Protocol Synopsis (OBX-319)

Document ID: PROT-301
Version: 1.0
Change History: 1.0 — Initial issue.
Standard(s): ICH E6(R3)

Protocol Synopsis

• Compound: OBX-319

• Sponsor: Virtual Biopharma Inc.

• Indication: Systemic Lupus Erythematosus (moderate-to-severe active)

• Phase: 3

• Design: randomized, double-blind, placebo-controlled, allocation 1:1:1

• Planned N: 480 randomized (screened 900)

• Arms: OBX-319 High, OBX-319 Low, Placebo

• Primary endpoint: SRI-4 response at Week 52 (operationalised as low disease activity, SLEDAI-2K <= 4)

• Visit schedule (weeks): [0, 4, 12, 24, 36, 52]

• Primary analysis: ANCOVA at Week 52; α = 0.05, power = 0.9

• Study identifier: OBX319-301

• Molecule / modality: humanized IgG1 anti-CD19 × anti-CD20 bispecific monoclonal antibody, produced by Chinese hamster ovary (CHO) cell culture, administered subcutaneously

• Primary objective: demonstrate the superiority of each OBX-319 regimen over placebo, on background standard-of-care, in achieving low disease activity (SLEDAI-2K <= 4) at Week 52

• Key secondary endpoint: change from baseline in SLEDAI-2K at Week 52

• Study treatment: OBX-319 High-dose or Low-dose subcutaneous, or matched subcutaneous placebo, each added to stable background standard-of-care

• Planned enrolment per arm: approximately 160 subjects per arm under the 1:1:1 allocation of 480 randomized subjects

• Expected baseline disease activity: mean baseline SLEDAI-2K of approximately 11 (moderate-to-severe active disease)

• Treatment duration: 52-week double-blind period

• Multiplicity control: pre-specified hierarchical (fixed-sequence) testing across the primary and key secondary endpoints for each active regimen versus placebo

• Data monitoring: independent Data Safety Monitoring Board (DSMB) oversight throughout the treatment period

• Regulatory basis: biological product; marketing application submitted as a Biologics License Application (BLA) under 21 CFR Part 601

1 Introduction and Background

1.1 Disease Background and Unmet Medical Need

Systemic lupus erythematosus (SLE) is a chronic, relapsing–remitting multisystem autoimmune disease driven by loss of tolerance to nuclear self-antigens, production of pathogenic autoantibodies (notably anti–double-stranded DNA [anti-dsDNA]), immune-complex deposition, and complement consumption. Manifestations span the skin, joints, serosal surfaces, haematological lineages, central nervous system, and kidney. Patients with moderate-to-severe active disease frequently remain inadequately controlled despite standard of care (glucocorticoids, antimalarials such as hydroxychloroquine, and conventional immunosuppressants) and accrue irreversible organ damage together with cumulative glucocorticoid toxicity. B lymphocytes are central to SLE pathogenesis — as precursors of autoantibody-secreting cells, as antigen-presenting cells, and as producers of pro-inflammatory cytokines — so depletion of the autoreactive B-cell compartment is a mechanistically rational therapeutic strategy and the basis for the OBX-319 program.

1.2 Investigational Product and Mechanism of Action

OBX-319 is a humanized IgG1 bispecific monoclonal antibody that simultaneously engages two B-lineage surface antigens, CD20 and CD19. CD20 is expressed from the pre-B stage through mature and memory B cells but is largely absent on plasmablasts and plasma cells, whereas CD19 has a broader expression window that begins earlier in the B lineage and persists on plasmablasts and a subset of short-lived plasma cells. By co-targeting both antigens, OBX-319 is designed to achieve deeper and broader depletion of the pathogenic B-cell pool than single-antigen (anti-CD20 alone) approaches, including populations that may partially escape CD20-directed therapy. Depletion is mediated through the intact human IgG1 Fc via antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC). The product is administered subcutaneously, supporting outpatient dosing.

OBX-319 drug substance is produced by recombinant expression in a Chinese hamster ovary (CHO) cell line and purified using a platform downstream process comprising Protein A affinity capture followed by orthogonal polishing chromatography and dedicated viral-clearance steps. The control strategy addresses attributes characteristic of a bispecific IgG1 — correct chain pairing and quantitation of mispaired/half-antibody species, charge variants, aggregate/high-molecular-weight content, and Fc glycosylation (relevant to effector function and clearance). Quality aspects are developed consistent with ICH Q6B (specifications), ICH Q5C (stability), and ICH Q5A(R2) (viral safety of biotechnology products), with full description in Module 3 and the Quality Overall Summary in Module 2.3. Investigational-product handling, storage, and accountability for this study follow the study pharmacy manual.

