Antipyrine metabolism in acute hepatic porphyria in relapse and remission

Gas chromatography-mass spectrometry profiling of steroids in urine of patients with acute intermittent porphyria

OBJECTIVE

Acute intermittent porphyria (AIP) is an autosomal dominant disease that results from a deficiency of hydroxymethylbilane synthase, the third enzyme of the heme biosynthetic pathway. AIP carriers may present acute neurovisceral attacks with hepatic overproduction of heme-precursors. In some patients, remission of the acute symptoms leads to long-term hepatic metabolic abnormalities. In this study, gas chromatography-mass spectrometry (GC/MS) was used to investigate urinary steroid metabolome of AIP patients.

DESIGN AND METHODS

Steroid profiling in urine was performed in a group of AIP patients with biochemically active disease (n=22) and healthy controls (n = 20). Five asymptomatic AIP family carriers were also studied. Commonly used ratios for the evaluation of disturbances in the steroid metabolism were calculated.

RESULTS

We found that etiocholanolone/androsterone and tetrahydrocortisol/5α-tetrahydrocortisol (THF/5α-THF) metabolic ratios were significantly increased in the urine of AIP patients compared to controls (2.3 ± 0.3 vs 0.8 ± 0.1; p < 0.001 and 2.9 ± 0.7 vs 0.9 ± 0.1; p < 0.01). The (THF+5α-THF)/tetrahydrocortisone ratio was reduced among the AIP patients (p < 0.01). Quantification of the steroid absolute concentrations showed that these variations were due to a decrease of the 5α metabolites. Other ratios, like cortisol/cortisone and 6β-hydroxycortisol/cortisol in the free steroid fraction did not show differences between patients and controls. All ratios were normal among the family carriers.

CONCLUSION

A significant number of AIP patients present a basal decrease of steroid 5α-reductase activity in the liver. The deficiency may be related to malnutrition and hepatic energy misbalance associated with active AIP. Urinary steroid profiling by GC/MS may be a valuable tool to assess hepatic metabolome in AIP.

Clinical and biochemical characteristics and genotype-phenotype correlation in Finnish variegate porphyria patients

Variegate porphyria (VP) is an inherited metabolic disease resulting from the partial deficiency of protoporphyrinogen oxidase, the penultimate enzyme in the heme biosynthetic pathway. We have evaluated the clinical and biochemical outcome of 103 Finnish VP patients diagnosed between 1966 and 2001. Fifty-two per cent of patients had experienced clinical symptoms: 40% had photosensitivity, 27% acute attacks and 14% both manifestations. The proportion of patients with acute attacks has decreased dramatically from 38 to 14% in patients diagnosed before and after 1980, whereas the prevalence of skin symptoms had decreased only subtly from 45 to 34%. We have studied the correlation between PPOX genotype and clinical outcome of 90 patients with the three most common Finnish mutations I12T, R152C and 338G-->C. The patients with the I12T mutation experienced no photosensitivity and acute attacks were rare (8%). Therefore, the occurrence of photosensitivity was lower in the I12T group compared to the R152C group (P=0.001), whereas no significant differences between the R152C and 338G-->C groups could be observed. Biochemical abnormalities were significantly milder suggesting a milder form of the disease in patients with the I12T mutation. In all VP patients, normal excretion of protoporphyrin in faeces in adulthood predicted freedom from both skin symptoms and acute attacks. The most valuable test predicting an increased risk of symptoms was urinary coproporphyrin, but only a substantially increased excretion exceeding 1,000 nmol/day was associated with an increased risk of both skin symptoms and acute attacks. All patients with an excretion of more than 1,000 nmol/day experienced either skin symptoms, acute attacks, or both.

Limited heme synthesis in porphobilinogen deaminase-deficient mice impairs transcriptional activation of specific cytochrome P450 genes by phenobarbital

