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: Nephrotoxicity of combined cephalothin - gentamicin regimen .
### Output Text:
Nephrotoxicity of combined <span class="chemical">cephalothin</span> - <span class="chemical">gentamicin</span> regimen . 