Safe, yes, but I think "promising" is more aimed at the investors than a reflection of reality. Both Alkahest and Ambrosia's results are essentially ambiguous, which reflects the equally ambiguous animal data for plasma transfusion.<p>This isn't going to stop other people from trying analogous things.<p>The Conboys are trying apheresis to strip out harmful factors from old blood ( <a href="https://www.leafscience.org/conboy-interview/" rel="nofollow">https://www.leafscience.org/conboy-interview/</a> ), which seems to me unlikely to move the needle either, and other groups are trying to identify and deliver signals from young blood.<p>What all of these things have in common is that they are failing to address root causes. Why does signaling change in old blood and tissues? Because the cells that generate that signaling are either damaged or reacting to damage. Trying to force the signaling to be more youthful has very definite limits - it might compensate a little for one consequence of the damage, but that damage is still there, still producing all of the other problems that it causes. The "change the signaling" approach requires the research community to map every single one of potentially hundreds or thousands of individual changes, then understand which are relevant, and then run individual projects for each one. It just won't happen - look at the cancer research community for a guide as to how slowly that sort of scenario progresses.<p>The plausible outer limits of signaling adjustment are probably indicated by the effects of first generation stem cell therapies, which largely work through the signals introduced by the transplanted cells. Assume that something somewhat better than that can be achieved: meaning temporarily improved regeneration, less increased cancer risk than was originally suspected, but not much of an impact on aging and age-related disease, when considered in the context of the bigger picture of what is possible.<p>(What is possible if people would just stop tinkering with downstream consequences and start repairing the root cause damage. At least the flurry of interest in senolytics is a start in that direction, and should hopefully prove the point by being so very much better and more cost-effective than just about everything else on the market when it comes to treating age-related conditions).