Abstract

Growth of mouse fibroblasts is inhibited when cultured in vitro at a concentration of 6-MP of 10^-6 M. The toxicity is delayed, and the cells will divide a few times before dying. This characteristic delayed toxicity can be observed after exposing the cells to the drug for only one or two days, and then replacing the culture medium with a drug-free solution.

The toxic effects can be prevented by 4-amino-5-imidazole-carboxamide, 4-amino-5-imidazole- carboxamide riboside, hypoxanthine, inosine, inosinic acid, adenosine, and adenylic acid (2', 3' and 5'). Adenine is much less effective, and guanine, guanosine, xanthine, or a mixture of adenine-containing coenzymes are ineffective.

Variants of the normal, sensitive cell line have been obtained which are resistant to 6-MP at concentrations of 10^-6, 10^-5, 10^-4 and l0^-3 M. The resistant characteristic persists unaltered for many months in cells cultured in the absence of drug.

On incubation of the cells with 6-MP-8-C¹⁴, both the riboside and ribotide of 6-MP appear in the culture medium of the sensitive cell line. Only the riboside appears in the culture medium of the sub-line resistant to 10-4 M 6-MP. Under circumstances which force these cells to utilize exogenous purines, it can be shown that the resistant cell lines have a marked impairment of their ability to utilize hypoxanthine or inosine as compared to the normal cell, but can utilize inosinic acid.

It is concluded that this instance of resistance is due to an impaired capacity of these cells to form nucleotides from the free purine bases. It cannot yet be stated whether the particular enzyme involved is a nucleotide pyrophosphorylase or a nucleoside phosphokinase.

The possible mechanism of action of 6-MP is discussed.