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: Primary pulmonary hypertension is a rare , progressive and incurable disease , which has been associated with the intake of appetite suppressant drugs .
### Output Text:
Primary pulmonary hypertension is a rare , progressive and incurable disease , which has been associated with the intake of <span class="chemical">appetite suppressant</span> drugs . 