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: Use of antithymocyte globulin may be the optimal treatment of D - penicillamine - induced aplastic anemia .
### Output Text:
Use of <span class="chemical">antithymocyte globulin</span> may be the optimal treatment of <span class="chemical">D - penicillamine</span> - induced aplastic anemia . 