Gossamer Bio (GOSS) Phase 3 PROSERA miss offsets strong seralutinib subgroup data
Filing Impact
Filing Sentiment
Form Type
8-K
Rhea-AI Filing Summary
Gossamer Bio reported topline Phase 3 PROSERA results for seralutinib in pulmonary arterial hypertension. The trial did not meet its primary endpoint, with a Hodges‑Lehmann placebo‑adjusted improvement in six‑minute walk distance of 13.3 meters at Week 24 and a p‑value of 0.0320 versus a prespecified α of 0.025.
All four key secondary endpoints, including NT‑proBNP reduction, clinical improvement, time‑to‑clinical worsening and REVEAL Lite 2 risk score, numerically favored seralutinib, with especially strong effects in prespecified intermediate‑ and high‑risk patients and in connective tissue disease–associated PAH. Seralutinib was generally well tolerated, though liver enzyme elevations and cough were more frequent than with placebo.
The company plans to meet with the U.S. FDA to discuss a potential development path and is pausing enrollment in the SERANATA Phase 3 study while it assesses PROSERA, including regional differences in placebo response and forthcoming CT functional respiratory imaging substudy data.
Positive
- None.
Negative
- Seralutinib’s Phase 3 PROSERA trial missed its primary 6MWD endpoint under the prespecified α=0.025 threshold, creating a material setback and regulatory uncertainty for this late‑stage PAH program.
Insights
Phase 3 miss on primary endpoint is a material setback despite encouraging subgroup and biomarker data.
Gossamer Bio disclosed that seralutinib’s PROSERA Phase 3 trial in PAH failed its primary six‑minute walk distance endpoint under the stricter α=0.025 threshold. For a late‑stage asset, missing the primary endpoint is a significant negative inflection for regulatory probability and perceived asset value.
That said, multiple key secondary measures, including NT‑proBNP reductions and clinical improvement, favored seralutinib, with pronounced effects in prespecified intermediate‑ and high‑risk patients and in connective tissue disease–associated PAH. These signals support biological activity but, because the primary endpoint was not met, p‑values are nominal and less persuasive for regulators.
Safety appeared generally acceptable relative to placebo, though cough and liver enzyme elevations were clearly more frequent and will require careful benefit‑risk positioning. Management plans to pause SERANATA enrollment and engage the FDA after deeper analyses and CT functional respiratory imaging substudy readout, so the future development path and required additional studies remain uncertain.
UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549
FORM 8-K
CURRENT REPORT
Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934
Date of Report (Date of earliest event reported): February 23, 2026
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Item 8.01 Other Events.
On February 23, 2026, Gossamer Bio, Inc. (the “Company” or “Gossamer”) announced topline results from its Phase 3 PROSERA study of seralutinib in patients with pulmonary arterial hypertension (PAH). Seralutinib is an investigational, inhaled tyrosine kinase inhibitor targeting PDGFR, CSF1R, and c-KIT, delivered via dry powder inhalation for the treatment of pulmonary arterial hypertension.
The Phase 3 PROSERA Study enrolled 390 patients with WHO Functional Class (FC) II or III PAH, with 197 randomized to the seralutinib arm and 193 randomized to the placebo arm. 55% of the enrolled patients were on background triple or quadruple PAH therapy, and 61% were on background prostacyclin therapy. The treatment and placebo arms were generally well balanced.
At Week 24, patients receiving seralutinib had a median change of +28.2 meters in 6MWD from baseline, while patients receiving placebo had a median change from baseline in 6MWD of +13.5 meters. The estimated Hodges‑Lehmann treatment effect was +13.3 meters, with a p‑value of 0.0320, which did not meet the prespecified threshold on the primary endpoint (α = 0.025); therefore, p values for the key secondary endpoints cannot be evaluated for statistical significance. All p values herein are nominal. All four key secondary endpoints favored seralutinib versus placebo in the overall population.
