Atrium Therapeutics Schedule 13G discloses passive holdings by RA Capital and affiliates. The filing reports that RA Capital Healthcare Fund, L.P. directly holds 864,102 shares of Common Stock, representing 5.6% of the class based on 15,514,996 shares outstanding as of February 26, 2026. The filing explains that RA Capital serves as investment adviser and that voting and dispositive power over the Fund's shares are delegated to RA Capital; the Reporting Persons disclaim group status and disclaim beneficial ownership except as required for Section 13(d) purposes.
Positive
None.
Negative
None.
Insights
RA Capital reports a passive 5.6% stake via its healthcare fund.
The Schedule 13G states the Fund directly holds 864,102 shares and that the percentage is calculated from 15,514,996 shares outstanding as of February 26, 2026. The filing notes delegation of voting and dispositive power to the adviser and contains standard disclaimers about beneficial ownership under Section 13(d).
Cash-flow treatment or planned sales are not stated; subsequent filings would disclose any changes in ownership or voting arrangements.
Filing emphasizes advisory/control structure and disclaimers, not active coordination.
The report names RA Capital Management, L.P., RA Capital Healthcare Fund GP, LLC, and individuals Peter Kolchinsky and Rajeev Shah as Reporting Persons, and expressly disclaims "group" status. It describes delegated voting/dispositive authority and the 61-day revocation condition for the Fund's delegation.
Material governance items to watch would appear in future filings if voting arrangements or percent ownership change.
Key Figures
Fund holdings:864,102 sharesPercent of class:5.6%Shares outstanding:15,514,996 shares+1 more
4 metrics
Fund holdings864,102 sharesDirectly held by RA Capital Healthcare Fund, L.P.; as of March 31, 2026
Percent of class5.6%Calculated using 15,514,996 shares outstanding as of February 26, 2026
Shares outstanding15,514,996 sharesOutstanding as of <date>February 26, 2026</date>
Delegation notice period61 daysMinimum notice to revoke delegated voting and investment power described in filing
Key Terms
Schedule 13G, Beneficial ownership, Dispositive power
3 terms
Schedule 13Gregulatory
"The names of the persons filing this report (collectively, the "Reporting Persons") are:"
A Schedule 13G is a formal document that investors file with the government when they acquire a large ownership stake in a company, usually for investment purposes rather than control. It helps keep the public informed about who owns significant parts of a company's shares, which can influence how the company is managed and how investors make decisions. Filing this schedule is important for transparency and understanding the ownership landscape of publicly traded companies.
Beneficial ownershipfinancial
"Amount beneficially owned: Row 9 of each Reporting Person's cover page to this sets forth the aggregate number"
Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.
Dispositive powerregulatory
"Sole Dispositive Power 0.00 8 | Shared Dispositive Power 864,102.00"
Dispositive power is the authority to decide the final outcome of an asset, legal claim, contract, or corporate action — in effect the power to dispose of or resolve something. For investors it matters because whoever holds that authority can determine who gets paid, who controls an asset or vote, and how risks and returns are allocated; think of it like holding the key that lets you lock in the winner or loser in a deal.
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
UNDER THE SECURITIES EXCHANGE ACT OF 1934
Atrium Therapeutics, Inc.
(Name of Issuer)
Common Stock, par value $0.001 per share
(Title of Class of Securities)
04965N104
(CUSIP Number)
03/31/2026
(Date of Event Which Requires Filing of this Statement)
Check the appropriate box to designate the rule pursuant to which this Schedule is filed:
Rule 13d-1(b)
Rule 13d-1(c)
Rule 13d-1(d)
schemaVersion:
SCHEDULE 13G
CUSIP Number(s):
04965N104
1
Names of Reporting Persons
RA Capital Management, L.P.
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
DELAWARE
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
864,102.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
864,102.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
864,102.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
5.6 %
12
Type of Reporting Person (See Instructions)
IA, PN
SCHEDULE 13G
CUSIP Number(s):
04965N104
1
Names of Reporting Persons
Peter Kolchinsky
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
UNITED STATES
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
864,102.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
864,102.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
864,102.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
5.6 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
CUSIP Number(s):
04965N104
1
Names of Reporting Persons
Rajeev Shah
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
UNITED STATES
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
864,102.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
864,102.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
864,102.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
5.6 %
12
Type of Reporting Person (See Instructions)
HC, IN
SCHEDULE 13G
CUSIP Number(s):
04965N104
1
Names of Reporting Persons
RA Capital Healthcare Fund, L.P.
2
Check the appropriate box if a member of a Group (see instructions)
(a)
(b)
3
Sec Use Only
4
Citizenship or Place of Organization
DELAWARE
Number of Shares Beneficially Owned by Each Reporting Person With:
5
Sole Voting Power
0.00
6
Shared Voting Power
864,102.00
7
Sole Dispositive Power
0.00
8
Shared Dispositive Power
864,102.00
9
Aggregate Amount Beneficially Owned by Each Reporting Person
864,102.00
10
Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)
11
Percent of class represented by amount in row (9)
5.6 %
12
Type of Reporting Person (See Instructions)
PN
SCHEDULE 13G
Item 1.
(a)
Name of issuer:
Atrium Therapeutics, Inc.
(b)
Address of issuer's principal executive offices:
10578 Science Center Drive, Suite 125, San Diego, CA, 92121.
Item 2.
(a)
Name of person filing:
The names of the persons filing this report (collectively, the "Reporting Persons") are:
RA Capital Management, L.P. ("RA Capital")
Peter Kolchinsky
Rajeev Shah
RA Capital Healthcare Fund, L.P. (the "Fund")
The Reporting Persons expressly disclaim status as a "group" for purposes of this Schedule 13G.
