Notch-targeted agents that were initially intended for the treatment of Alzheimer disease are now being examined for their possible anticancer activity.
Aberrant Notch signaling can involve transcription factor CSL, coactivators (CoA), corepressors (CoR), MAML coactivator, and intracelluar domain (NICD).
A growing body of evidence suggests that aberrations in the Notch signaling pathway may contribute to the development of a variety of different human cancers, as well as potentially driving resistance to a number of targeted anticancer agents. Notch-targeted agents that were initially intended for the treatment of Alzheimer disease are now being examined for their possible anticancer activity. Toxicity issues have tempered success with the first generation of drugs, but a second generation is now waiting in the wings to enter clinical testing and may rejuvenate enthusiasm for this pathway.In humans, the Notch pathway is composed of four known receptors (Notch 1-4), which span the cell membrane and are activated by the binding of one of two different families of ligands, named for their homology to the Drosophila genes: delta-like and jagged. Since the ligands are also bound to cell membranes, receptor activation typically occurs through direct contact between two cells, one expressing the receptor and one expressing the ligand.
Ligand binding causes the receptor to be broken down into several pieces through the action of two different enzyme families, the α-secretase complex (primarily A disintegrin and metalloproteinase 10 [ADAM10]) and the γ-secretase complex. This process frees up a portion of the receptor that is inside the cell membrane, known as Notch intracellular domain (NICD), which then travels into the nucleus where it activates a number of different genes involved in many important processes.
Notch signaling plays a vital role in communication between cells and in determining the “fate” of cells and ensuring they assume their proper role, both in adult tissues and during embryonic development. Though a relatively simple pathway at first glance, significant levels of complexity have been uncovered in recent years, with a variety of mechanisms in place that precisely control the timing, intensity, and biological consequences of Notch signaling.As with other signaling pathways that control important cellular processes, when these pathways go awry it can often lead to the development of cancer. The first evidence of a role for Notch signaling in the development of cancer came in 1991 from patients with T-cell acute lymphoblastic leukemia; mutations in Notch1 later were observed in more than 50% of cases.
Although mutations in the component genes of the Notch signaling pathway are not always found to be directly causative of disease, they are often associated with poor prognosis and may define distinct clinical subtypes in a large number of different cancers. High levels of receptor or ligand expression have now been confirmed in pancreatic, breast, cervical, lung, and gastric cancers, among others, as well as in malignant gliomas and glioblastomas.
To add to the complexity of Notch signaling, researchers have discovered that while Notch pathway mutations have an oncogenic, tumor-promoting role in some cancers, they can have a tumor suppressive role in others, such as hepatocellular carcinoma and skin cancer. There is clearly still much to understand about the subtle nuances of Notch signaling in order to fully exploit therapies directed against it in as wide a range of cancers as possible.
Another reason for the interest in the Notch pathway is due to its role in determining the fate of stem cells. It is now widely accepted that there are populations of cancer stem cells (also dubbed tumorinitiating cells) within tumors that hold the key to their metastatic potential. Since Notch is involved in the maintenance of stem cells, drugs that target this pathway could help to eliminate cancer stem cells and prevent the recurrence of tumors that initially respond well to treatment.
Finally, the Notch signaling pathway has substantial crosstalk with other notorious signaling pathways that play a significant role in cancer, including the phosphatidylinositol-3-kinase (PI3K)/AKT pathway and the estrogen pathway. This is another driving factor behind the development of Notch-targeted cancer therapies, since combinatorial strategies could help to overcome resistance to other targeted drugs.The first Notch-targeting drugs to be developed were inhibitors of the enzyme γ-secretase in the 1990s, which work by blocking processing of the Notch receptor. The results of initial trials were disappointing, with gastrointestinal toxicities and diarrhea posing problems likely to limit dosing.