Heme is not only a very important prosthetic group that modulates the structure and activity of heme proteins but also a regulatory molecule that controls metabolic pathways and the biosynthesis of various proteins. However, investigation into heme regulatory effects in higher vertebrates has been hampered by the lack of a suitable animal model. A knockout mouse with targeted disruption of porphobilinogen deaminase, the third enzyme of the heme pathway, has been generated in our laboratory and used in the present study as an in vivo model of heme deficiency to explore diverse heme regulatory properties. In this model with a defined heme disturbance, we observed a superinductive response of delta-aminolevulinate synthase, the first enzyme in heme synthesis, after phenobarbital treatment. We also found that limited heme is associated with decreased induction of cytochrome P450 by phenobarbital as a consequence of impaired gene transcription. This inhibitory effect is isoenzyme-specific, being significant for cyp2a5. The activity and mRNA level of this particular cytochrome P450 are significantly lower in the phenobarbital-induced porphobilinogen deaminase-deficient mice (55% and 43%, respectively), but its expression can be restored to normal values when exogenous heme is administered. Other heme proteins, namely neuronal nitric oxide synthase and soluble guanylate cyclase, function normally in mice with limited heme. Our results demonstrate that the expression of various heme proteins is differentially regulated in conditions of reduced heme availability. Moreover, our findings emphasize the importance of heme protein function in the genesis of pathophysiological manifestations in acute intermittent porphyria.

Selective effect of liver disease on the activities of specific metabolizing enzymes: investigation of cytochromes P450 2C19 and 2D6

BACKGROUND AND OBJECTIVES

Drug metabolism is influenced by liver disease because of the central role that the liver plays in metabolic activities in the body. However, it is still unclear how activities of specific drug-metabolizing enzymes are influenced by the presence and severity of liver disease. As a consequence, alteration in metabolism of specific drugs cannot be easily predicted or appropriate dosage adjustment recommendations made.

METHODS

The activities of cytochromes P450 (CYP) 2C19 and 2D6 were investigated in a group of patients with mild or moderate liver disease (n = 18) and a group of healthy control subjects (n = 10). The disposition of racemic mephenytoin for CYP2C19 and debrisoquin for CYP2D6 were characterized in plasma and urine samples collected over 192 hours.

RESULTS

The elimination of S-mephenytoin was severely reduced among patients with liver disease, resulting in a 79% decrease in plasma clearance for all patients combined. This reduction was related to the severity of disease, patients with moderate disease being affected more severely than patients with mild disease. Similar differences were observed in the urinary excretion of 4'-hydroxymephenytoin metabolite. By contrast, there was no effect on the disposition of R-mephenytoin or debrisoquin.

CONCLUSION

These results show selectivity in the effect of liver disease on activities of specific metabolizing enzymes, CYP2C19 being more sensitive than CYP2D6. They suggest that recommendations for modification in drug dosage in the presence of liver disease should be based on knowledge of the particular enzyme involved in metabolism of the drug. The results emphasize the need for further studies of each specific drug-metabolizing enzyme in the presence of liver disease.

Clinical pharmacokinetics in patients with liver disease

From considerations of hepatic physiology and pathology coupled with pharmacokinetic principles, it appears that altered drug elimination in liver disease may result from the following mechanisms: reduction in absolute cell mass, in cellular enzyme content and/or activity, in portal vein perfusion due to extrahepatic/intrahepatic shunting, or of portal perfusion of hepatocyte mass due to decreased portal flow or sinusoidal perfusion; increase in arterial perfusion relative to portal perfusion; preferential perfusion of the sinusoidal midzone and terminal zones by arterioles; potential for direct mixing of arterial blood within the space of Disse; reduced exchange across the endothelial lining; and impaired diffusion within the space of Disse. In general, oxidative drug metabolism is impaired in liver disease and the degree of impairment of oxidisation differs between drugs but correlates best with the degree of sinusoidal capillarisation, i.e. the degree of access of the drug from the sinusoid to the hepatocyte. Drug conjugation appears to be relatively unaffected by liver disease, whereas elimination by biliary excretion correlates best with the degree of intrahepatic shunting and not with sinusoidal capillarisation. As the latter should impair hepatocyte access of all compounds similarly, a potentially important mechanism could be impaired access of oxygen to hepatocytes as oxidative metabolism is much more sensitive to oxygen supply than are conjugation or biliary excretion. This suggests a potentially important therapeutic role for agents which increase the hepatic oxygen supply. Useful adjunctive strategies may also derive from the oxygen limitation hypothesis. Anaemia should be targeted as a critically important variable, as should oxygen-carrying capacity, i.e. modification of the smoking habit. Additionally, enzyme inducers such as barbiturates may be used if overriding hypoxic constraints are removed by oxygen supplementation. Agents likely to seriously compromise arterial perfusion of the hepatic vascular bed should be avoided, e.g. those causing postural hypotension or vasospasm. Vasodilators can be used to actively promote arterial perfusion. While the effect of liver disease on drug handling is highly variable and difficult to predict, there are well recognised principles for modifying dosage. These include halving the dose of drugs given systemically (or of low clearance drugs given orally) and a 50 to 90% reduction in the dose of drugs with a high hepatic clearance given orally. Changes in the pharmacodynamic effects of drugs (either alone or in addition to pharmacokinetic changes) can also be profound, and awareness of this possibility should be increased.

