Two dimensions in targeting HER2

Organoids from patient biopsy samples can predict the response of BC patients to neoadjuvant chemotherapy

PROPOSE

Neoadjuvant chemotherapy has been widely used in locally advanced and inflammatory breast cancer. Generally, complete pathological response after neoadjuvant chemotherapy treatment predicts survival. Studies have shown that patient-derived organoids can be used in cancer research and drug development. Therefore, we aimed to generate a living organoid biobank from biopsy samples to predict the response of patients to neoadjuvant chemotherapy.

METHOD

We generated a living organoid biobank from locally advanced breast cancer patients receiving neoadjuvant chemotherapy. When the patient received neoadjuvant chemotherapy, the organoids were treated with similar drugs, thereby simulating the situation of the patient receiving treatment.

RESULT

We successfully constructed organoids from breast cancer biopsies, demonstrating that organoids can be generated from a small sample of tissue. The phenotype of breast cancer organoid often agreed with the original breast cancer according to the blinded histopathological analysis of H&E stain tissue and organoid sections. In addition, our data confirm that the patient's response to chemotherapy closely matches the organoids' response to drugs.

CONCLUSION

Our data indicate that patient-derived organoids can be used to predict the clinical response of breast cancer patients to neoadjuvant chemotherapy in vitro and to screen drugs that have different effects on different patients. Key messageComplete pathological response (pCR) after adjuvant chemotherapy can predict, survival, therefore, predicting patient response to neoadjuvant chemotherapy is critical.Patient-derived organoids (PDOs) matched the original tumour in terms of histopathology, hormone receptor levels and HER2 receptor status.Patient-derived organoids can predict the responsiveness of patient to neoadjuvant chemotherapy.

Defucosylated mouse‑dog chimeric anti‑HER2 monoclonal antibody exerts antitumor activities in mouse xenograft models of canine tumors

Human epidermal growth factor receptor 2 (HER2) overexpression has been reported in various types of cancer, including breast, gastric, lung, colorectal and pancreatic cancer. A humanized anti-HER2 monoclonal antibody (mAb), trastuzumab, has been shown to improve survival of patients in HER2-positive breast and gastric cancer. An anti-HER2 mAb, H₂Mab-77 (mouse IgG₁, kappa) was previously developed. In the present study, a defucosylated version of mouse-dog chimeric anti-HER2 mAb (H77Bf) was generated. H77Bf possesses a high binding-affinity [a dissociation constant (K_D): 7.5×10⁻¹⁰ M, as determined by flow cytometric analysis] for dog HER2-overexpressed CHO-K1 (CHO/dHER2) cells. H77Bf highly exerted antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) for CHO/dHER2 cells by canine mononuclear cells and complement, respectively. Moreover, administration of H77Bf significantly suppressed the development of CHO/dHER2 ×enograft tumor in mice compared with the control dog IgG. H77Bf also possesses a high binding-affinity (K_D: 7.2×10⁻¹⁰ M) for a canine mammary gland tumor cell line (SNP), and showed high ADCC and CDC activities for SNP cells. Intraperitoneal administration of H77Bf in mouse xenograft models of SNP significantly suppressed the development of SNP xenograft tumors compared with the control dog IgG. These results indicated that H77Bf exerts antitumor activities against dHER2-positive canine cancers, and could be valuable treatment regimen for canine cancers.

Inactivating amplified HER2: challenges, dilemmas, and future directions

The pharmaceutical inactivation of driver oncogenes has revolutionized the treatment of cancer replacing cytotoxic chemotherapeutic approaches with kinase inhibitor therapies for many types of cancers. This approach has not yet been realized for the treatment of HER2-amplified cancers. The monotherapy activities associated with HER2-targeting antibodies and kinase inhibitors are modest, and their clinical use has been in combination with, and not in replacement of cytotoxic chemotherapies. This stands in sharp contrast to achievements in the treatment of many other oncogene-driven cancers. The mechanism-based treatment hypothesis regarding the inactivation of HER2 justifies expectations far beyond what is currently realized. Overcoming this barrier requires mechanistic insights that can fuel new directions for pursuit, but scientific investigation of this treatment hypothesis, particularly with regards to trastuzumab, has been complicated by conflicting and confusing data sets, ironclad dogma, and mechanistic conclusions that have repeatedly failed to translate clinically. We are now approaching a point of convergence regarding the challenges and resiliency in this tumor driver, and I will provide here a review and opinion to inform where we currently stand with this treatment hypothesis and where the future potential lies.

