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: PURPOSE : To evaluate the response rate and toxic effects of vinorelbine ( VNB ) administered as a single agent in metastatic squamous cell esophageal carcinoma .
### Output Text:
PURPOSE : To evaluate the response rate and toxic effects of <span class="chemical">vinorelbine</span> ( <span class="chemical">VNB</span> ) administered as a single agent in metastatic squamous cell esophageal carcinoma . 