Drug elimination Important to Know

2 Facts about Drug elimination

Renal filtration accounts for most drug excretion. About one-fifth of the plasma reaching the glomerulus filtered through pores in the glomerular endothelium; nearly all water and most electrolytes passively and actively reabsorbed from the renal tubules back into the circulation.
However, polar compounds, which account for most drug metabolites, cannot diffuse back into the circulation and excreted unless a specific transport mechanism exists for their reabsorption (e.g., as for glucose, ascorbic acid, and B vitamins).

With ageing, renal drug excretion decreases; at age 80, clearance typically reduced to half of what it was at age 30.
The principles of transmembrane passage govern the renal handling of drugs.
Drugs bound to plasma proteins remain in the circulation; the only unbound drug in the glomerular filtrate. Unionised forms of drugs and their metabolites tend reabsorbed readily from tubular fluids.

Glomerular filtration: Unbound drug molecules of less than 20’000 Da filtered through the glomerulus with the primary urine.

Active tubular secretion: This mechanism is predominant in the proximal tubule. Several transporters are responsible for the tubular secretion of drugs: the P-glycoproteins (PGps), the multidrug resistance-associated proteins (MRPs), the organic anion transporters (OATs), the organic cation transporters (OCTs), etc. These transporters are not highly specific and may become saturated at high drug concentrations.