MYC: there is more to it than cancer

MYC is a pleiotropic transcription factor involved in multiple cellular processes. While its mechanism of action and targets are not completely elucidated, it has a fundamental role in cellular proliferation, differentiation, metabolism, ribogenesis, and bone and vascular development. Over 4 decades of research and some 10,000 publications linking it to tumorigenesis (by searching PubMed for "MYC oncogene") have led to MYC becoming a most-wanted target for the treatment of cancer, where many of MYC's physiological functions become co-opted for tumour initiation and maintenance. In this context, an abundance of reviews describes strategies for potentially targeting MYC in the oncology field. However, its multiple roles in different aspects of cellular biology suggest that it may also play a role in many additional diseases, and other publications are indeed linking MYC to pathologies beyond cancer. Here, we review these physiological functions and the current literature linking MYC to non-oncological diseases. The intense efforts towards developing MYC inhibitors as a cancer therapy will potentially have huge implications for the treatment of other diseases. In addition, with a complementary approach, we discuss some diseases and conditions where MYC appears to play a protective role and hence its increased expression or activation could be therapeutic.

MYC targeting by OMO-103 in solid tumors: a phase 1 trial

Among the 'most wanted' targets in cancer therapy is the oncogene MYC, which coordinates key transcriptional programs in tumor development and maintenance. It has, however, long been considered undruggable. OMO-103 is a MYC inhibitor consisting of a 91-amino acid miniprotein. Here we present results from a phase 1 study of OMO-103 in advanced solid tumors, established to examine safety and tolerability as primary outcomes and pharmacokinetics, recommended phase 2 dose and preliminary signs of activity as secondary ones. A classical 3 + 3 design was used for dose escalation of weekly intravenous, single-agent OMO-103 administration in 21-day cycles, encompassing six dose levels (DLs). A total of 22 patients were enrolled, with treatment maintained until disease progression. The most common adverse events were grade 1 infusion-related reactions, occurring in ten patients. One dose-limiting toxicity occurred at DL5. Pharmacokinetics showed nonlinearity, with tissue saturation signs at DL5 and a terminal half-life in serum of 40 h. Of the 19 patients evaluable for response, 12 reached the predefined 9-week time point for assessment of drug antitumor activity, eight of those showing stable disease by computed tomography. One patient defined as stable disease by response evaluation criteria in solid tumors showed a 49% reduction in total tumor volume at best response. Transcriptomic analysis supported target engagement in tumor biopsies. In addition, we identified soluble factors that are potential pharmacodynamic and predictive response markers. Based on all these data, the recommended phase 2 dose was determined as DL5 (6.48 mg kg-1).ClinicalTrials.gov identifier: NCT04808362 .

Therapeutic implications of the interplay between interferons and ER in breast cancer

The involvement of interferons (IFNs) in various diseases, including breast cancer, has sparked controversy due to their diverse roles in immunity and significant impact on pathological mechanisms. In the context of breast cancer, the heightened expression of endogenous IFNs has been linked to anti-tumor activity and a favorable prognosis for patients. Within the tumor tissue and microenvironment, IFNs initiate a cascade of molecular events involving numerous factors, which can lead to either cooperative or repressive interactions. The specific functions of IFNs in breast cancer vary depending on the two major disease phenotypes: hormone dependent (or responsive) and hormone independent (or unresponsive) breast cancer. Hormone dependence is determined by the presence of estrogen receptors (ERs). The interplay between the IFN and ER signaling pathways, and the involvement of intermediate factors such as NFκB, are areas that have been somewhat under-researched, but that hold potential importance for the understanding and treatment of breast cancer. This review aims to provide a comprehensive overview of the actions of IFNs in breast cancer, particularly in relation to the different breast cancer phenotypes and the significance of comprehending the underlying mechanisms. Furthermore, the use of IFN-based therapies in cancer treatment remains a topic of debate and has not yet gained widespread acceptance. However, emerging discoveries may redirect focus towards the potential of IFN-based therapies.

Reducing MYC's transcriptional footprint unveils a good prognostic gene signature in melanoma

MYC's key role in oncogenesis and tumor progression has long been established for most human cancers. In melanoma, its deregulated activity by amplification of 8q24 chromosome or by upstream signaling coming from activating mutations in the RAS/RAF/MAPK pathway-the most predominantly mutated pathway in this disease-turns MYC into not only a driver but also a facilitator of melanoma progression, with documented effects leading to an aggressive clinical course and resistance to targeted therapy. Here, by making use of Omomyc, the most characterized MYC inhibitor to date that has just successfully completed a phase I clinical trial, we show for the first time that MYC inhibition in melanoma induces remarkable transcriptional modulation, resulting in severely compromised tumor growth and a clear abrogation of metastatic capacity independently of the driver mutation. By reducing MYC's transcriptional footprint in melanoma, Omomyc elicits gene expression profiles remarkably similar to those of patients with good prognosis, underlining the therapeutic potential that such an approach could eventually have in the clinic in this dismal disease.

Abstract 3435: Identification of potential biomarkers of response to OMO-103, a first-in-modality pan-MYC inhibitor, in patients with advanced solid tumors

Background: MYC is a key transcription factor driving and maintaining human tumors. Since MYC has long been perceived as an “undruggable” target, to date, there is still no MYC inhibitor approved for clinical use. However, we designed and validated Omomyc, a MYC dominant negative mini-protein, demonstrating its potent therapeutic impact in various mouse models of cancer. Importantly, a Phase 1 study testing OMO-103, an Omomyc-based mini-protein developed by Peptomyc S.L., was successfully completed in 2022. Here, we present the main findings of the study and associated biomarker program.

Material and Methods: A phase I dose escalation study was performed in all-comers solid tumor patients, with a 3+3 design of 6 dose levels ranging from 0.48 to 9.72mg/kg, as a weekly 30-min i.v. infusion. Tumor and liquid biopsies were collected at screening, upon and at the end of treatment, to assess different biomarkers of drug activity.

