Impact on L-carnitine Homeostasis of Short-term Treatment with the Pivalate Prodrug Tenofovir Dipivoxil

Pivalate-generating prodrugs have been suggested to cause clinically significant hypocarnitinaemia. Tenofovir dipivoxil, a novel ester prodrug of tenofovir, can be used for treatment for hepatitis B and HIV infection and it was necessary to evaluate the effect of its treatment on carnitine homeostasis. We sought to investigate the effect of Class 1 drug tenofovir dipivoxil on endogenous L-carnitine level during a 72-hr test in healthy Chinese volunteers and to establish a suitable dose of L-carnitine nutritional supplement for patients who were administered short-term tenofovir dipivoxil tablets for treatment for hepatitis B and herpes simplex virus infection. Tenofovir dipivoxil was administered in one of eight dosing regimens (single dose 150, 300 and 600 mg, multiple dose 300, 450, and 600 mg, multiple dose 450 (600) mg tenofovir dipivoxil and 0.5 g L-carnitine) to gender-balanced groups of 84 healthy Chinese volunteers. Plasma concentrations of L-carnitine were quantified before, during and after treatment. Plasma L-carnitine concentrations fell during tenofovir dipivoxil dosing. The nadir in L-carnitine concentration was dependent on the dose of tenofovir dipivoxil and it decreased from 6.1 ± 0.6 to 4.4 ± 0.8 μg/ml, 6.1 ± 1.8 to 3.3 ± 1.2 μg/ml, 6.2 ± 0.6 to 2.5 ± 0.5 μg/ml for single doses of 150, 300, 600 mg tenofovir dipivoxil tablets and from 6.0 ± 1.4 to 2.1 ± 1.5 μg/ml, 6.2 ± 0.4 to 0.9 ± 0.5 μg/ml for multiple doses of 450, 600 mg tenofovir dipivoxil tablets, respectively. Short-term administration of tenofovir dipivoxil results in hypocarnitinaemia and increased losses of carnitine in resulting of minor adverse events of decreased food appetite, nausea, abdominal distention and muscle weakness.

Valproic Acid and topiramate induced hyperammonemic encephalopathy in a patient with normal serum carnitine

A 17-year-old female developed hyperammonemic encephalopathy 2 weeks after valproic acid (VPA), 500 mg twice a day, was added to her regimen of topiramate (TPM), 200 mg twice a day. She presented to the emergency department (ED) with altered mental status, hypotension, bradycardia, and lethargy. Laboratory analysis showed mild non-anion gap hyperchloremic acidosis, serum VPA concentration of 86 mg/L, and urine drug screen result that was positive for marijuana. She was admitted to the pediatric intensive care unit for persistent symptoms, prolonged QTc, and medical history. Blood ammonia concentrations were obtained because of her persistent altered mental status, initially 94 μmol/L and a peak of 252 μmol/L. A serum carnitine profile was obtained at the time of hyperammonemia and was found to be normal (results were available postdischarge). VPA and TPM were discontinued on day 1 and day 2, respectively, as the patient's blood ammonia concentration remained elevated. On day 3, her mental status had returned to baseline, and blood ammonia concentrations trended downward; by day 4 her blood ammonia concentration was 23 μmol/L. VPA has been associated with numerous side effects including hyperammonemia and encephalopathy. Recently, drug interactions with TPM and VPA have been reported; however, serum carnitine concentrations have not been available. We discuss the possible mechanisms that VPA and TPM may affect serum ammonia and carnitine concentrations and the use of levocarnitine for patients or treating toxicity.

