Keytruda and Chemotherapy: What to Expect

Keytruda is the brand name for pembrolizumab, an antibody that blocks PD-1, a switch on the surface of T cells that tumors use to shut those cells down. It does not kill cancer cells directly. It removes a brake from your own immune system.

The adult dose is fixed rather than weight-based, and is given either every 3 weeks or, at a larger amount, every 6 weeks. It runs as a 30-minute infusion through a line with a filter between 0.2 and 5 microns.

The prescribing information gives a sequencing rule that patients rarely hear stated out loud. When pembrolizumab and chemotherapy fall on the same day, pembrolizumab goes first. There is one notable exception in the label: with enfortumab vedotin for bladder cancer, pembrolizumab goes second. If your infusion nurse hangs things in a different order, it is fair to ask why.

The pairings are not interchangeable, and they came from separate trials.

For nonsquamous non-small cell lung cancer, KEYNOTE-189 used pembrolizumab with pemetrexed and either cisplatin or carboplatin, all on day 1 of a 21-day cycle for four cycles. After that, the platinum stops and pembrolizumab plus pemetrexed continue. Entry required tumors with no EGFR or ALK alteration.

For squamous non-small cell lung cancer, KEYNOTE-407 used carboplatin with either paclitaxel on day 1 or protein-bound paclitaxel on days 1, 8, and 15, for four cycles, then pembrolizumab alone.

For triple-negative breast cancer, KEYNOTE-355 paired pembrolizumab with paclitaxel, protein-bound paclitaxel, or gemcitabine plus carboplatin.

The biomarker rule also changes by cancer. In metastatic lung cancer the combination is approved regardless of PD-L1 level. In triple-negative breast cancer it is limited to tumors with a PD-L1 combined positive score of 10 or higher, measured by an FDA-authorized test. What a PD-L1 CPS means explains how that number is counted.

Chemotherapy in these regimens is finite. Four cycles, then it stops. Pembrolizumab runs on a different schedule: until the cancer grows, until side effects become unacceptable, or up to 24 months, whichever comes first.

Treatment before and after surgery has its own count. For high-risk early-stage triple-negative breast cancer, the label describes 24 weeks of pembrolizumab with chemotherapy before surgery, which is 8 doses on the three-weekly schedule or 4 on the six-weekly one, followed by up to 27 weeks of pembrolizumab alone afterward.

The trial randomized 616 people with untreated metastatic nonsquamous lung cancer, two to one. At the final overall survival analysis, median survival was 22.0 months with pembrolizumab added versus 10.6 months with chemotherapy alone, a hazard ratio of 0.56. Median progression-free survival was 8.8 months versus 4.9 months. The response rate was 48% versus 19%.

One number is worth reading carefully. Complete response, meaning no measurable cancer left on scans, occurred in 0.5% of people in both arms. The gain came from more partial responses and longer control, not from routine disappearance of disease. Stage 4 lung cancer: what now sets those figures against the wider picture.

Chemotherapy side effects follow a rhythm. Counts fall, then recover, and the pattern repeats each cycle. Immune side effects do not follow that rhythm. They can appear at any point, including months after the last dose.

Across 2,799 people treated with pembrolizumab, the label reports hypothyroidism in 8%, hyperthyroidism in 3.4%, immune-mediated pneumonitis in 3.4%, colitis in 1.7%, adrenal insufficiency in 0.8%, hepatitis in 0.7%, hypophysitis in 0.6%, nephritis in 0.3%, and type 1 diabetes in 0.2%. Pneumonitis was fatal in 0.1%. Systemic steroids were needed by 67% of those with pneumonitis. Everyone who developed type 1 diabetes needed insulin permanently.

In KEYNOTE-189, pembrolizumab was stopped for side effects in 20% of patients. The commonest reasons were pneumonitis at 3% and acute kidney injury at 2%. Doses were interrupted in 53%, most often for neutropenia at 13%. In KEYNOTE-407 the chemotherapy partner drove the difference: hair loss was 47% with pembrolizumab plus chemotherapy versus 36% with chemotherapy alone, and peripheral neuropathy 31% versus 25%. Those are paclitaxel effects, not immune ones.

The label sets numeric stopping rules. Pembrolizumab is held when AST or ALT rises above 3 times the upper limit of normal, and stopped for good above 8 times. Grade 3 or 4 pneumonitis means permanent discontinuation. Immune-related side effects covers how those grades are assigned.

All three trials excluded people with autoimmune disease that had needed systemic treatment within the past two years, anyone on immunosuppressive medicine, and anyone who had received more than 30 Gy of chest radiation in the previous 26 weeks. That is why a history of lupus, ulcerative colitis, or a transplant triggers a longer conversation before the first infusion.

• Call 911 or go to an emergency department if breathlessness or chest tightness comes on hard, or you are gasping at rest. Grade 3 or 4 pneumonitis ends pembrolizumab for good, and it can be fatal, so severe breathing trouble is not a phone-call-tomorrow problem.
• Phone your cancer team at once, day or night, if your temperature is 100.4°F (38°C) or higher. The chemotherapy half of this combination, not the pembrolizumab, is what empties your neutrophils about a week to ten days after each cycle, and the CDC classes a fever in that window as a medical emergency. MedlinePlus, reviewed in October 2024, uses 100.4 F; NCI's infection page, reviewed in January 2020, uses 100.5 F. Take the lower and newer figure. Speak to a person rather than leaving a message, and if you cannot reach the team quickly, go to an emergency department and tell them straight away that you are having chemotherapy.
• Call your care team the same day if a new cough, breathlessness, or chest tightness appears or worsens, even mildly. This is the pneumonitis question and it does not wait. Same day also covers 4 or more loose stools a day above your usual number, or any blood or mucus, and eyes or skin turning yellow, or urine turning dark.
• Call your care team within a day or two if you are suddenly very thirsty, urinating far more than usual, and losing weight. Type 1 diabetes from pembrolizumab is rare, but everyone who developed it in the label's safety set needed insulin permanently.

• https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=9333c79b-d487-4538-a9f0-71b91a02b287
• https://www.cancer.gov/about-cancer/treatment/side-effects/infection
• https://medlineplus.gov/ency/patientinstructions/000913.htm
• https://www.cdc.gov/cancer-preventing-infections/patients/fever.html