It has been widely recognised that in acute APAP overdose, saturation of sulfation and glucuronidation systems together with glutathione depletion results in accumulation of the toxic metabolite N-acetyl-p-benzoquinoneimine (NAPQI), which inhibits mitochondrial respiration and causes direct hepatocellular necrosis, resulting in severe lactic acidosis. On the other hand, chronic APAP ingestion, even within therapeutic doses, may nevertheless lead to toxicity in susceptible individuals by causing PGA. This is due to interruption of the γ-glutamyl cycle which governs the synthesis and metabolism of glutathione, probably via an ATP-depleting cycle of reactions as depicted in Fig. 1. In patients with debilitating illness and/or malnourishment who are on chronic APAP treatment, there may be depletions of both glutathione and cysteine stores. Glutathione depletion releases the negative feedback on γ-glutamyl-cysteine synthetase, which catalyses a two-step reaction for synthesis of γ-glutamyl-cysteine from glutamic acid. Because of concomitant cysteine depletion, such reaction cannot be completed. Instead, γ-glutamyl-phosphate is formed at the expense of ATP, and is spontaneously hydrolysed into pyroglutamic acid. 5-oxoprolinase, with consumption of ATP, metabolises pyroglutamic acid back into glutamic acid. The newly formed glutamic acid may then be converted into γ-glutamyl-phosphate and subsequently pyroglutamic acid again. As such, an ATP-consuming futile cycle is established, and with ATP depletion, there will be subsequent accumulation of pyroglutamic acid leading to RAGMA2.
Inherited forms of PGA are rare and mostly due to glutathione synthetase deficiency, which can manifest as severe metabolic acidosis, haemolytic anaemia and progressive encephalopathy. Acquired PGA, usually associated with chronic APAP intake, is more common and was first described in 1989. Additional risk factors for PGA include female sex, malnourishment, concomitant sepsis, hepatic and renal impairment3,4. These risk factors are prevalent and often co-existent in hospitalized patients. Therefore, PGA is likely to be an underrecognized and underdiagnosed condition. Concurrent use of the antibiotics flucloxacillin and netilmicin5, and the anticonvulsant vigabatrin6 have also been implicated in PGA. These drugs inhibit the enzyme 5-oxoprolinase, preventing breakdown of pyroglutamic acid into L-glutamate, resulting in accumulation of pyroglutamic acid5.
Urine pyroglutamic acid to creatinine (Cr) ratio in our patient was estimated to be 1716 µmol/mmol Cr as measured by liquid chromatography tandem mass spectrometry (LC-MS/MS). This high level of urine pyroglutamic acid was at comparable levels as in reported PGA cases in the literature, ranging from 700 to 11000 µmol/mmol creatinine7–9. Because measurement of pyroglutamic acid is only available in specialised metabolic laboratories, it can be difficult to confirm the diagnosis of PGA. On the other hand, urine anion gap (UAG) is a readily available test which may provide a clue to the diagnosis. As in our patient, a positive UAG indicates increased excretion of unmeasured anions in urine, including pyroglutamic acid. Though such finding is non-specific, it would nevertheless be useful if interpreted in a proper clinical context. In addition, it must be noted that a therapeutic level of APAP does not rule out the diagnosis of PGA as the pathogenesis is different from acute overdose.
Apart from chronic APAP ingestion, additional risk factors are often present in patients with PGA. Our patient had chronic infection, prolonged hospitalisation and malnutrition, probably leading to depletion of glutathione and cysteine stores. His renal impairment likely also reduced clearance of pyroglutamic acid and contributed to its accumulation. The patient was also on chronic voriconazole, which has also been reported to cause glutathione depletion and have potential hepatotoxicity based on an animal study10. Recently, a 7-year-old patient with leukaemia who was given chronic APAP, antibiotics and antifungals including voriconazole has been reported to develop PGA11.These suggest that voriconazole might be a contributing culprit. We would like to advise cautious use of APAP in susceptible individuals, particularly patients with multiple aforementioned risk factors. Prolonged use of APAP should be avoided and the dosages should be kept to a minimum, and alternative analgesics should be used where appropriate.
Treatment for PGA should be initiated based on clinical grounds and must not be delayed until laboratory confirmation. APAP and other offending agents should be withheld and alternative prescriptions should be employed where appropriate. NAC is widely used as an antidote for acute APAP poisoning and is the standard treatment for hereditary glutathione synthetase deficiency. Its mechanism of action is by regeneration of glutathione and cysteine stores12. Although there has been limited experience with its use in acquired PGA and the optimal dosage is not uncertain, it appears to be safe and effective according to the published case reports3,4,8,9. We adopted the treatment protocol as in acute APAP poisoning, and demonstrated rapid reversal of acidosis without noticeable adverse effects after NAC administration in our patient.
This case illustrates acute severe RAGMA due to PGA in a patient on therapeutic doses of APAP. Extra caution should be exercised when administering repetitive doses of APAP in hospitalized patients at risk of glutathione depletion. Prolonged use of high therapeutic APAP doses, such as 4g/day in our case, should be avoided. PGA should be considered when patients develop unexplained RAGMA. Elevation of urine anion gap reflect urinary excretion of unmeasured organic acids and may be an informative adjunct to the diagnosis of PGA in an appropriate clinical context. Diagnosis of PGA should be confirmed by measuring organic acids in the serum and/or urine. However, testing may not be readily available and results are often delayed. High clinical suspicion in at-risk patients is the key to a timely diagnosis and effective treatment by withholding the offending agents and giving NAC empirically.