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Human PD & QT Study Reports (OBX-319)

Document ID: CLINPHARM-003
Version: 1.0
Change History: 1.0 — Initial issue.
Standard(s): ICH E14/S7B

Human Pharmacodynamic & QT Study Reports — OBX-319

This report summarises the human pharmacodynamic (PD) and cardiac-repolarisation data for OBX-319, a humanized bispecific IgG1 monoclonal antibody that simultaneously engages CD19 and CD20 on the B-cell surface to produce deep B-cell depletion. OBX-319 is expressed in recombinant Chinese hamster ovary (CHO) cell culture, purified by a Protein A capture and polishing downstream train, and administered by subcutaneous (SC) injection. The pharmacodynamic characterisation centres on the mechanistic biomarker of the modality — circulating CD19+ B-cell depletion — together with the disease-relevant serologic markers (anti-dsDNA autoantibody and complement C3/C4) and the exposure–response relationship supporting the doses evaluated in the pivotal Phase 3 study OBX319-301 (randomized, double-blind, placebo-controlled, 1:1:1, 52 weeks; N = 480 [162 OBX-319 High, 158 OBX-319 Low, 160 Placebo]) in moderate-to-severe active SLE (baseline SLEDAI-2K ~11) on background standard of care. For a bispecific antibody, a dedicated thorough-QT study is generally not warranted; cardiac safety is addressed by concentration-QTc analysis and the clinical safety database. ICH E14/S7B; E14/S7B Q&A.

Overview and organisation of the pharmacodynamic program (CTD 5.3.4)

For an intact immunoglobulin, pharmacodynamics are defined by cellular target engagement and its downstream immunologic consequences rather than by a small-molecule receptor-occupancy surrogate. The primary PD biomarker for OBX-319 is the peripheral CD19+ B-cell count measured by a validated flow-cytometry method; the secondary PD markers are the SLE disease-activity serologies (anti-dsDNA autoantibody titre and complement components C3 and C4) and longitudinal serum immunoglobulins (IgG/IgM/IgA). Because the pharmacologic action of a chronic B-cell-depleting agent is immunomodulatory and would be neither interpretable nor ethically appropriate to induce in healthy volunteers, the human pharmacodynamics were characterised principally in the target patient population; freestanding healthy-subject pharmacodynamic reports (5.3.4.1) are therefore not a feature of the programme, and the substantive PD data reside in the patient studies (5.3.4.2). Pharmacodynamic samples from the Phase 1/2 experience and from OBX319-301 were integrated with the pharmacokinetic data into the population PK/PD and exposure–response analyses summarised in Module 2.7.2 and supported by the human PK study reports (5.3.3). Bioanalytical (drug and anti-drug-antibody) methods were validated in accordance with ICH M10.

Primary pharmacodynamic marker — CD19+ B-cell depletion

Circulating CD19+ B-cell counts were the primary pharmacodynamic biomarker. CD19 was selected as the enumeration antigen because it is expressed continuously across the B-cell maturation window and, importantly, because CD20-directed engagement by the drug can mask the CD20 epitope and confound CD20-based cell counting; a CD19 gate therefore provides an unambiguous measure of B-cell depletion during therapy. Both active regimens produced near-complete peripheral B-cell depletion (approximately 210 → approximately 7 cells/µL), whereas counts on placebo were essentially unchanged. The magnitude and consistency of depletion across the High and Low dose levels are the direct clinical readout of the dual-targeting mechanism: simultaneous engagement of CD19 and CD20 broadens coverage to B-cell subsets that can escape single-antigen therapy — including CD20-low or CD20-negative plasmablasts and CD19-expressing early B-lineage and antibody-secreting cells — yielding deeper and more uniform depletion together with redundancy against antigen-loss escape. Depletion is effected through the native, glycosylated human IgG1 Fc (antibody-dependent cellular cytotoxicity and phagocytosis and complement-dependent cytotoxicity), the mechanistic basis for which is presented in the nonclinical pharmacology (Module 4.2.1).

