Development of the Enzyme Replacement Therapy Asfotase Alfa

The discovery that HPP is caused by decreased TNSALP activity provided a straight-forward rational for enzyme replacement therapy [13, 19]. In a landmark contribution, researchers led by Drs. Crine, Whyte and Milan rationally engineered a recombinant fusion protein composed of a soluble TNSALP, the Fc region of human IgG gamma-1 for Protein A Sepharose purification, and ten acidic aspartate residues for skeletal targeting [20, 21]. Between 2008 and 2010 at least 4 clinical trials were initiated and asfotase alfa tested in patients (Table 2). In 2012, the Federal Drug Administration (FDA) granted a Breakthrough Therapy designation to asfotase alfa. Three years later, in 2015, asfotase alfa received FDA, Canada Health, and European Medicines Agency (EMA) approval for pediatric-onset HPP in the United States, Canada, and Europe, respectively, and Ministry of Health, Labor and Welfare (MHLW) approval for both pediatric and adult-onset HPP in Japan. The drug is manufactured by Alexion, AstraZeneca Rare Disease and has been branded as Strensiq [22].

Table 2 Registered* and completed clinical studies in the development of enzyme replacement therapy for HPP

The pre-approval development of asfotase alfa was accompanied by a single journal publication. In a seminal report, the efficacy of asfotase alfa was demonstrated in 11 infants and young children with life-threatening or debilitating perinatal or infantile HPP [23]. Ten patients completed the study [23]. A 12-months course of asfotase alfa resulted in healing of rickets at 6 months in 9 patients, accompanied by improvement in developmental milestones and pulmonary function as well as markedly improved overall survival [23]. Elevated plasma levels of PPi and PLP diminished with asfotase alfa therapy [23].

The Treatment EnvironmentSpecialists and Patient Referrals

Hypophosphatasia is a rare disease and best managed by experienced specialists that see high volumes of HPP cases and thus have an in-depth knowledge of HPP. The number of specialists in the United States is not known but likely small. A recent Delphi panel to build consensus on assessing HPP severity and disease progression in adult patients included only 31 health care providers and assembled 9 panelists [36]. Most specialists have not received formal training in treating HPP patients but rather developed an interest and expertise during their tenure. The authors believe that the field would greatly benefit from a Rare Bone Diseases Fellowship Program, as it would facilitate seamless transition of knowledge between generations of HPP doctors.

Specialist referrals are common in the early stages of HPP management. Nearly 45% of pediatric cases (n = 18) seen at the Connecticut Children’s Rare Bone Disease program are from family cascade testing. The primary care provider is the most common source of referral (60%) followed by clinical genetics (22%). Similarly, about 30% of adult cases (n = 155) at the Program for Metabolic Bone Disorders at Vanderbilt, stem from cascade family testing, however, referrals come mostly from endocrinology (29%) and clinical genetics (14%), and only occasionally from primary care providers (6%). Referrals from orthopedics (1%) or dentists (1%) are rare, which draws suspicion of undiagnosed patients not receiving care. Notably, about 7% of adult patients have self-diagnosed and seek care directly. The authors’ referral sources might be representative for specialized HPP centers, but there is no systematic documentation or published data on HPP referral sources. Understanding referral sources is warranted because known sources can be educated about HPP. This is important for two reasons. First it facilitates an accurate diagnosis of HPP. Misdiagnoses of HPP are common and can be consequential [5]. For example, in adults a misdiagnosis of osteoporosis can result in initiation of bisphosphonate treatment, which might induce fractures. Second, educated health care providers may recognize the disease at an early stage, helping to avoid unnecessary medical visits and testing, as well as reducing the significant delay between onset of symptoms and diagnosis in patients with HPP [5]. For instance, real-world data from the Global HPP Registry observed a diagnostic delay of 5.7 years in an adult cohort (n = 304) comprising both pediatric and adult-onset patients [11]. A separate analysis from the Global HPP Registry included 269 patients and found less than 1 week diagnostic delay in infants, while a median diagnostic delay of 24.5 years was seen for adults who had a first reported manifestation of HPP before age 18 years and a delay of 3.8 years for those with a first recorded manifestation at or after age 18 years [37].

