FormalPara Key Summary Points

Why carry out this study?

 Transforming growth factor beta (TGFβ) has been implicated as a key mediator of fibrogenesis and has been found to be upregulated in fibrotic diseases; however, systemic pan-TGFβ inhibition has demonstrated unacceptable toxicities

 This phase 1a study evaluated the safety, tolerability, pharmacokinetics, and pharmacodynamics of RO7303509, a high-affinity, TGFβ3-specific, humanized immunoglobulin G1 monoclonal antibody, in healthy adult volunteers

What was learned from the study?

 RO7303509 was well tolerated at single subcutaneous doses up to 1200 mg in healthy participants with favorable pharmacokinetic data that appeared to increase dose-proportionally

 Overall, these data support the further development of RO7303509 for the treatment of fibrotic diseases

Introduction

Fibrosis is defined as the progressive accumulation of fibrous connective tissue that occurs when the normal wound-healing process becomes dysregulated [1,2,3,4,5]. The excessive accumulation of extracellular matrix (ECM) components replaces normal tissues with nonfunctional fibrotic tissue, leading to permanent scarring, organ malfunction and failure, and death [2, 3]. Fibrosis is a key pathology in diseases such as systemic sclerosis (SSc) and idiopathic pulmonary fibrosis, in chronic organ-specific diseases, such as liver, kidney, and heart disease, and in chronic inflammatory diseases, such as rheumatoid arthritis, inflammatory bowel disease, and systemic lupus erythematosus [2, 3, 6].

Transforming growth factor beta (TGFβ) has been implicated as a key mediator of fibrogenesis and has been found to be upregulated in fibrotic diseases [7, 8]. The three TGFβ cytokines (TGFβ1, TGFβ2, and TGFβ3) are critical regulators of immune responses, fibroblast differentiation and tissue development, and epithelial function and wound repair [3, 6, 9]. Genetic deletion studies have revealed that each isoform has distinct roles in development and homeostasis [10,11,12,13,14,15,16]. Furthermore, TGFβ2 and TGFβ3 have activation patterns and cellular expression patterns that are distinct from those of TGFβ1 [17]. Once activated, the isoforms bind to a common receptor complex (TGFβR1, TGFβR2) and initiate similar signaling pathways. Despite their beneficial functions, these signaling pathways have been implicated in multiple fibrotic diseases and pathologic processes [9, 18,19,20,21,22,23,24], including the promotion of fibroblast differentiation into myofibroblasts whose activation leads to subsequent ECM deposition [3, 25].

TGFβ inhibition is a promising therapeutic approach for fibrotic diseases, including SSc, a rare and severe autoimmune disease of the connective tissues that is characterized by microvascular damage that leads to skin and internal organ fibrosis, resulting in reduced life expectancy and quality of life [26,27,28,29,30,31]. When patients with SSc received the pan-TGFβ inhibitor, fresolimumab, investigators observed improvements in the modified Rodnan skin score, a measure of skin fibrosis, reduced numbers of myofibroblasts, and decreases in TGFβ-regulated gene expression [32]. However, systemic pan-TGFβ inhibitors have demonstrated unacceptable toxicities, including risks of bleeding, development of skin lesions, and degeneration of cardiac valves and aorta [17, 32,33,34,35]. Patients with SSc who received the TGFβ1-specific inhibitor metelimumab (CAT-192) showed no evidence of clinical activity and had a higher rate of adverse events (AEs) and serious AEs (SAEs) compared with placebo-treated patients [36]. A TGFβ1/TGFβ3 inhibitor, AVID200, showed improvements in skin fibrosis and had an acceptable safety profile [37], but grade 2 and grade 3 anemia was observed in patients with SSc and advanced solid tumors [37, 38].

Emerging evidence has indicated that selective inhibition of TGFβ3 signaling may target fibrotic disease pathogenesis while circumventing the AEs associated with pan-TGFβ or TGFβ1 inhibition. As TGFβ is a ubiquitous pro-fibrotic cytokine, indiscriminate inhibition can lead to significant adverse reactions [39]. Therefore, targeting more specific isoforms, such as TGFβ3, could lessen the risks for an adverse reaction. Elevated TGFB3 gene expression in human lung and liver fibrotic tissue, and in the skin of patients with SSc, suggests its use as a more specific therapeutic target [17, 40]. Increased expression of TGFB3, but not TGFB1 or TGFB2, was significantly associated with higher levels of biomarkers of SSc disease severity and prognosis, including TGFβ target genes in the skin and systemic cartilage oligomeric matrix protein (COMP) and periostin [41].

