SHR-A1811: A Closer Look at This HER2-Targeted ADC

SHR-A1811 is an investigational antibody-drug conjugate that pairs trastuzumab, the well-known HER2-targeting antibody, with a novel topoisomerase I inhibitor payload called SHR169265. In early-phase clinical trials, it has produced response rates above 60% in HER2-low breast cancer and nearly 80% in HER2-positive breast cancer, while showing a notably low incidence of interstitial lung disease, one of the most feared complications of HER2-directed ADCs. The drug’s design choices and clinical results position it as a potential competitor in a rapidly evolving landscape, though it remains in clinical development without regulatory approval.

How SHR-A1811 Is Built

An antibody-drug conjugate works by attaching a potent cell-killing chemical to an antibody that homes in on a specific target on cancer cells. SHR-A1811 uses trastuzumab as its targeting antibody, the same antibody backbone that has been used in HER2-positive cancer treatment for over two decades. What distinguishes SHR-A1811 from other HER2-directed ADCs is its payload and its design ratio. The cytotoxic payload is SHR169265, a topoisomerase I inhibitor that interferes with DNA replication inside cancer cells. A cleavable chemical linker connects the payload to the antibody, designed to break apart preferentially inside tumor cells while staying intact in the bloodstream.

One of the deliberate engineering decisions behind SHR-A1811 is its drug-to-antibody ratio (DAR) of 6, meaning each antibody molecule carries an average of six payload molecules. This number was chosen to balance tumor-killing potency against toxicity. In preclinical work, a DAR of 6 provided strong antitumor activity and effective bystander killing, where the released payload can also destroy nearby cancer cells that may not express HER2 themselves, while potentially improving the safety profile compared to higher loading ratios.¹ Pharmacokinetic data from clinical trials confirmed that the linker remains stable in the bloodstream: the amount of free (unattached) payload detected in patients’ plasma was low across all dose levels, suggesting that normal tissues are largely spared from the cytotoxic effect during circulation.²

Breast Cancer Results

The strongest efficacy signal for SHR-A1811 has come from breast cancer. In the global phase 1 HORIZON-X trial, patients with HER2-positive breast cancer who had already received prior treatment showed confirmed response rates around 79%, with responses lasting a median of nearly two years. Median progression-free survival reached 20 to 25 months depending on the data cut analyzed.³ In the same trial, patients with HER2-low breast cancer, a group that was historically considered HER2-negative and only recently recognized as a treatable subpopulation, had a confirmed response rate of about 62% and a median progression-free survival of 11 months.⁴

Separate early data from a neoadjuvant setting, where SHR-A1811 was given before surgery in patients with hormone receptor-positive, HER2-low breast cancer, showed an objective response rate of about 74% in the first stage of a phase 2 trial. These results are still maturing but suggest the drug’s activity extends beyond the metastatic setting.

Activity in Lung and Gastric Cancers

HER2 abnormalities are not limited to breast cancer. In non-small cell lung cancer (NSCLC), HER2 mutations occur in a small but meaningful fraction of patients. SHR-A1811 was tested in a phase 1/2 study of patients with pretreated HER2-mutant NSCLC at a dose of 4.8 mg/kg. The objective response rate was about 42%, disease was controlled in over 95% of patients, and the median duration of response reached nearly 14 months with many responses still ongoing at the time of analysis. Median progression-free survival was roughly 8.4 months.²

In gastric and gastroesophageal junction cancers, early data from HORIZON-X showed a confirmed response rate of roughly 46% in HER2-positive patients, with a notably durable median duration of response approaching 16 months. The numbers are based on small patient cohorts, and progression-free survival in gastric cancer was shorter than in breast cancer, which is consistent with how aggressive these tumors tend to be. Still, the durability of responses in patients who did respond is encouraging.³

Across non-breast solid tumors more broadly, the confirmed response rate in HORIZON-X was about 40%, with a median duration of response of roughly 15 months.⁴ These are phase 1 results and the patient numbers are modest, but they point to activity across multiple tumor types where HER2 is expressed or mutated.

