Trying to create a custom calculated Attribute

Trying to create a custom calculated Attribute with the below conditions:

Attribute : Item Sub-Type 

Condition 1 : When Item Class is set to Type A, then the Item Sub Type dropdown should include options "Option 1 and Option 2"

Condition 2 : When Item Class is set to Type B, then the Item Sub Type dropdown should include options "Option 3, Option 4, Option 5"

 

We are looking for a solution which dynamically updates the List values associated with each Item Class, and letting users select only the ones appropriate for each Item Class