DPP4 and COVID-19: Moreover, it was suggested that numerous compounds (estradiol, spironolactone, isotretinoin, and retinoic acid) may down-regulate ACE2 receptors [50], and inhibitors of TMPRSS2 (bicalutamide, camostat mesylate, and nafamostat), inhibitors of DPP4, and inhibitors of endocytotic transport (chloroquine, hydroxychloroquine, amodiaquine, artemisinin and artesunate baricitinib, chlorpromazine, niclosamide, imatinib, and amiodarone) could be useful to treat COVID-19 [50].