First-pass Effect
First-pass Effect - Related Videos
01:12
First Pass Effect
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2023
Presystemic elimination, or the first-pass effect, is the metabolism of drugs that reduces their effective concentration at the site of action. Apart from the first-pass effect, the systemic bioavailability of the drug is also reduced by other factors, including incomplete absorption or chemical degradation of drugs. Depending on the route of administration, drugs can be metabolized in the liver, intestine, lungs, and vasculature. Orally administered drugs are first absorbed through the...
01:25
Single-pass Transmembrane Proteins
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2023
Integral membrane proteins are tightly associated with the cell membrane and play a crucial role in cell communication, signaling, adhesion, and transport of the molecules. Some integral membrane proteins are present only in the membrane monolayer. For example, the enzyme fatty acid amide hydrolase is present in the cytoplasmic side of the membrane monolayer. In contrast, another type of integral membrane protein, also known as a transmembrane protein, spans across the membrane. Transmembrane...
01:23
Factors Influencing Bioavailability: First-Pass Elimination
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2023
When a drug is taken orally, it undergoes a journey starting from the gastrointestinal (GI) tract, passing through the portal vein, reaching the liver, and finally entering the systemic circulation. This process involves the absorption of the drug across the GI tract. The liver is the primary site for metabolizing the drug, with some metabolism also occurring in the gut wall. This journey significantly reduces the quantity of the drug that reaches the systemic circulation, a phenomenon known as...
01:29
Insertion of Multi-pass Transmembrane Proteins in the RER
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2023
The rough ER membrane synthesizes, assembles, and embeds transmembrane proteins in diverse topologies. These proteins function as transporters or channels and can remain in the ER membrane or are sent to the Golgi complex, lysosome, and cell membrane. The multipass transmembrane proteins are the type IV integral membrane proteins with multiple topogenic sequences determining their spatial arrangement in the ER membrane. Nearly all multipass proteins lack a cleavable signal sequence and use...
01:26
Insertion of Single-pass Transmembrane Proteins in the RER
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2023
Integral membrane proteins are proteins adhered to the lipid bilayer of a cell organelle or membrane. They can be of two types: transmembrane integral proteins that span the lipid bilayer and monotopic proteins that are attached to either side of the membrane but do not pass through it. Integral transmembrane proteins possess transmembrane and extra membrane domains. The transmembrane domains are primarily made of 20-25 hydrophobic amino acids arranged in a helical secondary confirmation. These...
FAQs
Both sites can reduce the amount of an orally administered drug that reaches general circulation, but they contribute at different stages. Intestinal metabolism may modify the drug during or before absorption, whereas hepatic metabolism occurs after absorbed drug enters the portal vein. Considering both locations helps explain why the final bioavailable fraction can be substantially lower than the administered dose.
After gastrointestinal absorption, portal circulation directs the drug to the liver before it reaches general circulation. This arrangement exposes the absorbed dose to hepatic metabolic enzymes, including cytochrome P450 systems. If those enzymes chemically modify or inactivate part of the drug, less active drug becomes systemically available, which can influence the dose needed to produce the intended clinical effect.
Cytochrome P450 systems can chemically modify an absorbed drug during hepatic processing, and some of the administered dose may therefore lose activity before reaching general circulation. The extent of this metabolism affects bioavailability and can contribute to differences in clinical response. It also makes enzyme-related drug interactions important when clinicians evaluate oral treatment outcomes.
Dose selection must account for the fraction of an oral dose that may be lost before systemic circulation. When presystemic metabolism substantially reduces bioavailability, a medicine may require a higher oral dose to achieve an effective exposure. Clinicians therefore consider this effect when balancing the intended response with the amount administered.
An alternative route may be considered when presystemic metabolism significantly limits the amount of an oral drug reaching general circulation. Changing the route can provide a way to address reduced oral bioavailability and may support more consistent therapeutic exposure. The choice is clinically relevant when oral dosing does not provide the desired efficacy or requires an unsuitable dose.
Drug interactions deserve careful evaluation because changes affecting metabolic processing can alter how much of an oral medicine remains available after gastrointestinal absorption and hepatic handling. For drugs with substantial first-pass loss, even changes in presystemic metabolism may influence efficacy and patient-to-patient variability. This assessment supports more informed dose selection and clinical monitoring.