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: Since 1975 mitomycin C ( MMC ) has been suggested to be cardiotoxic , especially when combined with or given following doxorubicin .
### Output Text:
Since 1975 <span class="chemical">mitomycin C</span> ( <span class="chemical">MMC</span> ) has been suggested to be cardiotoxic , especially when combined with or given following <span class="chemical">doxorubicin</span> . 