Multidisciplinary Care

Management of patients with HPP includes treatment of acute symptoms, educating about preventive measures, and long-term treatment monitoring. There clearly is a rational for multidisciplinary care. Multidisciplinary care teams were proposed as early as 2017 and thought to be particularly important in managing the transition from pediatric to adult care [15]. Based on an individual patient’s complexity of symptoms and disease trajectory, these teams assemble specialists from a wide range of medical disciplines and coordinate care, an approach thought to benefit outcome. But multidisciplinary care teams are not without challenge. For example, care teams are typically located at centers specialized in HPP care, and for patients the travel distance to those centers might be substantial. This hurdle is amplified by current telehealth regulations that prevent remote patient consultations across state lines. Importantly, sustaining multidisciplinary care teams critically depend on funding and long-term institutional backing.

Access to Asfotase Alfa

In the United States, rare disease medicines including biologics such as asfotase alfa are often filled through rare disease pharmacies. These pharmacies provide patient support, adherence outreach, refills, and coordinated delivery of the drug to patients. Latter helps to assure treatment access independent of geographical location.

Medical treatment of HPP is associated with significant costs. Although there are no published health care costs for HPP and pricing for asfotase alfa, which is branded and sold as Strensiq, may vary considerably, retail pricing permits an estimate. The costs for 1 mg Strensiq as subcutaneous solution is $75 (https://www.drugs.com/price-guide/strensiq, accessed October 2024) and therefore annual standard dosing of a 80 kg patients would incur drug costs of approximately $1.8 million. Out of pocket, such costs are unsustainable for a vast majority of patients. Therefore, access to treatment almost always relies on reimbursements and programs that ease patients’ financial burden, including the proposed Medicare Part D limitations on annual out-of-pocket prescription drug costs, are essential for broad access to HPP treatment. However, FDA, Canada Health and EMA regulatory approval of asfotase alfa is restricted to pediatric-onset HPP and most insurance providers follow suit with coverage to these patients. This leaves adult patients often with the burden to prove pediatric-onset or adult off-label treatment.

Initiation of Asfotase Alfa Therapy

The decision when to initiate treatment remans challenging. At present, there are no evidence-based recommendations available. To provide some guidance and foster a broader discussion [38], the authors identified 26 important criteria in the three categories severity, genotype and manifestations (Fig. 1). For 9 criteria a positive and for 4 criteria a negative treatment recommendation is suggested. However, there is a gray zone for patients comprising manifestations like isolated pain, dental problems, kidney stones or abnormal gait. In these patients the number of manifestations could help to reach a treatment decision. Further, patients play an important role in the decision-making process and intention-to-treat may not always align with medical reasoning, as exemplified by patients requesting treatment based on knowledge of their ALPL variant. Lastly, even if a manifestation, such as chronic pain, would prompt a treatment decision, asfotase alfa may not be a cost-effective treatment.

Fig. 1

Eminence-based decision making for initiation of asfotase alfa therapy. A total of 26 criteria in three categories are evaluated. Rectangles indicate manifestations seen in both pediatric and adult patients. Hexagons indicate manifestations mostly seen in adults. Ovals indicate pediatric manifestations. Red color: Weak reason for asfotase alfa treatment. Green color: Strong reason for asfotase alfa treatment. *No US regulatory approval of asfotase alfa for adult-onset HPP. **Onset < 6 months of age. ***With or without biochemical signature. §In infancy. HPP, hypophosphatasia

Dosing of Asfotase Alfa

The prescribing information for asfotase alfa details the FDA recommended dosage regimen [22] which is weight-based and differentiates between patients with perinatal/infantile and juvenile onset. Perinatal/infantile dosage is 2 mg/kg administered subcutaneously three times per week, or 1 mg/kg administered subcutaneously six times per week. The dose can be increased to 3 mg/kg administered subcutaneously three times per week. Juvenile onset dosage is 2 mg/kg administered subcutaneously three times per week, or 1 mg/kg administered six times per week for both children and adults age 18 and older. No dose escalation is recommended for juvenile onset patients of any age [22].

Over-treatment may excessively lower PPi, potentially increasing the risk of vascular calcifications by diminishing its protective effect against ectopic mineralization. While this risk remains theoretical and lacks strong clinical evidence, factors like pre-existing cardiovascular disease, age, and baseline PPi levels could play a role. One reassuring case report demonstrated that 8 months of asfotase alfa showed no evidence of vascular calcifications or other concerning ectopic mineralization [39]. A challenge in safety monitoring is that PPi assays are not commercially available, and in vitro, asfotase alfa in a patient’s serum can continue to dephosphorylate PLP into pyridoxal, complicating assay results. Regular monitoring of biochemical markers such as calcium and phosphate is recommended during treatment.

