Targeting TRAIL: A Long and Winding Road to Efficacy

A hallmark of cancer cells is their ability to evade apoptosis,¹ a highly regulated form of cell death. Although investigators have become heavily invested in identifying ways to reactivate apoptosis in tumors, designing drugs that rejuvenate this process while avoiding indiscriminate killing of normal and cancerous cells has proved challenging.

A major barrier to this approach is exemplified by the debilitating toxicity associated with drugs targeting the tumor necrosis factor (TNF) superfamily of proteins, because its members play a significant role in apoptosis.²

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Now pharmaceutical companies are developing novel platforms to yield a new generation of drugs,⁷ with clinical trials ongoing across a range of solid and hematological malignancies (Table). Meanwhile, results from a first-in-human study into ABBV-621, a TRAIL receptor agonist, highlight the potential of focusing on TRAIL.^8-10

As development of TRAIL-targeting drugs moves forward, many questions about the signaling network and its function in cancer apoptosis continue to be unraveled. These include the nature of intrinsic and acquired resistance to TRAIL-targeted drugs.

Despite these hurdles, there is great optimism for this class of drugs as the TRAIL pathway comes of age.

The Apoptosis Connection

Apoptosis leads to clearance of unwanted or damaged cells without breaching the plasma membrane or releasing potentially damaging cellular contents.¹¹ Broadly speaking, 2 major pathways induce apoptosis. The extrinsic, or death receptor, and the intrinsic, or mitochondrial, pathways, as their names suggest, are triggered by extra- and intracellular cues, respectively.^3,11

The intrinsic pathway is predominantly triggered by the tumor suppressor p53 in response to cellular stress, whereas the extrinsic pathway is coordinated by the death receptor (DR) family. DRs and the ligands that activate them belong to the TNF superfamily, a group of structurally related transmembrane proteins.³

Among the key players is TRAIL, which serves as a ligand for 5 receptors: the membrane-bound receptors TRAIL-R1, -R2, -R3, and -R4 and the soluble receptor osteoprotegerin (OPG). TRAIL-R1 and -R2 are also known as DR4 and DR5, and they contain characteristic death domains through which they trigger apoptosis.^3,5,12

DCR1 and DCR2, the alternative designations for TRAIL-R3 and -R4, are thought to act as decoy receptors, competing for ligand binding and negatively regulating apoptosis. DCR1 completely lacks a death domain, whereas in DCR2, the domain is truncated and nonfunctional. OPG contains a death domain, but it binds to TRAIL with a much weaker affinity than the other 4 receptors and is thought to be largely unable to trigger apoptosis.^3,6

TRAIL is a trimeric protein found on the surface of a variety of cell types, including activated immune cells. It can bind 3 receptor molecules simultaneously, facilitating clustering of DRs that is critical for full activation of apoptosis. Clustering brings the DRs’ death domains together to form an assembly platform for other death domain—containing proteins, such as the adaptor protein Fas-associated death domain (FADD).^2,5

FADD also contains a death effector domain, through which it subsequently binds to procaspase-8 and -10. Together, TRAIL-R1 and -R2, FADD, and procaspase-8/10 form the death-inducing signaling complex (DISC) that promotes activation of caspase-8 and -10, which are released into the cytosol, where they activate the executioner caspases that drive apoptotic breakdown of the cell.^3,5,12,13

In some cells, dubbed type II cells, the formation of DISC and activation of caspase-8/10 is insufficient to induce apoptosis; signal amplification via mitochondria is required. Cross-signaling through the BCL-2 pathway activates the mitochondrial apoptotic machinery, and cell death induction is completed through the intrinsic pathway.^3,5,6 (Figure⁶).

Jekyll and Hyde?

The ability to evade apoptosis and grow unchecked amid oncogenic stresses is one of the hallmark abilities that allow the malignant transformation of a normal cell.¹ Targeting the central regulators of apoptotic signaling to restore the balance in cancer cells has long been recognized as an attractive therapeutic strategy.

