HER2-Driven Lung Adenocarcinoma Turned Aggressive After a Single Immunotherapy Exposure
A never-smoking woman with stage IVB lung adenocarcinoma carrying an ERBB2 exon 20 insertion and low-level copy-number gain followed an unusually sharp clinical arc: urgent first-line chemoimmunotherapy, explosive progression after one dose of ivonescimab, then a rapid partial response to the HER2-directed ADC trastuzumab rezetecan. The case captures a difficult bedside problem in metastatic NSCLC with actionable drivers — the need to treat immediately when disease burden is high, even before genomic results return, while trying not to miss a targetable oncogenic alteration that may dominate the biology of the tumor.
Her disease was extensive at presentation. Imaging showed a right upper-lobe mass with nodal disease and metastases to the brain, kidneys, bones, adrenal gland, and abdominal/retroperitoneal nodes. The patient arrived with dyspnea, chest tightness, back pain, headache, nausea, vomiting, and an enlarging occipital superficial mass. Performance status was already impaired at ECOG 1–2, and the tumor burden made delay untenable.
The molecular profile was highly relevant. A 919-gene next-generation sequencing panel identified ERBB2 p.Y772_A775dup, a canonical exon 20 insertion, with variant allele frequency of 58.4%. Copy-number gain was present but modest at 3.1, a level far below high-level amplification and not a substitute for an activating mutation. TP53 p.I195T and p.R249S were also detected. PD-L1 TPS was 40%, and tumor mutation burden was low at 3.31 mutations/Mb, a pattern that does not argue for a strongly inflamed, immunotherapy-sensitive phenotype.
Chemotherapy Started Before Biomarkers Returned
Treatment began the way many real-world metastatic cases do: under pressure. On August 31, 2025, the patient received pemetrexed plus carboplatin while biomarker testing was still pending. Ivonescimab was added in cycle 2 on September 23, 2025 after PD-L1 and genomic data became available. The sequencing mattered because the first cycle did not include immunotherapy, creating a natural reference period before checkpoint exposure entered the picture.
The subsequent turn in the disease course was abrupt.
At restaging on October 15, 2025, the right upper-lobe target lesion had expanded from 4.56 cm to 10.05 cm, a gain of more than 120%. New brain, renal, adrenal, and retroperitoneal metastatic sites appeared, and a large right pleural effusion developed. The patient’s condition worsened to ECOG 3–4, she became bedbound, pain control required high-dose fentanyl, and the occipital lesion enlarged to about 5 cm. The tempo fit hyperprogressive disease rather than ordinary progression, with acceleration emerging after the first and only ivonescimab exposure and not during the earlier chemotherapy-only interval.
A sudden jump in burden across multiple compartments is not the way most metastatic tumors evolve over a three-week window. In a case like this, the temporal relationship to immune checkpoint exposure becomes clinically hard to ignore.
Hyperprogression after Ivonescimab Exposure
Hyperprogressive disease was assessed using widely applied Champiat/Saâda-Bouzid criteria, including RECIST 1.1 progression at the first on-treatment evaluation plus a marked increase in tumor growth kinetics. The patient met those thresholds through multiple signals: a dramatic increase in target-lesion diameter, new metastatic deposits, and a steep functional decline. The first post-treatment assessment occurred about three weeks after ivonescimab was introduced, which aligns with the kind of early, paradoxical worsening described in HPD reports across several tumor types.
Biologically, the case reinforces the caution that immunotherapy can be a poor fit for some oncogene-driven cancers, even when PD-L1 is not negligible. HER2-mutant NSCLC often behaves as a molecularly defined disease rather than an immune-responsive one. Low tumor mutation burden and the presence of a dominant activating ERBB2 insertion are consistent with a tumor dependence on HER2 signaling rather than one primarily shaped by immune surveillance escape. In that setting, checkpoint blockade may fail to slow the driver lesion and, in rare circumstances, may coincide with accelerated progression.
