Topical pharmacokinetics for a rational and effective topical drug development process
Topical drugs are not developed by the same process as oral drugs. The process is more uncertain and contains gaps. This leads to a poor discharge of risks before going to the clinical phases. The topical drug development process is reviewed in the introduction of the thesis. In particular, past and current topical drug development practices are described and compared to the oral drug development process. The large risks taken during the topical drug development are pointed out. These risks are largely associated with a lack of pharmacokinetic's involvement prior to the drug candidate selection stage. Pharmacokinetics is considered after drug selection when it is often too late. Furthermore, the topical pharmacokinetic techniques available appear to be not suitable for three reasons: accessibility to the pharmacokinetic techniques, meaning of the data generated and reliability of these data. It concludes that the knowledge of target skin tissue concentration would be key for a more rational drug development process. To this end, the primary objective of this thesis is to define a way of measuring drug concentration in skin tissue after topical application that is reliable, effective and practical. A secondary objective is then from the knowledge of the skin tissue concentration, to develop a topical PharmacoKinetic/PharmacoDynamic model to predict likely efficacy for a topical drug candidate. First a direct skin tissue concentration approach is described that brings theoretical reliability into the pharmacokinetic data generated and improves throughput. However the pharmacokinetic data generated have limited use as total drug (bound + unbound) tissue concentration is measured while, pharmacodynamically, only the unbound fraction is of interest. An indirect skin tissue concentration determination is then proposed. It consists in predicting the in vivo unbound drug concentration in diseased skin tissues. Three steps are required: In the first step, the in vitro percutaneous flux is linked with the unbound drug concentration in the dermis. From there, the in vivo unbound drug concentration in all the skin tissues is defined using different physiological parameters. Finally, taking into account the effect of the skin disease on skin permeability and dermal capillary clearance, the in vivo unbound drug concentration in skin tissues in diseased skin is defined. The predicted concentration is therefore calculated from a constant (which is skin disease dependent) and from the in vitro percutaneous flux (which is an accessible and reliable experimental pharmacokinetic data). A PharmacoKinetic/PharmacoDynamic model is then built. This model delivers two types of information: -1- The "efficacy index" which is a prediction of efficacy for a drug candidate based on percutaneous flux and drug potency and -2- the "systemic safety index" which is an assessment of systemic exposure based on total systemic clearance and plasma protein binding. To check the validity of this new model, a validation exercise is run with the key eight topical drugs classes: NSAIDS, anaesthetics, retinoids, corticosteroids, vitamin D3 derivatives, antifungals, antibacterials for acne and immunomodulators. For seven out of the eight classes, the validation of the model is good. For the last class, the antibacterials for acne, the model underpredicts efficacy and it is suggested that the route of entry of antibacterial agents in acne occurs via the sebaceous duct as opposed to the more classic stratum corneum pathway. Finally, three pilot studies are conducted with the aim to improve the quality and relevance of the data generated with in vitro percutaneous flux studies as well as the access to this technique and throughput of this technique.