A first‐in‐human, randomized study of the safety, pharmacokinetics and pharmacodynamics of povetacicept, an enhanced dual BAFF/APRIL antagonist, in healthy adults

Abstract

Therapeutic agents targeting the tumor necrosis factor (TNF) superfamily cytokines B‐cell activating factor (BAFF, BLyS) and/or A PRoliferation Inducing Ligand (APRIL) have demonstrated clinical effectiveness in multiple autoimmune diseases, such as systemic lupus erythematosus, lupus nephritis, and immunoglobulin A nephropathy (IgAN). However, their clinical utility can often be limited by incomplete and/or prolonged times to clinical response and inconvenient dosing regimens, which may be improved by more potent dual inhibition of both cytokines. Povetacicept (ALPN‐303; TACI vTD‐Fc) is a crystallizable fragment (Fc) fusion protein of an engineered transmembrane activator and CAML interactor (TACI) domain which mediates more potent inhibitory activity than wild‐type TACI‐Fc or BAFF‐ or APRIL‐specific antibodies and demonstrates superior pharmacokinetic and pharmacodynamic activity in multiple preclinical disease models. In this first‐in‐human study in healthy adults, povetacicept was well‐tolerated as single ascending doses of up to 960 mg administered intravenously or subcutaneously. Dose‐dependent pharmacokinetics were observed. Coverage of BAFF and APRIL was observed for 2–3 weeks and ≥4 weeks after doses of 80 mg and ≥240 mg, respectively. Maximal pharmacodynamic effects were observed at dose levels ≥80 mg for a single dose, associated with on‐target reductions in antibody‐secreting cells as well as in all circulating immunoglobulin isotypes, including the IgAN disease‐related biomarker galactose‐deficient‐immunoglobulin A1 (Gd‐IgA1), and were superior to results reported for wild‐type TACI‐Fc. These data strongly support further development of povetacicept for the treatment of B‐cell‐mediated automimmune diseases.

Study Highlights.

• WHAT IS THE CURRENT KNOWLEDGE ON THE TOPIC?

Inhibitors of BAFF and/or APRIL have demonstrated promising results in various autoimmune diseases, but there remains room for improvement. Povetacicept (ALPN‐303; TACI vTD‐Fc) was engineered via directed evolution for enhanced dual BAFF/APRIL inhibition and has demonstrated activity superior to various B‐cell modulators in preclinical disease models, suggesting the possibility of a more convenient, yet more effective, dosing regimen.

• WHAT QUESTION DID THIS STUDY ADDRESS?

This study provides proof of mechanism for povetacicept and provides robust data relating PK and PD (e.g., reduction of immunoglobulins and antibody‐secreting cells), as well as data on initial safety and tolerability.

• WHAT DOES THIS STUDY ADD TO OUR KNOWLEDGE?

A strong relationship between povetacicept PK, serum Ig (including GdIgA1) reduction, and B‐cell modulation was observed. These results, coupled with initial safety and tolerability data, support further clinical evaluation of povetacicept in autoimmune/inflammatory diseases driven by pathogenic autoantibodies.

• HOW MIGHT THIS CHANGE CLINICAL PHARMACOLOGY OR TRANSLATIONAL SCIENCE?

The results support further clinical development of povetacicept in diseases induced by pathogenic autoantibodies, and the observed dose‐PK–PD relationships will guide dosing regimens in autoimmune and inflammatory diseases.

INTRODUCTION

B‐cell activating factor belonging to the tumor necrosis factor superfamily (TNFSF) (BAFF; also known as B lymphocyte stimulator [BLyS] and TNFSF13B) and A Proliferation Inducing Ligand (APRIL; also known as TNFSF13A) are pleiotropic cytokines which are implicated in the pathogenesis of multiple autoimmune and inflammatory diseases. ¹ , ² BAFF and APRIL interact with 3 TNF receptor superfamily (TNFRSF) members that are expressed at different but partially overlapping stages of B‐cell development: BAFF Receptor (BAFF‐R; TNFRSF13C), Transmembrane activator and calcium modulating ligand Interactor (TACI; TNFRSF13B), and B‐cell maturation antigen (BCMA; TNFRSF17). BAFF and APRIL both bind BCMA and TACI, and BAFF also binds BAFF‐R. ³ Activation of BAFF‐R contributes to survival and maturation of earlier stage transitional and naïve B cells whereas TACI is crucial for T‐cell‐independent B‐cell responses to certain antigens, B‐cell regulation, and immunoglobulin (Ig) class‐switch recombination. ² , ⁴ BCMA, which is upregulated in activated B cells, is important for the long‐term survival of plasma cells. ⁵

