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 compare the antihypertensive efficacy of a new angiotensin II antagonist , valsartan , with a reference therapy , amlodipine .
### Output Text:
OBJECTIVE : To compare the antihypertensive efficacy of a new <span class="chemical">angiotensin II</span> antagonist , <span class="chemical">valsartan</span> , with a reference therapy , <span class="chemical">amlodipine</span> . 