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Pharmacology: Pharmacokinetics: When Benzathine Benzylpenicillin is given by intramuscular injection ii forms a depot from which ii is slowly released and hydrolyzed to Benzylpenicillin, Peak plasma concentrations are produced in about 24hours and are lower than those following an equivalent dose of Benzylpenicillin Potassium or Sodium.
However, depending on the dose, Benzylpenicillin is usually detectable for up to 4 weeks. Distribution into the CSF is reported to be poor.
Due to the slow absorption form the site of injection, Benzylpenicillin has been detected in the urine for up to 12 weeks after a single dose.
Benzathine Benzylpenicillin is relatively stable in the presence of gastric juice, but absorption from the gastrointestinal tract is variable. Plasma concentrations of Benzylpencillin achieved following an oral dose are lower than those from the same dose of a soluble Penicillin; peak concentration are also produced less rapidly but may persist for longer.
Plasma Concentration: Benzathine Benzylpenicillin has been given every 4 weeks for secondary prophylaxis against rheumatic fever, although some advocate administration every 3 weeks to ensure adequate plasma concentrations of Benzylpenicillin. Typical concentrations achieved after single intramuscular injection of Benzathine Benzylpenicillin 900 mg have been cited as about 0.1, 0.02 μg per mL on days 1, 14 and 32 respectively. In one study adequate concentrations (defined as 0.02 μg or more per ml) were seen in more than 80%or serum samples at 3 weeks, but in only 36% at 4weeks. In a further study in which single dose of 900 mg, 1.35 g and 1.8 g were compared, it appeared that dose higher than900 mg of Benzathine Benzylpenicillin usually recommended might prolong the duration of protective plasma concentrations of Benzylpenicillin (defined as above 0.025 μg per ml) and improve the efficacy of dosing every 4 weeks for prophylaxis against rheumatic fever.
