What is the optimal prophylaxis regimen to use prior to a selinexor based regimen in the treatment of multiple myeloma for nausea and electrolyte abnormalities? 

The official Karyopharm guidelines for nausea are as follows: NK-1R antagonist OR a combination of 5-HT3 antagonist AND olanzapine. So the details are:

a) All patients should receive ondansetron 8 mg or equivalent, unless contraindicated, orally 1 hour before each dose of selinexor and q 8 hours for ...

A once-weekly (as opposed to twice-weekly) selinexor dosing will surely help to cut down on side effects, especially nausea and vomiting. It's also important to educate patients thoroughly that even though it is a pill, it is still once a week and not daily (unlike lenalidomide for example), and sho...