Results: 22 patients with advanced solid tumors were included and 18 patients were considered evaluable for response by CT scan. Of these, 9 achieved SD. The PK analysis revealed a plasma half-life of >40h. No ADAs were detected in any of the patients. Drug pharmacodynamics supported target engagement, as demonstrated by Digital Spatial Profiling analysis showing shut down of MYC transcriptional signature in patients’ tumor biopsies. In addition, a distinctive pharmacodynamic cytokine signature that correlated with stable disease was found through liquid biopsies already 3 to 4 weeks before CT scan. Importantly, a cytokine signature was also identified as being predictive of disease stabilization at baseline and could help stratify patients in upcoming additional clinical studies. Finally, several anti-tumor immune related markers were also found modulated upon OMO-103 treatment.

Conclusion: OMO-103 demonstrates a favorable safety profile, with encouraging signs of activity supported by predictive and pharmacodynamic biomarkers worthy of further investigation.

Citation Format: Marie-Eve Beaulieu, Elena Garralda, Sílvia Casacuberta-Serra, Sandra Sandra Martínez-Martín, Emiliano Calvo, Víctor Moreno, Sergio López-Estévez, Laia Foradada, Guzman Alonso, Elena Corral, Bernard Doger, Tatiana Hernández, Judit Grueso, Íñigo Íñigo González-Larreategui, Erika Serrano del Pozo, Hugo Thabussot, Virginia Castillo Cano, Mariano F. Mariano F. Zacarías-Fluck, Jastrinjan Kaur, Fabio Giuntini, Jonathan R. Whitfield, Josefa Morales, Manuela Niewel, Laura Soucek. Identification of potential biomarkers of response to OMO-103, a first-in-modality pan-MYC inhibitor, in patients with advanced solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3435.

MYC Inhibition Halts Metastatic Breast Cancer Progression by Blocking Growth, Invasion, and Seeding

MYC's role in promoting tumorigenesis is beyond doubt, but its function in the metastatic process is still controversial. Omomyc is a MYC dominant negative that has shown potent antitumor activity in multiple cancer cell lines and mouse models, regardless of their tissue of origin or driver mutations, by impacting on several of the hallmarks of cancer. However, its therapeutic efficacy against metastasis has not been elucidated yet. Here we demonstrate for the first time that MYC inhibition by transgenic Omomyc is efficacious against all breast cancer molecular subtypes, including triple-negative breast cancer, where it displays potent antimetastatic properties both in vitro and in vivo. Importantly, pharmacologic treatment with the recombinantly produced Omomyc miniprotein, recently entering a clinical trial in solid tumors, recapitulates several key features of expression of the Omomyc transgene, confirming its clinical applicability to metastatic breast cancer, including advanced triple-negative breast cancer, a disease in urgent need of better therapeutic options.

Significance

While MYC role in metastasis has been long controversial, this manuscript demonstrates that MYC inhibition by either transgenic expression or pharmacologic use of the recombinantly produced Omomyc miniprotein exerts antitumor and antimetastatic activity in breast cancer models in vitro and in vivo, suggesting its clinical applicability.

Between immunomodulation and immunotolerance: The role of IFNγ in SARS-CoV-2 disease

Interferons have prominent roles in various pathophysiological conditions, mostly related to inflammation. Interferon-gamma (IFNγ) was, initially discovered as a potent antiviral agent, over 50 years ago, and has recently garnered renewed interest as a promising factor involved in both innate and adaptive immunity. When new disease epidemics appear such as SARS-CoV (severe acute respiratory syndrome coronavirus), MERS-CoV (Middle East respiratory syndrome coronavirus), IAV (Influenza A virus), and in particular the current SARS-CoV-2 pandemic, it is especially timely to review the complexity of immune system responses to viral infections. Here we consider the controversial roles of effectors like IFNγ, discussing its actions in immunomodulation and immunotolerance. We explore the possibility that modulation of IFNγ could be used to influence the course of such infections. Importantly, not only could endogenous expression of IFNγ influence the outcome, there are existing IFNγ therapeutics that can readily be applied in the clinic. However, our understanding of the molecular mechanisms controlled by IFNγ suggests that the exact timing for application of IFNγ-based therapeutics could be crucial: it should be earlier to significantly reduce the viral load and thus decrease the overall severity of the disease.

The long journey to bring a Myc inhibitor to the clinic

The oncogene Myc is deregulated in the majority of human tumors and drives numerous hallmarks of cancer. Despite its indisputable role in cancer development and maintenance, Myc is still undrugged. Developing a clinical inhibitor for Myc has been particularly challenging owing to its intrinsically disordered nature and lack of a binding pocket, coupled with concerns regarding potentially deleterious side effects in normal proliferating tissues. However, major breakthroughs in the development of Myc inhibitors have arisen in the last couple of years. Notably, the direct Myc inhibitor that we developed has just entered clinical trials. Celebrating this milestone, with this Perspective, we pay homage to the different strategies developed so far against Myc and all of the researchers focused on developing treatments for a target long deemed undruggable.

The Wnt signaling receptor Fzd9 is essential for Myc-driven tumorigenesis in pancreatic islets

The huge cadre of genes regulated by Myc has obstructed the identification of critical effectors that are essential for Myc-driven tumorigenesis. Here, we describe how only the lack of the receptor Fzd9, previously identified as a Myc transcriptional target, impairs sustained tumor expansion and β-cell dedifferentiation in a mouse model of Myc-driven insulinoma, allows pancreatic islets to maintain their physiological structure and affects Myc-related global gene expression. Importantly, Wnt signaling inhibition in Fzd9-competent mice largely recapitulates the suppression of proliferation caused by Fzd9 deficiency upon Myc activation. Together, our results indicate that the Wnt signaling receptor Fzd9 is essential for Myc-induced tumorigenesis in pancreatic islets.

Abstract 1823: Translating Myc inhibition to the clinic in metastatic breast cancer

Breast cancer is a leading cause of cancer mortality in women due to the high frequency of metastatic disease, which, despite advances in therapeutic options, is still essentially incurable. The role of Myc in promoting tumorigenesis is beyond doubt, but there are contradictory reports in the literature on its role in the metastatic process. Using a Myc dominant negative termed Omomyc, we have demonstrated in various mouse models that Myc inhibition is a safe and effective therapeutic approach against several types of cancer, regardless of the tissue of origin or the driver oncogenic lesion. So far, Omomyc has only been tested in primary tumors. However, since many steps of the metastatic cascade have been reported to depend on Myc, we hypothesized that Omomyc could be extremely effective in both the prevention and treatment of metastasis too.

