What Is FAP Protein and Its Role in Cancer Treatment?

Fibroblast activation protein, usually called FAP, is a protein found on the surface of certain cells that shows up almost exclusively in diseased tissue, including most solid tumors, making it one of the more promising targets in modern cancer research. FAP sits on the outside of cancer-associated fibroblasts, the support cells that tumors recruit to build their protective microenvironment, and it helps those tumors grow, spread, and hide from the immune system. Because FAP is abundant in tumors but scarce in healthy adult tissue, researchers are exploiting it for everything from sharper imaging scans to experimental therapies that deliver radiation or toxic drugs straight to the cancer.

What FAP Actually Is

FAP is a type-II transmembrane serine protease, which in plain terms means it is a protein anchored in the cell membrane with an enzyme on its outer end that can cut other proteins. It belongs to the same enzyme family as DPP4, a protein many people know indirectly because drugs that block DPP4 are widely used to treat type 2 diabetes. But FAP has a trick DPP4 lacks: in addition to snipping short peptide chains, FAP can also slice through larger proteins like collagen, the main structural fiber in connective tissue. That dual ability is central to why tumors find FAP so useful.

In a healthy adult, FAP is essentially absent from most organs. It appears transiently during wound healing and embryonic development, then largely disappears. The protein resurfaces in pathological conditions such as fibrosis, arthritis, and cancer.¹ This near-exclusivity to diseased tissue is what makes FAP so attractive as a therapeutic target: if you aim a drug at FAP, you are unlikely to hit healthy cells along the way.

How FAP Helps Tumors Thrive

Tumors are not just clumps of cancer cells. They build an entire support ecosystem called the tumor microenvironment, and FAP-positive cancer-associated fibroblasts (CAFs) are among its most important architects. These fibroblasts remodel the extracellular matrix, the dense web of proteins that gives tissue its structure, creating physical barriers that make it harder for immune cells and drugs to penetrate the tumor. FAP-positive CAFs also promote the growth of new blood vessels feeding the tumor, a process called angiogenesis.² Mouse studies using a drug that inhibited FAP showed roughly a three-fold decrease in tumor blood vessel density compared with untreated controls, underscoring FAP’s role in keeping the tumor’s supply lines running.³

Perhaps most consequentially, FAP-positive fibroblasts help tumors suppress the immune response. They alter how macrophages behave, pushing them toward a tumor-friendly state, and they interfere with the ability of T cells to attack cancer cells.² This immunosuppressive shield is one of the core reasons that many immunotherapies struggle against solid tumors, and it is a major motivation for the effort to knock out FAP-positive cells or use them as a homing beacon for treatment.

FAP as a Prognostic Marker

High FAP expression in a tumor generally means worse news for the patient. A large meta-analysis pooling data across multiple cancer types found that patients with high FAP levels had roughly 50 percent higher odds of dying from their cancer compared with patients whose tumors expressed less FAP. The same analysis linked high FAP to about double the risk of lymph node spread and more than two-and-a-half times the risk of distant metastasis.⁴ In high-grade serous ovarian cancer specifically, FAP stood out as the only novel gene that significantly predicted both overall survival and progression-free survival, with high expression tied to poorer outcomes.⁵

The fact that FAP expression correlates so consistently with aggressive disease reinforces its value not just as a therapeutic target but as a way to identify patients who may benefit from more intensive or FAP-directed treatment. Still, FAP expression varies among fibroblast subtypes even within the same tumor. In non-small cell lung cancer, researchers identified at least two distinct fibroblast subsets with opposite prognostic implications: one FAP-positive subset was linked to worse outcomes, while a different FAP-negative subset was associated with better survival.⁶ This kind of heterogeneity complicates the picture and is one reason blanket assumptions about FAP in any single patient remain risky.

Sharper Imaging With FAPI PET Scans

One of the most clinically advanced uses of FAP is in diagnostic imaging. Traditional PET scans rely on FDG, a radioactive sugar tracer that lights up wherever cells are consuming a lot of energy. FDG works well for many cancers, but it also lights up in inflamed tissue, infections, and even active muscles, creating false positives. FAP-targeting tracers, called FAPIs, take a different approach: they bind specifically to FAP on tumor-associated fibroblasts, producing images with less background noise.

