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: Affinity profiles of ( + / - ) - PG - 9 for muscarinic receptor subtypes , determined by functional studies ( rabbit vas deferens for M1 , guinea pig atrium for M2 , guinea pig ileum for M3 and immature guinea pig uterus for putative M4 ) , have shown an M4 / M1 selectivity ratio of 10 . 2 that might be responsible for the antinociception and the anti - amnesic effect induced by ( + / - ) - PG - 9 through an increase in acetylcholine extracellular levels .
### Output Text:
Affinity profiles of ( + / - ) - <span class="chemical">PG - 9</span> for muscarinic receptor subtypes , determined by functional studies ( rabbit vas deferens for M1 , guinea pig atrium for M2 , guinea pig ileum for M3 and immature guinea pig uterus for putative M4 ) , have shown an M4 / M1 selectivity ratio of 10 . 2 that might be responsible for the antinociception and the anti - amnesic effect induced by ( + / - ) - <span class="chemical">PG - 9</span> through an increase in <span class="chemical">acetylcholine</span> extracellular levels . 