Introduction

If you are looking up the performance of sesn stock today, you might notice that the charts are frozen or completely empty. That is because Sesen Bio, Inc., which famously traded under the ticker symbol SESN, no longer exists as an active, independent entity. Once a highly watched player in the microcap biotech space, Sesen Bio underwent a dramatic transformation that serves as a textbook case study of clinical-stage regulatory risks, activist investor negotiations, and ultimate corporate transition.

To understand what happened to your investment in sesn stock, or to analyze the lessons of this speculative biotech play, you must trace a line from Sesen Bio's early clinical trials through its merger with Carisma Therapeutics, and finally to Carisma's subsequent struggles, delisting, and final wind-down. This comprehensive analysis breaks down the history of Sesen Bio (SESN), the structural mechanics of its 2023 reverse merger, the fate of the Contingent Value Rights (CVRs) issued to shareholders, and the ultimate position of the company.

The Science and Downfall of Sesen Bio: The Lead-Up to the Merger

At its peak, Sesen Bio was a late-stage clinical biopharmaceutical company focusing on designing, engineering, and commercializing targeted fusion protein therapeutics for cancer. The core value proposition of the company rested almost entirely on its lead product candidate, Vicineum (also known as oportuzumab monatox).

Vicineum was designed as a locally administered fusion protein targeting epithelial cell adhesion molecule (EpCAM) on cancer cells. Its primary indication was for the treatment of BCG-unresponsive non-muscle invasive bladder cancer (NMIBC)—a disease with historically limited treatment options where patients often face total bladder removal (radical cystectomy) if standard therapies fail.

The investor enthusiasm surrounding sesn stock in early 2021 was immense. The company was actively preparing for a commercial launch, hiring sales teams, and submitting its Biologics License Application (BLA) to the U.S. Food and Drug Administration (FDA). Biotech speculators drove the stock price up, expecting an easy approval and rapid market penetration given the significant unmet medical need.

However, the clinical-stage biotech sector is notoriously brutal, and the reality of regulatory approval struck a devastating blow on August 13, 2021. The FDA issued a Complete Response Letter (CRL) for Vicineum’s BLA, stating that it could not approve the drug in its current form. The FDA highlighted outstanding questions regarding trial design, chemistry, manufacturing, and controls (CMC), as well as clinical data integrity and safety profiles.

The fallout was immediate:

• Stock Price Crash: SESN stock plunged by more than 60% in a single trading session, wiping out hundreds of millions of dollars in shareholder value overnight.
• Trial Suspensions: Sesen Bio was forced to pause its clinical development plans for Vicineum in the United States and eventually halt its regulatory efforts in the European Union.
• Wind Down of Operations: To conserve capital, Sesen Bio initiated a massive wind-down of its manufacturing operations and paused business development partnerships.

With its lead asset effectively shelved and faced with the prospect of expensive, multi-year clinical trials to satisfy the FDA's requirements, Sesen Bio’s management was left with a single viable path forward: find a merger partner with a promising pipeline that could utilize Sesen's remaining cash reserves.

The Carisma Therapeutics Merger: Anatomy of the Deal

On September 21, 2022, Sesen Bio announced a definitive merger agreement with Carisma Therapeutics, Inc., a privately held clinical-stage biopharmaceutical company focused on pioneering chimeric antigen receptor macrophage (CAR-M) immunotherapies.

While the merger represented a strategic lifeline for both companies—giving Carisma access to Nasdaq public markets and Sesen's cash, while giving Sesen stockholders exposure to a novel oncology platform—the deal was initially met with fierce resistance from retail and institutional investors. A vocal group of Sesen stockholders felt the initial merger terms significantly undervalued Sesen's massive cash pile (which stood at over $160 million) while heavily diluting Sesen’s original owners.

Following intense pressure, proxy battles, and active negotiations with activist investor groups, Sesen Bio and Carisma amended their merger agreement in early 2023. The revised terms successfully appeased the majority of shareholders by offering immediate cash returns and future upside potential. On March 2, 2023, approximately 88% of Sesen Bio stockholders voted to approve the merger.

