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Module 2.6 — Nonclinical Summaries (OBX-319)

Document ID: M26
Version: 1.0
Change History: 1.0 — Initial issue.
Standard(s): ICH M3(R2), S6(R1), S5(R3), S7A; S2/S1/S7B addressed by waiver rationale

2.6 Nonclinical Written & Tabulated Summaries — OBX-319

These written summaries (2.6.2 Pharmacology, 2.6.4 Pharmacokinetics, 2.6.6 Toxicology) and their companion tabulated summaries (2.6.3/2.6.5/2.6.7) integrate the nonclinical evidence characterising OBX-319 in support of the marketing application and of the pivotal study OBX319-301. OBX-319 is a humanised IgG1 bispecific monoclonal antibody in which one Fab arm engages CD19 and the second engages CD20, directed at B-cell depletion for the treatment of moderate-to-severe active Systemic Lupus Erythematosus (SLE) and administered subcutaneously. The molecule is produced by recombinant Chinese hamster ovary (CHO) cell culture and purified by a Protein A capture and polishing downstream train; drug substance and drug product are manufactured and controlled per the quality guidance (ICH Q5A(R2), Q5C, Q6B) described in Module 3. The programme was designed under the biologics framework of ICH S6(R1): the cynomolgus monkey is the sole pharmacologically relevant toxicology species (no rodent cross-reactivity to the human CD19/CD20 epitopes), and standard genotoxicity, rodent carcinogenicity, and hERG/thorough-QT assessments were not conducted, with the scientific justification given in the relevant sub-sections below and in Module 2.4. Detailed study reports reside in Modules 4.2.1 (pharmacology), 4.2.2 (pharmacokinetics), and 4.2.3 (toxicology).

2.6.2 Pharmacology Written Summary

Primary and secondary pharmacodynamics and safety pharmacology for the bispecific antibody are summarised here; detailed reports are in Module 4.2.1.

2.6.2.1 Brief Summary

OBX-319 depletes B lymphocytes by simultaneously engaging two lineage-restricted surface antigens, CD19 and CD20, through a single bispecific IgG1. Retention of a fully effector-competent human IgG1 Fc enables antibody-dependent cell-mediated cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC) as the depletion mechanisms. In vitro potency, dual simultaneous binding, and effector function were characterised, and in vivo pharmacology (peripheral and tissue B-cell depletion) was demonstrated in the cynomolgus monkey. The mechanism provides the biological rationale for the reduction in autoantibody burden and normalisation of complement observed clinically in SLE.

2.6.2.2 Primary Pharmacodynamics

Molecular mechanism and target rationale. CD19 and CD20 are expressed across largely overlapping but non-identical windows of B-cell ontogeny. CD19 is expressed broadly from the pro-B stage through to the plasmablast, including populations that down-regulate or lack CD20, whereas CD20 is expressed from the late pre-B stage across mature naïve, memory, and germinal-centre B cells but is absent on the earliest progenitors and on terminally differentiated plasma cells. By co-targeting both antigens with a single molecule, OBX-319 is designed to achieve broader coverage of the pathogenic B-cell compartment and to reduce the potential for antigen-escape that can limit monospecific depletion. Simultaneous bivalent-bispecific engagement on the same cell increases functional avidity and stabilises the immune synapse that recruits Fc effector mechanisms.

Effector function. OBX-319 carries a native, glycosylated human IgG1 Fc; effector competence is deliberately retained because cell killing is the therapeutic mechanism (in contrast to Fc-silenced designs). The N-glycosylation profile, which modulates Fcγ-receptor engagement and therefore ADCC potency, is a critical quality attribute controlled in Module 3. Depletion is mediated by ADCC via FcγRIIIa (CD16) on natural-killer cells, ADCP by tissue macrophages, and CDC via C1q recruitment.

In vitro characterisation. Binding to human CD19 and to human CD20 was quantified by surface plasmon resonance for each arm, and true simultaneous dual antigen binding was confirmed. Functional potency was established in Fcγ-receptor and complement reporter assays, in primary natural-killer-cell ADCC assays, and in whole-blood and peripheral-blood-mononuclear-cell B-cell depletion assays, demonstrating concentration-dependent, near-complete depletion of target B cells.

