Atara Biotherapeutics increases ATM capacity $79.27M

Atara Biotherapeutics amended its sales-agreement prospectus to confirm it may continue at-the-market sales up to an aggregate offering price of $79,269,007 under the existing Sales Agreement with TD Securities (USA) LLC.

Rhea-AI Filing Summary

Atara Biotherapeutics amended its sales-agreement prospectus to confirm it may continue at-the-market sales up to an aggregate offering price of $79,269,007 under the existing Sales Agreement with TD Securities (USA) LLC. This supplement, dated May 12, 2026, relies on a staff interpretation addressing Form S-3 eligibility after a Section 10(a)(3) update.

The supplement states the company may offer and sell shares of Common Stock from time to time through the named sales agent, and that the full amount remaining under the Sales Agreement is available despite a change in the Form S-3 eligibility criterion referenced in General Instruction I.B.1 because the company qualifies under General Instruction I.B.6.

Insights

Equity Capital Markets Attorney

Confirms ATM capacity continuity under staff guidance

The supplement formally memorializes reliance on the Staff Interpretation permitting continued sales under an existing AT-the-market prospectus supplement. It specifies the available aggregate offering price as $79,269,007 and names TD Securities (USA) LLC as sales agent.

Critical dependencies include the Sales Agreement terms and the company's continued S-3 eligibility under General Instruction I.B.6; cash‑flow treatment is standard issuer proceeds from sold shares. Subsequent filings will reflect sales activity and any Section 10(a)(3) updates.

Key Figures

Registration No.: 333-275256 Available ATM capacity: $79,269,007 Common stock listing: NASDAQ: ATRA +2 more

5 metrics

Registration No. 333-275256 Form S-3 shelf registration

Available ATM capacity $79,269,007 <date>May 12, 2026</date> aggregate offering price remaining

Common stock listing NASDAQ: ATRA Company's Common Stock listed on the Nasdaq Global Market

Prospectus supplement date May 12, 2026 Date of this Second Supplement

Sales agent TD Securities (USA) LLC Successor to Cowen and Company under the Sales Agreement

Key Terms

at-the-market, Form S-3 General Instruction I.B.6, Section 10(a)(3) update, Sales Agreement Prospectus

4 terms

at-the-market market

"offer and sale of shares of common stock... through TD Securities"

"At-the-market" is a method for companies to sell new shares of stock directly into the open market over time, rather than all at once. It allows companies to raise money gradually, similar to selling slices of a pie instead of the entire pie at once, which can help manage the sale's impact on the stock price. This approach gives investors a steady supply of shares while providing companies with flexible funding options.

Form S-3 General Instruction I.B.6 regulatory

"remains eligible to use Form S-3 in reliance on General Instruction I.B.6"

Section 10(a)(3) update regulatory

"at the time of the company’s next Section 10(a)(3) update"

Sales Agreement Prospectus other

"Sales Agreement Prospectus, as previously amended and supplemented"

A sales agreement prospectus is the document a company files and provides to investors when it asks permission to sell new shares or other securities to the public through an ongoing arrangement with an underwriter or broker. It explains how sales will be executed, the limits and costs, and the potential impact on existing shareholders; think of it as a menu and rulebook that lets a company raise cash quickly while warning investors about possible dilution.

Offering Details

ATM

Offering

Offering Type ATM

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What does the May 12, 2026 supplement for ATRA do?

It confirms Atara may continue at-the-market sales up to $79,269,007 under its Sales Agreement. The supplement relies on a SEC staff interpretation allowing the previously filed prospectus supplement to remain effective after a Section 10(a)(3) update.

Who is the sales agent under the Sales Agreement for ATRA?

The named sales agent is TD Securities (USA) LLC (successor to Cowen and Company, LLC). The agent may sell Common Stock from time to time pursuant to the Sales Agreement and prospectus supplement.

Will Atara receive proceeds from these sales?

Yes. The sales are issuer primary offerings through an ATM sales agreement, so the company receives proceeds from shares sold under the arrangement. The supplement states the aggregate offering price available is $79,269,007.

