SARC025 arms 1 and 2: A phase 1 study of the poly(ADP-ribose) polymerase inhibitor niraparib with temozolomide or irinotecan in patients with advanced Ewing sarcoma.

BACKGROUND: In preclinical Ewing sarcoma (ES) models, poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors were identified as a potential therapeutic strategy with synergy in combination with cytotoxic agents. This study evaluated the safety and dosing of the PARP1/2 inhibitor niraparib (NIR) with temozolomide (TMZ; arm 1) or irinotecan (IRN; arm 2) in patients with pretreated ES. METHODS: Eligible patients in arm 1 received continuous NIR daily and escalating TMZ (days 2-6 [D2-6]) in cohort A. Subsequent patients received intermittent NIR dosing (cohort B), with TMZ re-escalation in cohort C. In arm 2, patients were assigned to NIR (days 1-7 [D1-7]) and escalating doses of IRN (D2-6). RESULTS: From July 2014 to May 2018, 29 eligible patients (23 males and 6 females) were enrolled in arms 1 and 2, which had 7 dose levels combined. Five patients experienced at least 1 dose-limiting toxicity (DLT) in arm 1 (grade 4 [G4] neutropenia for >7 days or G4 thrombocytopenia), and 3 patients experienced at least 1 DLT in arm 2 (grade 3 [G3] colitis, G3 anorexia, or G3 alanine aminotransferase elevation). The maximum tolerated dose was NIR at 200 mg every day on D1-7 plus TMZ at 30 mg/m2 every day on D2-6 (arm 1) or NIR at 100 mg every day on D1-7 plus IRN at 20 mg/m2 every day on D2-6 (arm 2). One confirmed partial response was observed in arm 2; the median progression-free survival was 9.0 weeks (95% CI, 7.0-10.1 weeks) and 16.3 weeks (95% CI, 5.1-69.7 weeks) in arms 1 and 2, respectively. The median decrease in tumor poly(ADP-ribose) activity was 89% (range, 83%-98%). CONCLUSIONS: The combination of NIR and TMZ or IRN was tolerable, but at lower doses in comparison with conventional cytotoxic combinations. A triple-combination study of NIR, IRN, and TMZ has commenced.

LRRC15 antibody-drug conjugates show promise as osteosarcoma therapeutics in preclinical studies.

BACKGROUND: Osteosarcoma (OS), the most common bone tumor in children and adolescents, has high rates of metastasis leading to poor survival. Leucine-rich repeat containing 15 (LRRC15), a transmembrane protein whose expression is modulated by TGFß, was recently shown to be highly expressed on the surface of OS tumor cells. Here, we evaluate a novel antibody-drug conjugate (ADC) targeting LRRC15 in OS human cell lines and murine xenografts. We compare this new ADC, which is conjugated to the anthracycline derivative PNU-159682 (PNU), to a previously studied LRRC15 ADC that is conjugated to the tubulin inhibitor monomethyl auristatin E (MMAE), since anthracyclines are standard of care in OS. PROCEDURE: We evaluated LRRC15 expression in OS cells using Western blots and flow cytometry, and analyzed the epigenetic landscape of the LRRC15 locus using chromatin immunoprecipitation. Efficacy of ADCs on cell growth was analyzed by IncuCyte live cell imaging. Intramuscular xenograft tumor growth was assessed by bioluminescence imaging and hematoxylin and eosin staining. RESULTS: LRRC15-PNU is more effective at inhibiting growth in vitro and in vivo than an isotype antibody control or the LRRC15-MMAE ADC in two high LRRC15 expressing OS cell lines. Low expressing cell lines are not sensitive to either ADC. Importantly, cells with low LRRC15 expression are amenable to re-expression after TGFß treatment, suggesting a potential to sensitize insensitive OS cells to LRRC15 ADC treatment. In vivo, LRRC15-PNU had cure rates of 40-100% in OS xenograft models. CONCLUSIONS: Overall, LRRC15-directed ADCs are a promising new avenue for OS treatment.

Insights into pediatric rhabdomyosarcoma research: Challenges and goals.

