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: This observation is suggested to be an in vivo demonstration of the conclusion derived from in vitro experiments that MK - 801 binding requires agonist - induced opening of the channel sites of the NMDA receptor .
### Output Text:
This observation is suggested to be an in vivo demonstration of the conclusion derived from in vitro experiments that <span class="chemical">MK - 801</span> binding requires agonist - induced opening of the channel sites of the <span class="chemical">NMDA</span> receptor . 