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Discussion on Allergies and hydroxyzine vs. cetirizine

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Amy Leonardi
Username: jamie111

Post Number: 45
Registered: 9-2005
Posted on Tuesday, Jun 16, 2009 - 12:10 pm:   Edit PostPrint Post

I've been trying to research the use of cetirizine (generic of Zyrtec) in horses with allergies. I found that there have indeed been some studies on cetirizine in horses that concluded it could be a useful drug. I also found the below abstract, which I don't understand. Could you help 'translate' what they are saying (this is specifically looking at the use in dogs)? Are they saying that hydroxyzine converts to cetirizine? And do you have any information on the effectiveness and potential side effects of cetirizine in horses? Thanks in advance for your help!

Department of Clinical Sciences, College of Veterinary Medicine, Veterinary Teaching Hospital, North Carolina State University, 4700 Hillsborough Street, Raleigh, NC 27606, USA.

Pharmacokinetic parameters of hydroxyzine and its active metabolite cetirizine were determined after oral and intravenous administration of 2 mg kg(-1) of hydroxyzine to six healthy dogs. Plasma drug levels were determined with high-pressure liquid chromatography. Pharmacodynamic studies evaluated the suppressive effect on histamine and anticanine IgE-mediated cutaneous wheal formation. Pharmacokinetic and pharmacodynamic correlations were determined with computer modelling. The mean systemic availability of oral hydroxyzine was 72%. Hydroxyzine was rapidly converted to cetirizine regardless of the route of administration. The mean area-under-the-curve was eight and ten times higher for cetirizine than hydroxyzine after intravenous and oral dosing, respectively. After oral administration of hydroxyzine, the mean peak concentration of cetirizine was approximately 2.2 microg mL(-1) and that of hydroxyzine 0.16 microg mL(-1). The terminal half-life for cetirizine varied between 10 and 11 h after intravenous and oral administration of hydroxyzine. A sigmoidal relationship was fit to the data comparing cetirizine plasma concentration to wheal suppression. Maximum inhibition (82% and 69% for histamine and anticanine IgE-mediated skin reactions, respectively) was observed during the first 8 h, which correlated with a plasma concentration of cetirizine greater than 1.5 microg mL(-1). Pharmacological modelling suggested that increasing either hydroxyzine dosages or frequencies of administration would not result in histamine inhibition superior to that obtained with twice daily hydroxyzine at 2 mg kg(-1). In conclusion, there was rapid conversion of hydroxyzine to cetirizine. The reduction of wheal formation appeared almost entirely due to cetirizine. Pharmacodynamic modelling predicted that maximal antihistamine effect would occur with twice daily oral administration of hydroxyzine at 2 mg kg(-1).
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Robert N. Oglesby DVM
Username: dro

Post Number: 23201
Registered: 1-1997
Posted on Wednesday, Jun 17, 2009 - 7:42 am:   Edit PostPrint Post

Hello Amy,
Many (most?) drugs are administered as a metabolically inactive chemical and require a chemical conversion, usually in the liver, to the metabolically active form. Some drugs have many different metabolites and some may be active and some not. One advantage of giving only the active metabolite is that the base chemical or some of the other metabolites may be responsible for the undesirable side effects experienced. But administering the active metabolite is not always best as it may not be well absorbed or more rapidly broken down once in the body.

This study finds in the dog that hydroxyzine is almost entirely converted to cetirizine. It also finds that the antihistimine effects on preventing and treating wheal formation almost entirely do to the cetirizine concentration.

There is some initial work on using centrizine in horses and I have added this to our article on antihistimines, Treatments and Medications for Horses » Anti-inflammatories (NSAID's, Steroids, Arthritis Rx) » Antihistamine Use in Horses.
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Amy Leonardi
Username: jamie111

Post Number: 46
Registered: 9-2005
Posted on Wednesday, Jun 17, 2009 - 9:47 am:   Edit PostPrint Post

Thanks, Dr. O! That's very interesting. I appreciate the explanation, and I'm going to read the article now. I began researching cetirizine for my horse because I have a friend that swears by Zyrtec for alleviating her seasonal allergy symptoms. As you might recall, I've been struggling with my horse's allergies/headshaking for the last several spring/summer seasons. I'm VERY nervous about bad side effects from drugs after a colic incident last year, so I've been mostly concentrating on supplements and homeopathic and herbal remedies to try to help him. I know you won't be surprised when I say none of those things has helped him much :-)
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