Trodelvy Mechanism of Action: Trop-2 and SN-38 Delivery

Trodelvy (sacituzumab govitecan) works by combining a cancer-targeting antibody with a potent chemotherapy drug, delivering that drug directly into tumor cells while sparing much of the rest of the body. It belongs to a class of therapies called antibody-drug conjugates, or ADCs, and its specific design features set it apart from earlier drugs in that class. The story of how it kills cancer cells involves three linked components and at least one surprising trick that affects even neighboring tumor cells the antibody never touches.

What Trodelvy Is Made Of

Trodelvy is built from three distinct parts that work together. The first is a humanized antibody engineered to recognize a protein called Trop-2 on the surface of cells. The second is SN-38, a chemotherapy agent that poisons an enzyme cancer cells need to copy their DNA. The third is a specialized chemical connector called CL2A that links the antibody to SN-38 and controls when and where the drug gets released. Each of these components matters for how the drug behaves in the body, and each was chosen deliberately to solve problems that plagued earlier ADC designs.

One of Trodelvy’s distinguishing features is its unusually high drug-to-antibody ratio of 7.6, meaning each antibody molecule carries roughly eight molecules of SN-38 on average. It is also given at a relatively high dose of 10 mg/kg. Together, these design choices deliver a large amount of chemotherapy to tumor tissue with each infusion, compensating for the fact that SN-38 is considered only moderately potent compared to the ultra-toxic payloads used in some other ADCs.¹

Trop-2 and Why It Makes a Good Target

Trop-2 is a protein found on the surface of many cell types, but cancer cells tend to produce far more of it than healthy tissue does. This overexpression has been documented across a range of tumor types, including triple-negative breast cancer, urothelial cancer, and several other carcinomas. The sheer abundance of Trop-2 on tumor cells gives the Trodelvy antibody plenty of docking sites, increasing the amount of drug that reaches the right place.²

The catch is that Trop-2 is not exclusively a cancer protein. It appears at meaningful levels on several normal tissues as well, including skin, lung, and parts of the gastrointestinal tract. This overlap is the main reason Trodelvy causes side effects like diarrhea and skin rashes; the antibody does not perfectly distinguish between a cancer cell and a healthy cell that also expresses Trop-2.³ Research into next-generation Trop-2 antibodies has explored whether tweaking the antibody’s binding site or its affinity for Trop-2 can improve this selectivity. One study identified new antibody variants that recognize a different structural region of Trop-2 than sacituzumab does, with binding affinities ranging from about 10 to 26 nanomolar compared to sacituzumab’s tighter 1.4 nanomolar grip.⁴ Whether a looser or tighter binding antibody performs better in the body is still an open question, and tighter affinity does not always translate to better tumor penetration, since a very sticky antibody can get trapped in the outer layers of a tumor instead of reaching its core.

How SN-38 Kills Cancer Cells

SN-38 is the active form of irinotecan, a chemotherapy drug that has been used in oncology for decades. On its own, SN-38 is too toxic and too quickly cleared from the bloodstream to be given directly as a standalone treatment at useful doses. By packaging it inside an ADC, Trodelvy essentially smuggles a concentrated dose of SN-38 into the tumor.

Once inside a cell, SN-38 targets an enzyme called topoisomerase I. This enzyme normally helps untangle DNA strands so they can be copied during cell division. SN-38 locks topoisomerase I onto the DNA strand, creating a stable complex that the cell cannot easily resolve. When the cell’s replication machinery crashes into these stalled complexes, the result is double-stranded DNA breaks, which are catastrophic damage that typically triggers cell death.⁵

Interestingly, SN-38 does not only damage cells that are actively dividing. Research has shown it can also cause immediate chromosome breaks independent of DNA replication, meaning even cells in a resting state are vulnerable to some degree. This dual activity may help explain why Trodelvy can be effective against tumors with a mix of rapidly dividing and slower-growing cells.⁶

The CL2A Linker and Drug Release

The linker connecting the antibody to its drug payload is one of the most carefully engineered parts of any ADC. In Trodelvy’s case, the CL2A linker is designed to be hydrolyzable, meaning it can be gradually broken down by water, particularly in the mildly acidic environment found inside cellular compartments after the ADC is internalized. This is a deliberate design choice rather than a flaw. Once Trodelvy binds to Trop-2 on a cancer cell and gets pulled inside, the acidic conditions accelerate linker cleavage, releasing free SN-38 right where it needs to be.