Efficacy of Asfotase Alfa Therapy

The efficacy of asfotase alfa was first demonstrated in clinical trials in children [23, 27]. Pharmacokinetic and -dynamic measures demonstrated a favorable bioavailability of the drug and effective metabolism of the substrates PPi and PLP [23]. In addition to biochemical testing, the early trials also recorded changes in disease manifestation, including survival, time on respiratory support and growth rate for severely affected pediatric patients, radiography for the assessment of bone mineralization and healing, a variety of functional tests for mobility, motor and muscle function, and patient reported outcomes (PROs) [23, 25, 27, 29]. These means remain important efficacy measures of asfotase alfa.

Pediatric

Two long-term studies, both initiated prior to approval of asfotase alfa, have recently reported important post-marketing evidence of the efficacy of asfotase alfa. Trial NCT01205152 comprised 10 children with perinatal or infantile HPP who were part of the initial phase 2 trial (NCT00744042) and followed them over 7 years [28]. The early improvements reported in 2012 [23] were sustained including improved skeletal mineralization, ongoing catchup growth in height and weight, respiratory function improvements that led to independence of respiratory support in all patients by four years of treatment, and notable ongoing improvements in developmental milestones including gross and fine motor function and cognition [28]. Data from the larger NCT01176266 trial in 69 children age 5 or younger reported that most (72%), but not all, infants/young children given asfotase alfa had early radiographic and clinical improvement that were sustained up to 6 years [27].

Adult

Data from a phase 2 trial (NCT01163149) provided initial evidence of sustained disease control in 19 (13 patients age 18 or older) pediatric-onset adolescents and adults treated with asfotase alfa [25]. Initiated in 2010 the study encompassed a primary treatment period of 6 months followed by an extension phase of 4.5 years. In contrast to the standard dose of 6 mg/kg/week used in almost all asfotase alfa studies, the first 6 month were dose at 2.1 or 3.5 mg/kg/week, followed by 6 months of 3.5 mg/kg/week and, at one year, start of the standard dose for the remaining 4 years of the study [25]. Pharmacodynamic data collected at 6 months and pooled from both initial dosings showed a statistically significant reduction in circulation PLP but not PPi in patients treated with asfotase alfa (n = 13) as compared to no treatment controls (n = 6) [25]. However, within subject data at study end revealed a significant reduction of both PLP and PPi compared to baseline [25]. Importantly, the reduced PPi level was within the lower normal range, thus limiting unwanted ectopic tissue mineralization. Despite the underdosing in the first year of treatment, functional improvements were observed. At 6 months, the 6-min walk test (6MWT) was significantly increased both in meters walked and percentage predicted for age; these improvements were sustained over the 5 year duration of the study [25]. Similar improvements have been previously reported in pediatric patients [24].

The subsequent phase 2 trial NCT02797821 further investigated asfotase alfa pharmacology in a cohort of 27 pediatric-onset adult patients. Pharmacodynamic data demonstrated that treatment with asfotase alfa normalized serum PPi, which is critical for improving bone mineralization, and also reduced the elevated level of PLP [31]. Importantly, the study confirmed the effectiveness of the standard dose of 6 mg/kg/week, which was developed mostly in children, for treatment of adults. Further, it confirmed maintenance of physiological PPi plasma levels in patients treated with asfotase alfa; notwithstanding, asymptomatic ectopic calcifications, in particular in the eye, have been repeatedly observed in HPP patients treated with asfotase alfa [24, 25, 31]. A twin report on pharmacokinetics revealed that the drug has a relatively long half-life of about 5 days, enabling less frequent dosing while maintaining stable therapeutic concentrations after about one months of treatment [30].

Three recent reports from the observational study NCT03418389 addressed bone mineralization and physical function. One report analyzed 21 adults with pediatric-onset HPP over 24 months [33]. Asfotase alfa treatment induced changes in bone turnover and mineral metabolism markers, suggesting that treatment-mediated mineralization may enable remodeling and bone turnover on previously unmineralized surfaces [33]. Another report recorded physical function in 22 patients over a period of 24 months [32]. Asfotase alfa treatment improved physical measures and reduced pain and fatigue [32]. A third report analyzed 14 pediatric-onset adult patients over 12 months and confirmed that asfotase alfa treatment enhances physical performance, as reflected in improved 6MWT and Timed Up and Go (TUG) scores, alongside better health-related QoL [34].

