Effect of gentamicin dosing interval on therapy of viridans streptococcal experimental endocarditis with gentamicin plus penicillin

This study compares the effects of a total daily dose of gentamicin given once a day (q.d.) or three times a day (t.i.d.) in the therapy of experimental endocarditis in rabbits caused by penicillin-susceptible, penicillin-tolerant, or penicillin-resistant viridans streptococci. Four isolates were used in vivo: one penicillin susceptible (MIC < or = 0.03 microgram/ml), one penicillin tolerant (MBC/MIC, < or = 0.03/ > 32 micrograms/ml), and two penicillin resistant (MICs = 0.5 and 2 micrograms/ml). Animals were infected with one of the four isolates and assigned to one of the following treatment regimens: no treatment, procaine penicillin at 1.2 million IU intramuscularly (i.m.) t.i.d., procaine penicillin plus gentamicin at 1 mg/kg of body weight i.m. t.i.d., procaine penicillin plus gentamicin at 3 mg/kg i.m. q.d., or procaine penicillin plus gentamicin at 1 mg/kg i.m. q.d. (only animals infected with the penicillin-susceptible isolate). Serum drug concentrations measured 30 min after administration of 1.2 million IU of penicillin and 1 or 3 mg of gentamicin per kg were 22.6, 3.8, and 8.5 micrograms/ml, respectively. The reduced total daily dose of gentamicin was ineffective among animals infected with penicillin-susceptible viridans streptococci; treatment with 1 mg of gentamicin per kg per day plus penicillin was less effective (P < 0.05) than was treatment with 3 mg of gentamicin per kg per day plus penicillin. The 1-mg/kg/day gentamicin treatment regimen was not further studied. The gentamicin dosing interval did not significantly affect (q.d. versus t.i.d., P > 0.05) the relative efficacy of penicillin plus gentamicin for treatment of experimental endocarditis among animals infected with each of the four isolates tested.