Safety and Tolerability

The side-effect profile of any ADC matters enormously, and for HER2-targeted ADCs in particular, interstitial lung disease (ILD) is the toxicity that draws the most attention. ILD is a potentially life-threatening inflammatory condition of the lungs that has been a significant concern with existing HER2-directed ADCs. In the HORIZON-X trial of SHR-A1811, ILD occurred in about 2.5% of patients. Most of those cases were low-grade, and only three were grade 3 or higher, including one fatal event in a patient receiving the highest dose (8.0 mg/kg).⁵ A separate analysis of the broader phase 1 program reported an ILD rate of about 2.6%.⁶

The more common side effects were hematologic. In the global phase 1 program, the most frequent grade 3 or higher adverse events were drops in neutrophil count (about 39% of patients) and white blood cell count (about 23%).⁶ Nausea and vomiting were very common at any grade but rarely severe. Serious adverse events occurred in about 23% of patients, and deaths related to any cause occurred in about 4% across the trial. The recommended doses selected for phase 2 studies were 4.8 mg/kg and 6.4 mg/kg, chosen based on the balance between efficacy and tolerability observed across the dose-escalation cohorts.

These tolerability numbers come from a heavily pretreated population, many of whom had received multiple prior lines of therapy. It is worth noting that combination regimens involving SHR-A1811 with other agents like pyrotinib have shown higher rates of certain toxicities, such as diarrhea affecting all patients at any grade and over half at grade 3 or higher, which reflects the added burden of combining active drugs rather than a problem inherent to SHR-A1811 alone.

Early Comparisons With Established ADCs

The elephant in the room for any new HER2-targeted ADC is trastuzumab deruxtecan (T-DXd), which has reshaped treatment in HER2-positive and HER2-low breast cancer as well as other tumor types. Head-to-head randomized trials between SHR-A1811 and T-DXd have not yet been reported, so all comparisons are indirect and should be interpreted cautiously.

A retrospective real-world analysis comparing HER2-directed ADCs in metastatic breast cancer found that median progression-free survival was about 25.8 months with SHR-A1811, compared with 9.3 months for T-DXd, 7.6 months for T-DM1, and 3.2 months for RC48.⁷ That PFS gap is striking, but real-world comparisons like this are heavily confounded by differences in patient selection, lines of therapy, and the fact that newer drugs tend to be used in patients with different treatment histories than established ones. Researchers who have published cross-trial comparisons in NSCLC, where SHR-A1811 and T-DXd show broadly similar response rates in HER2-mutant disease, have cautioned explicitly against reading too much into between-study comparisons given differences in trial design and patient characteristics.²

Where the comparison becomes more concrete is safety. The ILD rate of roughly 2.5% with SHR-A1811 in phase 1 data is in a range that has caught the attention of the field, particularly because ILD management and monitoring have become central challenges in the use of existing HER2-directed ADCs. Whether this lower rate holds up in larger randomized trials, and whether it reflects the optimized DAR, the specific linker chemistry, or patient selection in early trials, remains to be established.

Combination Strategies Under Investigation

Researchers are already exploring whether SHR-A1811 works better when combined with other agents. A phase 1b/2 study has been evaluating SHR-A1811 alongside adebrelimab, an anti-PD-L1 checkpoint inhibitor, in patients with unresectable or metastatic triple-negative breast cancer, a particularly aggressive subtype that does not express hormone receptors and was historically not considered a candidate for HER2-directed therapy.⁸ The same study platform is also testing SHR-A1811 in combination with pyrotinib (a pan-HER tyrosine kinase inhibitor), pertuzumab (another HER2-targeted antibody), and albumin-bound paclitaxel in various breast cancer populations.

The rationale for combining ADCs with immunotherapy is that the cell damage caused by the cytotoxic payload may make tumors more visible to the immune system, potentially amplifying the effect of checkpoint inhibitors. Combining with HER2-targeted small molecules like pyrotinib, on the other hand, aims to block HER2 signaling through a different mechanism while the ADC delivers its cytotoxic blow. These are early-stage investigations, and the combination safety data reported so far suggest that managing overlapping toxicities, especially hematologic side effects and gastrointestinal symptoms, will be a central challenge as these regimens move forward.

Tracking Response Through Circulating Tumor DNA

One intriguing line of investigation has been whether blood-based biomarkers can help predict which patients will benefit most from SHR-A1811 and how long that benefit will last. In the NSCLC study, researchers measured circulating tumor DNA (ctDNA) in a subset of patients and found that changes in ctDNA levels tracked closely with tumor shrinkage. Among patients whose tumors were shrinking on imaging, over 93% also showed a drop in ctDNA levels from baseline.⁹

More practically, patients who cleared ctDNA from their blood within six months of starting treatment had significantly longer progression-free survival than those who did not, and they were also less likely to develop drug resistance. ctDNA levels were significantly higher in patients whose cancers were growing resistant to the drug compared to those still responding. If validated in larger studies, ctDNA monitoring could serve as an early warning system, flagging treatment resistance before it shows up on a scan and potentially prompting earlier changes in therapy.

