Fentanyl is a synthetic opioid agonist at the mu receptor. A transdermal delivery system is available as 25, 50, 75 and 100 microgram per hour matrix patches; the bioavailability is 90 per cent.1

Buprenorphine is a potent mu receptor partial agonist with some antagonist activity at kappa sites. There were initial concerns about the possibility of its avid binding to mu receptors, leading to non-naloxone reversible ventilatory depression, increased emesis, liability to abuse and reversal of morphine analgesia leading to pain and opioid withdrawal. These fears have been unfounded, but because of them, use of the drug has remained limited in some countries.2

Transdermal buprenorphine was introduced to clinical practice in 2001 as a matrix patch, allowing controlled release for about 72 hours. This is available in 35, 52.5, and 70 microgram per hour patches. A more recent innovation is a patch that delivers 5, 10, or 20 microgram per hour for a period of seven days.

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This agonist-antagonist has a dosage ceiling, making it difficult to use in patients with large-dose opioids (300mg oral morphine per day). It has a pronounced anti-hyperalgesic effect, which may make it a good agent in managing neuropathic pain. There is some evidence of buprenorphine being effective in managing allodynia and hyperalgesia.2

Adjusting dosage
Fentanyl patches are applied every three days; buprenorphine patches are applied every three or seven days. These matrix type patches can be cut to give a smaller dose, although the manufacturers do not recommend this practice (the older style reservoir patches cannot be cut because the drug leaks out). Cutting patches has been used for children, the elderly, and those with sensitivity to opioids; however, data are limited and care is needed with dose accuracy.

Initial dose titration is slow, because transdermal drugs take a long time to achieve steady-state plasma concentration (at least three half-lives). Once this is attained, patches often prevent ‘clock watching’ and many breakthrough pain problems are improved. It may also allow patients to carry on with their lives and forget about medication.

Patches can be very useful in patients who cannot take oral medication, for example, those with GI problems or when there is intestinal hurry or malabsorption.

Care should be taken when converting to the transdermal route because sub-therapeutic dosage can lead to uncontrolled pain during the initial titration period. One study recommends 2mg per day oral morphine to 1 microgram per day transdermal fentanyl as safe and effective.3 Recent research investigating a 3:1 ratio had to convert to a 2:1 ratio within 48 hours, because the pain relief was not acceptable.4

Older studies suggest a ratio of morphine to transdermal buprenorphine of about 70:1. A recent retrospective cohort study suggested a 100:1 ratio and this has been substantiated by an open label study.4

Practical issues
In terms of patient selection, indications for transdermal drugs include persistent, moderate to severe cancer or non-cancer pain. Patients should be thoroughly physically and psychologically evaluated before initiating strong opioid therapy. Other evidence-based pain management strategies should be initiated and maintained. Maximum efficiency is achieved by good adhesion (see Box 1). Patients who are using transdermal opioid patches should be monitored regularly for pain control, function in its widest sense, drug use, and side-effects. Tolerance is not usually a major problem once the correct dose is achieved. Increased dose requirement should prompt investigation for disease progression.

Disadvantages of the transdermal route
Disadvantages of patches include difficulty in varying the dose quickly in response to changing circumstances; this route should not be used in those with pain that is changing quickly or for incident or breakthrough pain.

If side-effects occur, they are often not quickly reversed, because a drug depot remains under the skin for many hours after the patch has been removed.