Why do I vocally support rejuvenation research based on the Strategies for Engineered Negligible Senescence (SENS) over other forms of longevity science? Why do I hold the view that SENS and SENS-like research should be prioritized and massively funded? The short answer to this question is that SENS-derived medical biotechnology has a much greater expected utility - it will most likely produce far better outcomes, and at a lower cost - than other presently ongoing lines of research into creating greater human longevity.
What is SENS?
But firstly, what is SENS? It is more an umbrella collection of categories than a specific program, though it is the case that narrowly focused SENS research initiatives run under the auspices of the SENS Research Foundation. On the science side of the house, SENS is a synthesis of existing knowledge from the broad mainstream position regarding aging and the diseases of aging: that aging is caused by a stochastic accumulation of damage at the level of cells and protein machinery in and around these cells. SENS is a proposal, based on recent decades of research, as to which of the identified forms of damage and change in old tissues are fundamental - i.e. which are direct byproducts of metabolic operation rather than cascading effects of other fundamental damage. On the development side of the house, SENS pulls together work from many subfields of medical research to show that there are clear and well-defined ways to produce therapies that can repair, reverse, or make irrelevant these fundamental forms of biological damage associated with aging.
(You can read about the various forms of low-level damage that cause aging at the SENS Research Foundation website and elsewhere. This list includes: mitochondrial DNA mutations; buildup of resilient waste products inside and around cells; growing numbers of senescent and other malfunctioning cells; loss of stem cells; and a few others).
Present arguments within the mainstream of aging research are largely over the relative importance of damage type A versus damage type B, and how exactly the extremely complex interaction of damage with metabolism progresses - but not what that damage actually is. A large fraction of modern funding for aging research goes towards building a greater understanding this progression; certainly more than goes towards actually doing anything about it. Here is the thing, however: while understanding the dynamics of damage in aging is very much a work in progress, the damage itself is well known. The research community can accurately enumerate the differences between old tissue and young tissue, or an old cell and a young cell - and it has been a good number of years since anything new was added to that list.
If you can repair the cellular damage that causes aging, it doesn't matter how it happens or how it affects the organism when it's there. This is the important realization for SENS - that much of the ongoing work of the aging research community is largely irrelevant if the goal is to get to human rejuvenation as rapidly as possible. Enough is already known of the likely causes of aging to have a reasonable expectation of being able to produce laboratory demonstrations of rejuvenation in animal models within a decade or two, given large-scale funding.
Comparing Expected Values
Expected value drives human endeavor. What path ahead do we expect to produce the greatest gain? In longevity science the investment is concretely measured in money and time, and we might think of the expected value in terms of years of healthy life added by the resulting therapies. The cost of these therapies really isn't much of a factor - all major medical procedures and other therapies tend to converge to similar costs over time, based on their category: consider a surgery versus an infusion versus a course of pills, for example, where it's fairly obvious that the pricing derives from how much skilled labor is involved and how much care the patient requires as a direct result of the process.
On the input side, there are estimates for the cost in time and money to implement SENS therapies for laboratory mice. For the sake of keeping things simple, I'll note that these oscillate around the figures of a billion dollars and ten years for the crash program of fully-funded research. A billion dollars is about the yearly budget of the NIA these days, give or take, which might be a third of all research funding directed towards aging - by some estimates, anyway, though this is a very hard figure to verify in any way. It's by no means certain the that the general one third/two thirds split between government and private research funding extends to aging research.
On the output side, early SENS implementations would be expected to take an old mouse and double its remaining life expectancy - e.g. produce actual rejuvenation, actual repair and reversal of the low-level damage that causes aging, with repeated applications at intervals producing diminishing but still measurable further gains. This is the thing about a rejuvenation therapy that works; you can keep on applying it to sweep up newly accruing damage.
So what other longevity science do we have to compare against? The only large running programs are those that have grown out of the search for calorie restriction mimetic drugs. So there is the past decade or so of research into surtuins, and there is growing interest in mTOR and rapamycin analogs that looks to be more of the same, but slightly better (though that is a low bar to clear).
In the case of sirtuins, money has certainly flowed. Sirtris itself sold for ~$700 million, and it's probably not unreasonable to suggest that a billion dollars has gone into broader sirtuin-related research and development over the past decade. What does the research community have to show for that? Basically nothing other than an increased understanding of some aspects of metabolism relating to calorie restriction and other adaptations that alter life span in response to environmental circumstances. Certainly no mice living longer in widely replicated studies as is the case for mTOR and rapamycin - the sirtuin results and underlying science are still much debated, much in dispute.
The historical ratio of dollars to results for any sort of way to manipulate our metabolism to slow aging is exceedingly poor. The thing is, this ratio shouldn't be expected to get all that much better. Even if marvelously successful, the best possible realistic end result of a drug that slows aging based on what is known today - say something that extracts the best side of mTOR manipulation with none of the side-effects of rapamycin - is a very modest gain in human longevity. It can't greatly repair or reverse existing damage, it can't much help those who are already old become less damaged, it will likely not even be as effective as actual, old-fashioned calorie restriction. The current consensus is that calorie restriction itself is not going to add more than a few years to a human life - though it certainly has impressive health benefits.
(A sidebar: we can hope that one thing that ultimately emerges from all this research is an explanation as to how humans can enjoy such large health benefits from calorie restriction, commensurate with those seen in animals such as mice, without also gaining longer lives to match. But if just eating fewer calories while obtaining good nutrition could make humans reliably live 40% longer, I think that would have been noted at some point in the last few thousand years, or at least certainly in the last few hundred).
From this perspective, traditional drug research turned into longevity science looks like a long, slow slog to nowhere. It keeps people working, but to what end? Not producing significant results in extending human longevity, that's for sure.
The cost of demonstrating that SENS is the right path or the wrong path - i.e. that aging is simply an accumulation of damage, and the many disparate research results making up the SENS vision are largely correct about which forms of change in aged tissue are the fundamental forms of damage that cause aging - is tiny compared to the cost of trying to safely eke out modest reductions in the pace of aging by manipulating metabolism via sirtuins or mTOR.
The end result of implementing SENS is true rejuvenation if aging is caused by damage: actual repair, actual reversal of aging. The end result of spending the same money and time on trying to manipulate metabolism to slow aging can already be observed in sirtuin research, and can reasonably be expected to be much the same the next time around the block with mTOR - it produces new knowledge and little else of concrete use, and even when it does eventually produce a drug candidate, it will likely be the case that you could do better yourself by simply practicing calorie restriction.
The expectation value of SENS is much greater than that of trying to slow aging via the traditional drug discovery and development industry. Ergo the research and development community should be implementing SENS. It conforms to the consensus position on what causes aging, it costs far less than all other proposed interventions into the aging process, and the potential payoff is much greater.