Structure-activity relationships in the myotoxicity of ring-methylated p-phenylenediamines in rats and correlation with autoxidation rates in vitro

Toxicity of para-phenylenediamine (PPD;1, 4 diaminobenzene) on isolated human lymphocytes: The key role of intracellular calcium enhancement in PPD-induced apoptosis

Para-phenylenediamine (PPD) is a derivative of benzene used as an ingredient in dyes, a photographic developing agent, and a component of engineered polymers. The carcinogenicity of PPD, which has been documented in several studies, may be related to its toxic effects on different compartments of the immune system. The main goal of this research was to evaluate the mechanism of the toxicity of PPD on human lymphocytes by exploiting the accelerated cytotoxicity mechanism screening (ACMS) technique. Lymphocytes were isolated from the blood of healthy persons using a Ficoll-Paque PLUS standard method. Assessment of cell viability was carried out 12 h following treatment of human lymphocytes with 0.25-1 mM PPD. For determination of cellular parameters, isolated human lymphocytes were incubated with 1/2 the IC50 (0.4 mM), the IC50 (0.8 mM), and twice the IC50 (1.6 mM) for 2, 4, and 6 h. Half maximal inhibitory concentration (IC50) is the concentration that reduces cell viability approximately 50% following treatment. The results of this study demonstrated that PPD-associated apoptosis in human lymphocytes was mainly through the enhancement of intracellular calcium, oxidative stress, and following adverse effect on lymphocyte organelles (like mitochondria and lysosomes). Lipid peroxidation, activation of caspase-3, and stimulation of cytokines (IL₂, interferon-gamma (IFN-γ), and TNF-alpha) production were also observed in PPD-treated lymphocytes. Considering the results of this study, we can suggest an association between PPD carcinogenicity and its toxic effects on different compartments of the immune system.

Plasma 2-hydroxyglutarate, a promising prognostic biomarker candidate for skeletal muscle injury in Fischer 344 rats

Previously, we have demonstrated the potential of plasma 2-hydroxyglutarate (2HG) as an easily detectable biomarker for skeletal muscle injury in rats. Here, we examined whether plasma 2HG was superior to conventional skeletal muscle damage biomarkers, including aspartate aminotransferase (AST), creatine kinase (CK), and skeletal muscle-type CK isoenzyme (CK-MM) levels, in rats. Skeletal muscle injury was induced in 4- or 9-week-old male Fischer 344 rats by cerivastatin (CER) or tetramethyl-p-phenylenediamine (TMPD) administration. Plasma 2HG levels were measured on days 4, 8, and 11 (CER group) and at 6 and 24 hr post-administration (TMPD group). Plasma AST, CK, and CK-MM activities and histopathological changes in the rectus femoris muscle were evaluated at the study endpoints. In the CER group, AST, CK, and CK-MM increased in 4- and 9-week-old rats, whereas increases in CK (4- and 9-week-old rats) and CK-MM (4-week-old rats) were not obvious in the TMPD group. In both 4- and 9-week-old rats, plasma 2HG increased on day 8 and at 24 hr post-administration in the CER and TMPD groups, respectively. Histopathological analysis revealed myofiber vacuolation and necrosis in both groups. The histopathological damage to the rectus femoris muscle was more severe in the CER than in the TMPD group. Increased plasma 2HG was associated with CER- and TMPD-induced skeletal muscle injuries in rats and was not affected by age differences or repeated blood collection. The results suggest that plasma 2HG is superior to CK and CK-MM as a biomarker for mild skeletal muscle injury.

Plasma 2-hydroxyglutarate and hexanoylcarnitine levels are potential biomarkers for skeletal muscle toxicity in male Fischer 344 rats

To identify new candidate biomarkers for skeletal muscle toxicity, an unbiased metabolomic analysis was performed in rats treated with two distinct myotoxicants, cerivastatin (CER) and tetramethyl-p-phenylenediamine (TMPD). Skeletal muscle toxicity was induced in male Fischer 344 rats by administering CER or TMPD and monitored using established endpoints, such as increased plasma creatine kinase (CK) activity and histopathology, and a metabolomic analysis of skeletal muscle and plasma samples. Plasma CK levels in CER-treated rats were markedly elevated at Day 11; however, those in TMPD-treated rats showed a statistically significant decrease at 24 hr after dosing. Light microscopy revealed that vacuolated or necrotic fibers were evident in all CER-treated rats on Day 11, and slightly vacuolated fibers were observed in TMPD-treated rats at 6 and 24 hr after dosing. Metabolomic analysis of the rectus femoris indicated increases in 2-hydroxyglutarate (2HG) in CER-treated rats and hexanoylcarnitine in CER- and TMPD-treated rats. There were also increases in plasma 2HG in CER-treated rats on Days 8 and 11 and in TMPD-treated rats at 24 hr after dosing and increases in plasma hexanoylcarnitine in CER-treated rats on Day 11 and in TMPD-treated rats at 6 and 24 hr after dosing. These experiments demonstrated the potential of plasma 2HG and hexanoylcarnitine as specific and easily detectable biomarkers for skeletal muscle toxicity in rats and demonstrated the value of metabolomics for biomarker detection and identification in toxicological studies.

