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: Subtenon carboplatin is not free of toxicity , and its use is best restricted to specific indications .
### Output Text:
Subtenon <span class="chemical">carboplatin</span> is not free of toxicity , and its use is best restricted to specific indications . 