PACIFIC Trial: Durvalumab for Stage III Lung Cancer

The PACIFIC trial is a phase 3, randomized, placebo-controlled study that demonstrated a significant survival benefit when the immunotherapy drug durvalumab was given as maintenance treatment after chemoradiotherapy in patients with unresectable stage III non-small cell lung cancer (NSCLC). Published results showed that durvalumab reduced the risk of death by roughly 30% and nearly tripled the time patients lived without their disease progressing. The trial reshaped the standard of care for a large group of lung cancer patients who previously had limited treatment options after completing chemoradiation, and its influence continues to drive new research years later.

Why the Trial Mattered

Stage III NSCLC accounts for about a third of all lung cancer diagnoses. “Unresectable” means the tumor cannot be fully removed by surgery, often because of its size, location, or involvement of nearby structures. For decades, the standard treatment for these patients was concurrent chemoradiotherapy, a demanding regimen combining platinum-based chemotherapy with daily radiation. After completing that treatment, patients simply waited and watched. Five-year survival rates with chemoradiotherapy alone ranged between about 15% and 25%, and long-term data were rarely reported.¹ The gap between finishing chemoradiation and disease recurrence felt uncomfortably short for many patients and their oncologists.

Durvalumab is a monoclonal antibody that blocks PD-L1, a protein some tumors use to hide from the immune system. The idea behind PACIFIC was that chemoradiation might prime the immune system by releasing tumor-specific signals, and following up with an immune checkpoint inhibitor could extend or deepen that response. The trial was designed to test whether giving durvalumab after chemoradiation could delay or prevent cancer from returning.

How the Trial Was Set Up

PACIFIC enrolled patients aged 18 or older who had histologically confirmed stage III, unresectable NSCLC and had completed at least two cycles of platinum-based concurrent chemoradiotherapy without disease progression afterward. Patients were randomly assigned in a 2:1 ratio to receive either intravenous durvalumab at 10 mg/kg or a matching placebo every two weeks for up to 12 months. Treatment had to begin within 1 to 42 days after finishing chemoradiotherapy. The randomization was stratified by age, sex, and smoking history, and the trial was double-blind, meaning neither patients nor investigators knew who was getting the active drug.² The two primary endpoints were progression-free survival (PFS), meaning how long patients lived without their cancer worsening, and overall survival (OS).

The Headline Results

The initial OS analysis, at a median follow-up of about 25 months, found that the two-year survival rate was roughly 66% in the durvalumab group compared with about 56% in the placebo group. Durvalumab significantly reduced the risk of death, with a hazard ratio of 0.68. PFS was even more striking: the median time before the disease progressed was 17.2 months with durvalumab versus 5.6 months with placebo.³ In practical terms, durvalumab roughly tripled the duration patients went without their cancer growing or spreading, and it meaningfully extended life at a point when there had been no approved follow-up therapy.

Five-Year Follow-Up

Long-term data solidified the initial findings. At a median follow-up of about 34 months for all patients (and nearly 62 months for those still being tracked), the estimated five-year overall survival rate was roughly 43% with durvalumab compared with about 33% with placebo. Five-year PFS rates were about 33% versus 19%. Median overall survival was 47.5 months with durvalumab versus 29.1 months with placebo, an absolute difference of more than a year and a half.⁴ Compare those five-year figures against the historical range of 15% to 25% with chemoradiotherapy alone, and the shift is dramatic. A meaningful fraction of patients who received durvalumab were alive and disease-free five years later.

Safety and Side Effects

Combining immunotherapy with prior chemoradiation raises obvious concerns about toxicity, especially to the lungs. A post-hoc analysis of the PACIFIC data looked specifically at immune-mediated adverse events. Immune-related pneumonitis (lung inflammation driven by the immune system) occurred in about 9.4% of durvalumab-treated patients at any severity grade, with serious (grade 3 or 4) pneumonitis in about 1.9%. Other immune-mediated side effects outside the lungs occurred in roughly 10.7% of patients, but severe versions were uncommon at about 1.7%. Fatal immune-mediated events occurred in 0.8% of patients receiving durvalumab, and all were cases of pneumonitis.⁵

Those numbers are notable for being relatively modest given that these patients had just received intensive radiation to their chests. The overall safety profile was manageable enough that regulatory agencies approved durvalumab for this setting. That said, pneumonitis remains the side effect clinicians watch most closely. Real-world data from the PACIFIC-R study found that about 9.5% of patients discontinued durvalumab because of pneumonitis or interstitial lung disease.⁶ A separate real-world analysis identified that a higher mean radiation dose to the lungs and a prior history of chronic obstructive pulmonary disease (COPD) were both associated with a higher risk of significant pneumonitis during durvalumab treatment.⁷ Clinicians therefore pay particular attention to radiation planning and lung dose when a patient is expected to receive durvalumab afterward.

