Acquired Cross-Resistance in Small Cell Lung Cancer due to Extrachromosomal DNA Amplification of MYC Paralogs

Small cell lung cancer (SCLC) presents as a highly chemosensitive malignancy but acquires cross-resistance after relapse. This transformation is nearly inevitable in patients but has been difficult to capture in laboratory models. Here, we present a preclinical system that recapitulates acquired cross-resistance, developed from 51 patient-derived xenograft (PDX) models. Each model was tested in vivo against three clinical regimens: cisplatin plus etoposide, olaparib plus temozolomide, and topotecan. These drug-response profiles captured hallmark clinical features of SCLC, such as the emergence of treatment-refractory disease after early relapse. For one patient, serial PDX models revealed that cross-resistance was acquired through MYC amplification on extrachromosomal DNA (ecDNA). Genomic and transcriptional profiles of the full PDX panel revealed that MYC paralog amplifications on ecDNAs were recurrent in relapsed cross-resistant SCLC, and this was corroborated in tumor biopsies from relapsed patients. We conclude that ecDNAs with MYC paralogs are recurrent drivers of cross-resistance in SCLC.

SIGNIFICANCE

SCLC is initially chemosensitive, but acquired cross-resistance renders this disease refractory to further treatment and ultimately fatal. The genomic drivers of this transformation are unknown. We use a population of PDX models to discover that amplifications of MYC paralogs on ecDNA are recurrent drivers of acquired cross-resistance in SCLC. This article is featured in Selected Articles from This Issue, p. 695.

Human lung cancer harbors spatially organized stem-immunity hubs associated with response to immunotherapy

The organization of immune cells in human tumors is not well understood. Immunogenic tumors harbor spatially localized multicellular 'immunity hubs' defined by expression of the T cell-attracting chemokines CXCL10/CXCL11 and abundant T cells. Here, we examined immunity hubs in human pre-immunotherapy lung cancer specimens and found an association with beneficial response to PD-1 blockade. Critically, we discovered the stem-immunity hub, a subtype of immunity hub strongly associated with favorable PD-1-blockade outcome. This hub is distinct from mature tertiary lymphoid structures and is enriched for stem-like TCF7+PD-1+CD8+ T cells, activated CCR7+LAMP3+ dendritic cells and CCL19+ fibroblasts as well as chemokines that organize these cells. Within the stem-immunity hub, we find preferential interactions between CXCL10+ macrophages and TCF7-CD8+ T cells as well as between mature regulatory dendritic cells and TCF7+CD4+ and regulatory T cells. These results provide a picture of the spatial organization of the human intratumoral immune response and its relevance to patient immunotherapy outcomes.

Role of Adjuvant Chemotherapy in Early-Stage Combined Small Cell Lung Cancer

BACKGROUND

The role of adjuvant therapy in completely resected primary tumors that have components of both non-small cell lung cancer and small cell lung cancer (combined SCLC) is poorly understood. We sought to determine the potential benefits of adjuvant chemotherapy in patients who undergo complete resection for early-stage combined SCLC.

METHODS

Overall survival of patients with pathologic T1-2 N0 M0 combined SCLC who underwent complete resection in the National Cancer Database from 2004 to 2017, stratified by adjuvant chemotherapy vs surgery alone, was evaluated by multivariable Cox proportional hazards modeling and propensity score-matched analysis. Patients treated with induction therapy and those who died within 90 days of surgery were excluded from analysis.

RESULTS

Of 630 patients who had pT1-2 N0 M0 combined SCLC during the study period, 297 patients (47%) underwent complete R0 resection. Adjuvant chemotherapy was administered to 63% of patients (n = 188), and 37% of patients underwent surgery alone (n = 109). In unadjusted analysis, the 5-year overall survival was 61.6% (95% CI, 50.8-70.7) for patients who underwent surgery alone and 66.4% (95% CI, 58.4-73.3) for patients who underwent adjuvant chemotherapy. In multivariable and propensity score-matched analysis, there were no significant differences in overall survival between adjuvant chemotherapy and surgery alone (adjusted hazard ratio, 1.16; 95% CI, 0.73-1.84). These findings were consistent when limited to patients who underwent lobectomies or to healthier patients who have at most 1 major comorbidity.

CONCLUSIONS

In this national analysis, patients with pT1-2 N0 M0 combined SCLC treated with surgical resection alone have similar outcomes to those who undergo adjuvant chemotherapy.

Concordance of ASCL1, NEUROD1 and POU2F3 transcription factor-based subtype assignment in paired tumour samples from small cell lung carcinoma

AIMS

Small cell lung carcinoma (SCLC) can be classified into transcription factor-based subtypes (ASCL1, NeuroD1, POU2F3). While in-vitro studies suggest intratumoral heterogeneity in the expression of these markers, how SCLC subtypes vary over time and among locations in patients remains unclear.

METHODS AND RESULTS

We searched a consecutive series of patients at our institution in 2006-22 for those with greater than one available formalin-fixed paraffin-embedded SCLC sample in multiple sites and/or time-points. Immunohistochemistry for ASCL1, NeuroD1 and POU2F3 was performed and evaluated using H-scores, with subtype assigned based on the positive marker (H-score threshold >10) with the highest H-score. The 179 samples (75, lung; 51, lymph nodes; 53, non-nodal metastases) from 84 patients (74 with two, 10 with more than two samples) included 98 (54.7%) ASCL1-dominant, 47 (26.3%) NeuroD1-dominant, 15 (8.4%) POU2F3-dominant, 17 (9.5%) triple-negative and two (1.1%) ASCL1/NeuroD1 co-dominant samples. NeuroD1-dominant subtype was enriched in non-lung locations. Subtype concordance from pairwise comparison was 71.4% overall and 89.7% after accounting for ASCL1/NeuroD1-dual expressors and technical factors including <500 cells/slide, H-score thresholds and sample decalcification. No significant difference in subtype concordance was noted with a longer time lapse or with extrathoracic versus intrathoracic samples in this cohort.

CONCLUSIONS

After accounting for technical factors, transcription factor-based subtyping was discordant among multiple SCLC samples in ~10% of patients, regardless of sample locations and time lapse. Our findings highlighted the spatiotemporal heterogeneity of SCLC in clinical samples and potential challenges, including technical and biological factors, that might limit concordance in SCLC transcription factor-based subtyping.

