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Lecture 3. III. Drug Metabolism. The aim of drug metabolism is to convert lipid soluble (non polar) drugs to polar metabolites easily excreted in urine. The liver is the principle organ of drug metabolism. The drug metabolite may be:

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iii drug metabolism
III. Drug Metabolism

The aim of drug metabolism is to convert lipid soluble (non polar) drugs to polar metabolites easily excreted in urine.

The liver is the principle organ of drug metabolism.


The drug metabolite may be:

Inactive: metabolism results in termination of drug action e.g. most drugs.

Active like the parent drug: e.g. diazepam (sedative and hypnotic).

Active and the parent drug is inactive: (prodrug)

e.g. methotrexate

iv drug excretion
IV. Drug Excretion

Kidney is the major rout of excretion of drugs or their metabolites.

Other channels of excretion include the lungs, intestine, skin and milk.

Drugs are eliminated from the body either unchanged as the parent drug or as metabolites (a changed form of the drug).


Since drugs are small particles dissolved in the blood, they are usually filtered into the kidneys and then reabsorbed back into the bloodstream.

  • In order for the kidney to eliminate drugs from the body, the drug must somehow be prevented from being reabsorbed from the urine into the bloodstream.
  • The drug must be chemically changed into a compound that is less fat-soluble and therefore less capable of being reabsorbed. 

Drug Product (Delivery system):

The dosage form that contains the therapeutically active agent (s), generally but not necessarily in association with inactive ingredients (e.g. tablet, capsule, suspension, ….. ).

Drug product selection:

The process of selecting dosage form in which the drug product will be manufactured.

The duration of drug therapy:

The time necessary for the treatment of a disease with a drug.


Dosage regimen:

  • Is the manner in which a drug is taken.
  • It comprises calculation of the required dose (size of the dose) and adjusting dosing interval (dosing frequency) for patients.

Therapeutic Objective:

  • It may be either the treatment, mitigation or the prevention of the disease.
  • It is evident that both the duration of drug therapy and the dosage regimen depend on the therapeutic objective.

Minimum Effective Concentration (MEC):

  • Many drugs have no demonstrable therapeutic effect or do not produce a desired degree of pharmacologic response unless a minimum concentration is reached at the receptors (sites of action).
  • The MEC represents the minimum concentration of a drug needed at the sites of action (receptors) to produce the desired pharmacologic effect.
minimum toxic concentration mtc
Minimum Toxic Concentration (MTC):

It reflects the drug concentration at which a toxic effect will appear.









Therapeutic concentration range

(Therapeutic window):

  • It is the range of drug concentration between the MEC and the MTC.
  • In other words, it is a region associated with therapeutic effect.
  • An optimal dosage regimen might be defined as one that maintains the plasma concentration of a drug within its therapeutic window.

Drug with narrow therapeutic window:

  • These drugs require conc.

Therapeutic drug Toxic

concentration monitoring.

  • Examples: Therapeutic Window

e.g.1 Digoxin

e.g.2 WarferinIneffective

e.g.3 Theophyllinetime


Therapeutic effectiveness:

  • Is the difference between effective therapy and toxic effects.


  • Is the study of the effect of various pharmaceutical formulation variables on the delivery of the drug to the body under normal and pathologic conditions (impaired physiological function).


  • Is the rate (amount/time) and extent (total amount) to which the active drug ingredient or therapeutic moiety is absorbed from a drug product and becomes available at the receptor (site of action).

When a medication is administered intravenously, its bioavailability is 100%.

  • However when a medication is administered via other routes (orally), its bioavailability generally decreases (due to incomplete absorption and first-pass metabolism) or may vary from patient to patient.
  • Bioavailability must be considered when calculating dosages for non-intravenous routes of administration.

Pharmaceutic equivalents:

  • These are drug products that are identical in:
  • Active drug ingredient
  • Strength or concentration
  • Dosage form
  • Route of administration
  • They may differ in some characteristics e.g. colour, shape, packing, expiration time and inactive ingredients.

Bioequivalent drug products:

  • These are pharmaceutical equivalents whose rate and extent of absorption are not different (i.e. have similar bioavailability).
  • When given in the same concentration (molar dose) under similar experimental conditions).

Pharmaceutic alternatives:

  • These are drug products that contain the same therapeutic moiety but not necessarily as the same chemical structure.

e.g.1Betamethasone Acetate and BetamethasoneValerate

e.g.2Tetracyclin Hydrochloride and Tetracyclin Phosphate


Also, different dosage forms containing the same active ingredients and produced by a single manufacturer are pharmaceutic alternatives.

e.g.1 Sustained-release dosage form (Olfen SR)R versus a standard immediate-release drug product of the same ingredient (Olfen)R.

e.g.2Diclofenac Sodium (Declophen)R ampoule and Diclofenac Sodium (Declophen)R tablet.