Sean A. McCarthy, D.Phil. CEO & Chairman, CytomX Therapeutics
Sean A. McCarthy, D.Phil., is the CEO and Chairman of CytomX Therapeutics, a San Francisco–based, clinical-stage biopharmaceutical company he has led since 2011. A cancer biologist by training, McCarthy brings more than 25 years of experience across biologics discovery, oncology drug development, and biotech leadership. Before joining CytomX, he held scientific and business roles at Millennium Pharmaceuticals and SGX Pharmaceuticals and served as a venture partner at Pappas Ventures.
CytomX is advancing its proprietary Probody® platform — a protease-activated masking technology designed to keep antibody-based drugs inactive until they reach the tumor microenvironment, widening the therapeutic window and unlocking previously undruggable cancer targets. Its lead program, Varseta-M (CX-2051), an EpCAM PROBODY® antibody-drug conjugate, is currently in Phase 1 development for late-stage metastatic colorectal cancer and has shown early clinical results the company describes as remarkable.
In this Q&A, McCarthy reflects on the origin of CytomX's differentiated scientific vision, the clinical and strategic milestones that define the company's progress, what it takes to sustain a high-performing team through uncertainty, and why he remains optimistic about the future of oncology innovation despite a challenging external environment.
May 18, 2026
Your Story
Q1. What was the defining moment that led you to take the helm of CytomX?
I spent the early part of my career managing biologics discovery programs and large pharma partnerships, then moved into business development and eventually venture capital. While I enjoyed investing, I wanted to get back into an operating role. Through those investor relationships, I was introduced to the founders of CytomX just as they were getting the company off the ground. The science immediately intrigued me — masking antibodies so they only activate in the right environment was a fundamentally new way of thinking about how to make drugs work better, a true paradigm shift. I joined as Chief Business Officer and became CEO about eight months later. I've been here ever since, and I still get energized by the work every day. As a cancer biologist, there's nothing more meaningful than trying to make a real difference for patients.
Q2. In plain terms, what does CytomX do — and what changes for patients if you succeed?
We're developing cancer therapies designed to activate specifically within tumors, enabling more precise and effective treatment. Our masking technology keeps drugs inactive until they reach the tumor microenvironment, delivering potent therapies while minimizing damage to healthy tissue. This approach also opens the door to targeting molecules that were previously considered undruggable. If we can improve both the precision and tolerability of cancer treatment, we can meaningfully shift the risk-benefit equation — making therapies more effective and potentially expanding access to treatment for patients who currently have few good options.
The Science & The Strategy
Q3. What sets CytomX apart from others working in oncology?
We've stayed deeply committed to a truly differentiated idea — our Probody masking platform — at a time when it had no established precedent or proof of concept. The concept of masking an antibody so it only becomes active in the tumor was a real intuitive leap. We've been willing to take risks and have remained consistent in our pursuit for many years. Transformative science takes time to mature and reach real impact. My view has always been: if you want to make a genuine difference, you have to actually do something different, and sometimes be willing to take a leap of faith, learning continuously along the way.
Q4. What has been your most important milestone to date, and what's the next one you're working toward?
Our most important milestone has been the clinical validation of our Probody platform through our ongoing Phase 1 study with Varseta-M (CX-2051) in late-stage metastatic colorectal cancer, which began in 2024. This program builds on more than a decade of research at CytomX. We recently released updated data showing remarkable anti-cancer activity in one of the hardest-to-treat patient populations in oncology, and it's incredibly rewarding to see that the years of hard work are paying off. The next critical step is aligning on a registrational study design, with the goal of filing for approval as soon as possible. We'll be sharing additional data later in 2026, and those results will inform which path we take.
Q5. What is the biggest challenge you're facing right now, and how are you tackling it?
Our biggest challenge is also our biggest opportunity: moving as fast as possible toward a registrational study for Varseta-M. Patients are waiting. There are multiple potential paths forward, all of them promising, and the specific path we choose will be informed by the next wave of clinical data we expect by the end of the year. At the same time, we need to strike the right balance in maintaining sharp focus on our most important value drivers while making thoughtful investments to broaden the program and pipeline, all with careful attention to capital allocation.
Leadership & Ecosystem
Q6. What has surprised you most about being a CEO in life sciences?
It's a remarkably multi-faceted and endlessly fascinating job. I learned early on that it's fundamentally about the team — aligning performance to goals and cultivating the right culture. Building high-performing teams and then getting out of the way to let great people do their best work is key to long term success. The other aspect I'd highlight is that running a public biotech carries a unique tension: you're doing R&D, which is inherently unpredictable, in full view of the public markets. But when markets function well, they're an extraordinary resource, as they provide us access to the capital we need to pursue the science. Learning to operate effectively within this dynamic has been one of the most interesting parts of the role.
Q7. How would you describe the culture you've built at CytomX, and why does it matter for the science?
From the very beginning, we've been a company driven by vision, mission, and values. Over the years, we've navigated clinical setbacks, funding challenges, and real uncertainty, but the team has consistently stayed focused on the end goal of helping patients. That continuity, that belief in the science, coupled with an unwavering focus on execution, is critical because this kind of work takes years. You need a team that can stay the course through both the highs and the lows. We've built a culture of resilience and long-term commitment at CytomX — and we also like to have fun along the way. Work hard, play hard.
Looking Ahead
Q8. Where do you see CytomX in three years, and what has to go right to get there?
We've climbed a lot of mountains to get here, and I'm extraordinarily proud of what this team has accomplished. In three years, I see CytomX continuing to advance Varseta-M through later-stage clinical development and standing on the verge of becoming a commercial-stage company. To get there, we need to keep executing on strong clinical data, align with regulators on the right registrational path, and secure the resources to run larger trials. Our momentum is tangible — and now it's on us to stay on track toward delivering Varseta-M, and in time other important therapies, to the patients who need them.
Q9. What is one thing about the life sciences industry you would change — and what gives you optimism?
We need a policy and regulatory environment that is genuinely conducive to scientific innovation. In recent years that environment has been challenged — whether through regulatory uncertainty, headwinds on drug pricing, or a broader “anti-science” sentiment at the political level. There's also a tendency within drug development to favor incremental progress over bold risks. At CytomX, we believe we owe it to patients to take big swings, and that requires the right external conditions. What gives me optimism is the evidence that truly differentiated science can still break through, even in a crowded and uncertain landscape. When you're willing to pursue something genuinely novel, you can stay ahead and make meaningful progress. That's what we've always tried to do.
Financing & Strategy
Q10. CytomX has navigated multiple industry downturns. How has your financing strategy evolved?
Very early on, we recognized that our innovative platform could serve as the foundation for R&D alliances. The capital raised through those non-dilutive partnerships has been strategically critical, helping us navigate periods when our equity cost of capital wasn't where we needed it to be. We're now entering a new phase with line of sight to the market for our first potential product, so our focus has shifted. We've recently returned to the public equity markets to fund our next clinical trials. But the core lesson remains constant: always be creating optionality for financing, because the one thing we know for certain is that the market is unpredictable.
About The Big4Bio CEO Weekly Q&A
Every Monday, Big4Bio spotlights a life sciences CEO from one of our eight coverage regions — Boston, San Francisco Bay Area, San Diego, Philadelphia, New York City, the Capital Region, Los Angeles, and Seattle. Each feature is promoted across all eight Big4Bio daily newsletters, reaching 30,000+ life sciences professionals. CEO participation is complimentary and editorial — every CEO approves the final Q&A before publication.
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