## Absorption Rate

The rate at which a drug enters the body after administration is called the absorption rate, and is represented by the symbol ka. This is probably one of the simplest pharmacokinetic (PK) parameters to explain and understand. Let’s consider the case of oral administration first, then we can discuss other routes of administration. A drug […]

Topics: PK/PD Modeling & Simulation

## Bioavailability and Your Pay Stub

Bioavailability is another primary pharmacokinetic parameter (like Clearance and Volume of Distribution) that describes the fraction of administered drug that reaches the systemic circulation. As a fraction, bioavailability can take any real value between 0 and 1 (e.g. 0.54). Sometimes this is reported as a percentage instead of a decimal (e.g. 54%). To explain this, […]

Topics: PK/PD Modeling & Simulation

## What is a Half-life?

One of the most incorrectly used but most often quoted parameters is half life. The symbol for half-life is t1/2. The reason for the multitude of errors with half-life is that in most cases, the assumptions required to use half-life correctly are rarely valid. And when the assumptions are false, the derived interpretations are also […]

Topics: PK/PD Modeling & Simulation

## What Does Clearance Mean?

Drug clearance is an extremely important topic in the science of pharmacokinetics. Drug clearance defines how much drug should be administered, how frequently to dose a patient, and how two interacting drugs will affect a patient. The primary PK parameter clearance is very similar to its friend, volume of distribution. Clearance (CL) is a proportionality […]

Topics: PK/PD Modeling & Simulation

## Understanding Volume of Distribution

One of the most misunderstood pharmacokinetic (PK) parameters is volume of distribution. First of all it has numerous abbreviations (V, Vd, Vz, Vss, V1, Vc, V2, etc.), and to make matters worse, many people incorrectly define the parameter. But, once you understand the meaning behind volume of distribution, you will have a solid grasp on […]

Topics: PK/PD Modeling & Simulation

## What is Pharmacodynamics?

Starting with definitions is always helpful… so after defining pharmacokinetics, a definition for pharmacodynamics is in order. The definition of pharmacodynamics (PD) is much less controversial than the definition for pharmacokinetics. The word pharmacodynamics is from two Greek words (see wikipedia for more): pharmakon: Drug dynamikós : force or power Thus, pharmacodynamics is the study […]

Topics: PK/PD Modeling & Simulation

## What is Pharmacokinetics?

You might think this is a simple question, especially for an expert in pharmacokinetics. Well, this question is full of controversy with each scientist having their own opinion. I think this is because the definition of pharmacokinetics is simple, yet pharmacokinetics science is broad in scope. The word pharmacokinetics is from two greek words (see […]