There's a growing recognition that the molecular profile of a primary tumor can differ significantly from its metastases. To guide treatment more accurately, the preferred practice is to biopsy an accessible metastatic lesion when possible, as this better reflects the biology of the active disease being treated.

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.

For HER2+ metastatic colorectal cancer, experts choose HER2-targeted therapies like TDXD or tucatinib/trastuzumab over standard second-line chemotherapy (FOLFIRI/BEV), despite label constraints. The rationale is the significantly higher response rate from targeting the oncogenic driver directly.

For RAS wild-type metastatic colorectal cancer, oncologists may prefer starting with a trastuzumab/tucatinib regimen over TDXD. This sequencing strategy preserves TDXD as a later option, as there is currently no data supporting tucatinib's efficacy after a patient has progressed on TDXD.

In the rare scenario of colorectal cancer with both HER2 amplification and a KRAS G12C mutation, US-based experts might prioritize KRAS-directed therapy. This preference is driven by durable data for KRAS inhibitors, even though choosing between targets is difficult without direct comparative studies.

For BRAF V600E mutated colorectal cancer, data argues against the common practice of starting with chemotherapy and saving targeted therapy for later. Hitting the specific tumor biology hard and early with combination targeted agents leads to significantly better outcomes, including doubling overall survival.

While clinicians often ponder how to prioritize treatments for patients with multiple actionable biomarkers, this scenario is exceedingly rare in practice. The guiding principle, if it does occur, is to choose the therapy with the strongest supporting clinical trial data, though this remains an infrequent dilemma.

In HER2-positive colorectal cancer, the choice of targeted therapy depends on RAS mutation status. The tucatinib/trastuzumab combination is effective only in RAS wild-type patients. In contrast, the antibody-drug conjugate trastuzumab deruxtecan (TDXD) shows efficacy regardless of whether a RAS mutation is present.

A common clinical pitfall is treating RAS/BRAF wild-type anal cancer with anti-EGFR antibodies, extrapolating from rectal adenocarcinoma protocols. Retrospective data shows this approach has only modest efficacy (4-5 month PFS) and is not a recommended strategy, highlighting a key difference between the two diseases.