TRex Bio, Inc.

S-11 ck0002139097-20260918.htm S-1

As filed with the Securities and Exchange Commission on September 18, 2026.

Registration No. 333-

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM S-1

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

TREX BIO, INC.

(Exact name of registrant as specified in its charter)

269 East Grand Avenue, Suite 300

South San Francisco, CA 94080

(650) 567-5582

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

M. Johnston Erwin, Jr.

Chief Executive Officer

269 East Grand Avenue, Suite 300

South San Francisco, CA 94080

(650) 567-5582

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale to the public:

As soon as practicable after the effective date of this Registration Statement.

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, check the following box.☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.☐

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

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 7(a)(2)(B) of the Securities Act.☐

The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933, as amended, or until the Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

The information in this preliminary prospectus is not complete and may be changed. These securities may not be sold until the registration statement filed with the Securities and Exchange

Commission is effective. This preliminary prospectus is not an offer to sell nor does it seek an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

Subject to completion, dated September 18, 2026

Preliminary Prospectus

Shares

This is TRex Bio, Inc.’s initial public offering. We are selling shares of our common stock.

We expect the public offering price to be between $ and $ per share. Currently, no public market exists for the shares. We have applied to list our common stock on The Nasdaq Global Select Market (“Nasdaq”) under the symbol “TRXB” and this offering is contingent upon obtaining approval of such listing.

We are an “emerging growth company” and a “smaller reporting company” as defined under the federal securities laws and, as such, have elected to comply with certain reduced reporting requirements in this prospectus and may elect to do so in future filings.

Investing in our common stock involves a high degree of risk. See the section titled “Risk Factors” beginning on page 11 of this prospectus.

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

(1) See the section titled “Underwriting” for additional information regarding underwriting compensation.

The underwriters may also exercise their option to purchase up to an additional shares from us, at the initial public offering price, less underwriting discounts and commissions, for 30 days after the date of this prospectus.

The underwriters expect to deliver the shares of our common stock on or about , 2026.

Eli Lilly and Company (“Lilly”) has indicated an interest in purchasing at the initial public offering price shares of common stock in this offering in an amount, when combined with shares of common stock beneficially owned by Lilly prior to the initial public offering, that will not exceed 19.9% of the number of shares of common stock issued and outstanding after this offering. Because this indication of interest is not a binding agreement or commitment to purchase, Lilly may determine to purchase more, less or no shares in this offering or the underwriters may determine to sell more, less or no shares to Lilly. The underwriters will receive the same underwriting discounts and commissions on any of our shares of common stock purchased by Lilly as it will from any other shares of common stock sold to the public in this offering. The number of shares of common stock available for sale to the public will be reduced to the extent that Lilly purchases shares of common stock in the offering.

The date of this prospectus is , 2026.