Renal Drug Recovery & Excretion Pathways : Mechanism

Renal drug recovery is expressed as a percent of fraction or absolute amount of dose that is recovered in the urine as the unchanged drug. Now if a drug has a high urinary recovery. For example, greater than 80 percent. It indicates that the drug is primarily excreted unchanged by the kidneys Typical example is aminoglycoside. If the drug has a high urinary recovery, then it would require dosing adjustment in renal failure. A good estimate of renal drug recovery does require greater than five half-lives of urine collection. I mean five half-lives of the drug. Renal excretion of drugs happen by filtration by the glomerulus. Active secretion in the proximal tubule which is saturable active transport.

And reabsorption in the distal tubule which is a passive diffusion process. This graph shows. The Green Dot shows the filtration. The black dot shows the active secretion which is saturable. In the net excretion, notice here the reabsorption is not shown because it is a negative value. Mechanism of renal clearance. Let’s summarize the mechanisms of renal clearance. For example, if we have a renal clearance of 50, which is less than GFR. That means the drug is filtered. The drug may be secreted. But the drug is definitely reabsorbed to a greater extent than the active secretion. If renal clearance is for example 150, which is greater than GFR. And the drug is filtered, is definitely secreted.

It may be also reabsorbed, but we know active secretion is much greater or greater than reabsorption. If it’s 100. If the renal clearance of the drug is 100 the numbers is equivalent to GFR. Then the drug could be filtered only. Or it could be active secreted as well and it could be reabsorbed as well. Except that the active secretion, the magnitude of active secretion is the same as the magnitude of reabsorption.