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 drugs commonly used are cyclophosphamide and chlorambucil ( alkylating agents ) , azathioprine ( purine analogue ) , and methotrexate ( folic acid analogue ) .
### Output Text:
The drugs commonly used are <span class="chemical">cyclophosphamide</span> and <span class="chemical">chlorambucil</span> ( <span class="chemical">alkylating agents</span> ) , <span class="chemical">azathioprine</span> ( <span class="chemical">purine</span> analogue ) , and <span class="chemical">methotrexate</span> ( <span class="chemical">folic acid</span> analogue ) . 