Efficacy and Tolerability of ALK/MET Combinations in Patients With ALK-Rearranged Lung Cancer With Acquired MET Amplification: A Retrospective Analysis

Introduction

MET amplification is a potentially actionable resistance mechanism in ALK-rearranged (ALK+) lung cancer. Studies describing treatment outcomes of this molecular subgroup are lacking.

Methods

We assembled a cohort of patients with ALK+ lung cancer and acquired MET amplification (identified by tissue or plasma) who received regimens targeting both ALK and MET. Efficacy and safety were assessed using the Response Evaluation Criteria in Solid Tumors version 1.1 and Common Terminology Criteria for Adverse Events version 4.03, respectively.

Results

A total of 12 patients were included in the series. MET amplification was detected after a median of 1.5 (range 1-5) lines of therapy. Four distinct regimens were implemented to address MET amplification: crizotinib (n = 2), lorlatinib plus crizotinib (n = 6), alectinib plus capmatinib (n = 3), and alectinib plus crizotinib (n = 1). Partial responses were observed in five (42%) of 12 patients, including patients who received crizotinib (n = one of two), lorlatinib plus crizotinib (n = three of six), and alectinib plus capmatinib (n = one of three). Primary progression was observed in four patients (33%). Grades 1 to 2 peripheral edema, occurring in seven (58%) patients, was found with both crizotinib and capmatinib. One patient required dose reduction of capmatinib plus alectinib for persistent grade 2 edema. Across the regimens, one patient discontinued therapy for toxicity, specifically neurocognitive toxicity from lorlatinib plus crizotinib. At progression on ALK+ MET therapy, potential resistance mechanisms included MET copy number changes and ALK kinase domain mutations.

Conclusions

Combined ALK and MET inhibition is associated with moderate antitumor activity in patients with ALK+ NSCLC with concurrent MET amplification. Prospective studies are indicated to confirm activity and identify individuals most likely to benefit from the treatment.

MCT4-dependent lactate secretion suppresses antitumor immunity in LKB1-deficient lung adenocarcinoma

Inactivating STK11/LKB1 mutations are genomic drivers of primary resistance to immunotherapy in KRAS-mutated lung adenocarcinoma (LUAD), although the underlying mechanisms remain unelucidated. We find that LKB1 loss results in enhanced lactate production and secretion via the MCT4 transporter. Single-cell RNA profiling of murine models indicates that LKB1-deficient tumors have increased M2 macrophage polarization and hypofunctional T cells, effects that could be recapitulated by the addition of exogenous lactate and abrogated by MCT4 knockdown or therapeutic blockade of the lactate receptor GPR81 expressed on immune cells. Furthermore, MCT4 knockout reverses the resistance to PD-1 blockade induced by LKB1 loss in syngeneic murine models. Finally, tumors from STK11/LKB1 mutant LUAD patients demonstrate a similar phenotype of enhanced M2-macrophages polarization and hypofunctional T cells. These data provide evidence that lactate suppresses antitumor immunity and therapeutic targeting of this pathway is a promising strategy to reversing immunotherapy resistance in STK11/LKB1 mutant LUAD.

Acquired Cross-resistance in Small Cell Lung Cancer due to Extrachromosomal DNA Amplification of MYC paralogs

Small cell lung cancer (SCLC) presents as a highly chemosensitive malignancy but acquires cross-resistance after relapse. This transformation is nearly inevitable in patients but has been difficult to capture in laboratory models. Here we present a pre-clinical system that recapitulates acquired cross-resistance in SCLC, developed from 51 patient-derived xenografts (PDXs). Each model was tested for in vivo sensitivity to three clinical regimens: cisplatin plus etoposide, olaparib plus temozolomide, and topotecan. These functional profiles captured hallmark clinical features, such as the emergence of treatment-refractory disease after early relapse. Serially derived PDX models from the same patient revealed that cross-resistance was acquired through a MYC amplification on extrachromosomal DNA (ecDNA). Genomic and transcriptional profiles of the full PDX panel revealed that this was not unique to one patient, as MYC paralog amplifications on ecDNAs were recurrent among cross-resistant models derived from patients after relapse. We conclude that ecDNAs with MYC paralogs are recurrent drivers of cross-resistance in SCLC.

SIGNIFICANCE

SCLC is initially chemosensitive, but acquired cross-resistance renders this disease refractory to further treatment and ultimately fatal. The genomic drivers of this transformation are unknown. We use a population of PDX models to discover that amplifications of MYC paralogs on ecDNA are recurrent drivers of acquired cross-resistance in SCLC.

Brief Report: Declining Rates of SARS-CoV-2 Vaccine Uptake Among Patients With Thoracic Malignancies

•Patients with primary thoracic malignancies are at increased risk of complications and death from COVID-19. Multiple studies have demonstrated humoral immune responses to SARS-CoV-2 vaccinations in patients with cancer, though the degree of immunogenicity varies. Over the last year the United States CDC has issued recommendations for multiple additional ‘booster’ vaccine doses for patients with cancer for protection against severe disease. •In this study, we found that among 242 patients with primary thoracic malignancies who underwent initial vaccination against SARS-CoV-2, there was a marked decline in uptake of each subsequent additional vaccine dose. Specifically, among patients who received an initial mRNA-based vaccination series (mRNA-1273 or BNT162b2), 75% of eligible patients received the recommended third dose, 39% received the recommended fourth dose, and 5% received the recommended fifth dose at the time of data cutoff. Of note, we assessed serologic responses in a subset of patients receiving booster vaccinations and found that additional vaccinations increased humoral immunity, as expected. •With the recent CDC recommendation of novel bivalent mRNA-based vaccine booster doses for all individuals over the age of 12, our findings highlight the need for further understanding of the reasons behind decreased vaccine uptake and emphasize the importance of counseling by providers regarding public health recommendations for patients with lung cancer.

