HICKORY Phase III Failure Exposes Adult HPP as Efzimfotase Alfa's Hardest Evidentiary Target

AstraZeneca's Efzimfotase Alfa Misses Primary Endpoint in Phase III HICKORY Trial

AstraZeneca's enzyme replacement therapy candidate, efzimfotase alfa, failed to meet its primary endpoint in the Phase III HICKORY study for hypophosphatasia (HPP). The trial, conducted by Alexion Pharmaceuticals, compared efzimfotase alfa against placebo in adults and adolescents with HPP who had not received prior Strensiq treatment. While efzimfotase alfa showed a 17.05-meter improvement in the six-minute walk test (6MWT) after 25 weeks, this was not statistically significant compared to the "better-than-expected" placebo response. Despite this, AstraZeneca plans to continue development, citing positive findings in post-hoc analyses and significant unmet patient needs.

• The Phase III HICKORY trial for efzimfotase alfa in hypophosphatasia (HPP) missed its primary endpoint, which was the six-minute walk test (6MWT). Although the drug showed a 17.05-meter improvement from baseline after 25 weeks, this was not statistically significant due to a higher-than-anticipated improvement observed in the placebo group, which AstraZeneca described as "better-than-expected."
• A post-hoc analysis revealed that 45.2% of patients treated with efzimfotase alfa achieved a minimum clinically important difference (MCID) in the 6MWT, compared to 30% in the placebo arm. This indicates a 2.5-fold higher chance of a clinically meaningful response for treated patients, with MCID defined as 31.1 and 43-meter improvements for adults and adolescents, respectively.
• Beyond the primary endpoint, efzimfotase alfa demonstrated significant improvements in patients' self-reported levels of fatigue over placebo. Additionally, a nominally significant 35.6-meter benefit in 6MWT values was observed in a prespecified subgroup of patients with paediatric-onset HPP. In adults with adult-onset HPP, the drug showed clinically meaningful improvement in the Lower Extremity Functional Scale (LEFS) and nominally significant and clinically meaningful improvement in FACIT-Fatigue.
• Despite the trial's primary endpoint miss, AstraZeneca intends to pursue development of efzimfotase alfa due to the limited treatment options for HPP, with Strensiq (asfotase alfa) being the only approved drug in the US. The HPP market is also seeing competitive activity, highlighted by BioMarin's recent acquisition of Alesta Therapeutics and its candidate ALE1 for $490 million, which is currently in Phase I/IIa development.

Unpacking Efzimfotase Alfa's HICKORY Trial Results in HPP

Several recent clinical investigations have advanced the evidence base for asfotase alfa in hypophosphatasia across age groups and disease severities. A Phase 2 open-label study enrolling 69 patients aged ≤5 years with severe HPP evaluated asfotase alfa administered subcutaneously at 2 mg/kg three times per week or 1 mg/kg six times per week over a median of 2.3 years. Radiographic Global Impression of Change (RGI-C) scores improved significantly at Month 6 (+2.0), Year 1 (+2.0), and Last Assessment (+2.3; P < 0.0001 for all), with 46% of patients who required respiratory support at baseline no longer needing it. Height and weight Z-scores generally increased. Nine patients (13%) died, and all patients experienced at least one adverse event, with pyrexia the most common. Radiographic nonresponders — comprising 28% of the cohort — had more severe disease at baseline and a higher rate of neutralizing antibodies at Last Assessment.

A separate single-arm, open-label Phase 2 extension trial followed 11 infants and young children with life-threatening perinatal or infantile HPP for approximately 7 years on asfotase alfa (1 mg/kg three times per week subcutaneously, adjusted to 3 mg/kg three times per week if required). Skeletal healing was sustained over the full treatment period; all evaluable patients had RGI-C scores of at least +2 at year 6 (median +2·0 [range 2·0–3·0]) and year 7 (median +2·3 [2·0–3·0]). No patient who completed the study required respiratory support after year 4, and weight Z-scores improved to within normal range from year 3 onward. Age-equivalent scores on gross motor, fine motor, and cognitive subscales of the Bayley Scales of Infant and Toddler Development also improved. All 11 patients experienced at least one treatment-emergent adverse event; the most common were pyrexia (73%), upper respiratory tract infection (73%), craniosynostosis (64%), and pneumonia (64%). Serious adverse events related to asfotase alfa occurred in 3 patients (27%), including severe chronic hepatitis, a moderate immediate post-injection reaction, and severe craniosynostosis with severe conductive deafness.

