The landscape of metastatic breast cancer treatment is facing a fundamental rethink, with the oncology field laser-focused on one of its greatest clinical challenges: how to break immune resistance and stop metastatic spread. Despite a wave of checkpoint inhibitor approvals and initial enthusiasm, progress in metastatic breast cancer—especially for subtypes like triple-negative and HER2-positive disease—has been incremental at best. Recent data from Kazia Therapeutics at SABCS 2025, however, has breathed new life into hopes for combination immunotherapy, providing the field with a fresh rationale and clear clinical signals that the “cold tumor” problem may finally be solvable.

What makes metastatic breast cancer so difficult to treat is its ability to outmaneuver both the immune system and the latest wave of targeted drugs. Monotherapies such as pembrolizumab (Keytruda) have shown flashes of promise, but durable responses remain elusive and are often limited to a small fraction of patients. The majority either fail to respond or quickly relapse as the disease finds new biological escape routes—CTC clusters, T-cell exhaustion, and epigenetic rewiring among them. It’s against this backdrop that the buzz around next-generation combination regimens is now growing, with biotech innovators like Kazia Therapeutics, G1 Therapeutics, and Syndax Pharmaceuticals pushing the field forward.

What scientific breakthroughs are enabling the new wave of combination immunotherapy strategies for advanced breast cancer?

The “why now?” for combination immunotherapy rests on two pivotal insights. First, recent advances in liquid biopsy and immune phenotyping have finally exposed the twin culprits of metastatic resilience: therapy-resistant circulating tumor cell (CTC) clusters and profound immune exhaustion. Second, new drugs and trial designs are directly targeting these resistance mechanisms, with the hope that adding a second or third mechanism of action will crack the code where monotherapy fails.

Kazia Therapeutics’ work with paxalisib, a brain-penetrant PI3K/mTOR inhibitor, is at the epicenter of this trend. Early results unveiled at SABCS 2025 showed that when paired with pembrolizumab and chemotherapy in triple-negative breast cancer, paxalisib drove a 76% reduction in tumor volume for the first trial patient and sharply curtailed CTC clusters—an achievement not seen with pembrolizumab alone. Mechanistically, the drug appeared to not only dismantle the metastatic machinery at a cellular level but also reinvigorate immune function by reversing T-cell exhaustion and activating inflammatory pathways.

It’s not just about one molecule. Other biotechs are attacking the problem from multiple angles. G1 Therapeutics is combining CDK4/6 inhibitors with immunotherapy. Syndax Pharmaceuticals and Ikena Oncology are leveraging epigenetic and metabolic modulators. The shared ambition is clear: make cold tumors hot, suppress escape routes, and deliver deeper, more durable responses for patients who have exhausted standard therapies.

How do circulating tumor cell clusters and immune exhaustion influence metastatic spread and therapy resistance?

For years, clinical researchers focused on the primary tumor. The science, however, has shifted: it’s now the circulating tumor cell clusters—the engine room of metastatic disease—that are capturing the sector’s attention. These clusters, often marked by a mesenchymal phenotype (think Vimentin+, Snail+, NRF2+), are inherently resistant to most standard-of-care agents and act as seedbeds for new metastases. Advanced profiling tools like liquid biopsy are now making it possible to track and even quantify these clusters in real time, offering both a new biomarker for drug response and a potential target for direct intervention.

Immune exhaustion is the second half of the problem. Many patients, especially those with heavily pre-treated disease, have immune systems that have simply run out of steam—cytotoxic T-cells are “burned out,” and immune checkpoints are firmly in place. Checkpoint inhibitors alone cannot revive these cells if the underlying immune machinery is too depleted. That’s where drugs like paxalisib come in, by shifting the tumor microenvironment from “cold” (immune silent) to “hot” (immune responsive) and restoring a critical level of anti-tumor immunity.

What challenges do next-generation combination trials face, and what is the sector doing to overcome them?

The push toward more effective combinations is not without risk. Previous attempts at combining checkpoint inhibitors with chemotherapy or targeted agents in breast cancer have produced only incremental benefits, and toxicity remains a real concern—PI3K and mTOR inhibitors, in particular, have a history of tough safety profiles. Trial design has also been a moving target, as regulatory agencies and key opinion leaders debate which endpoints—overall survival, progression-free survival, or liquid biopsy-based metrics—should drive approval and reimbursement.

That said, there’s a noticeable pivot in the way new trials are being built. Adaptive trial designs, biomarker-enriched cohorts, and real-time monitoring are becoming standard. Biotechs are increasingly focusing on identifying the right patient at the right time, using liquid biopsy to select those with high CTC burden or specific immune signatures. Regulatory agencies, including the U.S. Food and Drug Administration, are showing more openness to novel endpoints and surrogate markers when the science is strong and the unmet need is urgent.

Which companies and combinations should investors and clinicians watch in 2025 and beyond?

Kazia Therapeutics is clearly one to watch, with its paxalisib program now expanding into multiple subtypes and moving beyond breast cancer to brain metastases and rare pediatric tumors. The mechanistic data on CTC cluster disruption and immune reactivation is positioning the company as a potential leader in the combination space.

But it’s not a one-horse race. G1 Therapeutics is making waves by combining trilaciclib with immunotherapy in small cell lung cancer and breast cancer. Syndax Pharmaceuticals is pushing its dual epigenetic and immune modulation strategy, while Big Pharma players like Merck & Co. and AstraZeneca are testing their checkpoint inhibitors with everything from PARP inhibitors to metabolic drugs. Diagnostic innovators such as Guardant Health and Freenome are also shaping the field by enabling real-time response monitoring and patient selection.

Investor sentiment across the sector is mixed but trending positive, especially as more mature data readouts arrive and pivotal studies get underway. Analysts expect that the next 18 months will see a winnowing of approaches, with a handful of combinations moving rapidly toward registrational trials.

What could finally change the prognosis for patients with metastatic breast cancer—and what comes next?

The holy grail for metastatic breast cancer remains durable, progression-free survival with manageable toxicity. While the past decade has been defined by incremental gains, there is growing optimism that the convergence of new science, smarter clinical trial design, and real-time patient monitoring can shift the curve more dramatically.

Expert sentiment, as echoed by industry analysts and clinical leaders, suggests that if combination regimens like those involving paxalisib can continue to deliver on both mechanistic and clinical endpoints, the standard of care for metastatic breast cancer could look very different in just a few years. The next big catalysts? Pivotal trial launches, regulatory feedback on surrogate biomarkers, and successful demonstration of meaningful survival benefits in broad patient populations.

For now, the sector is bracing for a wave of data that will determine whether next-gen immunotherapy combos can truly crack the code—or whether metastatic breast cancer will remain the ultimate proving ground for innovation in oncology.

  breast cancer, checkpoint inhibitors, combination therapy, immunotherapy, Kazia Therapeutics, metastatic breast cancer, paxalisib