Excretion is the passage out of systemically absorbed drug. Drugs and their metabolites are excreted in:
EXCRETION
Excretion
is the passage out of systemically absorbed drug. Drugs and their metabolites
are excreted in:
Through the kidney. It is the most important channel of
excretion for majority of drugs (see
below).
Apart from the unabsorbed fraction, most of the drug present in faeces is derived from bile. Liver actively transports into bile organic acids (especially drug glucuronides by OATP and MRP2), organic bases (by OCT), other lipophilic drugs (by Pgp) and steroids by separate nonspecific active transport mechanisms. Relatively larger molecules (MW > 300) are preferentially eliminated in the bile. Most of the drug, including that released by deconjugation of glucuronides by bacteria in intestines is reabsorbed (enterohepatic cycling) and ultimate excretion occurs in urine. Drugs that attain high concentrations in bile are erythromycin, ampicillin, rifampin, tetracycline, oral contraceptives, phenolphthalein.
Certain drugs are excreted directly in colon, e.g. anthracene purgatives, heavy metals.
Gases and volatile
liquids (general anaesthetics,
paraldehyde, alcohol) are eliminated by lungs, irrespective of their lipid
solubility. Alveolar transfer of the gas/vapour depends on its partial pressure
in the blood. Lungs also serve to trap and extrude any particulate matter
injected i.v.
These are of minor importance for drug
excretion. Lithium, pot. iodide, rifampin and heavy metals are present in these
secretions in significant amounts. Most of the saliva along with the drug in
it, is swallowed and meets the same fate as orally taken drug.
The excretion of drug in milk is not important for the
mother, but the suckling infant inadvertently receives the drug. Most drugs enter
breast milk by passive diffusion. As such, more lipid soluble and less protein
bound drugs cross better. Milk has a lower pH (7.0) than plasma, basic drugs
are somewhat more concentrated in it. However, the total amount of drug
reaching the infant through breast feeding is generally small and majority of
drugs can be given to lactating mothers without ill effects on the infant.
Nevertheless, it is advisable to administer any drug to a lactating woman only
when essential. Drugs that are contraindicated during breast feeding or need
special caution are given in Appendix3 at the end of the book.
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