Overcoming drug resistance: targeting more than one site.

Activation of the molecular machinery of programmed cell death (apoptosis) is a convergence point for many cytotoxic agents, irrespective of the primary mechanism of action. Indeed, apoptosis is induced in response to DNA strand breaks caused by such mechanistically diverse anti-leukemic drugs as the nucleoside cytosine arabinoside (ara-C), anthracyclines (daunorubicin, idarubicin), and agents directed against topoisomerase I (camptothecins) and II (etoposide, mitoxantrone). The identification of apoptosis on both the molecular and cellular levels has led to the finding that the mitochondrion plays a pivotal role in governing the decision and execution of the apoptotic process (reviewed in [1]). It is now clear that the mitochondrion acts as a repository for diverse proand anti-apoptotic proteins, many of which are located in mitochondrial membranes at the interface between the organelle and the cytosol [2,3]. In the response to cellular stress, such as from genotoxic anti-cancer agents, changing the mitochondrial membrane potential and pore size releases mitochondrial factors including cytochrome c and apoptosis inducing factor (AIF). These factors, in turn, lead to the enhanced caspase activity that results in cleavage of critical cellular substrates [1–3]. This pro-apoptotic pathway is kept in check by two protein groups— namely, the BCL-2 protein family in the mitochondrion and inhibitor of apoptosis (IAP) family in the cytosol. The balance between these proand anti-apoptotic components influences the ultimate cellular decision between life and death. The development of new anti-cancer agents is based on the premise that therapeutic ratios can be favorably altered by understanding and exploiting differences between normal and malignant cells. In this regard, mitochondrial membrane components seem like ripe targets to exploit for the purpose of enhancing the cell’s ‘stress response’ to chemotherapy. Of particular interest, are components that determine membrane pore size and permeability—the so-called mitochondrial permeability pore transition complex (PTPC) that determines whether or not cytochrome c will be released in response to a potentially cytotoxic stimulus [5]. One such target for manipulation is the peripheral benzodiazepine receptor (pBzR) [6], an outer mitochondrial membrane protein that is a PTPC constituent and acts similarly to BCL-2 and BCL-XL by interacting with the pore-forming protein voltage-dependent anion channel (VDAC) in a way that keeps the channel closed, thereby blocking mitochondrial permeabilization and abrogating apoptosis [3–6]. Importantly, pBzRs are expressed in both normal and leukemia hematopoietic cells and can protect those cells from undergoing apoptosis in response to oxidative stress [7]. While the apoptotic machinery is the final determinant of stress-induced programmed cell death, malignant cells call into action multiple factors from the cell surface to the genome that confer an uncanny ability to survive in an adverse and stressful environment. One such factor is the multidrug resistance protein known as MDR-1 or P-glycoprotein (PgP), a membrane transport pump located at the cell surface that promotes efflux of exogenous toxins and prevents intracellular accumulation at high-enough concentrations or for long-enough periods of time to inflict lethal damage (reviewed in [8]). Tumor cell MDR-1 expression has been linked to clinical drug resistance and poor clinical outcome in diverse malignancies, including acute myelogenous leukemias (AMLs). The fact that anti-AML anthracyclines and etoposide are MDR-1 substrates and the availability of agents that inhibit MDR-1 activity have led to numerous trials of MDR-1 inhibitors in AML, particularly in * Corresponding author.