ROR1: A Therapeutic Target in Cancer Treatment

ROR1 is a surface protein found on many types of cancer cells but virtually absent from healthy adult tissues, making it one of the more promising therapeutic targets to emerge in oncology over the past decade. It was originally identified for its role in embryonic development, where it helps guide cell migration and organ formation. After birth, ROR1 expression essentially shuts off in most tissues. But a wide range of cancers reactivate it, and when they do, ROR1 does more than just sit on the cell surface. It actively drives tumor growth, promotes the spread of cancer to distant organs, and helps maintain the population of cancer stem cells that resist conventional therapy. Several drugs aimed at ROR1 are now in clinical trials, spanning antibody-drug conjugates, CAR-T cell therapies, bispecific antibodies, and small-molecule inhibitors.

What ROR1 Actually Does

ROR1 belongs to a family of receptor tyrosine kinases, but calling it an “orphan” receptor reflects the fact that for years researchers did not know what molecule activated it. That mystery has been largely solved: ROR1 serves as a receptor for Wnt5a and related Wnt proteins, which are signaling molecules that guide how cells move, orient themselves, and organize into tissues during fetal development.¹ During embryogenesis, ROR1 helps regulate cell polarity, neural patterning, and organogenesis. Mice engineered to lack ROR1 are born with skeletal abnormalities and show stunted growth after birth, underscoring the receptor’s importance in normal development.²

After development is complete, ROR1 is expressed by virtually no normal adult tissues.³ That distinction is critical. Most proteins that help tumors grow are also present on some healthy cells, which limits how aggressively you can target them without harming the patient. ROR1’s near-absence in adults creates a wider therapeutic window than many other cancer targets offer.

Which Cancers Express ROR1

The list of cancers where ROR1 has been detected is long and growing. In blood cancers, the strongest and most consistent expression occurs in chronic lymphocytic leukemia (CLL) and hairy cell leukemia, where virtually all tumor cells carry ROR1 on their surface. Other blood cancers like mantle cell lymphoma, marginal zone lymphoma, and diffuse large B-cell lymphoma also show ROR1 expression, though at lower and more variable levels.⁴ In CLL specifically, the proportion of ROR1-positive cells stays stable in patients whose disease is not progressing, but rises when the disease worsens.⁴

Among solid tumors, a large tissue analysis found ROR1 expressed most frequently in triple-negative breast cancer (TNBC), where about 57% of samples tested positive, with the majority showing strong, homogeneous staining across the tumor. Lung adenocarcinomas came next, with about 42% testing positive for ROR1. In contrast, HER2-positive breast cancers showed no ROR1 expression at all, and only about 12% of hormone receptor-positive breast cancers and squamous cell lung cancers were ROR1-positive.⁵ Ovarian cancer, endometrial cancer, and pancreatic cancer have also shown ROR1 expression in various studies, though the consistency of expression varies across tumor subtypes.

How ROR1 Fuels Cancer Progression

ROR1 is not just a passive marker sitting on tumor cells. When Wnt5a binds to it, ROR1 activates signaling cascades that push cancer cells toward more aggressive behavior. In acute lymphoblastic leukemia, for example, Wnt5a-ROR1 signaling boosts cell proliferation through RhoA and Rac1, a pair of molecular switches that control cell movement and division.⁶

One of ROR1’s most consequential roles involves a process called the epithelial-to-mesenchymal transition, or EMT. In normal biology, EMT allows stationary epithelial cells to become mobile, a process essential during wound healing and embryonic development. Cancer cells co-opt EMT to detach from the primary tumor, invade surrounding tissue, and seed metastases in distant organs. Tumors with high ROR1 expression show elevated levels of EMT-associated genes and proteins, along with reduced expression of the adhesion molecules that normally keep cells anchored in place. When researchers silenced ROR1 in aggressive breast cancer cell lines, the cells reverted: they regained epithelial adhesion markers and lost mesenchymal markers associated with invasiveness.⁷ The reverse experiment confirmed this: forcing ROR1 expression in normally ROR1-negative breast cancer cells pushed them toward a mesenchymal, invasion-prone state.⁷

ROR1 also plays a role in maintaining cancer stem cells, the subset of tumor cells thought to be responsible for treatment resistance and disease relapse. Higher ROR1 expression in breast cancer has been linked to faster disease relapse and shorter survival, and silencing ROR1 impairs the ability of cancer cells to migrate, invade, and form new tumors.⁸ There is even evidence that ROR1-positive tumor cells release small membrane-bound vesicles, called extracellular vesicles, that carry ROR1 to other cells and promote cancer spread. In animal models, extracellular vesicles depleted of ROR1 were less likely to distribute to organs where breast cancer typically metastasizes.⁹

Antibody-Drug Conjugates Targeting ROR1

The most clinically advanced ROR1-targeting therapy is zilovertamab vedotin, an antibody-drug conjugate (ADC). The concept is straightforward: an antibody that latches onto ROR1 on the tumor cell surface is chemically linked to a potent cell-killing payload. Once the antibody binds, the whole complex gets pulled inside the cancer cell, where the toxic payload is released. Zilovertamab vedotin uses monomethyl auristatin E, a drug that disrupts the cell’s internal scaffolding and prevents it from dividing.¹⁰