Spatial analysis of human lung cancer reveals organized immune hubs enriched for stem-like CD8 T cells and associated with immunotherapy response

The organization of immune cells in human tumors is not well understood. Immunogenic tumors harbor spatially-localized multicellular 'immunity hubs' defined by expression of the T cell-attracting chemokines CXCL10/CXCL11 and abundant T cells. Here, we examined immunity hubs in human pre-immunotherapy lung cancer specimens, and found that they were associated with beneficial responses to PD-1-blockade. Immunity hubs were enriched for many interferon-stimulated genes, T cells in multiple differentiation states, and CXCL9/10/11 + macrophages that preferentially interact with CD8 T cells. Critically, we discovered the stem-immunity hub, a subtype of immunity hub strongly associated with favorable PD-1-blockade outcomes, distinct from mature tertiary lymphoid structures, and enriched for stem-like TCF7+PD-1+ CD8 T cells and activated CCR7 + LAMP3 + dendritic cells, as well as chemokines that organize these cells. These results elucidate the spatial organization of the human intratumoral immune response and its relevance to patient immunotherapy outcomes.

Biology and impact of lineage plasticity in ALK-positive NSCLC: a narrative review

Background and Objective

Lineage transformation is a known mechanism of acquired resistance to targeted therapies in non-small cell lung cancer (NSCLC). Transformation to small cell and squamous carcinoma and epithelial-to-mesenchymal transition (EMT) have all been identified as recurrent but rare events in ALK-positive NSCLC. However, centralized data informing our understanding of the biology and clinical implications of lineage transformation in ALK-positive NSCLC are lacking.

Methods

We performed a narrative review by searching the PubMed and clinicaltrials.gov databases for articles published in English from August, 2007 until October, 2022 and reviewing the bibliographies of key references to identify important literature related to lineage transformation in ALK-positive NSCLC.

Key Content and Findings

In this review, we aimed to synthesize the published literature describing the incidence, mechanism(s), and clinical outcomes of lineage transformation in ALK-positive NSCLC. Lineage transformation as a mechanism of resistance to ALK TKIs in ALK-positive NSCLC is reported at a frequency of <5%. Available data across molecular subtypes of NSCLC suggest that the process of lineage transformation is likely to be driven by transcriptional reprogramming rather than acquired genomic mutations. Retrospective cohorts including tissue-based translational studies together with clinical outcomes make up the highest level of evidence that exists to inform treatment approach for patients with transfomed ALK-positive NSCLC.

Conclusions

The clinicopathologic features of transformed ALK-positive NSCLC as well as the biologic mechanisms underling lineage transformation remain incompletely understood. Prospective data are needed to develop improved diagnostic and treatment algorithms for patients with ALK-positive NSCLC that undergo lineage transformation.

Targeting EGFR Exon 20 Insertions in Non-Small Cell Lung Cancer: Recent Advances and Clinical Updates

Approximately 10% of EGFR-activating mutations occur as in-frame insertion mutations in exon 20 of the EGFR kinase domain (EGFR ins20). EGFR ins20 mutations have not demonstrated the same sensitivity to early generations of EGFR tyrosine kinase inhibitors (TKI) as canonical activating EGFR mutations such as del19 and L858R. Development of effective therapies for this subset of patients has been challenging, but recent years have seen more rapid progress in these efforts. In this review, we describe the molecular and clinicopathologic features of EGFR ins20 mutations and summarize recent data on emerging therapies for patients with this subtype of EGFR-mutant non-small cell lung cancer (NSCLC). SIGNIFICANCE: When activating mutations in EGFR were first discovered in lung cancer, the lack of sensitivity of tumors harboring EGFR ins20 mutations to early-generation EGFR TKIs resulted in this subset of EGFR-mutant tumors being initially classified as an untargetable or intrinsically resistant subpopulation. In addition, the diversity of mutations within EGFR exon 20 and resultant challenges identifying them on routine clinical genotyping tests led to underestimation of their frequency. However, recent scientific progress in targeting EGFR ins20 mutations as well as more effective identification of this clinical cohort has enhanced our ability to develop effective therapies for patients with this subtype of EGFR-mutant NSCLC.

High Sensitivity of Plasma Cell-Free DNA Genotyping in Cases With Evidence of Adequate Tumor Content

Plasma cell-free DNA (cfDNA) sequencing is a compelling diagnostic tool in solid tumors and has been shown to have high positive predictive value. However, limited assay sensitivity means that negative plasma genotyping, or the absence of detection of mutation of interest, still requires reflex tumor biopsy.

METHODS

We analyzed two independent cohorts of patients with advanced non-small-cell lung cancer (NSCLC) with known canonical driver and resistance mutations who underwent plasma cfDNA genotyping. We measured quantitative features, such as maximum allelic frequency (mAF), as clinically available measures of cfDNA tumor content, and studied their relationship with assay sensitivity.

RESULTS

In patients with EGFR-mutant NSCLC harboring EGFR T790M, detection of driver mutation at > 1% AF conferred a sensitivity of 97% (368/380) for detection of T790M across three cfDNA genotyping platforms. Similarly, in a second cohort of patients with EGFR or KRAS driver mutations, when the mAF of nontarget mutations was > 1%, sensitivity for driver mutation detection was 100% (43/43). Combining the two NSCLC patient cohorts, the presence of nontarget mutations at mAF > 1% predicts for high sensitivity (> 95%) for identifying the presence of the known driver mutation, whereas mAF of ≤ 1% confers sensitivity of only 26%-54% across platforms. Focusing on 21 false-negative cases where the driver mutation was not detected on plasma next-generation sequencing, other mutations (presumably clonal hematopoiesis) were detected at ≤ 1% AF in 14 (67%).

CONCLUSION

Plasma cfDNA genotyping is highly sensitive when adequate tumor DNA content is present. The likelihood of a false-negative cfDNA genotyping result is low in a sample with evidence of > 1% tumor content. Bioinformatic approaches are needed to further optimize the assessment of cfDNA tumor content in plasma genotyping assays.

Coronavirus Disease 2019 Infection in a Patient Population with Lung Cancer: Incidence, Presentation, and Alternative Diagnostic Considerations

Introduction

Lung cancer is associated with severe coronavirus disease 2019 (COVID-19) infections. Symptom overlap between COVID-19 and lung cancer may complicate diagnostic evaluation. We aimed to investigate the incidence, symptoms, differential diagnosis, and outcomes of COVID-19 in patients with lung cancer.

Methods

To determine an at-risk population for COVID-19, we retrospectively identified patients with lung cancer receiving longitudinal care within a single institution in the 12 months (April 1, 2019 to March 31, 2020) immediately preceding the COVID-19 pandemic, including an “active therapy population” treated within the last 60 days of this period. Among patients subsequently referred for COVID-19 testing, we compared symptoms, laboratory values, radiographic findings, and outcomes of positive versus negative patients.

