p-Alkoxyphenols, a new class of inhibitors of mammalian R2 ribonucleotide reductase: possible candidates for antimelanotic drugs

S Potsch, H Drechsler, B Liermann, A Graslund and G Lassmann

Max Delbruck Center of Molecular Medicine, Berlin-Buch, Germany.

The inhibition by different p-alkoxyphenol derivatives of the growth- regulating enzyme ribonucleotide reductase (RR) in purified Escherichia coli and mouse R2 protein preparations was studied by EPR spectroscopy. The inhibitor-induced inactivation of the catalytic subunit protein R2 was measured at 77 degrees K by observing the decrease of the typical EPR signal from the functionally essential protein-linked tyrosyl free radical. p-Methoxy-, p-ethoxy-, p-propoxy-, and p-allyloxyphenol were about 2 orders of magnitude more effective in inhibiting mouse R2, compared with E. coli R2. Among the p-alkoxyphenols studied, p- propoxyphenol was the most effective inhibitor of mouse R2 (IC50, 0.7 microM) and p-methoxyphenol was the least effective (IC50, 11 microM); p-ethoxy- and p-allyloxyphenol were intermediate. The observed half- maximal inhibition values characterized p-alkoxyphenols as a new class of strong inhibitors of the R2 protein of mammalian RR. p-Propoxy-, p- ethoxy-, and p-allyloxyphenol could be considered as new candidates for anticancer drugs. A special cellular inhibition assay of RR in proliferating tumor cells, in which the tyrosyl radical of R2 at natural concentration was monitored by EPR, showed that the four para- substituted alkoxyphenols also inhibited the enzyme with high efficiency in tumor cells (IC50, between 0.5 microM and 5 microM). Our results with inactivation of protein R2 of RR imply that the cytostatic effect of p-alkoxyphenols on melanoma cells, which has been hitherto explained by inhibition of tyrosinase [Melanoma Res. 2:295-304 (1992)], may be caused at least partly by inhibition of RR. Protein R2 of RR may be considered as an additional target that could be used for future cancer chemotherapy.

Volume 45, Issue 4, pp. 792-796, 04/01/1994
Copyright © 1994 by American Society for Pharmacology and Experimental Therapeutics