Capsaicin for osteoarthritis

Let’s begin with a patient case.  An elderly patient is being treated with acetaminophen 650 mg po q6hr prn osteoarthritis pain of the hands.  She takes all four doses on most days and does not feel this relieves her symptoms adequately.  She has multiple comorbidities and is looking for some therapy with improved efficacy.  She wants to know if Capzasin® over-the-counter would be a good choice.

Primary osteoarthritis is a common condition in the aging population that can affect multiple joints with varying severity.  Management of osteoarthritis can include a combination of patient education, nonpharmacologic, pharmacologic, and surgical interventions.  While there is no single best medication for any given patient, drug selection should consider individual agent’s toxicity, cost, symptom severity, and presence of comorbid conditions. 

• Acetaminophen is generally considered a first-line agent and should be dosed up to 3-4 grams per day in divided doses (different manufacturers have different maximum daily doses which have been decided relatively arbitrarily). 
• NSAIDs and COX-2 inhibitors are another oral option and may provide improved efficacy in moderate to severe osteoarthritis versus acetaminophen.  Responses to these agents, dosing frequency, and adverse event frequency vary among these agents yet they all have risks for gastrointestinal, renal, hepatic, and coagulation disturbances precluding them for use in many patients.  Topical formulations of NSAIDs also exist.

Topical capsaicin is another option for treatment of osteoarthritis and it is typically reserved for those patients who have unsatisfactory relief or are unable to tolerate the oral therapy above.  Its use is limited to osteoarthritis of the hands or knees making it a viable choice in our patient above.  The mechanism of action for capsaicin, however, requires particular dosing and expectations for relief of pain.

Mechanism of action

Capsaicin’s efficacy works through counter-irritation, a process whereby activation of nerve fibers in the skin results in desensitization over time.  Nerve degeneration occurs only after initially being stimulated by capsaicin where a depletion of substance P also occurs.  The drug then prevents the reaccumulation of substance P, but only following multiple daily dosing that is continued for a period of weeks.  Studies have shown that fewer than three applications per day fail to prevent the synthesis and transport of substance P.  During the initial days to weeks of use, patients will often experience significant pain causing them to discontinue use and never make it to the therapeutic onset.

Since capsaicin is the enzyme that makes chili peppers hot, extreme care must be taken to wash hands after use and to avoid touching the eyes or any mucous membranes. 

*There is also a topic patch containing 8% capsaicin (Qutenza®) that requires a one hour application in a physician’s office once every three months.  The pain during this application is so severe that local anesthetics are required to tolerate the hour long application.  The patch needs to be applied and removed in a particular way in order to minimize the risk of accidental exposure to unintended locations and aerosolization of the capsaicin.  This patch is indicated for neuropathic pain associated with postherpetic neuralgia.

Back to the patient case

Capsaicin could be an option as additional therapy for the above patient who is already taking adequate doses of acetaminophen without satisfactory relief.  Her multiple comorbid conditions may preclude the use of NSAIDs and tramadol (if not, these would likely be the best second choice after acetaminophen).  If choosing capsaicin, she must be counseled on the appropriate dosing, the expected initial pain, and the long time of onset of benefit.

Take home points:

• Capsaicin needs to be dosed 3-4 times daily, not prn, for effective denervation and analgesia
• Capsaicin will not have a therapeutic effect for up to 6 weeks of daily use and will likely cause additional pain during the initial phase
• Capsaicin is only for superficial joints in osteoarthritis such as hands and knees

References:

1.  Altman RD, Barthel HR.  Topical Therapies for Osteoarthritis. Drugs 2011; 71(10):1259-1279.

2.  Burkhart C, Morrell D, Goldsmith L. Chapter 65. Dermatological Pharmacology. In: Brunton LL, Chabner BA, Knollmann BC, eds. Goodman & Gilman's The Pharmacological Basis of Therapeutics. 12nd ed. New York: McGraw-Hill; 2011. 

3.  Ling SM, Ju YL. Chapter 116. Osteoarthritis. In: Halter JB, Ouslander JG, Tinetti ME, Studenski S, High KP, Asthana S, eds. Hazzard's Geriatric Medicine and Gerontology. 6th ed. New York: McGraw-Hill; 2009.

4.  Qutenza Prescribing Information. Acorda Therapeutics 2009.

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