Results

Between April 1, 2019 and March 31, 2020, a total of 696 patients received longitudinal care, including 406 (58%) in the active therapy population. Among 55 patients referred for COVID-19 testing, 24 (44%) were positive for COVID-19, representing a cumulative incidence of 3.4% (longitudinal population) and 1.5% (active therapy population). Compared with patients who were COVID-19 negative, those who were COVID-19 positive were more likely to have a supplemental oxygen requirement (11% versus 54%, p = 0.005) and to have typical COVID-19 pneumonia imaging findings (5 versus 56%, p = 0.001). Otherwise, there were no marked differences in presenting symptoms. Among patients who were COVID-19 negative, alternative etiologies included treatment-related toxicity (26%), atypical pneumonia (22%), and disease progression (22%). A total of 16 patients positive for COVID-19 (67%) required hospitalization, and seven (29%) died from COVID-related complications.

Conclusions

COVID-19 was infrequent in this lung cancer population, but these patients experienced high rates of morbidity and mortality. Oncologists should maintain a low threshold for COVID-19 testing in patients with lung cancer presenting with acute symptoms.

Acquired resistance to targeted therapies in NSCLC: Updates and evolving insights

While significant advancements have been made in the available therapies for metastatic non-small cell lung cancer (NSCLC), acquired resistance remains a major barrier to treatment. We have not yet achieved the ability to cure advanced NSCLC with systemic therapy, despite our growing understanding of many of the oncogenic drivers of this disease. Rather, the emergence of drug-tolerant and drug-resistant cells remains the rule, even in the face of increasingly potent targeted therapies. In this review, we provide a broad overview of the mechanisms of resistance to targeted therapy that have been demonstrated across molecular subtypes of NSCLC, highlighting the dynamic interplay between driver oncogene, bypass signaling pathways, shifting cellular phenotypes, and surrounding tumor microenvironment.

Improved Prognosis and Increased Tumor-Infiltrating Lymphocytes in Patients Who Have SCLC With Neurologic Paraneoplastic Syndromes

BACKGROUND

Approximately 10% of patients with SCLC develop a paraneoplastic syndrome (PNS). Neurologic PNS are thought to improve prognosis, which we hypothesized is related to increased tumor-infiltrating lymphocytes and immune recognition.

METHODS

We queried 2,512,042 medical records from a single institution to identify patients who have SCLC with and without PNS and performed manual, retrospective chart review. We then performed multiplexed fluorescence immunohistochemistry and automated quantitative analysis (AQUA Technology) on tumors to assess CD3, CD4, and CD8 T cell infiltrates and programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) interactions. T cell infiltrates and PD-1/PD-L1 interaction scores were compared among patients with neurologic PNS, endocrinologic PNS, and a control group without PNS. Clinical outcomes were analyzed using the Kaplan-Meier method and Cox proportional hazards models.

RESULTS

We evaluated 145 SCLC patients: 55 with PNS (25 neurologic and 30 endocrinologic) and 90 controls. Patients with neurologic PNS experienced improved overall survival compared to patients with endocrinologic PNS and controls (median overall survival of 24 months versus 12 months versus 13 months, respectively). Of the 145 patients, we identified tumor tissue from 34 patients that was adequate for AQUA analysis. Among 37 specimens from these 34 patients, patients with neurologic PNS had increased T cell infiltrates (p = 0.033) and PD-1/PD-L1 interaction (p = 0.014) compared to tumors from patients with endocrinologic PNS or controls.

CONCLUSIONS

Tumor tissue from patients with SCLC with neurologic PNS showed increased tumor-infiltrating lymphocytes and PD-1/PD-L1 interaction consistent with an inflamed tumor microenvironment.

Effective Cancer Genotyping-Many Means to One End

Precision cancer medicine requires effective genotyping of every patient's tumor to optimally design treatment plans. Despite its imperfect sensitivity, the rapidity and convenience of cell-free DNA sequencing makes it an essential complement to tumor genotyping, which, when used appropriately, can aid the pursuit of effective genotyping for all patients.See related article by Leighl et al., p. 4691.

Abstract 1025: Clinical outcomes and differential tumor immune microenvironment in patients with small cell lung cancer and paraneoplastic syndromes

Background

Among patients with small cell lung cancer (SCLC), approximately 20% develop a paraneoplastic syndrome (PNS). Neurologic PNS are immune-mediated phenomena predicated on host recognition of an onconeural antigen, while endocrinologic PNS are attributed to ectopic tumor secretion of normal hormones. The presence of neurologic PNS improves prognosis and endocrinologic PNS worsens prognosis in patients with SCLC. We hypothesized that tumors from patients with neurologic PNS may have increased TILs and PD-1/PD-L1 expression compared to tumors from patients with endocrinologic PNS and this improved immune recognition accounts for prognostic differences.

Methods

We searched electronic medical record text stored in the Vanderbilt University Medical Center (VUMC) clinical data warehouse to identify SCLC patients with and without a PNS. We obtained clinical information through manual, retrospective chart review using an IRB-approved protocol. Overall survival (OS) and progression free survival (PFS) were compared using a log rank test. Archived formalin fixed, paraffin embedded samples were obtained from the Vanderbilt University Pathology Tissue Repository. We performed multiplexed fluorescence immunohistochemistry combined with automated quantitative analysis (AQUA® Technoloy; Navigate BioPharma Services, Inc.) to assess PD-1, PD-L1, CD4, and CD8 expression. A PD-1/PD-L1 interaction score was calculated by measuring the total area of PD-1 positive cells within the proximity of PD-L1 positive cells. This area was then divided by the total area of all non-tumor nucleated cells in the image and multiplied by a factor of 10,000. CD4, CD8, and PD-1/PD-L1 interaction scores were compared using a two sample t-test or Wilcoxon Rank Sum test.

