Interferon appears only drug that might slow progression but some people can have severe side effects. Ropeginterferon, which hopefully will be available next year, is better tolerated and (I thought) more efficacious.
They say the focus is on finding better drugs for MF rather than slowing progression but don’t mention RG7388 which does appear to target a serious mutation (MDM2) that could speed up progression (by stopping p53 gene from working properly). I guess the respondent is referring to finding the as yet unknown prime driver. Hence at the moment drugs (such as RG7388) are targeting secondary mutations that speed progression. Still very useful but not yet universal one shot ‘cure’
Re Celebrating new arrival 
Have I progressed to PV?
Is this a sign of progression from ET to MF?
ET progression to MF - anxiety issues