Consistent with the Phase 2 TORREY Study, seralutinib delivered a compelling signal in the prespecified intermediate‑ and high‑risk subgroup (n = 234), as defined by the REVEAL 2 Lite Risk Score ≥ 6 at screening, with a +20.0m placebo‑adjusted improvement in 6MWD (p = 0.0207). Three of four key secondary endpoints also demonstrated a p-value below 0.0125, underscoring seralutinib’s activity in patients with higher risk.
A key secondary endpoint, change in NT‑proBNP at Week 24, demonstrated an estimated location shift of ‑120.4 ng/L compared with placebo (p=0.0002) in the overall population, with separation between the arms favoring seralutinib observed starting at Week 4 (-96.0 ng/L; p=0.0002). Key secondary endpoints time-to-clinical worsening (TTCW), clinical improvement and proportion of patients with a one point or greater reduction in REVEAL Lite 2 Risk Score all favored seralutinib as compared to placebo in the overall population.
In a prespecified subgroup analysis of patients with REVEAL Lite 2 score ≥6 at screening, corresponding to intermediate- and high-risk patients, seralutinib demonstrated a pronounced and clinically meaningful response profile across the primary and key secondary endpoints. All key secondary endpoints favored seralutinib, with placebo‑adjusted effects including NT‑proBNP at Week 24 (location shift = -265.8 ng/L; p=0.0002), ≥1‑point improvement in REVEAL Lite 2 risk score at Week 24 (odds ratio = 2.033; p=0.0083), clinical improvement at Week 24 (odds ratio = 3.318; p=0.0101), and TTCW through Week 48 (hazard ratio = 0.744; p=0.4360).
In patients with connective tissue disease–associated PAH (CTD‑APAH), seralutinib demonstrated a robust improvement in six‑minute walk distance, achieving a placebo‑adjusted gain of +37.0 meters at Week 24 (n=87; p=0.0104), indicating a strong treatment effect in this clinically challenging subgroup.
Overall, seralutinib was generally well tolerated in the PROSERA Study. Treatment‑emergent adverse events (TEAEs) were reported in 86.5% of patients receiving seralutinib and 80.5% of patients receiving placebo. Treatment-emergent serious adverse events (SAEs) occurred in 16.0% of patients receiving seralutinib and 18.9% of patients receiving placebo. Transaminase elevations of three times or greater of the upper limit of normal were observed in 13% of patients receiving seralutinib and 1% of patients receiving placebo. The most frequently reported adverse event in patients treated with seralutinib was cough, reported in 37.0% of patients.
Based on these results, Gossamer expects to meet with the U.S. FDA to discuss the path forward for seralutinib in pulmonary arterial hypertension. The Company is pausing enrollment into the SERANATA Study to evaluate the impact of PROSERA results, particularly regional discrepancies in placebo response.
Additionally, PROSERA results from the CT functional respiratory imaging (FRI) substudy are expected in the coming weeks and are anticipated to provide additional insight into seralutinib’s treatment effect, including pulmonary blood volume distribution.
The slides attached as Exhibit 99.1 to this Current Report contain certain additional information related to the clinical data results discussed above and are incorporated herein by this reference. The Company intends to present the slides during a conference call and live webcast with the investment community on February 23, 2026, at 8:30 a.m. ET.