The transaction officially closed on March 7, 2023, and was structured with several highly complex corporate actions that permanently altered the landscape of sesn stock:

The 1-for-20 Reverse Stock Split

To meet the minimum bid price requirements of the Nasdaq Global Market and prepare for the post-merger share structure, Sesen Bio implemented a 1-for-20 reverse stock split. Before the market opened on March 8, 2023, every 20 outstanding shares of SESN common stock were automatically consolidated into 1 share of the combined company.

The Ticker and Name Change

Upon the closing of the merger, Sesen Bio, Inc. changed its name to Carisma Therapeutics Inc. Sesen Bio common stock (SESN) was officially delisted, and the combined company began trading on the Nasdaq Global Market under the ticker symbol CARM on March 8, 2023.

The Special Cash Dividend

To return capital directly to the original Sesen stockholders, the Sesen Board declared a one-time, special cash dividend totaling $75 million. For pre-split Sesen Bio shareholders of record as of March 7, 2023, this translated to a cash payout of approximately $0.361 per share. The dividend was paid out on March 8, 2023.

The Sesen Bio Contingent Value Right (CVR)

To preserve potential upside from Sesen's legacy assets, pre-merger stockholders of record as of March 7, 2023, also received one non-transferable Contingent Value Right (CVR) for each share of SESN stock held. This CVR represented a contractual right to receive additional cash payments if certain events occurred prior to March 31, 2027.

Understanding the Sesen Bio CVR: Legacy Assets and Value Potential

For many historical holders of sesn stock, the Contingent Value Right (CVR) is the most critical lingering asset. When Sesen Bio transitioned to Carisma Therapeutics, the new management team made it clear they would not actively pursue the development of Vicineum. Instead, they sought to monetize Sesen's legacy intellectual property to maximize cash returns for the CVR holders.

Under the terms of the CVR agreement, holders are entitled to receive net cash proceeds from:

• The Sale of Vicineum: Sesen Bio engaged a financial advisor to market and sell the clinical and preclinical assets associated with Vicineum.
• The Roche Asset Purchase Agreement: Sesen had a pre-existing agreement with Roche that included a potential $30 million milestone payment. If this milestone is met and paid by Roche before the CVR expiration date, the proceeds (minus transaction costs and taxes) will be distributed to CVR holders.
• Other Preclinical Assets: Any licensing or sale of Sesen's remaining early-stage assets prior to the expiration date.

The timeline for these CVR payments was originally set to expire on December 31, 2023. However, during the final amendment of the merger agreement, Sesen successfully negotiated an extension of the CVR term to March 31, 2027. This extension gave the company significantly more runway to find strategic buyers, navigate complex technology transfers, and potentially trigger the Roche milestone payouts.

Investors should note that the CVR is non-transferable. It cannot be bought, sold, or traded on any public exchange. It sits in the brokerage accounts of record-date shareholders as a placeholder security (often designated by a custom CUSIP or a placeholder symbol like 'CONTRA CVR SESN BIO' or 'US817CVR0389'). If any eligible monetization event occurs before March 31, 2027, the cash distribution will be automatically deposited into the corresponding brokerage accounts. If no events occur by that date, the CVRs will expire worthless.

The Transition to CARM: What Happened After the Merger?

To understand the full scope of what happened to your investment, we must look at the trajectory of the successor company, Carisma Therapeutics (CARM), after the merger took effect in March 2023.

Initially, Carisma enjoyed significant clinical attention. The company’s proprietary cell therapy platform was unique because it utilized engineered macrophages and monocytes rather than traditional T-cells (such as those used in CAR-T therapy). Because macrophages naturally home to solid tumors and can alter the immunosuppressive tumor microenvironment, Carisma’s technology was seen as a potential breakthrough for solid tumor oncology.

