Safety, Pharmacokinetics and Pharmocodynamics of Recombinant Human Porphobilinogen Deaminase in Healthy Subjects and Asymptomatic Carriers of the Acute Intermittent Porphyria Gene Who Have Increased Porphyrin Precursor Excretion

Acute intermittent porphyria is an autosomal dominant disorder caused by deficient activity of the third enzyme in the haem biosynthetic pathway, porphobilinogen deaminase. It is characterised by acute, potentially life-threatening neurological attacks that are precipitated by various drugs, reproductive hormones and other factors. During acute attacks, the porphyrin precursors 5-aminolevulinic acid and porphobilinogen accumulate and are excreted at high concentrations in the urine. Current treatment is based on glucose loading and parenteral haem replenishment, which reduce the accumulation of 5-aminolevulinic acid and porphobilinogen. Recently, a new form of treatment based on porphobilinogen deaminase enzyme replacement therapy has been shown to be effective in an acute intermittent porphyria mouse model which, during phenobarbital (phenobarbitone) induction of haem biosynthesis, mimics the biochemical pattern of acute porphyric attacks. The objective of the present study was to investigate the safety, pharmacokinetics and pharmacodynamics of recombinant human porphobilinogen deaminase (P 9808), administered to healthy subjects and asymptomatic porphobilinogen deaminase-deficient subjects with high concentrations of porphobilinogen, the substrate of porphobilinogen deaminase.

Forty individuals participated in this two-part study: 20 asymptomatic porphobilinogen deaminase-deficient subjects (both male and female) with ≥4 times the upper reference urinary porphobilinogen level, and 20 healthy male subjects. Four different doses of recombinant human porphobilinogen deaminase were studied (0.5, 1, 2 and 4 mg/kg bodyweight). Part A included 12 asymptomatic porphobilinogen deaminase-deficient subjects, and the enzyme was administered in an open-label, single-dose design. Part B included 20 asymptomatic porphobilinogen deaminase-deficient subjects and 20 healthy subjects. The same enzyme dosages were administered as divided doses every 12 hours for 4 consecutive days in a randomised, double-blinded, placebo-controlled design. The washout period between Parts A and B was 2 weeks.

The concentrations of recombinant human porphobilinogen deaminase and titres of antibodies against recombinant human porphobilinogen deaminase were analysed by ELISA. Plasma porphobilinogen and 5-aminolevulinic acid concentrations were analysed using a novel liquid chromatography-tandem mass spectrometry method. Urinary porphobilinogen, 5-aminolevulinic acid and porphyrin concentrations, as well as plasma porphyrin concentrations, were analysed using standard methods. The pharmacodynamic effect of the enzyme was studied through changes in plasma porphobilinogen concentrations.

No serious adverse events were observed. Seven subjects (four healthy men and three asymptomatic porphobilinogen deaminase-deficient subjects) developed antibodies against recombinant human porphobilinogen deaminase but did not experience allergic manifestations. The mean elimination half-lives of the highest doses of recombinant human porphobilinogen deaminase ranged between 1.7 and 2.5 hours for both healthy men and asymptomatic porphobilinogen deaminase-deficient subjects. The area under the plasma concentration-time curve was proportional to the respective dose. In asymptomatic porphobilinogen deaminase-deficient subjects, plasma porphobilinogen concentrations decreased below measurable levels almost instantaneously after administration of any dose of the enzyme. The effect lasted for approximately 2 hours, after which the plasma porphobilinogen concentration slowly increased, reaching about 70% of the initial values 12 hours after administration. There was no effect on plasma 5-aminolevulinic acid concentrations, and there was a transitory increment in porphyrin concentrations. The corresponding concentrations of metabolites in the urine reflected the pattern observed in the plasma.

The recombinant human porphobilinogen deaminase enzyme preparation was found to be safe to administer and effective for removal of the accumulated metabolite porphobilinogen from plasma and urine. The pharmacokinetic profile of recombinant human porphobilinogen deaminase showed dose proportionality, and the elimination half-life was about 2.0 hours for the two highest doses. Thus, clinical grounds were established for investigation of the therapeutic efficacy of the enzyme during periods of overt disease in patients with acute intermittent porphyria.