2
Forward-Looking Statements
The Company cautions you that statements contained in this current regarding matters that are not historical facts are forward-looking statements. These statements are based on the Company’s current beliefs and expectations. Such forward-looking statements include, but are not limited to, statements regarding: the therapeutic potential and market opportunity of seralutinib in PAH or PH-ILD, plans to put the ongoing Phase 3 SERANATA Study on hold, the expected plan to discuss topline results with the FDA and the potential to identify a development path forward for seralutinib. The inclusion of forward-looking statements should not be regarded as a representation by the Company that any of its plans will be achieved. Actual results may differ from those set forth in this press release due to the risks and uncertainties inherent in Gossamer’s business, including, without limitation: topline results Gossamer reports is based on preliminary analysis of key efficacy and safety data, and such data may change following a more comprehensive review of the data related to the clinical trial and such topline data may not accurately reflect the complete results of a clinical trial; Gossamer may not be able to identify a development path forward for seralutinib, whether as a result of FDA feedback or otherwise, and any path forward may require additional capital and other resources or may limit the commercial opportunity for seralutinib; Gossamer may need to evaluate its current workforce in light of potential development paths for seralutinib; potential delays in the commencement, enrollment and completion of clinical trials; comparative safety information is not based on a head-to-head comparison and differences exist between study designs and subject characteristics which could confound the results; the Company’s dependence on third parties in connection with product manufacturing, research and preclinical and clinical testing; the results of preclinical studies and early clinical trials with seralutinib are not necessarily predictive of future results; the success of any future Gossamer’s clinical trials and preclinical studies for seralutinib; regulatory developments in the United States and foreign countries; unexpected adverse side effects or inadequate efficacy of seralutinib that may limit its development, regulatory approval and/or commercialization, or may result in clinical holds, recalls or product liability claims; Gossamer’s ability to obtain and maintain intellectual property protection for seralutinib; Gossamer’s ability to comply with its obligations in collaboration agreements with third parties or the agreements under which it licenses intellectual property rights from third parties; Gossamer may use its capital resources sooner than it expects; and other risks described under the heading “Risk Factors” in documents the Company files from time to time with the Securities and Exchange Commission. You are cautioned not to place undue reliance on these forward-looking statements, which speak only as of the date hereof, and Gossamer undertakes no obligation to update such statements to reflect events that occur or circumstances that exist after the date hereof. All forward-looking statements are qualified in their entirety by this cautionary statement, which is made under the safe harbor provisions of the Private Securities Litigation Reform Act of 1995.
Item 9.01 Financial Statements and Exhibits.
(d) Exhibits.
| Exhibit Number | Description | |||||||
| 99.1 | Slide Presentation entitled "Phase 3 PROSERA Topline Results" | |||||||
| 101 | Cover Page Interactive Data File (embedded within the Inline XBRL document) | |||||||
3
SIGNATURES
Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned thereunto duly authorized.
| GOSSAMER BIO, INC. | |||||||||||
| Date: February 23, 2026 | By: | /s/ Christian Waage | |||||||||
| Christian Waage | |||||||||||
| Executive Vice President, Technical Operations & Administration | |||||||||||
4
Phase 3 PROSERA Topline Results February 2026
