Percutaneous administration of theophylline in the preterm infant

Twenty Clinically Pertinent Factors/Observations for Percutaneous Absorption in Humans

At least 20 clinically relevant factors affect percutaneous absorption of drugs and chemicals: relevant physico-chemical properties, vehicle/formulation, drug exposure conditions (dose, duration, surface area, exposure frequency), skin appendages (hair follicles, glands) as sub-anatomical pathways, skin application sites (regional variation in penetration), population variability (premature, infants, and aged), skin surface conditions (hydration, temperature, pH), skin health and integrity (trauma, skin diseases), substantivity and binding to different skin components, systemic distribution and systemic toxicity, stratum corneum exfoliation, washing-off and washing-in, rubbing/massaging, transfer to others (human to human and hard surface to human), volatility, metabolic biotransformation/cutaneous metabolism, photochemical transformation and photosensitivity, excretion pharmacokinetics, lateral spread, and chemical method of determining percutaneous absorption.

Recent Advances in Neonatal Pharmacotherapy

Objective:

To provide commentary and reviews and brief discussions in controversial or innovative recent advances in neonatal pharmacotherapy. To discuss cutting edge drug delivery systems that may become useful in neonatal drug delivery in the future.

Data Sources:

Articles were identified through searches of MEDLINE (1990–October 2005), key articles in the authors' files, and in some cases, through data generated and/or published by the author of a particular topic.

Data Selection:

Article selection and relevance to the topics under discussion was determined by individual authors.

Data Synthesis:

Therapeutic strategies addressed in this review include the use of hematopoietic growth factors including a simulated amniotic fluid preparation containing these growth factors for neonates with selected gastrointestinal problems, erythropoietin for neuroprotection following perinatal asphyxia, drug therapy advances in treatment of patent ductus arteriosus (PDA), evaluation of advances in transdermal drug delivery, and its potential application to neonates and advances in the treatment of persistent pulmonary hypertension (PPHN) of the newborn.

Conclusions:

Despite being over 30 years old, the practice of neonatology is as much of an art as a science. Advances in the basic science research have improved our understanding of use of pharmacologic agents in the premature and full-term neonate including drug disposition pathways. Expanding our knowledge on issues such as physiology of hematopoietic factors, the pharmacologic responses of conditions such as PDA and PPHN, and newer technologies for drug administration, as well as other pharmacologic reponses in the neonate are vital in the development of safe and efficacious treatments for neonates. Many questions remain unanswered, and every clinician must make an effort to contribute to the knowledge and understanding of pharmacotherapy in this patient population.

Using skin for drug delivery and diagnosis in the critically ill

Skin offers easy access, convenience and non-invasiveness for drug delivery and diagnosis. In principle, these advantages of skin appear to be attractive for critically ill patients given potential difficulties that may be associated with oral and parenteral access in these patients. However, the profound changes in skin physiology that can be seen in these patients provide a challenge to reliably deliver drugs or provide diagnostic information. Drug delivery through skin may be used to manage burn injury, wounds, infection, trauma and the multisystem complications that rise from these conditions. Local anaesthetics and analgesics can be delivered through skin and may have wide application in critically ill patients. To ensure accurate information, diagnostic tools require validation in the critically ill patient population as information from other patient populations may not be applicable.

Effective use of transdermal drug delivery in children

Transdermal administration offers a non-invasive and convenient method for paediatric drug delivery. The competent skin barrier function in term infants and older children limits both water loss and the percutaneous entry of chemicals including drugs; but the smaller doses required by children eases the attainment of therapeutic concentrations. Transdermal patches used in paediatrics include fentanyl, buprenorphine, clonidine, scopolamine, methylphenidate, oestrogens, nicotine and tulobuterol. Some patches have paediatric labelling supported by clinical trials whereas others are used unlicensed. Innovative drug delivery methods, such as microneedles and sonophoresis are being tested for their safety and efficacy; needleless injectors are primarily used to administer growth hormone; and two iontophoretic devices were approved for paediatrics. In contrast, the immature and rapidly evolving skin barrier function in premature neonates represents a significant formulation challenge. Unfortunately, this population group suffers from an absence of approved transdermal formulations, a shortcoming exacerbated by the significant risk of excessive drug exposure via the incompletely formed skin barrier.

