How should back pain and myalgia be managed in a patient receiving ripretinib?

Management of Ripretinib-Associated Back Pain and Myalgia

Myalgia is a common adverse effect of ripretinib, occurring in approximately 26-27% of patients, and should be managed symptomatically with standard analgesics while continuing ripretinib at the prescribed dose, as these musculoskeletal symptoms rarely require dose modification. 1, 2

Understanding Ripretinib-Related Musculoskeletal Toxicity

Myalgia ranks among the most frequent adverse reactions with ripretinib, appearing in the top six most common side effects. The FDA label specifically identifies myalgia as occurring in ≥20% of patients receiving ripretinib for advanced gastrointestinal stromal tumors 1. Real-world data from the FAERS database confirms this pattern, with fatigue (8.4%) and myalgia being prominent complaints 3.

The mechanism appears to involve mitochondrial impairment in skeletal muscle cells. Research demonstrates that ripretinib depletes ATP levels, reduces mitochondrial DNA copy number, causes loss of mitochondrial membrane potential, and inhibits electron transport chain complexes in muscle tissue 4. This mitochondrial damage provides the biological basis for the muscle pain and fatigue patients experience.

Practical Management Algorithm

Initial Assessment

• Severity grading: Determine if myalgia is Grade 1-2 (mild to moderate) versus Grade 3-4 (severe, limiting self-care)
• Timing: Document when symptoms began relative to ripretinib initiation
• Pattern: Distinguish localized back pain from generalized myalgia
• Impact: Assess functional limitations and quality of life effects

Treatment Approach

For Grade 1-2 Myalgia (Most Common):

• Continue ripretinib at 150 mg once daily without dose modification 1
• Initiate NSAIDs for symptomatic relief (short-term use given side effect profile)
• Consider acetaminophen as alternative analgesic
• Avoid opioids given addiction risk and lack of evidence for chronic use in this setting
• Monitor for progression but expect symptoms to stabilize

For Grade 3 Myalgia (Rare):

• The FDA label does not specify dose modifications for isolated myalgia 1
• Consider temporary withholding if symptoms are severely limiting self-care
• Resume at same dose once symptoms improve to Grade ≤2
• Dose reduction to 100 mg daily may be considered if recurrent Grade 3 events occur

Important Caveats

Do not confuse with other ripretinib toxicities requiring dose modification:

• Palmar-plantar erythrodysesthesia syndrome (hand-foot syndrome) affects 30% and may require dose holds 5, 2
• Hypertension requires blood pressure monitoring and may necessitate dose adjustment 1
• Cardiac dysfunction (rare) requires permanent discontinuation if Grade 3-4 1

The absence of correlation between drug concentration and adverse events is notable. Chinese multicenter data showed no significant relationship between peak concentrations (Cmax) and adverse reactions, including myalgia 5. This suggests symptomatic management rather than dose reduction is the appropriate strategy.

Monitoring Strategy

• No routine laboratory monitoring is required specifically for myalgia
• Continue standard ripretinib monitoring per FDA label (cardiac function, blood pressure, dermatologic evaluation)
• Reassess at each clinic visit for symptom progression
• Document functional impact to guide treatment decisions

What NOT to Do

• Do not discontinue ripretinib for mild-moderate myalgia alone—this compromises cancer control without clear benefit
• Do not empirically reduce dose in absence of Grade 3+ toxicity
• Do not use muscle relaxants (not recommended for chronic pain and lack evidence in this setting) 6
• Do not pursue extensive workup for degenerative spine changes unless red flag symptoms present (the correlation between imaging findings and pain is poor) 7

The key principle: ripretinib-associated myalgia is common, mechanistically explained by mitochondrial effects, generally manageable with standard analgesics, and rarely requires treatment interruption. The benefit of continued cancer-directed therapy outweighs the discomfort of mild-moderate musculoskeletal symptoms in most patients.