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: Valsartan , a new angiotensin II antagonist for the treatment of essential hypertension : a comparative study of the efficacy and safety against amlodipine .
### Output Text:
<span class="chemical">Valsartan</span> , a new <span class="chemical">angiotensin II</span> antagonist for the treatment of essential hypertension : a comparative study of the efficacy and safety against <span class="chemical">amlodipine</span> . 