Development of Small Molecules Activating TRAIL Apoptosis Pathway for Cancer Therapies

It has been thirty-eight years since tumor necrosis factor- (TNF) was isolated and found with cancer therapeutic potential in 1975 [1]; however, all the efforts have failed in the development of TNF and its family ligands as cancer drugs [2] first due to the lethal toxicity of TNF and Fas ligand (FasL) [3,4]. In the middle 1990s, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL; Apo2 ligand, Apo2L) was identified as a member of the TNF ligand family [5,6] that can induce apoptosis in cancer cells without the toxicity as caused by TNF and FasL [7,8], leading to renewed efforts in development of this TNF family ligand as a cancer therapeutic. TRAIL is a type II membrane protein that is normally expressed on the surface of immune cells in execution of innate and adaptive immunity against cancers in immunosurveillance [9]. TRAIL has a short intracellular N-terminal tail and a long extracellular C-terminal with the receptor-binding domain [10]; thus, recombinant human TRAIL (rhTRAIL) of the C-terminal has been generated as a cancer drug (rhApoL/TRAIL; dulanermin). Phase I trials approved the safety of rhTRAIL in patients [11,12]; however, phase II trial revealed the drug resistance of human solid cancers [13]. TRAIL-induced apoptosis occurs through the binding of its cognate receptors, TRAIL receptor 1 (TRAIL-R1; death receptor 4, DR4) and TRAIL-R2 (DR5) on the surface of cancer cells; DR4 and DR5 become trimerized and recruit intracellular Fas-associate death domain (FADD) and apoptosis-initiating caspase-8 in assembly of a death-inducing signaling complex (DISC), where caspase-8 is dimerized, cleaved and activated in the initiation of the extrinsic apoptotic pathway [2]. In addition, caspase-8 enzymes cleave the Bid and activate the intrinsic mitochondrial pathway with the release of the apoptotic proteins including the second mitochondrial-derived activator of caspases (Smac). To target DR5, a mouse monoclonal antibody against human DR5 (TRA-8) was generated for antitumor activity but no hepatotoxicity [14]. This DR5 agonistic mouse monoclonal antibody was humanized as a therapeutic (CS-1008; tigatuzumab) [15] and passed the safety evaluation in a phase I trial [16]. Advances in antibody technology led to the generation of fully human monoclonal antibodies against DR4 and DR5 for activation of TRAIL apoptotic pathway in cancer cells [17]. Phase I trials evaluated the safety of the DR4 agonistic antibody HGS-ETR1 (mapatumumab), the DR5 agonistic antibodies HGS-ERT2 (lexatumumab) and PRO95780 (drozitumab) [18-20]. Once again, however, phase II trials failed to show clinical anticancer activity of HGS-ETR1 [21,22] and PRO95780, as reported at the 2010 Annual Meeting of the American Society of Clinical Oncology. Cleary, the drug resistance now is the roadblock in development of TRAIL agonists as cancer drugs. TRAIL is a natural cancer killer in immunosurveillance [9]; however, this notion also predicts that cancer occurs in patients by evasion of TRAIL-mediated immunity; thus, the drug resistance is the inherent nature of cancer. Cancer is a genetic disease and genetic alterations lead to its relentless growth through activation of growth pathways such as phosphatidylinositide-3-kinase (PI3K), AKT and extracellular signal-regulated kinase (ERK) on the one hand and inhibition of cell death pathways on the other hand; thus, current efforts are focused on the development of combination therapies that can block cell growth and activate cell death pathways in cancer cells [2]. From the pharmacokinetic point of view, however, rhTRAIL protein and DR4/DR5 agonistic antibodies as drugs suffer from short half-life and poor tissues distribution. To overcome these limits of protein-drugs, researchers have turned their attention to high-throughput drug screening for small molecules that can activate TRAIL apoptotic pathway in cancer cells. Biochemical and cell-based assays are the two mainstays in high-through drug screening; yet, cell-based assays are used more now because they represent specific responses of targeted cells. Published in the February