NAC is the drug of choice for APAP overdose. Unfortunately, NAC administration may induce a variety of side effects. Adverse effects for NAC include anaphylactic reaction, nausea, vomiting and diarrhea (Holdiness, 1991
; Bonfiglio et al., 1992
; Kao et al., 2003
; Kerr et al., 2005
). The latter GI side effect would ultimately diminish hepatic delivery of NAC. Consequently, other treatment modalities with fewer side effects would be beneficial. The major side effects associated with clinical use of SAMe at a dose of 1600 mg/day was a 20% incidence of headache and diarrhea but none of the individuals had to stop SAMe usage (Rosenbaum et al., 1990
Reports by other laboratories have shown that certain agents were beneficial at reducing APAP hepatotoxicity. Several agents such as clofibrate (Manautou et al., 1996
), ribose cysteine (Lucas et al., 2000
); L-cysteine glutathione mixed disulfides (Berkeley et al., 2003
), antioxidants (Oz et al., 2004
), 2(RS)-n-propylthiazolidine-4Rcarboxylic acid (Srinivsan et al., 2003) and NAC (Lauterburg et al., 1983
) reduced APAP hepatic toxicity. However, all these compounds were successful in reducing APAP hepatic toxicity when administered prior
to APAP exposure. Very few agents have been reported to reduce APAP hepatotoxicity when administered after
APAP exposure similar to what is needed for an antidote. NAC, the therapeutic antidote for APAP overdose, is one of the few agents to reduce toxicity when administered after APAP exposure to mice (Salminen et al., 1998
; James et al., 2003
). The effectiveness of NAC was inversely dependent on the time NAC was administered after APAP. NAC was most effective in reducing APAP toxicity in mice when administered within 1 h of APAP treatment and was least beneficial if administered 3 or 4 h after APAP overdose, respectively (Salminen et al 1998
; James et al., 2003
). Salminen and associates (1998)
also reported that covalent binding by NAPQI was not reduced if NAC was administered 1 h after APAP treatment but NAC still successfully reduced APAP hepatic toxicity. These findings would suggest that covalent binding by NAPQI is a component of APAP toxicity but additional events also contribute to hepatic damage. Furthermore, since NAC given 1 h after
APAP overdose still provides protection without reducing APAP mediated hepatic arylation (Salminen et al., 1998
; James et al., 2003
) the mechanism for protection involves processes independent of covalent binding when given after APAP exposure. It is also possible that the protection mediated by SAMe when administered after
APAP overdose is due to processes independent of covalent binding. Further studies will be necessary to determine whether this hypothesis is true.
In the clinical setting, NAC is administered every 4 h for as many as 17 doses over the course of 72 h following APAP overdose. Since APAP has a relatively short half-life of 2 to 4 h in humans, NAC treatment for multiple days must at least partially assist with cellular damage and increased oxidative stress. The mechanism for NAC in APAP recovery would be to mediate protection by providing cysteine for increased glutathione synthesis (Lauterburg et al., 1983
) as NAC administered after APAP injection does not prevent formation of APAP protein adducts (James et al., 2003
The current study showed that SAMe was effective in reducing APAP toxicity when it was administered after
APAP exposure. These results show that SAMe was similar to NAC in reducing APAP hepatotoxicity when administered 1 h after APAP treatment at equivalent mmol doses. Time course studies in mice treated with NAC at varying times after APAP overdose reported maximum protection occurred when NAC was administered within 1 h after APAP (James et al., 2003
). Consequently, our studies were designed to administer SAMe and NAC 1 h after APAP.
SAMe is normally present in the body and acts as a major methyl donor for transmethylation reactions. Transmethylation reactions are essential in maintaining normal cell function, especially in the liver, for transmethylation of phospholipids, proteins, nucleotides and neurotransmitters (Chang et al., 1996
; Lieber and Packer, 2002
). SAMe is also a substrate for the transsulfuration pathway which converts SAMe to homocysteine and ultimately to glutathione (Lu, 1998
). The mechanism for SAMe protection of APAP toxicity may involve both the transsulfuration and transmethylation pathways. The transsulfuration pathway would help assist in recovery of depleted hepatic GSH which would be similar to the mechanism for NAC. SAMe would also stimulate the transmethylation pathways which would assist in recovery by generating new intermediates within surviving hepatocytes. SAMe is a critical cofactor in transmethylation reactions of membrane phospholipids such as phosphatidyethanolamine to phosphatidylcholine (Stramentinoli et al., 1979
). Transmethylation reactions would be anticipated to increase membrane fluidity and reduce membrane damage.
SAMe provided protection of APAP mediated alterations in hepatic proteins due to oxidative stress. APAP induced an increase in oxidative stress as bands positive for protein carbonyls () and 4-HNE () were observed in hepatic tissue. 4-HNE positively stained bands between 40- 64 kDa were apparent with 4 h following APAP treatment (). 4-HNE, an aldehyde generated during lipid peroxidation, can impair cellular function. 4-HNE attaches to cellular proteins by a 1,4-Michael addition to specific amino acids including cysteine and lysine (Esterbauer et al., 1991
). The reduced intensity for the 4-HNE adducted proteins in the SAMe+APAP would indicate that SAMe reduced oxidative stress in hepatic tissue even when administered 1 h after APAP treatment.
It is unlikely that SAMe attenuation of APAP hepatic toxicity was due to inhibition of cytochrome P450 conversion of APAP to NAPQI. APAP has an approximate half-life of 1 h in mice (Fischer et al., 1981
) which would provide sufficient time to generate the toxic metabolite prior to SAMe administration. Based on the pharmacokinetics of APAP in mice, approximately 50% of APAP is already eliminated when SAMe was administered which would suggest that the mechanism for SAMe protection does not involve alteration of biotransformation.
In summary, the hepatic toxicity of APAP was reduced by equimolar doses (1.25 mmol/kg) of SAMe or NAC 1 h after administration of a toxic dose of APAP was effective in reducing hepatic damage. This study was also the first to provide a comparison of APAP hepatic toxicity when SAMe and NAC were administered 1 h after APAP treatment. These results suggest that SAMe, at least in our model, has some potential as an antidote for APAP toxicity.