Folate receptor alpha (FRα) testing is an immunohistochemistry-based assessment of tumor tissue that determines whether a cancer overexpresses a specific protein on its cell surface, guiding eligibility for targeted therapies like mirvetuximab soravtansine. The test matters most right now in platinum-resistant ovarian cancer, where it serves as a companion diagnostic, but the protein shows up across several other tumor types and is becoming a broader focus in oncology drug development. Getting the test right, interpreting it correctly, and understanding its limitations are all more nuanced than a simple positive-or-negative result might suggest.
Why FRα Is Worth Testing For
Folate receptor alpha is a protein anchored to the surface of certain cells. In healthy tissue, it sits mostly on the side of cells that faces inward, tucked away from the bloodstream and the immune system. It helps shuttle folate (a B vitamin) into cells, supporting normal DNA synthesis and repair. In normal ovaries, expression is actually quite low, but it’s present at higher levels in some other normal tissues like the fallopian tubes and kidneys, typically on surfaces that aren’t exposed to circulating drugs or immune cells.1PubMed Central. Expression of folate receptors alpha and beta in normal and cancerous gynecologic tissues: correlation of expression of the beta isoform with macrophage markers
Cancer changes this picture. Many malignant cells overexpress FRα on their outer surfaces, where it becomes accessible to drugs and imaging agents. This overexpression has been documented in ovarian cancer, non-small-cell lung cancer, colon cancer, endometrial cancer, and breast cancer, among others.2PubMed Central. Folate Receptor Alpha-A Novel Approach to Cancer Therapy The protein isn’t just a bystander marker sitting on the cell. It plays a functional role in tumor biology, contributing to cell proliferation and signaling pathways that drive cancer growth. That dual role, as both a functional player in tumorigenesis and a surface target accessible to drugs, is what makes it attractive for targeted therapy and, by extension, worth testing for.
A meta-analysis pooling data across cancer types found that higher FRα expression was associated with worse overall survival and worse disease-free survival in patients. The relationship was especially pronounced in breast cancer and endometrial carcinoma.3PubMed. A systematic review and meta-analysis of higher expression of folate receptor alpha (FOLR1) predicts poor cancer prognosis This means FRα isn’t just a drug-targeting handle; it’s also prognostically relevant. Tumors with lots of it tend to behave more aggressively, which makes identifying and treating them with FRα-directed agents all the more urgent.
How the Test Actually Works
The standard FRα test uses immunohistochemistry, or IHC, on a tissue sample. A pathologist stains tumor tissue with an antibody that binds specifically to FRα, then evaluates how much of the protein is present and where it’s located on the cells. The result is scored based on both the intensity of staining and the percentage of tumor cells that stain positive. Tumors are typically classified as FRα-high, FRα-medium, or FRα-low/negative.
The most widely used assay is the VENTANA FOLR1 (FOLR1-2.1) RxDx Assay, which functions as a companion diagnostic for mirvetuximab soravtansine. A real-world validation study of 313 cases found that three independent pathologists achieved excellent agreement, with concordance above 97% for both inter-observer and intra-observer readings.4Pathology – Research and Practice. Performance of the VENTANA FOLR1 Assay for folate receptor alpha: Real-world evidence from 313 Chinese participants That level of reproducibility is reassuring, because companion diagnostics need to give consistent results across different pathologists and laboratories if they’re going to reliably select patients for therapy.
Quality assurance programs are beginning to formalize this. Germany’s Quality Assurance Initiative for Pathology (QuIP) launched the first FRα testing proficiency trial to ensure that pathology labs across German-speaking countries can deliver reliable, reproducible results as demand for the test grows.5PubMed Central. Results of the First Folate Receptor Alpha Testing Trial by the German Quality Assurance Initiative in Pathology (QuIP®) As FRα-targeted drugs become more common, programs like these will matter for ensuring that a “high” result means the same thing regardless of where the test is performed.
