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 lack of any consistent protective effect noted with the alkylxanthines tested in the present study indicates that adenosine plays little , if any , pathophysiological role in gentamicin - induced ARF .
### Output Text:
The lack of any consistent protective effect noted with the <span class="chemical">alkylxanthines</span> tested in the present study indicates that <span class="chemical">adenosine</span> plays little , if any , pathophysiological role in <span class="chemical">gentamicin</span> - induced ARF . 