Therapy of the acute porphyrias

In the management of acute porphyria it is essential to be aware of the potential for many drugs to induce porphyrin synthesis and thus precipitate the acute porphyric crisis. In this review, lists of drugs unsafe and safe for use in the porphyrias are presented. In addition, therapeutic regimens are described which are appropriate to the porphyric subject. These include the use of high carbohydrate intake and the intravenous infusion of haem arginate.

Haem arginate improves hepatic oxidative metabolism in variegate porphyria

1. The elimination of antipyrine was investigated before and after intravenous administration of haem arginate (3 mg haem kg-1 day-1 on three or four successive days) to six patients with variegate porphyria in remission. 2. Haem arginate decreased the faecal content of protoporphyrin from 557 +/- 91 to 118 +/- 32 (mean +/- s.e. mean) and of coproporphyrin from 144 +/- 19 to 19 +/- 3 nmol g-1 dry weight. 3. Before haem treatment antipyrine elimination half-life was long (30.5 +/- 5.6 h), but the treatment decreased it to 6.3 +/- 0.8 h. Antipyrine clearance increased from 0.25 +/- 0.05 to 1.03 +/- 0.11 ml min-1 kg-1 (P less than 0.001), being 4.6 times higher after haem arginate infusions. 4. The volume of distribution of antipyrine did not change. 5. The severe impairment of hepatic mixed function oxidase activity even in the symptomless stage of porphyria indicates cautious dosage of drugs primarily eliminated by hepatic oxidative reactions.

Influences of diet and nutrition on clinical pharmacokinetics

The human diet represents a complex and variable mixture of nutrients, many of which have the potential for altering the disposition of drugs. This review highlights progress from a number of laboratories illustrating nutrient influences on drug dispositions and actions. Emphasis is placed on nutritional effects on hepatic drug metabolism studied in humans. Data from animals have sometimes been difficult to extrapolate to humans, as illustrated by reports on the influences of starvation, dietary lipids, and ascorbic acid. From studies in healthy subjects it is now clear that a number of specific dietary factors can influence drug metabolism by the mixed function oxidase system and conjugating enzymes. These include dietary protein, cruciferous vegetables, charcoal-broiled beef containing polycyclic aromatic hydrocarbons, and methylxanthines. The effects of such dietary components have been demonstrated for only a limited number of drug substrates for these enzyme systems. Effects of food on bioavailability have been more widely studied, and depend greatly upon the type of drug. Malnutrition can be associated with variable but potentially important effects on the bioavailability, binding, hepatic metabolism, and renal clearance of drugs. In malnourished patients it is generally difficult to recognise the roles of individual nutrient deficiencies on drug disposition, and clinical predictors of altered pharmacokinetics for various drugs in such patients are not well defined. It is likely that many important interrelationships between nutrition and new or already marketed drugs remain to be recognised, and therefore warrant further research. Nutritional effects on drug metabolising enzymes also have implications for endogenous substances such as hormones and environmental toxins and carcinogens which are metabolised by the same or related enzyme systems, and for diseases likely to be related to the actions of such chemicals.

Effect of haem arginate therapy on porphyrin metabolism and mixed function oxygenase activity in acute hepatic porphyria

The effect of haem arginate on porphyrin metabolism and haemoprotein function was studied during seven attacks of acute hepatic porphyria in 5 patients. In each attack it greatly reduced the overproduction of porphyrin precursors and repressed the overactivity of the rate-controlling enzyme of haem synthesis delta-aminolaevulinic acid (ALA) synthase measured in leucocytes. Antipyrine clearance, an index of the oxidative function of cytochromes P-450, the major group of hepatic haemoproteins, was increased during haem therapy. Thus, haem arginate not only suppresses the overproduction of haem precursors but also improves hepatic oxidative metabolism in acute porphyria.