Glucose monitoring in neonates: need for accurate and non-invasive methods

Neonatal hypoglycaemia can lead to devastating consequences. Thus, constant, accurate and safe glucose monitoring is imperative in neonatal care. However, point-of-care (POC) devices for glucose testing currently used for neonates were originally designed for adults and do not address issues specific to neonates. This review will address currently available monitoring options and describe new methodologies for non-invasive glucose monitoring in newborns.

Performance of transdermal therapeutic systems: Effects of biological factors

Transdermal drug delivery (TDD) is a technique that is used to deliver a drug into the systemic circulation across the skin. This mechanism of drug delivery route has many advantages, including steady drug plasma concentrations, improved patient compliance, elimination of hepatic first pass, and degradation in the gastrointestinal tract. Over the last 30 years, many transdermal products have been launched in the market. Despite the inherent advantages of TDD and the growing list of transdermal products, one of the major drawbacks to TDD is the occurrence of inter- and intraindividual variation in the absorption of the drug across the skin. A majority of these variations are caused by biological factors, such as gender, age, ethnicity, and skin hydration and metabolism. These factors affect the integrity and the barrier qualities of the skin, which subsequently result in the variation in the amount of drug absorbed. The main objective of this review article is to provide a concise commentary on the biological factors that contribute to the variation in transdermal permeation of drugs across human skin and the available transdermal therapeutic systems that may reduce the variations caused by biological factors.

Developmental pharmacokinetics

The advances in developmental pharmacokinetics during the past decade reside with an enhanced understanding of the influence of growth and development on drug absorption, distribution, metabolism, and excretion (ADME). However, significant information gaps remain with respect to our ability to characterize the impact of ontogeny on the activity of important drug metabolizing enzymes, transporters, and other targets. The ultimate goal of rational drug therapy in neonates, infants, children, and adolescents resides with the ability to individualize it based on known developmental differences in drug disposition and action. The clinical challenge in achieving this is accounting for the variability in all of the contravening factors that influence pharmacokinetics and pharmacodynamics (e.g., genetic variants of ADME genes, different disease phenotypes, disease progression, and concomitant treatment). Application of novel technologies in the fields of pharmacometrics (e.g., in silico simulation of exposure-response relationships; disease progression modeling), pharmacogenomics and biomarker development (e.g., creation of pharmacodynamic surrogate endpoints suitable for pediatric use) are increasingly making integrated approaches for developmentally appropriate dose regimen selection possible.

Development of an in vitro model for premature neonatal skin: Biophysical characterization using transepidermal water loss

The objective was to develop an in vitro model for the developing skin of the premature neonate. Barriers of different levels of efficiency were produced by differentially tape-stripping the stratum corneum (SC) from the skin of excised porcine ears, and were characterized by measurements of transepidermal water loss (TEWL). In this way, it was possible to express the recorded TEWL as a function of percentage SC thickness (F) generating the following relationship: TEWL = 2.7 + 41.exp [- 0.028.F]. These data were then compared to previously published in vivo measurements of TEWL obtained from a population of premature neonates at various post-conceptional ages (PCA). The latter conformed to a remarkably parallel relationship to that found in vitro with the porcine skin model, namely TEWL = 3.3 + 41.exp [-0.026.(PCA-160)]. It can be suggested, therefore, that the empirically adjusted PCA (i.e., PCA-160) correlates closely with the developing thickness of the neonate's SC. The corollary is that porcine skin, in vitro, tape-stripped to a particular level, can provide a barrier corresponding to a specific degree of neonate maturation and can serve, hence, as a useful tool with which to explore whether transdermal drug delivery in this unique patient population may be beneficial.

