Volume of Distribution (Vd) Volume of distribution (Vd) relates amount of drug in body to plasma concentration Vd = (amount of drug in the body) / (plasma drug concentration) Vd is changed in disease states that decrease plasma proteins login to view 6 more bullets Vd increased in disease states that increase total body water login to view 2 more bullets Vd predicts drug distribution in body login to view 11 more bullets Clearance (CL) CL = (rate of elimination of drug) / (plasma drug concentration) = Vd * Ke login to view 1 more bullet CL relates the rate of elimination to the plasma concentration Half-Life (t1/2) t1/2 = (0.7 * Vd)/CL half-life is time required for amount of drug to fall to 50% of an earlier measurement during elimination or during constant infusion login to view 1 more bullet for drugs eliminated by first-order kinetics, half-life is constant regardless of concentration # of half lives 1 2 3 4 Concentration 50% 75% 87.5% 93.75% Bioavailability (F) bioavailability is the fraction of administered dose that reaches systemic circulation bioavailability is defined as unity, or 100%, in case of IV administration bioavailability of drug administered by other routes is generally reduced by incomplete absorption, first-pass metabolism, and any distribution into other tissues that occurs before the drug enters systemic circulation login to view 1 more bullet Calculation of bioavailabillity (F) = 100% * (AUC-oral * Dose-IV) / (AUC-IV * Dose-oral), where AUC is the area under the curve of a pharmacokinetic plasma concentration versus time plot delayed release formulations will have slower rise and lower peak compared with rapid release formulations