Abstract
Aqueous extracts from leaves of the European mistletoe (Viscum album L.) are postulated to exert an anticancer efficacy by cytotoxic and/or immunological mechanisms of action. Although popular as an unconventional therapy modality, no controlled randomized clinical trials are available, reliably documenting a clinically beneficial antineoplastic potential of the various commercial mistletoe preparations. Since previous investigations have focused on the purified galactoside-specific lectin (Viscum album L. agglutinin, VAA) as major biological response modifier in the low-dose range, the objective of the present experimental study was to examine its effect on N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN)-induced carcinogenesis in the urinary bladder of rats, a suitable animal model for human disease. The carcinogen was fed by gavage in three fractionated low doses (150 mg/kg body weight each) to obtain low-grade and low-stage transitional cell carcinomas. From the onset of the experiment VAA was injected subcutaneously twice a week (1 ng/kg body weight) continuously for either 6 or 15 months. Following an experimental period of 6 months the incidence of bladder carcinomas was 10.2% in rats given exclusively BBN and 6.7% in those additionally treated with VAA. After an experimental time of 15 months 25.8% of the rats fed BBN only and 19.7% of the animals additionally receiving VAA had developed urothelial carcinomas. The differences of the tumor incidences did not reach the level of statistical significance, neither after an experimental duration of 6 (P = 0.88) nor of 15 months (P = 0.71). A difference was found in the size of the transitional cell carcinomas. They proved to be significantly larger (P = 0.02) in the rats additionally treated with VAA for 15 months (mean maximum diameter: 3.31 mm) than in those without lectin treatment (mean maximum diameter: 1.88 mm). Quantitative immunocytochemistry analyzing a panel of immune cells yielded no evidence for the ability of the lectin to provoke a substantial, biologically relevant local cellular immune response in the wall of tumor-free and tumor-bearing bladders. From the current experiment it is obvious that galactoside-specific mistletoe lectin failed to protect against, inhibit, delay or reduce development of chemically induced urothelial carcinomas of the urinary bladder even after long-term administration in the clinically recommended schedule. It seems highly unlikely that adjuvant treatment with mistletoe extracts or VAA might favorably influence bladder cancer in patients by immunological effector mechanisms.