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: Chronic pre - treatment with metformin reduces post - myocardial infarction cardiac dysfunction and suppresses inflammatory responses , possibly through inhibition of TLR4 activities .
### Output Text:
Chronic pre - treatment with <span class="chemical">metformin</span> reduces post - myocardial infarction cardiac dysfunction and suppresses inflammatory responses , possibly through inhibition of TLR4 activities . 