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: To our knowledge , there is no previous report regarding whether domperidone , a peripheral dopamine D2 receptor antagonist , can also induce or aggravate symptoms of RLS .
### Output Text:
To our knowledge , there is no previous report regarding whether <span class="chemical">domperidone</span> , a peripheral <span class="chemical">dopamine</span> D2 receptor antagonist , can also induce or aggravate symptoms of RLS . 