Treponemicidal levels of penicillin in the CSF were not obtained following intramuscular (IM) injections of 600,000 IU of penicillin G procaine along with 2% aluminum monostearate (PAM). Treponemicidal levels of penicillin in the CSF were achieved in 31 patients by probenecid, 500 mg/6 hr by mouth, with penicillin G, 500,000 IU by IM injection, and by probenecid every six hours by mouth and IM injections of penicillin G procaine once a day in two of three patients following injections of 600,000 IU, in four of five following 1.2 million IU, and in three (now all nine) following 2.4 million IU.
(JAMA 241:2538-2540, 1979)
Dunlop EMC, Al-Egaily SS, Houang ET. Penicillin Levels in Blood and CSF Achieved by Treatment of Syphilis. JAMA. 1979;241(23):2538-2540. doi:10.1001/jama.1979.03290490044025