1.3 Nonclinical Experience

The CD19 and CD20 epitopes engaged by OBX-319 are not conserved in rodents; there is no pharmacological cross-reactivity in rodent species, and the cynomolgus monkey is the sole pharmacologically relevant species. The nonclinical program was designed in accordance with ICH S6(R1) and comprised in-vitro binding and effector-function characterization, tissue cross-reactivity, pharmacokinetic/pharmacodynamic evaluation, and repeat-dose toxicology in the cynomolgus monkey. The principal findings were the anticipated on-target consequences of B-cell depletion and associated pharmacological immunomodulation, with recovery on cessation of dosing. Consistent with ICH S6(R1) and the monoclonal-antibody modality, genotoxicity, carcinogenicity, hERG/in-vitro cardiac ion-channel assays, and a dedicated thorough-QT study were not conducted, as they are not warranted for a large, target-specific protein therapeutic that does not distribute to the nucleus, is not expected to interact directly with cardiac ion channels, and is catabolized to endogenous amino acids. The nonclinical findings are summarized in Module 2.4/2.6 and support the doses and monitoring adopted in this protocol.

1.4 Clinical Experience and Rationale for Dose Selection

The pharmacokinetics of OBX-319 are governed by target-mediated drug disposition (TMDD): at low concentrations, binding to and internalization by CD19/CD20-bearing B cells contribute a saturable, nonlinear elimination pathway, which gives way to approximately linear (predominantly catabolic) clearance once target is saturated and the peripheral B-cell compartment is depleted. As an exogenous therapeutic protein, OBX-319 carries an inherent potential for immunogenicity; anti-drug antibodies (ADA), including neutralizing antibodies, are characterized with a tiered assay strategy, with assessment of any impact on exposure, pharmacodynamics, efficacy, and safety. The two subcutaneous regimens evaluated here — a High-dose and a Low-dose — were selected from the earlier first-in-human and dose-ranging experience to bracket the exposure range predicted to achieve and maintain near-complete peripheral B-cell depletion while preserving a subcutaneous, outpatient-compatible regimen; the two-dose confirmatory design allows characterization of the dose–response relationship for efficacy and safety.

1.5 Benefit–Risk Rationale for the Study

Profound, dual-antigen B-cell depletion is expected to reduce autoantibody production and disease activity in moderate-to-severe active SLE, addressing a clear unmet need in patients inadequately controlled on standard of care. The anticipated risks are those of therapeutic B-cell depletion — serious and opportunistic infection and hypogammaglobulinaemia as identified risks, and injection/administration reactions and immunogenicity as expected consequences of a subcutaneously delivered therapeutic protein — each of which is monitorable and manageable through the screening, surveillance, and risk-mitigation measures specified in Section 9. This benefit–risk framing, together with the nonclinical and earlier clinical experience, supports conduct of the confirmatory study described below.

2 Objectives and Endpoints

Primary objective. To demonstrate that OBX-319 (each of the High-dose and Low-dose regimens), added to background standard of care, is superior to placebo in the proportion of subjects achieving low disease activity (SLEDAI-2K <= 4) at Week 52, as the operational definition of the SRI-4 primary response.

Key secondary objective. To estimate the effect of each OBX-319 regimen versus placebo on change from baseline in SLEDAI-2K at Week 52.

Additional secondary objectives. To evaluate the effect of OBX-319 on glucocorticoid reduction, on organ-domain disease activity (BILAG-2004), on the frequency and time to disease flare, and on serological activity (anti-dsDNA, complement C3/C4).

Pharmacodynamic / biomarker objectives. To characterize target engagement through depletion and repletion of circulating CD19+ B cells and associated serological normalization.

Pharmacokinetic and immunogenicity objectives. To characterize systemic exposure to OBX-319 under TMDD and to assess the incidence, titre, and neutralizing capacity of anti-drug antibodies and their relationship to exposure, efficacy, and safety.

Safety objective. To characterize the safety and tolerability of OBX-319 over the 52-week double-blind period, with emphasis on the identified and potential risks of B-cell depletion.

