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Aug 9, 2018

Read the related article "Addition of Vincristine and Irinotecan to Vincristine, Dactinomycin, and Cyclophosphamide Does Not Improve Outcome for Intermediate-Risk Rhabdomyosarcoma: A Report From the Children's Oncology Group" on JCO.org

This JCO podcast provides observations and commentary on the JCO article “addition of vincristine, irinotecan to vincristine, dactinomycin and cyclophosphamide does not improve outcome for intermediate risk rhabdomyosarcoma a report from the Children’s Oncology Group by Hawkins, et al.” My name is Alberto Pappo and I am a pediatric oncologist and Head of the Division of Solid Tumors at St Jude Children’s Research Hospital in Memphis, Tennessee.

Investigators of the Children’s Oncology Group (COG) developed a prospective randomized study to improve the outcome of patients with intermediate risk rhabdomyosarcoma by comparing the addition of the doublet vincristine and irinotecan (which will be called the irinotecan arm) to standard vincristine actinomycin D and cyclophosphamide (which will be called the VAC arm) to VAC chemotherapy only. Intermediate risk disease comprises the largest subgroup of patients with rhabdomyosarcoma and comprises patients with embryonal histology who present with tumors that are non-metastatic and unresected and arise in unfavorable sites as well as patients who present with non-metastatic alveolar histology tumors. The authors nicely review prior failed strategies that were aimed at increasing the outcome of this group of patients including dose intensification of active agents as well as the  addition of novel agents such as ifosfamide, etoposide and topotecan. 1-3

Irinotecan is a prodrug that is converted to its active metabolite SN38 and inhibits topoisomerase I. In a front-line trial for patients with metastatic rhabdomyosarcoma demonstrated a high level of activity with a 70% early response rate and an 8% disease progression rate.4 Based strong preclinical and clinical data, this agent was incorporated into an upfront randomized trial for rhabdomyosarcoma testing the benefit of adding vincristine and irinotecan to standard VAC in intermediate risk rhabdomyosarcoma.

Eligibility criteria included patients with embryonal rhabdomyosarcoma who had stage II and III clinical Group 3 disease and any alveolar rhabdomyosarcoma without evidence of distant metastases.  During the first 12 weeks the two treatments were identical in duration of schedule with the exception of the substitution of irinotecan for dactinomycin and cyclophosphamide at week 4 and for cyclophosphamide at week 7 and 10 in the irinotecan arm.  During the next 30 weeks irinotecan replaced actinomycin D and cyclophosphamide at weeks 16, 19, 25, 31 and 37 in irinotecan arm.  Patients were evaluated for response at week 15, 30 and at the end of therapy.  Radiation therapy unlike the prior COG D9803 trial  started early at week 4 instead of 12 and the dose was determined by the clinical group and histology with doses ranging to 36 Gy for those with clinical group I and II to 50.4Gy for those with Group III disease. The study was designed with an 80% power to detect an overall increase in the long term event-free survival from 65% with VAC chemotherapy to 76% with the doublet VAC VI with a sample size of 486 patients.

 

Between December 2006 and December 2012 there were 481 patients enrolled on the study of whom 33 were ineligible.  Of the remaining 448 eligible patients 222 were randomized to VAC and 226 were randomized to the irinotecan arm.  The patient’s characteristics were similar between both arms and to other COG trials. There was a slight predominance of males, 61% of patients were between 1 and 10 years of age and 71% were Caucasian.  Embryonal rhabdomyosarcoma was the predominant histology seen in 53% of the patients and 86% had clinical Group III disease. The most common primary site was parameningeal followed by bladder prostate, extremity and retroperitoneum. 

 

With a median followup for surviving patients of 4.8 years the estimated 4 year event-free survival was 63% for the VAC arm and 59% for the irinotecan arm.  The estimated 4 year overall survival rates was 73% for the VAC arm and 72% for the irinotecan arm. There were no differences in radiographic response among clinical Group III patients as assessed by institutional report by week 15 and no evidence of differences in outcome by treatment arm in the histologic subgroup analysis.  When compared to the previous trial D9083 which had slightly different eligibility criteria, there were no differences in event and overall survival for patients with alveolar rhabdomyosarcoma. However, patients with embryonal tumors had an inferior 4 year event-free survival in this trial when compared to patients in D9803 although the 4 year overall survival rates were similar.  The vast majority of treatment failures were due to tumor progression or recurrence. The 4 year local failure rate was 22.4%, the 4 year regional lymph node failure rate was 5.7% and the 4 year distant failure rate was 18%.  Hematologic toxicity and febrile neutropenia were more commonly observed in the VAC arm whereas diarrhea and mucositis were more prevalent in the irinotecan arm. Hepatopathy was more common in the VAC arm and patients in the irinotecan arms who developed grade 3 4 diarrhea were more likely to carry the UGT1A1  7/7 genotype.

