A preclinical study done in 1982 showed that chloroquine was found in the highest concentration in lung tissue, leading to the proposition of chloroquine as a possible treatment for COVID-19 caused pneumonia. In vitro studies were then conducted using VeroE6 cells from the kidneys of African green monkeys. In these studies, hydroxychloroquine (EC50 = 0.72μM) appeared more effective than chloroquine (EC50=5.47μM) in countering COVID-19 in vitro. A recent study was done using the PBPK models. The maintenance dose, used to bring the concentration up in infected lung tissue, consist of a loading dose of 400mg of chloroquine followed by 200mg twice daily for four days. This is found to be more successful than if given 500mg twice daily for 5 days. EC90 of chloroquine against COVID-19 was 6.90μM in VeroE6 cells. The cytotoxic concentration (CC50) is 273.20μM for chloroquine and 249.50μM for hydroxychloroquine. Chloroquine was more effective in treating COVID-19 than hydroxychloroquine due to its higher selectivity index (CC50/EC50).