The Effect of Arginase on the Retardation of Tumour Growth

THE use of enzymes as chemotherapeutic agents for the control of tumour growth has found comparatively little favour with experimenters of the past. The reasons for this are indeed understandable, as pointed out by Bergel (1961) in his account of the control of enzyme activities in tissues. Difficulties which arise concern the availability of the enzyme preparations, the immunological incompatibility of " foreign " proteins and the problem of permeability of the cell membrane to the comparatively large enzyme molecules. The first difficulty could be overcome through the use of modern preparative methods and the second through counteracting agglutinations by treatment with adreno-cortical preparations. The problem of transport of the enzyme into the cell will be discussed below. Therapeutic experiments with enzymes, similar to the ones carried out in this work, have been reported before, when Haddow, de Lamirande, Bergel, Bray and Gilbert (1958) injected xanthine oxidase into tumour-bearing mice in an attempt to retard tumour growth. The choice of the enzyme in the present studies for the purpose of impeding or at least, retarding tumour growth was guided by the following previous observations: arginine had been shown to exert a stimulating effect on the mitosis of mouse carcinoma (Bach and Lasnitzki, 1947) and to be excessively utilised in the synthesis of creatine in rats bearing Jensen sarcoma (Bach and Maw, 1953). Therefore, an enrichment of the tissues with arginase could have been expected to reduce the level of arginine in these tissues and to diminish its value as a stimulating agent, particularly so, since the work of Bach and Lasnitzki (1947) had also indicated that slow-growing tumours contained less arginase than fast-growing tumours. Subsequently, it was indeed observed that arginase added to tissue cultures of fibroblasts and to cells of Jensen sarcoma completely inhibited mitosis in the metaphase at very low enzyme concentrations (Bach and Simon-Reuss, 1953). Thus an in vivo approach to the problem, i.e. injections of arginase into tumour-bearing animals was the logical choice of experiment. The authors received little encouragement for this project since Greenberg and Sassenrath (1953) failed to reproduce the favourable results obtained in vivo experiments by Vrat (1951), Wiswell (1951) and Irons and Boyd (1952). These authors had claimed to have succeeded in controlling tumour growth in mice by injecting crude arginase solutions. However, in the meantime arginase preparations of higher purity have been made available (Bach and Killip, 1958 and 1961; Bach, Hawkins and Swaine, 1963). Such highly purified preparations when used in the present work for injections into rats bearing Walker carcinomas, resulted in a considerable retardation of tumour growth.