RO7303509 (MTBT1466A), a high-affinity, TGFβ3-specific, humanized immunoglobulin G1 (IgG1) monoclonal antibody, is being developed as a potential treatment for fibrosis [42]. In preclinical mouse studies, a murine surrogate of RO7303509 significantly inhibited endogenous activation of TGFβ target genes in a model of fibrotic lung disease [17]. In this first-in-human phase 1a study reported here, we evaluated the safety, tolerability, and pharmacokinetics (PK) of single intravenous (IV) or subcutaneous (SC) doses of RO7303509 in healthy participants. Additionally, the study examined serum biomarkers of TGF-β pathway inhibition. The phase 1a data was used to develop a population PK model to characterize PK in healthy volunteers (HVs) and inform dose selection for future trials.

Methods

Study Design

This phase 1a randomized, double-blind, placebo-controlled study (ISRCTN13175485) evaluated the safety, tolerability, and PK of a single IV or SC dose of RO7303509 in healthy participants. Participants were enrolled into six single-dose cohorts, consisting of eight participants each (6 active: 2 placebo), that tested the following doses of RO7303509: 50 mg, 150 mg, and 240 mg (IV) and 240 mg, 675 mg, and 1200 mg (SC) (Fig. 1).

Fig. 1
figure 1

Study design. In each cohort, 2 additional participants were enrolled and treated with placebo (i.e., 6 participants, RO7303509; 2 participants, placebo). HV Healthy volunteers, SAD single ascending dose, IV intravenous, SC subcutaneous

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Participants were screened within 35 days of randomization (day − 35 to day − 1) and checked into the clinical research unit (CRU) on day − 1. On day 1, participants were randomized and received a single dose of RO7303509 or placebo. For each cohort, two sentinel participants were dosed on the first day (1 active:1 placebo), and the remaining participants in each cohort were dosed at least 48 h after dosing of the sentinel pair, if no safety concerns arose. Participants remained in the CRU from day − 1 to day 3, and follow-up visits occurred through day 85. Dose escalation, including from IV to SC administration, proceeded if the safety information and emerging PK data through day 43 from the prior cohort were favorable.

The clinical study protocol and associated documents were approved by an Institutional Review Board before study initiation. The study was conducted in full conformance with the ICH E6 guideline for Good Clinical Practice and the principles of the Declaration of Helsinki. The study complied with the requirements of the ICH E2A guideline (Clinical Safety Data Management: Definitions and Standards for Expedited Reporting) and U.S. Food and Drug Administration (FDA) regulations and applicable local, state, and federal laws. All participants provided written, informed consent before enrolling in the study.

Participants

Eligible participants were adults, 18–75 years old, with a body mass index (BMI) of 18–32 kg/m2, a body weight of 50–150 kg at screening, and healthy, as determined by medical history, physical examination, clinical laboratory tests, vital signs, and 12-lead electrocardiograms (ECG). Female participants were excluded if pregnant or breastfeeding. Potential participants were also excluded if they had treatment with an investigational therapy within 90 days before initiation of study drug; an infection requiring antibiotics within 14 days before screening or chronic infection requiring systemic treatment within 1 year before study drug initiation; a history of smoking or alcohol or drug abuse; were taking co-medications, except those approved by the medical monitor; a recent history (within 3 months) of root canal, unresolved dental infection, poor dentition, and/or dental implant placement; and any serious medical condition or abnormality that would preclude safe participation and completion of study.

Randomization and Blinding

After eligibility requirements were confirmed, participants were randomized on day 1 via an interactive voice or web-based response system. Study site personnel and participants were blinded to treatment assignment during the study. Sponsor personnel were blinded to treatment assignment, except for individuals who required access to treatment assignments to fulfill their job roles during the trial. PK and immunogenicity samples were collected from placebo-treated participants to maintain blinding; laboratories responsible for PK and anti-drug antibody (ADA) analyses were unblinded to identify appropriate samples for analysis. Placebo doses were matched in appearance and administration to the respective IV or SC doses.

Safety Outcomes and Assessments

The primary outcome was the incidence and severity of AEs. AE terms were mapped to the Medical Dictionary for Regulatory Activities (MedDRA, version 25) thesaurus terms, and severity was determined according to the World Health Organization Toxicity Grading Scale. The study also monitored the change from baseline in targeted vital signs, targeted clinical laboratory test results, and 12-lead ECG parameters.