Secondary pharmacodynamic and disease-activity biomarkers

In association with clinical response, anti-dsDNA autoantibody titres declined and the complement components C3 and C4 normalised, with no comparable change on placebo. These shifts are consistent with interruption of autoreactive B-cell drive and with reduced immune-complex-mediated complement consumption, and — because anti-dsDNA and low complement are established components of SLE disease-activity indices — they provide the serologic bridge from B-cell depletion to the SLEDAI-2K improvement observed clinically. Serum immunoglobulins were monitored longitudinally because sustained B-cell depletion carries a class risk of hypogammaglobulinaemia: IgM (drawn from short-lived, rapidly turning-over B-cell output) is expected to fall earliest and furthest, whereas IgG is relatively preserved in the near term owing to sparing of CD20-negative/CD19-low long-lived plasma cells, with cumulative decline reflecting attrition of the replenishing memory-B/plasmablast pool during prolonged therapy. Immunoglobulin trajectories were therefore interpreted alongside the identified risks of serious and opportunistic infection and hypogammaglobulinaemia.

Time course, magnitude, and reversibility of the pharmacodynamic effect

Depletion was rapid, with onset within days to a few weeks of the first SC doses, and was sustained throughout the dosing period on both active arms. Because CD19 and CD20 are absent from haematopoietic stem cells and early progenitors, the depleting effect spares the regenerative compartment, and peripheral B-cell repopulation is expected upon treatment cessation as marrow output resumes. This reversibility provides the pharmacodynamic basis for infection-risk mitigation (including immunoglobulin surveillance and management of B-cell reconstitution) and for re-treatment strategies, and it distinguishes the anticipated recovery profile from an irreversible ablative intervention.

Exposure–response (PK/PD) for the Week 52 clinical endpoint

The exposure–response analysis supports the dose selection for SRI-4 response at Week 52 (operationalised as low disease activity, SLEDAI-2K <= 4). Both dose levels separated clearly from placebo: 52.4% (76/145) for OBX-319 High, 33.8% (49/145) for OBX-319 Low, and 6.0% (9/150) for Placebo. The corresponding LS-mean change from baseline in SLEDAI-2K was -6.37 (High), -5.62 (Low), and -3.46 (Placebo), giving placebo-adjusted differences of -2.91 and -2.17. Although peripheral B-cell depletion was near-maximal at both dose levels, the dose-ordered clinical benefit indicates that maintenance of adequate systemic exposure and depletion of tissue and germinal-centre B-cell compartments — rather than the peripheral B-cell count alone — drive the incremental response at the High dose, consistent with the target-mediated (TMDD) disposition in which the target sink governs low-concentration clearance. Taken together, the exposure–response relationship supports selection of the OBX-319 High dose for the intended indication. Exposure was also examined against the class-relevant identified risks (serious and opportunistic infection, hypogammaglobulinaemia) and against injection/infusion reactions and immunogenicity; no exposure threshold indicating an unfavourable benefit–risk shift within the studied range was identified.

Effect of anti-drug antibodies on pharmacodynamics

Anti-drug antibodies (ADA) can increase clearance and reduce exposure and, at sufficient titre or with neutralising capacity, attenuate or abrogate the B-cell-depleting pharmacodynamic effect. Because OBX-319 is a bispecific, ADA responses were characterised against each binding domain/idiotype (the anti-CD19 and anti-CD20 arms) and against potential novel junctional epitopes at the bispecific interface, and their impact on the PD biomarker was assessed jointly with the exposure data. Notably, profound B-cell depletion may itself blunt de novo humoral ADA formation, a factor considered when interpreting observed immunogenicity rates and their pharmacodynamic consequences. The tiered, validated immunogenicity assay strategy and the integrated ADA impact assessment are detailed in the immunogenicity summary (IMMUNO-001) and Module 2.7.2.

