Multidrug Resistance Proteins Overexpression Worsen the Favorable Clinical Impact of an Elevated Spontaneous Apoptosis in Acute Myeloid Leukemia (AML).

Multidrug resistance (MDR) proteins (P-glycoprotein [PGP], lung resistance protein [LRP], multidrug related protein [MRP]) and apoptosis proteins (bcl-2, bcl-x_l, bax) expression represent key mechanisms explaining the high rate of treatment failure in AML, as demonstrated also by gene expression profiling studies (Wilson CS et al, 2006). Therefore, from 1995 to 2006, a large series of 386 AML de novo pts, median age 58 years, treated with intensive chemotherapy regimens, was tested. The principal aims of our study were:

1. to demonstrate that bax/bcl-2 ratio and MDR are critical and independent events; and

2. to clarify whether MDR is able to modify the favorable outcome of pts with high spontaneous apoptosis.

PGP, LRP, MRP, bcl-2 and bax proteins were determined by multicolor flow cytometry. Concurrent positivity for PGP, LRP and MRP defined the MDR positive subset (n=112). There were significant correlations between immature FAB classes (M0–M1) and lower bax/bcl-2 ratio (83/128; p<0.00001) or monocytic differentiation (M4-M5) and higher PGP expression (133/163; p<0.00001). A significant correlation was found between a higher PGP and a higher bax/bcl-2 ratio (153/224; p=0.02). MRP was more represented in pts with lower apoptosis (103/162; p=0.00005). A significant lower complete remission (CR) rate was found in pts with lower bax/bcl-2 ratio (41% vs 70%, p<0.00001) or higher MDR (35 % vs 74%, p<0.00001). Overall survival (OS) and disease free survival (DFS) were significantly shorter either in pts with lower bax/bcl-2 ratio (0% vs 20% at 3.5 years, p<0.00001; 0% vs 17% at 2.7 years; p=0.0001) or higher MDR (0% vs 27% at 2.5 years, p<0.00001; 5% vs 36% at 1.3 years; p=0.0001). Bax/bcl-2 ratio and MDR showed an additive prognostic impact, since higher bax/bcl-2 ratio plus lower MDR identified pts at better prognosis with regard to CR (89% vs 27%; p<0.00001), OS (41% vs 0% at 2.5 years; p<0.00001) and DFS (42% vs 0% at 1.2 years; p=0.0009). Noteworthy, also PGP, LRP and MRP showed an additive prognostic impact with regard to OS, since PGP+LRP+MRP+ pts (n=112) identified an AML subset (MDR) at worse outcome as compared to PGP+ or LRP+ or MRP+ alone (0% vs 7% or vs 6% or vs 3% at 2.5 years, respectively). In order to establish whether MDR overexpression is able to worsen the favorable clinical outcome of AML pts with elevated apoptosis levels, we analyzed the subgroup with higher bax/bcl-2 ratio (n=224). A lower CR rate was found in patients with higher MDR expression (47% vs 89%, p=0.0005). Equally, higher MDR expression was associated both with a shorter OS and DFS (5% vs 41% at 1.5 years, p<0.00001; 10% vs 42% at 1.3 years, p=0.003). The superior and independent prognostic value of bax/bcl-2 ratio over MDR proteins was confirmed in multivariate analysis with regard to CR (bax/bcl-2: p=0.0001; PGP: p=0.004), OS (bax/bcl-2: p=0.0001; LRP: p=0.02) and DFS (bax/bcl-2: p=0.0004; PGP: p=0.03; LRP: p=0.03). Therefore, low apoptosis and higher MDR in elevated apoptosis identify the two main different biologic AML subsets at worse prognosis. In fact, it has to be taken in account that the favorable prognostic impact of a higher bax/bcl-2 ratio may be greatly reduced by MDR positivity. Actually, we conclude that we have to focus future research and therapeutic strategies targeting in the first place apoptosis and then MDR proteins in AML.