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Introduction, Routes, Absorption, and Distribution, Apparent volume of…
Introduction, Routes, Absorption, and Distribution
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Introduction
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It is divided into several branches like
pharmacokinetics
pharmacodynamics
pharmacotherapeutics
chemotherapy
toxicology etc.
When a drug is administered to a person, it will exert some effect on the patient(pharmacodynamics)
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pH
If pH of the medium is equal to pKa, then the drug is 50% ionized and 50% un-ionized.
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For acidic drugs, ionized form increases and non-ionized form decreases.
For basic drugs, unionized form increases and ionized form decreases.
If the pH of the medium is less than pKa, opposite happens, i.e. acidic drugs will be in more un-ionized form and basic drugs be more ionized.
Bioequivalence
Many different pharmaceutical companies can manufacture same compound with same dose as well as dosage form
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If the difference in the bioavailability of these two preparations(same drugs, same dose, same dosage forms) is less than 20%, these are known to be bioequivalent.
As the term implies, these are biologically equal i.e.will produce similar plasma concentrations.
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AUC
By IV route, bioavailability is 100 percent.
Bioavailability can be calculated by comparing the AUC (area under plasma concentration time curve) for IV route and for that particular route.
AUC tells about the extent of absorption of the drug.
T max tells about the time to reach maximum concentration i.e. rate of absorption.
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Plasma protein binding
It is the free form of drug which is not bound to plasma proteins that is responsible for the action as well as the metabolism of a drug.
Therefore plasma protein binding makes a drug long acting by reducing its metabolism.
Many drugs bind to plasma proteins like albumin, alpha 1 acid glycoprotein,
This delays the metabolism of drug.
This property can also expose the drug to several drug interactions due to displacement from the binding site by other drugs.
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Oral route
safer and economical
several drugs are not effective by this route because of high first pass metabolism in the liver and intestinal wall
e.g. nitrates, lignocaine, propranolol and pethidine.
Rectal route
avoids first pass metabolism to 50% extent.
Diazepam is given by this route in children for febrile seizures.
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Intradermal route
given at the angle of 10-15 degree to skin
and a bleb must be formed.
BCG vaccination and drug testing for allergy is done by this route.
Transdermal route
used only for the drugs which are highly lipid soluble and can be absorbed through intact skin
By this route, there is constant release of the drug
and there is a constant level of the drug in the blood
may be administered less frequently.
Examples: nitroglycerine, nicotine, fentanyl and hyoscine
Nasal route
Drugs administered by nasal route are nafarelin (GnRH agonist), calcitonin and desmopressin.
Inhalational route
The drugs administered by this route include drugs for asthma.
E.g. salbutamol, ipratropium, cromoglycate and inhalational steroids
and inhalational anaesthetic agents like nitrous oxide.
Sublingual route
avoids first pass metabolism can be used in emergencies
can be self-administered
and also after getting the desired action, rest of the drug can be spitted.
E.g. nitroglycerine, isosorbide dinitrate
Pharmacokinetics
It is effect of body on the drug i.e. movement of the drug in, through and out of the body.
Pharmacokinetics is also called ADME study
as it deals with absorption, distribution, metabolism and excretion.
Absorption
Movement of drug from site of administration into the blood stream.
It depends on several factors.
Only lipid soluble drugs can cross the biological membranes.
So, if a drug is administered by oral route, it has to cross the membranes of GIT and blood vessels to reach the blood.
Therefore, it should be in lipid soluble form.
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Weak acids:
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NSAIDs e.g. aspirin, diclofenac.
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Bioavailability
It is the fraction of administered drug that reaches the systemic circulation in the unchanged form.
When we administer a drug orally, first it is absorbed into the portal circulation and reaches the liver.
During this passage, certain drugs get metabolised and or removed or inactivated before they reach systemic circulation.
This process is known as first pass metabolism.
The net result is decreased bioavailability and diminished therapeutic response of the drug.
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Distribution
After the drug reaches the blood, it may be distributed to various tissues.
This is determined by a hypothetical parameter, volume of distribution(Vd).
It is defined as reversible transfer of drugs between body - fluid compartments.
Redistribution
Highly lipid soluble drugs when given by IV or by inhalation initially gets distributed to organs with high blood flow.
Later less vascular but more bulky tissues take up the drug and plasma concentration falls and drug is withdrawn from these sites.
If the site of action of the drug was in one of the highly perfused organs, redistribution results in termination of drug action.
e.g. Thiopentone