Lysosomotropic active compounds — hidden protection against COVID-19 / SARS-CoV-2 infection ?

1 Furtwangen University, Medical and Life Sciences Faculty, Institute of Precision Medicine, Jakob-Kienzle-Str. 17, D-78054 Villingen-Schwenningen, Germany 2 Institute of Pharmaceutical Sciences, University of Freiburg, Albertstraße 25, 79104 Freiburg i. Br., Germany 3 Department of Intensive Care Medicine, University of Rostock, Schillingallee 35, D-18057 Rostock 4 Department of Intensive Care Medicine, Hospital of Magdeburg, Birkenallee 34, D-39130 Magdeburg 5 Fraunhofer Institute IZI, Leipzig, EXIM Department, Schillingallee 68, D-18057 Rostock, Germany


Introduction
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been identified as the disease-causing pathogen of Coronavirus disease 2019 (COVID-19) 1 . A total of 1 289 380 confirmed infections and 70 590 deaths worldwide were reported by the Johns Hopkins Center for Systems Science and Engineering, Baltimore, Maryland, USA, as of April 4, 2020. 2 As of March 11, 2020, the status of the Covid-19 outbreak was reclassified from an epidemic to a pandemic, thus posing serious challenges to the health care systems in the EU, the US, and many Asian countries.
Research to identify active compounds for the treatment of SARS-CoV-2 viral infection has focused to date on the virustatic agents ritonavir 3 (off-label use) and remdesivir 4,5 (GS-5734, compassionate use) or the antimalarial active compounds chloroquine 5-7 and hydroxychloroquine 6,8,9 (off-label use), both of which are well-known immune modulators.
In 1984, the effects of the weak base and lysosomotropic compound chloroquine was investigated against Sindbis virus infection in BHK-21 cells. In established infections, chloroquine was found to inhibit the synthesis of viral RNA when added early in the process of pathogenesis 10 .
Lysosomotropism is an often neglected biological characteristic of small molecules, which is present in addition to their intrinsic receptor-mediated or enzymatic pharmacological effects and is sometimes responsible for severe adverse effects [11][12][13][14][15] . Regardless of the medical indications for which they have been used, many (active) compounds possess lysosomotropic characteristics [16][17][18] and therefore are potential active compounds for treatment of SARS-CoV-2 viral infection of airway epithelial cells (type II pneumocytes), such as chloroquine in Sindbis virus infection 10 . Approved drugs comprising lysosomotropic compounds used for various medical indications offer a chance to treat the SARS-CoV-2 infection through more personalized approaches depending on drug profile.
Given the wide application of drugs with lysosomotropic characteristics (Table 1), statistical testing of the protective effects of various drugs in the current pandemic appears straightforward. Here, we outline possible prevention and treatment options based on recent findings on the COVID-19 disease, which would be easy to apply, even for patients at risk.  25 .

Mechanism of host cell entry
Both types of SARS-CoV engage their receptor, ACE2, on the host cell surface for host cell entry 20 . In cells that do not express trypsin-like proteases (human airway trypsin-like protease (HAT)) 26 25,29,31 .

Current therapy guidelines
The There is a need for action in the field of therapy and prevention of SARS-CoV-2 infection, owing to the rapidly increasing number of cases. Therapies recommended to date represent a first step toward solving this immense challenge. 6

Research on the lysosomotropic compound chloroquine in SARS-CoV(-2) infection
Several in vitro studies have been conducted to elucidate the entry route and therapy options for treating SARS-CoV infection 28,30,36,37

Lysosomotropic active compounds
Lysosomotropic compounds are small molecules selectively taken up by lysosomes, regardless of their chemical nature or mechanism of uptake 38 . Typically, they are weak organic bases (pKa >6, lipophilic) that easily penetrate uncharged the lysosomal membrane and are protonated and consequently trapped in the lysosome lumen 16,17 .
Lysosomotropism is therefore a biological characteristic of active compounds that is independent of their pharmacological effects.

