The bioequivalence of nizatidine (Axid) in two extemporaneously and one commercially prepared oral liquid formulations compared with capsule

Formulations for children: problems and solutions

Paediatric formulation design is complex as there is a need to understand the developmental physiological changes that occur during childhood and their impact on the absorption of drugs. Paediatric dose adjustments are usually based on achieving pharmacokinetic or pharmacodynamic profiles equivalent to those achieved in adult populations. However, differences in the way in which children handle adult products or the use of bespoke paediatric formulations can result in unexpected pharmacokinetic drug profiles with altered clinical efficacy. Differences in drug formulations need to be understood by healthcare professionals involved in the prescribing, administration or dispensing of drugs to children such that appropriate advice is given to ensure that therapeutic outcomes are achieved. This issue is not confined to oral medicines but is applicable for all routes of administration encountered in paediatric therapy.

Pediatric pharmacokinetics: human development and drug disposition

Human development is described by the various anatomic and physiologic changes that occur as the single-celled zygote matures into an adult human being. Concomitant with bodily maturation are changes in the complex interactions between pharmacologic agents and the biologic matrix that is the human body. Profound changes in the manner by which drugs traverse the body during development can have significant implications in drug efficacy and toxicity. Although not a replacement for well-conducted, pediatric, pharmacokinetic studies, an understanding of developmental biology and the mechanisms for drug disposition invariably assists the pediatric clinician with the judicious use of medications in children.

Preparation of medicines for children - a hierarchy of classification

There is some confusion about the types of paediatric pharmaceutical preparation (in a regulatory and pharmaceutical development context) that are acceptable for approval by medicines regulators. Some of the confusion relates to terminology which may mean different things to different stakeholders. It may not always be possible to provide authorised, commercially manufactured, age appropriate, ready-to-administer preparations. In terms of assurance of quality and bioavailability there is a continuum from this ideal through intermediate products through authorised compounding and manipulation of commercial dosage forms to ad hoc compounding using only the skills and experience of the individual pharmacist. Additionally, it is widely known that caregivers may manipulate medicines at home, for example by segmenting tablets and by addition to foods or liquids. The first intent of the manufacturer should be to provide for children an age appropriate, ready-to-administer preparation which is commercially manufactured and approved by the competent authorities. However, there will still be a place for providing other age appropriate preparations such as approved products that are 'intermediates' requiring reconstitution before use, or instructions for compounding or manipulation of a dosage form. If compounding or manipulation is likely to be required it is preferable that data are generated by Industry, approved by the competent authorities and provided in the Summary of Product Characteristics (SmPC). It is acknowledged however, that ad hoc compounding or manipulation may also take place in certain circumstances such as logistical difficulties or to satisfy the needs of the child who does not find the authorised product to be 'age appropriate'. This paper explores compounding and manipulation of medicines in relation to approval by medicines regulators and non-approved preparation to fulfil the needs of the individual patient. Definitions are proposed to provide a hierarchical classification based on assurances of quality and bioavailability.

Bioequivalence of dispersed stavudine: opened versus closed capsule dosing

BACKGROUND

Stavudine, a nucleoside reverse transcriptase inhibitor, is used commonly to treat HIV-infected children in the developing world. The paediatric liquid formulation presents major logistical difficulties in rural and resource-limited areas, and prescribers are frequently forced to employ off-label 'opened capsule' dosing methods using the adult capsule. The South African Department of Health (DoH) has advised that caregivers should be instructed to disperse the contents of an adult capsule in 5 ml water and then withdraw the required dose using a syringe. The bioavailability of stavudine using the opened capsule dosing method has not previously been validated.

METHODS

This was a randomized crossover pharmacokinetic study with each subject serving as his/her own control. A total of 28 healthy HIV-negative adult volunteers were randomized on a 1:1 basis to receive one of the two generic preparations typically available in state hospitals. They were then further randomized to receive either intact or opened 30 mg capsules. After 1 week, those who initially received intact capsules, were given opened capsules and vice versa. The capsule dispersion technique used was identical to that prescribed by the DoH. Serial blood samples were collected and plasma stavudine concentrations were assayed by liquid chromatography tandem mass spectrometry. Stavudine pharmacokinetics were analysed using non-compartmental methods and formulations were compared using ANOVA.

