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: At 550 mg / m2 / d , three patients experienced nonurologic toxicity ; confusion ( 1 ) , nausea ( 1 ) , and Grade 2 leukopenia ( 1 ) .
### Output Text:
At 550 mg / m2 / d , three patients experienced nonurologic toxicity ; confusion ( 1 ) , nausea ( 1 ) , and Grade 2 leukopenia ( 1 ) . 