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: For glyburide , a second - generation sulfonylurea , only two brief reports of hepatotoxicity exist .
### Output Text:
For <span class="chemical">glyburide</span> , a second - generation <span class="chemical">sulfonylurea</span> , only two brief reports of hepatotoxicity exist . 