PHARMACOKINETICS & PHARMACODYNAMICS

Pharmacokinetics is simply the study of how the body reacts to therapeutic agents or drugs over a period of time. It investigates what a therapeutic agent or drug does to the body after being administered. The pharmacokinetics of a particular drug describes alterations in the absorption, distribution, metabolism and the elimination or excretion of the drug from the body.

After their administration either parenterally or orally, therapeutic agents are first broken down and absorbed by their target body sites and then metabolized or bio-transformed before it is finally excreted or eliminated from the body especially after the drug must have accomplished its restorative function in vivo.

The pharmacokinetic properties of drugs are generally the mechanisms in which therapeutic agents are processed by the body i.e. what the body does to the drug. Drugs after their administration must reach certain therapeutic levels in the bloodstream or plasma for it to be effective against a target pathogen or infection.

Absorption, distribution, metabolism and excretion are the main factors or pathways that control the circulation and modification of drug in the body prior to the expression of their authentic antimicrobial activity.

An understanding of the pharmacokinetic properties of a drug (inclusive of drug absorption, drug distribution, drug metabolism and drug elimination or excretion) enables physicians and scientists alike to have basic knowledge of what drugs does to the body (aside their normal therapeutic property); and this will go a long way in assisting clinicians to administer the most effective drug as per the differences that exist in individuals physiological and biochemical makeup.

• Absorption: Absorption is the process by which a substance (e.g. a drug) is taken into the body. Drug absorption is usually the first pharmacokinetic property of a drug; and the absorption of drug from their initial site of administration allows the therapeutic agent to reach the bloodstream or blood plasma for further antimicrobial activity. However, the route of administration of a drug may affect the rate of its absorption by the body. While drugs administered parenterally may reach the bloodstream at a much faster rate than orally-administered drugs; therapeutic agents administered orally take some time to reach the bloodstream because they have to be dissolved in the GIT fluid before proceeding to the bloodstream for distribution.    

• Distribution: After their absorption into the body, it is critical that the absorbed drug becomes distributed across the body especially from the bloodstream to other body sites or fluids such as the intracellular fluids and interstitial tissues or extracellular fluids where their antimicrobial activity is expressed. 

• Metabolism: At this stage the drug is metabolized i.e. broken-down or bio-transformed into substances that are easily excretable; and this is mainly done by certain tissues of the body such as the liver or kidney and then processed for elimination from the body.

• Excretion: Drugs are eliminated from the body in various ways. Both the drug and its metabolites are eliminated from the body in urine or feaces. Drugs can also be eliminated via milk in nursing mothers and via the intestines.

PHARMACODYNAMICS

Pharmacodynamics is the study of the effects of drug on the body. It explains what therapeutic agents do to the body. The pharmacodynamic properties of a drug explore all the physiological and biochemical effects of therapeutic agents on not just the host taking the agent but on the target pathogenic organism in vivo.

Antimicrobial agents particularly those that target pathogens in vivo interfere with some key metabolic processes of these infectious agents thereby inhibiting there infectious processes. However, in the process of expressing their antimicrobial effect, therapeutic agents can leave some untoward effects on the recipient host aside destroying their target pathogenic microorganisms.

Both the pharmacokinetic and pharmacodynamic properties of therapeutic agents assist clinicians and physicians to effectively manage their patients, and thus ensure that the best therapeutic regimens are always administered on case by case basis.   

References

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Ashutosh Kar (2008). Pharmaceutical Microbiology, 1^st edition. New Age International Publishers: New Delhi, India. 

Axelsen P.H (2002). Essentials of antimicrobial pharmacology. Humana Press, Totowa, New Jersey, USA. Al-Jasser A.M (2006). Extended – Spectrum Beta – Lactamases (ESBLs): A Global Problem. Kuwait Medical Journal, 38(3):171-185.

Bisht R., Katiyar A., Singh R and Mittal P (2009). Antibiotic Resistance – A Global Issue of Concern. Asian Journal of Pharmaceutical and Clinical         Research, 2 (2):34-39.

Block S.S (2001). Disinfection, sterilization and preservation. 5^th edition. Lippincott Williams & Wilkins, Philadelphia and London.

Joslyn, L. J. (2000). Sterilization by Heat. In S. S. Block (Ed.), Disinfection, Sterilization, and Preservation (5th ed., pp. 695-728). Philadelphia, USA: Lippincott Williams and Wilkins.

Nally J.D (Ed.) (2007). Good manufacturing practices for pharmaceuticals. Sixth edition. Informa Healthcare USA, Inc, New York.