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: Therefore , we studied the hyperemic response to dipyridamole in seven open - chest anesthetized dogs after pretreatment with either pentoxifylline ( 0 , 7 . 5 , or 15 mg / kg i . v . ) or theophylline ( 3 mg / kg i . v . ) .
### Output Text:
Therefore , we studied the hyperemic response to <span class="chemical">dipyridamole</span> in seven open - chest anesthetized dogs after pretreatment with either <span class="chemical">pentoxifylline</span> ( 0 , 7 . 5 , or 15 mg / kg i . v . ) or <span class="chemical">theophylline</span> ( 3 mg / kg i . v . ) . 