In addition to their critical activities in B cells, BAFF, and APRIL also impact T cells, natural killer cells, monocytes, and dendritic cells. ⁶ BAFF can drive increased human CD4+ and CD8+ T‐cell proliferation and survival, modulate TACI and BAFF‐R expression on T cells, ⁷ and promote differentiation and expansion of Th17 cells. ⁸ Monocytes express TACI, but not BAFF‐R or BCMA, and BAFF can enhance survival and promote upregulation of TACI and CD14 on these cells. ⁹ BAFF can also promote dendritic cell maturation. ¹⁰

BAFF, APRIL, and/or their receptors are also reportedly expressed by some non‐hematopoietic cells, including adipocytes, ¹¹ keratinocytes, ¹² neurites, ¹³ and renal cells like glomerular mesangial cells and podocytes, ¹⁴ , ¹⁵ , ¹⁶ though their precise roles in these cell types are the subject of ongoing investigation. The expression of BAFF and APRIL increases under proinflammatory conditions, ¹⁷ and elevated serum levels of these cytokines have been correlated with disease severity in patients with B‐cell‐related autoimmune disease. ² , ³ , ¹⁸ , ¹⁹

Inhibitors of BAFF and/or APRIL have proven effective for the treatment of a variety of autoimmune or other B‐cell‐related diseases. An anti‐BAFF antibody, belimumab, has been approved in the United States, the European Union, and other regions/countries for the treatment of patients with systemic lupus erythematosus (SLE) or lupus nephritis (LN) who are receiving standard therapy (BENLYSTA® package insert, GlaxoSmithKline 2024) and telitacicept has been approved for SLE in China (telitacicept package insert, Rongchang Biopharmaceutical [Yantai] Co., Ltd. 2021). ²⁰ Favorable clinical data in IgA nephropathy (IgAN) have been observed with multiple inhibitors of BAFF and/or APRIL, including the APRIL‐only inhibitors zigakibart ²¹ and sibeprenlimab ²² ; the BAFF‐only inhibitor blisibimod ²³ ; and the wild‐type (WT) TACI‐Fc fusion proteins atacicept ²⁴ and telitacicept. ²⁵ Except for blisibimod, these inhibitors are under evaluation in phase III studies for IgAN. Several other autoimmune diseases continue to be actively investigated in clinical trials, including Sjögren's syndrome, systemic sclerosis, rheumatoid arthritis, myasthenia gravis, and neuromyelitis optica spectrum disorder.

Povetacicept (ALPN‐303; TACI vTD‐Fc) is a crystallizable fragment (Fc) fusion protein of a human TACI variant TNFR domain (vTD) designed to potently inhibit BAFF and APRIL. It is an Fc dimer of a TACI domain, derived by directed evolution to bind both BAFF and APRIL with high affinity, fused to an effector function negative IgG1 Fc. ²⁶ It has demonstrated activity superior to WT TACI‐Fc; BAFF‐, APRIL‐, or neonatal Fc receptor (FcRn)‐specific inhibitors; and/or anti‐CD20‐mediated B‐cell depletion in head‐to‐head studies in multiple preclinical disease models of autoimmune diseases, including SLE, ²⁶ LN, myasthenia gravis, ²⁷ and autoimmune encephalitis. ²⁸ Based upon these preclinical data, povetacicept has the potential to improve outcomes in patients suffering from severe antibody‐associated autoimmune/inflammatory diseases.

To enable patient‐based clinical studies with povetacicept, a first‐in‐human study was conducted to evaluate its safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) in healthy adult subjects after single intravenous (IV) infusion or subcutaneous (SC) injection.

METHODS

Study design

Overview

This phase I, randomized, double‐blind, placebo‐controlled, single‐ascending dose study in healthy adult subjects (RUBY‐1; NCT05034484) was sponsored by Alpine Immune Sciences, (Alpine) and approved by the Alfred Health Human Research Ethics Committee (Melbourne, Australia) in accordance with the National Statement on Ethical Conduct in Human Research. It was conducted between 01 November 2021 and 27 February 2023 at Nucleus Network Pty Ltd. and Q‐Pharm Pty Ltd. in compliance with ICH E6(R2), annotated with comments by the Australian Therapeutics Goods Administration (2021).

Subjects

Healthy adult subjects aged 18–65 years, with a body mass index (BMI) between 18 and 32 kg/m² were eligible to participate. All subjects provided written informed consent prior to enrollment. Key exclusion criteria included: any significant concomitant or history of clinically significant disease, condition, or treatment that could interfere with the conduct of the study; history of significant hepatic or renal impairment; or clinically significant abnormalities in laboratory or cardiovascular tests.

Dose regimens

Planned doses were selected based on the predicted human PK profile using PK modeling and allometric scaling from nonclinical PK data in cynomolgus monkeys. The starting dose of 2.4 mg IV was the minimum anticipated biological effect level (MABEL) based on the potential for hypercytokinemia as assessed in an in vitro cytokine release assay. The highest planned dose of 960 mg was estimated to be the maximum necessary dose to maintain target coverage for up to 8 weeks after a single dose. The study enrolled 72 subjects in 11 sequential cohorts consisting of 7 IV and 4 SC single‐ascending dose levels. The number of subjects was selected to allow initial evaluation of safety/tolerability and to be sufficient for PK of the single doses to be administered in this study. The sample size was consistent with standards of practice for phase I studies. The number of subjects per cohort is summarized in Table S2. Additional information regarding subject enrollment is provided in Methods S1.

