Updated Long-Term Results of a Randomized Comparison of Prophylactic and Pre-Emptive Imatinib Following Allogeneic Stem Cell Transplantation for Philadelphia Chromosome Positive Acute Lymphoblastic Leukemia (Ph+ALL)

Abstract 247 Background: The presence of minimal residual disease (MRD) after allogeneic stem cell transplantation (SCT) for Ph+ ALL is highly predictive of eventual relapse. Imatinib (IM) has very limited efficacy in hematologic relapse of Ph+ALL, but may prevent leukemia recurrence if started when the leukemia burden is still very low and detectable only by molecular techniques. The optimal time for starting IM post transplant and the prognostic relevance of different bcr-abl transcript levels in relation to time after SCT have not been established. Aims: To determine the impact of post-transplant IM, given either prophylactically or after detection of bcr-abl transcripts (pre-emptively), on the overall incidence of MRD, remission duration, long-term treatment outcome and tolerability in pts. who underwent SCT for Ph+ALL in complete remission. Study Design: In a prospective, randomized multicenter trial, previously transplanted Ph+ ALL pts. (n=55) were assigned to receive imatinib prophylactically (n=26) or pre-emptively (n=29). SCT was performed in CR1 in 23 pts. and 27 pts. in the two groups, respectively. Five pts.were transplanted in CR2. Serial assessment of bcr-abl transcripts was performed by quantitative RT-PCR and additionally by nested-RT-PCR if the sensitivity of the qRT-PCR was below the quantitative range. Confirmatory testing of a second independent sample was not required, to reduce the risk of treatment delays. Samples were considered PCR negative only if the ABL copy number exceeded 104. Imatinib administration was scheduled for one year of continuous PCR negativity. Results: IM was started in 24/26 pts. allocated to prophylactic IM and in 14/29 pts. in the pre-emptive arm. The majority of pts. received IM 400 mg/d (26/38 pts.), the other 12 pts. 600 mg IM daily. IM was started a median of 48 d after SCT in the prophylactic arm and 70 d after SCT with pre-emptive therapy. After a median follow-up of 30 mos. and 32 mos., respectively, 82% and 78% of pts. are alive in ongoing CR, 4 pts. died in CR. Five pts. transplanted in CR1 and 2/5 pts. transplanted in CR2 have relapsed (median follow-up 9 mos. and 10.5 mos., respectively). The frequency of MRD positivity was significantly lower in pts. assigned to prophylactic imatinib (10/26; 40%) than those in the pre-emptive treatment arm (20/29; 69%) (p=0.046 by chi2 test). Only 9 of 29 pts. assigned to pre-emptive imatinib remained continuously PCR negative after SCT, with a median follow-up of 32 months (18–46 months) after SCT. The median duration of sustained, uninterrupted PCR negativity after SCT is 26.5 months with prophylactic and 6.8 months with pre-emptive administration of imatinib (p=0.065). The probability of remaing in CHR after SCT was significantly lower in partients who remained MRD negative after SCT (p=0.0002). Analysis of the kinetics of molecular relapse showed that detection of bcr-abl transcripts within 100 days of transplant, despite rapid initiation of IM, was associated with a significantly inferior EFS compared to first detection of MRD positivity more than 100 days after SCT. IM was discontinued prematurely in 54% pts. receiving imatinib prophylactically and in 64% of pts. receiving imatinib pre-emptively, mostly due to gastrointestinal toxicity. Accordingly, the time to IM discontinuation was 245 d and 191 d in the prophylactic and the pre-emptive treatment arms, respectively. Despite this early discontinuation rate, overall survival in the two treatment groups was 80% and 74.5% after 5 years, with no significant difference by log rank test (p=0.84). Conclusions: Prophylactic administration of imatinib significantly reduces the incidence of molecular relapse after SCT. Both interventional strategies are associated with a low rate of hematologic relapse, durable remissions and excellent long-term outcome in patients with Ph+ ALL. The presence of MRD both prior to and early after SCT identifies a small subset of patients with a poor prognosis despite post-transplant imatinib, and warrants testing of alternative approaches to prevent hematologic relapse. Disclosures: Schuld: Novartis: Employment. Goekbuget:Micromet: Consultancy. Ottmann:Novartis Corporation: Consultancy; Bristol-Myers Squibb: Consultancy, Research Funding.