What Is Keytruda Pneumonitis and How Is It Treated?

Keytruda (pembrolizumab) pneumonitis is lung inflammation triggered by the drug’s immune-boosting mechanism rather than by an infection. It affects a small but meaningful fraction of patients receiving Keytruda, and the standard treatment is corticosteroids, typically starting at a dose scaled to the patient’s body weight and tapered over several weeks. The condition ranges from barely noticeable on a scan to life-threatening respiratory failure, and the outcome depends heavily on how quickly it is caught and how aggressively it is managed.

How Keytruda Causes Lung Inflammation

Keytruda belongs to a class of drugs that block a protein called PD-1 on immune cells. Normally PD-1 acts as a brake, keeping immune cells from attacking the body’s own tissues. By releasing that brake, Keytruda helps T cells recognize and destroy cancer cells. The tradeoff is that those newly unleashed T cells sometimes also attack healthy tissue, and the lungs are one of the organs most vulnerable to this friendly fire. The resulting inflammation, called checkpoint inhibitor pneumonitis, can damage the delicate air sacs where oxygen exchange happens.

A multi-institutional study of patients with lung cancer treated with PD-1 or PD-L1 inhibitors estimated the risk of pneumonitis directly attributable to the drug at about 2.5%.¹ That number sounds low, but given how widely Keytruda is prescribed across lung cancer, melanoma, bladder cancer, and other tumor types, a few percent translates to thousands of affected patients each year. The risk is also not evenly distributed: certain patient profiles face considerably higher odds.

Who Is at Higher Risk

Several factors raise the chance of developing pneumonitis. Reviews of the published data point to pre-existing lung disease, prior radiation to the chest, and treatment with Keytruda in combination with other drugs as consistent risk factors.² Lung cancer patients seem to be at higher risk than patients with other tumor types, likely because their lungs are already compromised by disease, smoking history, or prior radiation fields.

Combining Keytruda with chemotherapy also appears to raise the risk. An analysis of two large global safety databases found that pembrolizumab plus chemotherapy had roughly 1.8 to 1.9 times the reporting odds of pneumonitis compared to pembrolizumab alone.³ This matters because chemo-immunotherapy combinations are now standard first-line treatment for many lung cancers, meaning a large pool of patients is exposed to this elevated risk.

When Symptoms Appear

Pneumonitis does not follow a strict timetable. One review of anti-PD-1 therapy found the time to onset ranged from as early as nine days to as late as about 19 months after starting treatment.⁴ The median onset is around 2.8 months.⁵ That wide window is part of what makes the condition tricky: a patient can develop pneumonitis after just one infusion or after a year of uneventful treatment. Vigilance cannot let up even when the first several cycles have gone smoothly.

What It Looks and Feels Like

The classic symptoms are a new or worsening cough, shortness of breath, and sometimes fever. Case reports describe patients presenting with all three, sometimes within days of an infusion.⁶ Low-grade pneumonitis, however, can be silent, spotted only on routine imaging without the patient feeling anything different. This asymptomatic presentation is one reason many oncologists order periodic chest scans during treatment.

The severity is graded on a scale from 1 to 5 using the Common Terminology Criteria for Adverse Events (CTCAE). Grade 1 means imaging findings only, with no symptoms. Grade 2 adds symptoms that limit some daily activities. Grade 3 or higher means severe symptoms, possible need for supplemental oxygen, and sometimes hospitalization. Research commonly splits patients into “mild” (grade 2 or below) and “severe” (grade 3 and above) groups for analysis.⁷

How It Is Diagnosed

Diagnosis begins with a CT scan of the chest, which typically shows ground-glass opacities, the hazy patches that indicate fluid or inflammation in the lung tissue. Patients treated with immunotherapy tend to show a distinctive constellation of features on imaging, including consolidation, septal thickening, traction bronchiectasis, and sometimes pulmonary nodules.⁸ Several radiographic patterns have been described, including organizing pneumonia, nonspecific interstitial pneumonia, hypersensitivity pneumonitis, and an acute pattern resembling respiratory distress syndrome.⁹