Safety assessments

Assessments to measure the incidence of treatment‐emergent adverse events (AEs), serious adverse events (SAEs), and adverse events of interest (AEIs); clinically significant abnormal laboratory tests, including clinical chemistry, hematology, coagulation, and urinalysis (central lab); and clinically significant physical examination, vital signs, and electrocardiogram (ECG) abnormalities were conducted throughout the study. Vital signs and ECGs were obtained after the subject had been resting, preferably supine, for ~5 min. AEIs were pre‐defined as severe infections, severe hypogammaglobulinemia (i.e., IgG <3 g/L or <30% of baseline), infusion‐related reactions (including cytokine release syndrome [CRS]), and other AEs as identified by medical review. Exploratory safety laboratory assessments of 16 different circulating cytokines (GM‐CSF, IFNγ, IL‐2, IL‐3, IL‐4, IL‐5, IL‐6, IL‐7, IL‐8, IL‐10, IL‐18, MIP‐1α, MIP‐1β, MCP‐1, TNFα, and TNFβ) were determined via the high‐sensitivity multiplex immunoassay Human HMPCORE1 (Myriad RBM).

PK assessments

Total and free PK assays

Serial dense PK serum samples were collected at pre‐dose (Day 1) and post‐dose up to 28 days, and at 56 days post‐dose. Concentrations of free and total (f/t) povetacicept were measured at a contract laboratory (PPD, Richmond, VA, USA) using validated electrochemiluminescent assays (ECLIA), both with lower limits of quantification (LLOQ) of 5 ng/mL, using two proprietary mouse anti‐TACI antibodies with and without an acid dissociation step, respectively.

PD assessments

Serum immunoglobulins

Total serum Igs (IgA, IgM, IgE, IgG, and IgG subclasses) were determined at ACL Laboratories (Melbourne and Brisbane, Australia). Serum galactose‐deficient IgA1 (Gd‐IgA1) was quantified by a commercially available quantitative enzyme‐linked immunosorbent assay (ELISA; IBL cat #27600) at Alpine (Seattle, WA, USA).

Immunophenotyping of fresh PBMC

Flow cytometric immunophenotyping was performed as described in Methods S1.

Free and total APRIL and BAFF assays

Serum f/t APRIL were determined using commercially available reagents or kits (R&D Systems, cat #DY884B; MSD, cat #D219C‐3). F/t BAFF were quantified by ECLIA using proprietary monoclonal antibodies. Testing was performed at Alpine.

Antidrug antibody assessments

ADA and NAb activity were determined as described in Methods S1.

Non‐compartmental analysis and exploratory population PK modeling

Non‐compartmental analysis (NCA) of the PK parameters for povetacicept were calculated as described in Methods S1.

Statistical analysis

All statistical analyses for disposition, demographics, and safety were performed using SAS, version 9.4 (SAS Institute, Cary, NC, SAS System). For f/t APRIL, f/t BAFF, Gd‐IgA1, and soluble receptors, concentrations were determined using SoftMax Pro software (Version 7.1) with 4PL regression and 1/y² weighting. Values below the LLOQ for free APRIL and BAFF (50 and 391 pg/mL, respectively) were imputed as the LLOQ – 1. Unless otherwise noted, PD datasets were analyzed using GraphPad PRISM® software (Version 10.2.2) to determine change from baseline, mean and standard deviation, and statistical analyses (p‐values <0.05 were considered statistically significant for all tests). For concentration–time analysis, povetacicept PK Geomean concentrations and Geomean SD were also calculated using PRISM®.

RESULTS

Demographics and disposition

A total of 72 healthy adult male (N = 23; 32%) and female (N = 49; 68%) subjects were enrolled, and 72 subjects completed the study. Subject screening began on November 1, 2021, and the last subject last visit for database lock was on 27 February 2023, with final Ig collection on 15 May 2023. Randomization envelopes were maintained by the sites and available in the event that unblinding was needed. No participants were unblinded during the study. The database lock occurred on 14 Mar 2023. A restricted number of Sponsor employees were unblinded prior to database lock due to PK and PD assessments conducted during the study; however, no unblinded PK/PD data were shared with investigators during the first 28 days of the observation period. Demographic and baseline characteristics were well‐balanced between povetacicept and placebo groups (Table S1). Most subjects were Asian (19%) or White (64%) and not Hispanic or Latino (74%). The median age (range) of all subjects was 31.0 (18–64) years, and the mean (SD) height was 168.2 (8.28) cm, body weight was 70.2 (12.27) kg, and BMI was 24.7 (3.28) kg/m².

