Drugs and the acute porphyrias

Guide to drug porphyrogenicity prediction and drug prescription in the acute porphyrias

AIMS

This paper addresses two common problems in the care of carriers of acute porphyria: the choice of safe drugs for pharmacotherapy and the strategy to apply when potentially unsafe drugs cannot be avoided.

METHODS AND RESULTS

A technique is presented for prediction of risk that a certain drug may activate the disease in a gene carrier for acute porphyria. It is based on a model explaining the clinical manifestations as a result of the acute overloading of a deficient enzyme within the hepatic heme biosynthetic chain. The capacity of the drug for induction of the rate-limiting enzyme in heme biosynthesis, e.g. housekeeping 5-aminolevulinate synthase (ALAS1), is assessed by critical appraisal of reports of the outcomes of clinical use of the drug, and by theoretical criteria. The assessment occurs within the frame of a flow-scheme employing variables of increasing specificity, i.e. endocrine properties of the drug, structure and metabolism pointing to affinity to cytochrome P450, hepatic load in therapeutic use, recognized affinity to major CYP species, capacity for CYP-induction or irreversible inhibition, and capacity to activate or modulate the transduction mechanisms of nuclear receptors affecting ALAS1-gene transcription. It is proposed that in the absence of a safer alternative, an urgently needed drug not should be withheld on the grounds of potential porphyrogenicity. After risk-benefit analysis it should be prescribed, but individualized preventive measures adapted to patient vulnerability may be needed.

CONCLUSIONS

About 1000 therapeutic drugs categorized with regard to porphyrogenicity by the technique proposed are presented on the internet (http://www.drugs-porphyria.org).

Anesthetic considerations in porphyrias

Four hereditary types of porphyria are now classified as acute porphyrias. Enzymatic defects result in accumulation of porphyrin precursors (usually ALA and PGB). The quantity of these precursors may be normal or slightly increased in latent periods but increase to toxic levels during a porphyric crisis. Iatrogenic induction of ALA synthetase by administration of certain triggers (classically barbiturates) is only one of several factors which contribute to porphyric crisis. Signs and symptoms of acute porphyric attack consist primarily of neurologic dysfunction, which occurs secondary to neurotoxicity of ALA or diminished intraneuronal heme levels. Appropriate anesthetic management of porphyria requires knowledge of the type of porphyria (acute vs non-acute), assessment of latent versus active (crisis) phase, awareness of clinical features of porphyric attack, and knowledge of safe pharmacologic intervention.

Occurrence and partition of the beta-carboline norharman in rat organs

The beta-carboline norharman was determined in plasma, brain, liver, kidney, spleen, heart and lung of the rat using HPLC with fluorescence detection. In order to improve the speed and sensitivity of this assay an earlier published sample clean-up extraction procedure and HPLC method were adjusted. Norharman was found to be present in plasma as well as in all organs tested, concentrations in organs being about 80 times higher than those in plasma. Intraperitoneal injections of 2 and 100 mg/kg norharman showed that the partition of norharman between organs and plasma is about 3. Only the highest dose was found to have behavioural effects, viz. alerting reactions, a decrease in motor and exploratory activity, sedation, loss of righting reflex and after 30 min complete muscle relaxation, but no catatonia was observed. Norharman was found to be metabolized by the liver with a half live of about 20 min, whereas all other organs tested did not show any norharman clearing capacity. The results suggest that norharman is not likely the cause of psychosis, but a natural sedative and by- or coproduct of a more primary biochemical derangement.

Studies in laboratory animals to assess the safety of anti-inflammatory agents in acute porphyria

The safety of various anti-inflammatory drugs in acute porphyria was assessed by examining their effect on rat hepatic haem synthesis. Azapropazone, chloroquine, and gold increased delta-aminolaevulinic acid (ALA) synthase activity, indicating that they are liable to precipitate porphyric crises. Aspirin, ibuprofen, indomethacin, ketoprofen, flurbiprofen, phenylbutazone, naproxen, prednisolone, and penicillamine did not increase ALA synthase activity and should be safe in porphyria. Though these animal studies can be used as a guide to prescribing in patients with acute porphyria, some caution is still required as species may vary in their response to inducing agents.

