Ectopic Ocular Surface Calcification in Patients With Hypophosphatasia Treated With Asfotase Alfa

Optic disc drusen: Dystrophic calcification, a potential target for treatment

Optic disc drusen (ODD) are calcified, acellular bodies, seen in the optic nerve head of up to 2% of the population. Although seldomly affecting visual acuity, visual field defects are common, and severe, ischemic complications causing irreversible vision loss are known to occur. Different treatment strategies for ODD have been explored, but so far without success. This review focuses on the unique, calcified property of ODD, describing what we know about ODD pathogenesis and previously tried treatment strategies. In this context, we discuss current knowledge about calcium and pathological calcifications, including intracranial and ocular calcifications. We also explore some of the obstacles that must be addressed to develop a therapy centred on the concept of calcification, should calcification be identified as a pathogenic factor contributing to vision loss.

Gene Therapy Using Adeno-Associated Virus Serotype 8 Encoding TNAP-D10 Improves the Skeletal and Dentoalveolar Phenotypes in Alpl-/- Mice

Hypophosphatasia (HPP) is caused by loss-of-function mutations in the ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP), whose deficiency results in the accumulation of extracellular inorganic pyrophosphate (PP_i ), a potent mineralization inhibitor. Skeletal and dental hypomineralization characterizes HPP, with disease severity varying from life-threatening perinatal or infantile forms to milder forms that manifest in adulthood or only affect the dentition. Enzyme replacement therapy (ERT) using mineral-targeted recombinant TNAP (Strensiq/asfotase alfa) markedly improves the life span, skeletal phenotype, motor function, and quality of life of patients with HPP, though limitations of ERT include frequent injections due to a short elimination half-life of 2.28 days and injection site reactions. We tested the efficacy of a single intramuscular administration of adeno-associated virus 8 (AAV8) encoding TNAP-D₁₀ to increase the life span and improve the skeletal and dentoalveolar phenotypes in TNAP knockout (Alpl^-/- ) mice, a murine model for severe infantile HPP. Alpl^-/- mice received 3 × 10¹¹ vector genomes/body of AAV8-TNAP-D₁₀ within 5 days postnatal (dpn). AAV8-TNAP-D₁₀ elevated serum ALP activity and suppressed plasma PP_i . Treatment extended life span of Alpl^-/- mice, and no ectopic calcifications were observed in the kidneys, aorta, coronary arteries, or brain in the 70 dpn observational window. Treated Alpl^-/- mice did not show signs of rickets, including bowing of long bones, enlargement of epiphyses, or fractures. Bone microstructure of treated Alpl^-/- mice was similar to wild type, with a few persistent small cortical and trabecular defects. Histology showed no measurable osteoid accumulation but reduced bone volume fraction in treated Alpl^-/- mice versus controls. Treated Alpl^-/- mice featured normal molar and incisor dentoalveolar tissues, with the exceptions of slightly reduced molar enamel and alveolar bone density. Histology showed the presence of cementum and normal periodontal ligament attachment. These results support gene therapy as a promising alternative to ERT for the treatment of HPP. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Pharmacodynamics of asfotase alfa in adults with pediatric-onset hypophosphatasia

BACKGROUND

Hypophosphatasia (HPP) is the rare, inherited, metabolic bone disease characterized by low activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) leading to excess extracellular inorganic pyrophosphate (PPi) and pyridoxal 5'-phosphate (PLP). Asfotase alfa is the human recombinant enzyme-replacement therapy that replaces deficient TNSALP. However, there is limited information concerning the appropriate dose of asfotase alfa for adult patients with pediatric-onset HPP. Thus, we evaluated the pharmacodynamics and safety/tolerability of different doses of asfotase alfa in such patients.

METHODS

This 13-week, Phase 2a, open-label study enrolled adults (aged ≥18 years) with pediatric-onset HPP. They were randomized 1:1:1 to receive a single subcutaneous dose of asfotase alfa (0.5, 2.0, or 3.0 mg/kg) at Week 1, then 3 times per week (ie, 1.5, 6.0, or 9.0 mg/kg/wk) starting at Week 3 for 7 weeks. Key outcome measures included change from Baseline to before the third dose during Week 9 (trough) in plasma PPi (primary outcome measure) and PLP (secondary outcome measure).

