What Is TROP2 and Why Is It a Target in Cancer?

TROP2 is a protein that sits on the surface of cells in many tissues throughout the body, but cancer cells tend to produce far more of it than healthy cells do. That overexpression made TROP2 an appealing target for drug developers, and in recent years it has become the basis for a new class of cancer treatments called antibody-drug conjugates, or ADCs. Two TROP2-directed ADCs are already approved for use in breast and other cancers, with several more in clinical trials for lung, bladder, and gastrointestinal tumors.

What TROP2 Actually Is

TROP2, short for trophoblast cell-surface antigen 2, is a transmembrane glycoprotein encoded by the TACSTD2 gene.¹ It was originally identified in placental tissue (hence “trophoblast”), but it turns out to be expressed across a range of normal epithelial tissues, including skin, esophagus, lung, kidney, and bladder.² In healthy cells, TROP2 plays roles in cell growth, migration, and maintaining the structural integrity of tissues.³ It also participates in calcium signaling, acting as a kind of relay that helps cells respond to their environment.⁴

TROP2’s only known close relative is EpCAM, another cell-surface protein frequently discussed in cancer biology. The two share about 49% of their amino acid sequence and perform partly overlapping functions in stabilizing the junctions between epithelial cells.⁵ Understanding this family resemblance matters because it explains why TROP2 is not some obscure molecule that only appears in cancer. It is part of the normal machinery of epithelial tissues, which is both what makes it useful as a drug target and what creates the challenge of side effects.

Why Cancer Cells Overexpress It

The central reason TROP2 became a cancer target is the stark difference in how much of it sits on the surface of tumor cells compared with normal tissue. While normal epithelial cells express TROP2 at modest levels, many carcinomas crank it up dramatically.¹ A meta-analysis pooling data from 16 studies and over 2,500 patients found that high TROP2 expression was linked to roughly a 90% increase in the risk of death and more than double the risk of disease progression across multiple solid tumor types.⁶ In breast cancer specifically, TROP2 is detected across all subtypes, with especially high levels in hormone-receptor-positive/HER2-negative disease and in triple-negative breast cancer.⁴

TROP2 is not just a bystander that happens to be elevated. It actively pushes tumors toward more aggressive behavior through at least two well-studied signaling cascades. One involves the ERK pathway, where TROP2 triggers a chain of events that accelerates cell division by ramping up proteins that drive cells through their growth cycle.⁷ The other runs through the PI3K/AKT pathway and promotes not only proliferation but also a process called epithelial-mesenchymal transition, in which tumor cells loosen their attachments, become more mobile, and gain the ability to invade surrounding tissue and spread to distant organs.⁸ In plain terms, TROP2 helps cancer cells both multiply faster and escape the original tumor site, which is why higher expression tracks with worse outcomes.

How TROP2-Targeted Drugs Are Designed

TROP2 sits on the outside of the cell, which makes it physically accessible to an antibody circulating in the bloodstream. That accessibility is essential, because the primary strategy for exploiting TROP2 involves attaching a potent chemotherapy drug to an antibody that recognizes TROP2 on the cell surface. The resulting molecule, an antibody-drug conjugate, acts like a guided missile: the antibody homes in on TROP2-expressing cells, the whole complex gets pulled inside the cell, and the chemotherapy payload is released internally, killing the cell from within.

The two furthest-along TROP2 ADCs differ in meaningful ways. Sacituzumab govitecan (often called SG, sold as Trodelvy) uses the active metabolite of the chemotherapy drug irinotecan, called SN-38, connected to the antibody through a moderately stable linker. This was a deliberate departure from most earlier ADCs, which carried extremely potent toxins. SG instead carries a less toxic drug and loads roughly seven or eight molecules of it per antibody, relying on the higher dose per delivery to get the job done.⁹ Because the linker is not perfectly stable, some SN-38 leaks out in the vicinity of the tumor even before internalization, which can damage nearby cancer cells that may not express much TROP2 themselves, a phenomenon called the bystander effect.¹⁰

Datopotamab deruxtecan (Dato-DXd) takes a different engineering approach. It carries a different topoisomerase I inhibitor payload and uses a cleavable tetrapeptide linker designed to release the drug after being broken down by enzymes inside the cell. The drug-to-antibody ratio is lower, averaging about four payload molecules per antibody.¹¹ These engineering choices influence everything from how much drug gets into tumors, to what side effects patients experience, to how well the drug works against tumors with mixed TROP2 expression.