Here we show that Omomyc expression has a dramatic effect on colony formation capacity in human breast cancer cell lines representative of all the molecular subtypes of the disease. In MDA-MB-231 cells, not only did it impair their proliferation but also migration, invasion and their capacity to induce angiogenesis, key aspects of the metastatic process. We demonstrate that, in vivo, Omomyc reduces the growth of orthotopically-implanted human breast cancer cells in immunocompromised mice, induces regression of established metastases after primary tumor resection and impairs the development of lung metastases after tail vein injection. In the immunocompetent MMTV-PyMT transgenic model, Omomyc expression dramatically delays the formation and growth of mammary fat pad tumors, thereby preventing the appearance of lung metastases. When the purified Omomyc mini-protein is administered exogenously, we observe remarkable growth inhibition in vitro that recapitulates transgenic expression of Omomyc. Intravenous administration of the mini-protein reduces lung colonization and tumor growth in vivo.

We have demonstrated for the first time the applicability of Omomyc against metastasis, challenging the pre-established notion that Myc inhibition could potentiate, rather than inhibit, invasion. Finally, we have validated the use of the purified Omomyc mini-protein as the first directly-deliverable Omomyc-based drug for the treatment of metastatic breast cancer, providing a new therapeutic opportunity for patients suffering from this dreadful and incurable disease.

Citation Format: Daniel Massó-Vallés, Marie-Eve Beaulieu, Toni Jauset, Erika Serrano, Sandra Martínez-Martín, Laia Foradada, Virginia Castillo, Francisco Castillo, Génesis Martín, Sílvia Casacuberta-Serra, Mariano F. Zacarias-Fluck, Antonio Luque-García, Marta Escorihuela, Jonathan R. Whitfield, Joaquín Arribas, Laura Soucek. Translating Myc inhibition to the clinic in metastatic breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1823.

Intrinsic cell-penetrating activity propels Omomyc from proof of concept to viable anti-MYC therapy

Inhibiting MYC has long been considered unfeasible, although its key role in human cancers makes it a desirable target for therapeutic intervention. One reason for its perceived undruggability was the fear of catastrophic side effects in normal tissues. However, we previously designed a dominant-negative form of MYC called Omomyc and used its conditional transgenic expression to inhibit MYC function both in vitro and in vivo. MYC inhibition by Omomyc exerted a potent therapeutic impact in various mouse models of cancer, causing only mild, well-tolerated, and reversible side effects. Nevertheless, Omomyc has been so far considered only a proof of principle. In contrast with that preconceived notion, here, we show that the purified Omomyc mini-protein itself spontaneously penetrates into cancer cells and effectively interferes with MYC transcriptional activity therein. Efficacy of the Omomyc mini-protein in various experimental models of non-small cell lung cancer harboring different oncogenic mutation profiles establishes its therapeutic potential after both direct tissue delivery and systemic administration, providing evidence that the Omomyc mini-protein is an effective MYC inhibitor worthy of clinical development.

Abstract B16: Targeting Myc in metastatic breast cancer by Omomyc: From proof of principle to pharmacologic approach

Introduction and Hypothesis: Breast cancer is a leading cause of cancer mortality in women due to the high frequency of metastatic disease, which, despite advances in therapeutic options, is still essentially incurable. The role of Myc in promoting tumorigenesis is beyond doubt, but there are contradictory reports in the literature on its role in the metastatic process. Using a Myc dominant negative termed Omomyc, we have demonstrated in various mouse models that Myc inhibition is a safe and effective therapeutic approach against several types of cancer, regardless of the tissue of origin or the driver oncogenic lesion. So far, Omomyc has only been tested in primary tumors. However, since many steps of the metastatic cascade have been reported to depend on Myc, we hypothesized that Omomyc could be extremely effective in both the prevention and treatment of metastasis too.

Methods: We induced transgenic expression of Omomyc in a panel of 11 breast cancer cell lines and analyzed its effect on clonogenic capacity, proliferation, cell cycle progression, angiogenesis, migration, and invasion. To characterize the effect of Omomyc expression in vivo, we performed prevention and intervention studies in several mouse models of metastatic breast cancer: an orthotopic human cell line-derived model with surgical resection, a human cell line-derived lung colonization model, and the MMTV-PyMT transgenic model. Omomyc expression was induced at different stages of the disease, and tumor burden and metastatic spread were compared between groups at different time points. In parallel to this systemic modeling of Myc inhibition by transgenic expression of Omomyc, we are also validating the therapeutic utility of Omomyc-derived peptides as a pharmacologic approach. To this aim, we assessed the cell-penetrating capacity of the peptides in MDA-MB-231 cells by confocal microscopy and flow cytometry. To enhance its activity and to target metastases in vivo, Omomyc was conjugated with a metastasis-targeting sequence, and its efficacy compared with the one exerted by Omomyc alone in vitro. We selected the fusion peptide for in vivo studies and treated the orthotopic and lung colonization mouse models by several routes of administration.

Results: Here we show that Omomyc expression has a dramatic effect on colony formation capacity in human breast cancer cell lines representative of all the molecular subtypes of the disease. In MDA-MB-231 cells, not only did it impair their proliferation but also migration, invasion, and their capacity to induce angiogenesis, key aspects of the metastatic process.

We demonstrate that, in vivo, Omomyc reduces the growth of orthotopically implanted human breast cancer cells in immunocompromised mice, induces regression of established metastases after primary tumor resection, and impairs the development of lung metastases after tail vein injection. In the immunocompetent MMTV-PyMT transgenic model, Omomyc expression dramatically delays the formation and growth of mammary fat pad tumors, thereby preventing the appearance of lung metastases.

When the Omomyc peptide is administered exogenously, we observe remarkable growth inhibition that recapitulates transgenic expression of Omomyc. When conjugated with a metastasis-targeting sequence, its cell-penetrating capacity is increased and causes abundant cell death in vitro. In vivo, treatment with the fusion peptide reduces growth of mammary primary tumors and lung metastases.