Safety and tolerability

Povetacicept was generally well‐tolerated at all single IV or SC doses. Table S3 presents a summary of AEs reported during the study; a complete listing of treatment‐emergent AEs is provided in Table S4. No deaths, life‐threatening AEs, grade >3 AEs, serious infections, infusion‐related reactions, injection site reactions following SC administration, or SAEs occurred during the study. No events of cytokine release syndrome were reported, and no study‐drug related elevations in circulating cytokines were observed during serial assessments. The most common non‐Ig AEs observed in subjects treated with povetacicept included: headache or migraine (24% of subjects receiving povetacicept); infections (24%); or dizziness (including dizziness, dizziness postural, presyncope, or vertigo; 12%). The majority of the AEs were considered not related to study drug by the investigator (Tables S3 and S4), and most were grade 1 in severity. Grade 2 infections were reported in 8 (16%) of povetacicept‐treated subjects, including four events of upper respiratory tract infection (URTI) and one each of COVID‐19, staphylococcal infection, urinary tract infection, and varicella Zoster virus infection; in contrast to 2 (9%) of placebo subjects, including 1 each of COVID‐19 and URTI. No dose‐related infection risk can be concluded; however, with the most occurring at the 80 mg dose level (Table S4). Two grade 3 AEs were reported; both were blood creatine phosphokinase increased (1 subject each in the 960 mg SC povetacicept and placebo IV groups) and considered not related to study drug by the investigator. The most frequently reported treatment‐related AE was decreased Ig (15 [30%] subjects who received povetacicept) (Table S4). However, no subjects who received povetacicept developed circulating IgG below the lower limit of normal (<610 mg/dL).

Pharmacokinetics and immunogenicity of povetacicept

Free povetacicept demonstrated biphasic exposure typical of Fc fusion proteins with an early distribution phase (0–3 or 0–7 days post‐dose for IV or SC, respectively) and a later elimination phase (3–28 or 7–28 days post‐dose for IV or SC, respectively) at doses ≥240 mg (Figure 1 and Table 1). The elimination phase of doses ≥240 mg was linear out to Day 28. Doses ≤80 mg showed nonlinear elimination.

FIGURE 1.

Povetacicept (free and total) serum concentrations versus time following IV or SC dosing. Measurement of dose‐dependent free and total povetacicept serum concentrations after a single administration at doses ranging from 2.4–960 mg (IV) and 80–960 mg (SC). Data are presented as geomean (by cohort) ± SD.

TABLE 1.

Povetacicept PK parameters following single‐dose IV or SC administration in healthy adults.

Dosing cohort	Measure	Free povetacicept			Total povetacicept
		C max (μg/mL)	t 1/2 (days)	AUC0–∞ (days μg/mL)	C max (μg/mL)	t 1/2 (days)	AUC0–∞ (days μg/mL)
2.4 mg IV	N	4	4	4	4	0	0
	Geo Mean	0.86	0.4	0.68	0.78	NC	NC
	Geo CV%	29.9	11.7	24.4	18.6	NC	NC
8 mg IV	N	4	4	4	4	1	1
	Geo Mean	2.47	0.8	2.53	2.24	17.93	8.53
	Geo CV%	17.2	12.0	26.3	22.9	NC	NC
24 mg IV	N	4	4	4	4	2	2
	Geo Mean	9.85	1.71	16.58	8.07	10.88	31.82
	Geo CV%	17.9	25.8	37.6	21.7	33.4	12.6
80 mg IV	N	7	7	7	7	6	6
	Geo Mean	24.1	2.60	76.38	23.58	11.58	95.45
	Geo CV%	30.4	30.4	20.3	31.4	117.6	26.9
240 mg IV	N	4	4	4	4	4	4
	Geo Mean	76.61	5.44	362.66	75.36	9.51	381.63
	Geo CV%	32.3	34.2	31.5	34.8	20.3	27.8
480 mg IV	N	4	4	4	4	4	4
	Geo Mean	164.60	5.03	738.06	156.93	10.11	809.79
	Geo CV%	20.3	28.8	7.1	18.5	9.3	8.7
960 mg IV	N	4	4	4	4	4	4
	Geo Mean	328.57	7.58	1553.62	324.04	11.41	1651.16
	Geo CV%	11.9	38.9	33.7	11.1	18.4	35.0
80 mg SC	N	7	6	6	7	6	6
	Geo Mean	6.01	2.77	64.67	5.92	10.47	87.42
	Geo CV%	33.7	34.0	30.5	31.9	99.5	28.2
240 mg SC	N	4	4	4	4	4	4
	Geo Mean	18.35	5.86	267.67	16.44	8.20	276.12
	Geo CV%	18.4	31.8	8.2	17.8	21.9	4.7
480 mg SC	N	4	4	4	4	4	4
	Geo Mean	34.39	5.19	600.97	37.46	11.17	643.94
	Geo CV%	18.6	50.8	16.0	15.4	13.0	19.1
960 mg SC	N	4	4	4	4	4	4
	Geo Mean	82.00	6.72	1240.62	81.07	9.75	1323.06
	Geo CV%	19.2	40.0	25.7	20.4	15.7	24.7

