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MAPK Pathway Drugs and Ocular Toxicity

MAPK stands for Mitogen-Activated Protein Kinase. The name reflects the pathway's original discovery as a kinase cascade activated by mitogens — extracellular signals (such as growth factors) that stimulate cell division (mitosis).

The MAPK Cascade

The MAPK/RAS-RAF-MEK-ERK cascade is a highly conserved, three-tiered signaling pathway that transmits extracellular signals from the cell surface to the nucleus, regulating cell proliferation, differentiation, survival, and migration.

Step-by-Step Signal Transduction

  1. Extracellular stimulus → Receptor activation: Growth factors bind to receptor tyrosine kinases (RTKs) such as EGFR, triggering receptor dimerization and autophosphorylation.
  2. Adaptor protein recruitment → RAS activation: Phosphorylated RTKs recruit adaptor proteins (Shc, Grb2) and the guanine nucleotide exchange factor SOS, which converts RAS from inactive (RAS-GDP) to active (RAS-GTP). Three RAS isoforms exist: HRAS, KRAS, and NRAS.
  3. RAS → RAF (MAP3K tier): Active RAS-GTP recruits RAF kinases (ARAF, BRAF, CRAF) to the plasma membrane for activation. BRAF is the most potent activator and the most frequently mutated in cancer (V600E).
  4. RAF → MEK (MAP2K tier): Activated RAF phosphorylates MEK1/2. MEK is the only known physiological substrate of RAF.
  5. MEK → ERK (MAPK tier): Activated MEK1/2 phosphorylate ERK1/2 at the conserved TEY motif. Both phosphorylation events are required for full activation.
  6. ERK → Downstream effectors: ERK1/2 phosphorylate over 250 known substrates in the cytoplasm and nucleus, activating transcription factors (Elk-1, c-Fos, c-Myc, AP-1) that drive cell cycle progression.

Key Regulatory Features

  • Signal amplification: Each tier amplifies the signal from upstream to downstream
  • Negative feedback: ERK phosphorylates upstream components (SOS, RAF) to attenuate signaling
  • Scaffold proteins (KSR1/2, IQGAP1, MP1) organize cascade components into signaling complexes
  • Crosstalk: Cross-communication with the PI3K/AKT pathway and other MAPK cascades (JNK, p38)

Relevance to Cancer and Ocular Toxicity

Oncogenic mutations — most commonly in KRAS (~30% of all cancers) and BRAF V600E (~7% of all cancers, ~60% of melanomas) — constitutively activate this cascade. Ocular toxicity from MAPK pathway inhibitors arises because ERK signaling is critical for retinal pigment epithelium (RPE) homeostasis; ERK loss in RPE cells leads to decreased RPE65 expression and retinal degeneration.

Drugs in the MAPK/RAS-RAF-MEK-ERK Pathway With Ocular Toxicity

BRAF Inhibitors (Type I)

Drug (Brand Name) Primary Ocular Toxicities Key Details
Vemurafenib (Zelboraf) Uveitis (anterior uveitis, panuveitis), photosensitivity Uveitis in ~4% of patients; highest ocular signal among BRAF inhibitors
Dabrafenib (Tafinlar) Uveitis Lower incidence (~1%); favorable ocular profile in pediatric cohorts
Encorafenib (Braftovi) Uveitis (when combined with binimetinib) 4% uveitis incidence in COLUMBUS trial

MEK Inhibitors

Drug (Brand Name) Primary Ocular Toxicities Key Details
Trametinib (Mekinist) RPED, serous retinal detachment, chorioretinopathy, RVO FDA label warns of RPED and RVO
Cobimetinib (Cotellic) Serous retinopathy (MEKAR), blurred vision 17.9% serous retinopathy in integrated analysis; median onset 15 days
Binimetinib (Mektovi) Serous retinopathy, RVO, uveitis FDA label mandates visual symptom assessment at each visit
Selumetinib (Koselugo) MEKAR, blurred vision FDA-approved for pediatric [NF1](https://www.openevidence.com/rare-disease/neurofibromatosis-type-1); ~1.4% central serous retinopathy

Pan-RAF Inhibitors

Drug (Brand Name) Primary Ocular Toxicities Key Details
Tovorafenib (Ojemda) MEKAR, photosensitivity, periorbital edema Pan-RAF inhibitor; ocular profile overlaps with MEK inhibitors
Naporafenib (investigational) Expected class-related retinopathy Being studied in combination with trametinib
Sorafenib (Nexavar) Blurred vision, conjunctivitis First-generation pan-RAF inhibitor; less retinal toxicity than MEK inhibitors
CH5126766/VS-6766 (investigational, dual RAF-MEK) RPED (30%), blurred vision/color changes (49%) Very high ocular toxicity rate due to potent MAPK suppression

Eye Diseases Associated With Tovorafenib

Tovorafenib (Ojemda), a pan-RAF kinase inhibitor approved for pediatric [low-grade glioma](https://www.openevidence.com/rare-disease/low-grade-astrocytoma), is associated with several ocular adverse events:

  • MEK inhibitor-associated retinopathy (MEKAR) — serous retinal detachment and retinal pigment epithelium (RPE) changes*
  • Photosensitivity — patients are advised to wear sunglasses*
  • Periorbital edema*
  • Blurred vision*

These effects are considered a class effect of MAPK pathway inhibition, particularly due to downstream suppression of ERK signaling in the retinal pigment epithelium.

