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: Nelarabine neurotoxicity with concurrent intrathecal chemotherapy : Case report and review of literature .
### Output Text:
<span class="chemical">Nelarabine</span> neurotoxicity with concurrent intrathecal chemotherapy : Case report and review of literature . 