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: Many of these patients are taking pentoxifylline ( Trental ) , a methylxanthine derivative which may improve intermittent claudication .
### Output Text:
Many of these patients are taking <span class="chemical">pentoxifylline</span> ( <span class="chemical">Trental</span> ) , a <span class="chemical">methylxanthine</span> derivative which may improve intermittent claudication . 