Options for providing analgesia (pain relief) to cats outside of a veterinary hospital are limited. Currently, these options include oral non-steroidal anti-inflammatory drugs (NSAIDs), tramadol, transdermal fentanyl, and transmucosal buprenorphine. NSAIDs have a risk for toxicity in cats, and no drug is labeled in the United States for repeated administration in cats. Opioids are often considered as the first line of treatment for acute pain. Unfortunately, they are expected to have a short duration of action after oral administration, because of the so-called first-pass effect. Indeed, absorption through the gastrointestinal system results in delivery of the drug to the liver, which inactivates a large fraction of the drug. An alternative method of drug delivery is absorption through the oral mucosa.
This study evaluated transmucosal absorption and analgesic efficacy of four opioids in cats: oxymorphone, hydromorphone, methadone, and morphine. The goals of the study were to determine if the drugs were absorbed after administration in the mouth, and if they were increasing the cat’s tolerance to mild heat, which would suggest that the drug could be used to treat pain.
Following administration of each drug to the oral mucosa, levels of the drug were assessed. None of the four drugs demonstrated adequate absorption at the dosages administered. Only one of the drugs showed any ability to decrease pain, and for a short time only. The investigators concluded that higher doses of the drug are necessary to determine if oral mucosal administration of these pain medications is clinically useful. At the completion of the study, the cats were offered for adoption and placed in pet homes.
Transmucosal absorption & analgesic efficacy of opioids in cats: A pharmacokinetic/pharmacodynamic study; $15,963, Bruno H. Pypendop and Jan E. Ilkiw, University of California, Davis
The unique behavior repertoire in cats makes objective evaluation of analgesic benefits quite challenging. It is only in the evaluation of the pharmackinetics of analgesic drugs that we will adequately begin to understand which drugs and delivery methods will be adequate to provide such a critical aspect of patient care.
This study underlines the importance of looking for efficacy data on non-licensed routes of administration for drugs before prescribing them. Many drugs are available in transmucosal or transdermal formulations, but data on efficacy is lacking. We do our patients a disservice if we prescribe a drug in an ineffective form.