The extrinsic pathway is a particularly promising target because it activates apoptosis independently of the p53 protein. The gene encoding this protein is the most frequently mutated across all cancer types; thus, many cancer cells are inherently resistant to p53-induced apoptosis.^4,6,11

Attempts to target components of the extrinsic pathway have proved problematic, however. TNF superfamily proteins are logical targets, but therapeutic development was tempered by the significant toxicity associated with drugs targeting TNFα, CD95, and FAS, resulting from their broad expression across normal and cancerous cells.^2,3

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TRAIL-R1 and -R2 have been reported to be frequently highly expressed in a number of cancer types, including pancreatic, colon, breast, bladder, head and neck, cervical, and ovarian, in addition to non—small cell lung cancer (NSCLC) and glioblastoma.^2,12 In some cancer types, TRAIL-R1 has been described as the major inducer of apoptosis, whereas in others, TRAIL-R2 appears to play this role.¹²

The reason for the selective proapoptotic activity of TRAIL in cancer cells is not fully understood, but it may be due to higher expression of decoy receptors, which counteract the activity of TRAIL, on normal cells compared with tumor cells, where their expression is rare.^2,6,17

Targeting TRAIL

Two types of TRAIL-targeting drugs have been developed: recombinant forms of the TRAIL ligand and agonistic antibodies targeting the TRAIL-R1 and -R2 receptors.

Dulanermin (Amgen/Genentech), a recombinant protein, consists of an untagged 167-amino acid stretch of the extracellular portion of TRAIL that binds to both TRAIL-R1 and -R2.^3,14 It induced cancer cell death in preclinical models and progressed to clinical trials, where it was tested as monotherapy and combined with chemotherapy in patients with advanced NSCLC, non-Hodgkin lymphoma, colorectal cancer (CRC), and B-cell lymphomas. Although dulanermin was generally well tolerated, its antitumor efficacy was limited.^3,5

A randomized phase III trial of dulanermin in combination with vinorelbine and cisplatin as first-line therapy in patients with advanced NSCLC was carried out in China. The objective response rate with dulanermin was 46.8% compared with 30.0% in the placebo arm. Although progression-free survival (PFS) was significantly longer in the dulanermin arm, overall survival was not.¹⁸

The efficacy of dulanermin is thought to have been impeded by its short half-life in the circulation. Several forms of recombinant TRAIL with improved half-life have been evaluated in clinical trials, with limited success.^3-5 A possible exception is circularly permuted TRAIL (CPT), which is being developed by Beijing Sunbio Biotech Co Ltd.^3,6

Circular permutation involves joining the current ends of a protein with a linker and then cutting the protein elsewhere to introduce new ends, thus reorganizing the order of the amino acids.¹⁹ CPT reportedly has greater stability and a longer half-life compared with dulanermin.³ It has been tested in several phase II trials in patients with multiple myeloma.

Among 27 patients treated with CPT monotherapy, the overall response rate (ORR) was 33.3%, with 1 near complete response (CR) and 8 partial responses (PRs).²⁰ In a phase II study, the ORR was 38.3% with the addition of CPT to thalidomide plus dexamethasone compared with 25.0% for dexamethasone and thalidomide alone, and the median PFS was 6.7 months compared with 3.1 months.²¹ The Chinese Clinical Trial Registry lists a phase III clinical trial, but it is unclear whether the study is ongoing.²²

The first generation of agonistic antibodies predominantly targeted TRAIL-R2 and included conatumumab, lexatumumab, tigatuzumab, drozitumab, and LBY-135. One TRAIL-R1-targeting antibody, mapatumumab, was also developed.

As with dulanermin, despite positive indications in preclinical testing, none of these antibodies displayed significant anticancer activity in clinical trials. A small percentage of patients did respond, suggesting that predictive biomarkers could be useful for identifying patient populations who benefit. Although candidate biomarkers have shown promise in cultured cells, none have been confirmed to predict response in patients.^3,23

Continuing the Pursuit

ABBV-621

A significant impediment of the first generation of drugs directed at this pathway was their limited capacity to induce TRAIL receptor clustering, which is vital to optimal induction of apoptosis and may have resulted in inadequate activation. Pharmaceutical companies have subsequently focused on drug development strategies to address this.

Investigators recently reported first-in-human clinical trial results for ABBV-621, a fusion protein composed of TRAIL receptor binding domains linked to a human immunoglobulin G1 Fc domain.

