Rearrangement of valproate glucuronide in a patient with drug-associated hepatobiliary and renal dysfunction

Unbound valproate fraction in plasma and subcutaneous microdialysate in steady state and after a single dose in humans

The aim of the current study was to characterize the observed discrepancy between unbound plasma valproate (VPA) in single dose and steady state in humans. Unbound and total plasma VPA and subcutaneous microdialysate VPA concentrations were estimated in single dose (6 subjects, n = 33) and steady state (11 subjects, n = 110). Trough plasma samples from 14 patients with total VPA concentrations of 300 micromol/L and 14 patients with VPA concentrations ranging from 600 to 700 micromol/L were analyzed for the unbound VPA fraction and compared with the unbound VPA fraction in spiked plasma samples from healthy subjects containing similar total VPA concentrations. The unbound plasma VPA fraction was significantly higher (P < 0.001) in the steady-state group compared with the single-dose group. The unbound VPA fraction was significantly higher in steady state compared with spiked plasma samples at high and low total VPA concentrations (P < 0.001). The difference between microdialysate and unbound plasma VPA concentrations was significant in the steady-state group (P < 0.001), while no difference was observed in the single-dose group. The mean (+/- SD) subcutaneous microdialysate-to-unbound plasma ratio in the single-dose and steady-state groups was 1.08 (+/- 0.401) and 0.74 (+/- 0.123), respectively. The ratio difference between the groups was significant (P < 0.001). The results of the current study show that unbound plasma fractions of VPA are consistently higher in steady state compared with single dose. Together with the finding of higher unbound VPA fraction in steady state compared with spiked plasma samples, these results provide indirect evidence of displacement of VPA from plasma proteins by product(s) of VPA biotransformation. In addition, subcutaneous microdialysate VPA levels were consistently lower than unbound plasma levels in steady state but not after single dose. The mechanisms underlying this observation need to be studied further.

Anticonvulsant therapy in aged patients. Clinical pharmacokinetic considerations

Alterations in drug disposition that occur with aging are now becoming widely recognised, and there is an increasing number of drugs for which the approach to therapy in elderly patients can be based on pharmacokinetic data. Both healthy aging and comorbid disease can alter the responsiveness of the body to drugs and to their absorption, distribution and elimination. Altered absorption in the elderly has not been documented after oral ingestion of any anticonvulsant drug. Increased adipose tissue in the elderly may raise the apparent volume of distribution (Vd) of lipid-soluble drugs. An increased Vd in the elderly has been shown for diazepam and clobazam, but not midazolam. The data are inconclusive for phenytoin and valproic acid (sodium valproate). The decreased plasma protein binding that often occurs in the elderly has few clinical consequences. The reduced liver function that to occur with aging seems to affect the elimination of drugs that are mainly cleared by oxidative metabolism [e.g. carbamazepine, phenytoin and phenobarbital (phenobarbitone)]. Reduced clearances for methylphenobarbital (methylphenobarbitone), diazepam, midazolam and clobazam occur in elderly men, but not in women. The reduced renal function that is seen in old age affects the disposition of drugs that are eliminated mainly by direct renal excretion. Thus. the clearances of vigabatrin and gabapentin correlate with creatinine clearance. Such considerations may help guide anticonvulsant dosage in the elderly.

Studies on the reactivity of acyl glucuronides--III. Glucuronide-derived adducts of valproic acid and plasma protein and anti-adduct antibodies in humans

