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: In vivo characterization of a dual adenosine A2A / A1 receptor antagonist in animal models of Parkinson ' s disease .
### Output Text:
In vivo characterization of a dual <span class="chemical">adenosine A2A / A1 receptor antagonist</span> in animal models of Parkinson ' s disease . 