Tuesday, April 18, 2006

Oxycodone vs. morphine for visceral pain

Pain has e-published ahead of print an interesting pre-clinical study about oxycodone vs. morphine for visceral pain.  The purpose of the study was to compare morphine, an inert placebo, and oxycodone on pain tolerance in several different tissues.  It involved 24 young, healthy subjects who were (on three separate occasions) given 30mg of oral morphine, 15mg of oxycodone, or placebo, and then subjected to a variety of stimuli (thermal, mechanical, and electrical in the skin, muscles, and viscera).  Most notably, they had an experimental tube placed in their esophagus to apply different 'stimuli' to their lower esophageal sphinctor to induce visceral pain.  As an example, the tube allowed inflation of a cuff in the esophagus to stretch it (this was their mechanical visceral pain model).  Pain was induced in all the areas at 30, 60, and 90 minutes after blinded administration of the analgesics. 

Morphine and oxycodone provided similar analgesia to painful stimuli to the skin and muscles, better than placebo.  However--and this is why I'm blogging this study--oxycodone was more effective than morphine and placebo in the thermal and mechanical visceral pain model.  Due to the way the study was designed I don't have a sense of the magnitude of this difference; however it was significant and specific to visceral pain (and not the other modalities suggesting that the doses used were equianalgesic for the other pain modalities).  The authors query whether this is due to kappa-opioid receptor agonism on oxycodone's part (I don't really understand why this is other than some people believe kappa-opioid receptors play a special role in visceral nociception).  Is this practice changing?  No--but it's provocative, and one hopes for more research along these lines.  It is, I guess, a reason to consider opioid rotation to oxycodone for more difficult to manage visceral pain (why not?).

As an addendum, I'd like to add that the authors made this statement about their choice of using oral opioids:

Furthermore, we anticipated that oral administration would decrease the risk for addiction compared to intravenous administration.

Is there something I'm missing here?  Is 'creating addiction' really a concern in experimental pain research like this?  This strikes me as excessively cautious--to be polite about it--but I'd be curious if anyone knows of literature about this risk of pain research...

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