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 estimated the direct cost of treating clinically significant RIHA to be 170 per patient receiving combination therapy per 48 - week treatment course ( range 68 - 692 ) .
### Output Text:
We estimated the direct cost of treating clinically significant RIHA to be 170 per patient receiving combination therapy per 48 - week treatment course ( range 68 - 692 ) . 