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: Adenosine antagonists have been previously shown to be of benefit in some ischaemic and nephrotoxic models of acute renal failure ( ARF ) .
### Output Text:
<span class="chemical">Adenosine</span> antagonists have been previously shown to be of benefit in some ischaemic and nephrotoxic models of acute renal failure ( ARF ) . 