The Drug That Put FRα Testing on the Map
Mirvetuximab soravtansine (marketed as Elahere) is the first FDA-approved therapy specifically targeting FRα. It’s an antibody-drug conjugate: an antibody that homes in on FRα, carrying a potent cell-killing payload that gets released inside the tumor cell after the drug is internalized. Its approval made FRα testing a clinical necessity rather than a research curiosity.
The pivotal phase III trial, known as MIRASOL, compared mirvetuximab soravtansine to standard chemotherapy in patients with FRα-positive, platinum-resistant ovarian cancer. Patients receiving the drug had significantly longer progression-free survival (about 5.6 months versus 4.0 months) and a much higher response rate: roughly 42% responded to the drug compared with about 16% on chemotherapy. Overall survival was also longer, at a median of about 16.5 months versus 12.75 months.6PubMed. Mirvetuximab Soravtansine in FRα-Positive, Platinum-Resistant Ovarian Cancer
An earlier single-arm trial, SORAYA, had already shown the drug’s activity in a heavily pretreated population. Among 105 evaluable patients, about a third responded, including five complete responses, and the median duration of response was nearly seven months. Final overall survival in SORAYA came in at 15 months.7PubMed Central. Mirvetuximab soravtansine in folate receptor alpha (FRα)-high platinum-resistant ovarian cancer: final overall survival and post hoc sequence of therapy subgroup results from the SORAYA trial These results held regardless of what treatments patients had received previously, which is encouraging for a population that often has very few options left.
All of this depends on accurate FRα testing. Patients in these trials were selected based on high FRα expression. Without the companion diagnostic confirming that the tumor is FRα-high, neither the patient nor the oncologist knows whether mirvetuximab soravtansine is likely to work.
Where the Testing Gets Complicated
A biopsy captures one snapshot of one part of a tumor, and cancers aren’t always uniform. FRα expression can vary between the primary tumor and a metastatic deposit, or between a biopsy sample and the full surgical specimen. This matters because treatment decisions are often made based on a small tissue sample, not the entire tumor.
A study examining concordance in lung and breast cancer found that FRα results from biopsies matched the primary tumor about 83% of the time in lung adenocarcinomas, and roughly 80% of metastases also matched the primary. Breast cancer showed lower concordance: only 68% agreement between biopsies and the primary tumor, dropping to 60% between the primary and lymph node metastases.8PubMed Central. Concordance of folate receptor-α expression between biopsy, primary tumor and metastasis in breast cancer and lung cancer patients The false-positive rate on biopsies was low (under 5%), so a positive biopsy result tends to be trustworthy. But a negative result on a small sample doesn’t guarantee the whole tumor lacks FRα.
In ovarian cancer specifically, primary tumors showed higher FRα positivity rates than metastatic lesions (about 44% versus 32%), meaning a test on a metastatic site might underestimate what was present in the original tumor.9Pathology – Research and Practice. Performance of the VENTANA FOLR1 Assay for folate receptor alpha: Real-world evidence from 313 Chinese participants One reassuring finding: prior chemotherapy didn’t appear to significantly alter FRα positivity rates, so the test remains informative even after patients have been treated.
Blood Tests as an Alternative to Tissue
Not every patient can easily undergo a tissue biopsy, and researchers are exploring whether a blood test could supplement or eventually replace IHC for some purposes. FRα can be shed from tumor cells into the bloodstream as a soluble protein (sFRα), and its levels in the blood appear to track with what’s happening in the tumor.
In ovarian cancer patients, circulating sFRα was significantly higher than in healthy volunteers, and levels were associated with FRα expression on the tumor itself. Among patients whose tumors showed at least some FRα positivity on IHC, the average blood level was about 7.6 ng/mL, compared to 2.6 ng/mL in patients with FRα-negative tumors. When researchers used a statistical threshold to see how well the blood test predicted tissue status, it performed reasonably well, especially for tumors with high expression levels.10PubMed Central. Folate receptor alpha in ovarian cancer tissue and patient serum is associated with disease burden and treatment outcomes Blood levels also dropped as patients responded to standard chemotherapy, suggesting sFRα could serve as a real-time monitoring tool for disease burden.