Application of surface-enhanced laser desorption/ionization technology to the detection and identification of urinary parvalbumin-α: A biomarker of compound-induced skeletal muscle toxicity in the rat

In toxicity studies, compound-induced changes are typically evaluated using a combination of endpoints and there are often a number of potential markers in biological fluids which can indicate toxic change in tissues and organs. However, some biomarkers are not specific to the organ of injury and therefore there is a continuing search for more sensitive and specific indicators of target organ toxicity. In experiments to assess the potential diagnostic usefulness of surface-enhanced laser desorption/ionization (SELDI) ProteinChip technology, skeletal muscle toxicity was induced in Wistar Han rats by administering 2,3,5,6-tetramethyl-p-phenylenediamine (TMPD). The skeletal muscle toxicity was monitored using established endpoints such as increase in serum aldolase (Aldol), aspartate aminotransferase (AST) and histopathology, and also using SELDI retentate chromatography mass spectrometry of urine samples. Clear differences in urinary protein patterns between control and TMPD-treated animals were observed on the ProteinChip surfaces. Additionally a specific urine marker protein of 11.8 kDa was identified in TMPD-dosed rats, and the detection of the marker was related to the degree of skeletal muscle toxicity assessed by recognized clinical pathology endpoints. The 11.8 kDa protein was identified as parvalbumin-alpha. These experiments demonstrated the potential of urinary parvalbumin-alpha as a specific, noninvasive, and easily detectable biomarker for skeletal muscle toxicity in the rat and the potential of SELDI technology for biomarker detection and identification in toxicology studies.

Muscle necrosis in rats induced by 2-methoxy-p-phenylenediamine

p-Phenylenediamine, together with several of its amino and alkyl derivatives, are known to be myotoxic in animals and man. In the present study, it was found that 2-methoxy-p-phenylenediamine, a component of oxidative hair dyes, similarly causes necrosis of skeletal muscle (gastrocnemius, diaphragm and tongue) in rats. The myotoxicity of 2-methoxy-p-phenylenediamine was much greater than that of p-phenylenediamine itself. This observation is consistent with the relative efficacy of these substances in disrupting mitochondrial metabolism, an effect which is believed to be responsible for the selective toxicity of p-phenylenediamine derivatives to muscle.

Phenylenediamine induced hepatocyte cytotoxicity redox. Cycling mediated oxidative stress without oxygen activation

Muscle necrosis induced by various phenylenediamine derivatives has been correlated with their autoxidation rate. However, a more detailed investigation of the cytotoxic mechanism using a model system of isolated hepatocytes and 2,3,5,6-tetramethylphenylenediamine (DD) shows little oxygen activation as indicated by the absence of cyanide resistant respiration, lipid peroxidation and lack of cytoprotection by iron chelators, superoxide dismutase mimics and xanthine oxidase inhibitors. Cytotoxicity was however attributed to oxidative stress as GSH was not only rapidly oxidized to GSSG but mixed protein disulfide formation also occurred. Furthermore, the disulfide reductant dithiothreitol added some time after DD restored protein thiols and prevented further cytotoxicity. This oxidative stress was attributed to a futile two electron redox cycle involving oxidation of DD to the corresponding diimine by the mitochondrial electron transport chain and rereduction by DT diaphorase. Evidence suggesting this was that both diimine accumulation and the ensuing cytotoxicity were markedly increased by inactivating hepatocyte DT diaphorase but were prevented by a subtoxic concentration of the mitochondrial respiratory inhibitor cyanide. Furthermore, addition of NADH generating substrates such as lactate, sorbitol, xylitol or ethanol prevented DD induced GSH oxidation and cytotoxicity. This suggests that DD undergoes intracellular redox cycling without oxygen activation until the hepatocyte is unable to maintain redox homeostasis and mixed protein disulfide cytotoxicity ensues.

