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

Therapy Follows Diagnosis: Old and New Approaches for the Treatment of Acute Porphyrias, What We Know and What We Should Know

Heme, iron protoporphyrin IX, is one of life’s most central molecules. Hence, availability of the enzymatic machinery necessary for its synthesis is crucial for every cell. Consequently, inborn errors of porphyrin metabolism that compromise normal synthesis, namely the family of porphyrias, undermine normal cellular metabolism given that heme has functions in catalytic centers, signal transduction and functional regulation and its synthesis is fully integrated into the center of intermediary metabolism. Very often, diagnosis of porphyrias is difficult and therefore delayed. Therapy can be as complicated. Over the last 50 years, several strategies have been developed: because of its integration with other parts of intermediary metabolism, the infusion of glucose (glucose effect) was one of the first attempts to counterbalance the dysregulation of porphyrin synthesis in porphyrias. Since heme synthesis is impaired, infusional replacement of heme was the next important therapeutic step. Recently, siRNA technology has been introduced in order to downregulate 5-ALA-synthase 1, which contributes to the patho-physiology of these diseases. Moreover, other novel therapies using enzyme protein replacement, mRNA techniques or proteostasis regulators are being developed.

A challenging diagnosis for potential fatal diseases: recommendations for diagnosing acute porphyrias

Acute porphyrias are a heterogeneous group of metabolic disorders resulting from a variable catalytic defect of four enzymes out of the eight involved in the haem biosynthesis pathway; they are rare and mostly inherited diseases, but in some circumstances, the metabolic disturbance may be acquired. Many different environmental factors or pathological conditions (such as drugs, calorie restriction, hormones, infections, or alcohol abuse) often play a key role in triggering the clinical exacerbation (acute porphyric attack) of these diseases that may often mimic many other more common acute medical and neuropsychiatric conditions and whose delayed diagnosis and treatment may be fatal. In order to obtain an accurate diagnosis of acute porphyria, the knowledge and the use of appropriate diagnostic tools are mandatory, even in order to provide as soon as possible the more effective treatment and to prevent the use of potentially unsafe drugs, which can severely precipitate these diseases, especially in the presence of life-threatening symptoms. In this paper, we provide some recommendations for the diagnostic steps of acute porphyrias by reviewing literature and referring to clinical experience of the board members of the Gruppo Italiano Porfiria (GrIP).

Liver metabolism of porphyrins and haem

The liver is an active site for the biosynthesis of haem and porphyrinogens/porphyrins, which are intermediates of the haem biosynthetic pathway, because haem is required for functional activity of the cytochrome P 450 system and other critical hepatic haemoproteins. The production of hepatic haem is regulated primarily through the activity of aminolaevulinic acid synthase which is the first and normally rate-limiting enzyme of the pathway. This is, in turn, controlled by a putative regulatory haem pool. Hepatic haem can be repleted by the intravenous administration of haem, which is the basis for haem therapy in patients with acute porphyric attacks. The liver catabolizes haem to bilirubin through microsomal haem oxygenase activity and excretes haem into bile along with porphyrins. Biliary excretion of porphyrins increases significantly in patients with some types of porphyria. In protoporphyria this may cause liver damage as a result of protoporphyrin toxicity. The delineation of the pathway for protoporphyrin excretion into bile should facilitate therapy in protoporphyria by identifying ways in which protoporphyrin excretion can be enhanced.

Elevation of blood lactate and pyruvate levels in acute intermittent porphyria--a reflection of haem deficiency?

Blood lactate concentrations after glucose loading were significantly higher in 6 patients with acute intermittent porphyria in clinical remission than in 6 control subjects and the percentage rise in glucose pyruvate and lactate concentrations were greater in the porphyric subjects than in the control group. It is postulated that the raised lactate levels in the porphyric patient group may reflect haem deficiency affecting the cytochromes of the terminal respiratory chain.

Elevation of hormone-binding globulins in acute intermittent porphyria

Sex hormone-binding globulin (SHBG), thyroxine-binding globulin (TBG) and cortisol-binding globulin (CBG) were measured in plasma of 26 patients with acute intermittent porphyria (AIP). Twelve patients had clinically manifest disease and all had elevated SHBG levels. All but one of 14 patients with latent porphyria had normal SHBG levels. TBG was elevated in 9 of the patients with clinically manifest porphyria and CBG elevated in three. Levels of TBG and CBG were either normal or only slightly elevated in those with latent porphyria. In a prospective study of 30 attacks of AIP in 7 patients, SHBG levels fell between admission and discharge, the fall being significant in the group of 21 attacks treated with haem arginate (p less than 0.001). Our findings suggest that a close correlation exists between elevated SHBG and clinical expression of AIP.

[Anesthesia and hepatic porphyria]

Three of the acute hepatic porphyrias, acute intermittent porphyria, variegata porphyria and hereditary coproporphyria, are characterized by an idiosyncratic reaction to many common drugs; the resulting excessive excretion of porphyrin precursors is responsible for episodes of acute neurological dysfunction. This review aimed to focus the attention of the anaesthesiologist on the porphyrinogenic properties of all the drugs used in anaesthesia and intensive care. An outline of the chemistry of porphyrins and the enzymatic pathways were recalled, so as to place the acute porphyrias in their proper perspective. There follows a reminder of the clinical aspect of acute porphyric crises. The part played by drugs is then assessed from clinical and laboratory data concerning their porphyrinogenicity. Those drugs for which there is conflicting evidence regarding their safe use in porphyric patients are discussed: propofol, ketamine, benzodiazepines, etomidate, local anaesthetics. Recommendations supported by clinical and experimental data are given, especially the results obtained with the chick embryo liver model. Treatment of the acute crisis is provided, with particular emphasis on the use of haematin. The anaesthetic management of hepatic porphyric patients is described. Those drugs which are well-known porphyrinogenic compounds in the chick embryo liver must be excluded from use for anaesthesia in the porphyric patients, even if they have been observed to be innocuous in rare cases of asymptomatic patients. Finally, recommendations for anaesthesia in symptomatic cutanea porphyria are given.

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.