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: Several cases of cardiac adverse reactions related to vinorelbine ( VNR ) have been reported in the literature .
### Output Text:
Several cases of cardiac adverse reactions related to <span class="chemical">vinorelbine</span> ( <span class="chemical">VNR</span> ) have been reported in the literature . 