New drug therapies in veterinary dermatology (Proceedings)
Veterinarians are always in search of the "magic bullet" to treat their dermatologic patients. This desire to obtain better treatments has lead to the discovery of many new therapies and new dermatological applications for older medications. Many of the newer treatments that we will discuss been recognized in veterinary medicine because of successes being reported anecdotally. Many of these drugs are used without scientific studies to back them up. Despite the lack of scientific studies for many of these drugs and drug applications, these drugs deserve consideration and discussion.
Azithromycin (Zithromax)Azithromycin is an orally or parenterally administered azalide macrolide that has been a mainstay for treating upper respiratory bacterial infections in humans. Recently, this drug has been used in dogs and cats for respiratory and pyodermas. Azithromycin is a bacteriostatic antibiotic that inhibits protein synthesis by penetrating the cell wall and binding to the 50S ribosomal subunits in susceptible bacteria.
Azathithromycin has a relatively broad spectrum. It has in vitro activity (does not necessarily indicate clinical efficacy) against gram-positive organisms such as Streptococcus pneumonia, Staphylococcus aureus and gram-negative organisms such as Haemophilus influenzae, Bordatella sp., Mycoplasma pnenumoniae, Borrelia burgdorferi and Toxoplasma.
The pharmacokinetics of azithromycin has been described in cats and dogs. In dogs, the drug has excellent bioavailability after oral administration is 97%. Tissue concentrations apparently do not mirror those in the serum after multiple doses and tissue half-live in the dogs may be up to 90 hours. Greater than 50% of an oral dose is excreted unchanged in the bile. In cats, oral bioavailability is 58%. Tissue half-lives are less than in dogs, and range from 13 hours in adipose tissue to 72 hours in cardiac muscle. As with dog, cats excrete the majority of a given dose in the bile. When compared to erythromycin, azithromycin has better absorption characteristics, longer tissue half-lives, and higher concentrations in tissue and white blood cells.
Azithromycin with its relative broad spectrum and favorable pharmacokinetic profile may be useful for a variety of infections in veterinary species (probably not Mycoplasma haemofelis or Chlamydophilia felis in cats). It has been suggested as being useful for such infections as cryptosporidiosis, toxoplasmosis (with pyrimethamine), Lyme disease, or those caused by Mycobacterium leprae or Mycobacterium avium-intracellulare and in combination with atovaquone to treat babesiosis. However, little data is published at this time on the use of this drug in veterinary species.
Azithromycin is contraindicated in animals hypersensitive to any of the macrolides and should be used with caution in patients with impaired hepatic function. Although not proven scientifically, azithromycin is thought to be safe in pregnancy.
Azithromycin can cause vomiting in dogs at high doses. However, when compared to erythromycin, azithromycin probably will cause significantly fewer GI side effects. Other adverse effects, particularly those associated with liver, may become apparent in dogs and cats as more experience is attained. Although the intravenous medication is not commonly used in veterinary medicine, intravenous administration of azithromycin has been associated with local reactions.
Drug interactions between azithromycin and other drugs occur. Cisapride should not be administered concurrently with azithromycin in cats. In dogs and cats, azithromycin may potentially increase cyclosporine blood levels so cyclosporine doses should be adjusted down when both drugs are given at the same time. In addition, oral antacids may reduce the rate of azithromycin absorption so the dose for azithromycin should be increased when antacids are used.
Recommended drug doses for dogs and cats generally range from 5 to 15 mg/kg once to twice daily for 3 to 20 days. Subsequent doses every 3 to 5 days if continued treatment is required may be effective. As more experience is gained with this agent, the appropriate dose for azithromycin should be clarified. Cost has been a major issue for use in animals, a 250 mg tablet costs around $6 to 7, but the drug has just recently become available generically and so the cost should decrease.