Results

A total of 145 SCLC patients were identified, 55 with a PNS (25 neurologic and 30 endocrinologic) and 90 control patients. Patients with neurologic PNS exhibited significantly improved OS and PFS compared to patients with endocrinologic PNS and control patients (median OS 24mo, 95% CI 16.4mo-not reached (NR), vs 12mo, 95% CI 8.3mo-15.5mo, vs 13mo, 95% CI 12.2mo-16mo; median PFS 14mo, 95% CI 9.3mo-NR vs 6mo, 95% CI 4.6mo-9.5mo, vs 7mo, 95% CI 6.6mo-8.1mo, respectively). Tumors from patients with neurologic PNS (n=9) had statistically significantly higher PD-1/PD-L1 interaction scores (p=0.02), and increased CD4 (p=0.01) and CD8 (p=0.003) T cell infiltrates compared to tumors from patients with endocrinologic PNS (n=11).

Conclusion

Our study of tumor tissue from patients with SCLC and PNS demonstrated a statistically significant increase in immune modulation markers' expression in patients with neurologic PNS. Tumor immunomodulation may be the driver of the improved prognosis that has been observed in ours and other retrospective cohorts of patients with SCLC and neurologic PNS.

Citation Format: Wade T. Iams, Eileen Shiuan, Catherine B. Meador, Marc Roth, Jennifer Bordeaux, Christine Vaupel, Lucy L. Wang, Joseph T. Schneider, Jeremy L. Warner, Zhiguo Zhao, Christine M. Lovly. Clinical outcomes and differential tumor immune microenvironment in patients with small cell lung cancer and paraneoplastic syndromes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1025.

Longitudinal Cell-Free DNA Analysis in Patients with Small Cell Lung Cancer Reveals Dynamic Insights into Treatment Efficacy and Disease Relapse

INTRODUCTION

Patients with SCLC have a poor prognosis and limited treatment options. Because access to longitudinal tumor samples is very limited in patients with this disease, we chose to focus our studies on the characterization of plasma cell-free DNA (cfDNA) for rapid, noninvasive monitoring of disease burden.

METHODS

We developed a liquid biopsy assay that quantifies somatic variants in cfDNA. The assay detects single nucleotide variants, copy number alterations, and insertions or deletions in 14 genes that are frequently mutated in SCLC, including tumor protein p53 gene (TP53), retinoblastoma 1 gene (RB1), BRAF, KIT proto-oncogene receptor tyrosine kinase gene (KIT), notch 1 gene (NOTCH1), notch 2 gene (NOTCH2), notch 3 gene (NOTCH3), notch 4 gene (NOTCH4), phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha gene (PIK3CA), phosphatase and tensin homolog gene (PTEN), fibroblast growth factor receptor 1 gene (FGFR1), v-myc avian myelocytomatosis viral oncogene homolog gene (MYC), v-myc avian myelocytomatosis viral oncogene lung carcinoma derived homolog gene (MYCL1), and v-myc avian myelocytomatosis viral oncogene neuroblastoma derived homolog gene (MYCN).

RESULTS

Over the course of 26 months of peripheral blood collection, we examined 140 plasma samples from 27 patients. We detected disease-associated mutations in 85% of patient samples with mutant allele frequencies ranging from 0.1% to 87%. In our cohort, 59% of the patients had extensive-stage disease, and the most common mutations occurred in TP53 (70%) and RB1 (52%). In addition to mutations in TP53 and RB1, we detected alterations in 10 additional genes in our patient population (PTEN, NOTCH1, NOTCH2, NOTCH3, NOTCH4, MYC, MYCL1, PIK3CA, KIT, and BRAF). The observed allele frequencies and copy number alterations tracked closely with treatment responses. Notably, in several cases analysis of cfDNA provided evidence of disease relapse before conventional imaging.

CONCLUSIONS

These results suggest that liquid biopsies are readily applicable in patients with SCLC and can potentially provide improved monitoring of disease burden, depth of response to treatment, and timely warning of disease relapse in patients with this disease.

Abstract 4949: Longitudinal monitoring of cell-free DNA in patients with small cell lung cancer reveals dynamic insights into treatment efficacy and disease relapse

Introduction: Small cell lung cancer (SCLC) is a highly lethal neuroendocrine malignancy that accounts for approximately 10-15% of all lung cancers and is responsible for approximately 30,000 deaths annually in the United States and 200,000 deaths worldwide every year1. There is an urgent need to develop novel treatment strategies for patients with this disease. We sought to improve the quality of patient care by establishing a liquid biopsy assay for rapid, noninvasive monitoring of disease burden.

Design: The SCLC assay relies on targeted next-generation DNA sequencing of cell-free DNA (cfDNA) collected from patient plasma. The assay targets a panel of 14 genes that are frequently mutated in SCLC2. We examined a total of 141 plasma samples from a cohort of 27 patients. 11 patients had limited stage SCLC and 16 patients had extensive stage SCLC. The analyzed plasma samples were collected during the course of patient treatment and included time points before and after chemotherapy or immunotherapy.

Results: We detected somatic, disease-associated mutations in the cfDNA of 78% of patient samples (21/27). The allele frequency of cfDNA ranged from ≤0.5% to ≥85%. The most commonly mutated genes were TP53 and RB1, which were found in 17/27 and 10/27 samples, respectively. We also detected single nucleotide variants in PIK3CA (3/27) and PTEN (1/27) as well as copy number variants in MYC and MYCL1 (2/27). The observed mutant allele frequencies in longitudinal samples tracked closely with treatment responses. Strikingly, we found instances where the assay detected the reappearance of tumor-associated markers several weeks before clinical evidence of relapse was detected.

Conclusions: cfDNA sequencing allows for improved monitoring of disease burden, depth of responses to treatment, and timely warning of disease relapse in patients with SCLC.

References:

1. Society, A.C., Cancer Facts and Figures 2015. American Cancer Society, Atlanta, Ga, 2015.

2. George, J. et al., Comprehensive genomic profiles of small cell lung cancer, Nature 524:47

Citation Format: Christine M. Lovly, Karinna Almodovar, Wade T. Iams, Catherine B. Meador, Sally York, Leora Horn, Christopher K. Raymond, Jennifer Hernandez, Lee P. Lim. Longitudinal monitoring of cell-free DNA in patients with small cell lung cancer reveals dynamic insights into treatment efficacy and disease relapse [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4949. doi:10.1158/1538-7445.AM2017-4949

Abstract 3160: Sustained MAPK activation as a mechanism of resistance to osimertinib plus selumetinib in models of EGFR-mutant cancer

INTRODUCTION: Osimertinib is a third-generation, mutant-selective epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor (TKI) recently approved for the treatment of T790M-positive EGFR-mutant lung cancer. The mutant-selective nature of osimertinib improves its tolerability by limiting its inhibition of wild-type EGFR compared to first- and second-generation EGFR inhibitors, making it a good candidate for rationally-designed combination therapies. One such combination, osimertinib plus the MEK inhibitor selumetinib (AZD6244; ARRY-142886), is currently being studied as part of the phase 1b TATTON trial (NCT02143466). Previous work by our group demonstrated that selumetinib may delay or reverse resistance to osimertinib in some cases; however, we anticipate that resistance to this combination will ultimately develop in patients. Here, we investigate acquired resistance to osimertinib and selumetinib combination therapy in vitro and in vivo.

