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Friday, November 26, 2010

Drugs Therapeutic Index Therapeutic Ratio Narrow Therapeutic Index

Therapeutic Index (Therapeutic Ratio) of a drug?
What is Meant by Narrow Therapeutic Index? Which are the drugs with Narrow Therapeutic Index? Narrow Therapeutic Index brand name drugs and their generic versions.

Therapeutic index of a drug is a comparison of a drug, to asses its safety with respect to toxicity, it is a ratio of a concentration of a drug required to produce toxic effect in 50% population to the concentration of the drug required to produce required therapeutic effect in 50% of the population.

That means is a drug with higher therapeutic index is preferable over a drug having a lower therapeutic index (Narrow Therapeutic Index Drug / Critical-Dose drugs) so that a toxic concentration of a drug is not achieved due to any accidental overdose or due to poor elimination of a drug from the body of an individual with metabolic defects or lack of certain enzymes responsible for metabolism of that drug. There were many adverse events reported earlier due to accidental overdose of such Narrow Therapeutic Index Drug (NTI drugs).

The drugs with narrow therapeutic index which are required to be monitored for therapeutic drug concentration (pharmacodynamic monitoring ) in patients include Dimercaprol, Theophylline, Warfarin and Lithium Carbonate, Digoxin, Colchicine. Also some antibiotics too are monitored to minimize adverse effects are gentamicin, vancomycin, amphotericin B, and polymyxin B.

The monitoring the drugs therapeutic concentration in patient is carried out through a therapeutic drug monitoring protocol.

Following are the classes of drugs which are monitored through therapeutic drug monitoring protocol.
1.Aminoglycoside antibiotics (gentamicin)
2.Antiepileptics (such as carbamazepine, phenytoin and valproic acid)
3.Mood stabilisers, especially lithium citrate
4.Antipsychotics (such as pimozide and clozapine)

Narrow therapeutic index drugs are one of the factor for debates for substitution of banrd name drug with their generic drugs.There are products in which small changes in the dose and/or blood concentration could potentially result in clinically important changes in drug efficacy or safety. Usually, these drugs require frequent adjustments in the dose of the drug and careful patient monitoring irrespective of whether the drug is a brand or generic drug product. These drugs may sometimes be described in FDA approved drug labeling as narrow therapeutic range drugs.

Recently US FDA has asked to halt and remove from market all unapproved drugs containing colchicine due to its Narrow Therapeutic Index that had resulted in several fetal incidences.

Which brand name drugs are not required to be substituted with generic drugs is a matter of subject to requirements of different states requirements as well. In US there are different requirements of different states, but in general it is observed in pharmacists that they do not like to substitute a drugs with lower therapeutic index and a Transplant Drug or Anti-rejection Medications with generic drugs unless it is asked by the physician.

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