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: Extensive experimental and limited clinical data have shown that lometrexol has activity against tumours which are refractory to other drugs , notably methotrexate .
### Output Text:
Extensive experimental and limited clinical data have shown that <span class="chemical">lometrexol</span> has activity against tumours which are refractory to other drugs , notably <span class="chemical">methotrexate</span> . 