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: The number of patients with suppression of HIV p24 antigenemia in the combination and zidovudine groups was six ( 40 % ) and two ( 11 % ) at week 4 ( p = 0 . 10 ) and five ( 45 % ) and two ( 14 % ) at week 24 ( p = 0 . 08 ) , respectively .
### Output Text:
The number of patients with suppression of HIV p24 antigenemia in the combination and <span class="chemical">zidovudine</span> groups was six ( 40 % ) and two ( 11 % ) at week 4 ( p = 0 . 10 ) and five ( 45 % ) and two ( 14 % ) at week 24 ( p = 0 . 08 ) , respectively . 