In vivo pharmacology. In the cynomolgus monkey, the pharmacologically relevant species, OBX-319 produced near-complete depletion of circulating B cells and marked depletion of B-cell zones (germinal centres and follicles) in spleen, lymph nodes, and gut-associated lymphoid tissue, consistent with the intended mechanism. Because CD19/CD20 epitope recognition is species-restricted, disease models were interpreted within this constraint and the monkey served as the primary in vivo pharmacology and safety system.

Translational relevance. The nonclinical mechanism is corroborated by the clinical pharmacodynamic response in OBX319-301: near-complete peripheral CD19+ B-cell depletion on the active arms (approximately 210 to approximately 7 cells/µL) versus no meaningful change on placebo, accompanied by falling anti-dsDNA antibody titres and normalisation of complement C3/C4 in responders. These readouts confirm that peripheral and tissue B-cell depletion translates into the expected reduction in autoantibody production and immune-complex-driven complement consumption that underlies disease activity in SLE.

2.6.2.3 Secondary Pharmacodynamics

Tissue cross-reactivity was assessed by immunohistochemistry on a panel of human tissues, with a corresponding cynomolgus monkey panel; membrane staining was confined to the expected B-lymphoid distribution of CD19 and CD20, with no unexpected reactivity in non-lymphoid tissues. The potential for cytokine release was evaluated in vitro in human whole-blood and peripheral-blood-mononuclear-cell systems, appropriate to a B-cell-engaging bispecific antibody; the results informed the starting-dose and monitoring strategy for first-in-human use. No off-target pharmacology of clinical concern was identified.

2.6.2.4 Safety Pharmacology

Consistent with ICH S6(R1), core safety-pharmacology endpoints were integrated into the repeat-dose toxicity studies in the cynomolgus monkey rather than conducted as standalone studies. Cardiovascular assessment (blood pressure, heart rate, and quantitative electrocardiography including corrected QT), respiratory assessment (rate and pattern), and central-nervous-system observations (functional/neurobehavioural evaluation) were captured. A dedicated hERG assay and a standalone thorough-QT study were not performed: a monoclonal antibody does not interact with cardiac ion channels, and this ICH S7B/E14 waiver is scientifically justified for the modality, with cardiovascular safety instead addressed within the in vivo monkey studies.

2.6.2.5 Pharmacodynamic Drug Interactions

No mechanism-based pharmacodynamic drug interactions are expected for an antibody acting through B-cell surface antigens. In the clinical setting OBX-319 is administered on background standard-of-care; the additive immunomodulatory potential of concomitant immunosuppressants is addressed in the clinical modules and the risk-management plan rather than through nonclinical interaction studies, which are not warranted for this modality.

2.6.4 Pharmacokinetics Written Summary

Subcutaneous absorption with typical IgG bioavailability; target-mediated drug disposition (TMDD) producing non-linear PK at low concentrations; distribution largely confined to plasma and interstitial fluid; elimination by proteolytic catabolism and (in the target-mediated component) receptor-mediated clearance. Classical small-molecule ADME (mass balance, CYP/transporter) is not applicable to an intact IgG.

2.6.4.1 Brief Summary

The disposition of OBX-319 is that of a typical humanised IgG1 subject to target-mediated drug disposition. Subcutaneous dosing gives IgG-typical bioavailability with a delayed peak; systemic clearance is governed by two parallel routes — non-specific proteolytic catabolism, slowed by neonatal-Fc-receptor (FcRn)-mediated salvage that confers a long terminal half-life, and a saturable, target-mediated component that predominates at low concentrations and diminishes as the B-cell target pool is depleted. The cynomolgus monkey provided the nonclinical pharmacokinetic and toxicokinetic basis for human exposure projection.

2.6.4.2 Methods of Analysis

Serum concentrations of OBX-319 were measured with a validated ligand-binding assay (immunoassay format). Anti-drug antibodies were evaluated with a validated tiered strategy (screening, confirmatory, and titre assays, with a neutralising-antibody assay), enabling interpretation of exposure in the context of immunogenicity. Assay performance and validation follow the bioanalytical principles applied across the programme.

