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: OBJECTIVES : Posterior leukoencephalopathy due to calcineurin - inhibitor - related neurotoxicity is a rare but severe complication that results from treatment with immunosuppressive agents ( primarily those administered after a liver or kidney transplant ) .
### Output Text:
OBJECTIVES : Posterior leukoencephalopathy due to calcineurin - inhibitor - related neurotoxicity is a rare but severe complication that results from treatment with immunosuppressive agents ( primarily those administered after a liver or kidney transplant ) . 