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: In the present study , the effect of chronic pre - treatment with metformin on cardiac dysfunction and toll - like receptor 4 ( TLR4 ) activities following myocardial infarction and their relation with AMPK were assessed .
### Output Text:
In the present study , the effect of chronic pre - treatment with <span class="chemical">metformin</span> on cardiac dysfunction and toll - like receptor 4 ( TLR4 ) activities following myocardial infarction and their relation with AMPK were assessed . 