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: The in vivo characterization of a dual adenosine A ( 2A ) / A ( 1 ) receptor antagonist in several animal models of Parkinson ' s disease is described .
### Output Text:
The in vivo characterization of a dual <span class="chemical">adenosine A ( 2A ) / A ( 1 ) receptor antagonist</span> in several animal models of Parkinson ' s disease is described . 