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: This is important , since different controlled release formulations of bupropion release the active drug at different rates .
### Output Text:
This is important , since different controlled release formulations of <span class="chemical">bupropion</span> release the active drug at different rates . 