Transdermal Delivery of Etoposide Phosphate I: In Vitro and In Vivo Evaluation

Drug Exposure to Establish Pharmacokinetic-Response Relationships in Oncology

In the oncology field, understanding the relationship between the dose administered and the exerted effect is particularly important because of the narrow therapeutic index associated with anti-cancer drugs and the high interpatient variability. Therefore, in this review, we provide a critical perspective of the different methods of characterising treatment exposure in the oncology setting. The increasing number of modelling applications in oncology reflects the applicability and the impact of pharmacometrics on all phases of the drug development process and patient management as well. Pharmacometric modelling is a worthy component within the current paradigm of model-based drug development, but pharmacometric modelling techniques are also accessible for the clinician in the optimisation of current oncology therapies. Consequently, the application of population models in a hospital setting by generating close collaborations between physicians and pharmacometricians is highly recommended, providing a systematic means of developing and assessing model-based metrics as 'drivers' for various responses to treatments, which can then be evaluated as predictors for treatment success. Characterising the key determinants of variability in exposure is of particular importance for anticancer agents, as efficacy and toxicity are associated with exposure. We present the different strategies to describe and predict drug exposure that can be applied depending on the data available, with the objective of obtaining the most useful information in the patients' favour throughout the full drug cycle. Therefore, the objective of the present article is to review the different approaches used to characterise a patient's exposure to oncology drugs, which will result in a better understanding of the time course of the response and the magnitude of interpatient variability.

Cutaneous Penetration-Enhancing Effect of Menthol: Calcium Involvement

Menthol is a naturally occurring terpene used as a penetration enhancer in topical and transdermal formulations. Literature shows a growing interest in menthol's interactions with the transient receptor potential melastatin 8. A decrease in extracellular Ca²⁺ due to the activation of the transient receptor potential melastatin 8 receptor produces inhibition of E-cadherin expression that is responsible for cell-cell adhesion. Because calcium is present in the entire epidermis, the purpose of this study is to evaluate whether the aforementioned properties of menthol are also related to its penetration-enhancing effects. We formulated 16 gels: (i) drug-alone (diphenhydramine or lidocaine), (ii) drug with menthol, (iii) drug, menthol, and calcium channel blocker (CCB; verapamil or diltiazem), and (iv) drug and CCB. In vitro studies showed no effect of the CCB on the release of the drugs either with or without menthol. In vivo experiments were performed for each drug/menthol/CCB combination gel by applying 4 formulations on a shaved rabbit's dorsum on the same day. Dermis concentration profiles were assessed with microdialysis. The gels containing menthol showed higher penetration of drugs than those without whereas the addition of the CCB consistently inhibited the penetration-enhancing effects of menthol. In summary, these findings strongly support the involvement of calcium in the penetration-enhancing effect of menthol.

Pharmacokinetic applications of cutaneous microdialysis: Continuous+intermittent vs continuous-only sampling

INTRODUCTION

Microdialysis is a technique that allows interstitial-fluid sampling with minimal tissue-damage. In a microdialysis study, samples are collected serially (continuous microdialysis, CMD) and participant's movements are reduced for the entire study. Intermittent Cutaneous Microdialysis (IMD) is a modified version of CMD, which allows for unrestrained periods in between samples. However, in separate experiments, pharmacokinetic parameters estimated with IMD showed higher variability than with CMD. The purpose of this study is to simultaneously assess and compare the skin pharmacokinetic profiles obtained with a combination of CMD and IMD with those obtained with traditional CMD sampling only, in the same experiment.

METHODS

Two linear microdialysis (MD) probes were inserted into the shaved dorsal skin of three rabbits. Following the oral administration of three different doses (20, 40 and 80mg/kg) of ciprofloxacin (CPLX), for the first 2h, samples were collected from both probes according to traditional CMD in order to assess intrinsic differences between the two sites. After 2h, one of the probes was switched to IMD schedule. Skin-exposure parameters were estimated with non-compartmental analysis.

RESULTS

Two of the nine experiments showed a difference larger than 30% between the concentrations measured from the two probes when both were on the CMD schedule. Otherwise, the skin concentration profiles were almost superimposable. Pharmacokinetic parameters were not statistically different.

CONCLUSION

The results of this study show that skin pharmacokinetic parameters measured via a combination of CMD and IMD were not statistically different from those estimated via traditional CMD sampling alone.

Transdermal Delivery of Etoposide Phosphate II: In Vitro In Vivo Correlations (IVIVC)

A dependable in vitro in vivo correlation (IVIVC) is a vital tool to optimize drug formulation and expedite product development time. Although many IVIVC examples are available for oral delivery systems, IVIVC for transdermal delivery is far less common, especially for electrical-assisted delivery. The objective of this study was to develop an IVIVC for the iontophoretic delivery of the anticancer drug etoposide. Iontophoresis was performed at 4 current densities (100, 200, 300, and 400 μA/cm(2)) both in vitro using a standard Franz-cell apparatus with excised porcine skin as membrane, and in vivo in a rabbit model. There was strong correlation between the in vitro % permeated across porcine skin and in vivo absorption (AUC, Cmax) in the range 100-300 μA/cm(2). The correlation between in vitro flux and in vivo input rate (R0) permitted to predict the R0 from a different set of in vitro data (external validation). Convolution of such input rate accurately predicted in vivo plasma profiles (PE% <15) in the absorption phase, whereas the elimination phase was slightly under-predicted (PE% >20). In vivo absorption profiles obtained with deconvolution did not overlap directly with the in vitro profiles; however, correction for the lag time and the application of a scaling factor estimated from Levy' s plots resulted in excellent correlation.