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: Together , the data suggest that sigma2 receptor antagonists have the potential to attenuate some of the behavioral effects of cocaine , and further development of more selective , high affinity ligands are warranted .
### Output Text:
Together , the data suggest that sigma2 receptor antagonists have the potential to attenuate some of the behavioral effects of <span class="chemical">cocaine</span> , and further development of more selective , high affinity ligands are warranted . 