Roxithromycin (RU 965): effective therapy for experimental syphilis infection in rabbits.

Roxithromycin (RU 965), a new macrolide antibiotic, was shown to be effective for therapy of active syphilis in rabbits. Dark-field-positive lesions were produced in adult male rabbits by intradermal inoculation of approximately 10(6) Treponema pallidum organisms at each of 11 sites. Beginning 7 days after infection, six animals per group were treated with benzathine penicillin G (200,000 U, intramuscularly, weekly for 2 weeks) or roxithromycin (15 mg/kg of body weight, orally, twice daily for 15 days); six animals were not treated. Chancres in untreated animals were dark-field positive throughout the 16-day observation period; all benzathine penicillin-treated rabbits were dark-field negative 1 day after the initiation of therapy. Five of six animals treated with roxithromycin were dark-field negative on day 3 following the initiation of therapy; the sixth animal was dark-field negative by day 6. Lesions in untreated animals reached a mean (+/- standard deviation) maximum diameter of 14.7 +/- 1.91 mm compared with 8.4 +/- 3.6 mm for benzathine penicillin-treated (P less than 0.005) and 10.4 +/- 1.2 mm for roxithromycin-treated (P less than 0.001) animals. Ulceration occurred at 62 of 66 lesions in untreated animals compared with 0 of 66 lesions in each treated group. At 3, 6, and 12 weeks postinfection, Venereal Disease Research Laboratory antibody titers were significantly higher (P less than 0.05) in untreated than in treated animals. Titers in penicillin-treated versus roxithromycin-treated animals were significantly different at 6 weeks postinfection but not at 3 and 12 weeks postinfection. Transfer of tissue from treated rabbits to seronegative recipient animals did not reveal any evidence of persistent infection in the benzathine penicillin- or roxithromycin-treated animals. These findings indicate that benzathine penicillin and roxithromycin, at the doses indicated above, are effective in treating active syphilis in rabbit model.