Transdermal Gels For Veterinary Use

Transdermal delivery of medications to veterinary patients is becoming increasingly popular. An effective, non-invasive method of medicating animals is welcomed by veterinarians and owners alike.

Prospective Patients

The transdermal route is desirable in many feline patients. These cats fall into three basic categories.

First, those animals with behavioral problems that makes handling and administering oral medications difficult. Second, animals whose health is compromised and the stress of oral administration would further aggravate medical conditions. The last catergory is any animal whose owners are unable to "pill" or administer oral medications to their pet. Examples from the first and second category include transdermal Amitriptyline for behavioral problems in cats and Diltiazem for cardiac patients or Buprenorphine for post-op patients in a stress-free transdermal gel. The majority of patients fall in the third category.

Bioavailability and Dosing

Bio-availability data is not available for most compounded transdermal formulations. The lack of clinical studies documenting bioavailability, safety and efficacy means careful consideration of each therapeutic agent when deciding whether the transdermal dosage form is appropriate. Certain classes of drugs are not appropriate for transdermal application. These include chemotherapeutic and cytotoxic agents, drugs with narrow therapeutic margins, drugs with doses greater than 50mg, insulin, and drugs that are very unstable in acid pH. Drugs with extremely long terminal half-life, topical steroids, and antibiotics should be considered with caution. Once a determination has been made to use the transdermal route, a dose must be determined. Factors such as first pass hepatic extraction, oral vs. parenteral bioavailability, and half-life must be considered when determing an initial starting dose.

Vehicle

The vehicle most commonly used in compounded transdermal gels is pluronic lecithin organogel. This vehicle has both an aqueous and an oil phase, making it suitable as a vehicle for a wide range of pharmaceutical agents. It enhances the transdermal penetration of active ingredient. It is generally well-tolerated topically and is non-toxic if ingested orally.

Concentration

Surface area is important in the administration of transdermal gels. In order to achieve maximum absorption on the relatively small surface area of the cat's pinnae, the active ingredient is concentrated so that the desired dose is contained in 0.1ml. For example, if the desired dose of medication is 5mg, the product will be formulated to contain 50mg/ml or 5mg in 0.1ml. The gel is packaged in 1ml calibrated topical syringes allowing the veterinarian or owner to accurately measure a 0.1ml dose.

Application

The 0.1ml dose of gel is applied to the inside surface of the pinnae. This area is selected because it provides sufficient surface area with little fur, and is not easily reached during grooming. Using a gloved finger, spread the gel evenly over the entire surface to maximize absorption. Do not rub in. Never instill the gel into the inner ear canal. Cotton swabs are not recommended for application, as too much of the gel will adhere to the cotton. Avoid human contact with transdermal gels. Store the transdermal gel syringes at room temperature. Do not refrigerate as this may cause the gel to separate.

Monitoring

Because of the lack of clinical data, when the decision is made to prescribe a transdermal gel, it is important to identify both the goal of therapy and the signs of toxicity and then monitor the patient carefully for both.

Transdermal gels offer an excellent alternative for many patients that have been difficult to treat with conventional oral medication dosage forms. Following is a listing of the most commonly requested medications Wedgewood has compounded in a transdermal gel vehicle.

Medication	Strength	Size	Target Response / Signs of Toxicity(1)
Amitriptyline Transdermal Gel	50mg/ml	1ml	Cessation of undesirable behavior, cessation of cystitis; onset of action reported to be as early as 3-5 days Signs of Toxicity: Dry mouth, gastric distress, constipation, ataxia, tachycardia, weakness, sedation, urinary retention
Atenolol Transdermal Gel	62.5mg/ml	1ml	Reduction in pulse to 140-200bpm *Signs of Toxicity:* Hypotension, bradycardia, bronchospasm, cardiac failure, hypoglycemia
Buprenorphine Transdermal Gel	1.2mg/ml	1ml	Apparent analgesia, animal benefitting from pain management Signs of Toxicity: Respiratory depression
Cisapride Transdermal Gel	50mg/ml	1ml	Evidence of colonic motility with no constipation or obstruction *Signs of Toxicity:* Diarrhea, abdominal pain and cramping, arrhythmias from drug interactions
Clomipramine Transdermal Gel	30mg/ml	1ml	Cessation of undesirable behavior Signs of Toxicity: Excessive sedation, dry mouth, urinary retention
Cyproheptadine Transdermal Gel	20mg/ml	1ml	Improved appetitie, relief of pruritis, cessation of undesirable behavior *Signs of Toxicity:* Excessive sedation, dry mouth, urinary retention
Diltiazem Transdermal Gel	150mg/ml	1ml	Reduction in pulse rate to 140-200 bpm Signs of Toxicity: Bradycardia, vomiting, heart block
Enalapril Transdermal Gel	12.5mg/ml	1ml	Improvement of clinical signs of heart failure Signs of Toxicity: GI distress, hypotension
Enrofloxacin Transdermal Gel	180mg/ml	1ml	Eradication of bacterial infection *Signs of Toxicity:* Pupillary dilation, lameness, seizures, behavior change
Glipizide Transdermal Gel	50mg/ml	1ml	Reduction in blood glucose to <200 *Signs of Toxicity:* GI distress, hypoglycemia, icterus, increased ALT; hyperglycemia indicating therapeutic failure
Methimazole Transdermal Gel	50mg/ml	1ml	Reduction in serum T4 levels; improvement in clinical symptons Signs of Toxicity: Increased vomiting, dermal excoriations, leukopenias, hepatopathies, thrombocytopenia
Prednisone Transdermal Gel	50mg/ml	1ml	Cessation of inflammatory signs *Signs of Toxicity:* Epidermal or cartilage atrophy, signs of hyperadrenocorticism if used chronically, signs of diabetes mellitus

1. American College of Veterinary Pharmacists. 2002. Information-at-a-Glance, Considering Drugs for Transdermal Therapy Quick Reference Chart for Cats. Volume 05, Number 02.

About the Author

Renee Lupo is a registered pharmacist. She graduated from the Philadelphia College of Pharmacy and Science in 1976.

Renee began focusing her practice in the areas of pharmacy compounding and veterinary pharmacy in 1994 after joining Wedgewood Pharmacy.

She is currently a member of the International Academy of Compounding Pharmacists and is an Associate Fellow of the American College of Apothecaries.

The information contained on this site is general in nature and is intended for use as an informational aid. It does not cover all possible uses, actions, precautions, side effects, or interactions of the products shown, nor is the information intended as medical advice or diagnosis for individual health problems or for making an evaluation as to the risks and benefits of using a particular product. You should consult your doctor about diagnosis and treatment of any health problems. Information and statements have not been evaluated by the Food and Drug Administration ("FDA"), nor has the FDA approved the products to diagnose, cure or prevent disease.

Wedgewood compounded veterinary medicines are not intended for use in food and food-producing animals.

Transdermal Gels For Veterinary Use Renee Lupo R.Ph., FACA

Prospective Patients

Bioavailability and Dosing

Vehicle

Concentration

Application

Monitoring

About the Author

Transdermal Gels For Veterinary Use
Renee Lupo R.Ph., FACA