The protective effects of stable nitroxides, as well as their hydroxylamine and amine precursors, have been tested in Chinese hamster V79 cells subjected to H2O2 exposure at fixed concentration or exposure to ionizing radiation. Cytotoxicity was evaluated by monitoring the viability of the cells assessed by the clonogenic assay. The compounds tested at fixed concentration varied in terms of ring size, oxidation state, and ring substituents. Electrochemical studies were carried out to measure the redox midpoint potentials. The studies show that in the case of protection against H2O2 exposure, the protection was determined by the ring size, oxidation state, and redox midpoint potentials. In general the protection factors followed the order nitroxides > hydroxylamines > amines. Both the six-membered ring nitroxides and substituted five-membered ring nitroxides were efficient protectors. For six-membered ring nitroxides, the compounds exhibiting the lowest midpoint potentials exhibited maximal protection. In the case of X-radiation, nitroxides were the most protective though some hydroxylamines were also efficient. The amines were in some cases found to sensitize the toxicity of aerobic radiation exposure. The protection observed by the nitroxides was not dependent on the ring size. However, the ring substituents had significant influence on the protection. Compounds containing a basic side chain were found to provide enhanced protection. The results in this study suggest that these compounds are novel antioxidants which can provide cytoprotection in mammalian cells against diverse types of oxidative insult and identify structural determinants optimal for protection against individual types of damage.

N-(omega-Aminoalkyl)-2,2,5,5-tetramethyl-3-pyrroline- or -pyrrolidine-3-carboxamides were acylated on the primary amino group of the side chain by means of reactive acid derivatives (acid chlorides, activated esters, phthalic anhydrides, phthalimide, 2-alkyl-4H-3,1-benzoxazin-4-ones) or they were alkylated by forming the Schiff bases and subsequent sodium borohydride reduction. Other tetramethyl-3-pyrrolinecarboxamide compounds were synthesized by acylating the aminoalkyl compounds with 2,2,6,6-tetramethyl-3,5-dibromo-4-piperidinone in a reaction involving Favorskii rearrangement. Saturation of the double bond of some pyrroline derivatives furnished the pyrrolidinecarboxamides. The new compounds of each type were active against aconitine-induced arrhythmia and several of them had higher activity and better chemotherapeutic index than quinidine. A few selected examples from each type of the active new compounds showed strong activity against ouabain-induced arrhythmia; for comparison known drugs such as lidocaine, mexiletine, and tocainide were selected. The most potent compounds were oxidized to the paramagnetic nitroxides and the latter were reduced to the N-hydroxy derivatives; these products had no or only decreased antiarrhythmic effect.

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Knowledge of pKa's is necessary to calculate intracellular/intravesicular pH values from nitroxide accumulation in cells or vesicles as detected with electron spin resonance (ESR) spectroscopy. pKa values were confirmed in lipid vesicles of known internal pH. To help select probes that do not accumulate in lipid membranes, octanol/buffer partition coefficients of uncharged nitroxides were determined. As an application of selected probes, pH gradients and internal aqueous volumes were analyzed in mitochondria (one internal compartment) and in the cyanobacterium Synechococcus 6311 (two internal compartments). The combination of 3-carboxy-, 3-amino- and 3-aminocarbonyl-2,2,5,5-tetramethylpyrrolidin-1-yloxyl was found to be most satisfactory for determinations of internal pH and volumes.