Does PD-L1 Expression Predict Who Benefits?

This is one of the most debated aspects of the PACIFIC data. In the trial, durvalumab improved PFS across almost all PD-L1 subgroups. When tumor cells expressed PD-L1 on at least 1% of their surface, the benefit in both PFS and OS was clear. But in patients whose tumors expressed PD-L1 on less than 1% of cells, the OS hazard ratio crossed 1.0, meaning no survival benefit could be confidently shown in that subgroup.⁸

This split led to a concrete regulatory difference: the U.S. FDA approved durvalumab for this setting regardless of PD-L1 status, while the European Medicines Agency (EMA) restricted its approval to patients whose tumors express PD-L1 on at least 1% of tumor cells, based on the subgroup analysis.⁹ Real-world data from the PACIFIC-R study showed a consistent pattern: three-year OS rates were about 67% in patients with PD-L1 expression of at least 1% versus about 54% in patients with expression below 1%.¹⁰ In practice, this means the question of whether to offer durvalumab to PD-L1-negative patients depends partly on where you live and which regulatory framework your oncologist follows.

Quality of Life During Treatment

A survival benefit means less if the treatment makes patients miserable. The PACIFIC investigators tracked patient-reported outcomes covering cough, shortness of breath, chest pain, fatigue, appetite loss, physical functioning, and overall quality of life over 12 months. Across all of these measures, scores remained stable in both the durvalumab and placebo groups, with no clinically meaningful differences between the two arms. Time to worsening of key symptoms was also similar.¹¹ In short, durvalumab delivered its survival gains without making patients feel appreciably worse during the treatment year. For a drug given after an already grueling course of chemoradiation, that is a meaningful reassurance.

How the Results Held Up Outside the Trial

Clinical trials enroll carefully selected patients who meet strict eligibility criteria. Real-world practice is messier: patients are older, sicker, or have complications that would have excluded them from the trial. The PACIFIC-R study tracked nearly 1,400 patients across 11 countries who received durvalumab after chemoradiotherapy in routine clinical practice. The median real-world PFS was about 21.7 months, broadly consistent with the trial’s findings.¹² An interim OS analysis of the same cohort found a three-year survival rate of about 63%.¹³

The real-world data also confirmed some nuances. Patients who had received concurrent (simultaneous) chemoradiotherapy did better than those who received sequential treatment, with a median real-world PFS of about 23.7 months versus 19.3 months.¹⁴ This aligns with the original trial’s enrollment criteria, which focused on concurrent chemoradiotherapy, and suggests that the specific sequencing of treatment matters for getting the most out of durvalumab.

When to Start Durvalumab After Chemoradiation

The original PACIFIC protocol allowed durvalumab to be started anywhere from 1 to 42 days after completing chemoradiotherapy. A natural question emerged: does starting sooner versus later affect how well the drug works? A subgroup analysis from the trial itself showed that patients randomized 14 or more days after finishing radiation had a hazard ratio that crossed 1.0 for OS, hinting that earlier initiation might matter.¹⁵ But a more recent population-based analysis complicated this picture. That study found no significant survival difference when durvalumab was started within four weeks, and the survival benefit actually became significant when initiation was at five weeks or later. There was no deterioration in benefit when starting after six weeks.¹⁶

This is reassuring for patients and doctors in real-world practice, where logistical delays in insurance authorization, scheduling, or recovery from chemoradiation side effects can push the start of durvalumab past the 42-day window. The data suggest that a somewhat delayed start does not erase the benefit, and clinicians should not feel that missing the narrow early window means the opportunity is lost.