Trastuzumab/Pertuzumab combination therapy stimulates anti-tumor responses through complement-dependent cytotoxicity and phagocytosis

Standard-of-care treatment for advanced HER2+ breast cancers (BC) is comprised of two HER2-specific monoclonal antibodies (mAb), Trastuzumab (T) and Pertuzumab (P) with chemotherapy. While this combination (T+P) is highly effective, its synergistic mechanism of action (MOA) is not completely known. Initial studies had demonstrated that Pertuzumab suppressed HER2 hetero-dimerization as the potential therapeutic MOA, thus the improved outcome associated with the T+P combination MOA compared to Trastuzumab alone has been widely reported as being due to Pertuzumab-mediated suppression of HER2 signaling in combination with Trastuzumab-mediated induction of anti-tumor immunity. Unraveling this MOA may be critical to extend this combination strategy to other antigens or other cancers, as well as improving this current treatment modality. Using novel murine and human versions of Pertuzumab, we found it induced both Antibody-Dependent-Cellular-Phagocytosis (ADCP) by tumor-associated macrophages and suppression of HER2 oncogenic signaling. Most significantly, we identified that only T+P combination therapy, but not when either antibody used in isolation, allows for the activation of the classical complement pathway, resulting in both direct complement-dependent cytotoxicity (CDC) as well as complement-dependent cellular phagocytosis (CDCP) of HER2+ BC cells. Notably, we show that tumor expression of C1q was positively associated with survival outcome in HER2+ BC patients, whereas expression of complement regulators CD55 and CD59 were inversely correlated, suggesting the importance of complement activity in clinical outcomes. Accordingly, inhibition of C1 activity in mice abolished the synergistic therapeutic activity of T+P therapy, whereas knockdown of CD55 and CD59 expression enhanced T+P efficacy. In summary, our study identifies classical complement activation as a significant anti-tumor MOA for T+P therapy that may be functionally enhanced to augment therapeutic efficacy in the clinic.

Mechanisms of Therapeutic Antitumor Monoclonal Antibodies

Monoclonal antibodies (mAb) are a major component of cancer therapy. In this review, we summarize the different therapeutic mAbs that have been successfully developed against various tumor-expressed antigens and examine our current understanding of their different mechanisms of antitumor action. These mechanisms of action (MOA) largely center on the stimulation of different innate immune effector processes, which appear to be principally responsible for the efficacy of most unconjugated mAb therapies against cancer. This is evident in studies of mAbs targeting antigens for hematologic cancers, with emerging data also demonstrating the critical nature of innate immune-mediated mechanisms in the efficacy of anti-HER2 mAbs against solid HER2+ cancers. Although HER2-targeted mAbs were originally described as inhibitors of HER2-mediated signaling, multiple studies have since demonstrated these mAbs function largely through their engagement with Fc receptors to activate innate immune effector functions as well as complement activity. Next-generation mAbs are capitalizing on these MOAs through improvements to enhance Fc-activity, although regulation of these mechanisms may vary in different tumor microenvironments. In addition, novel antibody-drug conjugates have emerged as an important means to activate different MOAs. Although many unknowns remain, an improved understanding of these immunologic MOAs will be essential for the future of mAb therapy and cancer immunotherapy.

Tackling Refractory Metastatic Colorectal Cancer: Future Perspectives

Substantial improvements have characterized the systemic treatment of metastatic colorectal cancer (mCRC) over the past 20 years. Besides strong evidence that supports the use of RAS and BRAF status as prognostic and predictive indicators of disease and response, novel technologies have made possible the incorporation of emerging biomarkers for the management of mCRC. On one hand, the discovery of point mutations, amplifications, fusions, and gene expression profiles highlights the genomic and dynamic complexity of CRC. On the other, such discoveries are leading to newer biomarker-driven strategies that add to existing anti-epidermal growth factor receptor (EGFR) and anti-angiogenic approaches. In addition, the availability of a wide molecular profiling has relevant implications for patient prognosis and treatment benefits. Here, we will review the molecular underpinnings and clinical data supporting novel targeted treatments under development for refractory mCRC harboring BRAF mutations, KRAS G12C mutations, HER2 amplification, and less common molecular alterations, such as the re-arrangements of NTRK, ALK, and ROS1. Additionally, we will discuss novel strategies driving the rechallenge of EGFR antibodies and the incorporation of newer anti-angiogenic agents in the therapeutic armamentarium.