RESULTS

Twenty-seven adults received asfotase alfa 0.5 (n = 8), 2.0 (n = 10), and 3.0 (n = 9) mg/kg; all completed the study. Median (range) age was 45 (18-77) years; most patients were white (96%) and female (59%). Median plasma PPi and PLP concentrations decreased from Baseline to Week 9 in all 3 cohorts. Differences in least squares mean (LSM) changes in PPi were significant with 2.0 mg/kg (p = 0.0008) and 3.0 mg/kg (p < 0.0001) vs. 0.5 mg/kg. Differences in LSM changes in PLP were also significant for 2.0 mg/kg (p = 0.0239) and 3.0 mg/kg (p = 0.0128) vs. 0.5 mg/kg. Injection site reactions were the most frequent treatment-emergent adverse event (78%), showing increasing frequency with increasing dose.

CONCLUSIONS

Adults with pediatric-onset HPP receiving asfotase alfa at 6.0 mg/kg/wk (the recommended dose) or 9.0 mg/kg/wk had greater reductions in circulating PPi and PLP concentrations compared with a lower dose of 1.5 mg/kg/wk.

TRIAL REGISTRATION

Clinicaltrials.gov identifier NCT02797821.

Tissue-Nonspecific Alkaline Phosphatase-A Gatekeeper of Physiological Conditions in Health and a Modulator of Biological Environments in Disease

Tissue-nonspecific alkaline phosphatase (TNAP) is a ubiquitously expressed enzyme that is best known for its role during mineralization processes in bones and skeleton. The enzyme metabolizes phosphate compounds like inorganic pyrophosphate and pyridoxal-5′-phosphate to provide, among others, inorganic phosphate for the mineralization and transportable vitamin B6 molecules. Patients with inherited loss of function mutations in the ALPL gene and consequently altered TNAP activity are suffering from the rare metabolic disease hypophosphatasia (HPP). This systemic disease is mainly characterized by impaired bone and dental mineralization but may also be accompanied by neurological symptoms, like anxiety disorders, seizures, and depression. HPP characteristically affects all ages and shows a wide range of clinical symptoms and disease severity, which results in the classification into different clinical subtypes. This review describes the molecular function of TNAP during the mineralization of bones and teeth, further discusses the current knowledge on the enzyme’s role in the nervous system and in sensory perception. An additional focus is set on the molecular role of TNAP in health and on functional observations reported in common laboratory vertebrate disease models, like rodents and zebrafish.

Hypophosphatasia: Biological and Clinical Aspects, Avenues for Therapy

Hypophosphatasia (HPP) is a rare inherited systemic metabolic disease caused by mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. TNSALP is expressed in the liver, kidney and bone, and its substrates include TNSALP inorganic pyrophosphate, pyridoxal-5'-phosphate (PLP)/vitamin B6 and phosphoethanolamine (PEA). Autosomal recessive and dominant forms of the disease result in a range of clinical entities. Major hallmarks are low alkaline phosphatase (ALP) and elevated PLP and PEA levels. Very severe infantile forms of HPP cause premature death as a result of respiratory insufficiency and also present with hypo-mineralisation leading to deformed limbs with, in some cases, the near-absence of bones and skull altogether. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency are indicative of a poor prognosis. Craniosynostosis is frequent. HPP leads to an unusual presentation of rickets with high levels of calcium and phosphorus, resulting in hypercalciuria, nephrocalcinosis and low ALP levels. Hypercalcaemic crisis, failure to thrive and growth retardation are concerns in infants. Fractures are common in both infantile and adult forms of the disease, concomitantly occurring with unexplained chronic pain and fatigue. Dental clinical presentations, which include the premature loss of teeth, are also commonly found in HPP and specifically manifest as odontohypophosphatasia. A novel enzyme therapy for human HPP, asfotase alfa, which is specifically targeted to mineralised tissues, has been developed in the past decades. While this treatment seems very promising, especially for infantile HPP, many questions regarding its long-term effects, the management of treatment, and any potential secondary adverse effects remain unresolved.