CLINICAL PHARMACOLOGY
After oral administration, cycloserine is readily absorbed from the gastrointestinal tract, with peak blood levels occurring in 4 to 8 hours. Blood levels of 25 to 30 µg/mL can generally be maintained with the usual dosage of 250 mg twice a day, although the relationship of plasma levels to dosage is not always consistent. Concentrations in the cerebrospinal fluid, pleural fluid, fetal blood, and mother's milk approach those found in the serum. Detectable amounts are found in ascitic fluid, bile, sputum, amniotic fluid, and lung and lymph tissues. Approximately 65% of a single dose of cycloserine can be recovered in the urine within 72 hours after oral administration. The remaining 35% is apparently metabolized to unknown substances. The maximum excretion rate occurs 2 to 6 hours after administration, with 50% of the drug eliminated in 12 hours.
MICROBIOLOGY
Cycloserine inhibits cell-wall synthesis in susceptible strains of gram-positive and gram-negative bacteria and in Mycobacterium tuberculosis.
SUSCEPTIBILITY TESTS
Cycloserine clinical laboratory standard powder is available for both direct and indirect methods1 of determining the susceptibility of strains of mycobacteria. Cycloserine MICs for susceptible strains are 25 µg/mL or lower.
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