P-glycoprotein (PGP)-mediated multidrug resistance (MDR) and anti-apoptotic proteins overexpression represent key mechanisms explaining the high rate of treatment failure in AML. Expression profiling studies have identified genes involved in drug resistance (Vey N, 2004) and in apoptosis block (Kern W, 2004) as discriminator genes for a poor prognosis. The availability of third-generation PGP inhibitors and bcl-2 targeting drugs prompted us to evaluate the impact of a MDR phenotype and apoptosis on AML prognosis. Therefore, a large series of 355 de novo AML pts, except FABM3, treated with intensive chemotherapy regimens, was tested. The principal aims of our study were: 1) to correlate spontaneous apoptosis (bax/bcl-2 ratio) with a MDR phenotype (PGP) and 2) to demonstrate that apoptosis and MDR are clinically relevant and independent events. PGP, bcl-2 and bax proteins were determined by multicolor flow cytometry. Two hundred-three pts (57%) were bax/bcl-2 ratio positive (>0.3) and 229/355 (64.5%) were PGP positive (>20%). There was a close correlation between MDR positivity and higher CD71 (167/225; p<0.00001), showing that a MDR phenotype is strictly linked to an increased proliferation. Only a slight correlation was found between a higher bax/bcl-2 ratio and a higher PGP (141/229; p=0.025). A lower complete remission (CR) rate was found in pts with lower bax/bcl-2 ratio (42% vs 71%, p<0.00001) or higher PGP (49% vs 71%, p=0.0003). Overall survival (OS) was shorter either in pts with lower bax/bcl-2 ratio (0% vs 14% at 3.5 years; p<0.00001) or higher PGP (2% vs 13% at 7 years; p=0.0008). A longer disease free survival (DFS) was observed either in pts with higher bax/bcl-2 ratio (10% vs 0% at 2.7 years; p=0.0002) or lower PGP (12% vs 0% at 3.9 years; p=0.002). Bax/bcl-2 ratio and PGP showed additive prognostic properties, since higher bax/bcl-2 ratio plus lower PGP identified a subset at better prognosis with regard to CR (85% vs 33%; p<0.00001), OS (31% vs 0% at 2.4 years; p<0.00001) and DFS (50% vs 0% at 1.2 years; p=0.00002). In order to establish the independent prognostic value of bax/bcl-2 ratio from PGP, we investigated MDR + (229 pts) and MDR - (126 pts) subgroups. As a matter of fact, a lower CR rate was found in pts with lower bax/bcl-2 ratio either within MDR+ (32% vs 65%, p=0.00002) or within MDR- subset (56% vs 87%, p=0.0003). Lower bax/bcl-2 ratio was associated both with a shorter OS and DFS in MDR+ (0% vs 7% at 2.4 years, p=0.005; 0% vs 13% at 1.2 years, p=0.01) and, more significantly, in MDR- (0% vs 30% at 3.5 years, p<0.00001; 0% vs 31% at 2.7 years, p=0.0008) pts. On the contrary, PGP was not able to distinguish pts at different prognosis within the lower bax/bcl-2 ratio subgroup. The independent prognostic value of bax/bcl-2 ratio was confirmed in multivariate analysis with regard to CR (p=0.00002), OS (p=0.000007) and DFS (p=0.00001). In conclusion, the lack of an important correlation between bax/bcl-2 ratio and MDR confirms the independence of apoptosis from MDR. The capacity of bax/bcl-2 ratio of clearly identifying pts at different prognosis within the MDR+ and MDR- subgroups implies that apoptosis has an intrinsic more relevant clinical significance. That has to be considered focusing future strategies on apoptosis inducers and effectors in order to improve outcome in AML (Reed JC, 2005).