The challenge is that imaging alone cannot always distinguish pneumonitis from infection, tumor progression, or radiation injury. In ambiguous cases, bronchoscopy with a bronchoalveolar lavage (where fluid is washed into a segment of lung and then collected for analysis) can be informative. Studies of patients who underwent this procedure have consistently found a high proportion of lymphocytes in the lavage fluid, a pattern that supports an immune-driven process rather than a straightforward infection.¹⁰ Higher lymphocyte percentages in the lavage fluid have also been linked to more severe disease, and patients with pre-existing lung disease before starting immunotherapy were far more likely to develop high-grade pneumonitis.¹¹

Tissue biopsies, though not always performed, reveal patterns that match the imaging. The most common finding under the microscope is organizing pneumonia, where plugs of inflammatory tissue fill the small airways. This pattern tends to correspond with milder disease. When pathologists instead see diffuse alveolar damage, with hyaline membranes lining the air sacs, the outlook is considerably worse.¹² Another study confirmed these two broad histological categories, with the acute lung injury pattern found in patients with respiratory failure and organizing pneumonia found in those with milder symptoms.¹³

Blood Markers That May Help Catch It Early

Researchers have been looking for blood tests that could flag pneumonitis before it becomes severe. One study tracked several inflammatory markers in lung cancer patients starting checkpoint inhibitor therapy. Patients who developed pneumonitis showed significant rises in a cytokine called IL-6 and in the platelet-to-lymphocyte ratio, while their lymphocyte counts dropped. High IL-6 levels and low albumin at the time pneumonitis appeared were both associated with more severe disease and worse survival.¹⁴ Other work has identified rises in additional circulating signals coinciding with the development of symptomatic pneumonitis.¹⁵ None of these markers have reached the point of routine clinical use for screening, but they are active areas of investigation and may eventually allow oncologists to intervene before symptoms worsen.

Treating the Inflammation

Corticosteroids are the backbone of treatment. For grade 1 pneumonitis, the usual approach is to hold Keytruda, monitor closely, and wait. If the imaging worsens or symptoms develop, steroids are started. For grade 2 and above, treatment typically begins with oral prednisolone at roughly 1 mg per kilogram of body weight per day. One phase II trial tested a structured six-week taper: full dose for two weeks, then stepping down through 0.7, 0.5, 0.2, and 0.1 mg/kg over the following four weeks before stopping.¹⁶ Some clinicians use longer tapers of several months, though case reports suggest that shorter courses can also achieve good results.¹⁷

For more severe cases, especially grade 3 or higher, intravenous methylprednisolone at high doses is common, with the switch to oral steroids once the patient stabilizes. The total steroid course often runs several weeks, and tapering too quickly can cause the pneumonitis to flare back. One published case described a patient restarted on immunotherapy after a three-week high-dose steroid taper who relapsed within four days, requiring permanent discontinuation of the drug.¹⁸

When Steroids Do Not Work

A small but dangerous subset of patients do not respond to steroids. Data from one study found that among patients with steroid-refractory pneumonitis, nearly all required intensive care, and three-quarters died of the pneumonitis or its infectious complications. Additional immunosuppressants tried in these cases included intravenous immunoglobulin (IVIG) and infliximab, but outcomes remained poor. All five patients treated with infliximab in that cohort died.¹⁹ These numbers underscore how critical early detection and steroid initiation are: by the time pneumonitis becomes refractory, treatment options narrow sharply.

There are occasional success stories with more aggressive approaches. One case report described a patient with steroid-refractory pneumonitis who improved after a triple combination of high-dose corticosteroids, tacrolimus, and cyclophosphamide pulse therapy, a strategy borrowed from the treatment of a different autoimmune lung condition.²⁰ This remains an isolated report, not a standard protocol, but it reflects the kind of creative problem-solving clinicians are forced into when standard therapy fails.

