Compositions and methods for inhibiting human gamma-glutamyl transpeptidase (GGT) in vivo or in vitro and for inhibiting and killing neoplastic cancer cells, for example for the treatment, inhibition or prevention of tumors or malignant growths or other neoplasias in mammals, or sensitizing tumor to other therapies including radiation therapy. The GGT inhibitor compounds comprise a class of benzylthiadiazol benzenesulfoniamides. The compounds may also be used to treat a reversible airways obstruction in a mammal or a disease associated with reverse airways obstruction such as asthma, chronic obstructive pulmonary disease (COPD), allergic reaction, respiratory tract infection or upper respiratory tract disease. Other diseases or conditions which may be treated include, for example, degenerative diseases, renal diseases, liver diseases, cardiovascular disease and inner ear conditions or diseases.