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Hydroxyzine is a first‐generation antihistamine and cetirizine, a second‐generation antihistamine and active metabolite of hydroxyzine. Hydroxyzine is commonly used in performance horses and as such its use in closely regulated; however, there are no published studies suitable for establishing appropriate regulatory recommendations. In the current study, 12 exercised Thoroughbred research horses received a single oral administration of 500 mg of hydroxyzine. Blood and urine samples were collected prior to and up to 96 hr postdrug administration and concentrations of hydroxyzine and cetirizine determined using liquid chromatography‐tandem mass spectrometry. A joint parent/metabolite population 2‐compartment pharmacokinetic model with first‐order absorption and elimination was utilized to describe the pharmacokinetics of both compounds. Serum hydroxyzine and cetirizine concentrations were above the limit of quantitation (0.1 ng/ml) of the assay at 96 hr (the last time point sampled). The terminal half‐life was 7.41 and 7.13 hr for hydroxyzine and cetirizine, respectively. Findings from this study suggest that a prolonged withdrawal time should be observed if this compound is used in performance administered to performance horses and is classified as prohibited substance by the applicable regulatory body.

We measured the serum concentrations of moxalactam in 10 children receiving antibiotic prophylaxis for surgery and in 18 children treated for documented or suspected infections. Moxalactam was administered intravenously at a dose of 50 mg/kg every 8 h. After the first dose in 28 patients, mean moxalactam concentrations 5 min, 30 min, 2 h, and 8 h after infusion were 257, 177, 82.2, and 17.5 ,ug/ml, respectively. The mean half-life (T112) was 2.44 h (range: 0.55 to 7.96 h). The mean distribution volume (VD) was 0.30 liters per kg. No significant accumulation was observed with multiple doses. Prophylactic and therapeutic groups had similar serum levels, T112, and VD. The five infants less than 1 year of age had a lower mean 30-min level (P < 0.01), larger VD (P < 0.001) and longer T1/2 (P < 0.025) than the children older than 1 year. A dose of 50 mg/kg produced 30-min levels in excess of 64 ,ug/ml in all patients studied, but 8-h trough levels were below the minimal inhibitory concentration breakpoint of 16 ,g/ml in 32% of patients.Moxalactam is a broad-spectrum beta-lactam antibiotic which is gaining increasing use in the treatment of serious pediatric infections. Its activity against gram-negative bacteria, including Haemophilus influenzae (8), and its penetration into the cerebrospinal fluid (9, 10, 15, 17) make it a reasonable alternative for the treatment of some cases of meningitis. Its activity against Enterobacteriaceae (6,7,13,18) and anaerobes (3) makes it an appealing choice for the treatment of children with ruptured appendix and other intraabdominal infections.The pharmacology of moxalactam has been extensively studied in healthy adults and in adult patients with normal and impaired renal function (1,4,14,16). Since studies in children have been much more limited, we examined the pharmacology of moxalactam in healthy children receiving prophylaxis for abdominal surgery, mainly appendectomy, and in seriously ill children receiving therapy for acute infections. MATERILS AND METHODSChildren from birth to 16 years were eligible for entry into the study. Patients with a history of penicillin or cephalosporin allergy, a history of anaphylaxis to any drug, severe renal insufficiency, or life-threatening infection for which conventional therapy was available were not eligible. Written informed consent was obtained from parents or legal guardians in all cases.

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