Overall survival rates for pediatric patients with high-risk or relapsed rhabdomyosarcoma (RMS) have not improved significantly since the 1980s. Recent studies have identified a number of targetable vulnerabilities in RMS, but these discoveries have infrequently translated into clinical trials. We propose streamlining the process by which agents are selected for clinical evaluation in RMS. We believe that strong consideration should be given to the development of combination therapies that add biologically targeted agents to conventional cytotoxic drugs. One example of this type of combination is the addition of the WEE1 inhibitor AZD1775 to the conventional cytotoxic chemotherapeutics, vincristine and irinotecan.

Risk-Based Therapy for Localized Osteosarcoma.

BACKGROUND: The outcome of localized osteosarcoma has remained constant over the past 30 years. Histological response to preoperative chemotherapy is the best predictor of outcome. Strategies to alter treatment based on histological response have not resulted in increased survival. PROCEDURE: Patients with localized osteosarcoma received preoperative chemotherapy with cisplatin, doxorubicin, and methotrexate. Patients whose tumors had a good histological response (≥90% necrosis) continued with the same treatment postoperatively. Patients with poor histological response (<90% necrosis) received three courses of melphalan 100 mg/m(2) on day -4, cyclophosphamide 2,000 mg/m(2) on days -3, and -2 followed by stem cell infusion. RESULTS: Fifty-two patients were enrolled. Median age was 14 years, and 56% of patients were male. The femur was the most common site. Forty patients underwent limb salvage surgery and amputation was performed in six patients. Forty-eight percent of tumors showed good histological response. Forty patients were evaluable for outcome; 18 patients with poor histologic response received high-dose chemotherapy. The 5-year event-free survival (EFS) and overall survival (OS) for patients treated on the high-dose chemotherapy arm were 28% (95% confidence interval [CI], 10-49) and 48% (95% CI, 23-69), respectively. The 5-year EFS and OS for patients treated on the standard chemotherapy arm were 62% (95% CI, 36-80) and 74% (95% CI, 44-90), respectively. All patients who received high-dose chemotherapy developed grade 3 or higher hematological toxicity. There were no treatment-related deaths. CONCLUSIONS: Postoperative alkylator intensification with high-dose cyclophosphamide and melphalan in patients with localized osteosarcoma with poor histological response failed to improve survival.

Compressing drug development timelines in oncology using phase '0' trials.

The optimal evaluation of molecularly targeted anticancer agents requires the integration of pharmacodynamic assays into early clinical investigations. Phase '0' trials conducted under the new Exploratory Investigational New Drug Guidance from the US Food and Drug Administration can provide a platform to establish the feasibility of assays for target modulation in human samples, evaluate biomarkers for drug effects and provide pharmacokinetic data. Phase 0 trials could facilitate rational drug selection, identify therapeutic failures early, and might compress timelines for anticancer drug development. We expect that such trials will become a routine part of early-phase oncological drug development in the future.

A phase 2 trial of R1507, a monoclonal antibody to the insulin-like growth factor-1 receptor (IGF-1R), in patients with recurrent or refractory rhabdomyosarcoma, osteosarcoma, synovial sarcoma, and other soft tissue sarcomas: results of a Sarcoma Alliance for Research Through Collaboration study.

BACKGROUND: Insulin-like growth factor-1 receptor (IGF-1R) is implicated in the pathogenesis of rhabdomyosarcoma (RMS), osteosarcoma (OS), and synovial sarcoma (SS). The authors conducted a multi-institutional phase 2 trial of the monoclonal antibody R1507 in patients with various subtypes of recurrent or refractory sarcomas. METHODS: Eligibility criteria included age ≥ 2 years and a diagnosis of recurrent or refractory RMS, OS, SS, and other soft tissue sarcomas. Patients received a weekly dose of 9 mg/kg R1507 intravenously. The primary endpoint was the best objective response rate using World Health Organization criteria. Tumor imaging was performed every 6 weeks × 4 and every 12 weeks thereafter. RESULTS: From December 2007 through August 2009, 163 eligible patients from 33 institutions were enrolled. The median patient age was 31 years (range, 7-85 years). Histologic diagnoses included OS (n = 38), RMS (n = 36), SS (n = 23), and other sarcomas (n = 66). The overall objective response rate was 2.5% (95% confidence interval, 0.7%-6.2%). Partial responses were observed in 4 patients, including 2 patients with OS, 1 patient with RMS, and 1 patient with alveolar soft part sarcoma. Four additional patients (3 with RMS and 1 with myxoid liposarcoma) had a ≥ 50% decrease in tumor size that lasted for <4 weeks. The median progression-free survival was 5.7 weeks, and the median overall survival was 11 months. The most common grade 3/4 toxicities were metabolic (12%), hematologic (6%), gastrointestinal (4%), and general constitutional symptoms (8%). CONCLUSIONS: R1507 is safe and well tolerated but has limited activity in patients with recurrent or refractory bone and soft tissue sarcomas. Additional studies to help identify the predictive factors associated with clinical benefit in selected histologies such as RMS appear to be warranted.

