Pembrolizumab + Lenvatinib for Advanced pMMR/MSS Endometrial Carcinoma

Patient Selection

• For patients with advanced or metastatic endometrial carcinoma that is proficient‑mismatch‑repair/microsatellite stable (pMMR/MSS) and whose disease has progressed after platinum‑based chemotherapy, the combination of pembrolizumab (200 mg IV q3 weeks or 400 mg IV q6 weeks) plus lenvatinib (20 mg orally daily) is the recommended second‑line regimen, providing superior survival compared with chemotherapy. Level of evidence: Phase III trial data. [1][2]3

• The combination is indicated only for pMMR/MSS tumors that have progressed after any platinum‑containing regimen; it is not indicated for dMMR/MSI‑H disease. Level of evidence: Phase III trial cohort definition. [1][2]

• Patients who are candidates for curative surgery or definitive radiation are excluded from this regimen. Level of evidence: Trial eligibility criteria. [1][2]

• For dMMR/MSI‑H tumors, single‑agent immune checkpoint inhibition (pembrolizumab or dostarlimab) is preferred over the pembrolizumab + lenvatinib combination. Level of evidence: Phase III and phase Ib/II data. 3

• PD‑L1 expression should not be used to select patients for this therapy because it has limited predictive value. Level of evidence: Observational analysis. 4

Efficacy Outcomes (pMMR/MSS)

Level of evidence: Randomized Phase III (KEYNOTE‑775). [3][1]

• In the phase Ib/II KEYNOTE‑146 study, the combination yielded an ORR of 38 % at week 24, with a median PFS of 7.4 months and median OS of 16.7 months. Responses were observed irrespective of MSI status, PD‑L1 expression, or histologic subtype. Level of evidence: Early‑phase trial. [1][2]

Safety Monitoring

• Thyroid function tests should be obtained at baseline and then every 4–6 weeks during therapy. Level of evidence: Safety monitoring protocol. 1

• Liver enzymes (ALT, AST, bilirubin) and renal function (creatinine, proteinuria) should be checked at baseline and monitored regularly throughout treatment. Level of evidence: Safety monitoring protocol. 1

• Complete blood count should be performed regularly to detect cytopenias. Level of evidence: Safety monitoring protocol. 3

Dose Modifications & Management

• Approximately 65 % of patients require lenvatinib dose reductions and 72 % need temporary dose interruptions due to toxicity. Level of evidence: Trial safety data. [1][2]

• Pembrolizumab dose reductions are not recommended; dose holds are used for immune‑related adverse events. Level of evidence: Trial dosing guidelines. 2

• Lenvatinib dose‑reduction steps follow a stepwise schedule: 20 mg → 14 mg → 10 mg → 8 mg daily. (Standard protocol; citation not required for the stepwise schema.)

• Supportive medications include antihypertensives (ACE inhibitors or ARBs), antiemetics (5‑HT₃ antagonists ± NK1 antagonists), antidiarrheals, and thyroid hormone replacement as needed. Level of evidence: Trial supportive‑care recommendations. 1

Alternative Options After Progression

• If the pembrolizumab + lenvatinib regimen is intolerable or contraindicated, single‑agent chemotherapy (e.g., doxorubicin or paclitaxel) may be used. Level of evidence: Comparative arm of Phase III trial. [2][3]

• For estrogen‑receptor‑positive endometrioid histology, letrozole + palbociclib provides a median PFS of 8.3 months. Level of evidence: Subgroup analysis. 1

Immunotherapy for dMMR/MSI‑H Tumors

• Pembrolizumab monotherapy achieves an ORR of ~46 % in dMMR/MSI‑H endometrial cancer. Level of evidence: Phase III/II data. 3

• Dostarlimab monotherapy yields an ORR of ~42.3 % in the same molecular subgroup. Level of evidence: Phase III/II data. 3

• Both agents produce durable responses, with the median duration of response not reached in dMMR tumors. Level of evidence: Long‑term follow‑up data. [1][3]

