Abstract: International Journal of Antimicrobial Agents 55 (2020) 106007 Contents lists available at ScienceDirect International Journal of Antimicrobial Agents journal homepage: www.elsevier.com/locate/ijantimicag Excessive lysosomal ion-trapping of hydroxychloroquine and azithromycin Hartmut Derendorf University of Florida a r t i c l e i n f o Article history: Received 22 March 2020 Accepted 28 April 2020 Editor: Jean-Marc Rolain a b s t r a c t A recent report identified significant reductions or disappearance of viral load in COVID-19 patients given a combination of hydroxychloroquine and azithromycin. The present communication discusses some common pharmacokinetic properties of these two drugs that may be linked to a potential underlying mechanism of action for these antiviral effects. The physicochemical properties of both hydroxychloroquine and azithromycin are consistent with particularly high affinity for the intracellular lysosomal space, which has been implicated as a target site for antiviral activity. The properties of both drugs predict dramatic accumulation in lysosomes, with calculated lysosomal drug concentrations that exceed cytosolic and extracellular concentrations by more than 50 0 0 0-fold. These predictions are consistent with previously reported experimentally measured cellular and extracellular concentrations of azithromycin. This is also reflected in the very large volumes of distribution of these drugs, which are among the highest of all drugs currently in use. The combination of hydroxychloroquine and azithromycin produces very high local concentrations in lysosomes. The clinical significance of this observation is unclear; however, the magnitude of this mechanism of drug accumulation via ion-trapping in lysosomes could be an important factor for the pharmacodynamic effects of this drug combination. © 2020 Elsevier B.V. and International Society of Chemotherapy. All rights reserved. Recent reports identified significant reduction or disappearance of viral load in COVID-19 patients given a combination of hydroxychloroquine and azithromycin [1,2]. However, other clinical studies could not confirm these early findings [3] and there are considerable concerns about the safety of this drug combination [4]. The present communication discusses some common pharmacokinetic properties of these two drugs that may be linked to a potential underlying mechanism of action for their pharmacological activity. 1. Chemical structures Fig. 1 shows the chemical structures of hydroxychloroquine and azithromycin. Although the two compounds are from two chemically distinct classes, they have a structural similarity that is pharmacokinetically relevant. Both compounds are multibasic amines with pKa values that are susceptible to protonation in the physiological pH range. Azithromycin has two nitrogens with pKa values of 8.1 and 8.8 [5]. Hydroxychloroquine has three nitrogens with pKa values of 4.0, 8.3 and 9.7 [6]. However, only the two nitrogens with the higher values (shown in circles in Fig. 1) are protonated under physiological conditions. E-mail address: hartmut@ufl.edu 2. Lysosomal ion-trapping If the pH of the molecular environment is lower (more acidic), more nitrogens are protonated, which in turn hinders the nowcharged moieties from crossing membranes. This is particularly relevant for the intracellular distribution of basic drugs crossing between the cytosol (pH approx. 7.4) and the acidic lysosomal space (pH approx. 5.0). Fig. 2..