Defects in succinate dehydrogenase in gastrointestinal stromal tumors lacking KIT and PDGFRA mutations.

Carney-Stratakis syndrome, an inherited condition predisposing affected individuals to gastrointestinal stromal tumor (GIST) and paraganglioma, is caused by germline mutations in succinate dehydrogenase (SDH) subunits B, C, or D, leading to dysfunction of complex II of the electron transport chain. We evaluated the role of defective cellular respiration in sporadic GIST lacking mutations in KIT or PDGFRA (WT). Thirty-four patients with WT GIST without a personal or family history of paraganglioma were tested for SDH germline mutations. WT GISTs lacking demonstrable SDH genetic inactivation were evaluated for SDHB expression by immunohistochemistry and Western blotting and for complex II activity. For comparison, SDHB expression was also determined in KIT mutant and neurofibromatosis-1-associated GIST, and complex II activity was also measured in SDH-deficient paraganglioma and KIT mutant GIST; 4 of 34 patients (12%) with WT GIST without a personal or family history of paraganglioma had germline mutations in SDHB or SDHC. WT GISTs lacking somatic mutations or deletions in SDH subunits had either complete loss of or substantial reduction in SDHB protein expression, whereas most KIT mutant GISTs had strong SDHB expression. Complex II activity was substantially decreased in WT GISTs. WT GISTs, particularly those in younger patients, have defects in SDH mitochondrial complex II, and in a subset of these patients, GIST seems to arise from germline-inactivating SDH mutations. Testing for germline mutations in SDH is recommended in patients with WT GIST. These findings highlight a potential central role of SDH dysregulation in WT GIST oncogenesis.

Mechanisms of sarcoma development.

Sarcomas are a rare and diverse group of tumours that are derived from connective tissues, including bone, muscle and cartilage. Although there are instances of hereditary predisposition to sarcomas, the overwhelming majority of such tumours are sporadic. In the past decade, we have gained much insight into the genetic abnormalities that seem to underlie the pathogenesis of these tumours. This information has already led to new classification of many sarcomas, as well as to successful therapies that are targeted at specific genetic abnormalities. It is likely that this approach will lead to continued refinements in classification and treatment of these tumours.

Phase I trial of Ganitumab plus Dasatinib to Cotarget the Insulin-Like Growth Factor 1 Receptor and Src Family Kinase YES in Rhabdomyosarcoma.

PURPOSE: Antibodies against insulin-like growth factor (IGF) type 1 receptor have shown meaningful but transient tumor responses in patients with rhabdomyosarcoma (RMS). The SRC family member YES has been shown to mediate IGF type 1 receptor (IGF-1R) antibody acquired resistance, and cotargeting IGF-1R and YES resulted in sustained responses in murine RMS models. We conducted a phase I trial of the anti-IGF-1R antibody ganitumab combined with dasatinib, a multi-kinase inhibitor targeting YES, in patients with RMS (NCT03041701). PATIENTS AND METHODS: Patients with relapsed/refractory alveolar or embryonal RMS and measurable disease were eligible. All patients received ganitumab 18 mg/kg intravenously every 2 weeks. Dasatinib dose was 60 mg/m2/dose (max 100 mg) oral once daily [dose level (DL)1] or 60 mg/m2/dose (max 70 mg) twice daily (DL2). A 3+3 dose escalation design was used, and maximum tolerated dose (MTD) was determined on the basis of cycle 1 dose-limiting toxicities (DLT). RESULTS: Thirteen eligible patients, median age 18 years (range 8-29) enrolled. Median number of prior systemic therapies was 3; all had received prior radiation. Of 11 toxicity-evaluable patients, 1/6 had a DLT at DL1 (diarrhea) and 2/5 had a DLT at DL2 (pneumonitis, hematuria) confirming DL1 as MTD. Of nine response-evaluable patients, one had a confirmed partial response for four cycles, and one had stable disease for six cycles. Genomic studies from cell-free DNA correlated with disease response. CONCLUSIONS: The combination of dasatinib 60 mg/m2/dose daily and ganitumab 18 mg/kg every 2 weeks was safe and tolerable. This combination had a disease control rate of 22% at 5 months.