In adults, a multinational, randomized, open-label study evaluated asfotase alfa in patients aged 13–66 years with HPP over a 6-month primary treatment period and a 4.5-year extension phase. Changes from baseline in plasma pyridoxal 5'-phosphate (PLP) and inorganic pyrophosphate (PPi) concentrations — coprimary efficacy measures — were greater in the combined asfotase alfa group versus control, reaching statistical significance for PLP (P = 0.0285) but not for PPi (P = 0.0715) at 6 months; however, within-subject changes in both PLP and PPi over 5 years of treatment were significant (P < 0.05). On the 6-Minute Walk Test, median distance walked increased from 355 meters before treatment to 450 meters at 5 years (P < 0.05). There were no deaths during this study, and the most common adverse events were mild to moderate injection site reactions. Complementing these findings, a prospective observational study in adults with pediatric-onset HPP treated with asfotase alfa for 24 months reported significant improvements in Timed Up and Go test time at 12 (P = 0.003) and 24 months (P = 0.005), chair rise time at 12 (P = 0.003) and 24 months (P < 0.002), and Short Form 36 Physical Component Summary scores at 12 (P = 0.012) and 24 months (P = 0.005). Injection site reactions occurred in 86.4% of participants, with no severe side effects or safety findings identified.

The Persistent Unmet Need in Hypophosphatasia Treatment

Despite the approval of asfotase alfa as the first enzyme replacement therapy for HPP, significant clinical, diagnostic, and therapeutic challenges persist across the disease spectrum. Treatment limitations span from practical administration burdens to unresolved efficacy gaps, underscoring the continued unmet need in this rare metabolic disorder.

• Frequent injections and injection site reactions: Asfotase alfa carries a short elimination half-life of 2.28 days, necessitating frequent subcutaneous injections. Injection site reactions are the most consistently reported adverse event across clinical studies — occurring in 78–86.4% of participants in pharmacodynamic and observational studies — and include lipohypertrophy in some pediatric patients.

• Delayed and missed diagnosis: Diagnostic delay is a pervasive problem. A rheumatology case series reported a median diagnostic delay of 13 years, and adult HPP is frequently misdiagnosed as osteoporosis, rheumatologic disease, or other musculoskeletal conditions. The nonspecific nature of adult presentations — chronic pain, fractures, muscle weakness — contributes to prolonged diagnostic pathways and inappropriate treatment with contraindicated agents such as bisphosphonates, denosumab, and potent antiresorptive drugs.

• Contraindicated standard-of-care therapies: Pharmaceutical therapies commonly used for fracture management and osteoporosis — including bisphosphonates, denosumab, and vitamin D supplementation in certain contexts — are contraindicated in adults with HPP, substantially narrowing the therapeutic toolkit and complicating management of comorbid bone disease.

• Neutralizing antibodies and treatment refractoriness: Cases of neutralizing antibody (NAb) development against asfotase alfa have been documented, with NAb expression correlating with recurrence of rickets and reduced therapeutic effect. Optimal dosing in the context of NAb positivity remains uncertain, with clinical findings guiding dose adjustments on a case-by-case basis.

• Ectopic calcification as a treatment-associated complication: Asfotase alfa treatment has been associated with the development of conjunctival calcification in all seven subjects evaluated in one retrospective analysis, attributed to disinhibition of hydroxyapatite crystal propagation. While the lesions were asymptomatic and nonprogressive, this finding represents an unresolved safety signal requiring ongoing monitoring.

• Unresolved efficacy in pain and adult-onset disease: Despite improvements in functional outcomes and quality of life, no significant change in pain level was observed at 12 or 24 months in one prospective observational study of adults with pediatric-onset HPP. Treatment options for adult-onset HPP specifically are described as not well established, and long-term prognosis data remain limited.

• Limitations of second-generation therapies still under evaluation: Efzimfotase alfa (ALXN1850), a second-generation TNSALP enzyme replacement therapy, demonstrated acceptable safety and pharmacokinetics in a Phase 1 dose-escalation study, with absolute bioavailability of 28.6%–36.8% over the subcutaneous dose range of 15–90 mg. However, its clinical efficacy in broader adult and pediatric populations remains to be established in further trials.

Shifting Dynamics in the Hypophosphatasia Treatment Landscape

Asfotase alfa, a hydroxyapatite-targeted recombinant tissue-nonspecific alkaline phosphatase (TNSALP) enzyme replacement therapy (ERT) that received its first regulatory approval in 2015, has remained the central pillar of HPP management and has accumulated a substantially broader evidence base over the past several years. Data from the Global HPP Registry — an observational, prospective, multinational study — demonstrated that among 190 adults treated with asfotase alfa for ≥ 6 months, mean distance on the 6-Minute Walk Test increased from 404 m at baseline to 484 m at Month 12, with improvements in pain severity scores ranging from −0.72 to −1.13 across all time points and Physical Component Summary scores of the SF-36v2 sustained through Month 36. Parallel prospective single-center data in adults with pediatric-onset HPP over 24 months showed significant improvements in the Timed Up and Go test at 12 (P = 0.003) and 24 months (P = 0.005), usual gait speed at 12 (P = 0.003) and 24 months (P = 0.015), and dominant and nondominant hand grip strength at 24 months (P = 0.029 and P = 0.019, respectively), with functional gains observed as early as 3 months after treatment initiation. These findings collectively reinforced that asfotase alfa's therapeutic benefits extend across pediatric and adult populations regardless of age of disease onset.

