# Liquid Biopsy for HER2-Positive Resistance Liquid biopsy uses a blood sample to detect tumour-derived material, most often **circulating tumour DNA (ctDNA)**. In HER2-positive metastatic breast cancer, it may help identify some resistance mechanisms without requiring a new tissue biopsy. {% hint style="warning" %} A negative liquid-biopsy result does not rule out resistance. Some mechanisms are poorly captured in ctDNA, especially protein-level and RNA-level changes. {% endhint %} ### What liquid biopsy may detect well DNA-based ctDNA testing is often most useful for: * **PIK3CA mutations** * **KRAS, NRAS, and BRAF mutations** * **EGFR amplification** * **MET amplification** * **FGFR amplification** * **CCNE1 amplification** * some forms of **RB1 loss** These are the kinds of changes that many next-generation sequencing panels are already designed to capture. ### What liquid biopsy may miss or undercall Standard DNA-based ctDNA panels are less reliable for: * **HER2 truncations such as p95HER2** when the problem is proteolytic cleavage rather than a DNA alteration * **PTEN loss** when it is functional, epigenetic, or protein-level rather than a clear mutation or deletion * **IGF1R overexpression** without gene amplification * pure **phenotypic shift** without an accompanying genomic change These may require tissue biopsy, RNA-based testing, proteomic methods, or broader clinical interpretation. ### Alteration-by-alteration breakdown #### EGFR amplification **Detectable in liquid biopsy?** Usually yes. ctDNA can reveal EGFR copy-number gain, especially with NGS-based assays. Sensitivity falls when tumour shedding is low, but improves with higher tumour fraction or more sensitive methods such as digital PCR. **Main limitation:** false negatives become more likely in low-shedding disease. #### PIK3CA activating mutations **Detectable in liquid biopsy?** Yes. Hotspot PIK3CA mutations are among the most reliably detected breast-cancer alterations in ctDNA panels and often show high concordance with tissue findings. **Main limitation:** performance still depends on adequate tumour DNA in the sample. #### PTEN loss **Detectable in liquid biopsy?** Partial only. Some PTEN deletions or truncating mutations can be seen in ctDNA, but many clinically relevant PTEN losses are epigenetic or protein-level events rather than clear DNA calls. **Main limitation:** ctDNA may miss functional PTEN loss unless a specific mutation or deep deletion is present. #### KRAS, NRAS, and BRAF pathway mutations **Detectable in liquid biopsy?** Yes. Point mutations in KRAS, NRAS, and BRAF are standard ctDNA targets and are usually detectable with good sensitivity when tumour fraction is high enough. **Main limitation:** sensitivity falls when circulating tumour DNA levels are low. #### HER2 truncations such as p95HER2 **Detectable in liquid biopsy?** Limited. Most p95HER2 biology reflects proteolytic cleavage rather than a straightforward DNA alteration. Standard DNA-based ctDNA assays often miss this unless there is an associated genomic rearrangement or splice-related event detectable through RNA-based methods. **Main limitation:** DNA-based assays are poorly suited to purely proteolytic truncations. #### RB1 loss **Detectable in liquid biopsy?** Often yes. Homozygous deletions or inactivating RB1 mutations may be detectable through copy-number or mutation analysis, especially when tumour fraction is adequate. **Main limitation:** shallow sequencing may miss focal deletions. #### CCNE1 amplification **Detectable in liquid biopsy?** Yes. Cyclin E amplification can be detected as a copy-number gain in ctDNA and has been reported in plasma analyses across tumour types including breast cancer. **Main limitation:** detection still depends on assay depth and tumour fraction. #### MET amplification **Detectable in liquid biopsy?** Yes. MET amplification is a recognised resistance marker and is usually detectable by modern NGS-based liquid-biopsy assays when tumour shedding is sufficient. **Main limitation:** low-shedding disease can still reduce sensitivity. #### FGFR1, FGFR2, and FGFR3 amplification **Detectable in liquid biopsy?** Yes. FGFR amplifications are routinely captured in many ctDNA panels and can be clinically relevant as bypass drivers. **Main limitation:** copy-number calling is less reliable when tumour fraction is low. #### IGF1R amplification or overexpression **Detectable in liquid biopsy?** Possible, but incomplete. IGF1R amplification may be seen in DNA-based assays. Overexpression without gene amplification is much harder to infer from ctDNA alone. **Main limitation:** RNA-based or protein-based methods are better for true overexpression states. #### Phenotypic shift or loss of HER2 dependence **Detectable in liquid biopsy?** Indirectly at best. ctDNA may suggest this if HER2 amplification falls away and alternative drivers emerge, but a true phenotypic switch without a strong genomic signal may not be visible in plasma DNA. **Main limitation:** this often requires paired interpretation, repeat tissue biopsy, or RNA-level profiling. ### What ctDNA does best DNA-based liquid biopsy is usually strongest for: * point mutations such as **PIK3CA**, **KRAS**, **NRAS**, and **BRAF** * copy-number alterations such as **EGFR**, **MET**, **FGFR**, **CCNE1**, and some **RB1** losses ### What ctDNA does less well Standard ctDNA is less reliable for: * proteolytic HER2 truncations such as **p95HER2** * pure protein overexpression without amplification * some forms of **PTEN** loss * phenotype change without a clear genomic driver ### Why this matters after HER2CLIMB If cancer progresses on tucatinib, trastuzumab, and capecitabine, ctDNA testing may help identify whether the tumour has developed a bypass route such as: * PI3K-pathway activation * EGFR amplification * MET or FGFR amplification * cyclin E amplification * RAS/RAF pathway activation That information may help guide discussion about next-line HER2-targeted therapy, pathway inhibitors, or clinical-trial options. ### Practical limits Liquid biopsy performance depends heavily on **tumour shedding**. Sensitivity falls when: * total tumour burden is low * disease is mainly in the CNS * the tumour is not shedding much ctDNA * the alteration is copy-number subtle or protein-driven rather than DNA-driven In rapidly progressive metastatic disease, a broad NGS-based ctDNA panel with both **mutation** and **copy-number** analysis can still reveal many plausible resistance drivers. A negative result, however, does **not** rule out resistance mechanisms acting at the protein or RNA level. ### Good practical use of liquid biopsy Liquid biopsy is often most useful when used alongside: * imaging * current treatment history * prior tumour biology * tissue biopsy when feasible * clinical pattern of progression Complementary approaches may include: * repeat tissue biopsy * RNA-based liquid biopsy * proteomic testing * broader functional profiling where available ### Questions to discuss with the care team * whether a broad ctDNA panel is available * whether the assay includes copy-number analysis as well as hotspot mutations * whether low tumour shedding could limit the result * whether a negative result would still leave a role for tissue biopsy or RNA-based testing ### Related pages * [HER2CLIMB Guide](/myhealingcommunity-docs/breast-cancer/her2-positive/her2climb/her2climb-guide.md) * [HER2CLIMB Resistance Mechanisms](/myhealingcommunity-docs/breast-cancer/her2-positive/her2climb/her2climb-resistance-mechanisms.md) * [Natural Compounds and CYP3A4 — HER2CLIMB Considerations](/myhealingcommunity-docs/breast-cancer/her2-positive/her2climb/natural-compounds-and-cyp3a4-her2climb-considerations.md) ### References Abstract PD8-06: Acquired resistance to tucatinib\ Treating advanced breast cancer: a spotlight on tucatinib\ Potential Resistance Mechanism to Tucatinib in HER2+ Breast Cancer\

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