Estimand for the primary endpoint. The primary estimand contrasts each active regimen against placebo, on background standard of care, in the target population of adults with moderate-to-severe active SLE (treatment attribute), for the binary variable of low disease activity (SLEDAI-2K <= 4) at Week 52 (variable attribute). Intercurrent events of premature discontinuation of randomized treatment or absence of an evaluable Week 52 assessment are handled by a composite strategy in which affected subjects are classified as non-responders (intercurrent-event attribute). The population-level summary is the difference between active and placebo in the proportion of responders (population-level summary attribute), consistent with ICH E9(R1).

3 Study Design

3.1 Overall Design

OBX319-301 is a Phase 3, randomized, double-blind, placebo-controlled, three-arm study evaluating two subcutaneous regimens of OBX-319 (High-dose and Low-dose) versus matched placebo, each added to stable background standard of care, in adults with moderate-to-severe active SLE. Approximately 900 subjects are screened to randomize 480 subjects in a 1:1:1 ratio (approximately 160 per arm). Treatment continues for a 52-week double-blind period, after which subjects enter protocol-specified safety follow-up for monitoring of B-cell repletion and immunoglobulin recovery.

3.2 Randomization and Blinding

Eligible subjects are randomized centrally through an interactive web/response system in a 1:1:1 ratio to OBX-319 High-dose, OBX-319 Low-dose, or placebo. Randomization is stratified by baseline disease activity and background therapy to balance prognostic factors across arms. Double-blinding is preserved by use of a matched subcutaneous placebo identical in presentation and administration to active OBX-319; subjects, investigators, site staff, and the sponsor study team remain blinded to individual treatment assignment. Emergency unblinding is available only through the interactive response system when knowledge of the assignment is essential to subject management, with the event documented. Details of allocation concealment and code management are provided in the randomization/IWRS plan.

3.3 Study Duration and Visit Schedule

Each subject participates through a screening period, a 52-week double-blind treatment period, and a post-treatment safety follow-up period. On-treatment clinic visits with efficacy, pharmacodynamic, pharmacokinetic, immunogenicity, and safety assessments occur at Weeks 0, 4, 12, 24, 36, and 52, with Week 52 serving as the primary-endpoint and end-of-treatment visit.

4 Study Population

The study enrols adults with moderate-to-severe active SLE inadequately controlled on standard of care. The population is expected to present with a mean baseline SLEDAI-2K of approximately 11.

4.1 Key Inclusion Criteria

• Age ≥ 18 years at screening.
• Diagnosis of SLE meeting established classification criteria, with autoantibody positivity (e.g., antinuclear antibody and/or anti-dsDNA) documented at screening or in medical history.
• Moderate-to-severe active disease, defined as SLEDAI-2K ≥ 6 at screening.
• On stable background standard of care (antimalarial and/or immunosuppressant and/or oral glucocorticoid at a prednisone-equivalent dose ≤ 40 mg/day) for a protocol-defined interval before randomization.
• Willing and able to provide written informed consent and to comply with the visit schedule and study procedures.

4.2 Key Exclusion Criteria

• Active, severe, or unstable neuropsychiatric or rapidly progressive renal lupus requiring therapy not permitted by the protocol.
• Active, chronic, or recurrent serious infection; evidence of latent tuberculosis without adequate management; active or inadequately treated hepatitis B or hepatitis C; known HIV infection.
• Prior B-cell–depleting therapy within a protocol-defined washout, or persistent peripheral B-cell depletion at screening.
• Receipt of a live or attenuated vaccine within the protocol-defined interval before randomization.
• Clinically significant hypogammaglobulinaemia, cytopenia, or other laboratory abnormality that would place the subject at undue risk.
• Pregnancy or breastfeeding; women of childbearing potential unwilling to use protocol-specified contraception.
• Any condition that, in the investigator's judgement, would compromise subject safety or the interpretation of study data.

4.3 Screening for Latent Infection

Given the infection risk of B-cell depletion, screening includes evaluation for latent tuberculosis and for hepatitis B and hepatitis C, with appropriate management before randomization. Vaccination status is reviewed and updated where feasible before treatment; live/attenuated vaccines are prohibited during the treatment period.

5 Study Intervention

5.1 Investigational Product

OBX-319 is supplied as a sterile solution for subcutaneous injection and administered at the assigned dose level (High-dose or Low-dose) according to the protocol dosing schedule during the 52-week double-blind period, with in-clinic administration and observation at scheduled visits. Matched placebo is administered by the same route and schedule to maintain the blind. Product presentation, storage conditions, preparation, and accountability follow the study pharmacy manual; the drug product is manufactured and controlled consistent with ICH Q6B/Q5C and the Module 3 control strategy.

