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 is thus an active single agent in this patient population , with a 3 - hour infusion proving comparably effective to a 24 - hour infusion and superior in terms of the incidence of hematologic and nonhematologic toxicity .
### Output Text:
<span class="chemical">Paclitaxel</span> is thus an active single agent in this patient population , with a 3 - hour infusion proving comparably effective to a 24 - hour infusion and superior in terms of the incidence of hematologic and nonhematologic toxicity . 