Question

Can Steady State Pharmacokinetic concept be applied similarly on drugs that obey zero and first order kinetics? Explain your answer.

Answer 1

ANSWER

Pharmacokinetics is currently defined as the study of the time course of drug absorption, distribution, metabo- lism, and excretion. Clinical pharmacokinetics is the application of pharmacokinetic principles to the safe and effective therapeutic management of drugs in an individual patient.

The fundamental difference between zero and first-order kinetics is their elimination rate compared to total plasma concentration.

Zero order kinetics is a way of describing how the body uses and breaks down some medicines. While the rate at which the body eliminates most drugs is proportional to the concentration administered, known as first order kinetics, drugs that work by zero order kinetics work at a predictable, constant rate.

Zero-order kinetics

• Zero order kinetics is a way of describing how the body uses and breaks down some medicines.
• Zero-order kinetics undergo constant elimination regardless of the plasma concentration, following a linear elimination phase as the system becomes saturated.
• Drugs that work by zero order kinetics work at a predictable, constant rate. Knowing that a medicine works by zero order kinetics lets the person administering it know exactly how long to expect that it will work.
• With zero order kinetics, the available rate of elimination is a constant no matter how much the blood concentration is. The most commonly used example is the drug Alcohol.
• A few substances are eliminated by zero-order elimination kinetics, because their elimination process is saturated.
• Knowing about zero order kinetics can help a medical professional predict how long a medicine will last for a patient.

One example of a medicine that works by zero order kinetics is warfarin, or its trade name Coumadin. This medicine is a blood thinner, which can be very important for a person who has had a history of blood clots or heart attacks. Doctors who know how fast the body breaks down a warfarin dose can tell a person to take a dosage that's enough medicine to last them the whole day.

First order kinetics

• First order kinetics occur when a constant proportion of the drug is eliminated per unit time. Rate of elimination is proportional to the amount of drug in the body. The higher the concentration, the greater the amount of drug eliminated per unit time.
• 95% of the drugs in use at therapeutic concentrations are eliminated by first order elimination kinetics.
• first order kinetics are drugs that the body breaks down by first order kinetics break down at a percentage rate based on how much a person has been given. This percentage rate is known as a half-life; about 50 percent of the drug is used each hour.

For example, if a person took 100 milligrams of a drug that works by first order kinetics, after an hour, there would be 50 milligrams left in his system. The next hour, there would be 25 milligrams left, and the next, 12.5 milligrams. But if a person took the same drug and it worked by zero order kinetics at a rate of 10 milligrams an hour, his body would use the exact same amount every hour. After the first hour, there would be 90 milligrams left in his body, then 80 milligrams after the next hour, and so on. After 10 hours, the zero order drug would be all gone!

Gentamicin is a model of first-order elimination kinetics.This drug is not metabolised by any saturable enzymes. Its elimination is nothing fancy: it just ends up in the urine at a rate which is proportional to its concentration.

Other Examples are Phenytoin, Salicylates, Cisplatin, Fluoxetin, Omeprazol.

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