Asfotase Alfa (Strensiq)

Asfotase alfa is an enzyme-replacement therapy for hypophosphatasia, an ultra-rare inherited disease in which a missing enzyme lets bone-forming minerals go where they shouldn't and leaves the skeleton unable to harden where it should — in its most severe infant-onset forms, historically fatal in roughly half to all cases from respiratory failure caused by an undermineralized, too-soft rib cage. FDA-approved 23 October 2015 as the first treatment for the disease of any kind, it's since become a real case study in the highest-cost end of rare-disease pricing: an average US list price of roughly $285,000 a year, a UK cost regulator that initially rejected paying for it in adults, and eventual managed-access deals in both countries that got it to patients anyway.

A missing enzyme, and a skeleton that can't harden

Hypophosphatasia (HPP) is a rare inherited metabolic disease caused by loss-of-function mutations in the gene for tissue-nonspecific alkaline phosphatase (TNSALP), an enzyme normally essential for mineralizing bone and teeth. Without enough active TNSALP, a mineralization-blocking molecule called inorganic pyrophosphate accumulates instead of being broken down, leaving bones undermineralized and prone to fracture and deformity. In its most severe perinatal- and infantile-onset forms — symptoms appearing before birth or before six months of age — HPP has historically killed a large share of affected infants, typically from respiratory failure caused by a rib cage too soft and undermineralized to support normal breathing; published natural-history data cited in HPP's regulatory record put perinatal-onset mortality as high as around 100% and infantile-onset mortality around 50% without treatment. Alexion Pharmaceuticals developed asfotase alfa, a recombinant, bone-targeted TNSALP fused to a deca-aspartate peptide tag that concentrates the enzyme at sites of active bone mineralization, and the FDA approved it as Strensiq on 23 October 2015 — the first treatment for hypophosphatasia of any kind, anywhere, after the agency granted it Breakthrough Therapy designation in 2012.

The evidence: a real survival benefit against historical controls

Because perinatal- and infantile-onset HPP is so rare and so often fatal in infancy, Alexion's pivotal evidence couldn't take the form of a placebo-controlled trial withholding treatment from dying infants — instead, two open-label, multinational Phase 2 studies treating 37 infants with asfotase alfa (median treatment duration 2.7 years) were compared against 48 closely matched historical-control infants with the same disease severity who had not received the drug. Treated infants survived significantly longer: 95% of treated infants were alive at age 1, versus 42% of historical controls, and 84% versus 27% at age 5 (P<0.0001 by log-rank test). Among the sickest infants — those who needed invasive ventilator support — 76% (16 of 21) of treated, ventilated infants survived and 75% of those survivors were later weaned off the ventilator, versus just 5% (1 of 20) survival among historical-control infants who needed ventilation (Whyte MP, Rockman-Greenberg C, Ozono K, et al., "Asfotase Alfa Treatment Improves Survival for Perinatal and Infantile Hypophosphatasia," J Clin Endocrinol Metab 2016;101(1):334-342, PMID 26529632, DOI: 10.1210/jc.2015-3462).¹ An earlier, related trial in the same open-label program documented direct radiographic evidence of the drug's mechanism working: independent, blinded radiologist review found 69% (9 of 13) of treated older children (ages 5-12) reached "substantial healing" of their rachitic bone lesions on X-ray after six months, versus 6% (1 of 16) of historical controls (P=0.007), alongside improved growth and respiratory function (Whyte MP, Greenberg CR, Salman NJ, et al., "Enzyme-Replacement Therapy in Life-Threatening Hypophosphatasia," N Engl J Med 2012;366(10):904-913, PMID 22397652, DOI: 10.1056/NEJMoa1106173).² Neither trial had a randomized, blinded, placebo-controlled design — a genuine limitation the FDA weighed against the alternative of running one in a disease this severe and this rare, and one the drug's approval documents themselves acknowledge rather than paper over.

Monitoring craniosynostosis: a disease feature, not a proven drug effect

One of the highest list prices in medicine — and a real reimbursement fight over it

At launch, Alexion set Strensiq's average US list price at roughly $285,000 per patient per year (dosing, and so cost, scales with body weight, so actual costs vary considerably by patient size). In the UK, the NHS cost-effectiveness body NICE evaluated the drug at a reported list price of about £366,000 per year and, in its original 2017 guidance (HST6), recommended coverage for perinatal- and infantile-onset disease but rejected coverage specifically for adults with juvenile-onset HPP, citing the drug's high cost and what it called insufficiently justified evidence of long-term benefit in that population — a decision Alexion publicly and sharply disputed. That wasn't the end of the story: NICE formally reviewed the guidance again and, on 1 March 2023, published updated guidance (HST23) that replaces HST6 and now recommends asfotase alfa for a substantially wider population — children with juvenile-onset disease who haven't met expected motor milestones or have ongoing musculoskeletal pain unresponsive to two different painkillers, and adults with juvenile-onset disease who have current or recurrent fractures, limited mobility, or significant pain unresponsive to specialist-recommended painkillers — under a simple discount patient access scheme with the NHS. The six-year gap between the original adult rejection and the 2023 reversal is a real, documented instance of a rare-disease reimbursement decision changing as more real-world and trial evidence accumulated, not a one-time dispute that simply faded away.

Current status

Strensiq remains FDA-approved and actively marketed by Alexion Pharmaceuticals, now an AstraZeneca subsidiary following AstraZeneca's 2021 acquisition of Alexion. It remains the only FDA-approved enzyme-replacement therapy for hypophosphatasia; no competing product or biosimilar has reached the US market as of this review. Patients and prescribers weighing it do so against a disease with a genuinely severe, often fatal natural history in its infant-onset forms and a treatment with real, if non-randomized, survival evidence behind it — set against one of the highest recurring drug costs in medicine, a tension this page's reimbursement history section describes concretely rather than in the abstract.