Carisma's lead candidate, CT-0508 (an anti-HER2 CAR-Macrophage), was evaluated in Phase 1 clinical trials. The company also secured a prestigious multi-target research collaboration with Moderna to develop in vivo CAR-M therapeutics, combining Carisma's macrophage expertise with Moderna's mRNA and LNP delivery technologies.

Despite the promising science, the macroeconomic environment for clinical-stage, cash-burning biotech companies deteriorated significantly. Developing advanced cell therapies is an incredibly capital-intensive endeavor, and clinical trial progress is slow.

By early 2025, Carisma found itself in a severe cash crunch:

• Strategic Pipeline Restructuring: On March 31, 2025, Carisma announced a drastic change in its operating plan. The Board of Directors approved a revised strategy to focus entirely on exploring strategic alternatives while aggressively reducing operational cash burn.
• Workforce Reductions: The company terminated the majority of its workforce, retaining only essential personnel required to manage its remaining assets and evaluate transaction proposals.
• Discontinuation of Lead Programs: Carisma paused its clinical pipeline programs, including its lead oncology assets, and focused its efforts on licensing its technology and monetizing its platform.

The Final Demise: Nasdaq Delisting and Wind Down

The financial pressure culminated in late 2025, marking the final chapter for the corporate remnants of Sesen Bio.

On October 10, 2025, Carisma received a formal delisting determination from Nasdaq due to non-compliance with the exchange's minimum bid price and market value requirements. Given its depleted cash reserves and lack of active clinical trials, Carisma decided not to appeal the decision.

Trading of CARM stock on the Nasdaq Global Market was officially suspended on October 13, 2025. The company’s stock was subsequently relegated to the over-the-counter (OTC) market tier, trading under the same symbol (CARM) in a highly illiquid, penny-stock environment.

To finalize its dissolution and save administrative costs, Carisma's Board approved a voluntary delisting and SEC deregistration on December 5, 2025:

• Form 25 Filing: Filed on December 15, 2025, officially removing CARM from Nasdaq listing.
• Form 15 Filing: Filed to suspend and terminate the company's reporting obligations under the Securities Exchange Act of 1934.
• Orderly Wind Down: Throughout 2026, the company continues its orderly wind-down, aiming to liquidate any remaining physical assets, settle debts, and potentially monetize intellectual property licensing to satisfy creditors and remaining obligations.

This collapse means that the common stock (CARM), which was once sesn stock, has effectively lost all of its equity value. The shares trade at a nominal fraction of a cent or are entirely frozen in retail brokerage accounts as the wind-down process completes.

Actionable Guide: What Should Former SESN Shareholders Do?

If you held sesn stock through the 2023 merger, you are likely wondering what your options are today. Here is a practical checklist of what you should check:

1. Locate Your Transaction History: Check your brokerage statements from March 2023. Look for the transition of your SESN shares. You should see a line item showing the 1-for-20 consolidation, the deposit of the $0.361 per share special cash dividend, and the receipt of your CVRs.
2. Review Your CVR Status: Look at your current portfolio holdings. You should see a non-transferable security listed under a placeholder symbol or CUSIP. If your broker does not display it, contact their customer support to verify that your Sesen Bio CVRs are properly registered in your name.
3. Assess Tax Impact: The Sesen Bio corporate actions in 2023 had distinct tax implications. The special cash dividend is generally treated as a taxable distribution (either as a qualified dividend or a return of capital, depending on the company's accumulated earnings and profits). The reverse stock split was a tax-free event that adjusted your cost basis per share. Talk to a certified public accountant (CPA) to ensure you have accurately reported these transactions on your historical tax returns.
4. Manage Expectations on the CVR: While the Sesen Bio CVR agreement is legally active until March 31, 2027, the severe operational decline and wind-down of Carisma Therapeutics significantly lower the probability of a major asset sale or licensing payout. However, because the CVR remains active, any potential licensing of Vicineum or milestone triggers from Roche prior to the 2027 deadline would still flow to the registered holders. Do not expect an immediate windfall, but keep the placeholder in your account until the formal expiration date.