DESIGN: In cells sensitive to osimertinib plus selumetinib (hereafter referred to as combination-sensitive cells), ERK phosphorylation was monitored via Western blot over the course of a 7-day treatment with 200 nM osimertinib plus 1 µM selumetinib. To detect RAS-GTP, an active RAS precipitation was performed in combination-sensitive and combination-resistant cell lines following treatment with osimertinib. In combination-resistant cells, pharmacologic inhibitors of various MAPK pathway components were used to assess whether they could restore potency. Western blot analysis was used to confirm on-target effects. Lastly, an analysis of differences in gene expression between four isogenic sets of combination-sensitive and combination-resistant cell lines is currently ongoing.

RESULTS: In cell lines that are sensitive to growth inhibition by the combination of osimertinib plus selumetinib, we observed complete re-activation of the MAPK pathway after 5 days of continuous exposure to both inhibitors. Additionally, RAS activity was elevated in combination-resistant cell lines, and these cells remained sensitive to the ERK inhibitor SCH772984. Likewise, the addition of a pan-RAF inhibitor restored the growth inhibitory effects of osimertinib plus selumetinib in combination-resistant cells, suggesting that incomplete inhibition of MAPK is responsible for resistance in these cells. Furthermore, an alternative MEK inhibitor, trametinib, was efficacious in combination with osimertinib in cell lines that were resistant to osimertinib plus selumetinib.

CONCLUSION: These data identify a potential mechanism of resistance to a combination therapy that is currently being tested in the clinic. Specifically, these data demonstrate that re-activation of the MAPK pathway is a mechanism of resistance to osimertinib plus selumetinib and that this pathway is still targetable in combination-resistant cells.

Citation Format: David Westover, Catherine B. Meador, Eiki Ichihara, Hayden F. Byrd, Cath Eberlein, Yingjun Yan, Darren A. Cross, Christine M. Lovly. Sustained MAPK activation as a mechanism of resistance to osimertinib plus selumetinib in models of EGFR-mutant cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3160. doi:10.1158/1538-7445.AM2017-3160

SFK/FAK Signaling Attenuates Osimertinib Efficacy in Both Drug-Sensitive and Drug-Resistant Models of EGFR-Mutant Lung Cancer

Mutant-selective EGFR tyrosine kinase inhibitors (TKI), such as osimertinib, are active agents for the treatment of EGFR-mutant lung cancer. Specifically, these agents can overcome the effects of the T790M mutation, which mediates resistance to first- and second-generation EGFR TKI, and recent clinical trials have documented their efficacy in patients with EGFR-mutant lung cancer. Despite promising results, therapeutic efficacy is limited by the development of acquired resistance. Here we report that Src family kinases (SFK) and focal adhesion kinase (FAK) sustain AKT and MAPK pathway signaling under continuous EGFR inhibition in osimertinib-sensitive cells. Inhibiting either the MAPK pathway or the AKT pathway enhanced the effects of osimertinib. Combined SFK/FAK inhibition exhibited the most potent effects on growth inhibition, induction of apoptosis, and delay of acquired resistance. SFK family member YES1 was amplified in osimertinib-resistant EGFR-mutant tumor cells, the effects of which were overcome by combined treatment with osimertinib and SFK inhibitors. In conclusion, our data suggest that the concomitant inhibition of both SFK/FAK and EGFR may be a promising therapeutic strategy for EGFR-mutant lung cancer. Cancer Res; 77(11); 2990-3000. ©2017 AACR.

A workshop on leadership for senior MD-PhD students

Leadership skills are essential for a successful career as a physician-scientist, yet many MD-PhD training programs do not offer formal training in leadership. The Vanderbilt Medical Scientist Training Program (MSTP) previously established a 2-day leadership workshop that has been held biennially since 2006 for students in the first and second years of the graduate school portion of combined MD and PhD training (G1/G2 students). Workshop attendees have consistently rated this workshop as a highly effective experience. However, opportunities for structured training in leadership competencies during the subsequent 3-5 years of MD-PhD training are limited. Given the success of the G1/G2 leadership workshop and the need for continuity in this model of leadership training, we developed a half-day workshop for MSTP students in the clinical years of medical school (M3/M4 students) to foster continued training in leadership. Our workshop curriculum, based in part on original cases drafted by Vanderbilt MSTP students, provides concrete strategies to manage conflict and navigate leadership transitions in the physician-scientist career path. The curriculum emphasizes both short-term competencies, such as effective participation as a member of a clinical team, and long-term competencies, such as leadership of a research team, division, or department. Our inaugural senior leadership workshop, held in August, 2015, was judged by student participants to be well organized and highly relevant to leadership concepts and skills. It will be offered biennially in our training curriculum for M3 and M4 MSTP students.

Afatinib plus Cetuximab Delays Resistance Compared to Single-Agent Erlotinib or Afatinib in Mouse Models of TKI-Naïve EGFR L858R-Induced Lung Adenocarcinoma

PURPOSE

The EGFR tyrosine kinase inhibitors (TKIs), erlotinib and afatinib, have transformed the treatment of advanced EGFR-mutant lung adenocarcinoma. However, almost all patients who respond develop acquired resistance on average approximately 1 year after starting therapy. Resistance is commonly due to a secondary mutation in EGFR (EGFR(T790M)). We previously found that the combination of the EGFR TKI afatinib and the EGFR antibody cetuximab could overcome EGFR(T790M)-mediated resistance in preclinical models. This combination has shown a 29% response rate in a clinical trial in patients with acquired resistance to first-generation TKIs. An outstanding question is whether this regimen is beneficial when used as first-line therapy.