A Phase II Trial of Guadecitabine in Children and Adults with SDH-Deficient GIST, Pheochromocytoma, Paraganglioma, and HLRCC-Associated Renal Cell Carcinoma.

PURPOSE: Succinate dehydrogenase (dSDH)-deficient tumors, including pheochromocytoma/paraganglioma, hereditary leiomyomatosis and renal cell cancer-associated renal cell carcinoma (HLRCC-RCC), and gastrointestinal stromal tumors (GIST) without KIT or platelet-derived growth factor receptor alpha mutations are often resistant to cytotoxic chemotherapy, radiotherapy, and many targeted therapies. We evaluated guadecitabine, a dinucleotide containing the DNA methyltransferase inhibitor decitabine, in these patient populations. PATIENTS AND METHODS: Phase II study of guadecitabine (subcutaneously, 45 mg/m2/day for 5 consecutive days, planned 28-day cycle) to assess clinical activity (according to RECISTv.1.1) across three strata of patients with dSDH GIST, pheochromocytoma/paraganglioma, or HLRCC-RCC. A Simon optimal two-stage design (target response rate 30% rule out 5%) was used. Biologic correlates (methylation and metabolites) from peripheral blood mononuclear cells (PBMC), serum, and urine were analyzed. RESULTS: Nine patients (7 with dSDH GIST, 1 each with paraganglioma and HLRCC-RCC, 6 females and 3 males, age range 18-57 years) were enrolled. Two patients developed treatment-limiting neutropenia. No partial or complete responses were observed (range 1-17 cycles of therapy). Biologic activity assessed as global demethylation in PBMCs was observed. No clear changes in metabolite concentrations were observed. CONCLUSIONS: Guadecitabine was tolerated in patients with dSDH tumors with manageable toxicity. Although 4 of 9 patients had prolonged stable disease, there were no objective responses. Thus, guadecitabine did not meet the target of 30% response rate across dSDH tumors at this dose, although signs of biologic activity were noted.

The Childhood Cancer Data Initiative: Using the Power of Data to Learn From and Improve Outcomes for Every Child and Young Adult With Pediatric Cancer.

Data-driven basic, translational, and clinical research has resulted in improved outcomes for children, adolescents, and young adults (AYAs) with pediatric cancers. However, challenges in sharing data between institutions, particularly in research, prevent addressing substantial unmet needs in children and AYA patients diagnosed with certain pediatric cancers. Systematically collecting and sharing data from every child and AYA can enable greater understanding of pediatric cancers, improve survivorship, and accelerate development of new and more effective therapies. To accomplish this goal, the Childhood Cancer Data Initiative (CCDI) was launched in 2019 at the National Cancer Institute. CCDI is a collaborative community endeavor supported by a 10-year, $50-million (in US dollars) annual federal investment. CCDI aims to learn from every patient diagnosed with a pediatric cancer by designing and building a data ecosystem that facilitates data collection, sharing, and analysis for researchers, clinicians, and patients across the cancer community. For example, CCDI's Molecular Characterization Initiative provides comprehensive clinical molecular characterization for children and AYAs with newly diagnosed cancers. Through these efforts, the CCDI strives to provide clinical benefit to patients and improvements in diagnosis and care through data-focused research support and to build expandable, sustainable data resources and workflows to advance research well past the planned 10 years of the initiative. Importantly, if CCDI demonstrates the success of this model for pediatric cancers, similar approaches can be applied to adults, transforming both clinical research and treatment to improve outcomes for all patients with cancer.

Phase II study of sequential gemcitabine followed by docetaxel for recurrent Ewing sarcoma, osteosarcoma, or unresectable or locally recurrent chondrosarcoma: results of Sarcoma Alliance for Research Through Collaboration Study 003.

