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: However , the initial clinical development of lometrexol was curtailed because of severe and cumulative antiproliferative toxicities .
### Output Text:
However , the initial clinical development of <span class="chemical">lometrexol</span> was curtailed because of severe and cumulative antiproliferative toxicities . 