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 treatment with metformin suppresses toll - like receptor 4 signaling and attenuates left ventricular dysfunction following myocardial infarction .
### Output Text:
Chronic treatment with <span class="chemical">metformin</span> suppresses toll - like receptor 4 signaling and attenuates left ventricular dysfunction following myocardial infarction . 