Excretion renal clearance equation

Digoxin is not extensively metabolized in humans almost two thirds is excreted unchanged by the kidneys. Its renal clearance is proportional to creatinine clearance and the half-life is 36-40 hours in patients with normal renal function. Equations and nomograms are available for adjusting digoxin dosage in patients with renal impairment. 

It can be seen from Equation 7.11 that oral doses of nonrestrictively metabolized drugs should not need to be adjusted in response to changes in hepatic blood flow. Equation 7.11 also forms the basis for using AUCoral measurements to calculate so-called oral clearance" as an estimate of fuCLint. However, if renal excretion contributes to drug elimination, it will reduce AUCorai and lead to over estimation of fuCLinf unless the contribution of renal clearance is accounted for (2). 

Assuming that the renal clearance of BSP is independent of the plasma concentration and that the plasma concentration after a standard dose falls exponentially over the period of excretion into the urine with a decay constant a, the amount of BSP (U) excreted by the kidney during the time t is given by equation... 

Conventionally, renal clearance has been equated to urinary clearance. In this case, the amount of the plasma solute excreted in urine per minute is given by the product of urine flow V (in ml/min) and the concentration of the solute in urine (U). If P denotes the steady concentration... 

Drugs can be cleared from the body by metabolism as well as renal excretion, and when this occurs it is not possible to measure directly the amount cleared by metabolism. However, the total clearance rate (TCR), or total body clearance, of the drug can be calculated from its pharmacokinetic parameters using the following equation ... 

And if the victim drug s clearance were mediated entirely by transport (e.g., biliary or renal excretion), then Equation (12.21) would simplify to its Equation... 

In Equation 1.36, Dg is the adjusted maintenance dose for the patient with renal failure, D is the maintenance dose for a normal individual,/is the fraction excreted, and C/r is the patient s creatinine clearance, which can be determined from the serum creatinine value using the Cockcroft and Gault Equation 1.42. If a new dosing interval for the patient with renal failure (x ) is calculated, a reciprocal of the bracketed term in Equation 1.45 is used and the new equation becomes Equation 1.46. ... 

Transporters are involved in biliary and renal excretions that are the two common routes of drug elimination. In the liver, a drug is first taken up into hepatocytes, then either secreted back to the systemic circulation or excreted to the bile in an intact form or as metabolites via Phase I and/or Phase II enzymes. Given the involvement of transporters in both uptake at the sinusoidal membrane and efflux at the sinusoidal and canulicular membranes (Fig. 6.1c), the hepatic clearance can be expressed based on well-stirred model as the following equation (Shitara et al., 2005 Yamazaki et al., 1996) ... 

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