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: One hundred and fifty - one patients intolerant to zidovudine ( AZT ) received didanosine ( ddI ) to a maximum dose of 12 . 5 mg / kg / day .
### Output Text:
One hundred and fifty - one patients intolerant to <span class="chemical">zidovudine</span> ( <span class="chemical">AZT</span> ) received <span class="chemical">didanosine</span> ( <span class="chemical">ddI</span> ) to a maximum dose of 12 . 5 mg / kg / day . 