Conclusions: We have demonstrated for the first time the applicability of Omomyc against metastasis, challenging the pre-established notion that Myc inhibition could potentiate, rather than inhibit, invasion. Finally, we have validated a metastasis-targeting fusion peptide as the first directly deliverable Omomyc-based drug for the treatment of metastatic breast cancer, providing a new therapeutic opportunity for patients suffering from this dreadful and incurable disease.

Citation Format: Daniel Massó-Vallés, Marie-Eve Beaulieu, Toni Jauset, Erika Serrano, Sandra Martínez-Martín, Laia Foradada, Virginia Castillo, Sílvia Casacuberta-Serra, Mariano F. Zacarias-Fluck, Génesis Martín, Antonio Luque-García, Marta Escorihuela, Jonathan R. Whitfield, Joaquín Arribas, Laura Soucek. Targeting Myc in metastatic breast cancer by Omomyc: From proof of principle to pharmacologic approach [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B16.

Abstract 3956: Omomyc-based cell-penetrating peptides: From proof of concept to a clinically viable anti-Myc therapy

Deregulation of the MYC oncoprotein drives tumorigenesis in most–if not all–cancers and generally correlates with poor prognosis, suggesting that its inhibition would be a useful therapeutic strategy. Indeed, we have shown that Myc inhibition displays extraordinary therapeutic benefit in various transgenic mouse models of cancer (i.e., skin, lung, pancreatic cancer and glioma), without eliciting resistance to therapy, and causes only mild, well-tolerated and reversible side effects in normal tissues. For these studies we employed a dominant negative inhibitor of Myc, called Omomyc, which is an effective inhibitor of Myc transactivation function both in vitro and in vivo. Omomyc has so far been utilized exclusively as a transgene, as a successful proof of principle whose application was believed to be solely limited to gene therapy. Here, though, we show that the purified Omomyc polypeptide itself spontaneously transduces into cancer cells and effectively interferes with MYC transcription, abrogating cell cycle and promoting apoptosis in different cancer cells, independently of their mutational profile. Efficacy of the Omomyc polypeptide in two complementary murine models of non-small cell lung cancer (NSCLC) establishes its therapeutic potential through both direct tissue delivery (intranasal) and systemic intravenous administration, providing for the first time evidence that the Omomyc polypeptide is an effective MYC inhibitor worthy of clinical development.

Citation Format: Marie-Eve Beaulieu, Toni Jauset, Daniel Massó-Vallés, Sandra Martínez-Martín, Peter Rahl, Löika Maltais, Mariano F. Zacarias-Fluck, Sílvia Casacuberta-Serra, Erika Serrano del Pozo, Christopher Fiore, Laia Foradada, Virginia Castillo Cano, Matthew Guenther, Eduardo Romero Sanza, Marta Oteo, Cynthia Tremblay Tremblay, Génesis Martín, Danny Letourneau, Martin Montagne, Miguel Ángel Morcillo Alonso, Jonathan R. Whitfield, Pierre Lavigne, Laura Soucek. Omomyc-based cell-penetrating peptides: From proof of concept to a clinically viable anti-Myc therapy [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 3956.

Abstract 3351: Myc inhibition by Omomyc impairs melanoma growth and progression through genome-wide gene expression reprogramming

Myc encodes for a transcription factor deregulated in the vast majority of human cancers, mainly by amplification or constant upstream oncogenic signaling. Previous work from our laboratory showed that targeting Myc by transgenic expression of the Myc inhibitor Omomyc is an effective strategy to inhibit tumor progression and cause tumor regression in a wide array of mouse cancer models, without any evidence of toxicity in normal tissues. Our goal now is to assess whether Myc inhibition can be an effective approach to treat melanoma, the most dangerous form of skin cancer. For this purpose, we transfected human melanoma cell lines with a doxycycline-dependent vector expressing Omomyc-RFP and characterized the effect of Myc inhibition on proliferation and colony formation, as well as the expression of different cell cycle related proteins. We observed that Omomyc significantly reduced proliferation of a wide variety of cell lines, regardless of their driving mutations. In p53 wild type cells, Omomyc expression was accompanied by p53 stabilization and concomitant p21 upregulation, and in BRaf or NF1 mutated melanoma cells, by downregulation of cyclin D1. In order to elucidate the mechanism of action underlying this anti-tumorigenic effect, we performed a microarray analysis of A375 melanoma cells in the presence or absence of Omomyc-RFP expression. This analysis showed that Omomyc affected genome-wide gene expression. More in detail, Gene Set Enrichment Analysis showed that Omomyc significantly blunts the expression of both Myc and E2F targets, as well as RNA biogenesis, DNA replication and different cell cycle checkpoints, among many others relevant gene sets. Importantly, Omomyc significantly reduced the expression of genes related to melanoma metastases. Finally, in order to validate Omomyc therapeutic impact in vivo, A375 and SkMel147 cells were s.c. implanted into nude mice and the effect of Omomyc expression on tumor progression was evaluated. Our results show that Omomyc expression significantly impairs the growth of both cell lines, increasing the survival of tumor-bearing mice. These results combined clearly show that Omomyc-mediated Myc inhibition is an effective means to impair melanoma progression, by directly targeting Myc, inhibiting cell cycle and decreasing melanoma aggressiveness.

Citation Format: Mariano F. Zacarias-Fluck, Génesis Martín, Daniel Massó-Vallés, Laia Foradada, Jonathan R. Whitfield, Marie-Eve Beaulieu, Laura Soucek. Myc inhibition by Omomyc impairs melanoma growth and progression through genome-wide gene expression reprogramming [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 3351.

BET inhibition is an effective approach against KRAS-driven PDAC and NSCLC

Effectively treating KRAS-driven tumors remains an unsolved challenge. The inhibition of downstream signaling effectors is a way of overcoming the issue of direct targeting of mutant KRAS, which has shown limited efficacy so far. Bromodomain and Extra-Terminal (BET) protein inhibition has displayed anti-tumor activity in a wide range of cancers, including KRAS-driven malignancies. Here, we preclinically evaluate the effect of BET inhibition making use of a new BET inhibitor, BAY 1238097, against Pancreatic Ductal Adenocarcinoma (PDAC) and Non-Small Cell Lung Cancer (NSCLC) models harboring RAS mutations both in vivo and in vitro. Our results demonstrate that BET inhibition displays significant therapeutic impact in genetic mouse models of KRAS-driven PDAC and NSCLC, reducing both tumor area and tumor grade. The same approach also causes a significant reduction in cell number of a panel of RAS-mutated human cancer cell lines (8 PDAC and 6 NSCLC). In this context, we demonstrate that while BET inhibition by BAY 1238097 decreases MYC expression in some cell lines, at least in PDAC cells its anti-tumorigenic effect is independent of MYC regulation. Together, these studies reinforce the use of BET inhibition and prompt the optimization of more efficient and less toxic BET inhibitors for the treatment of KRAS-driven malignancies, which are in urgent therapeutic need.