ERK Inhibitors

Drug (Brand Name) Primary Ocular Toxicities Key Details
Ulixertinib/BVD-523 (investigational) Subretinal fluid (MEKAR-like), intraretinal edema 100% bilateral, 95% foveal involvement; all cases reversible
ATG-017 (investigational) Retinopathy (grade 3 DLT), blurred vision Dose-limiting ocular toxicity at higher doses
LY3214996 (investigational) Expected class-related retinopathy Preclinical data show ERK loss in RPE leads to retinal degeneration

KRAS Inhibitors

Drug (Brand Name) Primary Ocular Toxicities Key Details
Sotorasib (Lumakras) Conjunctivitis (11% in mCRC combination arm) Minimal retinal toxicity; no MEKAR-type events
Adagrasib (Krazati) No significant ocular toxicity reported Primary toxicities are GI and hepatic

Key Patterns by Pathway Target

  • MEK inhibitors carry the highest ocular risk (MEKAR, serous retinopathy, RVO), occurring in up to 90% of patients on subclinical OCT
  • BRAF inhibitors predominantly cause uveitis rather than retinopathy
  • ERK inhibitors produce MEKAR-like retinopathy with additional intraretinal edema, but events appear self-limited
  • KRAS inhibitors have minimal ocular toxicity

References

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  2. Barbosa R, Acevedo LA, Marmorstein R. The MEK/ERK Network as a Therapeutic Target in Human Cancer. Mol Cancer Res. 2021;19(3):361-374. PMID: 33139506
  3. Roberts PJ, Der CJ. Targeting the Raf-Mek-Erk Mitogen-Activated Protein Kinase Cascade for the Treatment of Cancer. Oncogene. 2007;26(22):3291-3310. PMID: 17496923
  4. Roskoski R. ERK1/2 MAP Kinases: Structure, Function, and Regulation. Pharmacol Res. 2012;66(2):105-143. PMID: 22569528
  5. Food and Drug Administration. MEKTOVI (binimetinib) prescribing information. 2025. FDA Label*
  6. Food and Drug Administration. Mekinist (trametinib) prescribing information. 2026. FDA Label*
  7. Food and Drug Administration. LUMAKRAS (sotorasib) prescribing information. 2025. FDA Label*
  8. Mettler C, Monnet D, Kramkimel N, et al. Ocular Safety Profile of BRAF and MEK Inhibitors: Data From the World Health Organization Pharmacovigilance Database. Ophthalmology. 2021;128(12):1748-1757. PMID: 34000304
  9. Huang S, Guo Z, Wang M, et al. Ocular Adverse Events Associated With BRAF and MEK Inhibitor Combination Therapy: A Pharmacovigilance Disproportionality Analysis of the FDA Adverse Event Reporting System. Expert Opin Drug Saf. 2023. PMID: 36896641
  10. Zhang Z, Wu Q, Wang Y, et al. Adverse Events Associated With Dabrafenib, Trametinib, and Their Combination Therapy: A Disproportionality Analysis of the FDA Adverse Event Reporting System (FAERS) Database. Pharmacoepidemiol Drug Saf. 2025.
  11. Barteselli G, Goodman GR, Patel Y, et al. Characterization of Serous Retinopathy Associated With Cobimetinib: Integrated Safety Analysis of Four Studies. Drug Saf. 2022;45(12):1541-1555. PMID: 36310331
  12. Guo C, Chénard-Poirier M, Roda D, et al. Intermittent Schedules of the Oral RAF-MEK Inhibitor CH5126766/VS-6766 in Patients With RAS/RAF-mutant Solid Tumours and Multiple Myeloma: A Single-Centre, Open-Label, Phase 1 Dose-Escalation and Basket Dose-Expansion Study. Lancet Oncol. 2020;21(11):1478-1488. DOI
  13. Francis JH, Canestraro J, Haggag-Lindgren D, et al. Clinical and Morphologic Characteristics of Extracellular Signal-Regulated Kinase Inhibitor-Associated Retinopathy. Ophthalmol Retina. 2021;5(12):1200-1215. PMID: 34102344
  14. Sullivan RJ, Infante JR, Janku F, et al. First-in-Class ERK1/2 Inhibitor Ulixertinib (BVD-523) in Patients With MAPK Mutant Advanced Solid Tumors: Results of a Phase I Dose-Escalation and Expansion Study. Cancer Discov. 2018;8(2):184-195. PMID: 29247021
  15. Wahlroos S, Teng C, Tran B, et al. Results of a first-in-human, dose-escalation phase 1 study of the ERK1/2 inhibitor ATG-017 in patients with advanced solid tumors. 2024 ASCO Annual Meeting. ClinicalTrials.gov: NCT04305249
  16. Sammons RM, Ghose R, Tsai KY, Dalby KN. Targeting ERK beyond the boundaries of the kinase active site in melanoma. Mol Carcinog. 2019;58(9):1551-1570. PMID: 31148235
  17. Shang J, Lu S, Jiang Y, Zhang J. Allosteric modulators of MEK1: drug design and discovery. Chem Biol Drug Des. 2016.
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  22. Pyakurel A, Balmer D, Saba-El-Leil MK, et al. Loss of Extracellular Signal-Regulated Kinase 1/2 in the Retinal Pigment Epithelium Leads to RPE65 Decrease and Retinal Degeneration. Mol Cell Biol. 2017.