Pharmacokinetic Outcomes and Assessments

The secondary PK objective was to characterize the PK profile of a single dose of RO7303509 based on the serum concentration of RO7303509 at specified time points and the PK parameters.

Serum samples for PK analyses were collected on day 1 (predose and at 1, 2, 4, and 6 h after the start of infusion), at multiple timepoints throughout the study, and at the study completion visit on day 85. RO7303509 serum concentrations were measured using a sandwich enzyme-linked immunosorbent assay (ELISA), where capture antibody (clone 2A10.47; Genentech, Inc., South San Francisco, CA, USA) was coated on a microtiter plate to bind to RO7303509, followed by a biotinylated antibody (clone 2A10.37; Genentech Inc.) bound to the immobilized analyte and was detected by a horseradish peroxidase (HRP)-streptavidin conjugate.

Serum PK was summarized by estimating total exposure (area under the concentration–time curve; AUC), maximum serum concentration (Cmax), total clearance (CL) or apparent clearance (CL/F), volume of distribution (V) or apparent volume of distribution (V/F), time to maximum concentration (Tmax), terminal half-life (t1/2), and bioavailability (as appropriate for data collected). PK parameters were analyzed using non-compartmental methods in Phoenix WinNonlin (version 8.3, Certara, Princeton, NJ, USA).

Dose proportionality was assessed using a log-transformed power model. For full details see Eisenblaetter et al. [43].

Population Pharmacokinetic Analysis

A population pharmacokinetics (popPK) model was developed to describe serum RO7303509 concentration–time profiles. Various structural models were tested, including one-compartment, two-compartment, and three-compartment models, with the results showing that the PK of RO7303509 in the tested clinical dose range was best described by a two-compartment model. We used data from days 0–84 from the six cohorts in the study with single doses of RO7303509 (the IV cohorts received doses of 50, 150, 240 mg, and the SC cohorts received doses of 240, 675, 1200 mg). The PK population included  participants who received at least one dose of RO7303509 and had at least one evaluable PK sample. The SC/IV popPK model included a depot compartment for SC with first-order SC absorption kinetics (Electronic Supplementary Material [ESM] Fig. 1). RO7303509 PK data were analyzed by nonlinear mixed-effects modeling (NONMEM) using the first-order conditional estimation method (FOCE). Models were evaluated based on the likelihood objective function value (OFV) provided by NONMEM (version 7.5.0; [44]), careful evaluation of goodness-of-fit plots, and precision of the parameter estimates (relative standard error [RSE]).

Immunogenicity Outcomes and Assessments

The secondary immunogenicity objective was to evaluate the immune response to RO7303509 by measuring the prevalence of ADAs at baseline and the incidence of ADAs during the study.

Serum samples for ADA analyses were collected on day 1, day 15, day 29, day 57, and at the study completion visit on day 85. ADA sample analysis was performed using a homogenous bridging ELISA. Biotin-conjugated drug and digoxigenin-conjugated drug were co-incubated overnight with diluted samples. The mixture was transferred to a streptavidin-coated high-binding plate, and the signal was detected using an HRP-conjugated murine anti-digoxin monoclonal antibody (Jackson ImmunoResearch Inc., West Grove, PA, USA).

When determining post-baseline incidence, subjects were considered to be ADA positive if they were ADA negative at baseline but developed an ADA response following study drug exposure (treatment-induced ADA response), or if they were ADA positive at baseline and the titer of one or more post-baseline samples was at least 0.60 titer units greater than the titer of the baseline sample (treatment-enhanced ADA response). Subjects were considered to be ADA negative if they were ADA negative or had missing data at baseline and all post-baseline samples were negative, or if they were ADA positive at baseline but did not have any post-baseline samples with a titer that was at least 0.60 titer units greater than the titer of the baseline sample (treatment unaffected).

Exploratory Outcomes and Assessments

The exploratory biomarker objective was to evaluate serum levels of the TGFβ pathway-associated biomarkers periostin and COMP after treatment with RO7303509 to provide evidence of RO7303509 activity. Samples for biomarker analyses were collected at baseline on day − 1, and after dosing on day 1, day 5, day 8, day 15, day 29, and day 85. Periostin was tested at Microcoat Biotechnologie GmbH (Bernried am Starnberger See, Germany) using the COBAS Elecsys system (Roche Diagnostics Corp., Indianapolis, IN, USA) with proprietary antibodies, and COMP was tested at Frontage Laboratories, Inc. (Exton, PA, USA) using the research-grade ProteinSimple Ella platform (SPCKB-PS-000756; ProteinSimple, San Jose, CA, USA). Absolute levels of periostin and COMP were graphed as the mean ± standard error (SE).