5.2 Background Standard of Care

All subjects continue background standard-of-care therapy (antimalarials, oral glucocorticoids, and/or conventional immunosuppressants) per protocol. Background therapy is to remain stable during the early controlled interval to allow attribution of treatment effect, after which a protocol-defined glucocorticoid taper toward a prednisone-equivalent target of ≤ 7.5 mg/day is encouraged for subjects with adequate disease control.

5.3 Concomitant and Prohibited Therapy

Medications required for subject safety and management of intercurrent conditions are permitted and recorded. Other B-cell–depleting or B-cell–targeted biologics, investigational agents, and live/attenuated vaccines are prohibited during the treatment period. Changes to background immunosuppression outside protocol-defined windows are discouraged and, where they occur, are documented as they may constitute intercurrent events for the primary estimand.

5.4 Treatment Compliance and Accountability

Study-drug administration is performed or supervised by site staff and documented; drug accountability is reconciled at each relevant visit per the pharmacy manual.

6 Discontinuation and Withdrawal

Subjects may discontinue study intervention for adverse events, intercurrent illness, pregnancy, investigator judgement, or withdrawal of consent. Discontinuation of study intervention does not, by default, require withdrawal from the study; subjects who discontinue treatment are, wherever possible, retained for scheduled assessments and safety follow-up, including monitoring of B-cell repletion and immunoglobulins. Handling of treatment discontinuation for the primary and key secondary endpoints follows the estimand strategy in Section 2 and the statistical analysis plan.

7 Study Assessments and Procedures

The schedule of assessments below summarizes procedures across the study; laboratory handling and sample logistics follow the study laboratory manual.

7.1 Efficacy Assessments

Disease activity is measured with SLEDAI-2K (basis for the primary low-disease-activity endpoint and the key secondary change-from-baseline endpoint), the Physician Global Assessment, and BILAG-2004 for organ-domain activity; together these support the SRI-4 composite response. Disease flare is captured through the treatment period.

7.2 Pharmacokinetic Assessments

Serum OBX-319 concentrations are collected per the schedule of assessments to characterize exposure under target-mediated drug disposition, in which nonlinear, target-driven clearance predominates before, and catabolic clearance after, saturation of the depleting B-cell mass. Concentration data support the population pharmacokinetic and exposure–response analyses described in the clinical pharmacology summaries.

7.3 Pharmacodynamic and Biomarker Assessments

Target engagement is monitored by enumeration of circulating CD19+ B cells by flow cytometry, together with anti-dsDNA titres and complement C3/C4, at Weeks 0, 4, 12, 24, 36, and 52 and during safety follow-up for repletion. These biomarkers provide mechanistically coherent, target-engagement-linked evidence to support interpretation of the efficacy endpoints.

7.4 Immunogenicity Assessments

Anti-drug antibodies (binding and neutralizing) are evaluated using a validated tiered strategy (screening, confirmatory, titre, and neutralizing-antibody tiers). Because immunogenicity can alter the exposure of a TMDD-governed antibody, ADA status is integrated into the population pharmacokinetic and safety analyses.

7.5 Safety Assessments

Safety is assessed through adverse-event and concomitant-medication surveillance, vital signs, physical examination, 12-lead electrocardiography, and clinical laboratory testing (haematology, chemistry, urinalysis), with directed monitoring of infections, serum immunoglobulins, and injection-site/administration reactions as detailed in Section 9.

8 Statistical Considerations

Full detail is provided in the statistical analysis plan (SAP-301); the following summarizes the key features.

8.1 Sample Size and Power

A total of 480 randomized subjects (approximately 160 per arm under the 1:1:1 allocation) provides at least 90% power (power = 0.9) at a two-sided significance level of α = 0.05 to detect the anticipated difference between each active regimen and placebo on the primary low-disease-activity endpoint at Week 52, allowing for the expected non-responder handling of discontinuations and missing assessments.

8.2 Analysis Sets

The Full Analysis Set (all randomized subjects, analysed as randomized) is the primary set for efficacy; the Per-Protocol Set supports sensitivity analyses; the Safety Analysis Set (all randomized subjects who receive at least one dose) is used for safety; and dedicated pharmacokinetic, pharmacodynamic, and immunogenicity sets support those analyses.

8.3 Primary Analysis

The primary responder endpoint (low disease activity, SLEDAI-2K <= 4, at Week 52) is analysed as the proportion of responders per arm, with subjects who discontinue randomized treatment or lack an evaluable Week 52 assessment classified as non-responders in accordance with the primary estimand; risk differences versus placebo are presented with 95% confidence intervals. The continuous change from baseline in SLEDAI-2K is analysed by ANCOVA at Week 52 with treatment and baseline SLEDAI-2K as covariates, yielding least-squares mean changes and treatment differences with 95% confidence intervals.

