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: We report on the development of 8 serious cases of ischemic colitis in patients with migraine treated with sumatriptan .
### Output Text:
We report on the development of 8 serious cases of ischemic colitis in patients with migraine treated with <span class="chemical">sumatriptan</span> . 