Pharmacokinetics Language English Format: PDF Price - Tutorsindia

PharmacokineticsAbsorption: Absorption rate constant,...The key part of the definition is ‘before it ever reaches the systemic circulation’. A drug molecule that enters the generalcirculation and is then metabolized is not considered to have undergone first pass metabolism as it has spent some timein the general circulation and has had an opportunity to exert its therapeutic effect. You should now appreciate why theterm ‘systemic blood circulation’ is incorporated into the definition of bioavailability given in Section 2.3.1 – a drug’sarrival in the hepatic portal vessel does not necessarily constitute bioavailability.The liver is not the only place where first pass metabolism may occur. During absorption, drug molecules must passthrough the epithelial cells of the gut and these cells contain the same sort of drug metabolizing enzymes that are foundin the liver. Consequently, first pass metabolism can also occur in the lining of the gut.Table 2.2 shows average oral bioavailbility values for a variety of drugs. The list generally focuses on drugs withproblematically low bioavailabilities as these are of special interest. In fact a wide range of drugs do have oral bioavailabilitiesof 100% (or very nearly so).Drug Oral bioavailability (%)Gentamicin