C Chronopharmacokinetics: Aging and Sex C. Maldonado, R. Eiraldi and M. Vázquez Pharmaceutical Sciences Department, Faculty of Chemistry, Universidad de la República, Montevideo, Uruguay Definition The active process of entrainment ensures that the biological clock is stably synchronized to its zeit- gebers. Pharmacological treatments have the aim of curing or relieving diseases in the most effica- cious and less toxically manner and may interact with this process in a bidirectional manner. In this continuum, age and sex might be parameters changing chronopharmacokinetic responses, yet how they evolve could also be modified by the responses of the circadian clock. Chronopharmacokinetics and Aging Aging is a worldwide phenomenon, and the understanding of processes affected by aging should be of major concern as life expectancy increases and so does the chronicity of different pharmacologically treated diseases [1]. Rhythms accompany us since the beginning of our life; however, during aging other components arise that combine circadian rhythms with the aging process and the medicines taken. Therefore, the elderly population could be more highly impacted by the modification of biological rhythms, and there is urgent need of critical analysis of the research in the field. Our internal clock evolves from the moment we are born, though its changes have been more commonly studied from adolescence on. The cir- cadian phenotype is age dependent and can be better understood by the incorporation of hor- monal concentration and timing [2]. In young people (16–25 years of age), cortisol concentra- tion reaches its minimum (and growth hormone its maximum) at about 1 a.m., approximately 1 h later than in the elderly (70 years). These endo- crine factors affect the structure of sleep [2]. The end of adolescence is defined based on biological and sociological variables, and it coincides with the sharp maximum of lateness at around the age of 20 [3]. When chronotype stops delaying and starts advancing, adolescence has come to an end and individuals enter adulthood. With increasing age a dyssynchronization pro- cess begins in awakening, temperature control, and hormone release, among others, and has been stated that this phenomenon could be a sign in the progression of neurodegenerative diseases such as Alzheimer, Parkinson, and Huntington [4]. Metabolic, cardiovascular, and chronic inflammation also rise their prevalence in the elderly and are nowadays related to synchroniza- tion impairment as well [5]. © Springer Nature Switzerland AG 2021 A. Talevi (ed.), The ADME Encyclopedia, https://doi.org/10.1007/978-3-030-51519-5_162-1