Human Notch1 (ligand binding region)
Merck’s MK-0752 and Roche’s RO4929097, are now showing promising signs of clinical activity in phase I/II clinical trials in patients with a range of different cancers and have been generally well tolerated. Experts are cautiously optimistic as optimal dosing and scheduling remains somewhat unclear. Furthermore, given the context-dependent action of components of the Notch pathway as both oncogenes and tumor suppressors, careful management of these and other drugs in development is needed in order to avoid additional cancer risks.
Other types of Notch-targeted anticancer drugs are also beginning to enter the clinic. These include monoclonal antibodies directed against the different Notch receptors and against the Notch ligand delta-like 4 (anti-DLL4), which are undergoing phase I testing.
A number of other strategies to target the Notch pathway are in their infancy but may enter the clinic in the near future. There is a monoclonal antibody targeting α-secretase in preclinical development, INCB3619 from Incyte Corporation, which is a selective ADAM inhibitor. Others include mastermind-like protein 1 (MAML1)-stapled peptide, which forms a complex with the NICD when it enters the nucleus and aids in activating the transcription of target genes. Stapled peptides are synthetic miniproteins that are locked into their active structure through the use of a hydrocarbon “staple.” They compete with the native protein and prevent it from binding. Genetic strategies such as RNAi and microRNAs are also under investigation, but will require significant technological improvements to allow for efficient delivery in humans before they are ready for the clinic.In addition to the development of new agents, research is under way to find biomarkers that could help to identify patients who would most likely respond to these drugs to maximize their efficacy. In a phase I study of MK-0752, a nine-gene microarray signature from hair follicles was identified as a potential biomarker.
Notch agents are also being combined with a variety of other targeted drugs in clinical trials to enhance the efficacy of these drugs and help to overcome resistance. For example, the inhibitor RO4929097 is being examined in combination with the Hedgehog inhibitor GDC-0449 (Genentech), in patients with metastatic carcinoma. Both the Notch and Hedgehog pathways play important roles in stem cell maintenance, providing the rationale for this combination.
This compound is currently being investigated in phase I and phase I/II studies in patients with breast, pancreatic, and a range of other advanced cancers. In a recent phase I study involving 103 patients, weekly dosing was generally well tolerated, significant inhibition of Notch signaling was observed, and signs of clinical activity were seen predominantly in patients with gliomas.
NCT00106145, NCT01098344, NCT01295632
Roche/National Cancer Institute
This agent, also known as R4733, is undergoing phase I and phase I/II clinical testing in patients with breast, prostate, and colorectal cancers, as well as melanoma, glioma, non-small cell lung cancer, sarcoma, and a range of advanced solid tumors. Recently reported results from a phase I study of 110 patients with refractory, metastatic, or locally advanced solid tumors demonstrated preliminary evidence of clinical activity in patients with colorectal cancer and metastatic melanoma, as well as prolonged stable disease in patients with several other tumor types.
NCT01208441, NCT01193868, NCT01270438,
NCT01189240, NCT01192763, NCT01154452
This agent is currently being evaluated in phase I trials in patients with advanced solid tumors and T-cell lymphoblastic leukemia.
NCT01292655, NCT01653470, NCT01363817
Pfizer's offering is also undergoing phase I testing in patients with advanced solid tumors and leukemia. NCT00878189
OncoMed Pharmaceuticals/ GlaxoSmithKline
A monoclonal antibody directed against the Notch2 and Notch3 receptors, OMP-59R5 is undergoing phase I/II testing in combination with gemcitabine in patients with previously untreated stage IV pancreatic cancer), in addition to a phase I dose escalation study in patients with solid tumors.
OncoMed Pharmaceuticals/ GlaxoSmithKline
This anti-Notch1 antibody has just recently entered clinical trials, with a phase I dose escalation study in patients with relapsed or refractory lymphoid malignancies. NCT01703572
MEDI0639 is a monoclonal antibody directed against the Notch ligand delta-like 4 (DLL4) and is being examined in phase I trials in patients with advanced solid tumors. NCT01577745
Source note: NCT numbers refer to clinical trials described at www.ClinicalTrials.gov.
Jane de Lartigue, PhD, is a freelance medical writer and editor based in Davis, California.