2  A brief introduction to the concept of lysosomotropism in pharmacology
The concept of lysosomotropism was first introduced by De Duve et al. in 1974 [17]. The term was originally proposed for all substances taken up by lysosomes, regardless of their chemical structure or mechanism of action. Interestingly, the importance of the potential antiviral properties of lysosomotropic agents was emphasized in the original publication [17]. Moreover, some of the original concepts of lysosomotropism, such as the possibility to make virtually any substance lysosomotropic by suitable coupling with an appropriate chemical carrier, are now considered to be common principles in drug design. In general, most substances with weakly basic and lipophilic properties are believed to demonstrate lysosomotropism to some extent. These properties enable lysosomotropic drugs to diffuse passively through the endosomal membrane and undergo protonation-based trapping in the lumen of the acidic vesicle. From a chemical standpoint, most drugs that demonstrate lysosomotropic properties are cationic substances that usually belong to the group of primary, secondary or tertiary amines; from a pharmacological point of view, these compounds are often described by very large volumes of distribution and long residual effects [18]. Moreover, lysosomotropic agents inhibit endosomal maturation and disrupt endolysosomal trafficking, and these effects are of particular interest in the context of viral infection, as described above. The exact mechanisms by which different lysosomotropic agents induce these changes are still not well understood; however, modulation of pH and interaction with different molecular systems involved in pH regulation of endolysosomal vesicles seem to be the most important. For deeper understanding of the current concept of lysosomotropism, the reader is referred to the informative review by Marceau et al. [18]. Antimalarial drugs, such as CQ, which have demonstrated clinical efficacy in the treatment of COVID-19 [11] are often referred to as classic lysosomotropic agents because their primary mechanism of action relies on their lysosomotropic properties. CQ works by sequestration in digestive vacuoles that resemble endolysosomal compartments, and increases their pH to stop the nutrient supply [18]. Another interesting example of lysosomotropic pharmaceuticals are drugs used for the treatment of tuberculosis, where the drug has to reach both extracellular bacilli in the interstitial compartments and caseum, and intracellular pathogens residing in phagolysosomes of immune cells [18,19]. Consequently, retention of antituberculosis drugs in infected phagolysosomes has been considered an important factor in maximization of their efficiency [19], and this property has been investigated actively in the course of development of novel antituberculosis agents [20,21]. The importance of phagolysosomal sequestration of lysosomotropic agents in the context of Mycobacterium tuberculosis infection is also reflected by the fact that lysosomal neutralization with bafilomycin A or ammonium chloride inhibits the tuberculostatic effect of some drugs [22]. Interestingly, moderate antituberculosis activity has been shown in vitro for some standard lysosomotropic drugs, such as fluoxetine and sertraline, and this effect was absent in their non-protonable analogues that cannot undergo ion trapping [22]. However, apart from drugs that are well-known lysosomotropic substances, many pharmacological agents from other therapeutic classes belong to this group of chemicals based on both their physico-chemical properties and their effect on endolysosomal function in vitro and in vivo. It is suggested that these drugs should be examined in the context of their potential use in COVID-19 treatment protocols, as most of them could potentially be used to target both viral replication and dissemination and symptoms of the disease. Moreover, some drugs can produce similar effects on the endolysosomal pathway by targeting endosomal compartment pH through different mechanisms such as inhibition of vacuolar-type H+-ATPase (V-ATPase). In the following text, these compounds are considered as potential candidate drugs with antiviral effects (Fig. 2 ). As many pharmacological agents have shown endolysosomal pH-modulating activity both in vitro and in vivo, this review focused solely on drugs that satisfied several criteria that are considered important in this context: existing evidence of endolysosomal pH modulation, good safety profile, wide availability, low cost, and additional therapeutic benefits apart from the potential preventive and therapeutic antiviral effect (Table 1 ).
Fig. 2 Possible mechanisms of endolysosomal pH-modulation-mediated effects on the structure and function of endolysosomal vesicles with potential consequences for viral fusion, trafficking and shedding. (A) Drugs affecting activity of vesicular acidification mechanisms neutralize endolysosomal compartments. The illustration shows the general structure of a proton pump inhibitor; this might neutralize endolysosomal compartments through inhibition of vacuolar-type H+-ATPase (V-ATPase). (B) Weakly basic and lipophilic properties of lysosomotropic compounds enable them to diffuse passively through the endosomal membrane and undergo protonation-based trapping and subsequent neutralization of acidic vesicles. (C) The acidic environment enables conformational changes that activate viral fusion proteins. Moreover, H+ interacts with the viral matrix and activates mechanisms implicated in later stages of viral fusion and/or uncoating. (D) Acidic pH of endolysosomal compartments activates proteases involved with viral fusion processes. (E) The acidic luminal environment activates endolysosomal trafficking regulatory proteins [56].
Table 1 Drugs used in coronavirus disease 2019 (COVID-19) clinical trials with potential endolysosomal pH-mediated effect on severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Clinicaltrials.gov was searched for COVID-19, and pharmaceutical intervention data were analysed on 20th May 2020. Both observational and interventional trials were included in the table.