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 the present study , the effects of three alkylxanthines with different potencies as adenosine antagonists 8 - phenyltheophylline , theophylline and enprofylline , were examined in rats developing acute renal failure after 4 daily injections of gentamicin ( 200 mg kg - 1 ) .
### Output Text:
In the present study , the effects of three <span class="chemical">alkylxanthines</span> with different potencies as <span class="chemical">adenosine</span> antagonists <span class="chemical">8 - phenyltheophylline</span> , <span class="chemical">theophylline</span> and <span class="chemical">enprofylline</span> , were examined in rats developing acute renal failure after 4 daily injections of <span class="chemical">gentamicin</span> ( 200 mg kg - 1 ) . 