The absence of positive signal between tenofovir alafenamide treatment[
[19],
[21]] and COVID-19 outcome is intriguing because tenofovir disoproxil fumarate and tenofovir alafemanide both yield to the same active compound, namely tenofovir. While the former concentrates in plasma, the latter concentrates in the cells, in particular lymphatic cells [
[34]].
These pharmacokinetics differences may provide an advantage for tenofovir disoproxil fumarate in reaching SARS-CoV-2 infected cells, in particular the endothelium [
[35]] and highly vascularized organs such as the kidneys or the lungs. Tenofovir disoproxil fumarate and tenofovir alafenamide have a very different lipid metabolism and lipids play a fundamental role in viral replication [
[36]].
Finally, tenofovir disoproxil fumarate has immunomodulatory effects[
[37],
[38]], which could be of importance in limiting the cytokine storm during the second week of COVID-19, known as the "inflammatory" phase of the disease. For example, the administration of tenofovir disoproxil fumarate plus emtricitabine for 30 days was associated with a significant decrease of HLA-DR+ CD38+ co-expression on CD8+ T-cells, a marker of immune activation amongst 19 healthy volunteers [
[39]]. Of note, HLA-DR+ CD38+ co-expression on CD8+ T-cells was significantly associated with COVID-19 severity in a cohort of 125 COVID-19 patients [
[40]].