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: A 6 - week open study of the introduction of isradipine treatment was conducted in general practice in Hong Kong .
### Output Text:
A 6 - week open study of the introduction of <span class="chemical">isradipine</span> treatment was conducted in general practice in Hong Kong . 