A natural question is whether the linker also breaks apart prematurely in the bloodstream, releasing SN-38 before it ever reaches a tumor. Detailed tracking experiments using fluorescent labels showed that within the first 24 hours, no detectable drug release occurred outside of cells. By 48 hours, about a quarter of the labeled drug appeared to have separated from the antibody, but further analysis attributed this to linker cleavage happening in the blood circulation rather than in the space immediately around tumor cells. Because Trop-2 is internalized relatively quickly once the antibody binds to it, the amount of drug lost to premature release in the tumor neighborhood is considered negligible compared to what gets delivered directly through receptor-mediated uptake.⁷

In practical terms, this means Trodelvy maintains reasonable stability in circulation while still releasing its payload efficiently once it reaches tumor cells. The trade-off is that some systemic SN-38 exposure does occur, which contributes to side effects like neutropenia and diarrhea. But the design strikes a balance: a more stable linker would reduce side effects yet also slow drug release inside the tumor, potentially making the drug less effective.

Getting Inside the Cell

Binding to Trop-2 on the cell surface is only the first step. For Trodelvy to work, the entire antibody-drug complex needs to be pulled inside the cell through a process called receptor-mediated internalization. Trop-2 turns over on the cell surface at a measured half-life of roughly four hours, meaning the protein is regularly recycled into the cell interior whether or not an antibody is attached.⁸ When Trodelvy hitches a ride on this recycling process, it gets shuttled into acidic compartments inside the cell where the linker breaks down and SN-38 is freed.

The speed of this internalization matters. Because Trop-2 cycles inward relatively quickly, the window during which the ADC sits exposed on the cell surface is short. This limits the amount of drug that could theoretically leak off the antibody while it is still outside the cell. It also means each Trop-2 molecule on the surface can effectively ferry multiple rounds of ADC molecules into the cell over the course of a treatment cycle, amplifying the total dose of SN-38 delivered internally.

The Bystander Effect

One of Trodelvy’s most clinically significant features is its ability to kill tumor cells that the antibody never directly binds to. After SN-38 is released inside a Trop-2-positive cell, some of it diffuses back out through the cell membrane into the surrounding tumor tissue. Because SN-38 is a small, membrane-permeable molecule, it can enter nearby cells regardless of whether those cells express Trop-2 at all.⁹

This bystander killing effect is particularly important in tumors where Trop-2 expression is uneven. Real tumors are not uniform sheets of identical cells. Some patches may express high levels of Trop-2, while neighboring areas may express little or none. Without a bystander effect, those low-expressing patches would be largely untouched by the therapy. The ability of SN-38 to leak out and damage surrounding cells helps Trodelvy cover more of the tumor than its targeting antibody alone would suggest. The moderately potent nature of SN-38, combined with the high drug-to-antibody ratio and the hydrolyzable linker, all work together to make this bystander killing meaningful rather than negligible.

Why Your Genetics Can Affect Side Effects

SN-38 is broken down in the body by a liver enzyme called UGT1A1. Genetic variation in the gene that codes for this enzyme can significantly affect how quickly or slowly a person clears SN-38 from their system. People who carry two copies of a variant called UGT1A1*28 have reduced enzyme activity, which means SN-38 lingers in their blood at higher levels for longer. This raises their risk of severe neutropenia, a dangerous drop in white blood cell counts.