Abstract 2167: Preclinical validation of an Omomyc cell-penetrating peptide as a viable anti-Myc therapy

Deregulation of the MYC oncoprotein promotes tumorigenesis in most, if not all, cancers and is often associated with poor prognosis. However, targeting MYC has long been considered impossible based on the assumption that it would cause catastrophic side effects in normal tissues. Despite this general preconceived notion, we showed that MYC inhibition exerts extraordinary therapeutic impact in various genetic mouse models of cancer, and causes only mild, well-tolerated and reversible side effects. For these studies we employed the systemic and conditional expression of a dominant negative of MYC, called Omomyc, which we designed and validated, and that can inhibit MYC transactivation function both in vitro and in vivo. To date, Omomyc has only been considered a proof of principle, with any potential clinical application limited to gene therapy. Here we actually show that the 11 kDa Omomyc polypeptide spontaneously transduces into cancer cells, demonstrating unexpected cell-penetrating ability. Once inside the nuclei, the polypeptide effectively blocks MYC binding to its

target DNA sites, interfering with MYC transcriptional regulation and halting cell proliferation. Moreover, intranasal (i.n.) administration of the Omomyc polypeptide in mice results in its rapid and persistent distribution to lungs, as well as to other organs (i.e. intestine, liver, kidneys and brain). Importantly, i.n. treatment of mice bearing either Non-Small-Cell-Lung-Cancer (NSCLC) or glioblastoma (GBM) with the Omomyc cell-penetrating peptide (OmomycCPP) significantly reduces tumor burden compared to their control counterparts. Notably, tumor regression is accompanied by significant reprogramming of the tumor microenvironment and tumor immune response. In summary, our data indicate that this novel generation of polypeptides represents a new opportunity to potentially inhibit MYC pharmacologically in a variety of malignant diseases.

Citation Format: Marie-eve Beaulieu, Toni Jauset, Daniel Massó-Vallés, Peter Rahl, Sandra Martinez-Martin, Loika Maltais, Mariano F. Zacarias-Fluck, Silvia Casacuberta, Erika Serrano del Pozo, Christopher Fiore, Laia Foradada, Matthew Guenther, Eduardo Romero Sanz, Marta Oteo Vives, Cynthia Tremblay, Martin Montagne, Miguel Ángel Morcillo Alonso, Jonathan R. Whitfield, Pierre Lavigne, Laura Soucek. Preclinical validation of an Omomyc cell-penetrating peptide as a viable anti-Myc therapy [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 2167. doi:10.1158/1538-7445.AM2017-2167

Strategies to Inhibit Myc and Their Clinical Applicability

Myc is an oncogene deregulated in most-perhaps all-human cancers. Each Myc family member, c-, L-, and N-Myc, has been connected to tumor progression and maintenance. Myc is recognized as a "most wanted" target for cancer therapy, but has for many years been considered undruggable, mainly due to its nuclear localization, lack of a defined ligand binding site, and physiological function essential to the maintenance of normal tissues. The challenge of identifying a pharmacophore capable of overcoming these hurdles is reflected in the current absence of a clinically-viable Myc inhibitor. The first attempts to inhibit Myc used antisense technology some three decades ago, followed by small molecule inhibitors discovered through "classical" compound library screens. Notable breakthroughs proving the feasibility of systemic Myc inhibition were made with the Myc dominant negative mutant Omomyc, showing both the great promise in targeting this infamous oncogene for cancer treatment as well as allaying fears about the deleterious side effects that Myc inhibition might have on normal proliferating tissues. During this time many other strategies have appeared in an attempt to drug the undruggable, including direct and indirect targeting, knockdown, protein/protein and DNA interaction inhibitors, and translation and expression regulation. The inhibitors range from traditional small molecules to natural chemicals, to RNA and antisense, to peptides and miniproteins. Here, we briefly describe the many approaches taken so far, with a particular focus on their potential clinical applicability.

Abstract B23: Pushing Myc inhibition towards the clinic by direct delivery of cell-penetrating peptides

Inhibiting Myc has long been regarded as a promising cancer treatment. However, clinical Myc inhibition was considered unfeasible due to its central role in normal proliferation and the difficulties of targeting a nuclear transcription factor. The expression of Omomyc (a Myc inhibitor derived from the dimerization and DNA-binding domain of Myc) in the KRasG12D non-small cell lung cancer (NSCLC) mouse model challenged these assumptions, as it resulted in dramatic tumor clearance with only limited and well tolerated side effects in normal tissues (Soucek et al., 2008 and 2013). Omomyc expression proved equally potent in several other mouse models of cancer, revealing the huge potential of this inhibitory approach against multiple cancer types including papilloma, pancreas and glioma (Soucek et al., 2004; Sodir et al., 2011; Annibali et al., 2014). Recently, Max*, a b-HLH-LZ peptide derived from Myc's obligate protein partner Max, was shown to spontaneously enter cells (Montagne et al., 2012). As Omomyc and Max* display high structural homology, we hypothesized that Omomyc could also behave as a cell-penetrating peptide and thus recapitulate the effects of its transgenic counterpart. Our preliminary results show that the Omomyc peptide is well folded in solution; it transduces into cancer cells and effectively stops their proliferation in a dose-dependent manner.In vivo, nasal instillation of fluorescently-labeled Omomyc peptide leads to its rapid distribution to lungs and brain, as well as to other organs (G.I. tract, liver), as observed by IVIS® imaging and immunohistochemistry. Finally, a short treatment with the Omomyc peptide reduces the tumor size and number of Ki67 positive cells in the KRasG12D-induced NSCLC mouse model. In summary, the Omomyc cell penetrating peptide represents a new opportunity to pharmacologically inhibit Myc in a variety of malignant diseases.