This is still investigational. A blood test won’t replace the companion diagnostic IHC assay for selecting mirvetuximab soravtansine candidates today. But for patients where tissue is unavailable or where clinicians want to track FRα status over time without repeat biopsies, the liquid approach has genuine potential.
FRα in the Operating Room
Testing for FRα isn’t limited to treatment selection. It’s also being used during surgery to help surgeons find and remove cancerous tissue they might otherwise miss. Pafolacianine (marketed as Cytalux) is a fluorescent dye that binds to FRα. Injected before surgery, it causes FRα-positive tumors to glow under near-infrared light, making small or hidden deposits visible in real time.
The FDA approved pafolacianine in late 2021 for use in ovarian cancer surgery.11PubMed Central. Intraoperative Tumor Detection Using Pafolacianine A phase III trial showed that near-infrared imaging with pafolacianine identified additional cancer in about a third of patients, tissue that the surgeon had not planned to remove based on standard visual inspection and palpation alone. Among patients undergoing interval debulking surgery (after initial chemotherapy), the detection rate was even higher, approaching 40%. The sensitivity for detecting ovarian cancer was 83%.12PubMed Central. A Phase III Study of Pafolacianine Injection (OTL38) for Intraoperative Imaging of Folate Receptor-Positive Ovarian Cancer (Study 006)
The practical implication is substantial. In ovarian cancer, the completeness of surgical removal is one of the strongest predictors of long-term outcomes. Residual tumor deposits left behind can seed recurrence. A tool that lights up cancer the surgeon wouldn’t have otherwise seen translates directly into more complete operations. The false-positive rate was about 25%, meaning some non-cancerous tissue also glowed, but confirming those sites with a quick biopsy during surgery is straightforward.
Ocular Side Effects and What Patients Should Know
FRα-targeted antibody-drug conjugates carry a distinctive side-effect profile that patients and clinicians need to manage proactively. Mirvetuximab soravtansine and other agents in this class have been linked to ocular toxicities, including keratopathy (corneal damage), blurred vision, and dry eye.13SpringerLink. Management of ocular toxicity in patients with gynecologic cancer receiving novel antibody-drug conjugates: a narrative review These effects likely arise because normal corneal and conjunctival cells also express some FRα, making the eye a bystander target for the drug’s payload.
For patients, this means regular eye exams before and during treatment are essential, along with preventive measures such as lubricating eye drops and avoiding contact lenses during therapy. Eye-related side effects can affect adherence if they aren’t managed early. Most are reversible with dose modifications and supportive care, but they can be uncomfortable enough to interfere with daily life. Anyone considering mirvetuximab soravtansine should expect their oncology team to involve an ophthalmologist in their care plan.
When Treatment Stops Working
Even in patients whose tumors test FRα-high, resistance to FRα-directed therapy can develop. The mechanisms are varied. At the simplest level, tumors can lose FRα expression or become heterogeneous, with low-expressing clones expanding under the selective pressure of treatment. FRα-positive cells can also alter how they internalize the drug, reducing the amount of payload that actually reaches the interior of the cell. And some cells activate drug efflux pumps that push the payload back out before it can do damage.14International Immunopharmacology. Folate receptor-α targeted therapies in ovarian cancer: recent advances and emerging therapeutic strategies
This has implications for retesting. A patient whose tumor was FRα-high at diagnosis might have lower expression after several lines of therapy, not necessarily because chemotherapy wiped out FRα, but because the clonal composition of the tumor shifted. Retesting at relapse, when possible, gives a more current picture and can prevent futile use of an FRα-targeted drug on a tumor that no longer expresses the target adequately.
