This is highly unlikely to be a significant issue so long as therapy does not need to be continuous. Even if Imetelstat needs to be administered in repeated courses to maintain clinical response, so long as there is a gap between them, it's highly unlikely that this mechanism will do long-term damage to the immune system (and I'd add it's far from guaranteed even if therapy *is* continuous). From first principles, any form of chemotherapy should be written off as far too horrendously damaging according to this kind of logic. We still use it, and deal with the side effects, since they generally recede post-treatment, and because it works.
Honestly, the main risk is still that the disease-modifying activity simply fails to replicate in large trials. That's what we need to be worried about.