Abbreviations: AUC_0–∞, area under the concentration versus time curve calculated from time zero to infinity; AUC_0–lastd, area under the concentration versus time curve calculated from the time of dosing to the time of the last measurable concentration; CL, total body clearance of drug after IV administration; C _max, observed maximum concentration; CV, coefficient of variation; Geo, geometric; IV, intravenous; NC, not calculated; PK, pharmacokinetics; SC, subcutaneous; t _1/2, elimination half‐life; T _max, time to C _max; V_z, apparent volume of distribution during the terminal phase after intravascular administration; V_z/F, apparent volume of distribution during the terminal phase.

Total povetacicept demonstrated triphasic exposure with an early distribution phase (0–3 or 0–7 days post‐dose for IV or SC, respectively), an elimination phase (3–56 or 7–56 days post‐dose IV or SC, respectively) consistent with FcRn recycling at doses ≥240 mg, and a sustained exposure phase (56–112 days post‐dose). The elimination phase of doses ≥240 mg was linear with similar half‐lives/clearance. Doses ≤80 mg showed nonlinear elimination, which also transitioned into a sustained exposure phase starting after 7–28 days out to the last day sampled (28 days for 2.4–24 mg doses and 112 days for the 80 mg dose).

Antidrug antibodies (ADA) against povetacicept were observed in 9/50 (18%) of subjects, of whom 8 (16%) were positive by neutralizing antibody (NAb) assessment. There were no positive ADA results in subjects who received placebo. No apparent impact on exposure or safety of ADA or NAb in this single‐dose study was observed.

Non‐compartmental analysis

After single‐dose administration, povetacicept demonstrated dose‐proportional peak exposure (i.e., C _max) across all doses tested, and dose‐proportional systemic exposure (i.e., AUC) at doses of ≥240 mg (Table 1 and Table S5). For systemic exposure (AUC_0–∞), povetacicept showed generally greater than dose proportionality for doses ≤80 mg, while dose levels ≥240 mg were dose‐proportional (Table 1 and Table S5). The ≤80 mg doses showed nonlinear elimination corresponding with increasingly shorter half‐lives and greater clearance. The 80 mg and 240 mg SC doses exhibited half‐lives and bioavailability of 2.8 days and 83%, and 5.9 days and 72%, respectively.

Exploratory PopPK model and analysis

Povetacicept PK was best described as a two‐compartment model with first‐order absorption and parallel linear and nonlinear clearance accounting for saturable target‐mediated drug disposition (TMDD), particularly at doses ≤80 mg (Figure S1). Based on exploratory popPK analysis, PK was impacted by body weight; however, the absolute impact of this covariate was limited to a <30% difference between the predicted exposure at median body weight and the predicted exposures at the 5th and 95th body weight percentiles by both C _max and AUC_0–last. The percentage of the nonlinear clearance component relative to total clearance is >90% at doses <24 mg and <20% at doses >240 mg (Figure S1). All other covariates/factors tested (see Section 2) did not impact exposure, including race/ethnicity.

Pharmacodynamics of povetacicept

Target coverage

Povetacicept treatment was associated with transient reduction of circulating free APRIL for all doses tested, with the reduction occurring by Day 4 in all cohorts (Figure 2a and Figure S2A). The magnitude of reduction in cohorts ≤24 mg ranged between 43% at the 2.4 mg IV dose to 99% at the 24 mg dose. Time to baseline recovery was dose‐dependent in cohorts ≤24 mg, occurring between Day 8 and Day 22. The reduction of free APRIL was saturated (greater than 95%) at the 80 mg dose with a duration of response through Day 22 and recovered to baseline by Day 57. Duration of response for doses ≥240 mg was saturated out to Day 29 and 57, with recovery to baseline occurring between Day 57 and 113.

FIGURE 2.

Povetacicept administration is associated with dose‐dependent reductions in free BAFF and APRIL. Serum levels of free and total APRIL and BAFF after single doses of povetacicept. Percent change from baseline and percent of baseline data for APRIL and BAFF, respectively are presented as mean (by cohort) ± SD.

Povetacicept was associated with a transient reduction of circulating free BAFF by Day 4 for all doses tested (Figure 2b and Figure S2B). The magnitude of reduction in cohorts below 80 mg ranged from 78% at the 2.4 mg dose to 90% at the 24 mg dose. Time to baseline recovery was dose‐dependent in cohorts ≤80 mg occurring between Day 8 and 22 days. The reduction of free BAFF was observed to be saturated (greater than 95%) at the 80 mg dose level through Day 15. The saturated reductions of free BAFF were sustained through 28 days post‐dose at doses of ≥240 mg. Concentrations of free BAFF recovered to at least baseline levels between Day 57 and 113.

