When a biliary tract tumor has both an FGFR2 fusion and HER2 positivity, oncologists may prioritize targeting the FGFR2 fusion. They reason that fusions are often early, clonal, and homogenous driver events, making them a more reliable therapeutic target than HER2, which can be expressed heterogeneously.

Dr. Bardia emphasizes that ESR1 is an 'acquired alteration,' meaning the mutation can develop during treatment. This necessitates a shift from one-time diagnostic testing to a dynamic, serial testing model. Repeat testing is critical to identify these actionable mutations as they arise, allowing patients to access newly approved targeted therapies.

Retesting for biomarkers with liquid biopsy in the third-line setting is crucial. It can uncover new, actionable mutations that have emerged during treatment or confirm the absence of resistance mutations, potentially allowing patients to benefit from re-challenging with a previously used targeted therapy.

An individual tumor can have hundreds of unique mutations, making it impossible to predict treatment response from a single genetic marker. This molecular chaos necessitates functional tests that measure a drug's actual effect on the patient's cells to determine the best therapy.

When GIST progresses on therapy like imatinib, resistance is often heterogeneous. Different metastatic sites within the same patient can develop distinct secondary resistance mutations (e.g., an exon 13 mutation in the liver and an exon 17 in the peritoneum). This complicates subsequent treatment selection and underscores the value of comprehensive testing like ctDNA.

Despite mutation testing being a critical first step for effective treatment planning in gastrointestinal stromal tumors (GIST), a significant number of patients in the United States still do not receive this essential diagnostic. This highlights a major gap between established best practices and real-world clinical application.

The presence of heterogeneous resistance mutations, some of which may be below detection limits, suggests a new strategy. Using a potent, broad-spectrum combination therapy upfront in the second-line setting, rather than sequential monotherapies, could eradicate more resistant clones and give patients a better chance at long-term survival or even a cure.

The standard of care for GIST is evolving to mandate molecular testing at two key points: initial diagnosis and at the time of progression on first-line therapy. Using ctDNA at progression is now deemed critical to identify acquired resistance mechanisms, which directly informs the selection of subsequent, more effective therapies and avoids ineffective treatments.

Exploratory analysis of the INTRIGUE study reveals the power of ctDNA profiling at progression. Selecting repretinib for patients with exon 17/18 resistance mutations resulted in a 44% response rate, versus a 0% response rate for sunitinib in that same subset. This highlights how liquid biopsy can dramatically improve outcomes by matching the right drug to the right resistance mechanism.