An Interventional, Prospective Open-Label Study of Immunosuppressive Therapies to Mitigate Immune-Mediated Loss of Therapeutic Response to Asfotase Alfa (STRENSIQ®) for Hypophosphatasia (RESTORE)

The primary objective of this study is to evaluate the effect of **immunosuppressive therapies (IST)** in participants treated with **asfotase alfa** who demonstrate immune-mediated loss of effectiveness (LoE) in the context of **hypophosphatasia**. This is clinically relevant as it aims to address the challenge of immune-mediated resistance to treatment, which can significantly impact therapeutic outcomes in patients with this rare metabolic bone disorder.

Secondary objectives include:

• Assessing the impact of immunogenicity in participants treated with IST undergoing asfotase alfa treatment for the duration of the study.
• Evaluating the effect of IST on pharmacokinetic/pharmacodynamic (PK/PD) biomarkers of hypophosphatasia.
• Evaluating the safety of IST use in participants with immune-mediated LoE.
• Assessing the impact of IST on B cells with rituximab treatment.

The clinical trial involves a total of **7 participants** diagnosed with **hypophosphatasia**, a rare metabolic bone disease. The study population includes both male and female subjects, aged between **2 and 18 years**, with open epiphyseal growth plates. Participants were selected based on specific criteria, including a reoccurrence or worsening of rickets for at least the past three months in those who initially responded to asfotase alfa treatment. The presence of anti-drug antibodies (ADAs), with or without neutralizing antibodies (NAbs), is also a requirement. The trial population is considered vulnerable, and participants or their legal guardians must be capable of providing informed consent or assent. Lifestyle factors such as diet and physical activity are not specified, but female participants of childbearing potential and male participants with partners of childbearing potential must adhere to protocol-specified contraception guidance.

The clinical trial is designed as a **randomized**, **double-blind**, and controlled study to evaluate the effect of immunosuppressive therapies in participants with **hypophosphatasia** who demonstrate immune-mediated loss of effectiveness to **asfotase alfa**. The trial will span an estimated duration from February 2024 to August 2027, with a primary objective to assess the number and percentage of participants achieving a complete response to immunosuppressive therapy at Week 100. The study will involve multiple visits, starting with an inclusion visit to screen participants based on specific criteria, including age, presence of antibodies, and clinical evidence of disease progression.

Participants will be involved in the study for a maximum treatment period of 104 weeks. The study visits will include regular follow-up assessments to monitor the efficacy and safety of the treatment, with evaluations of antibody titers, radiographic evidence, and clinical safety parameters. The end-of-study visit will conclude the participant's involvement, where final assessments will be conducted to determine the overall response to the treatment. Conditions that may lead to early termination from the study include non-compliance with the protocol, adverse events, or withdrawal of consent by the participant or their legal guardian.

The trial will utilize a combination of **intravenous** and **subcutaneous** administration routes for the investigational products, including **rituximab**, **bortezomib**, and **methotrexate disodium**. The study will also monitor secondary endpoints such as the incidence of treatment-emergent adverse events, plasma concentrations of therapeutic markers, and enumeration of CD19 B cells. Participants will be required to adhere to protocol-specified contraception guidance if applicable. The trial is categorized as a phase 4 study, focusing on the potential of authorized investigational medicinal products to mitigate immune-mediated loss of effectiveness in enzyme-replacement therapy.

The clinical trial involves the administration of several **experimental medications** and **non-experimental treatments**. The primary experimental medication is **Asfotase Alfa**, a recombinant human tissue non-specific alkaline phosphatase, administered subcutaneously. It is provided in a pharmaceutical form coded as PHF00231MIG. The maximum daily dose is 2 mg/kg, with a total maximum dose of 624 mg over a treatment period of up to 104 weeks. This medication is designated as an orphan drug for the treatment of **Hypophosphatasia**.

**Human Normal Immunoglobulin (IV)**, also known as IVIg, is administered intravenously. It is a structurally diverse substance derived from blood, with a maximum daily dose of 500 mg/kg and a total maximum dose of 12,000 mg over 104 weeks. This medication is used as an auxiliary treatment in the study.

**Rituximab**, marketed as Rixathon, is administered as a concentrate for solution for infusion. It is a protein-based medication used intravenously, with a maximum daily dose of 375 mg/m² and a total maximum dose of 6,000 mg over 104 weeks. Rituximab is classified under antineoplastic agents and is used as a test treatment in the trial.

**Bortezomib**, marketed as Bortezomib Hikma, is administered as a solution for injection either intravenously or subcutaneously. It is a chemical substance with a maximum daily dose of 1.3 mg/m² and a total maximum dose of 15.6 mg over a treatment period of 72 weeks. Bortezomib is also classified under antineoplastic agents and serves as a test treatment in the study.

**Methotrexate Disodium** is administered in two forms: as MTX HEXAL tablets and Methotrexat-GRY® solution for injection. The oral form has a maximum daily dose of 15 mg and a total maximum dose of 1,500 mg over 104 weeks. The injectable form is administered subcutaneously with the same dosing parameters. Methotrexate Disodium is classified as a chemical substance and is used as a test treatment in the trial.

Additionally, **Folic Acid** is administered orally as an auxiliary treatment. It is provided in a pharmaceutical form coded as PHF00006MIG, with a maximum daily dose of 5 mg and a total maximum dose of 520 mg over 104 weeks. This treatment is used to support the primary and test treatments in the study.

Participant compliance with the dosing schedules is monitored throughout the trial to ensure adherence to the treatment protocols. The trial aims to evaluate the effect of immunosuppressive therapies in participants treated with Asfotase Alfa who demonstrate immune-mediated loss of effectiveness.

Efficacy in this clinical trial will be assessed using both primary and secondary endpoints. The primary endpoint is the number and percentage of participants who achieve a complete response to immunosuppressive therapy (IST) at Week 100. A complete response is defined as a decrease in anti-drug antibody (ADA) or neutralizing antibody (NAb) titer from baseline by at least two titer steps or becoming negative, along with radiographic evidence of improvement on the Rickets Severity Score (RSS) by at least one point from baseline.

Secondary endpoints include the incidence and response categories of ADA, ADA titer, NAb incidence, and NAb titer in participants treated with IST while undergoing treatment with **asfotase alfa**. Additionally, plasma concentrations of tissue non-specific alkaline phosphatase (TNSALP) will be measured by asfotase alfa enzyme activity, along with pyridoxal phosphate (PLP) and inorganic pyrophosphate (PPi) at prespecified time points. Safety assessments will include the incidence of treatment-emergent adverse events (TEAEs) and serious adverse events (TESAEs), clinical safety laboratory test results, vital signs, 12-lead ECGs, physical examination findings, and for participants treated with bortezomib, echocardiography and the pediatric-modified Total Neuropathy Scale. Enumeration of CD19 B cells will also be conducted for participants treated with IST throughout the study duration.