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: Two patients developed acute tubular necrosis , characterized clinically by acute oliguric renal failure , while they were receiving a combination of cephalothin sodium and gentamicin sulfate therapy .
### Output Text:
Two patients developed acute tubular necrosis , characterized clinically by acute oliguric renal failure , while they were receiving a combination of <span class="chemical">cephalothin sodium</span> and <span class="chemical">gentamicin sulfate</span> therapy . 