Zepzelca (lurbinectedin) is a chemotherapy drug, not an immunotherapy. The FDA classifies it as an alkylating drug, a category of traditional cytotoxic chemotherapy that works by directly damaging cancer cell DNA. While Zepzelca is sometimes used alongside immunotherapy drugs, it belongs firmly in the chemotherapy category based on how it kills cancer cells.
How Zepzelca Works
Zepzelca attacks cancer cells by binding to their DNA and disrupting the machinery that reads genetic instructions. Specifically, it latches onto guanine bases in the DNA strand, forming chemical bonds that bend and distort the double helix. This distortion stalls a key enzyme called RNA polymerase II, which cells need to copy genetic instructions into proteins. Once that enzyme gets stuck on the DNA, the cell tags it for destruction and breaks it down. The result is DNA breaks that accumulate until the cell triggers its own death.
This is fundamentally different from how immunotherapy works. Immunotherapy drugs like atezolizumab (Tecentriq) block proteins that cancer cells use to hide from the immune system, essentially removing the brakes so your immune cells can recognize and attack the tumor. Zepzelca doesn’t involve the immune system at all. It’s a direct, chemical assault on the cancer cell’s ability to function.
Why the Confusion With Immunotherapy
The confusion likely comes from the fact that Zepzelca is now FDA-approved in combination with an immunotherapy drug. In October 2025, the FDA approved Zepzelca paired with atezolizumab as a maintenance treatment for extensive-stage small cell lung cancer (ES-SCLC). In this combination, Zepzelca is the cytotoxic (cell-killing) component layered on top of ongoing immunotherapy. The two drugs work through completely separate mechanisms, but they’re given together.
This combination approach reflects a broader trend in SCLC treatment. Checkpoint inhibitors like atezolizumab produce real, long-term benefit in only about 15% of SCLC patients. The other 85% get little or no benefit from immunotherapy alone. Adding a cytotoxic agent like Zepzelca during the maintenance phase is one strategy for improving those numbers.
What Zepzelca Is Approved For
Zepzelca is used to treat small cell lung cancer, an aggressive cancer that tends to respond initially to treatment but often returns quickly. The drug has two roles in the current treatment landscape. First, the National Comprehensive Cancer Network lists single-agent Zepzelca as a preferred option for patients whose SCLC has progressed after earlier treatment. Second, the 2025 FDA approval allows it to be used alongside atezolizumab as maintenance therapy after first-line treatment with a combination of atezolizumab, carboplatin, and etoposide.
How Treatment Is Given
Zepzelca is delivered as an intravenous infusion over 60 minutes, once every 21 days. The standard dose is 3.2 mg/m² (calculated based on your body size). Treatment continues on this three-week cycle until the cancer progresses or side effects become unmanageable. Whether you’re receiving Zepzelca alone or in combination with atezolizumab, the dosing schedule stays the same.
Side Effects to Expect
Because Zepzelca is a chemotherapy drug that damages DNA, it affects fast-dividing healthy cells too, particularly blood cells and bone marrow. The most common side effects reflect this:
- Fatigue affects about 77% of patients, with roughly 12% experiencing severe fatigue.
- Low neutrophil counts (a type of white blood cell critical for fighting infection) occur in 71% of patients. Nearly half, 46%, develop severely low counts, which is the side effect most likely to require dose adjustments or treatment delays.
- Anemia (low red blood cells) shows up in about 74% of patients, though severe cases are less common at around 10%.
The high rate of neutropenia means your medical team will monitor blood counts closely throughout treatment. If counts drop too low, your next infusion may be delayed to give your bone marrow time to recover. This pattern of blood count monitoring and possible dose adjustments is typical of cytotoxic chemotherapy and is one of the practical differences patients notice compared to immunotherapy, which tends to cause immune-related side effects like skin rashes, thyroid problems, or inflammation in various organs.