BACKGROUND: Gemcitabine and docetaxel have a broad spectrum of clinical activity in patients with carcinoma. The Sarcoma Alliance for Research Through Collaboration conducted a phase II trial of gemcitabine in combination with docetaxel in children and adults with recurrent Ewing sarcoma (EWS), osteosarcoma (OS), or unresectable or recurrent chondrosarcoma. The primary objective was to determine the objective response rate using response evaluation criteria in solid tumors (RECIST). METHODS: Gemcitabine (675 mg/m2 i.v. over 90 minutes on days 1 and 8) was administered in combination with docetaxel (75 mg/m2 i.v. over 1 hour on day 8) every 21 days. All patients received filgrastim or pegfilgrastim. A Bayesian formulation was used to determine the probability of achieving the target response rate for each subtype-0.35 for EWS and OS or 0.20 for chondrosarcoma. If the probability of achieving the target response rate was <0.05, the combination was considered inactive. Toxicity was graded according to common terminology criteria for adverse events (CTCAE), version 3.0. RESULTS: Fifty-three eligible patients were enrolled in the three subtype groups-OS (n = 14), EWS (n = 14), and chondrosarcoma (n = 25). Toxicities included neutropenia, thrombocytopenia, fatigue, dyspnea, bronchospasm, edema, neuropathy, and liver function abnormalities. Dose modification for toxicity was required for eight patients during cycle 1 and 16 patients in subsequent cycles. Seven patients withdrew from therapy as a result of toxicity. No complete responses were observed. Partial responses were observed in OS (n = 1), EWS (n = 2), and chondrosarcoma (n = 2) patients. CONCLUSION: Gemcitabine in combination with docetaxel was associated with a probability of reaching the target 35% response rate of <5% in OS patients and 5.6% in EWS patients; the probability of reaching a 20% response rate in chondrosarcoma patients was 14%. DISCUSSION: The Bayesian formulation permitted estimation of the probability of achieving the target response rate for each subtype after each response evaluation. By allowing multiple looks at the data, this design stopped the trial after considering the probability of achieving the target response rate and accrual rate. Because this design did not specify a rule for declaring the treatment as "active", a direct comparison with a standard two-stage phase II design is not appropriate. The decision to close the EWS and chondrosarcoma subtype arms was based, in part, on slow accrual and was supported by the low probability of achieving the target response rate. The rate of enrollment, rather than the statistical design, had a significant effect on the trial duration.

Lymphopenia and interleukin-2 therapy alter homeostasis of CD4+CD25+ regulatory T cells.

CD4(+)CD25(+) regulatory T (T(reg)) cells have a crucial role in maintaining immune tolerance. Mice and humans born lacking T(reg) cells develop severe autoimmune disease, and depletion of T(reg) cells in lymphopenic mice induces autoimmunity. Interleukin (IL)-2 signaling is required for thymic development, peripheral expansion and suppressive activity of T(reg) cells. Animals lacking IL-2 die of autoimmunity, which is prevented by administration of IL-2-responsive T(reg) cells. In light of the emerging evidence that one of the primary physiologic roles of IL-2 is to generate and maintain T(reg) cells, the question arises as to the effects of IL-2 therapy on them. We monitored T(reg) cells during immune reconstitution in individuals with cancer who did or did not receive IL-2 therapy. CD4(+)CD25(hi) cells underwent homeostatic peripheral expansion during immune reconstitution, and in lymphopenic individuals receiving IL-2, the T(reg) cell compartment was markedly increased. Mouse studies showed that IL-2 therapy induced expansion of existent T(reg) cells in normal hosts, and IL-2-induced T(reg) cell expansion was further augmented by lymphopenia. On a per-cell basis, T(reg) cells generated by IL-2 therapy expressed similar levels of FOXP3 and had similar potency for suppression compared to T(reg) cells present in normal hosts. These studies suggest that IL-2 and lymphopenia are primary modulators of CD4(+)CD25(+) T(reg) cell homeostasis.

The new kid on the block(ade) of the IGF-1 receptor.

The insulin-like growth factors (IGF) system, and particularly the IGF-1 receptor, has recently become the subject of major interest in the arena of cancer research. Its involvement in cancer cell growth and survival makes the system an excellent target as potential adjunct therapy to standard chemotherapy.

Gastrointestinal stromal tumor: psychosocial characteristics and considerations.

