In Vivo Assessment of Histamine Pharmacodynamics

Overview

A critical first step to address the "pharmacodynamic information gap" in pediatrics resides with the development of methods capable of accurately measuring the time dependent effects of xenobiotic exposure in a manner sufficient to enable an integrative analysis of drug action which incorporates both pharmacokinetic and pharmacogenetic determinants. The proposed study will evaluate laser Doppler flowimetry, a necessary initial step in developing an "ideal" pharmacodynamic surrogate endpoint that ultimately could be used in infants, children and adolescents to fully characterize the impact of development on the pharmacokinetic (PK) / pharmacodynamic (PD) / pharmacogenetic (PG) determinants of drug effect for agents capable of modulating the cellular response to histamine.

The overall goal of this research is to develop and validate non-invasive approaches suitable for use in pediatrics that are sufficient to directly evaluate genotype-phenotype associations as determinants of the concentration-effect profile of drugs capable of altering microvascular function. The specific goal of this project will be to assess whether microvascular blood flow determined by a previously validated laser Doppler technique can be used as a reliable surrogate endpoint capable of functionally discriminating the effects of polymorphisms in the genes which are quantitatively important for histamine biotransformation. The investigators hypothesize that microvascular blood flow velocity can be used as a valid surrogate marker to discriminate the functional consequences of allelic variants in the enzymes primarily responsible for the biotransformation of histamine. The project includes two parts: 1. Validation of histamine iontophoresis (with measurement of microvascular blood flow by laser Doppler flowimetry) in a pediatric cohort as a pharmacologic provocation test capable of reliably characterizing the normal physiologic response of the cutaneous microvasculature to this xenobiotic addressed by a study in 50 children with a diagnosis of allergic disease or other "allergic" conditions where treatment with antihistamines represents the medical "standard of care". Subjects will concomitantly (on contralateral extremities) receive histamine challenges by the "gold standard" cutaneous prick test (time dependent response assessed by serial assessment of wheal and flare reactions) and by a previously described iontophoresis technique (time dependent response by serial assessment of microvascular blood flow using a validated laser Doppler technique).Additionally, DNA will be isolated from these subjects for genotyping as described in part 2). 2. Assessment of microvascular response to histamine challenge as a surrogate marker capable of functionally discriminating genotype-phenotype associations for polymorphically expressed genes that are quantitatively important in regulating the biotransformation and pharmacologic inactivation of histamine addressed first by genotyping 150 additional subjects who have an established diagnosis of allergic disease where treatment with antihistamines represents the medical "standard of care". Genotype data for candidate genes of primary interest, histamine-N-methyltransferase (HNMT) and diamine oxidase (DAO), will thus be available from a total of 200 subjects. Based upon known genotype-phenotype associations for HNMT (the enzyme primarily responsible for histamine biotransformation), two cohorts of subjects will be selected from those studied using an extreme discordant phenotype approach: subjects predicted, based upon genotype, to have reduced HMNT activity (i.e., those with 1 or 0 functional alleles) and subjects predicted to have normal enzyme activity (i.e., those with two functional alleles). These study participants will then receive a transcutaneous histamine challenge delivered by iontophoresis, followed by repeated assessment of microvascular reactivity reflected by time-averaged measurement of blood flow velocity using an established laser Doppler technique. Statistical analysis of blood flow velocity and other parameters from the laser Doppler assessment will be used as the primary outcome measurement for comparison of the two study cohorts with respect to expression of HNMT and DAO and the functional consequences thereof, using histamine response as the surrogate assessment of phenotype.

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Eligibility

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Outcomes