Penicillin benzathine benzylpenicillin salts

Benzathine benzylpenicillin is a very insoluble salt. It is administered i.m. in "long-acting" formulations that contain one-half procaine benzylpenicillin (procaine penicillin) and one-half benzathine benzylpenicillin. Any clinical effect is from the procaine benzylpenicillin (procaine penicillin) portion of these formulations since the benzathine penicillin provides persistent but sub-therapeutic plasma concentrations. 

Benzylpenicillin is rapidly absoibed and rapidly excreted However, certain sparingly soluble salts of benzylpenicillin (benzathine, benethamine and procaine) slowly release penicillin into the circulation over a period of time, thus giving a continuous high concentration in the blood. Simultaneous administration of benzylpenicillin (see Fortified Proeaine Penieillin, BP) may be given initially. 

The principal penicillins of present importance are benzylpenicillin sodium or potassium salt (penicillin G) procaine benzylpenicillin benzathine penicillin benethamine penicillin phenoxymethylpenicillin, potassium or calcium salt (penicillin V) methicillin sodium phenethicillin sodium penicillamine hydrochloride. 

In contrast to the sodium or potassium salt of benzyl penicillin, which is very water soluble and has a short duration of action, the procaine and benzathine salts (or more accurately, the ion-pair complexes) shown in Figure 8.27 are only sparingly soluble in water and very lipophilic. They are dissolved in an oily vehicle, such as ethyl oleate or sesame oil, and administered by intramuscular injection to create a depot from which the penicillin anion is slowly released into the bloodstream, achieving a sustained-release profile. An added advantage of these depot penicillins is their superior stability compared to benzylpenicillin. 

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