OBJECTIVE: Gastrointestinal stromal tumors (GIST) are very rare, especially within the pediatric population. The National Institutes of Health initiated a multidisciplinary clinic to bring together a worldwide group of healthcare providers with experience in the study and treatment of GIST in order to better understand the disease and to examine whether this is a population at psychosocial risk. METHODS: Seven GIST clinics have been held to date. Participants completed a GIST Psychosocial Assessment containing items covering demographic factors, family stressors, general health, psychosocial concerns, psychiatric history, and self-identified needs. Our sample consists of 60 adult patients (ages 18-66) and 18 pediatric patients (ages 9-17). RESULTS: Living with pain at least a few days a week was endorsed by those whose self-reported health status was good to excellent (49% of the adult cohort and 25% of the pediatric cohort). Pain was significantly associated with anxiety, difficulties with family relationships, behavior problems, and psychotropic medication use. Body image and appearance concerns were endorsed by over half of the cohort. Post-trauma symptoms were prevalent for those newly diagnosed and those living with GIST for over 15 years. CONCLUSIONS: This is the first study to explore the psychosocial impact of GIST. Individuals living with GIST experience chronic pain, post-trauma symptoms and significant anxiety along with an expressed need for interventions to help them manage their anxiety. Due to the chronic nature of this disease, the GIST population is at risk for long-term psychological distress. Psychosocial interventions in pediatric and adult care settings are offered.

Expression profiling identifies the cytoskeletal organizer ezrin and the developmental homeoprotein Six-1 as key metastatic regulators.

Patients presenting with metastatic rhabdomyosarcoma (RMS), the most common soft-tissue sarcoma in children, have a very poor clinical prognosis. This is due, in large part, to our rudimentary knowledge of the molecular events that dictate metastatic potential. We used cDNA microarray analysis of RMS cell lines, derived from Ink4a/Arf-deficient mice transgenic for hepatocyte growth factor/scatter factor (HGF/SF), to identify a set of genes whose expression was significantly different between highly and poorly metastatic cells. Subsequent in vivo functional studies revealed that the actin filament-plasma membrane linker ezrin (encoded by Vil2) and the homeodomain-containing transcription factor Six-1 (sine oculis-related homeobox-1 homolog) had essential roles in determining the metastatic fate of RMS cells. VIL2 and SIX1 expression was enhanced in human RMS tissue, significantly correlating with clinical stage. The identification of ezrin and Six-1 as critical regulators of metastasis in RMS provides new mechanistic and therapeutic insights into this pediatric cancer.

The membrane-cytoskeleton linker ezrin is necessary for osteosarcoma metastasis.

Metastatic cancers, once established, are the primary cause of mortality associated with cancer. Previously, we used a genomic approach to identify metastasis-associated genes in cancer. From this genomic data, we selected ezrin for further study based on its role in physically and functionally connecting the actin cytoskeleton to the cell membrane. In a mouse model of osteosarcoma, a highly metastatic pediatric cancer, we found ezrin to be necessary for metastasis. By imaging metastatic cells in the lungs of mice, we showed that ezrin expression provided an early survival advantage for cancer cells that reached the lung. AKT and MAPK phosphorylation and activity were reduced when ezrin protein was suppressed. Ezrin-mediated early metastatic survival was partially dependent on activation of MAPK, but not AKT. To define the relevance of ezrin in the biology of metastasis, beyond the founding mouse model, we examined ezrin expression in dogs that naturally developed osteosarcoma. High ezrin expression in dog tumors was associated with early development of metastases. Consistent with this data, we found a significant association between high ezrin expression and poor outcome in pediatric osteosarcoma patients.

Synergism between INK4a/ARF inactivation and aberrant HGF/SF signaling in rhabdomyosarcomagenesis.

Rhabdomyosarcoma (RMS) is the most common soft-tissue sarcoma in children, yet molecular events associated with the genesis and progression of this potentially fatal disease are largely unknown. For the molecules and pathways that have been implicated, genetic validation has been impeded by lack of a mouse model of RMS. Here we show that simultaneous loss of Ink4a/Arf function and disruption of c-Met signaling in Ink4a/Arf(-/-) mice transgenic for hepatocyte growth factor/scatter factor (HGF/SF) induces RMS with extremely high penetrance and short latency. In cultured myoblasts, c-Met activation and Ink4a/Arf loss suppress myogenesis in an additive fashion. Our data indicate that human c-MET and INK4a/ARF, situated at the nexus of pathways regulating myogenic growth and differentiation, represent critical targets in RMS pathogenesis. The marked synergism in mice between aberrant c-Met signaling and Ink4a/Arf inactivation, lesions individually implicated in human RMS, suggests a therapeutic combination to combat this devastating childhood cancer.