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: Treatment was comprised of VNB , 25 mg / m ( 2 ) , plus GEM , 1000 mg / m ( 2 ) , both on Days 1 , 8 , and 15 every 28 days .
### Output Text:
Treatment was comprised of <span class="chemical">VNB</span> , 25 mg / m ( 2 ) , plus <span class="chemical">GEM</span> , 1000 mg / m ( 2 ) , both on Days 1 , 8 , and 15 every 28 days . 