Free carbenicillin is the predominant pharmacologically active fraction of Geocillin. Carbenicillin exerts its antibacterial activity by interference with final cell wall synthesis of susceptible bacteria.

Geocillin is acid stable, and rapidly absorbed from the small intestine following oral administration. It provides relatively low plasma concentrations of antibiotic and is primarily excreted in the urine. After absorption, Geocillin is rapidly converted to carbenicillin by hydrolysis of the ester linkage. Following ingestion of a single 500 mg tablet of Geocillin, a peak carbenicillin plasma concentration of approximately 6.5 mcg/ml is reached in 1 hour. About 30% of this dose is excreted in the urine unchanged within 12 hours, with another 6% excreted over the next 12 hours.

In a multiple dose study utilizing volunteers with normal renal function, the following mean urine and serum levels of carbenicillin were achieved:

Microbiology

The antibacterial activity of Geocillin is due to its rapid conversion to carbenicillin by hydrolysis after absorption. Though Geocillin provides substantial in vitro activity against a variety of both gram-positive and gram-negative microorganisms, the most important aspect of its profile is in its antipseudomonal and antiproteal activity. Because of the high urine levels obtained following administration, Geocillin has demonstrated clinical efficacy in urinary infections due to susceptible strains of:

Escherichia coli
Proteus mirabilis
Proteus vulgaris
Morganella morganii (formerly Proteus morganii)
Pseudomonas species
   Providencia rettgeri (formerly Proteus rettgeri)
Enterobacter species
Enterococci (S. faecalis)

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