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


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 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.


Wide therapeutic window

A

B

Toxicity

Efficacy

Response

Drug concentration (log scale)


Narrow therapeutic window

Efficacy

Toxicity

Response

Drug concentration (log Scale)


  • 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


  • 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


  • 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


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.


  • 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.


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!


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


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


High Performance Liquid Chromatography


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