ROR1: From Developmental Regulator to Emerging Star in Cancer Therapy

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is an oncofetal surface antigen crucial for embryonic development. It exhibits extremely low expression in normal adult tissues but is aberrantly overexpressed in various hematologic malignancies and solid tumors, closely associated with tumor proliferation, invasion, metastasis, and poor prognosis. Due to its highly tumor-restricted expression profile, ROR1 has emerged as a promising next-generation target in tumor immunotherapy, with related therapies such as CAR-T cells, antibody-drug conjugates (ADCs), and bispecific antibodies now in critical stages of clinical development.

ROR1 is a member of the receptor tyrosine kinase (RTK) ROR family and is a type I single-pass transmembrane glycoprotein. Its structure includes:
Extracellular domain: Contains immunoglobulin (Ig)-like domains, a cysteine-rich Frizzled-like domain, and a Kringle domain, which mediate binding to ligands such as Wnt5a.
Transmembrane domain
Intracellular domain: Contains a tyrosine kinase-like domain (with degraded kinase activity) and a proline-rich domain, involved in downstream signal transduction.

In embryonic development: ROR1 plays a critical role in the development of organs such as the heart, skeleton, nervous system, and pancreas, primarily by binding to non-canonical Wnt (e.g., Wnt5a) and activating signaling pathways (e.g., Rho GTPases, PI3K/AKT) that regulate cell polarity, migration, and differentiation.
In adult tissues: Functional ROR1 is either absent or expressed at very low levels in the vast majority of healthy adult tissues.
In tumors: Various cancer cells reactivate ROR1 expression through mechanisms such as epigenetic reprogramming. The ROR1-Wnt5a axis then promotes:
Tumor cell proliferation and survival: Activates pro-survival signals such as PI3K/AKT, CREB, and STAT3.
Epithelial-mesenchymal transition (EMT): Enhances cancer cell invasion and migration.
Metabolic reprogramming: Supports tumor metabolic demands.
Maintenance of cancer stem cell-like properties: Linked to disease recurrence and drug resistance.

The high expression of ROR1 in multiple refractory tumors makes it a broad-spectrum potential therapeutic target.

Chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL):
Highest association: Over 95% of CLL patients exhibit high ROR1 expression in leukemia cells, making it a characteristic marker.
Clinical significance: ROR1 expression levels directly correlate with disease progression, drug resistance, and poor prognosis.
Mantle cell lymphoma (MCL): Universally overexpressed, serving as an important diagnostic and prognostic biomarker.
Acute lymphoblastic leukemia (ALL): Certain subtypes (especially Ph-like ALL) express ROR1.
Diffuse large B-cell lymphoma (DLBCL): Some subtypes express ROR1, associated with poor prognosis.

Breast cancer: Highly expressed in triple-negative and hormone receptor-positive breast cancer, promoting metastasis and drug resistance.
Lung cancer: In non-small cell lung cancer, particularly associated with EGFR-TKI resistance and poor prognosis.
Ovarian cancer: Linked to tumor metastasis and reduced patient survival.
Pancreatic cancer: Overexpressed, contributing to tumor progression.
Elevated ROR1 expression is also observed in melanoma, colorectal cancer, and other solid tumors.

The tumor-restricted expression pattern of ROR1 makes it an ideal therapeutic target, avoiding the "off-target toxicity" associated with targeting widely expressed antigens.

Mechanism: Genetically engineered patient T cells express chimeric antigen receptors targeting ROR1, enabling precise elimination of ROR1-positive tumor cells.
Advantages and challenges:
Advantages: Demonstrates preclinical and early clinical efficacy against hematologic malignancies like CLL and MCL, as well as some solid tumors.
Core challenges: Requires fine-tuning of CAR-T activity to avoid potential toxicity against normal tissues with minimal ROR1 expression (e.g., certain fibroblasts, adipocyte precursors). Additionally, overcoming the barriers of the solid tumor microenvironment remains a hurdle.

Mechanism: Highly specific anti-ROR1 monoclonal antibodies are conjugated to potent cytotoxic agents (e.g., microtubule inhibitors, DNA-damaging agents) for targeted delivery.
Representative drug: Zilovertamab vedotin (VLS-101, an ROR1-targeted ADC) has shown objective responses in clinical trials for ROR1-positive MCL, DLBCL, and triple-negative breast cancer, making it one of the most advanced ROR1-targeted therapies.

Bispecific antibodies: Simultaneously bind ROR1 and CD3 on T cells, redirecting T cells to tumor cells for cytotoxicity.
Monoclonal antibodies: Some naked antibodies exert antitumor effects by blocking ROR1 signaling or mediating antibody-dependent cellular cytotoxicity (ADCC).

Biomarker: Detection of ROR1 expression via immunohistochemistry, flow cytometry, or liquid biopsy can aid in diagnosis, risk stratification, and prognosis.
Treatment response prediction: High ROR1 expression may indicate better potential response to certain targeted therapies (e.g., specific ADCs).

Targeting safety: Despite restricted expression, further research is needed to clarify ROR1-expressing normal tissue types and their physiological functions to accurately assess potential treatment-related toxicity.
Tumor heterogeneity and antigen escape: Treatment pressure may lead to ROR1 downregulation or expansion of ROR1-negative clones, resulting in resistance.
Solid tumor delivery barriers: Infiltration and persistence of CAR-T cells in solid tumors remain unresolved challenges.

Development of novel drug formats: Next-generation ADCs, bispecific antibodies, and CAR-T products with wider therapeutic windows and enhanced penetration.
Combination strategies: Integrating ROR1-targeted therapies with chemotherapy, immune checkpoint inhibitors, or other targeted drugs to overcome resistance and improve efficacy.
Exploration of non-cancer applications: Preliminary studies suggest ROR1 may play a role in pathologies like organ fibrosis, warranting further investigation.
Precision patient selection: Developing more sensitive companion diagnostics to identify patients most likely to benefit from ROR1-targeted therapies.

ROR1 epitomizes the characteristics of an ideal therapeutic target as an "oncofetal antigen"—critical in development, silent in healthy adults, and reactivated in tumors. From hematologic malignancies like CLL to solid tumors such as breast and lung cancer, ROR1's broad yet specific expression profile transcends the limitations of traditional targets, serving as a "common vulnerability" linking diverse refractory cancers. With breakthroughs like zilovertamab vedotin among ADCs and ongoing optimization of CAR-T and bispecific antibody technologies, ROR1-targeted therapies are transitioning from concept to clinic, offering new hope for patients with limited treatment options and reshaping the landscape of cancer therapy.