Sir,

The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) is pandemic. There is no vaccine nor effective drug against the virus, but based on in vitro mechanism of action, several therapeutic options have been tested in vivo and in a few clinical studies. At present, the most promising drugs against SARS-CoV-2 are chloroquine (CQ) and hydroxychloroquine (HCQ).^1,2 Although high-quality studies are required to support wide clinical use, it is urgently needed to define a pharmacodynamic target that allows the calculation of rational dosing schedules required for at least three clinical scenarios: (a) prophylaxis for health workers; (b) early or mild symptomatic patients, when <5% of cells are infected; and (c) moderate or severe acute respiratory syndrome per se, meaning that >5% of cells are infected.

In virology, the Multiplicity of Infection (MOI) is a frequently used term to refer to the number of virions added per cell to induce the infection on cell cultures. It is not uncommon to overlook that MOI determines the percentage of cells infected or not during the experiments. Notably, there is a big difference between an MOI of 0.01 (1 virus per 100 cells) and 0.05 (5 viruses per 100 cells or 1 virus per 20 cells) as can be appreciated in Table 1. In fact, only an MOI equal or higher to 0.05 should be useful during the design of dosing regimens for treatment, while an MOI of 0.01 could be considered for prophylactic regimens.

Recently, Wang et al.³ and Yao et al.⁴ determined the susceptibility of chloroquine and hydroxychloroquine in vitro against SARS-CoV-2. They measured the 50% effective concentration values (EC₅₀) and, in one case, the 50% cytotoxic concentration (CC₅₀) and the selectivity index (SI: relation between toxicity or CC₅₀ and efficacy or EC₅₀) (Table 2). According to their results, both drugs had good in vitro activity and CQ exhibited a high SI. Yao et.al. suggested HCQ is more potent (8-fold based on EC₅₀) than CQ against coronavirus for a potential treatment, but the MOI used was 0.01. Disregarding this fact, they simulated the concentrations in the lungs for both drugs using a physiologically based pharmacokinetic (PBPK) model, obtaining data from in silico and animal models. Assuming that both CQ and HCQ are accumulated in human lungs as they are in rats, all dosing schedules attained increasing R_LTEC (ratio of free lung tissue trough concentration/EC₅₀) from day 1 (R_LTEC CQ = 2.38 and HCQ=21-33) to day 5 (R_LTEC CQ = 19 and HCQ =85-103). However, it is difficult to extrapolate these results because no human lung pharmacokinetics is published and the relevance of pulmonary tissue concentration for the treatment of pneumonia is unknown (in contrast to serum or epithelial lining fluid concentrations). Another limitation is that PBPK models suffer from parameter identifiability and redundancy, especially important when only in silico and in vivo data are considered, and a large number of parameters are included.⁵

Population pharmacokinetic-pharmacodynamic (popPK-PD) modeling based on human data could be necessary to design a better dosing schedule than suggested by PBPK models. However, popPK-PD requires a defined pharmacodynamic target related to efficacy. In anti-infective pharmacology, the minimal inhibitory concentration or the EC₅₀ have been accepted as the more useful PD parameters. Unfortunately, basic researchers may have difficulties to share their results with clinical scientists because the units used to measure drug concentrations are different. For example, the reported EC₅₀ is expressed in µM and the concentrations of CQ and HCQ are reported in ng/mL. A rule of thumb for easy conversion of these values is multiplying the molecular weight (g/mol) of the substance (e.g., 319.87 Da for CQ) by the EC₅₀: the product is a concentration useful for clinicians (e.g., 361 µg/L equivalent to ng/mL). For CQ, a plasma level >361 ng/mL should be necessary to reach the EC₅₀. Interestingly, the current dosing schedules used for malaria hardly achieve those plasma levels⁶, reinforcing the idea that a better dosing regimen is urgently needed to improve the potential of this drug in clinical trials. On the other hand, a plasma level of 242 ng/ml of HCQ (molecular weight of 335.87 Da) would be necessary to reach the EC₅₀, an achievable value with approved clinical doses.⁷ Finally, further studies are required to determine the PK/PD index (like C_max/EC₅₀, C_min/EC₅₀, AUC/EC₅₀ or T>EC₅₀) and its optimal magnitude against SARS-CoV-2.