Newer agents are already in preclinical development. One example, OBI-992, uses yet another payload and a different linker chemistry designed for even greater stability in the bloodstream. In lab tests, it released less than 2% of its payload in human serum over 21 days, compared with about 8% for Dato-DXd.¹² Whether that translates into better safety or efficacy in patients remains to be seen, but it illustrates how rapidly the engineering behind these drugs is evolving.

Clinical Evidence in Breast Cancer

The strongest data for any TROP2-directed therapy comes from the ASCENT trial, which tested sacituzumab govitecan against standard chemotherapy in patients with metastatic triple-negative breast cancer who had already been treated with at least two prior regimens. In the final analysis, patients receiving SG had a median progression-free survival of about 5 months compared with under 2 months for chemotherapy, and their median overall survival reached nearly 12 months versus about 7 months for chemotherapy.¹³ Those numbers may sound modest in absolute terms, but in a disease where patients had already exhausted other options, roughly doubling both time to progression and survival was a meaningful advance.

Subgroup analyses showed the benefit held up across demographic lines. Patients aged 65 and older, Black patients, and those with various prior treatment histories all showed consistent improvements with SG over chemotherapy.¹⁴ The exception was patients with brain metastases, who had better progression-free survival with SG but no clear overall survival advantage, consistent with the difficulty most drugs face crossing into the brain.

Results in Lung Cancer

Datopotamab deruxtecan has been tested most extensively in non-small cell lung cancer. The phase III TROPION-Lung01 trial compared Dato-DXd to the standard chemotherapy drug docetaxel in patients whose cancer had progressed after prior treatment. Overall, Dato-DXd improved progression-free survival (about 4.4 versus 3.7 months), but the overall survival difference was not statistically significant.¹⁵ The picture sharpened when broken down by tumor type: in nonsquamous lung cancer, the benefit was more pronounced, with progression-free survival of 5.5 months versus 3.6 months. In squamous cell lung cancer, Dato-DXd actually performed worse than docetaxel, a reminder that not all lung cancers are the same biologically.

For patients with specific driver mutations like EGFR or ALK alterations who had run through targeted therapies, a phase II study (TROPION-Lung05) showed encouraging results. About 36% of these heavily pretreated patients had their tumors shrink meaningfully, rising to about 44% in those with EGFR mutations. The disease was controlled in roughly four out of five patients.¹⁶ For a population with essentially no standard next-line options, those numbers generated real enthusiasm.

Does TROP2 Level Predict Who Responds

You might expect that the more TROP2 a tumor expresses, the better these drugs would work. The reality is murkier than that. In the ASCENT biomarker analysis, patients with high TROP2 expression did derive the largest benefit from sacituzumab govitecan, with median overall survival around 14 months compared with about 7 months on chemotherapy. But patients with medium expression saw a similar survival advantage. Even those with low TROP2 expression had better outcomes on SG than on chemotherapy, though the gap was smaller.¹⁷

A smaller pilot study in HER2-negative metastatic breast cancer reinforced the ambiguity. TROP2 expression did not reliably distinguish excellent responders from non-responders, though a complete absence of TROP2 staining might predict a lack of benefit.¹⁸ This means clinicians currently cannot use a TROP2 test the way they use HER2 testing to decide whether to prescribe trastuzumab. The drugs are given based on tumor type and treatment history rather than TROP2 score, which is unusual for a targeted therapy and a source of active debate in the field.

Interestingly, TROP2 expression may carry prognostic meaning even outside the context of TROP2-directed therapy. In HER2-positive early breast cancer patients treated with standard anti-HER2 regimens that do not target TROP2 at all, high TROP2 expression was associated with dramatically lower rates of complete pathological response after neoadjuvant treatment.¹⁹ This suggests that TROP2 marks something about the biology of aggressive tumors beyond just being a docking site for ADCs.

Side Effects of TROP2-Directed ADCs

Because TROP2 exists on normal epithelial surfaces, not just tumor cells, these drugs inevitably hit some healthy tissue. A systematic review and meta-analysis of ADC monotherapy in non-small cell lung cancer found that the most common side effects of TROP2-targeting agents were nausea (affecting roughly 38% of patients at any severity), fatigue (about 34%), and neutropenia (about 25%).²⁰ TROP2 ADCs were also associated with higher rates of stomatitis (painful mouth sores) and eye-related toxicity compared with HER2-directed ADCs. Interstitial lung disease, an inflammation of lung tissue that can be serious, occurred in about 9% of patients receiving Dato-DXd in the TROPION-Lung01 trial, roughly double the rate seen with standard chemotherapy.¹⁵

On the flip side, severe (grade 3 or higher) side effects were less common with Dato-DXd than with docetaxel in that same trial, about 26% versus 42%. The overall tolerability profile is part of what makes these ADCs attractive compared with conventional chemotherapy, despite the specific toxicities they introduce. Managing stomatitis and monitoring for lung inflammation are now routine parts of treatment protocols for patients on these drugs.

