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: Because caffeine is an adenosine receptor antagonist , these results suggest that some panic disorder patients may have abnormalities in neuronal systems involving adenosine .
### Output Text:
Because <span class="chemical">caffeine</span> is an <span class="chemical">adenosine</span> receptor antagonist , these results suggest that some panic disorder patients may have abnormalities in neuronal systems involving <span class="chemical">adenosine</span> . 