The major metabolite of the anti-epileptic agent valproic acid (VPA) is its acyl glucuronide conjugate (VPA-G), which undergoes non-enzymic, pH-dependent rearrangement via acyl migration to a mixture of beta-glucuronidase-resistant forms (collectively VPA-G-R). We have compared the reactivity of VPA-G and VPA-G-R towards covalent VPA-protein adduct formation by incubation in buffer, human serum albumin (HSA) and fresh human plasma at pH 7.4 and 37 degrees. In all three media, the predominant reaction of VPA-G over 30 hr was rearrangement to VPA-G-R (ca. 24%). Hydrolysis was quite minor (ca. 2%) and covalent adduct formation negligible (when protein was present). On the other hand, both hydrolysis (ca. 27%) and adduct formation (ca. 7%) were extensive when VPA-G-R was incubated with HSA or plasma. These data do not support a transacylation mechanism for VPA-protein adduct formation, since this pathway should be much more highly favoured by VPA-G (an acyl-substituted acetal) than VPA-G-R (simple esters). VPA-protein adducts were found in the plasma of epileptic patients taking VPA chronically (mean 0.77 +/- SD 0.63 microgram VPA equivalents/mL, N = 17). An enzyme linked immunosorbent assay was developed, using HSA modified by incubation with VPA-G-R, to test the immunoreactivity of the patients' plasma. Of 57 patients tested, nine showed measurable levels of antibodies to these adducts, but the titres were very low, with no difference in response to modified and unmodified protein detectable at plasma dilutions of 1:16 or greater. These results suggest that the VPA-protein adducts have little immunogenicity, and are in agreement with clinical observations that drug hypersensitivity responses have not been associated with VPA therapy. Thus, although the in vitro data show that VPA-G is an example of a relatively unreactive acyl glucuronide, covalent VPA-plasma protein adducts and anti-adduct antibodies are nonetheless formed in vivo, at least in some patients on chronic therapy with the drug.

Studies on the reactivity of acyl glucuronides--I. Phenolic glucuronidation of isomers of diflunisal acyl glucuronide in the rat

Diflunisal (DF) is metabolized primarily to its acyl glucuronide (DAG), phenolic glucuronide (DPG) and sulphate (DS) conjugates. Whereas DPG and DS are stable at physiological pH, DAG is unstable, undergoing hydrolysis (regeneration of DF) and rearrangement (intramolecular acyl migration to the 2-, 3- and 4-O-acyl-positional isomers). We have compared the in vivo disposition of DAG with that of an equimolar mixture of its three isomers after i.v. administration at 10 mg DF equivalents/kg to conscious, bile-exteriorized rats. After dosing with DAG, excretion in urine and bile (46% as DAG), hydrolysis (as assessed by recovery of 9% DPG and 8% DS resulting from reconjugation of liberated DF) and rearrangement (17% recovery as isomers of DAG) were important pathways. Highly polar metabolites excreted almost exclusively in bile and accounting for 13% of the dose were identified as an approximate 4:1 mixture of the 2- and 3-O-isomers of DAG which had been glucuronidated at the phenolic function of the salicylate ring i.e. "diglucuronides" of DF. Evidence for trace quantities only of the phenolic glucuronides of the 4-O-isomer of DAG, and of DAG itself, was found. After dosing rats with an equimolar mixture of the isomers, 52% was recovered (as the isomers) in urine and bile in 6 hr. Hydrolysis was less important--less than 3% (total) of the dose was recovered as DPG and DS. The phenolic glucuronides of the 2- and 3-O-isomers (ratio ca. 3:7) accounted for 37%. Evidence for appreciable formation of the phenolic glucuronide of the 4-O-isomer was not found. In one rat dosed with DPG, there was no evidence for further glucuronidation of the salicylate ring at its carboxy function. The data suggest that the 2- and 3-O-isomers of DAG, but not the 4-O-isomer, DAG itself or DPG, are good substrates for further glucuronidation.

