br Although there are growing

Although there are growing bodies of research dealing with diverse non-imidazole based compounds, they are not free from obstacles in their development pipeline and hence the design of these compounds is complicated by various factors briefly discussed below. One of the problems in designing H3R antagonist/inverse agonists is the affinity of these agents towards the hERG K+ channel, resulting in cardiotoxicity and originated from the similarity between the H3R pharmacophore and hERG K+channels (Gemkow et al., 2009; Lazewska & Kiec-Kononowicz, 2010; Łażewska & Kieć-Kononowicz, 2014; Tiligada et al., 2009). Phospholipidosis is an additional concern for non-imidazole-based compounds containing two basic sites (Gemkow et al., 2009; Lazewska & Kiec-Kononowicz, 2010; Łażewska & Kieć-Kononowicz, 2014). Prolonged duration of action as a consequence of high CNS penetration, lipophilicity, 93 3 mg protein binding and residence time, is a drawback for some non-imidazole compounds, leading to insomnia derived from a wake-promoting side-effect. This is one of the reasons for withdrawal of H3R ligands from clinical trials (Łażewska & Kieć-Kononowicz, 2014; Singh & Jadhav, 2013). Therefore, receptor occupancy should be considered in pre-clinical studies as occupancy over 80% induces insomnia and in this context paying attention to dose schemes of drug candidates in clinical investigations is critical (Kuhne et al., 2011). Another approach to resolve this obstacle may be the characterization of ligands concerning the duration of H3R blockade and to develop ligands with short to intermediate receptor residence times (Mocking, Verweij, Vischer, & Leurs, 2018). While screening of drug-target residence times is currently emerging to give additional information about the time-course of receptor-dissociation and the duration of target-occupancy, only some of the above-mentioned H3R ligands have been recently characterized in this context (Mocking et al., 2018; Reiner & Stark, 2019; Riddy et al., 2019). Being substrates for P-glycoprotein is a problem reported for some non-imidazole-based compounds (Gemkow et al., 2009; Lazewska & Kiec-Kononowicz, 2010). Apart from this, it should be kept in mind that developing H3R antagonist/inverse agonists is greatly affected by the complex pharmacology of H3Rs. Molecular heterogeneity of different splice variants, constitutive activity, receptor oligomerization, differential signaling pathways and species-related discrepancies are determinant parameters that debilitate the design process (Bhowmik et al., 2012; Esbenshade et al., 2008; Lazewska & Kiec-Kononowicz, 2010; Łażewska & Kieć-Kononowicz, 2014; Plancher, 2011; Riddy et al., 2017; Tiligada et al., 2011; Wijtmans et al., 2007). Therefore, based on the available structural information responsible for shortcomings and failures of H3R antagonists/inverse agonists, special notice should be paid in the rational design of such compounds. Additionally, for the appropriate assessment of new chemical entities, it has been suggested that the biological assays be performed in different functional platforms containing multiple H3R isoforms (Lazewska & Kiec-Kononowicz, 2010). Multi-targeting agents are of major interest in modern drug design and discovery. Design of such compounds can be applied to H3R antagonists for the purpose of achieving optimum efficiency as performed for some preclinical candidates such as recently for contilisant (Bautista-Aguilera et al., 2017; Bautista-Aguilera et al., 2018). To this end, different pharmacophores can be combined for developing hybrid and multi-target therapeutic agents useful in the treatment of several diseases with reduced side-effects (Khanfar et al., 2016; Lazewska & Kiec-Kononowicz, 2010; Nikolic et al., 2014; Sander et al., 2008; Stark, 2003; Wijtmans et al., 2007). In context of those poly-pharmacological agents, further research is prompted by recent findings of additional potent sigma 1 receptor binding behavior of some clinically investigated H3R ligands such as ABT-239, S 38093 and PF-3654746 as well as pitolisant (Riddy et al., 2019). Such multi-target directed ligands can be more effective in neurological diseases with multifactorial nature through different embedded pharmacophores required for interactions with multiple targets. Last, but not least, whether or not H3R antagonists/inverse agonists can be used as monotherapy or add-on therapy is a debatable issue, which requires more intensive clinical trial studies containing larger population sizes (Berlin et al., 2011; Celanire et al., 2005; Lazewska & Kiec-Kononowicz, 2010; Tiligada et al., 2009). Overall, in spite of the difficulties faced by the development of H3R antagonists/inverse agonists, there are several drug candidates advanced to different phases of clinical trials and it is expected to witness the marketing of H3R ligands in the near future, in addition to pitolisant (Wakix)®, the only H3R antagonist/inverse agonist approved by the European Medicines Agency and marketed in the European Union for the treatment of narcolepsy.