ACE2 and chronic obstructive pulmonary disease: Bromhexine is a kind of expectorant, which is mainly used in the treatment of chronic bronchitis, emphysema, bronchiectasis, and chronic obstructive pulmonary disease.[17–19] As a potent inhibitor of TMPRSS2, a key protease in the infection and transmission of novel coronavirus SARS-CoV-2, bromhexine has the advantage of low price and greater safety.[20] Also, bromhexine and its metabolites can competitively bind to cellular receptor angiotensin-converting enzyme 2 (ACE2).