First-Pass Metabolism and Effect

First-pass metabolism, also called the first-pass effect or presystemic elimination, is the loss of an orally administered drug to metabolism before it reaches the systemic circulation. Drug absorbed from the gut travels through the intestinal wall and the liver via the portal vein, and enzymes in these tissues can metabolise a substantial fraction on this first pass, lowering oral bioavailability sometimes dramatically.

Definition

First-pass metabolism is the presystemic biotransformation of an orally absorbed drug during its initial passage through the gut wall and liver, which reduces the fraction of the dose that reaches the systemic circulation and thus lowers oral bioavailability.

Scope

The entry covers the anatomical route that creates the first-pass effect, the gut-wall and hepatic enzymes responsible, the way extraction reduces oral bioavailability, and the variability that makes the effect drug- and patient-dependent. It treats first-pass metabolism as a mechanistic concept in biopharmaceutics and pharmacokinetics, not as dosing guidance.

Core questions

• Why does an orally absorbed drug pass through the liver before reaching the rest of the body?
• Which tissues and enzymes contribute to presystemic loss?
• How does first-pass extraction translate into reduced oral bioavailability?
• Why does the magnitude of the first-pass effect vary between drugs and people?

Key concepts

• Portal circulation and hepatic transit
• Gut-wall (enterocyte) metabolism
• Hepatic extraction ratio
• Presystemic loss and oral bioavailability
• Routes that bypass first pass
• Interindividual variability in first-pass metabolism

Mechanisms

After oral absorption, drug crosses the intestinal epithelium and enters the portal vein, which carries it to the liver before it reaches the systemic circulation. Metabolising enzymes in the enterocytes and hepatocytes can biotransform a portion of the drug on this single passage, so the fraction escaping presystemic elimination equals one minus the combined gut and hepatic extraction. Drugs with a high hepatic extraction ratio lose a large share of the dose on the first pass and therefore have low and often variable oral bioavailability, whereas routes such as intravenous, sublingual, or transdermal administration partially or wholly bypass this loss. Because the responsible enzymes vary in expression and activity, the first-pass effect differs substantially between individuals.

Clinical relevance

The first-pass effect explains why some drugs require much larger oral than intravenous doses, why certain agents are given by non-oral routes, and why oral exposure can vary widely between people. It is a reference concept for interpreting route- and patient-dependent exposure; it characterises drug disposition and is not a basis for individual dosing decisions.

Evidence & guidelines

The conceptual framework of first-pass elimination and its relationship to the hepatic extraction ratio was synthesised by Pond and Tozer, whose review remains a standard reference, and is codified in pharmacokinetic textbooks such as Rowland and Tozer. Tam reviewed the substantial interindividual variation in presystemic metabolism, highlighting why first-pass-dependent drugs show variable oral exposure.

History

As quantitative pharmacokinetics developed, the observation that some drugs had far lower oral than intravenous exposure was attributed to metabolism during the obligatory passage through gut and liver. Pond and Tozer's 1984 review consolidated the basic concepts and clinical consequences of first-pass elimination, and later work, including Tam's review, extended attention to its marked interindividual variability.

Key figures

• Susan Pond
• Thomas Tozer
• Malcolm Rowland
• Yun Tam

Seminal works

• pond-tozer-1984
• tam-1993

Frequently asked questions

Why does first-pass metabolism lower oral bioavailability?

Because an orally absorbed drug must pass through the gut wall and liver before reaching the systemic circulation, enzymes there can metabolise part of the dose on this first pass, so less of it reaches the rest of the body.

Which routes of administration avoid the first-pass effect?

Routes that do not deliver drug to the portal circulation first, such as intravenous, sublingual, and transdermal administration, partially or wholly bypass presystemic hepatic metabolism.