Single
amino acid arginine deprivation is a promising strategy in modern metabolic anticancer
therapy. Its potency to inhibit
tumor growth warrants the search for rational chemo- and radio-therapeutic approaches to be co-applied. In this report, we evaluated, for the first time, the efficacy of
arginine deprivation as anticancer
therapy in three-dimensional (3D) cultures of human
tumor cells, and propose a new combinatorial metabolic-chemo-radio-treatment regime based on
arginine starvation, low doses of
arginine natural analog
canavanine and irradiation. A sophisticated experimental setup was designed to evaluate the impact of
arginine starvation on four human epithelial
cancer cell lines in 2D monolayer and 3D spheroid culture. Radioresponse was assessed in colony formation assays and by monitoring spheroid regrowth probability following single dose irradiation using a standardized spheroid-based test platform. Surviving fraction at 2 Gy (SF(2Gy)) and spheroid control dose(50) (SCD(50) ) were calculated as analytical endpoints.
Cancer cells in spheroids are much more resistant to
arginine starvation than in 2D culture. Spheroid volume stagnated during
arginine deprivation, but even after 10 days of
starvation, 100% of the spheroids regrew. Combination treatment, however, was remarkably efficient. In particular, pretreatment of
cancer cells with the
arginine-degrading
enzyme arginase combined with or without low concentration of
canavanine substantially enhanced cell radioresponse reflected by a loss in spheroid regrowth probability and SCD(50) values reduced by
a factor of 1.5-3. Our data strongly suggest that
arginine withdrawal alone or in combination with
canavanine is a promising antitumor strategy with potential to enhance
cancer cure by irradiation.