In early-phase trials for blood cancers, the results have been encouraging. Among patients with non-Hodgkin lymphoma who had already failed prior treatments, the overall response rate was about 37%, including complete responses in some patients, with a median duration of response around eight months.¹⁰ Responses were particularly notable in mantle cell lymphoma, where nearly half of patients responded, and diffuse large B-cell lymphoma, where three of five patients had objective responses.¹¹

Solid tumors have proven much harder. A phase 2 study testing zilovertamab vedotin in patients with metastatic solid tumors yielded an objective response rate of just 1%, with only a single partial response among all enrolled patients. Median progression-free survival was roughly two months. Of the few tumor samples that could be tested for ROR1 by tissue staining, only three of seventeen showed any ROR1 expression, and none of those patients responded.¹² That last finding is telling: it suggests many patients enrolled in the trial may not have had tumors that actually expressed the target, which underscores the need for reliable methods to select the right patients before giving them ROR1-directed drugs.

CAR-T Cell Therapy Against ROR1

Another approach reprograms a patient’s own immune cells to recognize and attack ROR1-positive tumor cells. In a phase I study of ROR1-targeted CAR-T cells, two of three CLL patients achieved partial responses. One patient had rapid CAR-T cell expansion accompanied by complete clearance of circulating tumor cells from the blood, along with signs of massive tumor breakdown. A second patient received two rounds of CAR-T cells and achieved complete remission that was still holding seven months after the second infusion, with no detectable CLL cells in the bone marrow.¹³

These are early results in a small number of patients, but the depth of response, especially the clearance of tumor cells from bone marrow, is the kind of outcome that keeps researchers invested in the approach. CAR-T therapy does carry its own risks, though. Cytokine release syndrome, where the activated immune cells flood the body with inflammatory signals, and neurotoxicity remain challenges that need to be managed carefully.¹⁴

Bispecific Antibodies and T-Cell Engagers

Bispecific antibodies offer a different way to harness the immune system. These engineered molecules have two binding arms: one grabs ROR1 on the tumor cell, and the other grabs CD3 on T cells, physically bringing killer immune cells into contact with cancer cells. A ROR1/CD3 bispecific T-cell engager tested in preclinical models of triple-negative breast cancer triggered T-cell activation, proliferation, and potent killing of TNBC cells in a manner that depended on ROR1 being present. In mouse models, the drug redirected T cells to attack TNBC tumors and caused significant tumor shrinkage, with the bispecific antibody showing an extended half-life that could allow for less frequent dosing in patients.¹⁵

Separately, a comparative study of different ROR1-targeting bispecific formats found that the exact spot on ROR1 where the antibody binds matters a great deal. An antibody arm targeting a region close to the cell membrane proved most effective at recruiting T cells and killing tumor cells both in the lab and in animal models.¹⁶ These kinds of structural details will likely influence which bispecific formats ultimately advance to the clinic.

Small Molecule Inhibitors

While antibodies and cell therapies dominate the ROR1 pipeline, small-molecule inhibitors are also in development. The first ROR1 kinase inhibitor, KAN0439834, was reported in 2018. A second-generation compound, KAN0441571C, showed stronger cell-killing potency in lab tests and a substantially longer half-life in animal models.¹⁷ More recently, a structure-based drug design effort identified a new class of ROR1 inhibitors, with the best compound binding ROR1 tightly and killing lung and breast cancer cells at submicromolar concentrations in the lab.¹⁸

Small molecules have practical advantages that biologics lack: they can be taken orally, are cheaper to manufacture, and can reach tumor sites that large antibody molecules struggle to penetrate. But none of these ROR1 small-molecule inhibitors have reached human trials yet, so their real-world performance remains unknown.

The Safety Question and On-Target Toxicity

ROR1’s near-absence in healthy adults is the reason it draws so much attention as a drug target, but “near-absence” is not the same as “total absence.” Low levels of ROR1 have been detected in adipose tissue and parts of the gastrointestinal tract, raising the concern that drugs designed to kill ROR1-positive cells could cause collateral damage to those normal tissues.¹⁹ So far, early clinical trials of zilovertamab vedotin have not flagged unexpected toxicities. The most common treatment-related side effects in the solid tumor trial were fatigue and nausea, each in roughly a third of patients, while peripheral neuropathy, a known effect of the MMAE payload rather than of ROR1 targeting itself, occurred in about 8%.¹² That said, those trials enrolled patients whose tumors often had little or no detectable ROR1, so the safety picture under conditions of genuine high-target engagement remains to be fully characterized.

For CAR-T therapies, the safety concerns are somewhat different. Because CAR-T cells can expand explosively once they encounter their target, cytokine release syndrome and neurotoxicity are inherent risks of the platform, not specific to ROR1. Managing those effects is an area of active work across all CAR-T programs, not just ROR1-directed ones.

