I think it's all but certain that FX-322 actually functions as it is intended to in terms of restoring hair cells, but the critical issue to be addressed is delivery of the drug throughout the cochlea.
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I am going to assume the above graphic is a satisfactory representation of the frequency ranges within the cochlea since I got it from Britannica.
In this case, if your issue is say within the 4 kHz range, you would need to get enough injections for the drug to work its way down there. It has been stated before in this thread that the drug doesn't act upon intact cells, so further injections could push unused portions of FX-322 further down to address areas that need regeneration.
I don't think any of us can say for sure yet how "difficult" it will be to manage the lower frequencies, but that is a key issue that Frequency Therapeutics is not only aware of but actively trying to address in research.
This is also assuming that if your tone is a certain frequency then that implies you have damage to that specific region. I'm not sure if this is conclusive but rather assumed.