RESULTS

Plasma drug exposure after stavudine administration as a solution using an opened capsule dosing method was found to be bioequivalent to intact capsule administration. This was true for both generics tested.

CONCLUSIONS

The opened capsule dosing technique is bioequivalent to intact capsule dosing for stavudine in HIV-seronegative adults.

The H2 receptor antagonist nizatidine is a P-glycoprotein substrate: characterization of its intestinal epithelial cell efflux transport

The aim of this study was to elucidate the intestinal epithelial cell efflux transport processes that are involved in the intestinal transport of the H(2) receptor antagonist nizatidine. The intestinal epithelial efflux transport mechanisms of nizatidine were investigated and characterized across Caco-2 cell monolayers, in the concentration range 0.05-10 mM in both apical-basolateral (AP-BL) and BL-AP directions, and the transport constants of P-glycoprotein (P-gp) efflux activity were calculated. The concentration-dependent effects of various P-gp (verapamil, quinidine, erythromycin, ketoconazole, and cyclosporine A), multidrug resistant-associated protein 2 (MRP2; MK-571, probenecid, indomethacin, and p-aminohipuric acid), and breast cancer resistance protein (BCRP; Fumitremorgin C) inhibitors on nizatidine bidirectional transport were examined. Nizatidine exhibited 7.7-fold higher BL-AP than AP-BL Caco-2 permeability, indicative of net mucosal secretion. All P-gp inhibitors investigated displayed concentration-dependent inhibition on nizatidine secretion in both directions. The IC(50) of verapamil on nizatidine P-gp secretion was 1.2 x 10(-2) mM. In the absence of inhibitors, nizatidine displayed concentration-dependent secretion, with one saturable (J(max) = 5.7 x 10(-3) nmol cm(-2) s(-1) and K(m) = 2.2 mM) and one nonsaturable component (K(d) = 7 x 10(-4) microL cm(-2) s(-1)). Under complete P-gp inhibition, nizatidine exhibited linear secretory flux, with a slope similar to the nonsaturable component. V(max) and K(m) estimated for nizatidine P-gp-mediated secretion were 4 x 10(-3) nmol cm(-2) s(-1) and 1.2 mM, respectively. No effect was obtained with the MRP2 or the BCRP inhibitors. Being a drug commonly used in pediatrics, adults, and elderly, nizatidine susceptibility to efflux transport by P-gp revealed in this paper may be of significance in its absorption, distribution, and clearance, as well as possible drug-drug interactions.

Validation of a New High-Performance Liquid Chromatography Assay for Nizatidine

An expedient high-performance liquid chromatography (HPLC) assay for nizatidine measurement in human plasma was developed and validated. After deproteinization of 200 microL of plasma by filtration, nizatidine and 4-amino-antipyrine (internal standard) were separated (capacity ratio 3.0 and 6.63, respectively) on Nova-Pak C18 cartridge at room temperature (RT), and detected spectrophotometrically at 320 nm. The mobile phase, 0.02 mol/L disodium hydrogen phosphate, acetonitrile, methanol, and triethylamine (80:10:10:0.05 vol/vol), was delivered at 1.5 mL/min. Calibration curves were linear (r2 > or = 0.999) in the range 0.02 to 5 microg/mL, detection and quantification limits were 0.01 and 0.02 microg/mL, respectively, intra-run and inter-run coefficients of variation were < or = 3.5% and < or = 4.2%, respectively, and recovery was >90%. Nizatidine was stable for at least 4 hours at RT, 12 weeks at -20 degrees C, and 3 freeze-thaw cycles in plasma; 16 hours at RT and 48 hours at -20 degrees C in deproteinized plasma; and 6 hours at RT and 3 weeks at -20 degrees C in water.

