Methemoglobin (MHb) induction by 21a in dog blood precluded its development for clinical use. Attempts at dissociating 5-LPO inhibitory properties and MHb formation showed that MHb formation in vitro seemed to be related to the redox potential of the compounds whereas 5-LPO inhibition was not. This study led to a series of 4-(Nn-pentylcarbamoyl) indazolinones which maintained in vitro 5-LPO potency but did not induce MHb.