Dose Selection

We selected the first-in-human dose based on a PK/pharmcodynamic (PD)-guided approach, leveraging minimal pharmacological activity of RO7303509 in a bleomycin-induced mouse model of pulmonary fibrosis. The activity of RO7303509 on TGFβ3-induced target gene expression determined the minimal pharmacologically active dose, and exposure from this dose was used to predict the first-in-human dose of 50 mg IV [42].

Drug Administration

Genentech, Inc. provided the RO7303509 in solution. The solution used for IV administration was diluted in a manufacturer-provided diluent for injection and administered intravenously using syringe pumps. Undiluted study drug was administered by SC injection.

Ethical Approval

The clinical study protocol and associated documents were approved by an Institutional Review Board (Advarra IRB [Columbia, MD, USA] reference number: GA42285) before study initiation. The study was conducted in full conformance with the ICH E6 guideline for Good Clinical Practice and the principles of the Declaration of Helsinki. The study complied with the requirements of the ICH E2A guideline (Clinical Safety Data Management: Definitions and Standards for Expedited Reporting) and the U.S. FDA regulations and applicable local, state, and federal laws. All participants provided written, informed consent before enrolling in the study.

Statistical Analyses

Although no formal sample size calculations were performed, we screened a sufficient number of subjects to ensure eight participants at each dose level. Six participants dosed with active drug in each cohort was deemed sufficient to characterize safety, tolerability, and available PK data for RO7303509.

The safety analysis population included all participants who received one dose of study drug. The PK population comprised participants who received at least one dose of RO7303509 and had at least one evaluable PK sample. The immunogenicity analysis population included all participants with at least one post-dose ADA assessment. For all analysis populations, participants were grouped according to treatment received.

Results

Participant Disposition, Demographics, and Baseline Characteristics

The study enrolled participants at one investigational site in the USA from 29 September 2020 through 24 February 2022; the last participant’s last visit was on 20 July 2022. Ninety-eight participants were screened, of whom 49 failed screening mainly due to eligibility issues. Aside from not meeting eligibility criteria, the other main screen failure reasons were because the cohort closed (n = 8) or participants withdrew consent (n = 7). For those failing to meet inclusion criteria, the most common reason was an out-of-range BMI (n = 8). Of the 49 participants enrolled, 45 (91.8%) completed the study (Fig. 2). One placebo-treated participant was lost to follow-up and replaced. All participants in each cohort received one dose of either RO7303509 or placebo.

Fig. 2
figure 2

Study disposition. IV Intravenous, SC subcutaneous

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Twenty-seven (55.1%) participants in the study were male (Table 1). One participant (2.0%) was Hispanic or Latino, 39 (79.6%) participants were White, and eight (16.3%) participants were Black or African American. The median age was 39 (range 18–73) years, and the median baseline body weight was 78.7 (range 50.9–113.1) kg.

Table 1 Participant demographics/baseline characteristics
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Safety

During the study, there were no deaths or AEs leading to dose modification, treatment discontinuation, or study discontinuation (Table 2). There were no dose-limiting AEs, grade ≥ 3 AEs, or SAEs. The overall frequency of AEs was comparable between RO7303509-treated (10 [27.8%], 24 AEs) and placebo-treated (4 [30.8%], 6 AEs) groups. All observed AEs were mild, except for one grade 2 AE of hyperamylasemia in the 1200-mg SC cohort that was considered unrelated to RO7303509. AEs related to study drug occurred in eight (16.3%) participants. The most frequently reported AEs were injection site reaction (highest dose SC cohort) and infusion-related reaction (highest dose IV cohort). All of these AEs were grade 1, required no supportive care, were considered related to study drug, and were observed in both active and placebo-treated participants. There were no trends for clinically significant vital sign abnormalities, and no cardiac abnormalities were detected with electrocardiograms or echocardiograms.