8.4 Key Secondary Endpoints and Multiplicity

Type I error across the primary and key secondary endpoints for each active regimen versus placebo is controlled by a pre-specified hierarchical (fixed-sequence) testing procedure, so that formal significance of later endpoints is claimed only if preceding null hypotheses in the sequence are rejected at two-sided α = 0.05.

8.5 Handling of Intercurrent Events and Missing Data

Intercurrent events are handled consistent with the estimand definitions in Section 2 (composite/non-responder handling for the binary primary endpoint). Sensitivity and supplementary analyses under alternative missing-data assumptions are pre-specified in the SAP to assess robustness of the primary conclusions.

8.6 Interim Monitoring and Data Safety Monitoring Board

An independent DSMB reviews accumulating safety data at pre-specified intervals per its charter. No formal interim efficacy analysis for early stopping is planned unless specified in the SAP; any unblinded review is conducted through the DSMB with appropriate firewalls to preserve the integrity of the double blind.

9 Safety Management and Risk Mitigation

The safety strategy is directed at the identified and potential risks of B-cell–depleting therapy.

• Serious and opportunistic infections (important identified risk). Screening for latent tuberculosis and for hepatitis B and hepatitis C is required before randomization; live/attenuated vaccines are prohibited during treatment; infection surveillance continues throughout, with prompt evaluation and management of suspected serious or opportunistic infection.
• Hypogammaglobulinaemia (important identified risk). Serum immunoglobulins (IgG, IgM, IgA) are monitored during treatment and follow-up; persistent decline below the lower limit of normal is managed per protocol and captured in pharmacovigilance.
• Injection/administration and hypersensitivity reactions (expected). Subcutaneous administration is performed with on-site observation; local and systemic administration reactions are recorded and managed; provisions for treatment of hypersensitivity are in place.
• Immunogenicity (expected). Anti-drug antibodies are monitored as in Section 7.4, with assessment of any impact on exposure, efficacy, and safety.
• Haematological effects. Absolute neutrophil counts and other haematology parameters are monitored given the potential for cytopenias, including late-onset neutropenia, with B-cell–depleting antibodies.
• Cardiac safety. Routine 12-lead ECG monitoring is performed. Consistent with ICH guidance for biotechnology-derived products, a dedicated thorough-QT study and hERG assessment are not warranted for a monoclonal antibody of this class.
• Malignancy. Given the immunomodulatory mechanism, malignant events are monitored and malignancy is retained as a potential long-term risk for continued and post-marketing follow-up.

Adverse events are collected, graded for severity, and assessed for causality; serious adverse events are reported on an expedited basis in accordance with ICH E2A and the study safety management plan. The DSMB provides independent safety oversight per its charter.

10 Ethics and Regulatory Compliance

The study is conducted in accordance with the ethical principles of the Declaration of Helsinki and with Good Clinical Practice under ICH E6(R3), applicable ICH general-considerations guidance (ICH E8(R1)), and local regulatory requirements. The protocol, informed-consent form, and other required documents are reviewed and approved by the responsible Institutional Review Board/Independent Ethics Committee before initiation, and written informed consent is obtained from each subject before any study-specific procedure. OBX-319 is regulated as a biological product, and the marketing application is submitted as a Biologics License Application (BLA) under 21 CFR Part 601.

11 Data Handling and Quality

Data are captured, monitored, and managed under the study data-management and monitoring plans, with source-data verification, medical coding, and database quality control per those plans. Quality management follows a risk-based approach consistent with ICH E6(R3), and protocol deviations are recorded and reviewed. Records are retained per regulatory requirements.

12 References and Cross-References

Cross-references: Investigator's Brochure (clinical and nonclinical background); statistical analysis plan SAP-301 (analysis detail); randomization/IWRS plan (allocation and blinding); study pharmacy manual (investigational-product handling); study laboratory manual (sample logistics); safety management plan and DSMB charter (safety oversight); informed-consent form; Module 2.4/2.6 (nonclinical), Modules 2.7 (clinical summaries), and Modules 2.3/3 (quality).

ICH E6(R3); ICH E8(R1); ICH E9(R1); ICH E2A; ICH S6(R1); ICH Q5A(R2); ICH Q5C; ICH Q6B; Declaration of Helsinki; 21 CFR Part 601.