In a head-to-head study of 68 people with lung cancer, FAPI PET/CT substantially outperformed FDG PET/CT. Sensitivity was 99 percent with FAPI versus 87 percent with FDG, specificity was 93 percent versus 79 percent, and overall accuracy reached 97 percent compared with 85 percent.⁷ A separate study across 12 different tumor types found that FAPI-based PET detected primary tumors at a rate of about 98 percent versus 82 percent for FDG, and was far more sensitive for lymph node metastases and distant spread.⁸

These results are encouraging, but the field is not standing still. Several groups are developing next-generation FAPI tracers that stay in the tumor longer, clear from the kidneys faster, or offer better image contrast. One novel fluorine-18-labeled tracer showed superior tumor delineation compared with both FDG and the earlier FAPI-42 tracer in preclinical testing.⁹ Others are experimenting with technetium-99m-labeled dimeric FAPI compounds, which bind FAP at two points simultaneously to boost tumor uptake and specificity in SPECT imaging.¹⁰ The practical implication is that FAPI imaging may eventually supplement or partially replace FDG scanning for certain cancer types, particularly those where FDG is known to underperform, such as some brain tumors and certain gastrointestinal cancers.

Radioligand Therapy Aimed at FAP

If a tracer can find FAP in the body, the same molecule can carry a therapeutic dose of radiation instead of just a diagnostic signal. This “theranostic” approach, using the same targeting molecule for both imaging and therapy, is one of the most active areas in FAP research. Lutetium-177, yttrium-90, and actinium-225 have all been attached to FAPI molecules and tested in patients with advanced cancers who had run out of standard options. Across the studies reviewed to date, these agents show high tumor uptake and generally favorable safety profiles, with most side effects limited to mild fatigue, nausea, and low-grade drops in blood counts.¹¹ Severe toxicities have been rare. Disease control rates look promising, though the studies so far have been small and covered many different tumor types, making it hard to draw firm conclusions about which cancers respond best.

A persistent challenge is that some FAPI molecules wash out of tumors faster than ideal, limiting the radiation dose actually delivered. Researchers are tackling this by engineering tracers with longer retention. One approach attaches a maleimide group that locks onto albumin in the bloodstream, extending the tracer’s circulation time and increasing tumor uptake.¹² Others are testing tandem regimens that combine different radionuclides to hit the tumor from multiple angles.

Bispecific Antibodies and Immunocytokines

Another strategy uses FAP as an address label to redirect the immune system toward the tumor. Bispecific antibodies are engineered molecules that grip two targets at once. One arm grabs FAP on a cancer-associated fibroblast; the other arm grabs a receptor on a T cell, physically dragging the T cell into close contact with its target. OMTX305 is one such molecule: in patient-derived tumor samples from lung and ovarian cancers, it eliminated FAP-expressing fibroblasts and triggered bystander killing of neighboring tumor cells.¹³ The bystander effect matters because it means you do not need FAP on every cancer cell; destroying the supportive fibroblasts can expose and kill the cancer cells hiding behind them.

Some researchers are going further by combining two bispecific antibodies at once. One experimental approach pairs a FAP-targeting antibody that engages the CD28 co-stimulatory receptor on T cells with a second bispecific that targets a tumor surface marker called CD276 and engages CD3, the main T cell activation switch. The idea is to deliver both the “go” signal and the “keep going” signal to T cells simultaneously.¹⁴

Then there are immunocytokines, which fuse an FAP-targeting antibody with a modified immune-stimulating molecule. Simlukafusp alfa pairs an anti-FAP antibody with an engineered version of interleukin-2 that preferentially activates cancer-killing T cells and natural killer cells while largely avoiding the expansion of regulatory T cells that would dampen the immune response. In preclinical work, it boosted the effectiveness of multiple other immunotherapies, and a triple combination with a PD-L1 blocker and a CD40 agonist was the most potent regimen tested.¹⁵

Drug Conjugates That Use FAP as a Homing Signal

A separate class of FAP-directed treatments works by attaching a potent cancer-killing drug to a small molecule that binds FAP tightly, delivering the payload directly into the tumor. These small molecule-drug conjugates (SMDCs) are lighter and simpler than traditional antibody-drug conjugates and can penetrate tissue more easily. OncoFAP, an ultra-high-affinity FAP-binding molecule, has been linked to several payloads already used in approved cancer drugs, including MMAE (the payload behind Adcetris) and exatecan (related to the payload in Enhertu). In mice with FAP-positive tumors, OncoFAP conjugates carrying exatecan and MMAE were the most effective, and the MMAE version worked well even against tumors that only expressed FAP on their surrounding stroma rather than on the cancer cells themselves.¹⁶ That last point is especially relevant because in most human cancers, FAP is on the fibroblasts, not the cancer cells. A drug that only works when FAP is on the cancer cell itself would be useless for many patients.

