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: We wished to develop a more tolerable liposomal doxorubicin treatment regimen and document its level of activity in a well - defined patient population with platinum / paclitaxel - refractory disease .
### Output Text:
We wished to develop a more tolerable liposomal <span class="chemical">doxorubicin</span> treatment regimen and document its level of activity in a well - defined patient population with <span class="chemical">platinum</span> / <span class="chemical">paclitaxel</span> - refractory disease . 