The data bear this out concretely. Among patients treated with Trodelvy who had their UGT1A1 genotype tested, those homozygous for the *28 variant experienced grade 4 neutropenia at a rate of about 26%, compared to roughly 13% in patients carrying one copy and 11% in those with two normal copies. The *28 variant is not rare: it affects approximately 20% of Black or African American individuals, 10% of White individuals, and 2% of East Asian individuals.¹⁰

Despite this, routine genetic testing before starting Trodelvy is not standard practice. The current guidance is that patients already known to have reduced UGT1A1 activity should be monitored more closely, but testing everyone beforehand is not recommended. The right dose adjustment for people with this genetic profile has not been firmly established, so the approach remains individualized, typically involving more frequent blood count monitoring and readiness to reduce the dose or delay treatment cycles if neutropenia develops.¹¹

How Trodelvy Might Work with Immunotherapy

A growing area of research explores whether Trodelvy can enhance the effectiveness of immune checkpoint inhibitors, the drugs that help the immune system recognize and attack cancer. The rationale is that when an ADC kills tumor cells, it does not just quietly dissolve them. The dying cells release their internal contents, including proteins that can act as signals to the immune system. This process, sometimes called immunogenic cell death, can make the tumor more visible to immune cells that were previously ignoring it.

Preclinical and early clinical studies suggest that ADCs like Trodelvy may enhance the immune response in multiple ways: by releasing tumor-specific proteins that activate certain immune cells, by triggering antibody-dependent cellular cytotoxicity (where the antibody component recruits immune cells directly), and by making the tumor microenvironment more hospitable to immune attack. When combined with checkpoint inhibitors that remove the brakes on T-cell activity, the combined effect can be greater than either treatment alone.¹² Clinical trials testing these combinations are ongoing, and the evidence is still maturing, but the biological logic is compelling enough that it has become one of the most active research directions in breast cancer treatment.

Other Trop-2 ADCs and How They Differ

Trodelvy is not the only ADC targeting Trop-2. Datopotamab deruxtecan (Dato-DXd) is another Trop-2-directed ADC that has progressed through clinical trials, and comparing the two highlights how the same target can be approached with very different engineering choices. Dato-DXd uses a different antibody, a different linker (a cleavable tetrapeptide-based connector), and a different payload (a topoisomerase I inhibitor called DXd, which is more potent than SN-38 on a per-molecule basis). It also has a lower drug-to-antibody ratio of about 4, compared to Trodelvy’s 7.6.¹³

These differences translate to distinct clinical profiles. Trodelvy’s approach, a moderately potent payload carried in large quantities on a hydrolyzable linker, produces a significant bystander effect and a side-effect profile dominated by neutropenia and diarrhea. Dato-DXd’s approach, a more potent payload carried in smaller quantities on a more stable linker, tends to produce different toxicities, including more frequent mouth sores and skin reactions but generally less severe myelosuppression. Neither design is objectively superior; they represent different trade-offs between potency per molecule, amount of drug delivered, and how tightly the drug stays tethered until it reaches a cancer cell.

The existence of multiple Trop-2 ADCs also raises the question of sequencing, whether a patient whose cancer stops responding to one can still benefit from the other. This is an area of active investigation, and the answer likely depends on the specific resistance mechanisms at play. If resistance arises because the tumor loses Trop-2 expression, neither drug would work. If resistance involves the tumor learning to pump out the payload through drug efflux pumps, switching to a structurally different payload could potentially restore activity. If resistance involves changes to how topoisomerase I interacts with the drug, the picture gets more complicated, since both payloads target the same enzyme family.¹⁴

How It Was Developed

Sacituzumab govitecan was originally designated IMMU-132 during its preclinical and early clinical development. The drug was designed to solve a specific problem: irinotecan, the prodrug form of SN-38, had shown activity against many solid tumors but caused severe systemic toxicity because the body converts it to SN-38 unpredictably and throughout the entire body. By conjugating pre-formed SN-38 directly to an anti-Trop-2 antibody, the developers aimed to bypass irinotecan’s inefficient and toxic activation step and deliver the active drug more directly to tumors.

Early phase 1 trials enrolled patients who had already been through multiple rounds of prior therapy, and the drug showed encouraging activity across several tumor types. Pharmacokinetic studies during these trials confirmed that the ADC cleared from the body in a predictable way, and that both free SN-38 and its inactive metabolite SN-38G could be measured in the blood, confirming that the drug was behaving as designed: reaching tumors intact, releasing SN-38 there, and then the released SN-38 was being metabolized through the expected UGT1A1 pathway.¹⁵ The drug eventually received accelerated approval in 2020 for metastatic triple-negative breast cancer and has since gained approvals in additional settings, including urothelial cancer and hormone receptor-positive, HER2-negative breast cancer.