Trastuzumab Rezetecan Produced a Fast Partial Response
Trastuzumab rezetecan was initiated on October 16, 2025, immediately after HPD was recognized. The response was swift. By December 5, 2025, the lung lesion had shrunk to 5.68 cm, the pleural effusion had diminished, and intracranial disease showed partial improvement. The best overall response was partial response, and the early thoracic control suggested genuine activity against the dominant HER2-driven clone.
The speed of regression is clinically important. HER2-directed antibody–drug conjugates do more than block signaling; they deliver cytotoxic payloads into HER2-expressing tumor cells, which can produce meaningful tumor shrinkage even in heavily pretreated or biologically aggressive disease. In this case, the therapy reversed a dangerous post-immunotherapy trajectory within weeks, offering proof that the tumor remained vulnerable to HER2-directed targeting despite the earlier treatment setback.
Compartment-Specific Resistance Emerged Later
Disease control was not uniform across all sites. On March 1, 2026, thoracic disease and the pleural effusion remained controlled, while renal, adrenal, retroperitoneal nodal, skeletal, and intracranial lesions progressed. Meningeal enhancement raised concern for leptomeningeal spread, although lumbar puncture was declined. No cranial radiotherapy or surgery was given at any point.
This spatially heterogeneous relapse is a familiar challenge in HER2-driven metastatic disease. Different metastatic niches can respond differently to the same drug because drug exposure, clonal architecture, and microenvironmental pressure are not identical in lung, bone, kidney, and CNS compartments. The thorax remained sensitive longer than the brain and viscera, suggesting either pharmacologic sanctuary effects or outgrowth of resistant subclones in distant sites. In practical terms, the tumor did not fail everywhere at once; it failed selectively, compartment by compartment.
Progression-free survival was about 4.5 months, and duration of response was about 2.9 months. These are modest intervals in absolute terms, but they were achieved after a major disease flare and in the setting of widespread metastatic involvement. The pattern still favors a real drug effect rather than transient stabilization from prior chemotherapy.
Zongertinib Was Rational but Short-Lived
Zongertinib 120 mg once daily was started on March 2, 2026 as salvage HER2-targeted therapy after progression on the ADC. The choice was biologically coherent: once a tumor remains driven by ERBB2 signaling after ADC exposure, a HER2 TKI may still suppress the pathway through a different mechanism. Exposure was brief, however, because treatment stopped after seven days for financial reasons, leaving no objective response assessment possible.
The patient died on May 15, 2026, with overall survival of approximately 9.5 months. Her course reflects how quickly a HER2-mutant lung cancer can evolve when the disease is disseminated, treatment must begin before molecular profiling is complete, and access to sequential targeted therapy is limited.
Biomarker Turnaround and Access Shaped the Entire Sequence
This case is not only about HER2 biology. It is also about timing, access, and compartmental failure. The patient needed urgent therapy before genomic information returned, and that forced initial use of chemotherapy without a driver-matched agent. Once the mutation was known, HER2-directed treatment produced a meaningful response, but the earlier choice to add immunotherapy exposed the patient to a hyperprogressive course that may have worsened functional reserve before the targeted agent ever began.
The low-level ERBB2 copy-number gain should not be overread as equivalent to amplification. The dominant actionable event was the exon 20 insertion, and the clinical behavior fits an oncogene-addicted tumor in which HER2 pathway suppression matters more than immune activation. When a patient has metastatic NSCLC with a clear driver alteration, early molecular diagnosis can determine not just the order of therapies but whether the right therapy is reached before the disease crosses a functional threshold.
This case also leaves a hard question for routine practice: in HER2-driven NSCLC with substantial metastatic burden and pending molecular data, how often does urgent immunotherapy exposure buy time, and how often does it compromise the chance to deliver the targeted treatment that ultimately works? The answer will depend on better biomarker turnaround, broader access to HER2-directed agents, and more precise recognition of hyperprogression when a tumor accelerates immediately after checkpoint exposure.