"Pharmacology" word comes from two Greek words "Pharmacon" which means "Drug" and "logos" which means "study of", which means Pharmacology is the study of drugs. Or in other words, it deals with interaction of any administered chemical molecules with human body. Pharmacology covers all the aspects of drugs as well as their safe use for human well-being.
Pharmacodynamics (What the drug does to the body) is a combination of two greek words "pharmacon" which means "drug" and "dynamis" which means "power". It means the power of drug to affect our body. This includes physiological and biochemical effects of drugs and their mechanism of action at organ e.g-Paracetamol helps to lower the body temperature,
Pharmacokinetics (What the body does to the drug) is a combination of two Greek words "Pharmacon" which means "drug" and "Kinesis" which means "movement". It means in what manners body is treating drug.
This refers to movement of the drug in and alteration of the drug by the body; includes absorption, distribution, binding/localization/storage, biotransformation and excretion of the drug. e.g. paracetamol is rapidly and almost completely absorbed orally attaining peak blood levels at 30-60 min: 25% bound to plasma proteins, widely and almost uniformly distributed in the body; extensively metabolized in the liver,
Drug Absorption
Absorption is movement of the drug from its site of administration into the circulation.
Drug Absorption is important for obtaining desired action of drug
Some factors affecting the drug absorption
Solubility
Concentration
Surface area
Route of administration
1-Solubility::
Drugs given in solid form must dissolve in the aqueous biophase solution before they are absorbed
For poorly water soluble drugs (aspirin, griseofulvin) rate of dissolution governs rate of absorption.
Obviously, a drug given as watery solution is absorbed faster than when the same is given in solid form or as oily solution
2.concentration::
Passive diffusion depends on concentration gradient; drug given as concentrated solution is absorbed faster than from dilute solution.
3.Surface Area::
Area of absorbing surface Larger is the surface area, faster is the absorption.
4.Route of administration ::
Route of administration of the drug highly affects the rate and extent of absorption.
Like, if we are administering drug via oral route the rate of absorption will be slow because the drug has to cross the layers of the gastro-intestinal organs.
But if we are administering drug via sublingual route the absorption will be faster than oral administration because the drug has to cross just the mucous membrane in buccal cavity to reach out to the blood circulation.
Distribution
Once a drug reached to the blood stream, it gets distributed to other tissues that initially had no drug, concentration gradient being in the direction of plasma to tissues.
The extent and pattern of distribution of a drug depends on its:
#lipid solubility
#ionization at physiological pH (a function of its pKa)
#extent of binding to plasma and tissue proteins #presence of tissue-specific transporters
#differences in regional blood flow.
Movement of drug proceeds until an equilibrium is established between unbound drug in the plasma and the tissue fluids. Subsequently, there is a parallel decline in both due to elimination.
Biotransformation/Metabolism
Biotransformation means chemical alteration of the drug in the body.
It is needed to render nonpolar (lipid soluble) compounds into polar (lipid-insoluble) compounds so that they are not reabsorbed from urine in the renal tubules and are excreted into the urine.
Most hydrophilic drugs, eg, streptomycin, neostigmine, pancuronium, etc. are little bio- transformed and are largely excreted unchanged.
The primary site for drug metabolism is liver, others are kidney, intestine, lungs and plasma.
Biotransformation of drugs may lead to the following
1. Inactivation; most of the drugs and their active metabolites are converted to their inactive form after metabolism. For example paracetamol.
2. Active metabolite from an active drug: Many drugs have been found to be partially converted to one or more active metabolite, the effects observed are the sumtotal of that due to the parent drug and its active metabolite.
3. Activation of inactive drug: Few drugs are inactive as such and need conversion in the body to one or more active metabolites. Such a drug is called a prodrug.
Biotransformation reactions can be classified into:
(a) Non-synthetic/Phase I reactions: a functional group is generated or exposed metabolite may be active or inactive.
(b) Synthetic/Phase II reactions: an endogenous radical is conjugated to the drug metabolite is mostly inactive, except few drugs, eg glucuronide conjugate of morphine and sulfate conjugate of minoxidil are active.
Excretion
Excretion is the passage out of systemically absorbed drug. Drugs and their metabolites are excreted in:
1. Urine The kidney is responsible for excreting all water soluble substances.
The amount of drug or its metabolites ultimately present in urine is the sum total of glomerular filtration, tubular reabsorption and tubular secretion.
2. Faeces Apart from the unabsorbed fraction, most of the drug present in faeces is derived from bile.
Liver actively transports into bile organic acids, organic bases, other lipophilic drugs and steroids by distinct nonspecific active transport mechanisms. Relatively larger molecules are preferentially eliminated in the bile..
3. Saliva and sweat These are of minor importance for drug excretion. Lithium, pot, iodide, rifampicin and heavy metals are present in these secretions in significant amounts.
4. Milk The excretion of drug in milk is not important for the mother, but the suckling infant inadvertently receives the drug.
Most drugs enter breast milk by passive diffusion.
However, the total amount of drug reaching the infant through breast feeding is generally small and majority of drugs can be given to lactating mothers without ill effects on the infant.
