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 selective blockade of A2 adenosine receptor by DMPX ( 3 , 7 - dimethyl - 1 - propargylxanthine ) significantly enhanced cocaine - induced locomotor activity of animals .
### Output Text:
The selective blockade of A2 <span class="chemical">adenosine</span> receptor by <span class="chemical">DMPX</span> ( <span class="chemical">3 , 7 - dimethyl - 1 - propargylxanthine</span> ) significantly enhanced <span class="chemical">cocaine</span> - induced locomotor activity of animals . 