Forward Looking Statements This presentation contains forward-looking statements. All statements other than statements of historical facts contained in this presentation, including statements regarding our future results of operations and financial position, business strategy, prospective products, product approvals, research and development costs, timing and likelihood of success, plans and objectives of management for future operations, and future results of current and anticipated products, are forward-looking statements. In some cases, you can identify forward-looking statements by terms such as “may,” “will,” “should,” “expect,” “plan,” “anticipate,” “could,” “intend,” “target,” “project,” “contemplates,” “believes,” “estimates,” “predicts,” “potential” or “continue” or the negative of these terms or other similar expressions. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements. These known risks and uncertainties are described in detail in our filings with the Securities and Exchange Commission (the “SEC”) from time to time. Because forward-looking statements are inherently subject to risks and uncertainties, some of which cannot be predicted or quantified and some of which are beyond our control, you should not rely on these forward-looking statements as predictions of future events. The events and circumstances reflected in our forward-looking statements may not be achieved or occur and actual results could differ materially from those projected in the forward-looking statements. Except as required by applicable law, we do not plan to publicly update or revise any forward-looking statements contained herein, whether as a result of any new information, future events, changed circumstances or otherwise. All forward-looking statements are qualified in their entirety by this cautionary statement, which is made under the safe harbor provisions of the Private Securities Litigation Reform Act of 1995 and we undertake no obligation to revise or update this presentation to reflect events or circumstances after the date hereof. This presentation also contains estimates and other statistical data made by independent parties and by us relating to market size and growth and other data about our industry. This data involves a number of assumptions and limitations, and you are cautioned not to give undue weight to such estimates. In addition, projections, assumptions, and estimates of our future performance and the future performance of the markets in which we operate are necessarily subject to a high degree of uncertainty and risk. 2
Call Participants 3 Faheem Hasnain — Co-Founder, Chairman, and Chief Executive Officer Bryan Giraudo — Chief Operating Officer & Chief Financial Officer Richard Aranda, M.D. — Chief Medical Officer Bob Smith — Chief Commercial Officer Rob Roscigno, Ph.D. — Senior Vice President of Clinical Development Caryn Peterson – Executive Vice President, Regulatory Affairs
Phase 3 PROSERA Trial Design 4 Open Label Extension (OLE) Period Double Blind Placebo- Controlled Treatment Period (24 weeks) Patients with PAH Seralutinib 90 mg BID; n ≈ 175 Placebo BID; n ≈ 175 Seralutinib 90 mg BID Primary Endpoint (Δ in 6MWD) Evaluated at Week 24 Key Secondary Endpoints at Week 24: • Δ in NT-proBNP • Clinical Improvement • Reduction of REVEAL Lite Risk Score Key Secondary Endpoint (Time to Clinical Worsening) Evaluated out to 48 Weeks Placebo-Controlled Treatment Period (48 Weeks) Extended Double-Blind Treatment (up to an add’l 24 weeks) BID = twice-daily; 6MWD = 6-minute walk distance.
Patient Disposition 5 Placebo N=193 390 patients randomized and dosed Seralutinib N=197 Completed IP Week 24 177 (91.7%) Completed IP EOS 163 (84.5%) Completed IP Week 24 161 (81.7%) Completed IP EOS 154 (78.2%) Reasons for early withdrawal: • Progressive disease (12) • Adverse Event (9) • Death (2) • All Others (7) Reasons for early withdrawal: • Progressive disease (0) • Adverse Event (26) • Death (4) • All Others (13) Baseline characteristics included in appendix. IP = investigational product; EOS = end of study.
Results in Overall Patient Population 6
While Separation from Placebo is Apparent, Seralutinib Did Not Meet the Primary Endpoint of Δ in 6MWD at Week 24 (α = 0.025) 6MWD = 6-minute walk distance; CI = confidence interval. 7