Skin deep

Over the past 30 or so years there has been a considerable advance in our knowledge of the mechanisms of skin permeation. This has largely been brought about by the development of sophisticated biophysical techniques and increased computing powers. The advanced technology has clearly provided indications, at a molecular level, about routes of permeation and how the barrier function can be modulated by excipients with which actives are formulated. This publication reviews some of the advances that have been made and mathematical models that have been constructed to predict percutaneous penetration and transdermal delivery. The models also indicate the various enhancement strategies that can be used in dermal penetration. In the past, it has been difficult to identify precise mechanisms of action of the different classes of enhancer but a combination of appropriate biophysical techniques, mathematical modelling and chemometric analysis can help identify the contributing processes. The models can also be used to indicate rate control in transdermal delivery, whether it is in the applied delivery device or in the skin.

Transdermal drug delivery. Clinical aspects

The delivery of drugs into and through the skin is a recognized and effective means of therapy for dermatologic, regional, and systemic disease. The selection of drug candidates and the rational design of suitable formulations depends upon the biological make-up of the skin's barrier, and the physiochemical interactions between the membrane, the delivery system technology, and the active agent. This article summarizes the state of the art and examines more recent developments that are the subject of considerable research at this time. In addition, the potential to use the skin as a portal for noninvasive clinical chemistry (e.g., for glucose monitoring in diabetics) is discussed.

An in vitro study of diamorphine permeation through premature human neonatal skin

The permeation kinetics of diamorphine through human premature neonatal cadaver skin over a range of gestational ages between 24 and 36 weeks was investigated using small diffusion cells. A strong inverse correlation was noted between the apparent permeability coefficient and the gestational age of the skin (P < 0.01; n = 26). The calculated apparent permeability coefficients decreased with gestational age from 6.0 x 10(-2) cm.hr-1 at 24 weeks' gestation to 5.2 x 10(-6) cm.hr-1 at 36 weeks' gestation. The amount of diamorphine remaining bound within the skin at the end of the in vitro experiments did not change significantly with gestational age of the skin. Diamorphine was subject to degradation over the course of the in vitro experiments to produce significant amounts of 6-monoacetylmorphine and evidence is presented to suggest that this was due to residual skin esterase activity. It is calculated that the steady-state flux rate of diamorphine through neonatal skin observed in these experiments would be sufficient to obtain a therapeutic plasma concentration of morphine assuming a 2-cm2 area for application and a delivery rate of 15 micrograms hr-1 kg-1. However, the prolonged half-life of morphine in the premature neonate would result in a delay of some hours before the attainment of this level.

Percutaneous caffeine application in the treatment of neonatal apnoea

Topical application of caffeine for the treatment of neonatal apnoea was considered in 57 low birth weight infants (less than 1500 g birth weight). The rationale for the study was that transdermal absorption of drugs and chemical agents has been demonstrated in neonates depending on anatomical and functional immaturity of the epidermal barrier. Considering these issues we studied the efficacy of percutaneous application of caffeine using high pressure liquid chromatography (HPLC) for evaluation of its plasma levels. 2 x 7.5 mg (babies less than 1000 g, extremely low birth weight [ELBW] or 2 x 10 mg (babies greater than 1000 g, very low birth weight [VLBW]) of caffeine were applied transcutaneously in form of a gel to the abdominal skin (Standard dose = 0.06 g of gel equivalent to 10 mg of caffeine citrate). Gestational age of our patients was 29.4 +/- 1.7 weeks, mean birth weight 1025 +/- 240 g. Mean postnatal age at beginning of treatment was 25.5 +/- 18 h. Of the treated babies, 73% had serum levels in therapeutic range about 48 h after the first dose of caffeine application. After 10 doses 97% of patients had serum levels in the therapeutic range. We conclude that percutaneous caffeine application is a safe and useful approach for treatment of apnoea in VLBW and ELBW infants.