Povetacicept treatment was associated with a dose‐independent decrease in circulating total APRIL by Day 4 (Figure 2c and Figure S2C). The average maximum reduction of total APRIL was approximately 40%–60% in all cohorts, except for the lowest cohort (IV‐1, 2.4 mg). Time to baseline recovery was dose‐dependent and occurred between Day 15–113.

Povetacicept treatment was associated with a dose‐dependent and sustained elevation in circulating total BAFF for all doses tested (Figure 2d and Figure S2D). The magnitude of total BAFF increase in the lower dose cohorts ranged from ~15‐fold at the 2.4 mg dose to ~80‐fold at the 24 mg dose. At doses ≥80 mg, total BAFF levels increased from ~200‐fold to ~600‐fold of baseline and remained elevated through Day 113. Assessment of the increase in total BAFF beyond Day 29 was not available in cohorts <80 mg because samples were not collected.

Circulating lymphocytes

An initial increase in total B cells in whole blood associated with povetacicept administration was observed by Day 4 with maximum levels reached between Day 4 and 8 across all evaluated povetacicept dose cohorts before gradually returning to or below baseline between Day 29 and Day 113 (Figure S3). The magnitude of the increase in total B cells was not dose‐dependent, ranging at Day 4 from a minimum increase of 85.5% ± 78.4% (mean ± SD) in the 24 mg IV cohort to a maximum of 147.1% ± 75.8% (mean ± SD) in the 80 mg IV cohort. The increases in the SC cohorts fell within this range, with an average increase on Day 4 of 146.4% ± 102.7% (mean ± SD) as compared with Day 1. Changes in baseline in other populations were not generally observed and/or were also present in placebo‐treated subjects; total CD4+ and CD8+ cells remained largely unchanged (Figure S3).

Immunophenotyping of B‐cell subsets

Dose‐dependent reductions in naïve B cells and antibody‐secreting cells (ASC; plasma cells and plasmablasts) and increases in CD27⁺ memory B cells in isolated peripheral blood mononuclear cells (PBMC) were observed (Figure 3). Reductions in naïve B cells and ASC were generally observed with a delayed time to maximum reduction (≥Day 15) and slower recovery (≥Day 29) at povetacicept doses ≥80 mg; 80 mg IV cohort time to maximum reduction was Day 8 (Table S6). Percent reductions in naïve B cells in the 80–960 mg cohorts ranged from 23% ± 5% (mean ± SD; 80 mg IV) to 39% ± 14% (480 mg IV). Percent reductions in ASC did not exhibit clear dose dependency, with maximal reductions ranging from a 63% ± 15% (mean ± SD; 8 mg IV) to 76% ± 8% (80 mg SC) (Table S6). Complete reduction of ASC was not observed, though maximal observed reductions of 80%–89% were achieved with doses ≥80 mg. Reductions in ASC associated with povetacicept were confirmed using RNA sequencing (RNASeq) of whole blood (Figure S4).

FIGURE 3.

Povetacicept administration is associated with dose‐dependent reductions in circulating naive B cells and ASCs and increases in memory B cells. Percent change from baseline calculations were performed with frequency data, % naïve of CD19⁺ B cells (a), % CD27⁺ memory of CD19⁺ B cells (b), and % ASC of CD19⁺IgD⁻CD27⁺ B cells (c). Data are represented as mean (by cohort) ± SD.

The percentage of CD27⁺ memory B cells in PBMC increased in a dose‐related fashion in IV and SC cohorts (Figure 3). Time to maximum change was 6 to 8 days in cohorts receiving <80 mg, and 22 to 29 days at higher doses. Memory B cells returned to baseline by Day 29 in the 2.4 to 24 mg cohorts, but full recovery to baseline levels was not achieved in the 80–960 mg cohorts by Day 113. Dose‐dependent impacts on the magnitude of increase were less clear due to individual heterogeneity, particularly in the 80 mg SC cohort, but the largest individual increases were typically observed with ≥80 mg povetacicept.

Immunoglobulins and Gd‐IgA1

Povetacicept treatment was also associated with transient, dose‐related reductions in all serum Ig isotypes, comprising IgA (including serum Gd‐IgA1), IgG, IgM, and IgE (Figure 4 and Figure 5). Maximal nadirs for doses ≥80 mg IV or SC were observed to be approximately −60% in IgA; −30% in IgG; −80% in IgM; and −75% in IgE, occurring between Day 29 (80 mg) and Day 57 (24–960 mg) and then recovering toward baseline through Day 113. The IgG subclasses of IgG1, IgG2, IgG3, and IgG4 showed similar decreases as those observed for total IgG (Figure S5). Doses <24 mg showed minimal to no decreases in serum Igs compared with placebo. A single dose of 24 mg IV povetacicept demonstrated a transient decrease in serum Igs that nadired between 7 and 14 days and started to recover toward baseline by Day 29 (Figure 4).

