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 effect was attenuated by both the NK1 receptor selective antagonist RP 67 , 580 and the NK2 receptor selective antagonist SR 48 , 968 , given intra - arterially , suggesting that it was mediated by stimulation of both NK1 and NK2 receptors .
### Output Text:
The effect was attenuated by both the NK1 receptor selective antagonist <span class="chemical">RP 67 , 580</span> and the NK2 receptor selective antagonist <span class="chemical">SR 48 , 968</span> , given intra - arterially , suggesting that it was mediated by stimulation of both NK1 and NK2 receptors . 