Serum carnitine levels and levocarnitine supplementation in institutionalized Huntington's disease patients

Along with antioxidant properties, carnitine is an important regulator of lipid metabolism in humans. While beneficial effects of carnitine have been demonstrated in animal models of Huntington's disease (HD), metabolism of carnitine has not been studied in humans with this illness. In this retrospective database review from 23 patients admitted to a HD-specialized nursing home unit, we found a relatively high prevalence of hypocarnitinemia (6 cases, 26%). Our review suggests that catabolism and chronic valproate use predisposed our patients to develop hypocarnitinemia. The patients with low serum carnitine levels who received levocarnitine supplementation, during a mean period of 7.3 months, showed improvement in motor, cognitive and behavioral measures. We hypothesize that observed improvement related to the resolution of reversible metabolic encephalopathy and myopathy associated with secondary carnitine deficiency. In conclusion, notwithstanding its limitations, this is the first study to report measurements of carnitine levels in HD patients, revealing relatively high prevalence of hypocarnitinemia in our population. Our findings suggest that HD patients with hypocarnitinemia may benefit from low-dose levocarnitine supplementation. Further studies of carnitine metabolism and supplementation in HD patients are warranted.

Adverse drug reactions induced by valproic acid

Valproic acid is a widely-used first-generation antiepileptic drug, prescribed predominantly in epilepsy and psychiatric disorders. VPA has good efficacy and pharmacoeconomic profiles, as well as a relatively favorable safety profile. However, adverse drug reactions have been reported in relation with valproic acid use, either as monotherapy or polytherapy with other antiepileptic drugs or antipsychotic drugs. This systematic review discusses valproic acid adverse drug reactions, in terms of hepatotoxicity, mitochondrial toxicity, hyperammonemic encephalopathy, hypersensitivity syndrome reactions, neurological toxicity, metabolic and endocrine adverse events, and teratogenicity.

Ammonium metabolism in humans

Free ammonium ions are produced and consumed during cell metabolism. Glutamine synthetase utilizes free ammonium ions to produce glutamine in the cytosol whereas glutaminase and glutamate dehydrogenase generate free ammonium ions in the mitochondria from glutamine and glutamate, respectively. Ammonia and bicarbonate are condensed in the liver mitochondria to yield carbamoylphosphate initiating the urea cycle, the major mechanism of ammonium removal in humans. Healthy kidney produces ammonium which may be released into the systemic circulation or excreted into the urine depending predominantly on acid-base status, so that metabolic acidosis increases urinary ammonium excretion while metabolic alkalosis induces the opposite effect. Brain and skeletal muscle neither remove nor produce ammonium in normal conditions, but they are able to seize ammonium during hyperammonemia, releasing glutamine. Ammonia in gas phase has been detected in exhaled breath and skin, denoting that these organs may participate in nitrogen elimination. Ammonium homeostasis is profoundly altered in liver failure resulting in hyperammonemia due to the deficient ammonium clearance by the diseased liver and to the development of portal collateral circulation that diverts portal blood with high ammonium content to the systemic blood stream. Although blood ammonium concentration is usually elevated in liver disease, a substantial role of ammonium causing hepatic encephalopathy has not been demonstrated in human clinical studies. Hyperammonemia is also produced in urea cycle disorders and other situations leading to either defective ammonium removal or overproduction of ammonium that overcomes liver clearance capacity. Most diseases resulting in hyperammonemia and cerebral edema are preceded by hyperventilation and respiratory alkalosis of unclear origin that may be caused by the intracellular acidosis occurring in these conditions.

Valproate-induced hyperammonemic encephalopathy: a brief review

BACKGROUND

This brief review presents a comprehensive evaluation of valproate-induced encephalopathy (VHE) and also discusses potential mechanisms of the condition.

SCOPE

Sodium valproate (VPA) is an effective antiepileptic drug used in neurology as well as in psychiatry, in adults and children. VHE requires early diagnosis and management. Focused research efforts in understanding the condition will help decrease its incidence. Delay in recognition of VHE can result in the development of potentially life-threatening complications.

FINDINGS

Management options are described. Since VPA frequently causes a modest rise in plasma ammonia levels which is asymptomatic, it is important to recognize the symptoms of VHE promptly and to correlate them with the plasma ammonia levels.

CONCLUSIONS

Although there are several case reports on VHE, this review is a comprehensive evaluation of its causes and potential mechanisms. Rapid diagnosis and management will help in reducing VHE-related morbidity.