Introduction. Our prior phase 1 study (P9973) established the safety profile and suggested efficacy of imatinib in children with CP CML at doses varying from 260–570 mg/m2. The purpose of this phase 2 study was to define the rates of response in children with previously untreated CP CML.

Methods. Patients less than 22 years of age at study entry with newly diagnosed CP CML, with no prior therapy other than hydroxyurea, were eligible. Imatinib was administered orally at a dose of 340 mg/m2 daily, with courses defined as 28-day intervals. A hematological response (HR) was defined at the end of courses 1 and 2 as a reduction in the white-cell count to <10 x 109/L and in platelet count to <450 x 109/L, and was considered a complete response (CHR) when maintained for at least four weeks. Cytogenetic response is defined as follows, based on the absolute percent of Ph+ metaphase cells on marrow specimens: complete cytogenetic response (CCyR) 0% Ph+ cells; partial (PCyR)1–35%; minor 39–65%; minimal 66–95%; none 96–100%. Iterative cytogenetic analyses were performed every 3 months during therapy. Toxicities were reported prospectively using the NIH CTCv2.0 criteria.

Results. 50 children (42% boys), with a median age of 11.8 years (range 2.3–19.1) completed more than one course of therapy and were evaluable for response. Median number of courses delivered was 22.5 (range 1–43), with a median follow-up of 795 days. 96% of the calculated dose was administered. Eleven patients experienced 14 non-hematological grade 2–4 adverse events, and one patient discontinued therapy because of toxicity. The HR and CHR rates were 78% and 12%, at the end of course 1, and 20% and 78%, respectively, at the end of course 2. Only one patient was reported as a hematologic non-responder at the end of course 2. At the end of the third course, 33 patients were evaluated for cytogenetic response. Twelve (36%) children were in CCyR; 10 (30%) in PCyR; 5 in minor response; 4 in minimal response; 2 with no cytogenetic response. Six patients did not have cytogenetic evaluation; while in 11 (33%) the study was not possible due to insufficient sampling. Overall, 33 (66%) CCyRs were documented, at a median time of 5.6 months (91% documented by 9 months). Only 1 patient achieved a CCyR after course 10. Thirty-three children were removed from protocol, of which 23 underwent stem cell transplantation. One patient progressed to blast phase while on therapy, while six additional patients had cytogenetic progression. Of the 3 remaining patients, two patients had difficulty with taking medications and one had grade 4 liver toxicity. At 1 year, the estimated event free and overall survivals are 96% and 98%, respectively.

Conclusion. Imatinib is well tolerated in previously untreated children with CP CML and induces comparable rates of complete cytogenetic response to those observed in adults. Current evaluation of molecular response is being performed.

Disclosures: Supported in part by Novartis.

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