Studies on the muscle toxicant 2,3,5,6-tetramethyl p-phenylenediamine: effects on various biomarkers including urinary creatine and taurine

The effect of the specific muscle toxicant, 2,3,5,6-tetramethyl p-phenylenediamine (TMPD), on urinary creatine and taurine, markers of testicular and liver dysfunction, respectively, has been investigated in male Sprague-Dawley rats. Damage to the gastrocnemius and soleus muscles was accompanied by a rise in serum creatine kinase (predominantly the muscle-specific isoenzyme, CK-MM), alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Increases in serum alpha-hydroxybutyrate dehydrogenase (HBDH) and total lactate dehydrogenase (LDH) (mainly isoenzymes, LDH1 and LDH2), occurred but only minor damage to the heart and no rise in CK-MB, (heart muscle isoenzyme) was seen. Damage to stage XIV tubules in the testis was evident histologically after the highest dose. This was accompanied by an increase in LDH-C4 testis-specific isoenzyme and a decrease in serum testosterone. Apart from reduced serum albumin, no other serum parameters indicated liver damage and there was only slight liver steatosis in some animals at the highest dose. Urinary taurine was not significantly raised after any dose of TMPD, but there was a significant increase in urinary creatine after the highest dose. It can be concluded that in the presence of discrete muscle damage, the use of urinary taurine and urinary creatine as markers of liver and testicular dysfunction, respectively, is not confounded. However, a variety of different markers should be used in conjunction to fully delineate the tissue damage due to toxic chemicals.

Generation of superoxide radical and hydrogen peroxide by 2,3,6-triaminopyridine, a metabolite of the urinary tract analgesic phenazopyridine

2,3,6-Triaminopyridine, a metabolite of the widely-used drug phenazopyridine, has been shown to autoxidize at neutral pH, generating superoxide radical and hydrogen peroxide. Hydrogen peroxide was also detected in erythrocytes exposed to this substance, and these cells suffered oxidative damage, as reflected by methaemoglobin formation and glutathione depletion. The in vitro effects of 2,3,6-triaminopyridine are closely similar to those of the structurally-related compound, 1,2,4-triaminobenzene. The latter substance is known to be highly toxic in vivo by mechanisms which may involve free radical production and oxidative stress. It is possible, therefore, that triaminopyridine may be similarly toxic in animals and that this metabolite could be responsible for some of the harmful side-effects associated with phenazopyridine use.

In vitro cytotoxicity of methylated phenylenediamines

The acute cytotoxicities of methylated phenylenediamines (PDs) were evaluated with the neutral red assay, using BALB/c 3T3 mouse fibroblasts as the bioindicators. When the test agents were grouped according to their degree of methylation, good correlations were noted between their in vitro cytotoxicity and their in vivo myotoxicity to experimental animals, as well as to their in vitro autoxidation rates. For test agents of comparable methylation, the sequence of potency was ring-methylated p-PD > N-methylated p-PD >> N-methylated o-PD > N-methylated m-PD.

Mitochondrial oxidation of p-phenylenediamine derivatives in vitro: structure-activity relationships and correlation with myotoxic activity in vivo

A number of p-phenylenediamine derivatives are known to cause necrosis of skeletal and/or cardiac muscle when administered to experimental animals. Compounds of this type are oxidized to semiquinonedi-imines or quinonedi-imines by mitochondria in vitro, establishing alternative pathways for electron transport in the respiratory chain with concomitant decreases in respiratory control and ADP:O ratios. Muscle mitochondria were found to be particularly effective in promoting p-phenylenediamine oxidation in vitro and the magnitude of the mitochondrial effects of the various compounds tested correlated well with their ability to cause muscle necrosis in vivo. It is suggested that mitochondrial oxidation may be involved in the initiation of the myotoxic effects of these compounds and account for their target-site specificity.

Reactions of the Wurster's blue radical cation with thiols, and some properties of the reaction products

Formation of 1-electron oxidation products of aromatic amines in biological systems have been ascertained. The mechanisms of the toxic actions of the aminyl radicals and their corresponding detoxication reactions are much less established. During the studies of reactions of GSH with the N,N,N',N'-tetramethyl-p-phenylenediamine radical cation (TMPD) (Wurster's blue) two pathways were detected: (1) a slow second order reaction (k = 5 M-1.s-1) which gave the parent amine and (ultimately) GSSG, and (2) a fast, complex reaction which yielded 2-(glutathione-S-yl)-N,N,N',N'-tetramethyl-p-phenylenediamine (2-GS-TMPD). From kinetic reasons, this reaction was suggested to be composed of a rapid disproportionation reaction followed by a reductive 1,4-Michael-addition. This reaction pathway prevailed at GSH concentrations below 1 mM. At higher GSH concentrations formation of the thioether was suppressed. This hypothesis was confirmed when the reaction of the highly labile N,N,N',N'-tetramethyl-p-quinonediiminium dication (TMQDI++) with GSH was followed: In this case, thioether formation outweighed clearly reductive mechanisms, the latter yielding ultimately the amine and GSSG. Similar to N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), 2-GS-TMPD was also capable of producing ferrihemoglobin in a catalytic reaction. Its rate, however, was only 3% that observed with the parent amine. During this reaction the thioether was apparently oxidized to the corresponding quinonediiminium dication, which gave the corresponding quinonemonoimine on acidification.