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: Concerns about long - term methotrexate ( MTX ) neurotoxicity in the 1990s led to modifications in intrathecal ( IT ) therapy , leucovorin rescue , and frequency of systemic MTX administration in children with acute lymphoblastic leukemia .
### Output Text:
Concerns about long - term <span class="chemical">methotrexate</span> ( <span class="chemical">MTX</span> ) neurotoxicity in the 1990s led to modifications in intrathecal ( IT ) therapy , leucovorin rescue , and frequency of systemic <span class="chemical">MTX</span> administration in children with acute lymphoblastic leukemia . 