Why does the supplement cite a SEC staff interpretation?

The supplement cites a Staff Interpretation permitting continuation of an offering amount named in a prior prospectus supplement when a later Section 10(a)(3) update changes Form S-3 eligibility, provided the company meets conditions including reliance on General Instruction I.B.6.

Does this supplement change risk disclosures for ATRA?

No. The supplement directs readers to existing risk disclosures in the Sales Agreement Prospectus and documents incorporated by reference. It does not itself add new risk factors; it confirms available ATM capacity of $79,269,007.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Accepted by SEC EDGAR 05/12/2026 - 04:20 PM Learn about SEC filing dates

Filed Pursuant to Rule 424(b)(5)
Registration No. 333-275256

PROSPECTUS SUPPLEMENT

(To Prospectus dated November 13, 2023)

$79,269,007

Common Stock

This prospectus supplement no. 2 (this “Second Supplement”) amends and supplements the information in our sales agreement prospectus, dated November 13, 2023 (the “Sales Agreement Prospectus”), as previously amended and supplemented by the prospectus supplement dated March 16, 2026 (the “First Supplement”, and together with the Sales Agreement Prospectus, the “Prospectus”), contained in the shelf registration statement on Form S-3 (Registration No. 333-275256) of which the Prospectus is a part (the “Registration Statement”), relating to the offer and sale of shares of common stock, par value $0.0001 per share (“Common Stock”), of Atara Biotherapeutics, Inc. (the “Company”) from time to time through TD Securities (USA) LLC (as successor to Cowen and Company, LLC (“TD Cowen” or the “Sales Agent”)) pursuant to the sales agreement, dated November 1, 2023 (the “Sales Agreement”), between the Company and TD Cowen. This Second Supplement should be read in conjunction with the Prospectus, and is qualified by reference thereto, except to the extent that the information herein amends or supersedes the information contained in the Prospectus. This Second Supplement is not complete without, and may only be delivered or utilized in connection with, the Prospectus and any future amendments or supplements thereto.

Subsequent to the filing of the First Supplement, on March 19, 2026, the staff of the Division of Corporation Finance of the Securities and Exchange Commission (the “Staff”) published Question 116.26 of the Securities Act Forms Corporation Finance Interpretations (the “Staff Interpretation”). The Staff Interpretation provides that, where (i) a company has entered into a sales agreement with a named selling agent for an at-the-market offering of an amount of securities that the company reasonably expected to offer and sell, (ii) the company had an effective Form S-3 registration statement, was eligible to offer and sell securities in reliance on General Instruction I.B.1 of Form S-3, and filed a prospectus supplement for the offering, and (iii) at the time of the company’s next Section 10(a)(3) update, the company does not meet the $75 million public float requirement of General Instruction I.B.1 of Form S-3 but remains eligible to use Form S-3 in reliance on General Instruction I.B.6 of Form S-3, the Staff will not object if the company continues to offer and sell the full amount of securities covered by the prospectus supplement that was filed prior to the Section 10(a)(3) update, even if that amount would exceed the offering limits of General Instruction I.B.6 of Form S-3.

Consistent with the Staff Interpretation, the Company hereby supplements the Prospectus to reflect that it may offer and sell shares of Common Stock having an aggregate offering price equal to the full amount remaining available under the Sales Agreement and the Sales Agreement Prospectus. As of the date of this Second Supplement, the aggregate offering price of shares of Common Stock that remains available to be offered and sold under the Sales Agreement and the Prospectus, as supplemented by this Second Supplement, is $79,269,007.

Our Common Stock is listed on the Nasdaq Global Market under the symbol “ATRA”.

Investing in our Common Stock involves risks. Before buying any shares, you should read the discussion of material risks of investing in our Common Stock in “Risk Factors” beginning on page SA-5 of the Sales Agreement Prospectus, and in the risks discussed under similar headings in the documents incorporated by reference in this Second Supplement and the Prospectus, as they may be amended, updated or modified periodically in our reports filed with the Securities and Exchange Commission.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this Second Supplement and the Prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

TD Cowen

The date of this prospectus supplement is May 12, 2026.