Beyond Ovarian Cancer
While ovarian cancer is the clinical setting where FRα testing is most established, the receptor is overexpressed in other malignancies, and testing is expanding accordingly. In non-small-cell lung cancer, particularly adenocarcinomas, a majority of tumors show FRα positivity. Endometrial cancer, especially the aggressive uterine serous subtype, also frequently overexpresses the receptor. In one study, about 41% of uterine serous carcinoma specimens overexpressed FRα, and preclinical models of these tumors showed complete resolution with an FRα-targeting antibody-drug conjugate.15Molecular Cancer Therapeutics. In Vitro and In Vivo Activity of IMGN853, an Antibody–Drug Conjugate Targeting Folate Receptor Alpha Linked to DM4, in Biologically Aggressive Endometrial Cancers
Gastrointestinal and breast cancers are also under investigation for FRα-based strategies, though expression rates tend to be more variable. In breast cancer, the concordance challenges described earlier make testing particularly tricky, since a negative biopsy may not reflect the whole picture. As therapies targeting FRα move into trials for these other tumor types, the demand for standardized, high-quality testing will grow beyond the gynecologic oncology community.
Next-Generation Approaches in Development
The success of mirvetuximab soravtansine has prompted a wave of new FRα-targeting agents designed to improve on its limitations. One emerging approach involves dual-payload antibody-drug conjugates. DXC011, for example, is a next-generation ADC that carries two different cytotoxic mechanisms on a single antibody. In preclinical models, it showed activity across multiple FRα-expressing tumor types, including gastrointestinal cancers, while maintaining a favorable safety profile compared to older microtubule-inhibitor-based ADCs.16Cancer Research. Abstract 2395: DXC011, a novel dual-payload antibody-drug conjugate targeting folate receptor alpha The dual-payload strategy may also help address resistance to single-mechanism drugs, since a tumor cell would need to evade two killing pathways simultaneously.
CAR-T cell therapy targeting FRα represents a fundamentally different approach. Instead of delivering a chemical payload, these engineered immune cells recognize FRα on tumor surfaces and kill those cells directly. Early clinical work established the feasibility and safety of this strategy in ovarian cancer patients.17PubMed Central. A phase I clinical trial of adoptive transfer of folate receptor-alpha redirected autologous T cells for recurrent ovarian cancer More recent preclinical work has focused on improving CAR-T persistence. A newer design using a smaller antibody fragment showed reduced T-cell exhaustion and better long-term tumor control during repeated challenges with cancer cells, compared to the conventional CAR format.18PubMed. Human V(H)-antibody-based CAR T cells targeting folate receptor-alpha show enhanced persistence and reduced T-cell exhaustion against ovarian cancer
Each of these new approaches will require its own testing strategy. A dual-payload ADC might work at lower FRα expression thresholds than mirvetuximab soravtansine does. CAR-T cells might target tumors that an ADC can’t adequately penetrate. The companion diagnostic landscape will likely become more complex, with different tests or different scoring thresholds needed for different drugs, rather than a single binary positive-or-negative answer.
What the Biopsy Experience Is Like for Patients
FRα testing requires tissue, which means a biopsy. For patients with gynecologic cancers, research biopsies are common but not trivial. A survey of 129 patients with ovarian or endometrial cancer found that about 60% experienced some pain during the procedure, and a similar proportion reported physical discomfort. Serious complications were rare, with only one clinician-reported event (a small liver-area blood collection) among all biopsies taken. Despite the discomfort, 84% of patients said they would definitely or likely consent to another biopsy, and anxiety and depression scores actually improved slightly after the procedure compared to before it.19PubMed Central. Research biopsies in patients with gynecologic cancers: patient-reported outcomes, perceptions, and preferences
This matters for FRα testing because, as noted earlier, retesting at relapse may be valuable for confirming that the tumor still expresses the target. Patients who understand that the biopsy is safe, that it directly informs whether a specific drug is appropriate for them, and that the discomfort is temporary tend to be more willing to undergo the procedure. For patients who cannot safely undergo a tissue biopsy, the evolving blood-based sFRα assays described earlier may eventually offer an alternative, though they aren’t there yet for treatment-selection purposes.