2.6.4.3 Absorption

Following subcutaneous administration, absorption occurs largely via the lymphatic route, yielding bioavailability in the range typical of therapeutic IgG antibodies and a peak concentration reached over several days. Exposure increased with dose; consistent with target-mediated disposition, exposure rose less than dose-proportionally at low concentrations and approached dose-proportionality once target-mediated clearance was saturated.

2.6.4.4 Distribution

Distribution was confined largely to the plasma and interstitial fluid, with a volume of distribution approximating plasma volume plus interstitial space, as expected for a large, hydrophilic IgG that does not readily cross intact cell membranes. Radiolabelled tissue-distribution studies are not scientifically informative for an intact antibody and were not conducted; tissue exposure is instead inferred from the mechanism and from lymphoid pharmacodynamic effects.

2.6.4.5 Metabolism

OBX-319 is eliminated by catabolism to peptides and amino acids through proteolytic degradation in the reticuloendothelial system and within target cells after internalisation, with FcRn-mediated recycling protecting a fraction of the antibody from degradation. Cytochrome-P450 and phase-II metabolism and transporter-mediated processes are not involved and were not studied, in line with ICH S6(R1) for an intact immunoglobulin.

2.6.4.6 Excretion

Intact OBX-319 is not cleared renally (its molecular size precludes glomerular filtration) and is not subject to appreciable biliary excretion; catabolic products (amino acids and small peptides) enter endogenous pools. Dedicated mass-balance/excretion studies are therefore not applicable.

2.6.4.7 Pharmacokinetic Drug Interactions

No cytochrome-P450- or transporter-based drug-drug interactions are anticipated for an antibody. The principal determinant of non-linearity is the target (B-cell) burden itself: as circulating and tissue B cells are depleted, the target-mediated clearance component declines and total clearance shifts toward the slower catabolic route. Anti-drug antibodies, where they develop, may accelerate clearance and reduce exposure and were monitored to support interpretation of the pharmacokinetic and toxicokinetic data.

2.6.4.8 Other Pharmacokinetic Studies

Toxicokinetics were integrated into the pivotal repeat-dose toxicity studies to define systemic exposure (AUC and Cmax) at the no-observed-adverse-effect level; these exposures underpin the interspecies margin calculations reported in Module 4.2.3. Cynomolgus monkey pharmacokinetics were used to project human exposure and to support the subcutaneous clinical dose selection evaluated in OBX319-301.

2.6.6 Toxicology Written Summary

Repeat-dose toxicity was conducted in cynomolgus monkey as the sole pharmacologically relevant species (no rodent cross-reactivity); the integrated NOAEL and margins are in Module 4.2.3. Tabulated summaries (2.6.3/2.6.5/2.6.7) accompany each.

2.6.6.1 Brief Summary

The pivotal toxicology programme comprised GLP repeat-dose toxicity with integrated toxicokinetics and safety-pharmacology endpoints in the cynomolgus monkey (up to 26 weeks, subcutaneous clinical route), an enhanced pre- and post-natal development (ePPND) study, a tissue cross-reactivity study on human and monkey tissues, and local-tolerance evaluation within the repeat-dose studies. Findings were dominated by the expected, on-target pharmacology — B-cell depletion and its immunological sequelae — with no unexpected target-organ toxicity. Genotoxicity, rodent carcinogenicity, and hERG/thorough-QT studies were not conducted, as justified below.

2.6.6.2 Single-Dose Toxicity

A standalone single-dose lethality study was not conducted, consistent with ICH S6(R1) and M3(R2) for biologics; acute tolerability was characterised from the initial (first-dose) observations and toxicokinetics within the repeat-dose studies, where OBX-319 was well tolerated at the doses evaluated.

