At this year's RAADfest event, the interviewer noted here was taking an informal survey of optimistic versus pessimistic attitudes towards progress in the decades ahead. Apparently I was on the pessimistic end of the spectrum. Once past the present highly active development of senolytic therapies to remove senescent cells from old tissues, I think it quite plausible that we'll see a gap of a decade before the next class of SENS-like rejuvenation therapy arrives at the point of availability via medical tourism. The likely candidates include clearance of cross-links and restoration of the immune system via thymic regrowth.
Surprise progress in advance of the end of the 2020s seems implausible, with the exception of the discovery that an existing small molecule drug or otherwise widely available low cost compound breaks down significant amounts of some form of molecular waste, such as oxidized cholesterol or glucosepane cross-links. That is possible to engineer, given the resources, but so far as I know next to no-one is screening the compound libraries with this in mind. It is an expensive task with uncertain chances of success. This present state of the market, that there is a gulf of further required development ahead, is perhaps a little obscured by the excitement over senolytics. There is, however, a continued need for philanthropic support of lines of research that remain poorly funded. If senolytics are to be closely followed by the rest of the rejuvenation toolkit, then we still have a great deal of work to do.
What are the most promising near-term therapies that may actually turn back the clock on biological aging?
Senolytic treatment, obviously, is the one that is here already and is presently available. It is fortunate that some of the first drugs identified to have this effect are, to a significant degree, already widely used and cheap. The animal results are far better in terms of robustness and reproducibility than any of the calorie restriction mimetic and other stress response tinkering work. The first human data from formal trials will arrive late this year or in early 2019. These first-generation approaches are killing only about half of the senescent cells at best (and far fewer than that in some tissues) but are nonetheless very effective in comparison to any other approach to age-related inflammatory disease.
The next approach to arrive that will likely have a similar character and size of effect is breaking of glucosepane cross-links, but since that involves a completely new enzyme-based therapy, we're unlikely to see it in people any sooner than a decade from now. If there is interest in that field, someone might uncover a useful small molecule prior to then, but it seems unlikely.
Other than that, over that same timeframe: (a) advances in stem cell medicine, moving beyond the simple transplantation therapies that do little other than suppress inflammation towards ways to actually replace damaged populations and have them get to work; (b) removal of amyloids through means other than the immunotherapies that are the present staple of that field; (c) forms of immune system restoration, such as via thymic regrowth, replacement or enhancement of hematopoietic stem cells, and clearance of problem immune cells.
I'm not convinced that there is an enormous benefit to be realized from approaches to enhance mitochondrial function, such as NAD+ precursors and mitochondrially targeted antioxidants, that get a lot of hype and attention. They may have a small positive effect on metabolism in later life, which would make them worth taking when cheap and safe. They are not in any way reversing aging - they are forcing a damaged machine to work harder without addressing any of the causes of failure. One can paint the same picture when discussing ways to enhance stem cell function without addressing the underlying damage, such as telomerase therapies and the use of signaling molecules. It may meet the cost-benefit equation, but it also may not, since these are much more expensive propositions.
Why is breaking extracellular crosslinks so important?
This is important because cross-links cause stiffening of tissues. The stiffening of blood vessels is the cause of hypertension, and hypertension is (like inflammation) a major way in which low-level biochemical damage is translated into many different forms of structural damage: pressure damage to delicate tissues; rupture of capillaries in the brain; remodeling and weakening of the heart; increased risk of atherosclerotic lesions causing stroke or heart attack. High blood pressure is very damaging. It is so harmful that ways to reduce blood pressure that work by overriding signaling systems - which do absolutely nothing to eliminate the root cause, the biochemical damage of aging - can still produce large reductions in mortality risk.
All of that can be greatly reduced by cross-link breaking, and there is only one major class of cross-links in humans that needs targeting to obtain that benefit: those involving glucosepane. Thus, like senolytics, once there is some motion towards achieving this end, we should see a very rapid expansion of the industry and delivery of benefits to patients. Glucosepane is hard to work with, so very few groups have done anything meaningful - the first big advance that the SENS Research Foundation achieved in this field was to fund the creation of the tools needed to move forward at all in this part of the field. Even now, there is really only one group working earnestly on it that I know of, David Spiegel's team at Yale, with a couple of others doing some investigative work around the edges of the challenge. The Spiegel approach is to mine the bacterial world for enzymes that degrade glucosepane and then refine the successes into therapeutic drugs. His team is a fair way along, and work is progressing in a funded startup company at this point.