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: Paclitaxel 3 - hour infusion given alone and combined with carboplatin : preliminary results of dose - escalation trials .
### Output Text:
<span class="chemical">Paclitaxel</span> 3 - hour infusion given alone and combined with <span class="chemical">carboplatin</span> : preliminary results of dose - escalation trials . 