Molecular basis for hycanthone drug action in schistosome parasites.

Hycanthone (HYC) is a retired drug formerly used to treat schistosomiasis caused by infection from Schistosoma mansoni and S. haematobium. Resistance to HYC was first observed in S. mansoni laboratory strains and in patients in the 1970s and the use of this drug was subsequently discontinued with the substitution of praziquantel (PZQ) as the single antischistosomal drug in the worldwide formulary. In endemic regions, multiple organizations have partnered with the World Health Organization to deliver PZQ for morbidity control and prevention. While the monotherapy reduces the disease burden, additional drugs are needed to use in combination with PZQ to stay ahead of potential drug resistance. HYC will not be reintroduced into the schistosomiasis drug formulary as a combination drug because it was shown to have adverse properties including mutagenic, teratogenic and carcinogenic activities. Oxamniquine (OXA) was used to treat S. mansoni infection in Brazil during the brief period of HYC use, until the 1990s. Its antischistosomal efficacy has been shown to work through the same mechanism as HYC and it does not possess the undesirable properties linked to HYC. OXA demonstrates cross-resistance in Schistosoma strains with HYC resistance and both are prodrugs requiring metabolic activation in the worm to toxic sulfated forms. The target activating enzyme has been identified as a sulfotransferase enzyme and is currently used as the basis for a structure-guided drug design program. Here, we characterize the sulfotransferases from S. mansoni and S. haematobium in complexes with HYC to compare and contrast with OXA-bound sulfotransferase crystal structures. Although HYC is discontinued for antischistosomal treatment, it can serve as a resource for design of derivative compounds without contraindication.

Formation of promutagenic methylation damage in tissue-DNA of mice treated with antischistosomal agents.

The existence of the promutagenic methylation damage O6-MedG has been measured at various time intervals in different tissue DNAs of mice received a single therapeutic dose of various antischistosomal agents (hycanthone, oxaminiquine and metrifonate). Liver-DNA exhibited the highest levels of O6-MedG in all treated animals while, spleen DNA contained the lowest. The three antischistosomal agents tested seemed to exert the peak concentrations of their alkylating metabolites over a period of several hours following the administration. In mice which had received hycanthone, liver-DNA contained readily detectable amounts of O6-MedG by 6 h post-treatment (0.089 mol O6-MedG/mol dG) and by the end of 48 h, this was decreased by about 3-fold to reach a level of 0.026 mumol/mol dG. In intestinal-DNA, however, O6-MedG was formed more slowly and contained about half the level of that found in the liver-DNA. In the tissue-DNA of animals which had received oxaminiquine, the highest level of O6-MedG was observed at 6 h after administration and at a 24-h time point, the adduct dramatically decreased in the liver and intestine-DNA to undetectable values. In neither tissues was there any evidence for O6-MedG accumulation in the DNA at the end of a 48-h post-treatment. A pattern of O6-MedG, almost similar to that of oxaminiquine, was also observed in tissue-DNA of mice pretreated with metrifonate. These results demonstrate that treatment with antischistosomal agents leads to the formation of highly promutagenic alkylated lesions in the tissue-DNA. The implication of such existence for antischistosomal-induced toxicity and carcinogenicity are discussed.

Early years of the Salmonella mutagen tester strains: lessons from hycanthone.

The 1960s witnessed detailed studies on the genetic properties of a large number of histidine-requiring mutants of Salmonella typhimurium. The early 1970s saw development of selected strains, the Ames strains, for use in rapid, cheap, sensitive, and manipulable tests of chemicals and chemical mixtures for genotoxic activities. Our contribution during this latter period was an investigation into the mutagenicity of hycanthone and some of its analogues. Some lessons that this study provided are enumerated. Hycanthone is definitely a liver carcinogen in rodents predisposed by hepatic hyperplasia. Between 1969 and 1975, an estimated total of 100 kg of hycanthone was injected into some 1,000,000 humans with liver hyperplasia caused by infections with parasites. It may now be possible to assess directly the long-term impacts of hycanthone in man.

Hepatotoxicity of hycanthone in patients with metastatic breast cancer.