MMR/MSI‑Driven Immunotherapy Guidance for Advanced Endometrial Cancer

Biomarker Testing

• All patients with advanced or recurrent endometrial cancer should have mismatch‑repair (MMR) immunohistochemistry performed on tumor tissue as the first‑line biomarker test to determine dMMR status and guide therapy choice. 5
• When MMR IHC is unavailable or inconclusive, microsatellite instability (MSI) testing or next‑generation sequencing may be used as second‑line biomarker assessments. 5
• PD‑L1 expression should not be used to select patients for immunotherapy because it has limited predictive value and is heterogeneous across molecular subgroups. 6
• If loss of MLH1 protein is identified on IHC, immediate MLH1‑promoter methylation testing is required to differentiate sporadic (methylated) from hereditary (non‑methylated) cases. 7

Treatment Selection Based on MMR Status

• dMMR/MSI‑H tumors: patients who have progressed after prior therapy should receive pembrolizumab or dostarlimab as monotherapy. The Society for Immunotherapy of Cancer (SITC) recommends this approach. 6
• pMMR/MSS tumors: patients who have progressed after prior therapy should receive the combination of pembrolizumab + lenvatinib. This regimen is the SITC‑endorsed standard for this molecular subgroup. 6

Efficacy of Approved Regimens

dMMR/MSI‑H Monotherapy

• Pembrolizumab monotherapy yields an objective response rate (ORR) of ≈46 % with durable responses in dMMR endometrial cancer. 6
• Dostarlimab monotherapy produces an ORR of ≈42 % in dMMR tumors, comparable to pembrolizumab. 6

pMMR/MSS Combination Therapy

• Pembrolizumab + lenvatinib demonstrates superior outcomes versus chemotherapy: median progression‑free survival (PFS) 6.6 mo vs 3.8 mo (HR 0.60, p < 0.0001) and median overall survival (OS) 17.4 mo vs 12 mo (HR 0.68, p = 0.0001). 6

Molecular Rationale

• dMMR/MSI‑H tumors are hypermutated (median ≈ 541 predicted neoantigens) with high tumor‑mutational burden, explaining their high sensitivity to checkpoint‑inhibitor monotherapy. 5
• pMMR/MSS tumors have a low mutational burden (median ≈ 70 predicted neoantigens) and respond poorly to single‑agent checkpoint inhibition, necessitating anti‑angiogenic combination therapy. 5
• MMR status is the preferred biomarker over tumor‑mutational burden (TMB) for treatment selection, although exploratory data suggest TMB‑high tumors may benefit regardless of MSI status. 5

Toxicity Profile and Monitoring

• Grade ≥ 3 adverse events occur in ≈63–70 % of patients receiving pembrolizumab + lenvatinib; the most common are hypertension, fatigue, diarrhea, decreased appetite, nausea, and hypothyroidism. 8
• Immune‑related adverse events with the combination include hypothyroidism, rash, and less frequently colitis, hepatitis, pneumonitis, or nephritis. 8
• Continuous monitoring for immune‑related toxicities is required throughout treatment, with routine assessment of thyroid function, liver enzymes, renal function, and pulmonary symptoms. 8
• Adequate anti‑emetic prophylaxis (e.g., 5‑HT₃ antagonists ± NK1 receptor antagonists) should be provided when using the pembrolizumab + lenvatinib regimen because significant nausea is common. 8

Clinical Pitfalls

• Single‑agent pembrolizumab should not be used in pMMR/MSS disease; combination with lenvatinib is required for meaningful benefit. 6
• Approximately 40 % of patients with advanced recurrent endometrial cancer historically did not undergo MMR/MSI testing, resulting in missed opportunities for appropriate immunotherapy. 5
• Reliance on PD‑L1 expression for patient selection is unreliable across all molecular subtypes and should be avoided. 6
• Failure to perform MLH1‑promoter methylation testing after detecting MLH1 loss can lead to unnecessary germline testing and misclassification of hereditary versus sporadic disease. 7

Emerging First‑Line Strategies

• Phase III trials have shown that combining checkpoint inhibitors with carboplatin + paclitaxel improves outcomes in previously untreated stage III/IV or first‑recurrence endometrial cancer, especially in dMMR tumors; however, these regimens are not yet FDA‑approved (as of the 2023 SITC guideline). 6