The therapeutic impact of asfotase alfa also includes substantial pain reduction and improvements in fatigue, key symptoms that significantly impair the daily lives of HPP patients. A recent real-world study collected PROs as part of a patient support program [40]. Data from 50 adults emphasized that over a period of 12 months patients experienced considerable decreases in pain levels and reported reduced fatigue, which improved their ability to perform daily activities [40]. Similar findings were recorded in a recent 3-year analysis of PRO data from 190 patients enrolled in the Global HPP Registry [12]. Patients reported lasting benefits in terms of reduced pain and disability, contributing to better QoL [12].

Data from PRO studies consistently underscore the improvements in QoL following asfotase alfa treatment. Findings from both the clinical study NCT03418389 and the patient support program reported improvements in physical and emotional well-being, reduced limitations in mobility, and enhanced participation in social and personal activities [34, 40]. Adult burden of illness data from the Global HPP Registry further highlighted the emotional and socioeconomic burden of HPP, emphasizing the importance of long-term management strategies like asfotase alfa therapy to alleviate these challenges [11].

Above evidence collectively highlight the significant therapeutic benefits of asfotase alfa in treating adults with pediatric-onset HPP, and span improvements in bone health, physical function, pain reduction, and overall QoL. Together, these studies validate asfotase alfa as an effective and safe therapy for managing pediatric-onset HPP in adults, offering substantial improvements in bone health, physical function, pain, and QoL. The sustained benefits of the therapy, alongside its favorable pharmacokinetics and safety profile, suggest that early and continuous treatment is crucial for mitigating the long-term complications of this rare disease.

Treatment Disruption and Lack of Efficacy

Enzyme replacement therapy for HPP is based on the continues supply of the missing TNSALP activity and, in theory, ceased supply results in reoccurrence of HPP symptoms. Recent data from study NCT01163149 proved this assumption. Six adult patients treated with asfotase alfa for 61–68 months but who then had an abrupt discontinuation of treatment for 15–48 months experienced the expected deterioration in clinical symptoms [26]. Upon resumption of asfotase alfa, they all showed clinical improvement underscoring the importance of treatment continuation [26]. However, not all retreated patients achieved improvements to treatment-naïve baseline levels, emphasizing the negative impact of treatment discontinuation on efficacy [26]. Lastly, it is important to keep in mind that not all asfotase alfa treated patients experience a meaningful and sustained treatment response. Perceived lack of efficacy is likely to contribute to treatment discontinuation in up to 10% of asfotase alfa-treated patients [41].

Monitoring of Asfotase Alfa TherapyBiochemical Testing

Changes in TNSALP substrates, such as PPi and PLP, are valid measures of the response to enzyme replacement and, as described above, have been utilized during the clinical development of asfotase alfa. Furthermore, urine PEA is an emerging clinical marker of HPP [42, 43]. However, in clinical practice determining these substrates often has limited value because of (a) lack of commercial PPi tests, (b) the potential inaccuracy of vitamin B6 (PLP) measures, for example due to continued metabolism in the test tube, and (c) the not fully developed clinical protocols for PEA assays. Therefore, physicians should refrain from titrating asfotase alfa dosing based on biochemical parameters. In addition, clinicians should keep in mind that accurate ALP testing depends on the selection of an appropriate reference range and that ALP testing on enzyme replacement therapy does not reflect endogenous enzyme activity. In fact, unexplained lowering of ALP levels while on enzyme replacement therapy may signal a compliance or an antibody issue and further evaluation may be warranted. Lastly and probably most important there is currently no data that demonstrates changes in biochemical parameters have any effect on outcomes.

Amended Guidelines

Comprehensive, evidence-based consensus guidelines for monitoring of asfotase alfa therapy were first published in 2017 [15]. Since then, clinical practice has adopted several important changes based on clinical observations. First, bone densitometry, specifically dual-energy X-ray absorptiometry (DXA), is not always a reliable marker of disease severity or bone health in individuals with HPP. In HPP, the quality of bone, particularly its mineralization, is often compromised despite the bone density readings potentially being normal or even elevated in some cases [44]. Data showed that both the quality of bone and its structural integrity, which contribute to fracture risk, are not adequately captured by DXA [45, 46]. Second, bone biopsies are not generally advised anymore because they may destabilize the already compromised HPP skeleton, thus increasing fracture risk [47]. Third, mobility in adult patients is now routinely measured using TUG and a Five Times Sit-to-Stand Test (5TSTS) in addition to the established 6MWT. Both TUG and 5TSTS are widely used in clinical assessments of balance, strength, and mobility in other musculoskeletal and metabolic conditions, including osteoporosis, and are recommended because the administrative burden for staff in the clinic setting is greatly reduced compared to the 6MWT. Lastly, PRO measures, such as PHQ9 or SF-23v2, have been recognized as important tools for clinical decision making [40]. Table 3 provides up-dated recommendations for monitoring asfotase alfa therapy in HPP patients.

