marie,
ISO 10993-1, including sub-part ISO 10993-17 for performing toxicological risk assessments, applies to any medical device that has body contact, including combination products with an ancillary medicinal substance.
Thereby ISO 10993-1 pertains to the _final_ medical device, made of all constituents and having undergone all manufacturing processes. In this respect, the medicinal substance is regarded just as another "component" that makes up the final device. So yes, your medicinal substance could be a leachable in case it migrates to the patient during its intended use.
In the framework of ISO 10993-1, "testing shall be performed on the final medical device, or representative samples from the final device or materials processed in the same manner as the final medical device (including sterilization, if needed)." (quote from section 6.3.1 (a) ). So you would have to provide a rationale for why the drug part of your combination product is representative for the final device/drug combination product in its biological safety profile.
Specific additional tests on your ancillary substance depends on the amount and quality of already existing related toxicology data with respect to your intended use.
Hope this helps,