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.