The role of HER2 and HER3 in HER2-amplified cancers beyond breast cancers

HER2 and HER3 play key driving functions in the pathophysiology of HER2-amplified breast cancers, but this function is less well characterized in other cancers driven by HER2 amplification. This study aimed to explore the role of HER2 and HER3 signaling in other types of HER2-amplified cancer. The expression and signaling activity of HER2, HER3, and downstream pathway proteins were studied in cell panels representing HER2-amplified cancers of the breast, bladder, colon and rectal, stomach, esophagus, lung, tongue, and endometrium along with controls lacking HER2 amplification. We report that HER2-amplified cancers are addicted to HER2 across different cancer types and the depth of addiction is best linked with the expression level of HER2, but not with HER3 expression. We report that the expression and constitutive phosphorylation of HER3 are ubiquitous in HER2-amplified breast cancer cell lines, but much more variable in HER2-amplified cancer cells from other tissues. We observed the lapatinib-induced compensatory upregulation of HER3 signaling in many types of HER2-amplified cancers, although with much variability. We find that HER3 expression is essential for in vivo tumorigenic growth in some HER2-amplified tumors but not others. Importantly HER3 expression level does not correlate well with its functional importance. More biomarkers will be needed to guide the optimal use of HER3 inhibitors in HER2-amplified cancers from non-breast origin. Unlike oncogenes activated through mutational events, the activation of HER2 through overexpression represents a gradient of activities and depth of addiction and the response to inhibitors follows a similar gradient.

Efficacy of Combination Chemo-Immunotherapy as a First-Line Treatment for Advanced Non-Small-Cell Lung Cancer Patients With HER2 Alterations: A Case Series

Objective

Although the treatment of non-small-cell lung cancer (NSCLC) patients with human epidermal growth factor receptor 2 (HER2) alterations has been studied for years, the overall response rate (ORR) of these patients is still unsatisfactory, and more therapeutic strategies are needed. Little is known about the combination of chemo- and immunotherapy in HER2-altered lung cancer treatment.

Materials and Methods

We report five cases of advanced NSCLC with HER2 insertion mutation or amplification treated with immunotherapy combined with chemotherapy as the first-line treatment. The HER2 alteration type, duration of treatment and survival were also analyzed.

Results

The five advanced NSCLC patients, three with HER2 mutations and two with HER2 amplifications, received chemo-immunotherapy as the first-line treatment. The average patient age was 54.6 years. Three patients were females, and two were males. Among all the patients, only one had a smoking history. The immunotherapies used were as follows: two patients were treated with sintilimab, and three patients were treated with pembrolizumab. Only one patient had squamous carcinoma, and she was also the only patient with a complete response (CR). The progression-free survival (PFS) ranged from 2-12 months, with a median PFS of 8.0 months.

Conclusions

Chemo-immunotherapy may be a promising first-line treatment option for NSCLC patients with HER2 alterations. Further clinical trials are required to confirm this therapeutic option.

Immunotherapy as a partner for HER2-directed therapies

Introduction: Existing HER2-targeted therapies modulate the tumor microenvironment and the immunologic response cancer in a favorable way. While these therapies have made dramatic improvements in the treatment and prognosis of HER2-overexpressing malignancies, additional treatment options are still needed.Areas covered: This review covers the immunomodulatory effects of approved HER2-targeted therapies. We discuss the preclinical data that demonstrate an additive effect of the combination of trastuzumab or other HER2-targeting agents with immunomodulatory drugs. Finally, we report the initial studies on the combination of HER2-targeted agents together with immune checkpoint inhibitors or cancer vaccines in breast cancer.Expert opinion: Preclinical data suggest a synergistic effect of HER2-targeted therapy together with both checkpoint inhibitor and cancer vaccine immunotherapy. Results from initial trials with PD-1/PD-L1-blocking therapy together with HER2-targeted therapy have been negative, but responses were seen in patients with PD-L1+ breast cancer. Trastuzumab together with HER2-targeted cancer vaccination has shown benefits in triple negative breast cancer. Further trials are necessary and warranted to confirm the benefit of these combinations.

HER2-Positive Breast Cancer Patients with Pre-Treatment Axillary Involvement or Postmenopausal Status Benefit from Neoadjuvant Rather than Adjuvant Chemotherapy Plus Trastuzumab Regimens

BACKGROUND

No survival benefit has yet been demonstrated for neoadjuvant chemotherapy (NAC) against HER2-positive tumors in patients with early breast cancer (BC). The objective of this study was to compare the prognosis of HER2-positive BC patients treated with NAC to that of patients treated with adjuvant chemotherapy (AC).