Evidence has also expanded meaningfully in the domains of fracture healing and disease monitoring. Case-level and registry data documented that asfotase alfa promoted consolidation of nonunions and stress fractures that had failed to heal for extended periods — including one adult patient in whom complete femoral fracture healing and independent ambulation were achieved after 10 months of asfotase alfa at 2 mg/kg three times per week, and a pediatric case in which full radiological consolidation of a fifth metatarsal stress fracture was confirmed by cone beam computed tomography 13 weeks after treatment initiation, with bone mineral density in the fracture gap increasing by 36%. In parallel, a post hoc analysis of pooled clinical trial data in 19 children established that DXA BMD Z-scores have limited utility in assessing deficient bone mineralization in HPP, while bone mineral content Z-scores increased significantly from baseline to last assessment (P ≤ 0.0056), and improvements in the Radiographic Global Impression of Change scale correlated significantly with increases in whole body and lumbar spine BMC Z-scores (P < 0.05). The ALIGNOGRAM software further demonstrated that improvements in bone disorganization were detectable as early as 3 months after treatment initiation — before changes on X-ray or bone scintigraphy — positioning it as a potential early indicator of treatment efficacy.

Beyond the established ERT paradigm, the treatment landscape has begun to show early signals of evolution toward next-generation and adjunctive approaches. Preclinical data demonstrated that a single intramuscular injection of an adeno-associated virus vector serotype 8 harboring TNAP-D (AAV8-TNAP-D) at 3 × 10 vector genomes per body sustained serum alkaline phosphatase activity, reduced plasma inorganic pyrophosphate levels, and improved skeletal phenotype in both adult HPP and pseudo-HPP mouse models without promoting ectopic calcification, representing a potential path toward gene therapy. Clinically, a cannabidiol-predominant, full-spectrum prescription cannabis extract produced a reduction in pain from 8.5 to 2.5 on a numerical rating scale (NRS, 0–10) and enabled opioid discontinuation in one adult patient with HPP-related chronic pain refractory to asfotase alfa and multimodal management. A derivative of asfotase alfa, efzimfotase alfa, is noted to be undergoing clinical testing. Taken together, the evidence base accumulated over this period reflects a field that has moved from establishing survival benefit in severe pediatric HPP toward characterizing long-term functional, quality-of-life, and monitoring outcomes across the full disease spectrum, while early-stage research begins to explore alternatives to subcutaneous ERT.

HPP Therapy: A High Bar for New Entrants After Phase III Setback

The recent Phase III HICKORY study results for AstraZeneca's efzimfotase alfa in hypophosphatasia (HPP) mark a significant moment in the rare disease landscape. Despite a numerical improvement in the 6-minute walk test (6MWT), the enzyme replacement therapy candidate failed to achieve statistical significance against a 'better-than-expected' placebo response. This outcome underscores the formidable challenge of developing new therapies in an indication already served by a transformative treatment like asfotase alfa.

HPP is a debilitating genetic disorder characterized by deficient tissue-nonspecific alkaline phosphatase (TNSALP) activity, leading to impaired bone mineralization and a range of severe musculoskeletal and systemic issues. Asfotase alfa has set a high standard, demonstrating long-term efficacy in improving survival, bone quality, and crucial functional abilities, including significant and sustained improvements in 6MWT distance in both pediatric and adult patients. The 6MWT is a well-validated and reliable measure of physical function in HPP, making its failure as a primary endpoint particularly impactful for efzimfotase alfa.

For AstraZeneca, this setback necessitates a critical re-evaluation. While the company cites positive post-hoc analyses and unmet patient needs, pursuing development based on secondary findings after a primary endpoint miss carries inherent risks. The competitive landscape is dominated by an established therapy with a robust efficacy and safety profile. Any new entrant would need to demonstrate clear superiority, a distinct mechanism, or a compelling advantage in specific patient populations or administration to gain traction. The HICKORY trial's outcome also serves as a cautionary tale for rare disease clinical trial design, highlighting the potential for strong placebo effects in functional endpoints and the need for rigorous methodology to discern true therapeutic benefit. The path forward for efzimfotase alfa will likely involve navigating a complex regulatory and commercial environment, demanding a highly differentiated strategy to justify continued investment.