If You Invested in Turnstone Biologics Corp (TSBX)

Turnstone Biologics Corp. (Nasdaq: TSBX) has historically operated as a clinical-stage biotechnology company focused on research and development in biotechnology, specifically in the area of solid tumor oncology. The company has described its work as pioneering a differentiated approach to treat and potentially cure patients with solid tumors through selected tumor-infiltrating lymphocyte (Selected TIL) therapy, a next-generation form of TIL-based immunotherapy.

According to multiple company disclosures, Turnstone’s next-generation TIL therapy is based on the identification, selection and expansion of the most potent tumor‑reactive T cells, referred to as Selected TILs. This approach is designed to address limitations of first‑generation bulk TIL therapies, which have demonstrated objective responses only in a limited number of tumor types. Turnstone has stated that its Selected TIL process employs an unbiased identification and functional screening step intended to isolate and selectively expand a broad set of tumor‑reactive TILs from a patient’s tumor for more targeted tumor killing.

Clinical focus and development programs

Turnstone has described itself as a clinical-stage biotechnology company. Its lead clinical program, TIDAL‑01, has been evaluated in Phase 1 studies in patients with several solid tumor indications. Company communications state that TIDAL‑01 has been or was being studied in microsatellite stable metastatic colorectal cancer (MSS mCRC), head and neck cancer, and uveal melanoma, and at earlier points also in breast cancer and cutaneous melanoma, through Turnstone-sponsored STARLING trials and investigator-sponsored trials in collaboration with H. Lee Moffitt Cancer Center.

Turnstone has reported initial clinical data from its Phase 1 STARLING trial of TIDAL‑01 in MSS metastatic colorectal cancer. In its public updates, the company highlighted an observed overall response rate and disease control rate in the first evaluable patients, including a complete response in a heavily pre-treated late‑line patient with progression‑free survival extending beyond one year, and noted that these early findings were notable in a setting where patients have limited treatment options. The company has also reported that TIDAL‑01 manufacturing runs in colorectal cancer achieved the target cell dose in all manufactured products for patients with sufficient starting material.

In addition to clinical data, Turnstone has presented preclinical data at the Society for Immunotherapy of Cancer (SITC) Annual Meeting, including work on enrichment of neoantigen‑reactive TIL in gastric cancer and on methods to predict expansion of tumor‑reactive TIL using T cell repertoire overlap between blood and tumor. These studies were described as supporting the practicality of selecting and expanding tumor‑reactive TIL as a potential treatment option for patients with solid tumors.

Strategic shifts and restructuring

Throughout 2024, Turnstone reported a series of portfolio prioritization and corporate restructuring measures. The company announced that it would direct resources toward advancing TIDAL‑01 over other preclinical assets, execute a workforce reduction of approximately 60%, and streamline operations to extend its cash runway. These steps included organizational changes in clinical development, manufacturing, finance, and operations leadership, as disclosed in company press releases.

In early 2025, Turnstone announced that, after assessing its business and operations, it had decided to discontinue all clinical studies evaluating TIDAL‑01 and halt further development of the program. The company stated that manufacturing for its Selected TIL therapy required continued investment in process improvements and that, given capital requirements, future funding needs, and financial market conditions, it would conclude all TIDAL‑01 clinical studies in solid tumors. As a result, Turnstone’s management and board initiated a process to explore and review strategic alternatives focused on maximizing shareholder value, including potential transactions such as an acquisition, merger, business combination, sale of assets, licensing, or other transactions. Turnstone indicated that it engaged a financial advisor to assist in this strategic review and implemented further cost‑containment and cash conservation measures, including workforce reductions.

Acquisition and change in trading status

Subsequently, Turnstone Biologics Corp. entered into an Agreement and Plan of Merger with XOMA Royalty Corporation and a wholly owned subsidiary of XOMA. According to the company’s Form 8‑K filings, the transaction provided for a cash tender offer for all outstanding shares of Turnstone common stock, plus a non‑transferable contingent value right (CVR) per share under a CVR Agreement. Following completion of the tender offer, the merger was effected under Section 251(h) of the Delaware General Corporation Law, with the merger subsidiary merging with and into Turnstone, and Turnstone continuing as the surviving corporation and becoming a wholly owned subsidiary of XOMA Royalty Corporation.

In connection with the consummation of the tender offer and merger, Turnstone notified The Nasdaq Capital Market and requested suspension of trading of its common stock and the filing of Form 25 to effect the removal of TSBX from listing on Nasdaq and the deregistration of the shares under Section 12(b) of the Securities Exchange Act of 1934. A Form 25 filed by Nasdaq on August 11, 2025, identifies Turnstone Biologics Corp. as the issuer and confirms the removal of its common stock from listing and/or registration on Nasdaq. Following this, Turnstone filed a Form 15 to terminate registration of its common stock under Section 12(g) of the Exchange Act and to suspend its reporting obligations under Sections 13 and 15(d). The Form 15 indicates that, as of the certification date, there was approximately one holder of record.

These regulatory filings mean that TSBX is no longer listed on Nasdaq and the company has deregistered its common stock under the Exchange Act. Turnstone Biologics Corp. continues as a private entity within XOMA Royalty Corporation’s corporate structure, and its historical TSBX ticker now primarily serves as a reference for past trading and regulatory records.

Research and collaboration context

Turnstone’s public disclosures reference collaborations with research institutions, including H. Lee Moffitt Cancer Center and the University of Montreal Hospital Research Centre (CRCHUM), particularly around preclinical work on TIL selection methods and clinical manufacturing for TIDAL‑01. The company has also described preclinical pipeline efforts, such as TIDAL‑02 and a combination program pairing TIDAL‑01 with viral immunotherapy, although subsequent strategic updates focused primarily on TIDAL‑01 and, later, on the discontinuation of that program and exploration of strategic alternatives.

Regulatory and reporting history

While listed, Turnstone filed quarterly and annual reports with the U.S. Securities and Exchange Commission, including financial statements that detailed research and development expenses, general and administrative expenses, and net losses typical of a clinical-stage biotechnology company. The company’s press releases and SEC filings emphasize the risks and uncertainties inherent in clinical development and explicitly note that its product candidates had not received marketing approval from the U.S. Food and Drug Administration.

Status of TSBX stock

For investors researching TSBX stock, it is important to note that, based on the Form 25 and Form 15 filings and the completed merger with XOMA Royalty Corporation, TSBX is a former Nasdaq-listed ticker. The historical symbol TSBX now represents the period when Turnstone Biologics Corp. traded publicly and filed periodic reports, rather than an actively traded, exchange‑listed security.