Lessons for Biotech Investors from the SESN Saga

The story of sesn stock is a cautionary tale that highlights the structural complexities of investing in microcap clinical-stage biotechnology. For retail investors, several key takeaways emerge:

• The 'Binary Event' Risk is Real: In biotech, FDA approval is a binary gate. A positive decision can send a stock up 500%; a negative decision (like Sesen’s CRL in 2021) can erase the majority of your capital instantly. Always manage position sizing to ensure a single CRL cannot ruin your portfolio.
• The Value of Cash on the Balance Sheet: Sesen’s primary saving grace after its FDA rejection was its $160+ million cash reserves. This cash pile gave the company leverage to negotiate a merger and return $75 million directly to shareholders via the special dividend. When evaluating speculative biotechs, always check their cash runway.
• Read the Fine Print on CVRs: CVRs are often thrown into mergers as a sweetener to satisfy disappointed target shareholders. However, because they are non-transferable and highly speculative, they rarely achieve their maximum theoretical value. Investors should treat them as lottery tickets rather than guaranteed future income.

Frequently Asked Questions (FAQs)

Why is SESN stock no longer trading?

SESN stock (Sesen Bio, Inc.) ceased trading under its original ticker symbol on March 8, 2023, when the company completed its reverse merger with Carisma Therapeutics, Inc. Sesen Bio was renamed Carisma Therapeutics, and its ticker symbol became CARM.

What was the reverse split ratio for Sesen Bio?

As part of the merger with Carisma Therapeutics, Sesen Bio enacted a 1-for-20 reverse stock split. For every 20 shares of SESN stock you owned prior to March 8, 2023, you received 1 share of CARM stock.

How much was the Sesen Bio special cash dividend?

Pre-merger Sesen Bio stockholders of record as of March 7, 2023, received a special one-time cash dividend of approximately $0.361 per share (pre-reverse split). This dividend was paid out on March 8, 2023, totaling an aggregate distribution of $75 million.

Is the Sesen Bio CVR still active?

Yes, the Sesen Bio Contingent Value Right (CVR) remains legally active. During the merger amendments, the expiration date of the CVR agreement was extended to March 31, 2027. This means that if any monetization of Sesen's legacy assets (like Vicineum) or the $30 million Roche milestone occurs before March 31, 2027, CVR holders are still eligible to receive payments.

Can I buy or sell Sesen Bio CVRs?

No. The Sesen Bio CVRs are completely non-transferable. They cannot be sold, bought, or traded on any public exchange or over-the-counter market. They can only be held in your brokerage account until they either pay out or expire worthless on March 31, 2027.

Is Carisma Therapeutics (CARM) out of business?

Carisma Therapeutics is currently executing an orderly wind-down of operations. In late 2025, the company voluntarily delisted from Nasdaq and filed for SEC deregistration to minimize administrative costs, terminating its development programs to preserve remaining value for creditors and stakeholders.

How do I claim my unpaid Sesen Bio dividend or shares?

If you held physical share certificates of SESN stock before the merger or if your shares have not automatically transitioned, you must contact Carisma’s transfer agent, Computershare Trust Company, N.A., to submit your certificates and receive your cash distribution and adjusted stock assets. If your shares were held electronically in a brokerage account, the adjustments and dividend payments should have been handled automatically by your broker in March 2023.

Conclusion

The journey of sesn stock is a powerful reminder of how quickly fortunes can change in the high-stakes world of biopharmaceutical investing. What began as a promising bladder cancer play ended in an FDA rejection, an activist investor showdown, a complex reverse merger with Carisma Therapeutics, and ultimately a company wind-down.

For former Sesen Bio shareholders, the tangible assets remaining are the cash dividend paid back in 2023 and the active—though highly speculative—Contingent Value Rights (CVR) that run through March 31, 2027. By understanding these corporate mechanics, you can better navigate your tax reporting, track your lingering brokerage assets, and apply these valuable clinical-stage lessons to your future investment strategies.