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: Renal failure developed after a prolonged course of vancomycin therapy in 2 patients who were receiving tenofovir disoproxil fumarate as part of an antiretroviral regimen .
### Output Text:
Renal failure developed after a prolonged course of <span class="chemical">vancomycin</span> therapy in 2 patients who were receiving <span class="chemical">tenofovir disoproxil fumarate</span> as part of an antiretroviral regimen . 