FIGURE 4.

Povetacicept administration is associated with dose‐dependent reductions in all circulating Ig isotypes. Percent change from baseline calculations are presented as mean (by cohort) ± SD.

FIGURE 5.

Povetacicept administration is associated with dose‐dependent reductions in galactose‐deficient IgA1 (Gd‐IgA1). Percent change from baseline (upper graphs) and serum concentration data (ng/mL) (lower graphs) are presented as mean (by cohort) ± SD.

Povetacicept administration was also associated with dose‐dependent decreases in circulating Gd‐IgA1 concentrations; doses of ≥80 mg IV or SC led to a Gd‐IgA1 change from baseline of −70% by Day 29, which started to recover to baseline by Day 113 (Figure 5).

Soluble receptors and receptor expression on circulating B cells

Povetacicept administration was associated with increases in serum concentrations of soluble (s) TACI (Figure S6), though not in a clearly dose‐dependent manner. Elevated sTACI concentrations were observed in all treatment cohorts by Day 4, remaining elevated through Day 29, and increasing further in most cohorts at Day 113 ~ 25% to 250% above baseline. Povetacicept treatment was associated with dose‐related increases of sBCMA, though these increases were transient, with maximum concentrations observed at ~30%–100% above baseline on Day 8, nearing baseline by Day 29, and ~10%–40% lower than baseline by Day 113 in most cohorts (Figure S6). Povetacicept treatment was not associated with changes in circulating sBAFF‐R levels, except for a minimal, sustained reduction in the 960 mg IV cohort (Figure S6).

Povetacicept administration resulted in notable dose‐dependent increases in BAFF‐R expression on circulating CD19⁺ B cells, as compared with baseline expression and to placebo controls, and in dose‐dependent decreases in TACI expression on CD27⁺ memory B cells (Figure S7A); BCMA expression levels on B cells was too low to enable reliable tracking by flow cytometry (not shown). Povetacicept treatment was also associated with changes in transcripts encoding BAFF‐R (TNFRSF13C), TACI (TNFRSF13B), and BCMA (TNFRSF17) in whole blood collected from 80 mg subjects (Figure S7B).

DISCUSSION

In this study, povetacicept demonstrated several characteristics of a well‐behaved Fc fusion therapeutic, ²⁹ , ³⁰ , ³¹ including largely dose‐dependent PK and PD, excellent safety and tolerability, and extended (>2–4 weeks) target coverage after single low doses (≥80 mg). The latter is particularly notable, since povetacicept was specifically engineered for greater affinity for, and greater inhibitory activity against, human BAFF and APRIL, over and above WT TACI‐based therapeutics, which are currently dosed 150–160 mg weekly. The present findings indicate the potential for povetacicept to provide greater therapeutic coverage and effects at a significantly less frequent dosing interval(s) with comparable or lower doses, consistent with the preclinical and translational data which has demonstrated superior comparative PK and PD.

Indeed, povetacicept's >95% target coverage (2–3 weeks and 4–8 weeks for 80 and ≥240 mg, respectively) closely resembles that of conventional monoclonal antibodies, in contrast to WT TACI‐Fc's which appear unable to achieve such target coverage for even 1 week. ³² Povetacicept's dose‐normalized PK exposure by C _max and AUC are higher than those reported for WT TACI‐Fc's but was associated with significantly lower half‐lives, ³³ , ³⁴ presumably driven in part by TMDD attributable to povetacicept's higher target affinity. TMDD likely accounts for povetacicept's nonlinear clearance at doses ≤80 mg, in contrast to the linear clearance exhibited at doses ≥240 mg. This TMDD is further exacerbated by soluble BAFF, which appears in circulation more than 50‐fold over APRIL shortly after dosing, and likely forms drug–cytokine complexes since absolute total povetacicept and total BAFF levels appear quantitatively very similar 4–8 weeks after dosing (Figure 6). Similarly, povetacicept's PK exposure and half‐life appears lower than anti‐APRIL monoclonal antibodies at doses that achieve similar target coverage ³⁵ , ³⁶ ; this may similarly reflect povetacicept's improved APRIL affinity. Thus, povetacicept's engineering appears to have greatly enhanced the clinical potency and PK characteristics of TACI‐Fc.

FIGURE 6.

Povetacicept pharmacokinetics are associated with reciprocal and/or concordant changes in free and total APRIL and BAFF. Comparison of povetacicept (free and total) versus target (free and total APRIL and BAFF) molar concentrations in the 80 mg SC and the 240 mg SC cohorts. Molarity is reported as mean (by cohort).

