The Certara Blog

Author: Nathan Teuscher

Dr. Nathan Teuscher is the Vice President of Pharmacometric Solutions at Certara. He is an expert in clinical pharmacology, pharmacometrics, pharmacokinetics and pharmacodynamics and was trained by David Smith at the University of Michigan. Dr. Teuscher has held leadership positions in biotechnology, pharmaceutical and contract research companies. In 2008 he established the Learn PKPD.com website to share his knowledge with the community. Prior to coming to Certara, he was the Founder and President of PK/PD Associates.

Recent Posts

What is the Difference Between PK and TK?

What is the Difference Between PK and TK?

PK is the abbreviation for pharmacokinetics. TK is the abbreviation for toxicokinetics. Some consider these to be distinct specialties while others consider them to be the same. Let me explain and then I would like to see what you think. Pharmacokinetics generally deals with doses that are in a therapeutic range. Thus common dose ranges […]

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Why are PK Parameters Lognormally Distributed?

Why are PK Parameters Lognormally Distributed?

The statistical analysis of pharmacokinetic parameters is often overlooked and not always well understood. The disconnect between the pharmacokineticist and the biostatistician can often be a huge stumbling block that prevents the appropriate analysis of PK parameters. While I cannot solve all the disagreements between pharmacokineticists and biostatisticians in a single blog post, I hope […]

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What are Direct and Indirect Pharmacodynamic Models?

What are Direct and Indirect Pharmacodynamic Models?

When constructing pharmacodynamic (PD) models, you will often encounter the adjectives “direct” and “indirect” describing the associated PD model. This terminology was very confusing to me when I was learning about PD modeling. Hopefully a brief explanation will help you. Let’s start with the direct PD model. In this type of model, the drug is […]

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What is Modeling and Simulation?

What is Modeling and Simulation?

I am currently attending a short conference on modeling and simulation in pediatric clinical pharmacology, and I noticed that many people in the conference don’t have a good grasp on what “Modeling and Simulation” means. I think most of them think that it is an extremely complicated mathematical concoction that is intended to keep everyone […]

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Relationship Between AUC and Volume of Distribution

Relationship Between AUC and Volume of Distribution

A few days ago on the pharmacokinetics listserve PharmPK, the following question appeared: I’m having trouble wrapping my head around this question from an online pharmacology quiz.  The question is “The larger the volume of distribution, the smaller the AUC of a given drug.”  The answer is given as “False.” As I look at the […]

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Where Did the 80-125% Bioequivalence Criteria Come From?

Where Did the 80-125% Bioequivalence Criteria Come From?

Most people involved in clinical pharmacokinetics are familiar with the 80-125% criterion. This criterion is used to compare two treatments with the purpose of evaluating if the treatments are bioequivalent. But, where did this come from? Why 80-125%? Why not 90-110%? or why not 80-120%? Before we explain where 80-125% came from, let me explain […]

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Deciding on Which Drug-drug Interactions to Evaluate in the Clinic

Deciding on Which Drug-drug Interactions to Evaluate in the Clinic

Drug-drug interactions are a critical research area in pharmaceutical drug development. One of the most tragic examples of drug-drug interactions was the antihistamine terfenadine. Terfenadine (also known as Seldane) was a common antihistamine intended to block the effects of an allergic rhinitis. Upon administration terfenadine is metabolized to fexofenadine by the cytochrome P450 3A4 isoform. […]

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Generics and Bioequivalence

Generics and Bioequivalence

As the debate about health care in the United States continues forward, the term “generic drugs” has become rather commonplace. What are generic drugs? Are they safe to use? Why do we have them? And what is bioequivalence? All of these are common questions that I hope to answer with my post today. What are […]

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What are Compartmental Models?

What are Compartmental Models?

Almost everyone familiar with pharmaceuticals has heard a conversation like this before: Scientist 1: “What are the pharmacokinetics of Drug X?” Scientist 2: “Drug X follows a 1-compartment model in rats, but in monkeys it tends to have a distribution phase and seems to follow 2-compartment kinetics.” Scientist 1: Thinks to himself/herself …’What does a […]

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What are Drug-drug Interactions Anyway?

What are Drug-drug Interactions Anyway?

A current buzz phrase in pharmaceutical research right now is “drug-drug interaction” or simply “drug interaction”. The definition of a drug-drug interaction has fluctuated over the last few years, not because of changes in research, but because of misconceptions in the research community. The US Food and Drug Administration (FDA) has published a draft guidance […]

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Trial Designs—Non-inferiority vs. Superiority vs. Equivalence

Trial Designs—Non-inferiority vs. Superiority vs. Equivalence

The primary purpose of a clinical trial is to address a scientific hypothesis. To address a hypothesis, different statistical methods are used depending on the type of question to be answered. Most often the hypothesis is related to the effect of one treatment as compared to another. For example, one trial could compare the effectiveness […]

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Absorption Rate

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 […]

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Bioavailability and Your Pay Stub

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, […]

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What is a Half-life?

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 […]

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What Does Clearance Mean?

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 […]

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Understanding Volume of Distribution

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 […]

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What is Pharmacodynamics?

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 […]

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What is Pharmacokinetics?

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 […]

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Why Have a Blog About PK and PD?

Why Have a Blog About PK and PD?

Many blogs you will find on the internet are related to personal ventures, technology, or hobbies … all topics that are suited to the light-hearted fare that is common in the blogosphere. I believe that blogs can communicate knowledge in unique ways to a broad audience. Instead of developing a blog about my personal life, […]

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