WT1 is well-known to be a panleukemic marker that is expressed in 90% of acute myeloid leukemias (AML). Quantification of WT gene expression in bone marrow (BM) samples may be useful as a marker of minimal residual disease (MRD) during and after treatment for early prediction of relapse. We evaluated the validity of this AML-MRD marker after reduced intensity conditioning (RIC) allogeneic stem cell transplantation (SCT). The quantitative assessment of WT1 expression by real-time quantitative PCR (RQ-PCR) was measured in 25 patients (pts) with AML at diagnosis, at the time of RIC-SCT and after transplant at precise time points. All pts showed high WT1 levels at diagnosis with a mean of 4895 (SD 4462) and a median of 3679 (range 454-16,853) copies WT1/10(4) ABL. At transplant, 18/25 pts (72%) were in complete cytologic remission (CcR) and 7/25 (28%) had refractory AML. At the pre-SCT evaluation, BM samples from pts transplanted in CcR showed significantly lower WT1 expression levels compared to the samples from pts with refractory AML (p = 0.002). Median follow-up after RIC-SCT was 18 months (range 2-54). On 18 pts transplanted in CcR, those (17/18) who maintained CcR after RIC-SCT displayed a WT1 copy number persistently low during all the follow-up period. In patients who received RIC-SCT with active disease obtaining a sustained CcR after transplant (3/25), WT1 levels decreased to normal range in the first two months after RIC-SCT and remained low through the entire study period. All pts who relapsed after RIC-SCT (4/25) had a high WT1 copy number before the cytologic relapse. In 50% of these cases, an increase in WT1 expression was documented before molecular chimerism decreasing. With this experience, taking into account the limited number of pts, we confirmed a concordance between WT1 expression levels (measured by RQ-PCR at precise and sequential time points) and status of AML before and after RIC-SCT and we found a concordance between WT1 expression levels and hematopoietic chimerism status. Our data suggest that, in the RIC-SCT setting, the sequential and quantitative analysis of WT1 may be useful as a leukemia marker for monitoring MRD and as a predictor of overt AML cytologic relapse.

4941 Since the multidrug resistance phenotype was first observed 30 years ago, other 48 proteins, belonging to an evolutionary highly conserved family of transport proteins, named ABC proteins, has been identified and grouped in eight subclasses, according to divergent evolution. In eukaryotes, ABC proteins are involved in transport across the membrane to extracellular space or into intracellular organelles playing an important role in cell physiology. Almost three distinct subfamilies (B, C and G) have been recognized to transport a wide variety of amphipatic or hydrophobic molecules, including most of anticancer drugs compounds, affecting drug absorption, disposition, metabolism, excretion and toxicity (ADMEtox) and has been associated to chemotherapy failure in solid and hematologic neoplasia. In particular the role of ABCG2 has been pinpointed over the past years for its high expression on primitive hematopoietic and on progenitors cells and for its ability to confer to the cancer cell the properties of cancer stem cell, with increased survival capacity. At present over 40 single nucleotide polymorphisms of ABCG2 protein have been identified. Among them the 421C>A (Q141K) variant has been shown able to alter protein function and to modify in vitro sensitivity to many anticancer drugs, compared to the wild type protein. In the present paper we have evaluated the incidence and the impact on therapy outcome in a series of 100 caucasic patients with acute myeloid leukemia, receiving the same chemotherapy program including idarubicin. Q141K polymorphism was detected in 19/100 (19%) patients, without correlation with any clinical-biological characteristic at diagnosis (such as sex, age, peripheral blast count, FAB subtype, karyotytpe, CD34 expression). ABCG2 protein was overexpressed in 10/19 (52.6%) of mutated patients and in 37/83 (44.5%) of patients expressing the wt form of ABCG2 and no difference in intensity of ABCG2 expression was observed in the two groups. Again no difference in ABCG2 mRNA transcription was detected between patients carrying wt or polymorphic protein. Complete remission rate was comparable in wt and mutated patients (38/81, 45,7% vs 10/19, 52,6%). However patients with Q141K polymorphism (irrespective to the intensity of expression of ABCG2) protein) had a leukemia free survival comparable to patients overxpressing wt protein, and significantly shorter than patients without wt ABCG2 overepxression (chi square 12,2, p=0,002, logrank test). In conclusion our data demonstrated Q141K polymorphism of ABCG2 transporter protein identified a subset of patients with high probability of relapse when treated with idarubicin suggesting that other drugs should be considered in consolidation/maintenance treatment to improve patients outcome. Finally the impact of Q141K polymorphism on treatment toxicity is under investigation. Work supported by PRIN 2007WEYB34-004 Disclosures: No relevant conflicts of interest to declare.