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Blood concentration of amethocaine and its metabolite after topical application

Presented at the Neonatal Society 2002 Summer Meeting (programme).

Jain A1, Rutter N1, Gilmer J2

1 Department of Neonatal Medicine, City Hospital, Nottingham, UK
2 Department of Pharmaceutical Chemistry, Trinity College, Dublin, Ireland

Background: Topical amethocaine provides effective pain relief during venepuncture in the newborn infant (1). Little is known about its metabolism or safety following application in this age group.

Aim: To investigate the blood concentration of amethocaine and its metabolite p butylamino benzoic acid (BABA) following topical application in the newborn infant.

Design: Analysis of samples from a randomised placebo controlled trial.

Subjects: 44 newborn infants, gestation 27 to 41 weeks (median 33), age 2 to 17 days (median 7), undergoing routine venepuncture.

Method: 1.5g of 4% w/w amethocaine gel or placebo was applied to the skin under occlusion for one hour, then wiped away. Venepuncture was performed five minutes later. An additional 0.5ml of blood was collected into a sample bottle containing 0.5ml of HPLC grade acetonitrile and placed 'on ice'. Within 15 minutes the sample was centrifuged and then stored at -20 C. The samples were analysed by high pressure liquid chromatography. The analytical limit was 50ng/ml for amethocaine and BABA.

Results: 19 samples were taken following the application of amethocaine. In 9 out of 19 the concentration of amethocaine was < 50ng/ml. In the remainder the concentration ranged from 69.9 - 3345.6ng/ml. The concentration of BABA was < 50ng/ml in 16 out of 19 samples and 75ng/ml, 75ng/ml and 88ng/ml respectively in the remaining three. 8 of the 11 samples with a high amethocaine concentration had a corresponding BABA concentration of < 50ng/ml. In two out of three samples where blood was taken from a site distant to the site of application, the concentration of amethocaine was < 50ng/ml. No local or systemic adverse reactions were noted.

Conclusion: Following the application of topical amethocaine gel for one hour the concentration of BABA is below or close to the level of quantification. The pattern of a high amethocaine concentration paired with a low BABA concentration in some samples suggests contamination with amethocaine at venepuncture. This data supports the use of topical amethocaine gel prior to invasive procedures in the newborn infant.

1. A. Jain, N Rutter, 'Does topical amethocaine gel reduce the pain of venepuncture in newborn infants? A randomised double blind controlled trial'. Arch. Dis. Child. 2000:83:F207

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