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: BACKGROUND : Preclinical studies of intrathecal adenosine suggest it may be effective in the treatment of acute and chronic pain in humans , and preliminary studies in volunteers and patients with a Swedish formulation of adenosine suggests it may be effective in hypersensitivity states but not with acute noxious stimulation .
### Output Text:
BACKGROUND : Preclinical studies of intrathecal <span class="chemical">adenosine</span> suggest it may be effective in the treatment of acute and chronic pain in humans , and preliminary studies in volunteers and patients with a Swedish formulation of <span class="chemical">adenosine</span> suggests it may be effective in hypersensitivity states but not with acute noxious stimulation . 