Pharmacokinetics 2 - Absorption

Summary

This tutorial explores the pharmacokinetics of drug absorption, focusing on different routes of drug administration: oral, intravenous (IV), and inhalation. It explains how each route affects the drug's concentration over time, specifically Cmax (peak concentration) and Tmax (time to reach peak concentration). Oral administration is slower, IV is faster with no first-pass metabolism, and inhalation is the quickest. Despite these differences, the total drug exposure (AUC) remains the same. The video also touches on the excretion rates and half-life of drugs, which are consistent across routes. Future tutorials will cover drug metabolism in more detail.

Takeaways

• 😀 Oral drug administration involves absorption through the digestive system, hepatic portal system, liver, and then into the arterial circulation.
• 😀 Intravenous (IV) drug administration bypasses the digestive system and liver, delivering drugs directly into the bloodstream through the heart and lungs.
• 😀 Inhalation of drugs results in a very quick increase in drug concentration in the arterial circulation, as the drug bypasses both the digestive and hepatic systems.
• 😀 The concentration-time curve (Cmax and Tmax) differs significantly across routes: oral administration has a slow rise with a low Cmax and long Tmax, while IV and inhalation result in faster rises with higher Cmax and smaller Tmax.
• 😀 The area under the curve (AUC) remains constant for the same dose across all routes of administration, indicating similar overall drug exposure.
• 😀 Drugs administered intravenously reach higher concentrations faster than oral drugs, making them ideal for rapid therapeutic effects.
• 😀 Oral drugs may be safer for drugs with toxic side effects, as they are absorbed more gradually compared to intravenous administration.
• 😀 The rate of drug excretion depends on the current concentration, with higher concentrations leading to faster excretion (first-order kinetics).
• 😀 The half-life of a drug is a constant property and does not change based on the route of administration, meaning it takes the same time for the drug to reduce to half its peak concentration regardless of the method used.
• 😀 Cmax (peak concentration) and Tmax (time to peak concentration) are essential pharmacokinetic parameters that vary based on how the drug is administered, influencing the drug’s onset and duration of action.
• 😀 Different routes of drug administration are chosen based on the desired speed and effect of the drug in the body, balancing between rapid action and side effect risks.

Q & A

What is the main focus of this video tutorial?

The main focus of this tutorial is to explain the different routes of drug administration and how they affect the absorption, concentration, and excretion of drugs in the bloodstream.

Why is it recommended to watch the previous tutorial before this one?

The previous tutorial introduces important pharmacokinetic terminology that will be used and referenced in this video. Watching it first will help viewers understand the concepts discussed here.

What is the flow of blood in the body, and how does it relate to drug absorption?

Blood flows from the digestive system to the liver via the hepatic portal system, then to the Venus circulation, heart, lungs, and eventually the arterial circulation. This flow is crucial for drug absorption as it dictates how drugs are processed and enter the bloodstream.

How does oral drug administration affect blood concentration over time?

Orally administered drugs are absorbed through the digestive system, travel through the liver, and then enter the arterial circulation. This process is slower, leading to a gradual increase in drug concentration and a prolonged Tmax (time to reach peak concentration).

What happens when a drug is administered intravenously, compared to oral administration?

In intravenous (IV) administration, the drug is delivered directly into the bloodstream, bypassing the digestive system and liver, resulting in a faster rise in blood concentration, a higher Cmax (peak concentration), and a smaller Tmax.

Why does inhalation of drugs result in a faster onset of action compared to oral or intravenous administration?

Inhalation delivers drugs directly to the lungs, where they quickly enter the arterial circulation, leading to a rapid increase in drug concentration with a larger Cmax and a much smaller Tmax compared to oral or IV routes.

What is the significance of Cmax and Tmax in drug administration?

Cmax represents the peak concentration of the drug in the bloodstream, while Tmax refers to the time it takes to reach that peak. These values help determine the speed and intensity of a drug's effect, influencing how it is used for different therapeutic purposes.

What does it mean when the video mentions 'first pass metabolism'?

First pass metabolism refers to the initial processing of a drug by the liver before it reaches the systemic circulation. This is a key factor in oral drug administration, where some of the drug is metabolized and reduced in effectiveness before entering the bloodstream.

How does the area under the curve (AUC) differ across routes of drug administration?

The area under the curve (AUC), which represents the total drug exposure over time, is the same for all routes of administration because the total dose remains constant. However, the distribution of drug exposure across time varies, with some routes providing quicker or slower drug absorption.

Why does a drug administered intravenously have a faster rate of excretion than one administered orally?

The rate of excretion is proportional to the drug's current concentration in the blood. Since IV administration results in a higher concentration of the drug in the bloodstream, it is excreted more quickly than drugs administered orally, which have a slower rate of absorption and a lower peak concentration.

What is the role of the half-life of a drug in pharmacokinetics?

The half-life of a drug refers to the time it takes for its concentration to decrease by half. It is an inherent property of the drug, independent of the route of administration, and plays a key role in determining how long the drug stays active in the system.