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: She received a second cycle of nelarabine without additional IT prophylaxis one month later .
### Output Text:
She received a second cycle of <span class="chemical">nelarabine</span> without additional IT prophylaxis one month later . 