Hycanthone is an antischistosomal drug with promising antitumor activity against experimental animal tumors. In phase I clinical trials, hepatitis weas the dose-limiting toxic effect and a dose of 60-70 mg/m2/day for 5 days was reported as the maximum-tolerated dose. In a phase II study of hycanthone in patients with breast cancer we have recently observed severe hepatotoxicity, even at lower doses, which resulted in two drug-related deaths. From this study and the previously available data, we conclude that this drug is too toxic for human trials at the currently recommended doses. Its radiosensitizing effect may be worth exploring at a lower dose level.

Induction of hepatic neoplastic lesions in mice with a single dose of hycanthone methanesulfonate after partial hepatectomy.

Experiments were designed to determine whether hycanthone methanesulfonate (1-([2-(diethylamino)ethyl]amino)-4-(hydroxymethyl)thioxanthen-9-one monomethanesulfonate), an antischistosomal drug, and its analog, IA-4-N-oxide (8-chloro-2-[2-(diethylamino)ethyl]-2H-[1]benzothiopyrano[4,3,2-cd]indazole 5-methanol monomethanesulfonate), will induce neoplastic lesions in the livers of mice not infected with Schistosoma mansoni. All the mice received a single i.m. injection of hycanthone methanesulfonate (76 mg/kg), IA-4-N-oxide (80 mg/kg), or an equivalent volume of the solvent, 0.9% NaCl solution, 42 hr after partial hepatectomy. Of the mice receiving hycanthone methanesulfonate and living 200 days or longer, hepatocellular carcinoma was seen in 11.5% and liver sarcoma was seen in 4.2%. This type of malignant neoplasm was not seen in the animals receiving either IA-4-N-oxide or 0.9% NaCl solution. In addition, mice receiving hycanthone methanesulfonate showed a significantly higher incidence of both type 1 (43% compared to 21% in controls) and type 2 (21% compared to 12% in controls) hepatocyte neoplasms. Mice receiving IA-4-N-oxide showed no increased incidence of neoplasms.

Phase I study of hycanthone.

Hycanthone was given to 15 patients with metastatic cancer in order to determine the maximum tolerable dose. The drug was administered in 5-day courses at 3-week intervals. The starting dose was 30 mg/m2/day and the highest dose level reached was 90 mg/m2/day. The most common (13 patients) side effect was nausea and/or vomiting. The dose-limiting toxicity was toxic hepatitis manifested as elevation in serum transaminases in eight of 15 patients and an increase in serum bilirubin in three patients. Hepatotoxicity was dose-related and was observed in two of 25 courses given at the dose level of less than or equal to 70 mg/m2 compared to seven of nine courses given at the dose level of greater than or equal to 80 mg/m2. Because of an unacceptable incidence of hepatotoxicity at higher doses, 70 mg/m2/day x 5 appears to be a safe dose for phase II studies.

Hycanthone therapy in selected patients with S. mansoni and S. haematobium infections in the Sudan.

The results of a clinical trial of hycanthone in 601 selected Sudanese patients with S. mansoni and S. haematobium infections are reported. Hycanthone was given as a single intramuscular injection at a dose of 3.0 mg per kilogramme body weight. The commonest side effects were nausea and vomiting which occurred in 35 per cent and 32 per cent respectively. No incidence of acute hepatic damage and no deaths were encountered. About one third of patients reported for follow-up. Cure rate for S. mansoni was 90 per cent at 6 weeks of treatment and 97 per cent at three months and six months, while for S. haematobium infection it was 82 per cent at 6 weeks and 91 per cent at 3 months and 6 months. A significant reduction in egg excretion was achieved in those who were not cured.

Liver pathology in hycanthone hepatitis.

Histological examination of liver tissue from 3 patients who died 6, 4, and 9 days, respectively, after an intramuscular injection of 3 +/- 0.5 mg of hycanthone per kg of body weight for schistosomiasis, showed features of a toxic hepatitis with massive hepatic necrosis, many acidophil bodies and minimal inflammatory response in 2 patients, and confluent necrosis with early fibrosis in 1. Liver cells showed prominent fatty change. The former 2 patients had evidence of schistosomal granulomatous hepatitis. A follow-up liver biopsy in a 4th patient who had had hycanthone hepatitis with liver failure 1 year before showed good recovery with features of chronic persistent hepatitis.