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: Phosphorylated AMPKa ( p - AMPK ) in the myocardium was significantly elevated by 25mg / kg of metformin , slightly by 50mg / kg , but not by 100mg / kg .
### Output Text:
Phosphorylated AMPKa ( p - AMPK ) in the myocardium was significantly elevated by 25mg / kg of <span class="chemical">metformin</span> , slightly by 50mg / kg , but not by 100mg / kg . 