Resistance and Why Combinations May Be Needed

Even when a drug hits its target perfectly, cancers find ways to adapt. One emerging concern is that tumors can upregulate ROR1 as an escape route when other pathways are blocked. In ovarian cancer models treated with cell-cycle inhibitors, ROR1 expression rose along with activation of the PI3K/AKT survival pathway, suggesting that ROR1 can serve as a backup mechanism for tumors under therapeutic pressure.²⁰ Other potential resistance mechanisms include alternative splicing that might alter the ROR1 protein and barriers within the tumor microenvironment that suppress immune cell activity.¹⁹

This is why combination strategies are generating interest. In lab studies, combining a ROR1 small-molecule inhibitor with venetoclax, a drug already approved for CLL, produced synergistic cell killing in CLL cells that had become resistant to ibrutinib, another widely used CLL drug.²¹ Similarly, combining ROR1 bispecific T-cell engagers with checkpoint blockade (anti-PD-1) significantly improved tumor killing compared to either agent alone, partly by increasing the formation of long-lived anti-tumor memory T cells.²² In a field where monotherapy rarely produces lasting responses in solid tumors, these combination approaches may be essential to making ROR1-targeted treatment viable beyond blood cancers.

Patient Selection and the Push for Better Diagnostics

The disappointing results of zilovertamab vedotin in solid tumors highlighted a fundamental problem: if you do not know whether a patient’s tumor actually expresses ROR1, you are essentially treating blind. Unlike HER2 in breast cancer, where testing for the target protein is standard practice, there is no established clinical assay for selecting patients likely to benefit from ROR1-directed therapy. The phase 2 solid tumor trial’s finding that only three of seventeen tested samples showed any ROR1 expression suggests the enrolled population was poorly matched to the drug.¹²

Researchers are working on non-invasive approaches. A recent study developed radiolabeled peptide-based probes that bind to ROR1, enabling PET imaging to visualize where and how strongly ROR1 is expressed across a patient’s tumors. In preclinical models, these probes showed strong tumor targeting with good contrast between the tumor and surrounding normal tissue, and the signal cleared primarily through the kidneys.²³ If validated in humans, this kind of imaging could allow clinicians to map ROR1 expression in real time, without needing a tissue biopsy, and select patients most likely to respond to ROR1-targeted drugs.

ROR1 Versus ROR2

ROR1 has a close relative, ROR2, and assuming the two receptors do the same thing in cancer would be a mistake. In endometrial cancer, high ROR1 expression correlated with worse overall survival, while high ROR2 expression actually trended in the opposite direction, correlating with better outcomes. When researchers knocked down ROR1 in endometrial cancer cells, proliferation, migration, and invasion all decreased. Knocking down ROR2, by contrast, increased migration and invasion.²⁴ The implication is that ROR2 may act as a tumor suppressor in some contexts, which means therapies need to be carefully designed to hit ROR1 without inadvertently disrupting ROR2.

In pancreatic cancer, both receptors showed potential as prognostic markers when measured at the gene-expression level. But when the same team tried to confirm ROR1 protein expression using tissue staining, the results did not reliably predict survival, highlighting a broader challenge in the field: the method used to measure a biomarker can dramatically influence whether it appears useful.²⁵

The Naked Antibody Approach

Not every antibody targeting ROR1 needs a toxic payload attached. Zilovertamab, the antibody portion of zilovertamab vedotin, has been tested on its own. In lab experiments, zilovertamab alone inhibited the proliferation of ovarian and endometrial cancer cells, including models of platinum resistance. Adding standard chemotherapy drugs to zilovertamab did not generally improve the anti-proliferative effect, suggesting the antibody may work through a distinct mechanism rather than simply boosting chemo’s activity.²⁶ This raises the possibility that blocking ROR1 signaling, rather than using ROR1 as a homing beacon for a cell-killing drug, could be a viable therapeutic strategy in its own right, particularly for cancers where ROR1 drives aggressive behavior through its downstream signaling pathways.

What the Structural Biology Reveals

Understanding the three-dimensional shape of ROR1 has practical implications for drug design. Structural studies of ROR family proteins show that ROR1’s extracellular region contains a cysteine-rich domain (CRD) resembling those found on Frizzled receptors, the classical Wnt-binding receptors. Crystal structures have revealed that this CRD harbors a buried internal cavity containing a fatty acid molecule, similar to lipid-binding pockets seen in related receptor families.²⁷ The practical consequence is that each region of ROR1’s extracellular surface offers a different therapeutic opportunity. As the bispecific antibody work demonstrated, targeting a membrane-proximal epitope proved far more effective than targeting other regions, meaning that where on the protein a drug binds can matter as much as how tightly it binds.

In CLL, researchers have also identified a new protein partner for ROR1 called COBLL1. Patients with unmutated immunoglobulin genes and high COBLL1 expression had a particularly unfavorable disease course, with shorter overall survival. This interaction may influence how ROR1 signals inside the cell and could eventually serve as an additional marker for identifying patients with aggressive disease.²⁸