[8-K] Zymeworks Inc. Reports Material Event

Rhea-AI Filing Summary

Zymeworks Inc. (ZYME) announced positive topline Phase 3 results for its lead drug Ziihera® (zanidatamab-hrii). The HERIZON-GEA-01 trial evaluated Ziihera in combination with chemotherapy, with or without the PD‑1 inhibitor Tevimbra® (tislelizumab), as a first-line treatment for HER2‑positive locally advanced or metastatic gastroesophageal adenocarcinoma. This includes cancers of the stomach, gastroesophageal junction, and esophagus, an area of significant medical need. The positive topline outcome from this late‑stage study suggests Ziihera may have meaningful clinical activity in this setting, and further details are provided in the company’s November 17, 2025 press release.

Positive

• Positive Phase 3 topline data for Ziihera in HER2+ gastroesophageal adenocarcinoma reduces late‑stage clinical risk for a key pipeline asset and may support future regulatory steps.

Negative

• None.

Insights

Biotech and Clinical Trials Analyst

Positive Phase 3 topline results for Ziihera in HER2+ gastric cancer are a potentially significant value driver for Zymeworks.

The company reports positive topline data from the Phase 3 HERIZON‑GEA‑01 trial of Ziihera (zanidatamab‑hrii) in combination with chemotherapy, with or without Tevimbra (tislelizumab), as first‑line therapy for HER2‑positive locally advanced or metastatic gastroesophageal adenocarcinoma. Phase 3 success in this indication is typically a key step toward potential regulatory filings and future commercialization, though no regulatory steps are described here.

The trial targets HER2‑positive cancers of the stomach, gastroesophageal junction, and esophagus, which collectively represent a serious disease area with limited options. Positive topline results at this stage can materially influence the perceived value of a company’s pipeline because they reduce late‑stage clinical risk around the asset.

The detailed efficacy and safety data are not included in this summary and are instead contained in the attached press release dated November 17, 2025. The actual impact on Zymeworks will depend on the magnitude of benefit reported, safety profile, and subsequent regulatory and commercial developments described in future disclosures.

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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): November 17, 2025

Zymeworks Inc.

(Exact name of registrant as specified in its charter)

Delaware		001-41535		88-3099146
(State or other jurisdiction of incorporation)		(Commission File Number)		(IRS Employer Identification No.)

108 Patriot Drive, Suite A Middletown, Delaware		19709
(Address of principal executive offices)		(Zip Code)

(302) 274-8744

(Registrant’s telephone number, including area code)

Not Applicable

(Former name or former address, if changed since last report)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

☐ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

☐ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

☐ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

☐ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Securities registered pursuant to Section 12(b) of the Act:

Title of each class		Trading Symbol(s)		Name of each exchange on which registered
Common Stock, par value $0.00001 per share		ZYME		The Nasdaq Stock Market LLC

Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933 (§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter).

Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Item 8.01 Other Events.

On November 17, 2025, Zymeworks Inc. (the “Company”) issued a press release announcing positive topline results from the Phase 3 HERIZON-GEA-01 trial evaluating Ziihera^® (zanidatamab-hrii) in combination with chemotherapy, with or without the PD-1 inhibitor Tevimbra^® (tislelizumab), as a first-line treatment for HER2-positive (HER2+) locally advanced or metastatic gastroesophageal adenocarcinoma, including cancers of the stomach, gastroesophageal junction, and esophagus. A copy of this press release is attached as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated herein by reference.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

Exhibit No.		Description
99.1		Press Release, dated November 17, 2025
104		Cover Page Interactive Data File (embedded as Inline XBRL document).

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

				ZYMEWORKS INC.
				(Registrant)
Date: November 17, 2025				By:		/s/ Kenneth Galbraith
				Name: Title:		Kenneth Galbraith Chair, President and Chief Executive Officer

FAQ

What did Zymeworks Inc. (ZYME) announce in this 8-K filing?

Zymeworks Inc. announced positive topline results from the Phase 3 HERIZON‑GEA‑01 trial evaluating its drug Ziihera® (zanidatamab‑hrii) for HER2‑positive gastroesophageal adenocarcinoma.

Which drug is being evaluated in Zymeworks' Phase 3 HERIZON-GEA-01 trial?

The trial is evaluating Ziihera® (zanidatamab‑hrii) in combination with chemotherapy, with or without the PD‑1 inhibitor Tevimbra® (tislelizumab).

What condition is targeted in Zymeworks' HERIZON-GEA-01 Phase 3 study?

The study targets HER2‑positive locally advanced or metastatic gastroesophageal adenocarcinoma, including cancers of the stomach, gastroesophageal junction, and esophagus.

How is Ziihera being used in the HERIZON-GEA-01 trial?

Ziihera is being evaluated in combination with chemotherapy, and in some study arms with or without Tevimbra (tislelizumab), as a first‑line treatment option.

Where can investors find more details about the Phase 3 results for ZYME?

More detailed information is provided in the company’s press release dated November 17, 2025, which is attached to the report as Exhibit 99.1.

Does this 8-K filing indicate that Ziihera has regulatory approval?

No. The filing states that the company reported positive topline Phase 3 results; it does not state that Ziihera has received any regulatory approval.