Observed Mean Change in 6MWD Continued to Separate From Placebo After Week 24 in Sub-Population That Reached Week 48 ARSW w/MI = aligned rank stratified Wilcoxon test with multiple imputation; H-L Location Shift = Hodges-Lehmann location-shift estimate (treatment effect vs placebo); CI = confidence interval. 8 No. Observed Seralutinib 197 186 165 57 Placebo 193 186 176 65 Weeks after Randomization Seralutinib Placebo Observed Mean Observed Mean Imputed median (primary analysis – ARSW w/MI) Imputed median (primary analysis – ARSW w/MI) Primary Analysis at Week 24: H-L Location Shift: 13.298m 97.5% CI: -0.574, 27.169 p-value: 0.0320 Change in 6MWD Through Week 48 Δ i n 6 M W D ( m ) 0 10 20 30 40 50 60 0 12 24 36 48
PROSERA Had Elevated Placebo 6MWD Response vs. Other Pivotal Studies For Add-On Treatments in PAH Note: Seralutinib is an investigational agent not approved for use in any jurisdiction. Caution: cross-trial comparisons may be limited by differences such as patient populations and study design. Tre = Treprostinil; Pbo = placebo; ERA = endothelin receptor antagonist; PDE5 = phosphodiesterase type 5 (PDE5) inhibitor. 1) McLaughlin, et al. JACC 2010; 2) clinicaltrials.gov/study/NCT00325442; 3) clinicaltrials.gov/study/NCT00887978; 4) Sitbon, et al. NEJM 2015; 5) White, et al. AJRCCM 2020; 6) Hoeper, et al. NEJM 2023 9 3.0 4.8 11.0 (9.0) 8.0 1.0 13.5 (10.0) (5.0) 0.0 5.0 10.0 15.0 Reported Placebo Δ (m) in 6MWD in Select Pivotal PAH Studies Compound Inhaled Tre Oral Tre Oral Tre Selexipag Oral Tre Sotatercept Seralutinib Study TRIUMPH1 FREEDOM-C2 FREEDOM-C23 GRIPHON4 FREEDOM-EV5 STELLAR6 PROSERA Timepoint Week 12 Week 16 Week 16 Week 26 Week 24 Week 24 Week 24 Year 2007 2010 2010 2014 2018 2022 2026 Pbo-Adj. Δ 20m 11m 10m 12m 8m 41m 13m Background PAH Therapies PDE5 or ERA PDE5 + ERA PDE5 + ERA PDE5 + ERA PDE5 + ERA PDE5 + ERA + Prostacyclin PDE5 + ERA + Prostacyclin + Sotatercept
Placebo Effect was More Pronounced in Certain Regions, Particularly Latin America & Asia/Middle East 6MWD = 6-minute walk distance; CI = confidence interval. 10
Key Secondary Endpoints Favored Seralutinib in Overall Patient Population (all comparisons vs. placebo) OR = odds ratio; HR = hazard ratio; CI = confidence interval. Note: p-values nominal. 11 Time to Clinical Worsening Change in NT-proBNP Clinical Improvement Reduction in Risk Score 17% reduction (HR: 0.829, 97.5% CI: 0.393, 1.708, p-value: 0.5621) -120.4 ng/L (97.5% CI: -201.6, -39.1 p-value: 0.0002) 1.6x times more likely to improve (OR: 1.642, 97.5% CI: 0.863, 3.106 p-value: 0.0812) 1.4x times more likely to have reduction (OR: 1.420, 97.5% CI: 0.896, 2.248 p-value: 0.0877)
Results in Prespecified Intermediate- and High-Risk Subgroup 12
Seralutinib Demonstrated Meaningful Treatment Effect at Week 24 in Patients With Intermediate to High Risk, While Placebo Response Muted Effect in Cohort With Low Risk 13 Low Risk Sub-Group (REVEAL Lite 2 <6) Intermediate-to-High Risk Sub-Group (REVEAL Lite 2 ≥ 6) CI = confidence interval; 6MWD = six-minute walk distance. Note: p-values nominal.
The Tale of Two 6MWD Populations: Consistent Drug Effect, Differential Placebo 14 No. Observed Seralutinib 78 72 65 Placebo 78 75 72 Weeks after Randomization Seralutinib Placebo Observed Mean Observed Mean Imputed median (primary analysis) Imputed median (primary analysis) Low Risk Sub-Group (REVEAL Lite 2 <6) Change in 6MWD Through Week 24 0 5 10 15 20 25 30 35 40 45 50 0 12 24 0 5 10 15 20 25 30 35 40 45 50 0 12 24 119 114 100 115 111 104 Weeks after Randomization Seralutinib Placebo Observed Mean Observed Mean Imputed median (primary analysis) Imputed median (primary analysis) Intermediate-to-High Risk Sub-Group (REVEAL Lite 2 ≥ 6) Change in 6MWD Through Week 24 Primary Analysis at Week 24: H-L Location Shift: 4.548m 97.5% CI: -14.185, 23.280 p-value: 0.5561 Primary Analysis at Week 24: H-L Location Shift: 19.975m 97.5% CI: 0.549, 39.401 p-value: 0.0207 Δ i n 6 M W D ( m ) Δ i n 6 M W D ( m ) H-L Location Shift = Hodges-Lehmann location-shift estimate (treatment effect vs placebo); CI = confidence interval.