Pharmacodynamic interactions — vaccines and concomitant therapy

The principal extrinsic pharmacodynamic consideration is attenuation of humoral responses to vaccines during B-cell depletion; live and live-attenuated vaccines are to be avoided during therapy, and clinically indicated immunisations are best completed before initiation where feasible. Additive immunomodulation with background standard-of-care immunosuppressants was considered in the interpretation of infection-related pharmacodynamic risk. As an intact IgG1 that is not eliminated by cytochrome P450 enzymes or hepatic/renal transporters, OBX-319 has a low potential for classical pharmacokinetic drug–drug interactions, so the relevant interactions for this modality are pharmacodynamic (immunologic) rather than metabolic.

Cardiac electrophysiology and QT/QTc assessment

Twelve-lead electrocardiograms (ECGs) were collected at scheduled visits across the clinical programme, including OBX319-301, and were evaluated by central-tendency and categorical-outlier analyses of QT, heart-rate-corrected QTc (QTcF), heart rate, PR, and QRS. A concentration–QTc analysis relating OBX-319 plasma concentration to change-from-baseline QTcF (ΔQTcF) did not identify an exposure-dependent effect on cardiac repolarisation, and the ECG data showed no clinically relevant central-tendency change and no pattern of new outliers; the results were consistent with the absence of an effect approaching the 10-ms threshold of regulatory concern under ICH E14. These clinical observations are supported nonclinically by cardiovascular safety-pharmacology endpoints integrated into the GLP repeat-dose toxicity studies in the cynomolgus monkey — the sole pharmacologically relevant species — assessed by jacketed external telemetry (arterial blood pressure, heart rate, and quantitative electrocardiography including PR, QRS, RR, QT, and QTc), in which no test-article-related cardiovascular effects were identified (Module 4.2.1). No proarrhythmia signal was observed in the clinical or nonclinical data.

Rationale for the absence of a dedicated thorough-QT and in vitro hERG study

A dedicated thorough-QT (TQT) study and a nonclinical in vitro hERG (IKr) assay are not warranted for OBX-319. A large, hydrophilic IgG of approximately 150 kDa does not distribute intracellularly, does not access cardiac ion channels, and has no structural basis for direct channel block; the ICH S7B in vitro electrophysiology and integrated cardiac-risk framework is therefore not applicable to a monoclonal antibody, and the ICH E14/S7B question-and-answer framework supports that a dedicated TQT study is not needed for biologics of this class in the absence of a mechanistic cardiac liability. Cardiac safety is instead addressed by the routine 12-lead ECG surveillance and concentration–QTc analysis embedded in the clinical programme and by the QT/QTc endpoints captured in the cynomolgus telemetry. Consistent with the modality and with ICH S6(R1), the broader nonclinical waiver rationale (no genotoxicity or carcinogenicity assessment) is presented in the nonclinical documentation and is not a cardiac-safety concern for this product.

Conclusions

The human pharmacodynamics of OBX-319 are governed by near-complete, dose-consistent depletion of circulating CD19+ B cells (approximately 210 → approximately 7 cells/µL on both active arms versus unchanged placebo), accompanied by concordant improvement in the SLE disease-activity serologies (falling anti-dsDNA and normalising C3/C4) and interpreted against the monitored risk of hypogammaglobulinaemia. The exposure–response analysis links these pharmacodynamic effects to the Week 52 clinical endpoint (SLEDAI-2K <= 4: 52.4% High, 33.8% Low, 6.0% Placebo; LS-mean SLEDAI-2K change -6.37 / -5.62 / -3.46) and supports selection of the OBX-319 High dose. Cardiac-repolarisation assessment by routine ECG, concentration–QTc analysis, and nonclinical telemetry showed no signal of QTc prolongation, and a dedicated thorough-QT study and in vitro hERG assay are not warranted for a monoclonal antibody of this class.

Guidelines: ICH E14/S7B; E14/S7B Q&A; ICH M10; S6(R1) and S7A/B (as applicable to the modality).