Citation Format: Marie-Eve Beaulieu, Toni Jauset, Daniel Massó-Valles, Jonathan R. Whitfield, Erika Serrano del Pozo, Cynthia Tremblay, Loïka Maltais, Martin Montagne, Pierre Lavigne, Laura Soucek. Pushing Myc inhibition towards the clinic by direct delivery of cell-penetrating peptides. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B23.

Assessing the carcinogenic potential of low-dose exposures to chemical mixtures in the environment: the challenge ahead

Low-dose exposures to common environmental chemicals that are deemed safe individually may be combining to instigate carcinogenesis, thereby contributing to the incidence of cancer. This risk may be overlooked by current regulatory practices and needs to be vigorously investigated.

Lifestyle factors are responsible for a considerable portion of cancer incidence worldwide, but credible estimates from the World Health Organization and the International Agency for Research on Cancer (IARC) suggest that the fraction of cancers attributable to toxic environmental exposures is between 7% and 19%. To explore the hypothesis that low-dose exposures to mixtures of chemicals in the environment may be combining to contribute to environmental carcinogenesis, we reviewed 11 hallmark phenotypes of cancer, multiple priority target sites for disruption in each area and prototypical chemical disruptors for all targets, this included dose-response characterizations, evidence of low-dose effects and cross-hallmark effects for all targets and chemicals. In total, 85 examples of chemicals were reviewed for actions on key pathways/mechanisms related to carcinogenesis. Only 15% (13/85) were found to have evidence of a dose-response threshold, whereas 59% (50/85) exerted low-dose effects. No dose-response information was found for the remaining 26% (22/85). Our analysis suggests that the cumulative effects of individual (non-carcinogenic) chemicals acting on different pathways, and a variety of related systems, organs, tissues and cells could plausibly conspire to produce carcinogenic synergies. Additional basic research on carcinogenesis and research focused on low-dose effects of chemical mixtures needs to be rigorously pursued before the merits of this hypothesis can be further advanced. However, the structure of the World Health Organization International Programme on Chemical Safety ‘Mode of Action’ framework should be revisited as it has inherent weaknesses that are not fully aligned with our current understanding of cancer biology.

The effect of environmental chemicals on the tumor microenvironment

Potentially carcinogenic compounds may cause cancer through direct DNA damage or through indirect cellular or physiological effects. To study possible carcinogens, the fields of endocrinology, genetics, epigenetics, medicine, environmental health, toxicology, pharmacology and oncology must be considered. Disruptive chemicals may also contribute to multiple stages of tumor development through effects on the tumor microenvironment. In turn, the tumor microenvironment consists of a complex interaction among blood vessels that feed the tumor, the extracellular matrix that provides structural and biochemical support, signaling molecules that send messages and soluble factors such as cytokines. The tumor microenvironment also consists of many host cellular effectors including multipotent stromal cells/mesenchymal stem cells, fibroblasts, endothelial cell precursors, antigen-presenting cells, lymphocytes and innate immune cells. Carcinogens can influence the tumor microenvironment through effects on epithelial cells, the most common origin of cancer, as well as on stromal cells, extracellular matrix components and immune cells. Here, we review how environmental exposures can perturb the tumor microenvironment. We suggest a role for disrupting chemicals such as nickel chloride, Bisphenol A, butyltins, methylmercury and paraquat as well as more traditional carcinogens, such as radiation, and pharmaceuticals, such as diabetes medications, in the disruption of the tumor microenvironment. Further studies interrogating the role of chemicals and their mixtures in dose-dependent effects on the tumor microenvironment could have important general mechanistic implications for the etiology and prevention of tumorigenesis.

Abstract 396: Ibrutinib exerts potent antifibrotic activity in a mouse model of pancreatic adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a disease with a poor prognosis characterized by a dense stromal fibroinflammatory reaction that is a significant obstacle to effective therapy. The desmoplastic stroma comprises many inflammatory cells, in particular mast cells that are a key component of the PDAC microenvironment and whose infiltration often correlates with poor patient outcome. Reducing the stroma is of key importance for improving the standard of care. We have previously successfully utilized ibrutinib, a clinically-approved Bruton's Tyrosine Kinase inhibitor, as a mast cell blocker in a mouse model of β cell tumorigenesis. Here, we made use of a transgenic mouse model of PDAC and patient derived xenografts (PDXs) to assess ibrutinib's impact on the tumor microenvironment and animal survival. Across these various experimental pre-clinical models, we show that ibrutinib significantly diminishes tissue fibrosis, increases survival and improves the outcome of standard care. Our results suggest that ibrutinib could be deployed to one more type of cancer and open the way to new clinical trials assessing its therapeutic impact in PDAC patients.

Citation Format: Daniel Massó-Vallés, Toni Jauset, Erika Serrano, Nicole M. Sodir, Kim Pedersen, Nesrine I. Affara, Jonathan R. Whitfield, Marie-Eve Beaulieu, Gerard I. Evan, Laurence Elias, Joaquín Arribas, Laura Soucek. Ibrutinib exerts potent antifibrotic activity in a mouse model of pancreatic adenocarcinoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 396. doi:10.1158/1538-7445.AM2015-396

Ibrutinib exerts potent antifibrotic and antitumor activities in mouse models of pancreatic adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a dense stromal fibroinflammatory reaction that is a major obstacle to effective therapy. The desmoplastic stroma comprises many inflammatory cells, in particular mast cells as key components of the PDAC microenvironment, and such infiltration correlates with poor patient outcome. Indeed, it has been hypothesized that stromal ablation is critical to improve clinical response in patients with PDAC. Ibrutinib is a clinically approved Bruton's tyrosine kinase inhibitor that inhibits mast cells and tumor progression in a mouse model of β-cell tumorigenesis. Here, we show that ibrutinib is highly effective at limiting the growth of PDAC in both transgenic mouse and patient-derived xenograft models of the disease. In these various experimental settings, ibrutinib effectively diminished fibrosis, extended survival, and improved the response to clinical standard-of-care therapy. Our results offer a preclinical rationale to immediately evaluate the clinical efficacy of ibrutinib in patients with PDAC.