MATERIALS AND METHODS

We retrospectively analyzed disease-free (DFS) and overall survival (OS) in 202 HER2-positive patients treated with NAC and 701 patients treated with AC. All patients received trastuzumab in addition to chemotherapy. Patient data were weighted by a propensity score to overcome selection bias.

RESULTS

After inverse probability of treatment weights (IPTW) adjustment, no difference in DFS (p = 0.3) was found between treatments for the total population. However, after multivariate analysis, an interaction was found between cN status and chemotherapy strategy (IPTW-corrected corrected Hazard ratio cHR = 0.52, 95% CI (0.3-0.9), p interaction = 0.08) and between menopausal status and chemotherapy (CT) strategy (cHR = 0.35, 95%CI (0.18-0.7)) p interaction < 0.01). NAC was more beneficial than AC strategy in cN-positive patients and in postmenopausal patients. Moreover, after IPTW adjustment, the multivariate analysis showed that the neoadjuvant strategy conferred a significant OS benefit (cHR = 0.09, 95%CI [0.02-0.35], p < 0.001).

CONCLUSION

In patients with HER2-positive BC, the NAC strategy is more beneficial than the AC strategy, particularly in cN-positive and postmenopausal patients. NAC should be used as a first-line treatment for HER2-positive tumors.

CD47 blockade augmentation of trastuzumab antitumor efficacy dependent on antibody-dependent cellular phagocytosis

The HER2-specific monoclonal antibody (mAb), trastuzumab, has been the mainstay of therapy for HER2+ breast cancer (BC) for approximately 20 years. However, its therapeutic mechanism of action (MOA) remains unclear, with antitumor responses to trastuzumab remaining heterogeneous and metastatic HER2+ BC remaining incurable. Consequently, understanding its MOA could enable rational strategies to enhance its efficacy. Using both murine and human versions of trastuzumab, we found its antitumor activity dependent on Fcγ receptor stimulation of tumor-associated macrophages (TAMs) and antibody-dependent cellular phagocytosis (ADCP), but not cellular cytotoxicity (ADCC). Trastuzumab also stimulated TAM activation and expansion, but did not require adaptive immunity, natural killer cells, and/or neutrophils. Moreover, inhibition of the innate immune ADCP checkpoint, CD47, significantly enhanced trastuzumab-mediated ADCP and TAM expansion and activation, resulting in the emergence of a unique hyperphagocytic macrophage population, improved antitumor responses, and prolonged survival. In addition, we found that tumor-associated CD47 expression was inversely associated with survival in HER2+ BC patients and that human HER2+ BC xenografts treated with trastuzumab plus CD47 inhibition underwent complete tumor regression. Collectively, our study identifies trastuzumab-mediated ADCP as an important antitumor MOA that may be clinically enabled by CD47 blockade to augment therapeutic efficacy.

Immunooncology in Breast Cancer: Active and Passive Vaccination Strategies

Immunotherapies are set to become part of the therapeutic repertoire for breast cancer in the near future. Active vaccination is a promising strategy, especially in tumors that have a specific tumor-associated antigen. Although cellular immunotherapies have not yet shown efficacy, new technologies are on the way to improve this approach. Given the recent Food and Drug Administration approval of chimeric antigen receptor (CAR) T cells for leukemia, it is only a question of time before solid tumors will follow. However, not all breast cancer patients will respond to cellular or other immunotherapy. Hence, we must define subpopulations of breast cancer patients who benefit from this new approach.

TP53 mutations and protein immunopositivity may predict for poor outcome but also for trastuzumab benefit in patients with early breast cancer treated in the adjuvant setting

BACKGROUND

We investigated the impact of PIK3CA and TP53 mutations and p53 protein status on the outcome of patients who had been treated with adjuvant anthracycline-taxane chemotherapy within clinical trials in the pre- and post-trastuzumab era.

