Redefining Endocrine Therapy for Men: Inside Dr. Jose Pablo Leone’s ETHAN Trial
For decades, the treatment of male breast cancer has remained largely unchanged—quietly borrowing from protocols designed for women while rarely being questioned, tested, or refined for men themselves. At the Dana-Farber Cancer Institute, Dr. Jose Pablo Leone has spent much of his career confronting this imbalance. Through the ETHAN clinical trial, now actively underway at Dana-Farber and collaborating academic centers, he is leading one of the most consequential efforts to modernize endocrine therapy for men with breast cancer—not through extrapolation or assumption, but through direct, carefully designed clinical investigation.
Dr. Leone’s work sits at the intersection of clinical urgency and scientific discipline. Male breast cancer is rare, accounting for a small fraction of overall breast cancer diagnoses, yet rarity has come at a cost. While endocrine therapies for women have advanced rapidly—incorporating aromatase inhibitors, ovarian suppression strategies, and targeted agents such as CDK4/6 inhibitors—men have remained tethered to tamoxifen, a drug that has served as the default standard for decades.
“We’re still treating men the way we did 30 or 40 years ago,” Dr. Leone has observed. “Not because it’s the best option, but because we’ve never definitively tested anything else.”
That gap is precisely why ETHAN exists.
Why ETHAN Had to Be Built
Hormone receptor–positive disease represents approximately 90–95% of male breast cancer cases in the United States. In women, this subtype has been the focus of relentless innovation. Large trials reshaped care by demonstrating that aromatase inhibitors—alone or combined with ovarian suppression—could outperform tamoxifen in key settings. More recently, CDK4/6 inhibitors such as abemaciclib and ribociclib have extended disease-free survival even further.
For men, however, these advances created an uncomfortable paradox: better therapies exist, but clinicians lack the evidence to confidently apply them. “We’re stuck,” Dr. Leone explains, “because without male-specific data, every decision feels like a leap of faith.”
ETHAN was designed to replace that uncertainty with clarity.
A Trial That Watches Tumors Respond in Real Time
Rather than attempting to replicate massive, multi-thousand-patient trials—an unrealistic goal in a rare cancer—ETHAN uses a powerful alternative approach: a pre-surgical “window of opportunity” design.
Men with newly diagnosed, operable, ER/PR-positive, HER2-negative breast cancer are enrolled before any treatment begins. Participants are randomized to one of three initial endocrine strategies: tamoxifen alone, an aromatase inhibitor alone, or an aromatase inhibitor combined with gonadotropin-releasing hormone (GnRH) suppression. After just three weeks of therapy, a research biopsy is performed.
“The advantage of this design,” Dr. Leone explains, “is that we don’t have to guess. We can actually see what the tumor is doing.”
Following this window phase, patients continue endocrine therapy—with or without the addition of the CDK4/6 inhibitor abemaciclib—before proceeding to surgery. This two-by-two structure allows the study to answer several critical questions simultaneously: which endocrine backbone works best, whether CDK4/6 inhibition adds benefit, and which combinations make the most sense for men.
Measuring What Matters Most
ETHAN’s dual primary endpoints reflect its dual mission. The first is reduction in Ki-67, a marker of tumor proliferation, measured between diagnosis and the research biopsy. The second is the Residual Cancer Burden (RCB) index, assessed at surgery to quantify how much tumor remains after treatment.
“These aren’t redundant measures,” Dr. Leone emphasizes. “They tell us different things at different moments.”
Ki-67 captures early biological response—how aggressively the cancer slows when therapy begins. RCB reflects cumulative effect, translating treatment response into surgical reality.
Secondary endpoints broaden the lens further. Hormone levels, including estradiol and testosterone, are tracked to understand how each therapy reshapes male physiology. Safety, feasibility, and quality of life are monitored throughout, recognizing that effectiveness without tolerability is not a true solution.
A Translational Engine, Not Just a Clinical Trial
ETHAN is also notable for its depth of biological exploration. Tissue from diagnostic biopsies, window biopsies, and surgical specimens is collected alongside serial blood samples. Through collaborations with national research leaders, the study examines tumor genomics, germline mutations, circulating tumor DNA, and mechanisms of tumor cell death.
One focus is apoptosis—the programmed death of cancer cells—which may differ among CDK4/6 inhibitors. “There’s a hypothesis that abemaciclib may induce more tumor cell death than other agents,” Dr. Leone notes. “Believe it or not, this hasn’t been definitively studied even in women.”
The trial also seeks to identify endocrine response signatures—genetic patterns that predict which tumors will respond best to specific therapies. While women benefit from tools like Oncotype DX, no equivalent exists for men. ETHAN aims to change that.
“This is about understanding why endocrine therapy works beautifully in some men and not in others,” Dr. Leone says. “Once we understand that, everything changes.”
The Human Cost of Having Only One Option
Beyond efficacy, ETHAN confronts a hard truth: men often struggle to complete endocrine therapy. European studies suggest that only 40–50% of men finish five years of tamoxifen, compared with roughly 70% of women completing endocrine therapy overall.
Tamoxifen’s side-effect profile can be particularly challenging for men, including elevated blood-clot risk, sexual dysfunction, fatigue, and muscle pain. “If you only have one option,” Dr. Leone explains, “and that option causes significant side effects, the likelihood of stopping early is high.”
Aromatase inhibitors offer a different experience for men. With testosterone levels preserved, risks such as osteoporosis are minimal, and hot flashes are uncommon. “Even if these drugs work equally well,” he adds, “having another option is a huge win—for adherence, for quality of life, and for peace of mind.”
ETHAN allows clinicians to observe not only tumor response, but how men tolerate each approach—information that directly informs post-surgical treatment decisions.
Momentum, Collaboration, and Urgency
Enrollment in ETHAN is ongoing across major academic centers in the United States, with additional sites opening to expand geographic access. Progress has been steady, though slower than initially hoped—a reality Dr. Leone acknowledges candidly.
“This is still a success,” he says. “Eighteen men enrolled in a male-specific breast cancer trial is not trivial. But the community needs answers, and we want to get there as fast as possible.”
The trial’s momentum reflects extraordinary collaboration among academic institutions, patient advocates, and national organizations. Support from the male breast cancer advocacy community has been instrumental in expanding access and awareness.
“This work doesn’t happen in isolation,” Dr. Leone emphasizes. “It happens because patients, advocates, and researchers align around a shared goal.”
A Turning Point in Male Breast Cancer Care
ETHAN is not simply a study of drugs—it is a statement. It challenges the notion that men must accept hand-me-down standards of care. It insists that evidence matters, even in rare diseases. And it demonstrates that with thoughtful design, small trials can answer big questions.
“If all we learn is that these therapies work just as well as tamoxifen,” Dr. Jose Pablo Leone reflects, “that alone changes everything.”
For men facing a diagnosis that has long lived in the shadows of breast cancer research, ETHAN represents something profoundly overdue: clarity, choice, and progress grounded in evidence rather than assumption.
