The in vitro and in vivo testing of chemotherapeutic agents against pathogenic free living amebae : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Microbiology at Massey University, Palmerston North, New Zealand.
During the last ten years, there has been an increasing awareness of sporadic cases of Primary Amoebic Meningo-encephalitis (PAM) affecting primarily younger age groups and appearing in an acute fulminant form. The earliest positive case (Willaert, 1974) may have been in England in 1909 which shows that the disease has been with us for a long time. The pathogenic free-living amebae (PFLA), which comprises the genus Naegleria and the genus Acanthamoeba, are the causative organisms of PAM and AM*respectively. PAM is a rapidly fatal disease affecting the central nervous system (CNS),the treatment of which to date has been successful in only a small number of cases, and therefore the continual screening of suitable chemotherapeutic agents against amebae of the Naegleria spp. and Acanthamoeba spp., is of great importance. AM is also essentially confined to the CNS although it may take the form of chronic granulomata in the liver, spleen, uterus and kidneys (Martinez et al., 1977). Six chemotherapeutic agents: Amphotericin B, 5-Fluorocytosine, Kanamycin, Oxytetracycline, Tylosine and Levamisole were tested for activity against a non-pathogenic and a pathogenic species of Naegleria and a non-pathogenic and a pathogenic species of Acanthamoeba in axenic culture. For the Naegleria spp., Amphotericin B and Oxytetracycline were found to be active and the Acanthamoeba spp. were found to be only susceptible to Levamisole. The synergistic combinations of drugs against the amebae were also investigated in axenic culture. In preliminary trials Kanamycin together with Oxytetracycline showed promise against Naegleria fowleri (MsM) but this was later shown not to be the case. Amphotericin B in combination with 5-Fluorocytosine was also shown not to be synergistic, however Amphotericin B in combination with Oxytetracycline proved to be effective against N. fowleri. Amphotericin B was combined with 5-Fluorocytosine against A. culbertsoni (A-1) but was not found to be synergistically active. * Amebic meningitis caused by Acanthamoeba infections. Levamisole was also tested against N. gruberi (P1200f) and A. castellanii (0.1) at various stages in growth of the amebae (i.e. 24, 48 and 72 hour stock cultures) to determine the effect of using aged amebae. It was found that the age of the stock culture bore no relation to the activity of the drug. After axenic culture testing, the susceptibility of the pathogenic N. fowleri (MsM) and A. culbertsoni (A-1) to the agents which showed activity, was investigated in a vero cell culture system. For N. fowleri (MsM) the results of axenic culture testing were confirmed, with Amphotericin B and Oxytetracycline protecting the monolayer from the destructive effects of the amebae, both when used singly and at a greater efficiency when added together as a synergistic combination. Levamisole, although effective to some extent against Acanthamoeba spp. in axenic culture, failed to show any activity against the amebae in vero cell culture testing. In vivo animal protection studies were then performed using drugs that had been shown either in this or other studies to be effective against either Naegleria or Acanthamoeba spp. Chemotherapeutic agents tested on N. fowleri (MsM) included two imidazoles; Miconazole nitrate and Ketoconazole (previously known as R41,400), as well as Amphotericin B. The synergistic combination of Amphotericin B with either Tetracycline or Oxytetracycline was also investigated. For A. culbertsoni (A-1), 5-Fluorocytosine, and Polymyxin B were tried both singly and in combination. These drugs were injected by intraperitoneal (I.P.) and intraventricular (I.vent.) routes. The results were not promising, with none of the drugs offering significant protection even whilst using Amphotericin B which is considered the drug of choice. The question of adequate drug levels reaching the brain was tested out with two imidazoles, Ketoconazole and Miconazole. Serum samples were assayed against Candida pirapsilosis and C. pseudotropicalis respectively at various time intervals after innoculation with the drug, and a gradual increase and breakdown of the drug in the animal system could then be shown. These results showed that based on in vitro results, the levels of the imidazoles obtained in the serum after the first eight hours after injection, should have been sufficiently high to prevent amebic multiplication.