When Resistance Develops

Like most cancer therapies, TROP2-directed ADCs eventually stop working for many patients. One key mechanism is straightforward: the tumor cells reduce or eliminate TROP2 from their surface, removing the target the drug needs to bind. Lab models of Dato-DXd resistance have shown marked TROP2 downregulation, and clinical cases have identified specific TROP2 mutations that prevent the protein from localizing properly to the cell membrane, blocking drug binding.²¹ Other resistance pathways involve changes in how the cell processes the drug after internalization or activates efflux pumps that push the chemotherapy payload back out before it can do its work. Resistance remains one of the most active areas of research in the ADC field, and understanding it is essential for designing next-generation agents or rational combination strategies.

Combining TROP2 ADCs With Immunotherapy

One of the more promising frontiers is pairing TROP2 ADCs with immune checkpoint inhibitors. The rationale is not just “two drugs might be better than one.” Preclinical work has shown that Dato-DXd appears to activate dendritic cells within tumors, which in turn recruit CD8+ T cells that can kill cancer cells directly. When combined with PD-1 or PD-L1 inhibitors in mouse models, the antitumor effect exceeded what either treatment achieved alone.²² In essence, the ADC may transform an immunologically “cold” tumor into one the immune system can recognize and attack. Several clinical trials are now testing these combinations in patients, though mature efficacy data from randomized trials are still pending.

Research into dual-payload ADCs is also underway. One experimental design attaches two different types of chemotherapy to a single TROP2-targeting antibody, aiming to overwhelm the tumor’s ability to resist through any single mechanism. Early preclinical results in models with mixed TROP2 expression have shown these dual-payload agents achieving tumor clearance at low doses, including in models where expression was heterogeneous.²³

Beyond ADCs

ADCs are not the only way to weaponize TROP2. Researchers are developing CAR-T cells, a form of cellular immunotherapy, that are engineered to recognize TROP2 on tumor surfaces. A preclinical program for triple-negative breast cancer has built a second-generation CAR targeting TROP2 and demonstrated antitumor activity in both laboratory and animal models.²⁴ CAR-T cells have transformed the treatment of certain blood cancers, but making them work against solid tumors remains enormously difficult, and whether TROP2-targeting CAR-T cells can achieve meaningful clinical responses remains unknown.

Another approach uses bispecific antibodies, molecules designed to grab TROP2 on the tumor cell with one arm and CD3 on a T cell with the other, physically bridging the two and triggering the T cell to attack. An engineered TROP2/CD3 bispecific T-cell engager is being developed for gastric, pancreatic, and colorectal cancers, aiming to redirect the patient’s own immune cells toward TROP2-expressing tumors while minimizing damage to normal tissues.²⁵ Both of these approaches are years away from patient use, but they illustrate how many different ways the oncology field is trying to exploit a single surface protein.

The Cost Problem

There is an uncomfortable gap between the clinical promise of TROP2-directed ADCs and their real-world accessibility. Multiple cost-effectiveness analyses conducted in the Chinese healthcare context have reached the same conclusion: at current pricing, sacituzumab govitecan is not cost-effective. For triple-negative breast cancer, one analysis found the drug would need an 83% price reduction to clear cost-effectiveness thresholds.²⁶ Another analysis calculated that the cost of SG would need to drop to roughly $156 per unit to be considered good value within Chinese willingness-to-pay thresholds.²⁷ Even when the analysis was restricted to patients with high TROP2 expression, who derive the greatest clinical benefit, the cost per quality-adjusted life-year gained remained far above thresholds.²⁸

These analyses are specific to one country’s healthcare economics and do not directly translate to pricing negotiations in the United States, Europe, or elsewhere. But they highlight a broader challenge facing all ADCs: the manufacturing process is complex, the drugs are biologic products rather than small molecules, and the per-dose cost reflects that complexity. As more TROP2 ADCs enter the market and biosimilar competition eventually emerges, costs should come down. For now, though, access depends heavily on geography, insurance coverage, and national reimbursement policies, meaning many patients whose tumors express TROP2 cannot get the drugs designed to target it.