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: Treatment of previously treated metastatic breast cancer by mitoxantrone and 48 - hour continuous infusion of high - dose 5 - FU and leucovorin ( MFL ) : low palliative benefit and high treatment - related toxicity .
### Output Text:
Treatment of previously treated metastatic breast cancer by <span class="chemical">mitoxantrone</span> and 48 - hour continuous infusion of high - dose <span class="chemical">5 - FU</span> and <span class="chemical">leucovorin</span> ( <span class="chemical">MFL</span> ) : low palliative benefit and high treatment - related toxicity . 