Elderly is defined as 65 years of age or older. Physiological changes occur in aging which affect the ADME of drugs. The influence of age on pharmacokinetics and pharmacodynamics has been extensively reviewed in the literature [50-55]. In the elderly, the gastric pH is elevated, gastric emptying time slightly reduced; intestinal motility, muscular blood flow, plasma protein, and total body water are reduced; whereas, serum fatty acids and adipose tissue are increased . Kinirons and Crome  have summarized the following accepted principles for elderly population: decline in renal function with age, significant decline in liver size and mass, significant reduction in hepatic blood flow; decreased cardiac output, metabolic and renal clearance; in vitro content and activity of CYP450 enzymes or conjugation enzymes are not reduced with age. However, in vivo clearance of drugs metabolized by CYP3A4, 2C9, 2C19, and 1A2 have been reported to be reduced whereas, no reduction in clearance of drugs metabolized by CYP2D6 and Phase II enzymes has been reported. With regard to renal function, GFR, tubular secretion, and reabsorption are all reported to be reduced in the elderly population. Differences in sensitivity to drugs have also been reported with age for CNS and cardiovascular drugs [50, 52].
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