Nitrones as therapeutics

Abstract

Nitrones have the general chemical formula X-CH=NO-Y. They were first used to trap free radicals in chemical systems and then subsequently in biochemical systems. More recently several nitrones, including alpha-phenyl-tert-butylnitrone (PBN), have been shown to have potent biological activity in many experimental animal models. Many diseases of aging, including stroke, cancer development, Parkinson disease, and Alzheimer disease, are known to have enhanced levels of free radicals and oxidative stress. Some derivatives of PBN are significantly more potent than PBN and have undergone extensive commercial development for stroke. Recent research has shown that PBN-related nitrones also have anti-cancer activity in several experimental cancer models and have potential as therapeutics in some cancers. Also, in recent observations nitrones have been shown to act synergistically in combination with antioxidants in the prevention of acute acoustic-noise-induced hearing loss. The mechanistic basis of the potent biological activity of PBN-related nitrones is not known. Even though PBN-related nitrones do decrease oxidative stress and oxidative damage, their potent biological anti-inflammatory activity and their ability to alter cellular signaling processes cannot readily be explained by conventional notions of free radical trapping biochemistry. This review is focused on our studies and others in which the use of selected nitrones as novel therapeutics has been evaluated in experimental models in the context of free radical biochemical and cellular processes considered important in pathologic conditions and age-related diseases.

Figure 1

Chemical structures of nitrones, their reaction to trap free radicals and form spin adducts and the chemical structure of PBN.

Figure 2

The influence of PBN and its 3-hydroxy and 4-hydroxy derivatives on the parameters of preneoplastic lesions in the liver of rats administered these chemicals in a choline deficient and choline sufficient diet. The values were calculated from the data presented in Floyd et al [138].

Figure 3

(A) Representative T2-weighted images and histology (H&E, x4) staining of C6 gliomas from the different treatment groups. (i–ii) Untreated C6 glioma at day 18 following cell implantation. (iii–iv) PBN pre-treated C6 glioma at day 27. (v–vi) Post-tumor PBN treated C6 glioma undergoing regression, shown at day 32. (vii–viii) Nonresponsive post-tumor PBN treated C6 glioma shown at day 16. (B) Tumor volumes (mm³) measured at the last MRI timepoint, represented as means ± standard deviation.

Figure 4

(A) 2D vasculature projections in the horizontal plane of a representative non-treated C6 glioma at day 8 (i) and 20 (ii) after cell implantation, and of a PBN pretreated C6 glioma at day 9 (iii) and 39 (iv). The tumor boundaries are also represented (red). (B) 3D maximum intensity projection (MIP) rendering of the brain vasculature. (C) Blood volume ratios for each treatment group at the last time point (days 17–21 for untreated rats and non-responsive rats (PBN D+14), and days 39–40 for treated rats (PBN D−5, PBN D+14 responsive), represented as means ± standard deviation.

Figure 5

Figure 5A. Auditory brainstem response (ABR) threshold shifts averaged at frequencies of 2–8 kHz for control group, different 4-OHPBN dosage group, and different drug combination group. The threshold shifts of control group were reduced with increases of 4-OHPBN dosage and the number of drug combination. Asterisks of * and *** represent statistically significant differences in ABR threshold shifts between control group and each experimental group at p<0.05 and p<0.001, respectively. Asterisks of ***10, **20, ***20, **50, and ***50 indicate statistically significant differences in ABR threshold shifts between each experimental group (75 mg/kg, two, and three) and each different dosage of 4-OHPBN (10, 20, and 50 mg/kg) at p<0.001, p<0.01, p<0.001, p<0.01, and p<0.001, respectively. This figure has been published previously by Choi et al [184] and is reproduced with the permission of the publisher. Figure 5B. Percentage of missing OHC averaged at cochlear frequency regions corresponding to 2–8 kHz for four 4-OHPBN dosage groups, different drug combination groups, and control group. Outer hair cell (OHC) losses of control group were reduced with increases of 4-OHPBN dosage and the number of drug combination. Asterisks of ** and *** represent statistically significant differences in OHC loss between each experimental group and control group at p<0.01, and p<0.001, respectively. Asterisks of *10–20 indicate statistically significant differences in OHC loss between each experimental group and 4-OHPBN of 10–20 mg/kg at p<0.05. This figure has been published previously by Choi et al [184] and is reproduced with the permission of the publisher. Figure 5C. Auditory brainstem response (ABR) threshold shifts averaged at frequencies of 2–8 kHz for different antioxidant drugs and their combinations. Asterisks of ** and *** indicate statistically significant differences between control group and each experimental group at p<0.01 and p<0.001, respectively. The symbol # shown in the two-drug combination group indicates significant differences in averaged ABR threshold shifts between the two-drug combination and other experimental groups [4-OHPBN(20), NAC(325), ALCAR(100)] at p<0.001, respectively while the symbol # shown in the three-drug combination groups shows significant differences between the three-drug combination and other experimental groups [4-OHPBN(20), NAC(325), ALCAR(100)] at p<0.001 and the three-drug combination and an experimental group [4-OHPBN(50)] at p<0.05, respectively. Data for NAC (325 mg/kg) and ALCAR (100mg/kg) were excerpted from Coleman et al [189]. This figure has been published previously by Choi [184] and is reproduced with the permission of the publisher.