The influence of the free radical of diaziquone on inhibition of cell growth in vitro was studied using P388 murine leukaemia cells and a mixture of AZQH2 + AZQ. This mixture generates the AZQ free radical by the disproportionation-comproportionation reaction AZQH2 + AZQ in equilibrium with 2AZQH. 2. Reduced AZQ (AZQH2) was produced electrochemically under anaerobic conditions. This colourless solution was added anaerobically to a solution of AZQ at a 9:1 ratio of AZQH2 to AZQ. The free radical (AZQH) generated was detected by electron spin resonance (ESR) and amounted to about 7% of the concn of AZQH2 + AZQ. The AZQ free radical was stable enough to treat cells in culture. 3. Aerobic suspensions of P388 murine leukaemia cells treated with AZQH experienced a surge of free radicals lasting approx. 30 min at concn near 10 times that of the AZQ free radicals generated by P388 cells. 4. Cell growth inhibition indicates that the concentration of AZQ required to arrest the growth of P388 cells by 50% (IC50) is twice that of the AZQ free radical generating mixture. The mechanism of action of AZQ probably does not exclusively involve AZQ free radicals, but they appear to play an important role.
