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: Male Wistar rats were randomly assigned to one of 5 groups ( n = 6 ) : normal control and groups were injected isoproterenol after chronic pre - treatment with 0 , 25 , 50 , or 100mg / kg of metformin twice daily for 14 days .
### Output Text:
Male Wistar rats were randomly assigned to one of 5 groups ( n = 6 ) : normal control and groups were injected <span class="chemical">isoproterenol</span> after chronic pre - treatment with 0 , 25 , 50 , or 100mg / kg of <span class="chemical">metformin</span> twice daily for 14 days . 