Transdermal delivery of theophylline to premature infants using a hydrogel disc system

1. Preterm infants show incompletely developed skin with reduced barrier function. The possibility of transdermal delivery of theophylline from hydrogel discs swollen with choline theophyllinate has been investigated. 2. Drug loaded hydrogel discs 2 cm2 in area were applied to the abdomen and occluded. Serum theophylline concentrations were measured in twenty-one infants of less than 31 weeks gestation. 3. Therapeutic concentrations were achieved in 18 individuals, and maintained for up to 15 days after repeated application of discs. A correlation between maximum serum drug concentration and transepidermal water loss, gestation and birthweight was demonstrated.

Drug Disposition in the Pediatric Patient

The pediatric patient is a unique individual who undergoes continual physiologic change from the time of conception through adolescence. The pharmacokinetics of therapeutic agents are influenced by physiologic changes that ultimately affect drug therapy. The gastric absorption of many therapeutic agents is altered by changes in gastric pH, gastric emptying, intestinal motility, biliary function, pancreatic function, and regional blood flow. Intramuscular absorption is erratic and unpredictable because of reduced skeletal muscle mass, alterations in regional blood flow, and physical activity. Percutaneous absorption is increased in the neonate due to increased water content and decreased thickness of the stratum corneum. The distribution of many therapeutic agents is increased in the neonate and infant because of an increase in extracellular fluid and total body water, alterations in tissue binding, and decreased plasma protein binding. The metabolic capacity and elimination processes of the newborn are greatly reduced compared to the adult; hepatic function is approximately one-half that of adults and renal elimination is similarly reduced. Hepatic function in the infant and young child may actually exceed that in the adult due to the increase in hepatic metabolic surface area to body weight ratio. Renal function matures relatively quickly in the neonate and approaches adult drug renal elimination rates within the first year of life. There is a lack of clinical research that defines therapeutic guidelines in the pediatric patient for specific drugs. However, an understanding of these physiologic changes that take place during growth and development in the pediatric patient will facilitate optimal drug therapy in this patient population. Following the initiation of drug therapy, the continued physiologic changes taking place in the pediatric patient necessitate continual therapeutic drug monitoring and periodic dosage adjustments. These special considerations challenge and enhance the responsibility of the pharmacist as a key member in assuming safe and effective pediatric drug therapy.

Percutaneous absorption in preterm infants

The skin of preterm infants varies considerably in its level of maturity. To understand skin absorption in premature infants better, we report a technique for the assessment of percutaneous absorption at various gestational and postnatal ages using stable, isotope-labeled (13C6) benzoic acid. Our results indicate that in the preterm infant, this method detects enhanced skin absorption in the first postnatal days, which declines over three weeks to that expected of a full-term infant. This approach also indicates an inverse relationship between gestational age and skin absorption, as well as postnatal age and skin absorption. The reported technique is a safe and noninvasive method using a model skin penetrant for the study of percutaneous absorption in preterm infants from which basic data may be derived to add to our understanding of skin barrier function.

Skin permeability in the newborn

Skin permeability to drugs was assessed in the newborn infant using an in vitro method. Excised skin samples were studied in a Franz-type cell, and permeability to 0.1 M sodium salicylate was measured. Fourteen samples were studied, from infants of 25-41 weeks gestation and up to 8 days old. Gestation markedly affected skin permeability to salicylate, absorption being 10(2)-10(3) times greater in infants of 30 weeks gestation or less than in term infants. There are important implications for the high permeability of the preterm infant's skin; accidental poisoning from absorption of topically applied agents can easily occur, and the percutaneous route offers an alternative method of therapeutic drug administration.

High plasma urea concentrations in collodion babies

We describe two infants born with a collodion membrane; both were treated with a product containing 10% urea and 5% lactic acid and as a consequence were found to have a raised plasma urea concentration.