(b)
Address or principal business office or, if none, residence:
The address of the principal business office of each of the Reporting Persons is:
c/o RA Capital Management, L.P., 200 Berkeley Street, 18th Floor, Boston MA 02116
(c)
Citizenship:
RA Capital and the Fund are Delaware limited partnerships.
Dr. Kolchinsky and Mr. Shah are United States citizens.
(d)
Title of class of securities:
Common Stock, par value $0.001 per share
(e)
CUSIP Number(s):
04965N104
Item 3.
If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)
Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)
Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)
Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)
Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)
An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)
An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)
A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)
A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)
A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)
A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J),
please specify the type of institution:
(k)
Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.
Ownership
(a)
Amount beneficially owned:
Row 9 of each Reporting Person's cover page to this Schedule 13G sets forth the aggregate number of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference.
The Fund directly holds 864,102 shares of Common Stock.
RA Capital Healthcare Fund GP, LLC is the general partner of the Fund. The general partner of RA Capital is RA Capital Management GP, LLC, of which Dr. Kolchinsky and Mr. Shah are the controlling persons. RA Capital serves as investment adviser for the Fund and may be deemed a beneficial owner, for purposes of Section 13(d) of the Act, of any securities of the Issuer held by the Fund. The Fund has delegated to RA Capital the sole power to vote and the sole power to dispose of all securities held in the Fund's portfolio, including the shares of the Issuer's Common Stock reported herein. Because the Fund has divested voting and investment power over the reported securities it holds and may not revoke that delegation on less than 61 days' notice, the Fund disclaims beneficial ownership of the securities it holds for purposes of Section 13(d) of the Act. As managers of RA Capital, Dr. Kolchinsky and Mr. Shah may be deemed beneficial owners, for purposes of Section 13(d) of the Act, of any securities of the Issuer beneficially owned by RA Capital. RA Capital, Dr. Kolchinsky, and Mr. Shah disclaim beneficial ownership of the securities reported in this Schedule 13 other than for the purpose of determining their obligations under Section 13(d) of the Act, and the filing of this Schedule 13G shall not be deemed an admission that either RA Capital, Dr. Kolchinsky, or Mr. Shah is the beneficial owner of such securities for any other purpose.
(b)
Percent of class:
Row 11 of each Reporting Person's cover page to this Schedule 13G sets forth the percentages of the securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference. The percentage set forth in each Row 11 is based upon 15,514,996 shares of the Issuer's Common Stock that were outstanding as of February 26, 2026, as reported in the Issuer's Current Report on Form 8-K, filed with the Securities and Exchange Commission (the "SEC") on February 26, 2026.
(c)
Number of shares as to which the person has:
(i) Sole power to vote or to direct the vote:
Row 5 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference.
(ii) Shared power to vote or to direct the vote:
Row 6 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to vote or to direct the vote of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference.
(iii) Sole power to dispose or to direct the disposition of:
Row 7 of each Reporting Person's cover page to this Schedule 13G sets forth the sole power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference.
(iv) Shared power to dispose or to direct the disposition of:
Row 8 of each Reporting Person's cover page to this Schedule 13G sets forth the shared power to dispose or to direct the disposition of securities of the Issuer beneficially owned by such Reporting Person as of March 31, 2026 and is incorporated by reference.
Item 5.
Ownership of 5 Percent or Less of a Class.
Not Applicable
Item 6.
Ownership of more than 5 Percent on Behalf of Another Person.
Not Applicable
Item 7.
Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
Not Applicable
Item 8.
Identification and Classification of Members of the Group.
Not Applicable
Item 9.
Notice of Dissolution of Group.
Not Applicable
Item 10.
Certifications:
Not Applicable
SIGNATURE
After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.
RA Capital Management, L.P.
Signature:
/s/ Peter Kolchinsky
Name/Title:
By Peter Kolchinsky, Authorized Signatory
Date:
05/15/2026
Peter Kolchinsky
Signature:
/s/ Peter Kolchinsky
Name/Title:
Peter Kolchinsky
Date:
05/15/2026
Rajeev Shah
Signature:
/s/ Rajeev Shah
Name/Title:
Rajeev Shah
Date:
05/15/2026
RA Capital Healthcare Fund, L.P.
Signature:
/s/ Peter Kolchinsky
Name/Title:
By RA Capital Healthcare Fund GP, LLC, its General Partner, By Peter Kolchinsky, Manager
What stake does RA Capital report in Atrium Therapeutics (RNA)?
RA Capital Healthcare Fund, L.P. reports 864,102 shares, equal to 5.6% of the class based on 15,514,996 shares outstanding as of February 26, 2026. The filing attributes holdings and calculation to that outstanding share count.
Who are the reporting persons named in the Schedule 13G for RNA?
The filing lists RA Capital Management, L.P., RA Capital Healthcare Fund, L.P., and individuals Peter Kolchinsky and Rajeev Shah as Reporting Persons, with a joint filing agreement included as Exhibit 99.1.
Does the Schedule 13G indicate RA Capital actively controls voting of the shares?
The filing states the Fund delegated sole voting and dispositive power to RA Capital, and notes the delegation may not be revoked on less than 61 days' notice. It also includes disclaimers of beneficial ownership for Section 13(d) purposes.
Is the Schedule 13G filing indicating a formation of a group among the reporters?
No. The Reporting Persons explicitly disclaim status as a 'group' in the Schedule 13G and include language limiting admissions of beneficial ownership except as required under Section 13(d).
What outstanding share count is the 5.6% based on in the RNA filing?
The percentage is based on 15,514,996 shares outstanding as of February 26, 2026, as reported in the Issuer's Form 8-K cited in the Schedule 13G.