Why Starting Durvalumab During Chemoradiation Did Not Work

If durvalumab works well after chemoradiation, it seemed logical to ask whether giving it simultaneously with chemoradiation might work even better. The PACIFIC-2 trial tested exactly that: durvalumab given concurrently with chemoradiotherapy rather than afterward. The result was definitively negative. There was no statistically significant difference in either PFS or OS between patients receiving concurrent durvalumab and those receiving placebo. The two-year survival rates were nearly identical at about 58% and 60%, respectively.¹⁷ Objective response rates were also virtually identical.¹⁸

The failure of PACIFIC-2 underscored an important biological principle: the timing of immunotherapy in relation to radiation and chemotherapy is not interchangeable. The sequential approach, where chemoradiation goes first and immunotherapy follows, appears to depend on the immune-priming effects of radiation. Adding an immune checkpoint inhibitor while the immune system is simultaneously being suppressed by chemotherapy seems to blunt, rather than enhance, the interaction. This result has shaped how researchers design subsequent trials in this space.

Do EGFR and ALK Mutations Change the Picture?

Immune checkpoint inhibitors generally perform less impressively in NSCLC patients whose tumors harbor certain driver mutations, particularly in EGFR and ALK genes. In stage IV (metastatic) disease, these patients typically get targeted therapies rather than immunotherapy. But in stage III disease treated with chemoradiation, the question of whether durvalumab consolidation still helps is more nuanced. A Korean study compared outcomes of durvalumab consolidation in patients with EGFR or ALK mutations versus those without. The median PFS was about 21 months in both groups, with no statistically significant difference.¹⁹ This is somewhat surprising given the skepticism about immunotherapy in mutation-driven tumors, though the study was not a randomized comparison against placebo, so firm conclusions are harder to draw. Still, the finding provides some comfort for clinicians managing stage III patients who happen to carry these mutations.

The Cost Question

Durvalumab is expensive, and the economics of a 12-month course of immunotherapy have been scrutinized. An early U.S.-based cost-effectiveness analysis estimated an incremental cost-effectiveness ratio (ICER) of roughly $67,000 per quality-adjusted life year (QALY), which falls within the range many health economists consider acceptable for cancer treatments in the United States.²⁰ However, a later analysis using updated data estimated the ICER at about $139,000 per QALY, with durvalumab having roughly a 63% probability of being cost-effective at a willingness-to-pay threshold of $150,000 per QALY.²¹

When analyzed from an international perspective, the numbers look less favorable. A 2024 study modeling costs from the viewpoints of payers in the United States, Brazil, Singapore, and Spain found that durvalumab was cost-prohibitive in all four countries according to their respective willingness-to-pay thresholds. The U.S. ICER in that model was roughly $229,000 per QALY.²² The discrepancy between earlier and later estimates partly reflects different modeling assumptions and longer follow-up data. But the broader takeaway is consistent: durvalumab consolidation is clinically effective but financially burdensome, and its affordability depends heavily on drug pricing, the health-care system’s resources, and how much a society is willing to spend for an additional year of good-quality life. Biosimilars or negotiated pricing could shift this calculus in the future.

Next-Generation Combinations Building on PACIFIC

Researchers are now asking whether durvalumab’s results can be improved by adding a second immunotherapy agent. The COAST trial, a randomized phase 2 study, tested durvalumab alone against durvalumab combined with either oleclumab (which targets CD73, an enzyme tumors use to create an immune-suppressive environment) or monalizumab (which blocks NKG2A, a receptor that suppresses natural killer cells and certain T cells). Both combination arms showed higher objective response rates: about 30% with oleclumab and roughly 36% with monalizumab, compared with about 18% for durvalumab alone. PFS was also numerically longer with both combinations, and no new safety signals emerged.²³

Those results were encouraging enough to launch PACIFIC-9, an international phase 3 trial comparing durvalumab plus oleclumab, durvalumab plus monalizumab, and durvalumab plus placebo in patients with unresectable stage III NSCLC after concurrent chemoradiotherapy.²⁴ Updated data from the COAST trial continue to support the rationale for these combinations.²⁵ Whether the PACIFIC-9 results will eventually change standard practice is an open question, but the trajectory is clear: the PACIFIC framework, chemoradiation followed by immune consolidation, has become the platform on which the next generation of treatments is being built.