Association of SIBA treatment and a Met-depleted diet inhibits in vitro growth and in vivo metastatic spread of experimental tumor cell lines.

We have used 5'-deoxy-5'-S isobutyl-thioadenosine (SIBA), an analog of S-adenosylhomocysteine, alone or in association with a methionine-depleted diet in order to obtain an antitumoral effect in two different tumor models: a transplantable rat rhabdomyosarcoma (RMS-J1) induced by i.m. injection of nickel and the well-known Lewis lung carcinoma (3LL) of C57BL/6 mice. Since SIBA has been reported to inhibit the methyl group transfer from methionine to S-adenosylhomocysteine, among other activities, its association with a reduction of methyl donors, achieved by methionine depletion of the diet (in vivo) or the culture medium (in vitro), should logically lead to an additive effect. In vitro, 3LL and RMS-J1 were sensitive to the cytotoxic effect of SIBA and were methionine-dependent for their proliferation. Fibroblast proliferation was not affected by these two treatments alone or in association. In vivo, either SIBA treatment or a low methionine diet led to a significant decrease in the metastatic character of these two tumors; however, local tumor growth was not significantly affected. The median number of 3LL metastases counted in the lungs was reduced from 100 to 18 by SIBA treatment, and to 27 by the low methionine diet. No additive effect could be detected when the treatments were given simultaneously. RMS-J1-bearing rats treated with SIBA and fed a low Met diet underwent primary tumor excision. The median numbers of lung metastatic nodules were 27, 26, 14 and 8 for the control, SIBA-treated rats, methionine-deprived rats and rats receiving the combined therapy. Expressed as percentages 20 per cent were cured, 23 per cent showed a low number of lung metastases (P less than 10), whereas all the rats in the control group developed more than 10 pulmonary nodules. No cytotoxic effect could be observed on the treated rats. The role of SIBA and methionine depletion, as agents interfering with transmethylation processes, in regard to the control of tumor development, namely metastatic invasiveness, is discussed.

Decreased rat rhabdomyosarcoma pulmonary metastases in response to a low methionine diet.

Many Experimental and human tumor cell lines have been previously described as being dependent upon exogenous methionine for their in vitro proliferation. The rationale of the experiments described herein was to decrease the in vivo growth of malignant tumors by reducing the exogenous methionine available in diets fed to Wistar AG rats bearing the highly metastatic rhabdomyosarcoma, RMS-J1. The methionine content in the diet was reduced either by replacing casein (diet 1) with soybean protein (diet 4), or by lowering the amount of soybean protein in the diet (from 23 g/100 g to 12 g/100g) (diet 5), or by using a crystalline amino acid-defined mixture as the source of protein (diet 7). In the latter diet homocysteine replaced methionine and allowed the survival of the animals. Diet 4 significantly reduced the mean number of lung metastases without affecting the primary tumor growth. Treatment of RMS-J1 bearing rats with diet 5 led to the decrease of pulmonary invasion (78 and 21 median lung metastases, respectively, in control and treated groups). This diminished metastatic dissemination resulted from the reduced methionine consumption: the lowered casein content in diet 3 (10 g/100 g) as compared to diet 1 (23 g) did not alter primary tumor growth or the amplitude of lung invasion. Moreover, the addition of methionine to diet 5 prevented the diminution of the median number of lung metastases. Replacement of methionine with homocysteine in the crystalline amino acid-defined mixture (diet 7) fed to RMS-J1 bearing rats led to a limited retardation of primary tumor growth (less than 10%) and to a significant decrease in pulmonary invasion: the median number of pulmonary metastases was 28 and 9 for control and treated rats respectively.