Bioequivalence of two oral formulations of nizatidine capsules in healthy male volunteers

PURPOSE

The purpose of this randomized, crossover study was to compare the bioavailability of a generic and an innovator formulation of nizatidine 300 mg capsules under fasting conditions.

METHODS

Twenty blood samples per period were collected from 20 healthy, Arab male volunteers over 16 h, plasma nizatidine concentrations were determined by HPLC assay, and pharmacokinetic parameters were determined by the non-compartmental method.

RESULTS

Mean+/-SD C(max), T(max), AUC(0-->t), AUC(0-->infinity), and t1/2 were 2.96+/-0.54 and 3.28+/-0.68 microg/ml, 1.31+/-0.70 and 0.93+/-0.38 h, 9.04+/-1.66 and 9.03+/-1.94 microg x h/ml, 9.17+/-1.64 and 9.12+/-1.94 microg x h/ml, and 1.64+/-0.21 and 1.58+/-0.22 h for the generic and innovator formulation, respectively. The parametric 90% confidence intervals on the mean of the difference between log-transformed values were 98.06% to 103.21%, 98.74% to 103.71%, and 83.37% to 101.34%, for AUC(0-->t), AUC(0-->infinity), and C(max), respectively.

CONCLUSION

The results indicate that these two formulations are equivalent in the rate and extent of absorption.

Developmental pharmacokinetics and pharmacodynamics of nizatidine

OBJECTIVES

To characterize the impact of development on the pharmacokinetics and pharmacodynamics of nizatidine.

METHODS

Children (age range, 5 days-18 years) and adults (age range, 18-50 years) were enrolled in four open-label trials. Nizatidine formulation and dose were determined by age: infants received 2 or 4 mg/kg i.v., children 2.5 or 5 mg/kg in one of three oral liquid formulations, and adolescents and adults received a fixed 150-mg capsule. Nizatidine and N-desmethylnizatidine concentrations were measured in serial post-dose plasma samples by a high-performance liquid chromatographic assay with mass spectrometric detection. Intragastric pH was recorded during a 24-hour post-dose interval.

RESULTS

Data on 93 subjects were combined with previous values from 36 individuals to cover an age group not adequately captured and to control for formulation effects. Dose-normalized exposure estimates revealed no apparent age dependence; however, maximum plasma concentration (298.5 +/- 100.7 v 552.8 +/- 152.4 ng/mL per mg/kg dose) and AUC0-infinity (954.4 +/- 379.8 v 1,573.0 +/- 347.4 ng*hour/mL per mg/kg dose) were reduced in extemporaneous formulations in apple juice. The apparent modest age dependence observed for total body clearance (Cl/F) (r = 0.365) and Vss/F (r = 0.221) reflected a formulation-dependent decrease in bioavailability rather than a true age effect. The age-associated changes in lambda z observed for nizatidine and its metabolite were predictable and consistent with developmental acquisition of renal function. Mean and median pH, as well as fraction of time that the dosing interval remained above target pH values, were significantly greater with administration of the drug than without.

CONCLUSIONS

The biodisposition of nizatidine in children and adults is similar; however, response after a comparable weight-based dose is equal and potentially greater in children.

THEINTEGRATION OFPHARMACOKINETICS ANDPHARMACODYNAMICS: Understanding Dose-Response

Pharmacokinetic (PK) and pharmacodynamic (PD) studies have proven to be powerful and instructive tools, particularly in elucidating important aspects of human pharmacology. Nevertheless, they remain imperfect tools in that they only allow researchers to indirectly extrapolate, through computational modeling, the dynamic processes of drug action. Furthermore, neither tool alone provides a complete nor necessarily relevant picture of drug action. This review explores the utility and applications of PK and PD in the study of drugs, provides examples of lessons learned from their application to studies of human pharmacology, points out some of their limitations, and advances the thesis that these tools ideally should be employed together in an integrated approach. As we continue to apply these tools across the continuum of age and disease, they provide a powerful means to enhance our understanding of drug action, drug interactions, and intrinsic host factors that influence pharmacologic response.