Comparison of PROSERA 6MWD Δ vs. Approved PAH Add-On Therapies Note: Seralutinib is an investigational agent not approved for use in any jurisdiction. Caution: cross-trial comparisons may be limited by differences such as patient populations and study design. Pbo = placebo; Tre = Treprostinil; ERA = endothelin receptor antagonist; PDE5 = phosphodiesterase type 5 (PDE5) inhibitor. 1) McLaughlin, et al. JACC 2010; 2) clinicaltrials.gov/study/NCT00325442; 3) clinicaltrials.gov/study/NCT00887978; 4) Sitbon, et al. NEJM 2015; 5) White, et al. AJRCCM 2020; 6) Hoeper, et al. NEJM 2023; 7) SEC Filings. 15 8.0 10.0 11.0 12.0 13.3 20.0 20.0 40.8 0.0 10.0 20.0 30.0 40.0 50.0 Reported Pbo-Adj. Δ (m) in 6MWD in Select Pivotal PAH Studies Compound Oral Treprostinil Selexipag Seralutinib Inhaled Tre Sotatercept Study FREEDOM-EV5 FREEDOM-C23 FREEDOM-C2 GRIPHON4 PROSERA (Overall) PROSERA (Int & High-Risk Pop) TRIUMPH1 STELLAR6 Timepoint Week 24 Week 16 Week 16 Week 26 Week 24 Week 24 Week 12 Week 24 Year 2018 2010 2010 2014 2026 2026 2007 2022 Active Δ 16m 15m 14.5m 4m 28.2m 26.1m 21.6m 34.4m Placebo Δ 8m 11m 4.8m -9m 13.5m 3.9m 3.0m 1.0m Background PAH Therapies PDE5 + ERA PDE5 + ERA PDE5 + ERA PDE5 + ERA PDE5 + ERA + Prostacyclin + Sotatercept PDE5 or ERA PDE5 + ERA + Prostacyclin Most Recent TTM Sales7 $484mm $1.9bn NA NA $1.8bn $1.4bn
Likewise, Enhanced Effects Were Seen in Intermediate- to High-Risk Sub- Group for Key Secondary Endpoints (all comparisons vs. placebo) 16 Time to Clinical Worsening Change in NT-proBNP Clinical Improvement Reduction in Risk Score (>1 point) 26% reduction (HR: 0.744, 97.5% CI: 0.307, 1.728, p-value: 0.4360) -265.8 ng/L (97.5% CI: -446.2, -85.5 p-value: 0.0002) 3.3x times more likely to improve (OR: 3.318, 97.5% CI: 1.167, 9.43 p-value: 0.0101) 2.0x times more likely to have reduction in risk (OR: 2.033, 97.5% CI: 1.113, 3.713 p-value: 0.0083) OR = odds ratio; HR = hazard ratio; CI = confidence interval. Note: p-values nominal.