RESULTS

TP53 and PIK3CA mutations were found in 380 (21.5%) and 458 (25.9%) cases, respectively, including 104 (5.9%) co-mutated tumors; p53 immunopositivity was observed in 848 tumors (53.5%). TP53 mutations (p < 0.001) and p53 protein positivity (p = 0.001) were more frequent in HER2-positive and triple negative (TNBC) tumors, while PIK3CA mutations were more frequent in Luminal A/B tumors (p < 0.001). TP53 mutation status and p53 protein expression but not PIK3CA mutation status interacted with trastuzumab treatment for disease-free survival; patients with tumors bearing TP53 mutations or immunopositive for p53 protein fared better when treated with trastuzumab, while among patients treated with trastuzumab those with the above characteristics fared best (interaction p = 0.017 for mutations; p = 0.015 for IHC). Upon multivariate analysis the above interactions remained significant in HER2-positive patients; in the entire cohort, TP53 mutations were unfavorable in patients with Luminal A/B (p = 0.003) and TNBC (p = 0.025); p53 immunopositivity was strongly favorable in patients treated with trastuzumab (p = 0.009).

MATERIALS AND METHODS

TP53 and PIK3CA mutation status was examined in 1766 paraffin tumor DNA samples with informative semiconductor sequencing results. Among these, 1585 cases were also informative for p53 protein status assessed by immunohistochemistry (IHC; 10% positivity cut-off).

CONCLUSIONS

TP53 mutations confer unfavorable prognosis in patients with Luminal A/B and TNBC tumors, while p53 immunopositivity may predict for trastuzumab benefit in the adjuvant setting.

Phase Ib dose-finding trial of lapatinib plus pegylated liposomal doxorubicin in advanced HER2-positive breast cancer

Purpose

Combination of anthracyclines with trastuzumab is hampered by cardiotoxicity. Pegylated liposomal doxorubicin and lapatinib could represent a safer alternative to combination therapy.

Methods

In this phase Ib study with 3 + 3 dose escalation design, patients with HER2-positive advanced breast cancer received pegylated liposomal doxorubicin 30 mg/m² intravenously on day 1 plus lapatinib 1250 (level 1) or 1500 (level 2) mg/day orally on days 1–21 of each 21-day cycle. The aims were to establish the maximum tolerated dose at first cycle, and the activity and safety of multiple cycles.

Results

Nine patients out of 11 enrolled were evaluable: 3 at level 1 and 6 at level 2. No dose-limiting toxicities occurred at dose level 1, while 1 (grade 3 diarrhea) occurred at dose level 2, leading to the expansion of this cohort to 6 patients, with no further dose-limiting toxicities. Main grade 1–2 toxicities at first cycle were leucopenia, diarrhea, elevated transaminases, mucositis. Three patients had grade 3 toxicities at subsequent cycles, including colitis, anorexia, stomatitis plus hand-foot syndrome. One partial response, 5 disease stabilizations, and 3 disease progressions were reported.

Conclusions

Combination of pegylated liposomal doxorubicin and lapatinib is feasible and potentially active in pretreated HER2-positive advanced breast cancer patients.

Trial registration

NCT02131506 (ClinicalTrials.gov identifier).

Mass-spectrometry-based quantitation of Her2 in gastroesophageal tumor tissue: comparison to IHC and FISH

BACKGROUND

Trastuzumab has shown a survival benefit in cases of Her2-positive gastroesophageal cancer (GEC). Immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) currently determine eligibility for trastuzumab-based therapy. However, these low-throughput assays often produce discordant or equivocal results.

METHODS

We developed a targeted proteomic assay based on selected reaction monitoring mass spectrometry (SRM-MS) and quantified levels (amol/μg) of Her2-SRM protein in cell lines (n = 27) and GEC tissues (n = 139). We compared Her2-SRM protein expression with IHC/FISH, seeking to determine optimal SRM protein expression cutoffs in order to identify HER2 gene amplification.

RESULTS

After demonstrating assay development, precision, and stability, Her2-SRM protein measurement was observed to be highly concordant with the HER2/CEP17 ratio, particularly in a multivariate regression model adjusted for SRM expression of the covariates Met, Egfr, Her3, and HER2 heterogeneity, as well as their interactions (cell lines r (2) = 0.9842; FFPE r (2) = 0.7643). In GEC tissues, Her2-SRM protein was detected at any level in 71.2 % of cases. ROC curves demonstrated that Her2-SRM protein levels have a high specificity (100 %) at an upper-level cutoff of >750 amol/µg and sensitivity of 75 % at a lower-level cutoff of <450 amol/μg for identifying HER2 FISH-amplified tumors. An "equivocal zone" of 450-750 amol/µg of Her2-SRM protein was analogous to IHC2+ but represented fewer cases (9-16 % of cases versus 36-41 %).

CONCLUSIONS

Compared to IHC, targeted SRM-Her2 proteomics provided more objective and quantitative Her2 expression with excellent HER2/CEP17 FISH correlation and fewer equivocal cases. Along with its multiplex capability for other relevant oncoproteins, these results demonstrate a refined HER2 protein expression assay for clinical application.

