Delta-like protein 3 (DLL3) is a transmembrane protein belonging to the Delta/Serrate/Lag2 (DSL) family of ligands, typically involved in the Notch signaling pathway. Unlike most family members, DLL3 is considered an atypical Notch ligand due to its distinct functional characteristics. Its selective expression in disease states, contrasting with its minimal presence in most healthy adult tissues, makes it a compelling molecular target in medicine.
Defining the Biological Function
DLL3 primarily regulates the Notch signaling pathway, a cell-to-cell communication system that dictates cell fate, differentiation, and proliferation throughout development. Typically, a Notch ligand on one cell activates a receptor on an adjacent cell, triggering a cascade that alters gene expression. The function of DLL3 is unique because it generally acts as a powerful inhibitor of the pathway.
Instead of being presented on the cell surface, DLL3 is often retained inside the cell, primarily within the Golgi apparatus. Here, it interacts with the Notch receptor before the receptor can reach the cell surface, promoting its degradation and preventing signaling. This mechanism is known as cis-inhibition, where the ligand inhibits the receptor within the same cell.
This inhibitory role is important during embryonic development, specifically in the formation of somites, blocks of mesoderm tissue that give rise to the vertebrae, ribs, and skeletal muscles. The precise segmentation of these structures is controlled by Notch signaling, and DLL3 helps ensure the correct temporal and spatial organization of these boundaries.
The Link to Cancer and Genetic Disorders
Mutations in the \(DLL3\) gene are responsible for the rare congenital disorder Spondylocostal Dysostosis (SCD). This condition is characterized by severe skeletal malformations, including fused or missing vertebrae and ribs. The inherited mutation disrupts the precise control of the Notch pathway needed for proper segmentation, leading to defects in the vertebral column.
DLL3 gains medical prominence due to its aberrant and high expression in various cancers, most notably Small Cell Lung Cancer (SCLC). SCLC is an aggressive neuroendocrine tumor, and approximately 80% to 85% of these tumors exhibit elevated levels of DLL3 on the cancer cell surface. This tumor-restricted expression pattern makes DLL3 an attractive target for cancer therapy.
DLL3 overexpression is often regulated by the transcription factor ASCL1, associated with the neuroendocrine phenotype of the tumor. DLL3 expression suppresses Notch signaling, which promotes the growth and survival of SCLC cells. The high prevalence and selectivity of DLL3 establish it as a molecular marker for SCLC and other neuroendocrine carcinomas.
Therapeutic Strategies Against DLL3
The selective expression of DLL3 on cancer cells has enabled the development of highly targeted therapeutic strategies. The primary approach involves Antibody-Drug Conjugates (ADCs), which function like guided missiles designed to destroy DLL3-expressing tumor cells.
An ADC consists of three components:
- A monoclonal antibody that recognizes and binds specifically to the DLL3 protein.
- A potent cytotoxic chemical payload.
- A chemical linker that connects the two components.
Once the antibody binds to DLL3, the entire ADC is internalized through endocytosis. The ADC is then trafficked to the lysosome, where the linker is cleaved, releasing the cytotoxic drug payload inside the cell. This toxic drug disrupts the cell’s function, leading to programmed cell death (apoptosis). This mechanism maximizes drug concentration at the tumor site while minimizing damage to healthy tissues that lack DLL3.
Other advanced approaches are being explored, including Bispecific T-cell Engagers (BiTEs). These specialized antibody constructs simultaneously bind to DLL3 on the cancer cell and to the CD3 protein on the patient’s T-cells. This dual binding redirects the T-cell to the tumor cell, initiating a localized immune response to destroy the malignancy.
Chimeric Antigen Receptor (CAR) T-cell therapies are also under investigation. In this approach, the patient’s T-cells are genetically modified to express a synthetic receptor that specifically targets DLL3, further diversifying the treatment landscape.
Clinical Trials and Next Steps
The clinical landscape for DLL3-targeting agents began with the pioneering ADC, rovalpituzumab tesirine (Rova-T). Rova-T showed encouraging anti-tumor activity in early-phase trials for SCLC, particularly in patients with high DLL3 expression. However, subsequent Phase III trials revealed limitations in efficacy and a less favorable safety profile, leading to the discontinuation of its development.
The challenges encountered with Rova-T provided valuable lessons for the next generation of therapies, leading to a shift in focus toward bispecific T-cell engagers. Tarlatamab, a bispecific T-cell engager, has demonstrated significant clinical activity in relapsed or refractory SCLC and has received accelerated approval by the U.S. Food and Drug Administration (FDA).
Ongoing clinical trials are exploring the potential of DLL3-targeting agents in various stages and combinations. Studies are investigating tarlatamab as a first-line treatment combined with chemotherapy and immunotherapy for extensive-stage SCLC, and as an adjuvant therapy following chemoradiation for limited-stage disease. The future includes the continued development of newer-generation ADCs, CAR-T, and CAR-NK therapies, all aimed at improving efficacy and safety.
Optimizing patient selection remains a primary focus, as identifying patients whose tumors have high DLL3 expression is a key factor for maximizing treatment success. The investigation of DLL3 as a predictive biomarker, alongside the exploration of combination therapies, holds the potential to significantly improve outcomes for patients with neuroendocrine cancers like SCLC.