Current and achievable clinical impact of lysosomotropic compounds
To date, lysosomotropism has been of scientific interest for its association with the occurrence of adverse effects during the application of particularly active compounds.
Lysosomotropism in combination with dysfunction in elongation of very long-chain fatty acids is responsible for severe adverse effects when used orally or topically 15 , in some cases such as hydroxychloroquine (rash or itching) 14 , sertraline (exanthematous pustulosis) 12 , and terbinafine (Lupus erythematodes or exanthematous pustulosis) 11 .
Lysosomotropism appears at concentrations in the micromolar range; nevertheless, most drugs exhibit their desired primary pharmacological effects at low concentrations.
As described above, cathepsin L plays a crucial role in SARS-CoV-2 infection of host cells and subsequent dissemination. In general, cathepsin L can be inactivated through selective but not clinically approved inhibitors 43,44 , or alternatively by clinically approved lysosomotropic compounds (see lysosomotropic active compounds) in off-labeI use.
However, in the case of systemic administration of the active compound, the main indications must be considered, because they may represent undesired adverse effects in an anti-viral off-label use and therefore should not be ignored. According to current knowledge, inhibition of cathepsin L dependent viral entry (fusion) into host cells can be obtained only through off-label use of the active compounds listed in Table 1.  30 . A unique feature of glycopeptide antibiotics is that, if they are used off-label as lysosomotropic compounds, they retain their initial adverse effect profiles. In off-label use, the benefit-risk profile is indistinguishable from that in authorized applications.

SARS-Cov-2 infection is likely to cause both pulmonary and systemic inflammation, thus
leading to multi-organ dysfunction (e.g., acute respiratory distress syndrome (ARDS), myocarditis, septic shock, sepsis, sepsis after bacterial superinfection, acute liver injury, and hepatitis) in high risk populations 7 48 ; therefore, these findings suggest that a rapid, severe, and serious deterioration during SARS-Cov-2 infection is associated with CRS/cytokine storm syndrome 35,49 .
The hypothesis that chloroquine, owing to overactivation of the immune system triggered NB 06 and the clinically approved lysosomotropic compounds listed in Table 1 might therefore serve as valuable active compounds to prevent or mitigate the cytokine storm in the lungs after viral (SARS-CoV-2) infection of airway epithelial cells in COVID-19 (Fig. 2).

Local application of lysosomotropic compounds (off-label)
The respiratory tract is the gateway for SARS-CoV-2 infection and the primary affected  Table 1 is off-label; no guidelines or empirical data are currently available. Appropriate concentrations for inhalation therefore should be determined experimentally. If necessary, an application of mixtures of two lysosomotropic compounds for different indications may be used to avoid undesired pharmacological effects.

Prospects for the success of inhalative application
Inhalation therapy might be a simple method for managing COVID-19 and preventing SARS-CoV-2 infection in vulnerable individuals and those with undesirable systemic pharmacological effects. Individuals with COPD, asthma, or chronic bronchitis are usually familiar with the handling of inhalation devices. Numerous available active ingredients with lysosomotropic characteristics permit tailor-made therapy (dosage and composition). If required, a combination with clinically approved active compounds for inhalation could be used.

Systemic application (i.v.) of lysosomotropic compounds (off-label)
The use of glycopeptide antibiotics, such as teicoplanin, as lysosomotropic compounds is not covered by market authorization and would therefore be off-label. The adverse effect and benefit-risk profile, however, correspond to the specifications of the market authorization.

Systemic application (oral) of lysosomotropic compounds COVID-19 (off-label)
Conventional systemic therapy with lysosomotropic compounds (e.g., terbinafine) is likewise feasible. If systemic use is considered, a careful individual risk-benefit analysis must be performed.

Benefit-risk profile of (inhalative) treatment in COVID-19
Obtaining an efficacious blood level after inhalation is unlikely; if an efficacious blood level is obtained, the applied active compound triggers intrinsic pharmacological effects.
However, in certain circumstances, intrinsic pharmacological effects are undesirable adverse effects that are expected after lysosomotropic administration.
Because the application is temporary, no permanent alterations resulting from the lysosomotropic effects are expected. Well-known adverse effects, such as drug-induced (sphingo-)lipidoses or lupus erythematodes, are reversible upon termination of application [11][12][13] .

Hypothesis regarding SARS-CoV-2 carriers, spreaders and non-infectable humans
The lysosomes, and particularly cathepsin L, appear to play a key role in COVID-19 and in SARS-CoV-2 infection of host cells. To provide maximum cathepsin L activity and thus maximal cleavage capacity of viral S protein, properly functioning (endo)lysosomes are essential. If the lysosomal pH increases, cathepsin L activity diminishes, thereby decreasing the rate of cleavage, fusion, and infection.
Two scenarios can trigger an increase in lysosomal pH: a lysosomal proton pump breakdown or the administration/presence of lysosomotropic compounds (Fig. 3). To date, a breakdown of the vacuolar H + ATPase (V-ATPase) cannot be triggered by clinically approved active compounds. There is evidence, however, that in various skin disorders (e.g., psoriasis vulgaris, atopic dermatitis, exanthematous pustulosis, and pustular psoriasis), the function of the V-ATPase is more or less restricted by ATP deficiency caused by oxidative stress in cells (keratinocytes) 15