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: Patients with 200 to 500 CD4 cells / mm3 who tolerated < or = 12 weeks of prior zidovudine therapy received SC - 48334 ( 1000 mg every 8 h ) and zidovudine ( 100 mg every 8 h ) or zidovudine and placebo .
### Output Text:
Patients with 200 to 500 CD4 cells / mm3 who tolerated < or = 12 weeks of prior <span class="chemical">zidovudine</span> therapy received <span class="chemical">SC - 48334</span> ( 1000 mg every 8 h ) and <span class="chemical">zidovudine</span> ( 100 mg every 8 h ) or <span class="chemical">zidovudine</span> and placebo . 