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9056 Background: Merkel cell carcinoma (MCC, neuroendocrine carcinoma of the skin) frequently stains strongly for c-KIT (CD-117) expression. We therefore evaluated imatinib mesylate as a treatment for MCC. Methods: Eligibility included patients with measurable metastatic or unresectable MCC with CD117 expression by immunostaining. A Zubrod performance status of 0–2, adequate hematologic, renal, and hepatic function was also required. Imatinib 400 mg daily was administered orally in 28-day cycles. Results: A total of 25 patients were accrued to this trial from 13 institutions, with 6 accrued through Intergroup participation by the Eastern Cooperative Oncology Group. Two patients were ineligible (one was found not to express CD117 on central pathology review, and one lacked measurable disease). These 23 patients were included in the analysis. Imatinib was well tolerated with Grade 1 or 2 nausea, diarrhea, and hematologic toxicity as the most frequent side effects. There were no complete responses (0%) and 1 confirmed partial response (4%) in the 23 evaluable patients (4% objective response rate, CI 0 -22%). In addition, stable disease was observed in 3 patients (9, 4 and 3 months). Median progression-free survival was 1 month (95% CI: 1–2 months). Estimated 6 month PFS was 4%. Median overall survival was 5 months (95% CI 2–8 months). The estimated one-year overall survival was 17% (95% CI: 0% - 33%). There were three on-study deaths. All were attributed to tumor progression. One patient achieved a partial response and another had prolonged disease stabilization while receiving treatment. DNA sequencing of c-KIT was performed on tumor tissue from on a non- responding patient and the one patient with long-term stable disease (9 months). Neither demonstrated an activating mutation in c-KIT. Unfortunately, the patient with partial response withdrew consent for the study and DNA sequencing could not be performed. Conclusions: The majority of patients progressed rapidly within 1–2 cycles of treatment. The observed progression-free survival and overall survival were not adequate to conclude that this agent was active in advanced MCC. The planned second stage of patient accrual was not opened. No significant financial relationships to disclose.

Based on the lack of overall survival benefit, further evaluation of vitamin A as adjuvant therapy for melanoma does not appear warranted.

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