Contrasting systemic stabilities of the acyl and phenolic glucuronides of diflunisal in the rat

1. Diflunisal (DF) is metabolized in humans and rats primarily to its acyl glucuronide, phenolic glucuronide and sulphate conjugates. 2. After i.v. administration of DF acyl glucuronide to pentobarbitone-anaesthetized rats, DF and its phenolic glucuronide and sulphate conjugates appeared rapidly in plasma, indicating ready systemic hydrolysis of the acyl glucuronide and subsequent biotransformation of liberated DF. 3. Approximately 72% of the acyl glucuronide dose was recovered in bile and urine over 6 h: 52% as acyl glucuronide, 6% as phenolic glucuronide, 5% as sulphate, and 8% as isomers of the acyl glucuronide arising from intramolecular acyl migration. 4. Blockage of excretion routes by ligation of the ureters, bile duct, and both ureters and bile duct, decreased plasma clearance of the acyl glucuronide from 7.8 ml/min per kg to 6.0, 3.2 and 2.2 ml/min per kg respectively, and increased the apparent terminal plasma half-life of DF from 2.1 h to 2.6, 3.4 and 6.3 h, respectively. 5. By contrast, DF phenolic glucuronide was quite stable after i.v. administration at the same dose. 6. This study shows that systemic cycling between DF and its acyl glucuronide exists in the rat in vivo, with portions of each cycle of unstable acyl glucuronide through DF yielding stable phenolic glucuronide and (presumptively stable) sulphate conjugate.

Beta-oxidation of valproic acid. I. Effects of fasting and glucose in humans

The effect of fasting and glucose-infusion on valproate (VPA) disposition was investigated to determine the involvement of endogenous fatty acid (FA) beta-oxidation in the metabolism of VPA. Fifteen healthy volunteers received multiple oral doses of VPA to achieve steady state under both conditions. Depressed plasma FA concentrations in the glucose-infused state (median 51%, p less than 0.0001) were associated with lower unbound plasma VPA fractions (median 17%, p less than 0.0001). Unbound plasma VPA concentrations were notably lower in the glucose-infused state due to significantly higher (median 41%, p less than 0.0001) metabolic clearance, beta-oxidative metabolite formation clearance, representing the largest urinary dose fraction recovered, was significantly higher (median 60%, p less than 0.004) in the glucose-infused state. This finding is consistent with competition between endogenous FA and VPA for the enzymes of beta-oxidation modulated by conditions which affect FA mobilization to the site of catabolism.

Valproate-associated hepatotoxicity and its biochemical mechanisms

Intake of the anticonvulsant drug valproic acid, or its sodium salt, has been associated with occasional instances of severe and sometimes fatal hepatotoxicity. Probably at least 80 cases have occurred worldwide. The syndrome affects perhaps 1 in 10,000 persons taking the drug, and usually develops in the early weeks or months of therapy. Most instances have involved children, usually those receiving more than 1 anticonvulsant. Multiple cases have occurred in 2 families. The typical presentation is of worsening epilepsy, increasing depression of consciousness, and progressive clinical and biochemical evidence of liver failure. The liver has sometimes shown hepatocyte necrosis, and on other occasions widespread microvesicular steatosis, while cholestatic changes have also occurred. The appearances are interpreted as consistent with a drug toxicity reaction. During the hepatotoxicity increased amounts of unsaturated metabolites of valproate, notably 4-en-valproate, have been found in blood and urine. In 4 cases there has been evidence of impaired beta-oxidation of valproate with, in 1 case, accumulation of isomers of valproate glucuronide caused by intramolecular rearrangement of the conjugate. There are molecular structural similarities between 4-en-valproate and 2 known hepatotoxins (4-en-pentanoate and methylenecyclopropylacetic acid, the latter being responsible for hypoglycin poisoning). There are also clinical and histopathological similarities between valproate hepatotoxicity and both hypoglycin poisoning and certain spontaneous disorders of isoleucine metabolism (one pathway of valproate metabolism is analogous to oxidative degradation of isoleucine). Unsaturated metabolites of valproate, in particular 4-en-valproate, may contribute to the hepatotoxicity of the drug. However, since the hepatotoxicity appears to involve an element of idiosyncrasy, the primary defect in some cases may be an inherited or acquired deficiency in the drug's beta-oxidation. This defect may divert valproate metabolism towards omega-oxidation, with increased formation of the toxin 4-en-valproate, but may also allow increased formation of a toxic metabolite derived from isoleucine, since beta-oxidation of isoleucine derivatives will also be impaired.