2.6.6.3 Repeat-Dose Toxicity

In the cynomolgus monkey, subcutaneous OBX-319 for up to 26 weeks produced findings that were an extension of its intended pharmacology: near-complete depletion of peripheral B lymphocytes and corresponding depletion of B-cell-dependent zones (germinal centres and follicles) in spleen, lymph nodes, and gut-associated lymphoid tissue, with associated reductions in serum immunoglobulins (hypogammaglobulinaemia). T-cell and other haematological lineages were preserved, and there was no drug-related organ toxicity outside the immune system. During the recovery phase, peripheral B cells and immunoglobulins showed partial-to-substantial recovery, indicating reversibility once drug exposure declined and B-cell reconstitution resumed. The theoretical increase in susceptibility to infection accompanying B-cell depletion was monitored. The no-observed-adverse-effect level and the AUC/Cmax exposure margins over the clinical dose levels evaluated in OBX319-301 (OBX-319 High, OBX-319 Low, Placebo) are presented in Module 4.2.3.

2.6.6.4 Genotoxicity

Genotoxicity studies were not conducted. As a large recombinant protein, OBX-319 does not interact directly with DNA, and standard ICH S2 assays are neither applicable nor scientifically informative for this modality, consistent with ICH S6(R1).

2.6.6.5 Carcinogenicity

A conventional rodent carcinogenicity bioassay was not conducted; carcinogenic potential was addressed through a weight-of-evidence carcinogenicity risk assessment appropriate to a targeted biologic per ICH S6(R1). The assessment considered the immunomodulatory mechanism, the theoretical impact of sustained B-cell depletion on immune surveillance, the absence of any proliferative or pre-neoplastic finding in the chronic monkey study, and the accumulated class experience with B-cell-depleting therapeutics. A standard two-year rodent study would not have been scientifically meaningful given the lack of rodent pharmacological relevance.

2.6.6.6 Reproductive and Developmental Toxicity

An enhanced pre- and post-natal development (ePPND) study was conducted in the cynomolgus monkey in accordance with ICH S5(R3) and S6(R1). Because IgG placental transfer is greatest during the second and third trimesters, the design evaluated the potential for fetal and neonatal B-cell depletion; infant peripheral B-cell counts, lymphoid development, and post-natal reconstitution were assessed, together with maternal exposure and pregnancy outcomes. Effects on male and female reproductive organs were evaluated by histopathology within the repeat-dose studies; dedicated fertility studies are not required for this modality. There is no small-molecule-type embryotoxic or DNA-reactive concern for an antibody; the reproductive risk is confined to the expected on-target pharmacology in the developing immune system, which is reflected in the labelling and risk-management documentation.

2.6.6.7 Local Tolerance

Local tolerance at the subcutaneous injection site was evaluated within the repeat-dose toxicity studies rather than as a standalone study. Injection-site reactions are an expected consequence of subcutaneous antibody administration and were characterised for severity and reversibility to support the clinical presentation.

2.6.6.8 Other Toxicity Studies

Additional supportive assessments comprised the tissue cross-reactivity study (human and cynomolgus monkey tissue panels, confirming B-lymphoid-restricted binding with no unexpected reactivity), the in vitro cytokine-release evaluation, and immunogenicity monitoring. Anti-drug antibodies in the monkey were interpreted only for their effect on exposure and toxicokinetics; animal immunogenicity is not predictive of the clinical immunogenicity of a humanised antibody, and human immunogenicity is characterised in the clinical modules.

2.6.6.9 Discussion and Conclusions

The integrated nonclinical assessment supports the safety of OBX-319 at the clinical doses evaluated in OBX319-301. The toxicological profile is an extension of the intended pharmacology, with B-cell depletion, hypogammaglobulinaemia, and a consequent potential for serious and opportunistic infection identified as the key mechanism-based risks of the class; injection-site reactions and immunogenicity are expected. These risks are reversible in the nonclinical setting and are carried into the clinical risk-management strategy. No target-organ toxicity independent of the pharmacology, and no genotoxic, cardiac-electrophysiological, or unexpected reproductive signal, was identified. Adequate systemic-exposure margins at the cynomolgus monkey no-observed-adverse-effect level relative to the projected human exposures justify the doses studied, and no boxed warning is indicated for the class.

Guidelines: ICH M3(R2), S6(R1), S5(R3), S7A; S2/S1/S7B addressed by waiver rationale.