Myc inhibition is effective against glioma and reveals a role for Myc in proficient mitosis

Gliomas are the most common primary tumours affecting the adult central nervous system and respond poorly to standard therapy. Myc is causally implicated in most human tumours and the majority of glioblastomas have elevated Myc levels. Using the Myc dominant negative Omomyc, we previously showed that Myc inhibition is a promising strategy for cancer therapy. Here, we preclinically validate Myc inhibition as a therapeutic strategy in mouse and human glioma, using a mouse model of spontaneous multifocal invasive astrocytoma and its derived neuroprogenitors, human glioblastoma cell lines, and patient-derived tumours both in vitro and in orthotopic xenografts. Across all these experimental models we find that Myc inhibition reduces proliferation, increases apoptosis and remarkably, elicits the formation of multinucleated cells that then arrest or die by mitotic catastrophe, revealing a new role for Myc in the proficient division of glioma cells.

Myc has been implicated in the development of multiple types of cancer. Here, the authors explore the therapeutic potential and mechanism of action of Myc inhibition in mouse and human models of glioblastoma, an aggressive type of tumour that is often resistant to conventional therapy.

Inhibition of Myc family proteins eradicates KRas-driven lung cancer in mice

The principal reason for failure of targeted cancer therapies is the emergence of resistant clones that regenerate the tumor. Therapeutic efficacy therefore depends on not only how effectively a drug inhibits its target, but also the innate or adaptive functional redundancy of that target and its attendant pathway. In this regard, the Myc transcription factors are intriguing therapeutic targets because they serve the unique and irreplaceable role of coordinating expression of the many diverse genes that, together, are required for somatic cell proliferation. Furthermore, Myc expression is deregulated in most-perhaps all-cancers, underscoring its irreplaceable role in proliferation. We previously showed in a preclinical mouse model of non-small-cell lung cancer that systemic Myc inhibition using the dominant-negative Myc mutant Omomyc exerts a dramatic therapeutic impact, triggering rapid regression of tumors with only mild and fully reversible side effects. Using protracted episodic expression of Omomyc, we now demonstrate that metronomic Myc inhibition not only contains Ras-driven lung tumors indefinitely, but also leads to their progressive eradication. Hence, Myc does indeed serve a unique and nondegenerate role in lung tumor maintenance that cannot be complemented by any adaptive mechanism, even in the most aggressive p53-deficient tumors. These data endorse Myc as a compelling cancer drug target.

Abstract 4956: Pharmacological inhibition of Bruton's Tyrosine Kinase (BTK) as a therapy for insulinoma and pancreatic ductal adenocarcinoma

Myc, a pleiotropic transcription factor that is deregulated and/or over-expressed in most human cancers, instructs multiple extracellular programs that are required to sustain the complex microenvironment needed for tumor maintenance, including remodeling of tumor stroma, angiogenesis and inflammation. We previously showed in a model of pancreatic [[Unsupported Character - Symbol Font β]] cell tumorigenesis that acute Myc activation in vivo triggers rapid recruitment of mast cells to the tumor site and that this is absolutely required for angiogenesis and macroscopic tumor expansion. Moreover, systemic inhibition of mast cell degranulation with sodium cromoglycate induced death of tumor and endothelial cells in established tumors. Hence, mast cells are required both to establish and to maintain the tumors. While this intimates that selective inhibition of mast cell function could be therapeutically efficacious, cromoglycate is not a practical drug for systemic delivery in humans and no other systemic inhibitor of mast cell degranulation has hitherto been available.

Ibrutinib is a novel inhibitor of Bruton's tyrosine kinase (Btk) that blocks mast cell degranulation and is currently in clinical trials as a therapy for B cell non-Hodgkin's lymphoma. Here, we show that systemic treatment of insulinoma-bearing mice with Ibrutinib efficiently inhibits Btk, blocks mast cell degranulation and triggers collapse of tumor vasculature and tumor regression. We also show that pancreatic ductal adenocarcinoma (PDAC)-bearing mice treated with Ibrutinib survive longer and present reduced tumor stroma, suggesting that combinatorial therapy with Ibrutinib and standard of care is a feasible therapeutic approach. Conclusions: our data reinforce the notion that mast cell function is required for maintenance of certain tumor types and indicate that the Btk inhibitor Ibrutinib may be useful in treating pancreatic cancer.

Citation Format: Daniel Massó-Vallés, Erika Serrano, Jonathan R. Whitfield, Joseph J. Buggy, Nicole M. Sodir, Nesrine I. Affara, Lamorna Brown Swigart, Gerard I. Evan, Laura Soucek. Pharmacological inhibition of Bruton's Tyrosine Kinase (BTK) as a therapy for insulinoma and pancreatic ductal adenocarcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4956. doi:10.1158/1538-7445.AM2013-4956

Tumor microenvironment: becoming sick of Myc

Several years ago, we described Myc as "the oncogene from hell", since evidence had just emerged that Myc, aside from being responsible for cell-cycle progression and tumor expansion, was also able to induce genomic instability in culture, wreaking havoc in tumor cells and accelerating tumor progression (Soucek and Evan, Cancer Cell 1:406-408, 2002; Vafa et al., Mol Cell 9:1031-1044, 2002). In this review, we discuss recent publications that expand Myc's evil armory to include coordination of the crosstalk between tumor and microenvironment. Indeed, endogenous Myc, acting as a client for upstream oncogenic lesions, instructs the tumor stroma, engages a complex inflammatory response and induces angiogenesis, thus allowing the tumor to thrive. This is highly topical in light of the fact that Hanahan and Weinberg have recently redefined the hallmarks of cancer and pointed out that genomic instability and inflammation are essential for both their acquisition and development (Hanahan and Weinberg, Cell 144:646-674, 2011). Myc, it seems, is behind it all.