Safety & Tolerability Results 17
Overall Summary of Treatment-Emergent Adverse Events (Safety Population) TEAE = treatment-emergent adverse event; IP = investigational product; AESI = adverse event of special interest; SAE = serious adverse event. 18 Category Placebo (N=190) n (%) Seralutinib (N=200) n (%) Number of subjects with at least one: TEAE 153 ( 80.5) 173 ( 86.5) Severe TEAE 30 ( 15.8) 30 ( 15.0) TEAE leading to discontinuation of IP 11 ( 5.8) 30 ( 15.0) AESI 12 ( 6.3) 41 ( 20.5) SAE 36 ( 18.9) 32 ( 16.0) Fatal TEAE 3 ( 1.6) 4 ( 2.0) Number of SAEs 67 53
Incidence of SAEs by preferred term: ≥ 2 Seralutinib subjects (Safety Population) SAE = serious adverse event. 19 Preferred Term Placebo (N=190) n (%) Seralutinib (N=200) n (%) Number of subjects with a SAE 36 ( 18.9) 32 ( 16.0) Pneumonia 3 ( 1.6) 3 ( 1.5) Pulmonary arterial hypertension 5 ( 2.6) 3 ( 1.5) Right ventricular failure 4 ( 2.1) 3 ( 1.5) Acute kidney injury 0 2 ( 1.0) Device malfunction 2 ( 1.1) 2 ( 1.0) Lower gastrointestinal haemorrhage 0 2 ( 1.0)
Incidence of TEAEs by preferred term: ≥ 5% in Seralutinib arm (Safety Population) TEAE = treatment-emergent adverse event. 20 Preferred Term Placebo (N=190) n (%) Seralutinib (N=200) n (%) Number of subjects with a TEAE 153 ( 80.5) 173 ( 86.5) Cough 26 ( 13.7) 74 ( 37.0) Headache 27 ( 14.2) 32 ( 16.0) Alanine aminotransferase increased 1 ( 0.5) 29 ( 14.5) Aspartate aminotransferase increased 1 ( 0.5) 28 ( 14.0) Nausea 20 ( 10.5) 24 ( 12.0) Diarrhoea 26 ( 13.7) 23 ( 11.5) Upper respiratory tract infection 10 ( 5.3) 17 ( 8.5) Dizziness 14 ( 7.4) 13 ( 6.5) Dyspnoea 16 ( 8.4) 11 ( 5.5) Hypokalaemia 14 ( 7.4) 11 ( 5.5) Nasopharyngitis 17 ( 8.9) 11 ( 5.5) Vomiting 11 ( 5.8) 11 ( 5.5) Influenza 8 ( 4.2) 10 ( 5.0) Syncope 8 ( 4.2) 10 ( 5.0)
Conclusion and Next Steps 21
Overall Takeaways from PROSERA Topline Results • While study narrowly missed primary endpoint, we believe the overall results demonstrate clear evidence of clinical benefit in a heavily pretreated patient population • PROSERA confirmed observation from TORREY Phase 2 of enhanced separation from placebo in patients with more severe baseline disease • Safety and tolerability profile appear acceptable as on add-on therapy in PAH, with primary safety observations (cough and liver enzyme elevations) well understood by PAH treaters given profiles of existing therapies • We believe the PROSERA and TORREY results together support a positive risk-benefit profile for seralutinib, potentially offering a new mechanism of action for a progressive disease with significant unmet need 22
Next Steps CT FRI = computed tomography functional respiratory imaging. 23 • Complete our in-depth analyses of the PROSERA data across endpoints and subgroups, pending the results from the CT FRI substudy, expected in the coming weeks • Engage with the FDA to discuss the results to understand their perspective • Assess ramifications for seralutinib and Gossamer portfolio, including impacts on capital allocation
24 We are deeply grateful to the patients, investigators, and teams whose dedication advanced the development of seralutinib Thank you
Appendix