Table 2 Overview of adverse events and adverse events by preferred term in ≥ 2 healthy volunteers overall
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Pharmacokinetics

Pharmacokinetic data were sufficient to enable estimation of key PK parameters, and all 36 participants who received a single IV or SC dose of RO7303509 had evaluable serum concentration samples and were included in the PK analysis. For IV dosing, the median Tmax was 1–2 h, and the geometric mean (GM) t1/2 was similar across all cohorts, ranging from 20.0 to 25.7 days post-dose (Table 3). GM CL (147–275 mL/day) and GM Vz (5450–8730 mL) were also similar across IV cohorts. GM Cmax and AUC0–inf had moderate variability, with the percent coefficient of variation (%CV) ranging from 13.2–22.9 and 19.8–23.9, respectively. For SC dosing, the terminal phase of the observed PK profiles were generally comparable to those observed in participants after IV dosing (Fig. 3). The median Tmax after a single SC dose across cohorts was 5–7 (range 4–13) days, and the GM t1/2 was similar across all dose levels, ranging from 21.2 to 25.1 days post-dose. The GM CL/F (202–243 mL/day) and Vz/F (7080–7840 mL) were also similar across SC cohorts. We observed slightly higher variability for GM Cmax (%CV 23.1–37.2) and AUC0–inf (%CV 16.4–49.8) compared with the IV cohorts. The SC bioavailability was estimated to be 73%, based on the GM AUC0–inf from the IV cohort receiving 240 mg and the SC cohort receiving 240 mg.

Table 3 Pharmokinetic parameters of RO7303509 following single intravenous or subcutaneous doses
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Fig. 3
figure 3

Mean (± SD) RO7303509 serum concentrations over time for both the IV and SC cohorts. The profile for individual subjects receiving the 50-mg IV starting dose was previously reported in Yadav et al. [42]. IV Intravenous, SC subcutaneous, SD standard deviation

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Cmax and AUC0–inf appeared to increase proportionally with dose in the SC dose range of 240–1200 mg. After adjusting SC doses by the observed bioavailability of 73%, the mean serum systemic exposure (AUC0–inf) increased in an approximately dose-proportional manner over the 50-mg IV to 1200-mg SC dose ranges tested (Fig. 4).

Fig. 4
figure 4

Dose-proportionality for AUC0–inf vs. dose after single IV or SC administration of RO7303509. SC doses were adjusted by the observed bioavailability of 73%. Dose proportionality was assessed using a log-transformed power model. RO7303509 serum AUCinf_obs and the adjusted dose are presented on a natural logarithm scale. Horizontal lines, medians. AUC0–inf area under the concentration–time curve from time 0 to infinity, AUCinf_obs area under the concentration-time curve from time of dosing extrapolated to infinity, based on the last observed concentration, IV intravenous, SC subcutaneous

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Population Pharmacokinetics

A total of 36 participants received RO7303509, and 502 total PK samples were collected across all cohorts and timepoints. PK data across SC and IV cohorts was best characterized by a two-compartment model plus a depot compartment with first-order SC absorption kinetics. All parameters were well estimated by the model. Interindividual variability parameters were estimated for volume and clearance of the central compartment and absorption constant. A proportional error model was used. Bioavailability was estimated at 89.5%. Among the covariates explored (body weight, age, sex, race), body weight was found to be significant and was incorporated into the model by using fixed allometric exponents of 0.75 and 1 for clearance and volume, respectively. ESM Table 1 shows the model parameter estimates. Example goodness-of-fit plots are shown in ESM Figs. 2 and 3. No major bias was observed in residual plots. PopPK model predictions versus observed individual RO7303509 concentrations over time showed that popPK model captured individual level data in both SC and IV cohorts reasonably well (Fig. 5).

Fig. 5
figure 5

PopPK model predictions versus observed individual RO7303509 concentrations over time. Panels shown are selected individuals from each cohort. DV (Dependent variable), observed concentration, IPRED individual prediction, IV intravenous, PRED population prediction, SC subcutaneous

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Immunogenicity

At baseline, none of the participants tested positive for anti-RO7303509 ADAs (ESM Table 2). The incidence of treatment-emergent ADAs was 2.8% (1/36 participants) in the RO7303509 dosing arms. One RO7303509-treated participant in the 50-mg IV cohort was positive for ADAs (2.22 titer units) at one timepoint (day 15) after dosing, with no apparent impact on PK. Additionally, the observation of ADAs was not associated with any AEs.

Biomarkers

We also assessed whether RO7393509 would affect serum levels of periostin and COMP, both biomarkers associated with TGFβ pathway activation [45,46,47,48]. In all cohorts, with the exception of the 240-mg IV dose, the mean (standard error [SE]) serum periostin levels at baseline and throughout the study were comparable to placebo. The 240-mg IV cohort had higher levels of periostin than other cohorts at baseline and throughout the study (Fig. 6a). In the 50-mg IV, 240-mg IV, 675-mg SC, and 1200-mg SC cohorts, baseline COMP levels were slightly higher compared to placebo cohorts (Fig. 6). We observed a decrease in biomarker levels between pre-treatment visits on days − 1 and 1, demonstrating biomarker variability that was not related to drug treatment. Upon RO7393509 treatment IV or SC, we found no significant PD effects on serum COMP or periostin, compared to placebo cohorts.