EXPERIMENTAL DESIGN

Using mouse models of EGFR-mutant lung cancer, we tested whether the combination of afatinib plus cetuximab delivered upfront to mice with TKI-naïve EGFR(L858R)-induced lung adenocarcinomas delayed tumor relapse and drug-resistance compared with single-agent TKIs.

RESULTS

Afatinib plus cetuximab markedly delayed the time to relapse and incidence of drug-resistant tumors, which occurred in only 63.6% of the mice, in contrast to erlotinib or afatinib treatment where 100% of mice developed resistance. Mechanisms of tumor escape observed in afatinib plus cetuximab resistant tumors include the EGFR(T790M) mutation and Kras mutations. Experiments in cell lines and xenografts confirmed that the afatinib plus cetuximab combination does not suppress the emergence of EGFR(T790M).

CONCLUSIONS

These results highlight the potential of afatinib plus cetuximab as an effective treatment strategy for patients with TKI-naïve EGFR-mutant lung cancer and indicate that clinical trial development in this area is warranted.

EPHA2 Blockade Overcomes Acquired Resistance to EGFR Kinase Inhibitors in Lung Cancer

Despite the success of treating EGFR-mutant lung cancer patients with EGFR tyrosine kinase inhibitors (TKI), all patients eventually acquire resistance to these therapies. Although various resistance mechanisms have been described, there are currently no FDA-approved therapies that target alternative mechanisms to treat lung tumors with acquired resistance to first-line EGFR TKI agents. Here we found that EPHA2 is overexpressed in EGFR TKI-resistant tumor cells. Loss of EPHA2 reduced the viability of erlotinib-resistant tumor cells harboring EGFR(T790M) mutations in vitro and inhibited tumor growth and progression in an inducible EGFR(L858R+T790M)-mutant lung cancer model in vivo. Targeting EPHA2 in erlotinib-resistant cells decreased S6K1-mediated phosphorylation of cell death agonist BAD, resulting in reduced tumor cell proliferation and increased apoptosis. Furthermore, pharmacologic inhibition of EPHA2 by the small-molecule inhibitor ALW-II-41-27 decreased both survival and proliferation of erlotinib-resistant tumor cells and inhibited tumor growth in vivo. ALW-II-41-27 was also effective in decreasing viability of cells with acquired resistance to the third-generation EGFR TKI AZD9291. Collectively, these data define a role for EPHA2 in the maintenance of cell survival of TKI-resistant, EGFR-mutant lung cancer and indicate that EPHA2 may serve as a useful therapeutic target in TKI-resistant tumors.

Old Habits Die Hard: Addiction of BRAF-Mutant Cancer Cells to MAP Kinase Signaling

Dual and triple combination therapies with RAF inhibitors plus other targeted agents have demonstrated promising clinical utility in BRAFV600-mutant solid tumors. However, despite vertical inhibition at multiple nodes on the MAPK signaling pathway, resistant tumors emerge. Ahronian and colleagues show that in BRAF-mutant colorectal cancer, resistance involves reactivation of RAS/RAF/MEK/ERK signaling and may be overcome by newly emerging ERK inhibitors.

Acquired Resistance to the Mutant-Selective EGFR Inhibitor AZD9291 Is Associated with Increased Dependence on RAS Signaling in Preclinical Models

Resistance to targeted EGFR inhibitors is likely to develop in EGFR-mutant lung cancers. Early identification of innate or acquired resistance mechanisms to these agents is essential to direct development of future therapies. We describe the detection of heterogeneous mechanisms of resistance within populations of EGFR-mutant cells (PC9 and/or NCI-H1975) with acquired resistance to current and newly developed EGFR tyrosine kinase inhibitors, including AZD9291. We report the detection of NRAS mutations, including a novel E63K mutation, and a gain of copy number of WT NRAS or WT KRAS in cell populations resistant to gefitinib, afatinib, WZ4002, or AZD9291. Compared with parental cells, a number of resistant cell populations were more sensitive to inhibition by the MEK inhibitor selumetinib (AZD6244; ARRY-142886) when treated in combination with the originating EGFR inhibitor. In vitro, a combination of AZD9291 with selumetinib prevented emergence of resistance in PC9 cells and delayed resistance in NCI-H1975 cells. In vivo, concomitant dosing of AZD9291 with selumetinib caused regression of AZD9291-resistant tumors in an EGFRm/T790M transgenic model. Our data support the use of a combination of AZD9291 with a MEK inhibitor to delay or prevent resistance to AZD9291 in EGFRm and/or EGFRm/T790M tumors. Furthermore, these findings suggest that NRAS modifications in tumor samples from patients who have progressed on current or EGFR inhibitors in development may support subsequent treatment with a combination of EGFR and MEK inhibition.

Optimizing the sequence of anti-EGFR-targeted therapy in EGFR-mutant lung cancer

Metastatic EGFR-mutant lung cancers are sensitive to the first- and second-generation EGFR tyrosine kinase inhibitors (TKIs) gefitinib, erlotinib, and afatinib, but resistance develops. Acquired resistance to gefitinib or erlotinib occurs most commonly (>50%) via the emergence of a second-site EGFR mutation, T790M. Two strategies to overcome T790M-mediated resistance are dual inhibition of EGFR with afatinib plus the anti-EGFR antibody cetuximab (A+C), or mutant-specific EGFR inhibition with AZD9291. A+C and AZD9291 are now also being tested as first-line therapies, but whether these therapies will extend progression-free survival or induce more aggressive forms of resistance in this setting remains unknown. We modeled resistance to multiple generations of anti-EGFR therapies preclinically to understand the effects of sequential treatment with anti-EGFR agents on drug resistance and determine the optimal order of treatment. Using a panel of erlotinib/afatinib-resistant cells, including a novel patient-derived cell line (VP-2), we found that AZD9291 was more potent than A+C at inhibiting cell growth and EGFR signaling in this setting. Four of four xenograft-derived A+C-resistant cell lines displayed in vitro and in vivo sensitivity to AZD9291, but four of four AZD9291-resistant cell lines demonstrated cross-resistance to A+C. Addition of cetuximab to AZD9291 did not confer additive benefit in any preclinical disease setting. This work, emphasizing a mechanistic understanding of the effects of therapies on tumor evolution, provides a framework for future clinical trials testing different treatment sequences. This paradigm is applicable to other tumor types in which multiple generations of inhibitors are now available.