Select Baseline Demographics and PAH Disease Characteristics * PAH associated with anorexigen use, methamphetamine use, or pulmonary shunt. 26 Characteristic Placebo (N=193) Seralutinib (N=197) Total (N=390) Age (years) – mean (SD) 49.9 (13.87) 50.1 (13.89) 50.0 (13.86) Sex – n (%) Female 164 ( 85.0) 170 ( 86.3) 334 ( 85.6) PAH Classification – n (%)* Idiopathic 113 ( 58.5) 126 ( 64.0) 239 ( 61.3) Heritable 25 ( 13.0) 13 ( 6.6) 38 ( 9.7) Associated with CTD 42 ( 21.8) 45 ( 22.8) 87 ( 22.3) Other 13 ( 6.7) 13 ( 6.6) 26 ( 6.7) 6MWD at baseline (m) Mean (SD) 374.9 (65.27) 372.9 (73.13) 373.9 (69.27) Median 389.0 396.0 393.0 WHO FC at screening – n (%) Class II 49 ( 25.4) 51 ( 25.9) 100 ( 25.6) Class III 144 ( 74.6) 146 ( 74.1) 290 ( 74.4) NT-proBNP at baseline (ng/L) Mean (SD) 965.4 (1576.86) 1024.8 (1623.60) 995.4 (1598.86) Median 422.0 451.0 423.5
Select Baseline PAH Disease Characteristics PRA = prostacyclin receptor agonist. 27 Characteristic Placebo (N=193) Seralutinib (N=197) Total (N=390) Number of background PAH disease-specific medications – n (%) 1 9 ( 4.7) 13 ( 6.6) 22 ( 5.6) 2 76 ( 39.4) 78 ( 39.6) 154 ( 39.5) ≥ 3 108 ( 56.0) 106 ( 53.8) 214 ( 54.9) Prostacyclin/PRA – n (%) 120 (62.2) 117 (59.4) 237 (60.8) Inhaled 13 ( 6.7) 14 ( 7.1) 27 ( 6.9) Oral 50 ( 25.9) 49 ( 24.9) 99 ( 25.4) Parenteral 59 ( 30.6) 56 ( 28.4) 115 ( 29.5) Subcutaneous 37 ( 19.2) 30 ( 15.2) 67 ( 17.2) Intravenous (IV) 22 ( 11.4) 26 ( 13.2) 48 ( 12.3)
FAQ
What did Gossamer Bio (GOSS) report from the Phase 3 PROSERA trial?
Gossamer Bio reported topline Phase 3 PROSERA data for seralutinib in pulmonary arterial hypertension. The study showed a 13.3‑meter placebo‑adjusted six‑minute walk distance benefit at Week 24, with a p‑value of 0.0320 that did not meet the prespecified primary endpoint threshold.
Did seralutinib meet the primary endpoint in PROSERA for Gossamer Bio (GOSS)?
No, seralutinib did not meet the primary endpoint in PROSERA. The Hodges‑Lehmann treatment effect on six‑minute walk distance was 13.3 meters at Week 24 with a p‑value of 0.0320, above the prespecified α=0.025 required for statistical significance on the primary endpoint.
How did seralutinib perform in high-risk pulmonary arterial hypertension patients in PROSERA?
In the prespecified intermediate‑ and high‑risk subgroup, seralutinib showed stronger effects. The placebo‑adjusted improvement in six‑minute walk distance was 20.0 meters at Week 24, and key secondary endpoints such as NT‑proBNP reduction and REVEAL Lite 2 risk score improvement also favored seralutinib with nominally significant p‑values.
What safety profile did seralutinib show in the PROSERA Phase 3 study?
Seralutinib was generally well tolerated, but certain adverse events were more frequent than with placebo. Treatment‑emergent adverse events occurred in 86.5% of seralutinib patients versus 80.5% on placebo, with cough most common at 37.0% and liver enzyme elevations of at least three times normal in 13% versus 1% on placebo.
What are Gossamer Bio’s next steps for seralutinib after the PROSERA results?
Gossamer Bio plans to complete deeper analyses of PROSERA data, including a CT functional respiratory imaging substudy, and then meet with the U.S. FDA to discuss seralutinib’s path forward. The company is pausing enrollment in the SERANATA Phase 3 study while it evaluates PROSERA findings and regional placebo response differences.
How did seralutinib perform in connective tissue disease–associated PAH patients?
In patients with connective tissue disease–associated PAH, seralutinib showed a robust effect on exercise capacity. The drug achieved a placebo‑adjusted six‑minute walk distance gain of 37.0 meters at Week 24 in this subgroup of 87 patients, with a nominal p‑value of 0.0104 supporting a strong treatment effect.
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[S-3ASR] Gossamer Bio, Inc. SEC Filing
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[8-K] Gossamer Bio, Inc. Reports Material Event