Biomarkers for Predicting Response to Anti-HER2 Agents

The HER2 receptor is amplified or overexpressed in approximately 20% of all breast cancers, but despite significant efforts of the clinical research community and a growing number of anti-HER2 agents, a significant number of patients with HER2-positive breast cancer either progress or suffer disease relapse within 5-10 years. The development of robust biomarkers that predict response to anti-HER2 agents is therefore an important clinical need to prevent overtreatment and to enable earlier assignment of patients to more optimal therapies. Here we review some of the recent advances in the field by focusing on pathways mediating resistance to anti-HER2 therapies, and the role of the immune system and cancer stem cells in therapy response. We also review preoperative treatment strategies and research paradigms that show promise in identifying novel biomarkers of response while also enabling the delineation of the mechanisms underlying clinical benefit from anti-HER2 therapies.

Targeting HER2 aberrations as actionable drivers in lung cancers: phase II trial of the pan-HER tyrosine kinase inhibitor dacomitinib in patients with HER2-mutant or amplified tumors

BACKGROUND

HER2 mutations and amplifications have been identified as oncogenic drivers in lung cancers. Dacomitinib, an irreversible inhibitor of HER2, EGFR (HER1), and HER4 tyrosine kinases, has demonstrated activity in cell-line models with HER2 exon 20 insertions or amplifications. Here, we studied dacomitinib in patients with HER2-mutant or amplified lung cancers.

PATIENTS AND METHODS

As a prespecified cohort of a phase II study, we included patients with stage IIIB/IV lung cancers with HER2 mutations or amplification. We gave oral dacomitinib at 30-45 mg daily in 28-day cycles. End points included partial response rate, overall survival, and toxicity.

RESULTS

We enrolled 30 patients with HER2-mutant (n = 26, all in exon 20 including 25 insertions and 1 missense mutation) or HER2-amplified lung cancers (n = 4). Three of 26 patients with tumors harboring HER2 exon 20 mutations [12%; 95% confidence interval (CI) 2% to 30%] had partial responses lasting 3+, 11, and 14 months. No partial responses occurred in four patients with tumors with HER2 amplifications. The median overall survival was 9 months from the start of dacomitinib (95% CI 7-21 months) for patients with HER2 mutations and ranged from 5 to 22 months with amplifications. Treatment-related toxicities included diarrhea (90%; grade 3/4: 20%/3%), dermatitis (73%; grade 3/4: 3%/0%), and fatigue (57%; grade 3/4: 3%/0%). One patient died on study likely due to an interaction of dacomitinib with mirtazapine.

CONCLUSIONS

Dacomitinib produced objective responses in patients with lung cancers with specific HER2 exon 20 insertions. This observation validates HER2 exon 20 insertions as actionable targets and justifies further study of HER2-targeted agents in specific HER2-driven lung cancers.

CLINICALTRIALSGOV

NCT00818441.

Favorable response to trastuzumab plus irinotecan combination therapy in two patients with HER2-positive relapsed small-cell lung cancer

Small-cell lung cancer (SCLC) easily recurs with multidrug resistance phenotype. However, standard therapeutic strategies for relapsed-SCLC remain unestablished. Human epidermal growth factor receptor 2 (HER2) expression correlates with poor prognosis in extensive disease-SCLC. We have reported previously that HER2 expression is upregulated when HER2-positive SCLC cells acquire chemoresistance, and also demonstrated that trastuzumab exerts significant antitumor activity toward HER2-upregulated chemoresistant SCLC, mainly via antibody-dependent cell-mediated cytotoxicity mechanism. Based on these preclinical data, we treated two patients with HER2-positive SCLC by combination of trastuzumab (6 mg/kg, day 1) and irinotecan (80 mg/m(2), days 1 and 8) every 21 days as the third-line chemotherapy following two prior regimens, first-line carboplatin plus etoposide and second-line amrubicin. One patient achieved partial response after the first cycle and received 6 cycles in total without disease progression for 4.5 months. The other also received 4 cycles and kept stable disease for 3.5 months. This treatment can be continued safely at an outpatient clinic without any severe adverse event. In conclusion, trastuzumab plus irinotecan chemotherapy is promising and feasible against HER2-positive relapsed SCLC. Further clinical studies are encouraged to confirm the antitumor efficacy of trastuzumab in SCLC.