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: Increased extracellular concentrations of uridine ( Urd ) have been reported to reduce , in vitro , azidothymidine ( AZT ) - induced inhibition of human granulocyte - macrophage progenitor cells without impairment of its antihuman immunodeficiency virus ( HIV ) activity .
### Output Text:
Increased extracellular concentrations of <span class="chemical">uridine</span> ( <span class="chemical">Urd</span> ) have been reported to reduce , in vitro , <span class="chemical">azidothymidine</span> ( <span class="chemical">AZT</span> ) - induced inhibition of human granulocyte - macrophage progenitor cells without impairment of its antihuman immunodeficiency virus ( HIV ) activity . 