ABBV-621 consists of 3 receptor binding domains linked together in a single chain to mimic the natural TRAIL ligand and its ability to simultaneously bind 3 receptor molecules. Fusing the chain to an Fc domain leads to formation of a hexavalent drug when the Fc portions dimerize, yielding a molecule that can cluster multiple receptors without relying on cross-linking with Fc gamma receptors.⁷

ABBV-621 is being evaluated in an ongoing, phase I, multiarm trial (NCT03082209) as monotherapy and in combinations for select hematologic and solid malignancies. In the dose-escalation portion of the study, 57 patients with previously treated solid tumors received ABBV-621 as monotherapy; the maximum tolerated dose was not reached.⁹

Additional patients with similar characteristics were enrolled in further dose-optimization cohorts; as of April 2019, 48 patients in this study group had received ABBV-621 at a dose of 1.25 mg/kg, 3.75 mg/ kg, or 7.5 mg/kg. PRs were observed in 1 patient with CRC at the 1.25 mg/kg dose and in 1 with pancreatic cancer at the 3.75 mg/kg dose, and stable disease (SD) at 12 weeks was achieved in 20 patients (9 with CRC and 11 with pancreatic cancer).

Dose-limiting toxicities (DLTs) were experienced by 9.3% of patients. The most common any-grade treatment-related adverse events (AEs) included fatigue, increased alanine aminotransferase, stomatitis, increased aspartate aminotransferase, decreased appetite, diarrhea, nausea, vomiting, dysgeusia, and pyrexia. According to the investigators’ conclusions, all tested doses of ABBV-621 showed evidence of efficacy and acceptable safety.¹⁰

In another arm of the study, 17 patients with relapsed/refractory (R/R) acute myeloid leukemia (AML) or diffuse large B-cell lymphoma (DLBCL) received ABBV- 621 alone or with venetoclax (Venclexta), a BCL-2 inhibitor. Part of the rationale behind this drug combination was the prospect of inducing apoptosis through both the extrinsic (TRAIL-activated) and intrinsic (BCL-2regulated) pathways. The combination of ABBV-621 (3.75 mg/kg) and venetoclax led to CR in 1 patient with AML and SD in 1 with DLBCL. One patient with AML receiving the drug combination experienced 3 DLTs, and 16 across the study experienced AEs. The authors concluded that the combination therapy was tolerable and showed antitumor activity in patients with R/R AML.⁸

ONC201

Several other drugs are in development, including ONC201, a small molecule that was identified in a screen for compounds that could induce transcription of the TRAIL gene in colon cancer cells, regardless of p53 status. ONC201 binds to the dopamine receptors DRD2 and DRD3 and inhibits AKT and ERK.²⁴

ONC201 has shown some promise in clinical trials in patients with recurrent glioblastoma,²⁵ and numerous clinical trials are ongoing in high-grade gliomas, neuroendocrine tumors, and other solid and hematologic cancers. However, it is unclear how much TRAIL induction contributes to its anticancer mechanism of action. Recent study findings have revealed that ONC201 works by impairing cancer cells’ mitochondrial function via activation of ClpP, a mitochondrial protease.^26,27

In recent years, the relationship of TRAIL with cancer has been revealed to be increasingly complicated. TRAIL has numerous nonapoptotic functions within the cell—a prime example is activation of inflammatory NFκB signaling—that can have protumorigenic effects. Thus, there may be a duality of TRAIL pathway function in some cancers and a potential double-edged sword for TRAILtargeted therapies.^28-31

Jane de Lartigue, PhD, is a freelance medical writer and editor based in Gainesville, Florida.

The TRAIL ligand and its associated receptors have been on the radar for over 2 decades, thanks to TRAIL’s ability to selectively induce apoptosis in cancer cells, independently of p53 status, as well as demonstrations that recombinant human TRAIL could induce tumor regression in xenografts.^14-16

Nevertheless, 1 member of the TNF superfamily stands out for its unique ability to selectively induce apoptosis in cancer cells: TNF-related apoptosis-inducing ligand (TRAIL). Yet, although several types of agents targeting TRAIL were generally well tolerated in clinical trials, not a single drug aimed at this pathway has gained the FDA’s approval since its discovery more than 2 decades ago.^3-6

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