Acute intermittent porphyria : studies of chronic disease, recombinant enzyme therapy and late complications

Sammanfattning: This thesis comprise five studies on patients with acute intermittent porphyria (AIP) observed under different clinical conditions associated with the disorder. They have generally been monitored at the porphyria out-patient clinic in Stockholm South Hospital, and were biochemically monitored at Porphyria Centre Sweden. Acute intermittent porphyria is an autosomal dominant metabolic disorder with comparatively high prevalence in Sweden; 1:10 000. The disease is caused by a partial deficiency of porphobilinogen deaminase (PBGD), the third enzyme in heme biosynthesis. The clinical penetrance of the underlying gene mutation is relatively limited and characterized by attacks of painful and potentially life threatening symptoms, mainly from the nervous system, and accompanied by accumulation and excretion of the porphyrin precursors porphobilinogen (PBG) and the possibly neurotoxic 5-aminolevulinic acid (ALA). Estimation of the current extent of porphyrin precursor excretion is presently the only tool for assessment of an acute attack of porphyria in the patient with AIP. In this process, precise knowledge of the physiological and pathophysiological fluctuations of these markers is essential for the accurate evaluation of the clinical situation at hand. Beside the few recently reported liver transplantations, there is presently no available way to bring about permanent cure for the metabolic error in AIP. In wait for gene therapy, the obvious measure would be to compensate for the underlying enzyme deficiency by supplementation via administration of the deficient enzyme. In success, would not only acute attacks be inhibited, the lethal late manifestations from liver and kidneys would also be blocked. Our studies thus approach questions concerning the kinetics of PBG and ALA in AIP monitored also under conditions secondary to renal engagement and hepatic tumour. In order to be able to measure the low plasma concentrations of these metabolites, we developed a novel HPLC-MS procedure. One study defines the pharmacokinetic and pharmacodynamic variables of recombinant human PBGD administered in a clinical trial aimed at evaluation of the clinical efficacy of enzyme therapy. In the studies of late complications more than fifty patient case-books were surveyed with regard to beforehand selected variables connected with the development of renal engagement and primary liver cancer. Only patients with biochemical signs of active disease are studied, twenty four currently asymptomatic individuals with permanently increased urinary PBG and ALA being included in either study I or II. Additional three patients with four current attacks participate in study III, and in study IV further three patients burdened by recurrent attacks, chronic hypertension and end-stage renal disease take part. In study V twenty AIP patients with primary liver cancer are included. In study I we established the concentration patterns of PBG and ALA in plasma and urine, monitored during eight hours in ten currently asymptomatic AIP gene carriers. Within each individual, the pattern shows to be constant and there is strong correlation between these two metabolites in plasma and urine. Their renal clearances were in both cases about 70 mL/min. This study formed the basis for the studies II, III and IV. In studies I, II and III are observed significantly higher morning, than evening plasma concentrations of PBG, pointing to a circadian variation not previously reported, and not found for ALA. In Study II the safety, pharmacokinetics and pharmacodynamics of human recombinant PBGD used in substitution therapy of AIP, was studied. This work, done in collaboration with the company Zymenex A/S, aimed at repairing the PBGD deficient step in the heme biosynthetic pathway. The drug proved to be safe, and the pharmacokinetics and pharmacodynamics of the enzyme were elucidated. The drug effectively removed plasma PBG but showed no effect on the presumably neurotoxic ALA. In Study III we give evidence that plasma PBG is a more sensitive biomarker for the acute attack of porphyria, than plasma ALA, or urinary PBG and ALA. In Study IV is demonstrated that before hemodialysis PBG accumulates strongly in plasma, but is readily filtered by the hemodialysis membrane. The accumulation of PBG is less evident in the patient on peritoneal dialysis. Accumulated plasma porphyrins are not cleared by either of the dialysis procedures, and both patients studied developed skin lesions. The patient in predialysis showed only a mildly increased accumulation of porphyrins. Study V is a compilation of the clinical and biochemical presentations, histopathological characteristics and therapeutic outcomes of twenty patients with AIP that had developed primary liver cancer. The tumour was earlier recognized in an annual surveillance program, than via other measures, and all patients were detected after the age of 50 years and had increased porphyrin precursor excretion. In AIP patients with, as well as without symptoms in studies I, II and III, the concentrations of PBG in plasma and urine are about twice of those of ALA. This was not observed in study IV, where the patients had developed end stage renal failure. To conclude, through the thorough clinical and biochemical revisions of the patient materials of the study a greatly deepened insight is gained in the natural history of the disease caused by mutations in the gene for porphobilinogen deaminase, i.e. acute intermittent porphyria.