 The authors conclude that the addition of vincristine and irinotecan to a VAC backbone failed to improve the outcome of patients with intermediate risk rhabdomyosarcoma. However, the lower cumulative doses of cyclophosphamide in the irinotecan arm which could potentially reduce the risk of infertility in selected patients and the lower rates of hematologic toxicity in this regimen have provided a rationale for the COG to use a VAC irinotecan backbone in their current up front randomized trial for intermediate risk rhabdomyosarcoma.  

This trial highlights the lack of significant advances in the therapy of pediatric rhabdomyosarcoma despite the fact that irinotecan showed significant preclinical and clinical activity in metastatic patients with this disease.5,6 Similarly, in a previous study, the addition of another camptothecin, topotecan failed to improve the outcome of intermediate risk patients despite promising clinical activity in newly diagnosed metastatic patients 7suggesting that identification of novel agents based on their activity in  phase II window studies for high risk rhabdomyosarcoma is not an optimal method for selecting active compounds that could be incorporated into front-line studies for intermediate risk disease. As suggested by the authors of the paper just discussed, other novel mechanisms of drug testing such as randomized phase II studies in recurrent disease might offer alternative more effective strategies for identifying agents to be tested in intermediate risk patients. In contrast to this results, the European Pediatric Soft Tissue Sarcoma Study Group has recently reported at the ASCO 2018 meeting improved outcomes for a similar population of patients when maintenance therapy with low dose cyclophosphamide and vinorelbine is added to a backbone of vincristine, ifosfamide and actinomycin D (VAI). 8  In this trial, patients with non metastatic incompletely resected embryonal rhabdomyosarcoma arising in unfavorable sites and localized alveolar rhabdomyosarcoma without nodal metastases who achieved a complete remission after 9 cycles (27 weeks) of VAI with or without doxorubicin were randomized to stop treatment or receive maintenance chemotherapy with six 28 days cycle of vinorelbine and cyclophosphamide.  The study was initially designed with an 80% power to detect an increase in the 3 year EFS from 55% to 65% with a hazard ratio of 0.67 but was then amended to allow detection of a relativity reduction rate in the relapse rate of 50%.  This trial accrued 670 patients of whom 371 were eligible and 186 were assigned to the standard arm and 185 to the maintenance arm.  The clinical features were well balanced between the two groups. With median follow up of 5 years in surviving pts, the 3 year event-free survival and overall survival in the maintenance arm and the standard arm were  78.4% vs 72.3% (p value 0.061) and 87.3% vs. 77.4 (p value = 0.011).  The investigators concluded that the addition of maintenance therapy is a novel strategy that improves the outcome of this group of patients with rhabdomyosarcoma and establishes the new standard of care for patients in Europe.  The results of this trial however, need to be interpreted with caution and cannot be generalized. The follow-up of patients is relatively short and only those who achieved a complete radiographic response after chemotherapy were eligible to be randomized, no information was provided regarding outcomes of patients who were randomized to receive doxorubicin, there are only 9 cycles of chemotherapy given prior to randomization to maintenance therapy whereas the COG studies give 14 cycles of therapy and finally, there are slight differences in the eligibility criteria when compared to the previous COG  trials.   

In conclusion, the addition of an irinotecan backbone to standard VAC chemotherapy does not improve the outcome of patients with intermediate risk rhabdomyosarcoma. However, the irinotecan containing regimen offers potential advantages such as outpatient administration of chemotherapy, reduced hematologic toxicity and cumulative cyclophosphamide exposure and has therefore become the standard backbone for patients with intermediate risk rhabdomyosarcoma in the United States.  This concludes this JCO podcast.  Thank you for listening.