Drug Behavior in the Body

SHR-A1811 shows linear pharmacokinetics across the dose range tested, meaning that doubling the dose roughly doubles the drug exposure in the bloodstream. The half-life of the intact ADC ranged from about 5 to 7.5 days, and there was minimal accumulation with repeated dosing, with accumulation ratios between 1.2 and 1.5 for the ADC itself.¹⁰ The free payload, meanwhile, showed essentially no accumulation (ratios of 0.8 to 0.9), reinforcing the picture of a stable linker that releases its cargo mainly at the tumor site.

Immunogenicity, the risk that a patient’s immune system will develop antibodies against the drug and neutralize it, appears to be very low. In the NSCLC study, none of the 63 tested patients developed antibodies against SHR-A1811.¹⁰ The larger HORIZON-X trial confirmed this finding: no patient tested positive for treatment-induced anti-drug antibodies after baseline.⁵ This is a meaningful practical advantage, because anti-drug antibodies can reduce a therapy’s effectiveness over time and limit how long patients can stay on treatment.

Where Development Stands

All of the results described above come from phase 1 and phase 1/2 trials, which are designed primarily to establish safety and find the right dose, with efficacy measured as a secondary or exploratory goal. The response rates and progression-free survival numbers are promising, but phase 1 populations tend to be enriched for patients who tolerate treatment well, and the absence of a randomized control group makes it impossible to know how much of the observed benefit is due to the drug versus patient selection.

The investigators behind the HORIZON-X trial have stated that SHR-A1811’s overall safety profile supports moving into phase 3 trials with larger patient populations.³ Several randomized phase 3 trials are ongoing or planned, including studies in HER2-positive and HER2-low breast cancer and in gastric cancer. These trials will provide the kind of head-to-head or placebo-controlled data needed to understand whether SHR-A1811 offers a genuine improvement over existing options, or whether it occupies a similar efficacy tier with a potentially differentiated safety profile. Until those results mature, SHR-A1811 remains one of the most closely watched ADCs in development, particularly by clinicians looking for HER2-targeted options that might spare patients from the pulmonary complications that have complicated the use of earlier-generation agents.

1
PubMed Central. SHR-A1811, a novel anti-HER2 antibody-drug conjugate with optimal drug-to-antibody ratio, efficient tumor killing potency, and favorable safety profiles
2
PubMed Central. SHR-A1811 (antibody-drug conjugate) in advanced HER2-mutant non-small cell lung cancer: a multicenter, open-label, phase 1/2 study
3
PubMed Central. SHR-A1811, a novel HER2-targeting antibody-drug conjugate, in advanced solid tumors (HORIZON-X): a global phase 1 trial
4
Clinical Cancer Research. Efficacy and safety of SHR-A1811, an anti-HER2 antibody-drug conjugate (ADC), in 391 heavily pretreated multiple solid tumors with HER2-expression or mutations
5
Signal Transduction and Targeted Therapy. SHR-A1811, a novel HER2-targeting antibody-drug conjugate, in advanced solid tumors (HORIZON-X): a global phase 1 trial
6
PubMed. Safety, Efficacy, and Pharmacokinetics of SHR-A1811, a Human Epidermal Growth Factor Receptor 2-Directed Antibody-Drug Conjugate, in Human Epidermal Growth Factor Receptor 2-Expressing or Mutated Advanced Solid Tumors: A Global Phase I Trial
7
PubMed Central. Clinical outcomes and genomic landscape of anti-HER2 antibody-drug conjugates in HER2-positive and HER2-low metastatic breast cancer
8
Journal of Clinical Oncology. SHR-A1811 plus adebrelimab in unresectable or metastatic triple-negative breast cancer: Results from a phase 1b/2 expansion cohort
9
Signal Transduction and Targeted Therapy. SHR-A1811 (antibody-drug conjugate) in advanced HER2-mutant non-small cell lung cancer: a multicenter, open-label, phase 1/2 study
10
Signal Transduction and Targeted Therapy. SHR-A1811 (antibody-drug conjugate) in advanced HER2-mutant non-small cell lung cancer: a multicenter, open-label, phase 1/2 study – Section: Pharmacokinetics