Benzylpenicillin, carboxypenicillins and ureidopenicillins largely lose their properties when taken orally under the action of hydrochloric acid of gastric juice, for this reason they are administered only intramuscularly. Phenoxymethylpenicillin, oxacillin and aminopenicillins are more stable in an acidic environment, and therefore can be administered orally. A very high degree of absorption in the gastrointestinal tract is characterized by amoxicillin (75 % or more). Special soluble tablets (Flemoxin Solutab) have the highest degree of absorption (93%). The bioavailability of amoxicillin is independent of food intake. The absorption of phenoxymethylpenicillin is 40-60 %. Ampicillin (35-40 %) and oxacillin (25-30%) are slightly worse absorbed; food in the stomach significantly reduces their bioavailability. The absorption of the inhibitor (3-lactamase clavulanate) is 75 %, and it increases under the influence of food.
Benzylpenicillin procaine and benzylpenicillin benzatin are administered only by intramuscular injection. Gradually absorbed from the injection site, they provide lower levels of concentration in the blood serum compared to the sodium and potassium salts of benzylpenicillin, have a prolonged effect (they are combined under the name “depot-penicillins”). Therapeutic levels of benzylpenicillin procaine in the blood persist for 18-24 hours, and benzatin lpenicillin benzatin-up to 2-4 weeks.
Penicillins are distributed in many organs, tissues, and body fluids. The highest concentrations are observed in the lungs, kidneys, intestinal mucosa, reproductive organs, bones, and pleural fluid. In small amounts, they penetrate through the placenta into the fetal circulatory system and into breast milk. Poorly pass through the blood-ophthalmic barrier, as well as into the prostate gland. With inflammation of the brain membranes, the permeability through the blood-brain barrier increases compared to the norm. Oxacillin (up to 45 %) and ureidopenicillins (up to 30%) can undergo clinically significant transformation in the liver. Other penicillins are practically not metabolized and are excreted unchanged from the body. Most penicillins are excreted through the kidneys. Their half-life is on average about 1 hour (except for depo-penicillins) and, of course, increases greatly in renal failure. Almost all penicillins are completely removed during hemodialysis.
Penicillin, unfortunately, has not become a panacea for all types of infections, but it has been successfully treated and is being used to treat pneumonia, angina, blood poisoning (sepsis) and other diseases caused by exposure to pyogenic microbes. At the same time, for example, dysentery and tuberculosis sticks are insensitive to it, and yet such dangerous diseases as dysentery and tuberculosis still require the most careful attention. Other disadvantages of penicillins include the possibility of sensitization of the body and the development of allergic reactions up to anaphylactic shock; in addition, they are quickly eliminated from the body.