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 of intra - arterial PGE2 could not be blocked by intra - arterial RP 67 , 580 or SR 48 , 968 , which opens the possibility that the micturition reflex elicited by intra - arterial PGE2 was mediated by pathways other than the reflex initiated when the PG was given intravesically .
### Output Text:
The effect of intra - arterial <span class="chemical">PGE2</span> could not be blocked by intra - arterial <span class="chemical">RP 67 , 580</span> or <span class="chemical">SR 48 , 968</span> , which opens the possibility that the micturition reflex elicited by intra - arterial <span class="chemical">PGE2</span> was mediated by pathways other than the reflex initiated when the <span class="chemical">PG</span> was given intravesically . 