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: In a double - blind 6 - week trial , 458 patients with acute schizophrenia were randomly assigned to fixed - dose treatment with asenapine at 5 mg twice daily ( BID ) , asenapine at 10 mg BID , placebo , or haloperidol at 4 mg BID ( to verify assay sensitivity ) .
### Output Text:
In a double - blind 6 - week trial , 458 patients with acute schizophrenia were randomly assigned to fixed - dose treatment with <span class="chemical">asenapine</span> at 5 mg twice daily ( BID ) , <span class="chemical">asenapine</span> at 10 mg BID , placebo , or <span class="chemical">haloperidol</span> at 4 mg BID ( to verify assay sensitivity ) . 