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: The incidence of neurotoxicity may be reduced by employing lower doses of methotrexate in the presence of central nervous system leukemia , in older children and adults , and in the presence of epidural leakage .
### Output Text:
The incidence of neurotoxicity may be reduced by employing lower doses of <span class="chemical">methotrexate</span> in the presence of central nervous system leukemia , in older children and adults , and in the presence of epidural leakage . 