Beyond histology: translating tumor genotypes into clinically effective targeted therapies

Increased understanding of intertumoral heterogeneity at the genomic level has led to significant advancements in the treatment of solid tumors. Functional genomic alterations conferring sensitivity to targeted therapies can take many forms, and appropriate methods and tools are needed to detect these alterations. This review provides an update on genetic variability among solid tumors of similar histologic classification, using non-small cell lung cancer and melanoma as examples. We also discuss relevant technological platforms for discovery and diagnosis of clinically actionable variants and highlight the implications of specific genomic alterations for response to targeted therapy.

Abstract B10: Acquired resistance to afatinib plus cetuximab in EGFR-mutant lung adenocarcinoma may be mediated by EGFR overexpression and overcome by the mutant-specific EGFR inhibitor, AZD9291

EGFR mutant lung cancers are highly sensitive to first generation EGFR tyrosine kinase inhibitors (TKIs; gefitinib and erlotinib), but resistance invariably develops. In the majority of patients, disease progression is mediated by a second-site T790M mutation in EGFR. T790M-mediated acquired resistance can be overcome by the combination of the second-generation EGFR TKI, afatinib, with the anti-EGFR monoclonal antibody, cetuximab, in both preclinical models and humans. However, patients still develop acquired resistance to second-line anti-EGFR combination therapy. In order to develop therapeutic strategies for patients whose tumors progress on afatinib/cetuximab, we have modeled resistance using xenografts of PC-9/BRc1 cells (EGFR exon 19 deletion/T790M) treated chronically with the drug combination. Over the course of four months, 4 of 10 animals developed acquired resistance. Xenograft tumors still displayed features of adenocarcinoma only, and mutational analysis did not detect any common secondary hotspot mutations found in lung adenocarcinomas. Analysis of two cell lines derived from resistant tumors by array comparative genomic hybridization (aCGH) suggests that resistant tumors developed additional amplification at the EGFR locus. Fluorescence in situ hybridization (FISH) of all lines confirmed sustained amplification of EGFR in resistant cell lines, and immunoblotting of cell lysates showed increased EGFR protein expression relative to parental cells and vehicle-treated controls. Strikingly, all xenograft-derived cell lines displayed in vitro sensitivity to AZD9291, a third-generation, mutant-specific EGFR TKI. Finally, in vitro comparison of afatinib/cetuximab with AZD9291 in PC-9/BRc1 cells suggests that AZD9291 may even be more potent than afatinib/cetuximab in the setting of T790M-mediated acquired resistance to first and second-generation EGFR TKIs. Confirmatory xenograft experiments are planned, and experiments are ongoing, including with additional cell lines. Collectively, these data suggest that EGFR overexpression may be a mechanism of acquired resistance to afatinib/cetuximab combination therapy, and that resistance may still be overcome by mutant-specific EGFR small molecule inhibitors. Thus, patients with EGFR mutant lung cancers may benefit from sequential lines of therapy (erlotinib-&gt;afatinib/cetuximab-&gt;AZD9291) targeting EGFR. Further work is needed to discern specific indications for afatinib/cetuximab versus AZD9291 as therapy for patients with acquired resistance to EGFR TKIs.

Citation Format: Catherine B. Meador, Hailing Jin, Elisa de Stanchina, Valentina Pirazzoli, Marc Ladanyi, Lu Wang, Xi Chen, Rosana Eisenberg, Darren Cross, William Pao. Acquired resistance to afatinib plus cetuximab in EGFR-mutant lung adenocarcinoma may be mediated by EGFR overexpression and overcome by the mutant-specific EGFR inhibitor, AZD9291. [abstract]. In: Proceedings of the AACR-IASLC Joint Conference on Molecular Origins of Lung Cancer; 2014 Jan 6-9; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2014;20(2Suppl):Abstract nr B10.

Next-generation sequencing of paired tyrosine kinase inhibitor-sensitive and -resistant EGFR mutant lung cancer cell lines identifies spectrum of DNA changes associated with drug resistance

Somatic mutations in kinase genes are associated with sensitivity of solid tumors to kinase inhibitors, but patients with metastatic cancer eventually develop disease progression. In EGFR mutant lung cancer, modeling of acquired resistance (AR) with drug-sensitive cell lines has identified clinically relevant EGFR tyrosine kinase inhibitor (TKI) resistance mechanisms such as the second-site mutation, EGFR T790M, amplification of the gene encoding an alternative kinase, MET, and epithelial-mesenchymal transition (EMT). The full spectrum of DNA changes associated with AR remains unknown. We used next-generation sequencing to characterize mutational changes associated with four populations of EGFR mutant drug-sensitive and five matched drug-resistant cell lines. Comparing resistant cells with parental counterparts, 18-91 coding SNVs/indels were predicted to be acquired and 1-27 were lost; few SNVs/indels were shared across resistant lines. Comparison of two related parental lines revealed no unique coding SNVs/indels, suggesting that changes in the resistant lines were due to drug selection. Surprisingly, we observed more CNV changes across all resistant lines, and the line with EMT displayed significantly higher levels of CNV changes than the other lines with AR. These results demonstrate a framework for studying the evolution of AR and provide the first genome-wide spectrum of mutations associated with the development of cellular drug resistance in an oncogene-addicted cancer. Collectively, the data suggest that CNV changes may play a larger role than previously appreciated in the acquisition of drug resistance and highlight that resistance may be heterogeneous in the context of different tumor cell backgrounds.

Serological alterations in carcinogen-exposed teleosts: procedures for preparation and analysis of samples from small fish

To study the effects of environmental carcinogens on the immune system of Cyprinodon variegatus, we had to miniaturize or modify standard immunological procedures due to the small size of the fish. Modifications in standard bleeding procedures allowed collection of sufficient serum for most serological procedures. Serum electrophoresis showed considerable variation between exposed and unexposed fish as did qualitative immunoelectrophoresis techniques. We successfully adapted a bacteriophage neutralization procedure for use with the C. variegatus system to measure antiviral antibodies. The presence of antibody-forming cells in spleen suspensions from fish immunized with human type O erythrocytes was demonstrated by a modified immune rosette procedure. A capillary tube procedure was developed for separation of leukocytes from erythrocytes in blood from C. variegatus.