Concomitant
tumor resistance (CR) is a phenomenon in which a
tumor-bearing host is resistant to the growth of secondary
tumor implants and
metastasis. While former studies have indicated that T-cell dependent processes mediate CR in hosts bearing immunogenic small
tumors, the most universal manifestation of CR induced by immunogenic and non-immunogenic large
tumors had been associated with an antitumor serum factor that remained an enigma for many years. In a recent paper, we identified that elusive factor(s) as an equi-molar mixture of
meta-tyrosine and
ortho-tyrosine, two isomers of
tyrosine that are not present in normal
proteins and that proved to be responsible for 90% and 10%, respectively, of the total serum anti-
tumor activity. In this work, we have extended our previous findings demonstrating that a periodic
intravenous administration of
meta-tyrosine induced a dramatic reduction of lung and hepatic
metastases generated in mice bearing two different metastatic murine
tumors and decreased the rate of death from 100% up to 25% in
tumor-excised mice that already exhibited established
metastases at the time of surgery. These anti-metastatic effects were achieved even at very low concentrations and without displaying any detectable toxic-side effects, suggesting that the use of
meta-tyrosine may help to develop new and less harmful means of managing malignant diseases, especially those aimed to control the growth of
metastases that is the most serious problem in
cancer pathology.