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: The dose of D - penicillamine associated with a favorable response was as low as 2 to 5 mg / kg per day given over a period ranging from 15 to 53 months .
### Output Text:
The dose of <span class="chemical">D - penicillamine</span> associated with a favorable response was as low as 2 to 5 mg / kg per day given over a period ranging from 15 to 53 months . 