While topical gels may be a godsend for clients who dread trying to force a pill down their pet's throat, what do we actually know about the available drugs? Dr. Katrina Mealey, an assistant professor at Washington State University's College of Veterinary Medicine, made an effort to answer this question on July 13 at the AVMA Annual Convention.
"There are over 100 drugs advertised as products that can be administered topically to veterinary patients," she said. "But there are no data for most of these drugs."
Administering pills is difficult because few drug formulations on the market are suited for cats. Many caplets and tablets are too large, and solid and liquid medicines are often unpleasant tasting. Hiding medicine in food may work in some instances but can lead to the development of food aversions.
Veterinary compounding pharmacies have recently begun advertising topical gels as an alternative to pills. Although this appears to be an ideal solution for long-term administration of cat medications, the Food and Drug Administration has not approved these products, and data regarding their safety or efficacy are nonexistent.
According to Dr. Mealey, current studies suggest that a drug's absorption following topical administration varies substantially between patients and among drugs, and can be affected by the skin's health and thickness, which varies by location, hydration, and hair coat. Absorption can also be influenced by a drug's molecular weight, concentration, and lipid solubility; the surface area used for application; and the stability of the delivery vehicle. Other studies show that a drug delivered in a topical gel requires multiple doses before systemic levels can be detected.
If veterinarians decide to use topical gels, Dr. Mealey recommends limiting their use, reserving it for nonserious conditions with measurable endpoints, and starting with a low dose and increasing it as needed. They should not be prescribed for infectious diseases, she said, because only low drug concentrations may be attainable and this will encourage antimicrobial resistance. Veterinarians should also be aware that the vehicle used for most topical formulations contains soy lecithin, which is known to cause food allergies and asthma.
"We should avoid using them simply because it seems like state-of-the-art," she said. "Our knowledge of how effective they are has fallen way behind their use."