Possible Usefulness of Substituted Amino Acids for Tumor Growth Inhibition. ∗ †

Abstract

The avidity of transplanted sarcoma in rats for labelled tyrosine (1) and rat hepatoma for labelled alanine and glycine(2) suggested the possibility that amino acids containing a toxic substituent might prove useful for the inhibition of tumor growth. Although n-ace-tylation of tryptophane does not prevent its utilization as a growth promoter(3). n-iodoacetyl derivatives of l-tryptophane, l-leucine and d, l-phenylalanine were considered suitable for an exploratory study because the iodoacetyl group, an inhibitor of certain respiratory enzymes, is a toxic residue. Accordingly, these derivatives were prepared from corresponding chloroacetyl derivatives by treatment with sodium iodide in acetone(4). For preparation of the radioactive analogues Nal¹³¹ was substituted for sodium iodide(4). For assessment of their distribution in tissues, their general toxicity and their specific toxicity for tumor tissue, control observations with iodoacetamide were necessary. Radioactive iodoacetamide was similarly prepared from chloroacetamide (2).

The distribution of radioactivity in tumor, liver and blood of Swiss mice bearing transplanted sarcoma 37 was determined at intervals after injection of the radio-active analogues of the three amino acid derivatives and of iodoacetamide. With the possible exception of radioactive iodoacetamide in tumor (Fig. 2, 3, 4) there were no consistently significant differences among the 4 compounds in the concentrations of radioactivity in liver, blood and tumor. Although the concentration of radioactivity in blood and tissues in the initial stages is not significantly different after administration of these 4 compounds than it is after the administration of Nal¹³¹, the radioactivity from the latter disappears within 4 hours(5) while from the former small but significant amounts of radioactivity persist for considerably longer periods. This persistence may reasonably be explained by the assumption that at least a small fraction of radioactive iodoacetamide and iodoacetylamino acid derivatives had reached tissue cells.