In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: BACKGROUND : Treatment with angiotensin - converting enzyme ( ACE ) inhibitors reduces mortality and morbidity in patients with chronic heart failure ( CHF ) , but most affected patients are not receiving these agents or are being treated with doses lower than those found to be efficacious in trials , primarily because of concerns about the safety and tolerability of these agents , especially at the recommended doses .
### Output Text:
BACKGROUND : Treatment with <span class="chemical">angiotensin - converting enzyme ( ACE ) inhibitors</span> reduces mortality and morbidity in patients with chronic heart failure ( CHF ) , but most affected patients are not receiving these agents or are being treated with doses lower than those found to be efficacious in trials , primarily because of concerns about the safety and tolerability of these agents , especially at the recommended doses . 