Although clinical efficacy of povetacicept cannot be ascertained in this study in healthy adults, several biomarker observations suggest that its advantageous PK and PD characteristics may translate into greater therapeutic potency in relevant diseases. For instance, in contrast to the povetacicept results presented here, reductions in ASC have not been reported after single‐dose administration of anti‐BAFF, anti‐APRIL, or WT TACI‐Fc therapeutics. ³⁵ , ³⁷ , ³⁸ , ³⁹ This likely reflects the known high expression by ASCs of BCMA, ⁵ which could mediate relative resistance to therapeutic inhibition of BAFF or APRIL alone, as well as to WT TACI‐Fc’s, which inhibit APRIL less effectively than BAFF. ²⁶ Similarly, the observed reductions in circulating Ig associated with povetacicept treatment (Figure 4) exceed those reported for WT TACI‐Fc’s after single‐dose administration in healthy adults, ³⁷ , ³⁸ but are similar or modestly superior to those observed with anti‐APRIL antibodies ³⁵ , ³⁶ ; this suggests that circulating Ig of all isotypes, at least in healthy adults, are largely APRIL‐, not BAFF‐, dependent. ASCs account for the majority of pathogenic autoantibody production in multiple autoimmune diseases, ⁴⁰ and circulating Ig reductions associated with povetacicept treatment likely anticipate circulating pathogenic antibodies, as suggested by analogous reductions in Gd‐IgA1 (Figure 5). Thus, these findings altogether suggest that povetacicept may confer greater therapeutic efficacy in (auto)antibody‐related diseases, particularly via its effect on pathogenic ASCs and (auto)antibodies.

Notable observations in this study include the significant, sometimes dramatic, increases in total circulating BAFF, as well as circulating memory B cells, after povetacicept administration. These findings are not surprising; increases in BAFF have been observed after treatment with other B‐cell modulators, including rituximab, ⁴¹ telitacicept, ³⁴ and atacicept, ⁴² but not anti‐APRIL antibodies. ³⁵ Yet, BAFF activity is potently biologically inhibited by povetacicept, as indicated by low post‐dose free BAFF levels. This correlates with prolonged total drug PK that mirrors total BAFF, which together represent likely biologically inactive drug‐BAFF complexes (Figure 6). Similarly, elevated circulating memory B cells have been reported after treatment with anti‐BAFF antibodies, but these cells exhibit an anergic and hypoproliferative phenotype and their appearance likely reflects impaired tissue extravasation due to impaired trafficking. ¹⁷ , ⁴³ , ⁴⁴ , ⁴⁵ Thus, these seemingly paradoxical changes in BAFF and memory B cells after povetacicept administration likely reflect inhibition particular to the BAFF pathway and a significant lowering of pathological risk from these mechanisms.

Altogether, these findings support continued clinical, multi‐dose investigation of povetacicept in autoimmune diseases. Based upon pharmacodynamic target coverage and biomarker kinetics, a flat dose regimen of 80 mg or more, administered subcutaneously every 4 or more weeks, appears sufficient to confer clinically meaningful improvements in disease outcomes; given the magnitude of the impact of body weight on exposure, exploring dose modifications is not warranted. IgE‐related diseases, such as allergic asthma or environmental allergies, may be of additional future interest given the substantial IgE reductions which have not, to the best of our knowledge, been reported for other inhibitors of BAFF and/or APRIL. Currently, povetacicept is under active clinical investigation for multiple autoimmune diseases, specifically IgA nephropathy (NCT06564142; Phase III), primary membranous nephropathy, LN, and renal ANCA‐associated vasculitis (NCT05732402; Phase 1b/2); as well as immune thrombocytopenia, warm autoimmune hemolytic anemia, and cold agglutinin disease (NCT05757570; Phase 1b). ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ⁵⁰

AUTHOR CONTRIBUTIONS

R.D., S.L.P., A.E., A.G.C., L.B., and S.R.D. wrote the manuscript; R.D., S.L.P., A.E., A.G.C., and S.R.D. designed the research; J.L., K.M., A.G.C., L.B., N.W., T.B., and A.S. performed the research; R.D., S.L.P., A.E., A.G.C., L.B., N.W., T.B., H.M.T., and S.R.D. analyzed the data.

FUNDING INFORMATION

This work was funded by Alpine Immune Sciences, a Vertex Company, Seattle, WA, USA.

CONFLICT OF INTEREST STATEMENT

Rupert Davies, Stanford L. Peng, Amanda Enstrom, Allison G. Chunyk, Lori Blanchfield, NingXin Wang, Tiffany Blair, Heather M. Thomas, Alina Smith and Stacey R. Dillon are/were employees, shareholders, and/or officers (SLP) of Alpine Immune Sciences, Inc. Drs. Jason Lickliter and Kristi McLendon are employees of Nucleus Network.

Supporting information

Data S1

Davies R, Peng SL, Lickliter J, et al. A first‐in‐human, randomized study of the safety, pharmacokinetics and pharmacodynamics of povetacicept, an enhanced dual BAFF/APRIL antagonist, in healthy adults. Clin Transl Sci. 2024;17:e70055. doi: 10.1111/cts.70055