Restarting Keytruda After Pneumonitis

One of the most difficult decisions patients and oncologists face is whether to resume immunotherapy once pneumonitis has resolved. The cancer still needs treatment, and for many patients Keytruda was working well before the lung complication developed. Stopping it permanently can mean losing the best available weapon against the tumor.

The data here are mixed but somewhat encouraging. A retrospective cohort study found that about half of patients who resumed checkpoint inhibitor treatment experienced recurrence of pneumonitis, but those who were successfully rechallenged and continued on therapy had substantially better cancer outcomes than those who permanently stopped, with median overall survival roughly 34 months versus 13 months.²¹ A separate multi-center study reported a lower recurrence rate of about 20% after rechallenge, with patients who had more severe initial pneumonitis and poorer performance status at higher risk for recurrence.²² The wide range in recurrence rates across studies, from 20% to 50%, reflects differences in patient populations and how aggressively clinicians selected candidates for rechallenge. In general, patients whose initial pneumonitis was mild and resolved quickly are considered better candidates for restarting the drug.

Prognosis and Long-Term Outcomes

Pneumonitis does not just disrupt treatment temporarily; it can shape long-term survival. In one study of 170 patients receiving PD-1 inhibitors, the 16% who developed pneumonitis had markedly shorter overall survival, roughly 9 months versus 23 months, even though most of them recovered from the lung inflammation itself.²³ This survival gap is likely driven in part by the interruption of cancer therapy rather than by lasting lung damage, though both contribute.

A study tracking longer-term follow-up in 78 patients with checkpoint inhibitor pneumonitis found that two-thirds required at least one hospitalization related to the condition, and about a third experienced poor short-term outcomes including death, discharge to rehabilitation, or referral to hospice. At a median follow-up of about eight months, 37% were still alive. Among those well enough to receive additional cancer treatment afterward, about 55% were able to do so.²⁴ These numbers are sobering but also reflect a population that includes many patients with advanced cancer and significant comorbidities, so the pneumonitis outcomes are entangled with the underlying disease trajectory.

Recognizing Symptoms Early

The single most actionable thing a patient on Keytruda can do is learn what symptoms to report and report them immediately. Pneumonitis caught at grade 1 or 2 usually responds well to steroids, while grade 3 or above is far harder to treat. A large analysis of how immune-related side effects are detected found that pneumonitis falls into a “mixed” category: sometimes caught by routine tests like imaging, sometimes flagged by patient-reported symptoms like new cough or breathlessness.²⁵ This means both scheduled scans and open communication between patients and their care team matter. Nurses play a central role in this process, often serving as the first point of contact for symptom reports and educating patients on what to watch for.²⁶

The practical advice for anyone starting or currently receiving Keytruda: contact your oncology team if you develop a new cough, feel unusually winded with normal activity, run a fever without an obvious cause, or notice chest tightness. These symptoms overlap with many common conditions, and most of the time they will not turn out to be pneumonitis. But in the small percentage of cases where they are, the speed of your report can be the difference between a manageable steroid taper and an ICU admission.

How Pneumonitis Differs From Infection

One of the trickiest aspects of managing Keytruda pneumonitis is that it can look identical to a lung infection, and the two can even occur simultaneously. Case reports describe patients initially treated for pneumonitis whose workups revealed concomitant bacterial pneumonia, diagnosed by elevated procalcitonin levels even though standard infectious panels were negative.⁶ This overlap is clinically important because the treatment for pneumonitis, high-dose steroids, suppresses the immune system and can make an underlying infection worse. Oncologists typically send sputum cultures and sometimes order bronchoscopy specifically to rule out infection before committing to steroids, or they cover both bases by adding antibiotics alongside the steroid course until cultures come back clean.

Patients undergoing chemotherapy alongside Keytruda are especially susceptible to this diagnostic confusion, since chemotherapy itself lowers white blood cell counts and raises infection risk. The practical consequence is that a new lung infiltrate in a patient on chemo-immunotherapy triggers a broader workup than it would in someone on Keytruda alone, and the clinical team often has to manage both possibilities at once until the picture clarifies.