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: Design and analysis of the HYPREN - trial : safety of enalapril and prazosin in the initial treatment phase of patients with congestive heart failure .
### Output Text:
Design and analysis of the HYPREN - trial : safety of <span class="chemical">enalapril</span> and <span class="chemical">prazosin</span> in the initial treatment phase of patients with congestive heart failure . 