Effects of repeated intravenous administration of haem arginate upon hepatic metabolism of foreign compounds in rats and dogs

Haem arginate is a new haem compound, recently introduced for the treatment of acute hepatic porphyrias. Porphyrias are characterized biochemically by decreased formation of haem due to defects in certain enzyme activities involved in the haem biosynthesis. Haem is essential for cell respiration and oxidative biotransformation. Hepatic drug metabolism, haem biosynthesis and catabolism were investigated after repeated intravenous administration of haem arginate in connection with toxicity studies. The daily doses of haem for rats were 4, 12 and 40 mg kg-1 and for dogs 3 and 9 mg kg-1 for 30 days and for 28 days, respectively. Hepatic microsomes were used in the assay of the following drug metabolizing enzymes: cytochrome P-450 and b5, aminopyrine N-demethylase, ethoxyresorufin O-deethylase and UDP-glucuronyl transferase. The assay of NADPH-cytochrome C-reductase and the enzymes reflecting synthesis and metabolism of haem in the liver (delta-aminolaevulinic acid synthase, delta-aminolaevulinic acid dehydratase, uroporphyrinogen I-synthase, uroporphyrinogen decarboxylase, haem synthase, haem oxygenase and biliverdin reductase) were performed from 20,000 g supernatants. The lowest dose administered to rats and dogs did not cause any significant changes compared to controls in the parameters measured. The highest doses significantly increased the activities of haem oxygenase and uroporphyrinogen I-synthase but decreased concentrations or activities of other enzymes, e.g. cytochrome P-450 and ethoxyresorufin O-deethylase. The results show that it is important to avoid overdosage of haem when restoration of mixed function oxygenase activity is needed.

Barbiturate and ethanol sleeping times and pharmacokinetics of propranolol in mice after intravenous administration of haem arginate

Haem arginate is a new haem compound recently introduced for treatment of porphyrias. Previously haematin has been reported to increase certain hydroxylase activities in extrahepatic tissues, but even in therapeutic doses it impairs the microsomal foreign substance metabolism in the liver. Haem arginate at a dose equivalent to haem 10 mg/kg (threefold therapeutic dose) did not prolong the hexobarbital sleeping time of mice, 20 mg/kg did prolong the hexobarbital and possibly also the ethanol sleeping time. Haem arginate administered in high doses prior to oral propranolol did not alter the bioavailability of the latter. With regard to drug interactions haem arginate may be safer than haematin.

Effect of carbamazepine on haem biosynthesis in man

The effects of the enzyme-inducing anticonvulsant carbamazepine on haem biosynthesis in healthy male subjects is reported. A dose-dependent increase in the activity of leucocyte delta-aminolaevulinic-acid synthase, the rate-limiting enzyme of haem biosynthesis, was noted following 400 and 600 mg carbamazepine daily in the same eight subjects. This rise was maximal after 1 week's treatment (400 mg: 509% +/- 285 of baseline; 600 mg: 1062% +/- 170 of baseline; P less than 0.01). Values fell from this peak during the second week despite continuing carbamazepine administration. This pattern was confirmed in a further six subjects taking carbamazepine 400 mg daily for 3 weeks in whom more frequent enzyme measurements were made. The activity of uroporphyrinogen-1-synthase in the erythrocyte fell by 10-15% during the treatment periods (P less than 0.01). Uroporphyrin and penta-, hexa- and hepta-carboxylic porphyrins appeared in the urine of all subjects during CBZ therapy. Changes in daily urinary porphyrin and precursor excretion were inconsistent. CBZ is a porphyrinogenic drug which mimics the changes in enzyme activities and urinary porphyrin ester profile found in patients with latent acute intermittent porphyria, who have a genetic deficiency in uroporphyrinogen-1-synthase activity. Leucocyte delta-aminolaevulinic-acid synthase may provide a suitable in vivo system for testing the porphyrinogenicity of drugs in man.