Side-by-side preclinical comparisons have shown that both the small-molecule and antibody-based versions of these conjugates can deliver high concentrations of active drug selectively to the tumor site, producing strong antitumor activity.¹⁷ Several of these candidates are now moving toward or are already in clinical trials.

Combination Approaches and the Checkpoint Connection

Some of the most striking results in FAP-targeted therapy come from combining FAP agents with immune checkpoint inhibitors, the drugs that remove the brakes tumors put on the immune system. In a mouse model of pancreatic cancer, which is notoriously resistant to immunotherapy, a combination of a FAP-targeting CD40 agonist and a PD1-targeted IL-2 variant caused significant tumor regression and triggered the formation of new clusters of T cells and dendritic cells within the tumor.¹⁸ These clusters are thought to serve as local command centers for immune coordination, essentially warming up “cold” tumors that the immune system had previously ignored.

In another preclinical study, combining a lutetium-177-labeled FAPI molecule with a PD-L1 blocking antibody eliminated tumors entirely in mice, and every mouse that received the combination rejected new tumor cells when they were re-introduced, suggesting the treatment had generated lasting immune memory.¹⁹ These are mouse results and the leap to human patients is never straightforward, but the pattern is consistent: FAP-targeted therapies and checkpoint inhibitors appear to amplify each other.

The Safety Question

For all the excitement, there is an important caution in the literature. While FAP is rare in most healthy adult tissues, it is not entirely absent. A key mouse study found that FAP is expressed on multipotent bone marrow stromal cells, the cells that help maintain bone and blood-forming tissue. When researchers used T cells engineered to attack FAP-positive cells, the mice experienced severe weight loss (cachexia) and lethal bone damage, because those T cells destroyed the bone marrow stromal cells along with the tumor fibroblasts.²⁰ The same study confirmed that clinical-grade human bone marrow stromal cells also express FAP and were recognized by FAP-targeting T cells.

This finding does not mean all FAP-targeted therapies are dangerous. Radioligand therapies and drug conjugates deliver their effects locally at the tumor site and have shown manageable side-effect profiles in early clinical experience. But it does suggest that therapies involving a sustained, systemic immune attack on all FAP-positive cells, such as certain CAR-T designs, need careful engineering to avoid collateral damage. Researchers are exploring safety switches, dose-limiting strategies, and conditional activation mechanisms to mitigate this risk.

What FAP Does Beyond Its Role in Tumors

FAP’s enzymatic activity extends beyond just remodeling the tumor’s scaffolding. Studies have identified several natural hormone substrates that FAP can cleave, including neuropeptide Y, substance P, B-type natriuretic peptide, and peptide YY. Among these, neuropeptide Y is cut most efficiently.²¹ These peptides are involved in appetite regulation, pain signaling, and cardiovascular function, which may help explain some of the systemic effects seen when FAP activity is disrupted. Follow-up work confirmed that neuropeptide Y is the most efficiently cleaved FAP substrate in human blood plasma, and that the in vivo targets of FAP and its cousin DPP4 differ, which is relevant to ensuring that drugs designed to block one do not inadvertently affect the other.²²

Circulating levels of FAP in the blood have also drawn attention as a possible diagnostic biomarker. Counterintuitively, some cancer patients have lower blood levels of FAP than healthy people. In patients with thoracic cancers undergoing radiation, the median circulating FAP concentration was significantly lower than in healthy volunteers.²³ A similar pattern appeared in esophageal squamous cell carcinoma, where plasma FAP levels were significantly decreased in patients compared with controls, yielding moderate diagnostic accuracy.²⁴ This paradox, where tissue FAP is high but blood FAP is low, is not yet fully explained but may reflect the protein being sequestered in the tumor stroma rather than shed into the bloodstream.

FAP Beyond Cancer

Because FAP is expressed wherever activated fibroblasts are present, its relevance extends into other diseases. In rheumatoid arthritis, FAP is overexpressed on the fibroblast-like cells that line inflamed joints but is scarce in normal synovial tissue. FAPI PET scans can visualize joint inflammation with high precision and track how patients respond to treatment. Beyond joint disease, FAP imaging is being used experimentally to assess interstitial lung disease and cardiac fibrosis, conditions where the degree of fibroblast activation correlates with disease severity. Early work with FAP-targeted photodynamic therapy, which uses light-activated compounds to destroy FAP-positive cells, has shown the ability to selectively kill pathogenic fibroblasts in arthritis models.²⁵ The broader point is that FAP research developed in oncology is feeding back into other fields where fibroblast activity drives disease, potentially opening up therapeutic avenues for conditions that currently lack targeted treatments.