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: OBJECTIVE : To report two cases of a possible adverse interaction between clonidine and verapamil resulting in atrioventricular ( AV ) block in both patients and severe hypotension in one patient .
### Output Text:
OBJECTIVE : To report two cases of a possible adverse interaction between <span class="chemical">clonidine</span> and <span class="chemical">verapamil</span> resulting in atrioventricular ( AV ) block in both patients and severe hypotension in one patient . 