Benzylpenicillin lipophilicity

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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