Down-regulation of interleukin-12, interleukin-12R expression/activity mediates the switch from Th1 to Th2 granuloma response during murine Schistosomiasis mansoni

In murine schistosomiasis mansoni the worm egg-induced granulomatous inflammation is bi-phasic: an initial Th1 type is subsequently switched to a Th2 type response. Analysis of the cellular, molecular base of the Th1-associated response (5-6 weeks post infection) revealed mRNA messages for interleukin (IL)-12 p40, IL-12Rbeta2 and interferon (IFN)-gamma in the granulomatous livers. When the Th2 type granulomas matured (8 weeks post infection) message expression weakened or became extinct. Macrophages of the Th1 type granulomas produced maximal amounts of IL-12, but production diminished in the mature granulomas. A similar pattern of IL-12 responsiveness of granuloma lymphocytes was observed. In vitro IL-12 production by Th1 type granuloma macrophages was enhanced by tumour necrosis factor (TNF)-alpha and IFNgamma, whereas lymphocyte IL-12 responsiveness was boosted only by TNF-alpha. Both systems were down-regulated by IL-4 and IL-10 cytokines. Treatment of mice with anti-IL-10 monoclonal antibodies (MoAb) between 6 and 7 weeks of the infection enhanced mRNA expression for IFN-gamma and IL-12Rbeta2, but not for IL-12 p40. It is concluded that IL-12 and IL-12R expression and function regulate the Th1 phase of the liver granulomatous response. This phase is cross-regulated by type-2 cytokines especially IL-10.

Enhanced Th1 and dampened Th2 responses synergize to inhibit acute granulomatous and fibrotic responses in murine schistosomiasis mansoni

In murine schistosomiasis mansoni, CD4(+) Th1 and Th2 cells participate in the ovum-induced granulomatous inflammation. Previous studies showed that the interleukin-12 (IL-12)-induced Th1 response strongly suppressed the Th2-cell-mediated pulmonary granuloma development in naive or primed mice. However, liver granulomas were only moderately suppressed in egg-vaccinated, recombinant IL-12 (rIL-12)-treated infected mice. The present study shows that repeated rIL-12 injections given during early granuloma development at 5 to 7 weeks after infection prolonged the Th1 phase and resulted in gamma interferon-mediated suppression of liver granulomas. The timing is crucial: if given at 6 to 8 weeks, during the Th2-dominated phase of florid granuloma growth, the treatment is ineffective. Daily injections of rIL-12 given between 5 and 7.5 weeks during the period of granuloma growth achieved a somewhat-stronger diminution in granuloma growth with less deposition of collagen but caused 60% mortality and liver pathology. In contrast, combined treatment with rIL-12 and anti-IL-4-anti-IL-10 monoclonal antibody (MAb) injections given during the Th2 phase strongly inhibited liver granuloma growth without mortality. The diminished inflammatory response was accompanied by less deposition of collagen in the liver. Moreover, neutralization of endogenous IL-12 by anti-IL-12 MAbs effectively decreased the early Th1 phase (between 5 and 6 weeks after infection) but not the developing Th2 phase (5 to 7 weeks) of granuloma development. These studies indicate that the granulomatous response in infected mice can be manipulated by utilizing the Th1-Th2-subset antagonism with potential salutary results in the amelioration of fibrous pathology.

Endogenous IL-10 regulates IFN-gamma and IL-5 cytokine production and the granulomatous response in Schistosomiasis mansoni-infected mice

In murine Schistosomiasis mansoni circumovum, granuloma formation is regulated by pro- and anti-inflammatory cytokines. Among the latter, interleukin-10 (IL-10) has been shown to regulate the inflammatory response. In this study we examined the role of endogenously produced IL-10 in T-helper 1 (Th1)- and Th2-type cytokine production and granuloma formation. The dynamics of IL-10 production through the course of the infection were different in granuloma versus splenic cells. In the former, production peaked during the early developmental stage (6 weeks of infection) of the granuloma and then declined. In splenocytes production peaked at 12 weeks, before down-modulation of the granuloma response. In the developing granuloma both macrophages and T cells secreted IL-10. In anti-IL-10 monoclonal antibody (mAb)-supplemented granuloma cell cultures endogenous IL-10-mediated regulation of interferon-gamma (IFN-gamma) was manifest only at 6 weeks; that of IL-2 continued throughout the infection (6-20 weeks). IL-4 production was unaffected, but IL-5 production was regulated at the 6 and 8 weeks time point. Splenocytes showed regulation of IFN-gamma and IL-2 production at the peak of the granulomatous response (8 weeks). IL-4 production was not regulated, whereas IL-5 production was regulated only at 6 weeks. Repeated injections of anti-IL-10 mAb given to mice at 6, 12 or 20 weeks of the infection significantly enhanced liver and lung granuloma growth, tissue eosinophilia, and IFN-gamma, IL-5 production at the early developmental phase (6 weeks) of the lesions. Thus, in schistosome-infected mice endogenous IL-10 is shown to regulate Th1- and Th2-type cytokine production and granuloma formation during the early Th0/Th1 phase of the immune response.

Th2 cells are required for the Schistosoma mansoni egg-induced granulomatous response

The development of Schistosoma mansoni ova-induced granulomas is regulated by cytokines secreted by distinct Th cell subsets. To determine the importance of Th1 and Th2 cells in granuloma formation, we have studied the immune response to S. mansoni ova in Stat4- and Stat6-deficient mice, which lack Th1 and Th2 cells, respectively. Lymphocytes from both naive and infected Stat6-deficient mice produced minimal levels of Th2 cell cytokines and Ag-specific IgG1 and IgE, but showed enhanced production of IFN-gamma and Ag-specific IgG2a and IgG2b following schistosome egg injection. This shift away from a Th2 cell-mediated immune response was coupled with the development of pulmonary and hepatic granulomas that were greatly decreased in size compared with those in control littermates. Hepatic granulomas in Stat6-deficient mice were composed of predominantly mononuclear cells with very sparse appearance of eosinophils, and their diminished size was accompanied by decreased amounts of liver hydroxyproline content as a measure of collagen deposition. In contrast, lymphocytes from infected Stat4-deficient mice produced Th2 cell cytokines in amounts comparable to those produced by control littermates, but low levels of IFN-gamma. While infected Stat4-deficient mice developed pulmonary granulomas following schistosome egg injection that were modestly impaired in size, the granuloma size and amount of collagen deposition in the liver were equivalent to those seen in control littermates. These studies demonstrate that Th2 cells are required for the full development of the granulomas and tissue-destructive fibrotic pathology associated with the immune response to S. mansoni ova.