Table 3 Up-dated Monitoring Guidelines for Patients With HPP Treated With Asfotase Alfa*,§Management of Long-term Treatment with Asfotase AlfaAdverse Reactions

The prescribing information for asfotase alfa lists injection side reactions (ISRs) (63%), lipodystrophy (28%), ectopic calcification (14%), and hypersensitivity reactions (12%) as the most common side effects [22]. These incidences were based on approximately one hundred pediatric-onset HPP patients [22]. Recent data from the post-approval observational study NCT03418389 reported ISRs and lipodystrophy in 86% and 82% of 22 pediatric-onset adult patients, respectively [32]. A sub-group analysis of 14 patients from the same study found ISR rates of 79% and 93% after 3 and 12 months of asfotase alfa treatment, respectively, suggesting a temporal ISR increased [34]. Newer data from 216 HPP adults in the Global HPP Registry reported a rate of 12% ISR [12]. However, the stark difference between this rate and the ISR rates from clinical studies necessitate further validation of the real-world data.

Injection site reactions from asfotase alfa administration are generally transient and manifest as erythema, discoloration/hypopigmentation, pain/tenderness, pruritus/itching [22, 32]. Prevention of ISRs relies on rotation between 5 common injection sites [15, 22]. Management of patients prone to mild to moderate ISRs include administration of antihistamine plus acetaminophen or ibuprofen prior to asfotase alfa injection [15]. It is likely that there is an impact of ISRs on treatment adherence and that impact may increases with prolonged therapy, but data to support this notion is largely elusive.

Immunogenicity

Human anti-asfotase alfa antibodies were observed during early clinical development and the prescribing information for asfotase alfa states an anti-asfotase alfa antibody rate of 89%, with 57% of patients also tested positive for neutralizing antibodies (nAbs) [22, 23]. Recent studies NCT02797821 and NCT01176266 found anti-asfotase alfa antibody rates of 60% (n = 27) and 88% (n = 60), respectively [27, 31]. Despite the prevalence of anti-asfotase alfa antibodies, available data is limited. Study NCT01176266 provided the most detailed assessment. Consistent with previous observations, 67% of patients tested positive for nAbs over the course of a 6-year study [27]. No clear relationship was found between the presence of anti-asfotase alfa antibodies and adverse events nor were any adverse events suggestive of immune mediations [27]. It is well established, however, that nAbs can negate the clinical benefit of biologics, while non-neutralizing antibodies can also reduce efficacy, for example by affecting clearance, pharmacodynamics and pharmacokinetics [48]. Such adverse clinical effects are particularly relevant for long-term protein replacement therapy to treat monogenic disease [48]. Further investigation of the immune response elicited by asfotase alfa is needed and will aid mitigation strategies, such as dose escalation, immunomodulation, or asfotase alfa derivatives.

Development of Efzimfotase Alfa

At the time of writing, physicians have gained about a decade of clinical experience with asfotase alfa. Clinical data demonstrates that most treated patients experience improved symptoms with manageable side effects. This raises the prospect of a lifelong asfotase alfa therapy and thus sustained control of HPP symptoms. However, asfotase alfa requires between 3 and 6 weekly injections which frequently induce ISRs. These characteristics challenge patient compliance not only in the pediatric population but also in patients requiring lifelong treatment. Therefore, asfotase alfa derivatives with optimized administration characteristics are under development. Proprietary modifications include TNSALP mutations for increased catalytic activity, alternate Fc regions, and variations in the bone targeting motif [49]. A lead compound, termed ALXN1850 or efzimfotase alfa, has entered development. The TNSALP domain of efzimfotase alfa lacks two N-linked glycosylation sites and harbors a single point mutation (E108M). In addition, it comprises the Fc part of human IgG2/4 instead of the Fc part of human IgG gamma-1, while maintaining the deca-aspartate bone targeting motif. In vitro, these modifications resulted in the expected increase in enzymatic activity [