Fig. 6
figure 6

Serum periostin and COMP levels in IV (a) and SC (b) cohorts. COMP Cartilage oligomeric matrix protein, IV intravenous, SC subcutaneous

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Discussion

Patients with SSc have been shown to benefit from treatment with pan-TGFβ; however, treatment with systemic pan-TGFβ inhibition is associated with unacceptable toxicities [32]. The main objective of this study was to characterize the safety, tolerability, and PK of isoform-specific TGFβ-inhibition in HVs. RO7303509, an investigational, TGFβ3-specific, humanized IgG1 monoclonal antibody was administered in single IV (up to 240 mg) or SC (up to 1200 mg) doses.

RO7303509 was well tolerated and the safety profile was acceptable; all AEs were low grade and non-serious and there were no SAEs or deaths observed. There were few treatment-related AEs, with the most frequent being injection-site reactions and infusion-related reactions, which were observed in both RO7303509- and placebo-treated participants. There were no notable changes to vital signs or laboratory evaluations among participants.

Serum concentrations were quantifiable through day 85 (end of study) in all PK-evaluable participants and demonstrated dose-proportional increases in Cmax and AUC0–inf across single IV or SC doses. After a single SC administration, observed bioavailability was 73%. The long terminal half-life (21.2–25.1 days) of RO7303509 supports SC administration every 4 weeks for further development. A linear two-compartment model with first-order absorption kinetics best described the PK of RO7303509 for participants in all cohorts. Body weight was included as a covariate in the model using allometric scaling. The model will be further refined with additional data as they become available to characterize RO7303509 PK, target engagement, and PD data to inform dose selection in future trials.

There were no clear PD effects for periostin or COMP upon TGFβ3 inhibition. The lack of an effect on TGFβ pathway biomarkers likely indicates that TGFβ3 is inactive during homeostasis, which suggests that targeting this pathway may have a favorable safety profile. It may thus be more informative to investigate biomarker levels in a disease population with evidence of active signaling, such as in patients with SSc [41, 49]. Although measurement of target engagement PD biomarkers was attempted by quantifying the post-dose accumulation of mature TGFβ3 [50], systemic TGFβ3 was largely undetectable in serum (data not shown), as expected in healthy participants without active disease. We expect to observe mature TGFβ3 in patients with fibrotic diseases, which can be used as a target engagement biomarker.

This study supports further assessment of RO7303509 safety, tolerability, PK, and PD in response to repeated dosing. An ongoing study is exploring repeat-dosing in patients with SSc and will be better able to explore aspects of use expected for an antifibrotic therapy for a rheumatic disease (e.g., repeat-dose PK, immunogenicity, safety profile, and requirements for short- and long-term monitoring for AEs). Although this study did not contribute to our understanding of the activity of RO7303509, it did support the potential safety and tolerability of isoform-specific TGFβ inhibition. As TGFβ is recognized as a central mediator of fibrosis, our ongoing study will test the hypothesis that TGFβ3 serves a particularly important pathogenic role in fibrosis in patients with SSc, as indicated by our observations and those of others [40, 41].

While it is encouraging that single doses of RO7303509 were not associated with any of the toxicities observed with systemic pan-TGFβ inhibition, an important limitation of conducting the present study in HVs includes the low baseline levels of TGFβ3 present in such participants or in the absence of a fibrotic condition. Our ongoing study will continue to characterize the safety and tolerability of RO7303509 in patients and similarly allow for further understanding of PK/PD. Another limitation of this study is the inability to demonstrate PD activity of anti-TGFβ3 due to low baseline levels of periostin and COMP in healthy individuals. The ongoing study in patients with SSc (NCT05462522) has been designed to explore the activity of selective TGFβ3 inhibition on downstream TGFβ signaling.

Conclusion

Our study demonstrated that RO7303509 was well tolerated at single SC doses up to 1200 mg in healthy participants with favorable PK data that appeared to increase in a dose-proportion manner. Overall, these data support the further development of RO7303509 for the treatment of fibrotic diseases. A phase 1b trial (NCT05462522) is currently enrolling patients with SSc.