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: Definite , although limited , antineoplastic activity is observed in patients with well - defined platinum - and paclitaxel - refractory ovarian cancer .
### Output Text:
Definite , although limited , antineoplastic activity is observed in patients with well - defined <span class="chemical">platinum</span> - and <span class="chemical">paclitaxel</span> - refractory ovarian cancer . 