoring

Therapeutic drug Monitoring


Therapeutic drug monitoring

What is therapeutic drug monitoring (TDM)?

Individualization of drug doses by maintaining plasma/blood drug concentrations within a target range---- therapeutic range

therapeutic window.

Takes care of inter-individual variability.


Therapeutic drug monitoring

Therapeutic Window

Therapeutic failure results when either the concentration is too low, ineffective therapy, or is too high, producing unacceptable toxicity. Between these limits of concentration lies a region associated with therapeutic success – regarded as a Therapeutic window.


Therapeutic drug monitoring

Wide therapeutic window

A

B

Toxicity

Efficacy

Response

Drug concentration (log scale)


Therapeutic drug monitoring

Narrow therapeutic window

Efficacy

Toxicity

Response

Drug concentration (log Scale)


Therapeutic drug monitoring

  • Major sources of Variability:

  • Compliance

  • Age- neonates, children, elderly

  • Physiology- gender, pregnancy

  • Disease- Hepatic, renal, cardiovascular, respiratory

  • Drug interactions

  • Environmental influences on drug metabolism

  • Genetic polymorphisms


Therapeutic drug monitoring

  • For which drugs is monitoring helpful?

  • Marked pharmacokinetic variability

  • Concentration related therapeutic and adverse effects

  • Narrow therapeutic index

  • Defined therapeutic (target) concentration range

  • Desired therapeutic effect difficult to monitor


Therapeutic drug monitoring

  • TDM useful in 2 major situations:

  • Drugs used prophylactically to maintain absence of a condition--- seizures, cardiac arrhythmias. depressive/manic episodes, transplant rejection

  • To avoid serious toxicity--- Aminoglycoside antibiotics


Therapeutic drug monitoring

Sampling and drug analysis:

Plasma/ serum; cyclosporin- whole blood.

Timing: least variable point in dosing interval– predose/trough concentration.

Wait for steady state to be achieved---at least 5 half-lives. Exceptions are there! Drugs with long half-life.

HPLC, GLC, Immunoassays- sensitivity, specificity.


Therapeutic drug monitoring

  • Information required for interpretation:

  • Timing of sample in relation to last dose

  • Duration of treatment in with current dose

  • Age, gender

  • Other drug therapy

  • Relevant disease states

  • Reason for TDM- lack of effect, routine monitoring, suspected toxicity.


Therapeutic drug monitoring

Plasma protein binding:

Free drug vs total drug concentration.

Importance of plasma protein binding.

Remember that only total drug concentration is measured but only the free drug is active!


Therapeutic drug monitoring

Drugs commonly monitored:

DrugTherapeutic range (mg/L)

Amiodarone1.0-2.5

Digoxin0.5-2.1microgram/L

Quinidine2.0-5.0

Theophylline10-20

Phenytoin10-20

Carbamazepine5.0-12

Sodium valproate50-100

Phenobarbitone15-40

Gentamicin peak>5, trough<2

Amikacin peak>15, trough<5

Vancomycin peak20-40, trough<10

Lithium0.6-1.2mmol/L


Therapeutic drug monitoring

Target concentration intervention

ESTIMATE INITIAL DOSE

Target Dose

Loading Dose

Maintenance Dose

BEGIN THERAPY

ASSESS THERAPY

Patient Response

Drug Level

REFINE DOSE ESTIMATE

ADJUST DOSE


Therapeutic drug monitoring

High Performance Liquid Chromatography


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