The Interventions Testing Program (ITP) was established by the National Institute
on Aging (NIA) to investigate the potential of dietary interventions to promote healthy
aging (https://www.nia.nih.gov/research/dab/interventions-testing-program-itp). The
ITP uses a four-way cross genetically heterogeneous mouse model (UM-HET3) to reduce
the impact of strain-specific characteristics on outcomes (Nadon et al., 2008). Lifespan
tests are done in parallel, using the same protocol, at three independent sites to
increase robustness of the findings. Population sizes are large enough that the protocol
will detect a 10% change in mean lifespan, in either sex, with 80% power, pooling
data from as few as two sites. Standard operating procedures were designed to maintain
as much consistency as possible among the three sites, including caging, bedding,
food, and light/dark cycles; a more in-depth discussion of the SOP has been published
(Nadon et al., 2015). Interventions for testing are proposed by the research community
through an annual call-for-proposals, and proposed compounds have ranged from drugs
and dietary supplements to micronutrients and metabolic intermediates.
Before the ITP embarks on testing a compound, pilot studies are done to maximize the
chances of a successful test. Goals of the pilot studies include demonstrating that
the compound is stable in food and that it is uniformly mixed in the food, determining
blood levels after short-term treatment (bioavailability), showing evidence of an
effect from the short-term treatment (bioactivity), and in some cases, testing for
toxicity. The testing of rapamycin is a good case-in-point for analyzing stability
of the compound in the food. Pilot analysis showed that about 85% of the rapamycin
was degraded by the food preparation process, leading to the use of microencapsulation
to deliver stable doses of the compound in food (Harrison et al., 2009).
The list of all compounds tested by the ITP and in progress is on the ITP website
at https://www.nia.nih.gov/research/dab/interventions-testing-program-itp/compounds-testing.
To date, six compounds have shown significant extension of lifespan:
•
Aspirin – males only (Strong et al., 2008);
•
Rapamycin – males and females (females > males) (Harrison et al., 2009, Miller et
al., 2011, Miller et al., 2014);
•
17αEstradiol – males only (Harrison et al., 2014);
•
Acarbose – males and females (males > > females) (Harrison et al., 2014);
•
Nordihydroguaiaretic acid (NDGA) – males only (Strong et al., 2008, Harrison et al.,
2014);
•
Protandim® - males only (Strong et al., 2016).
The positive findings illustrate some important aspects for aging interventions research.
The effective interventions appear to include several disparate mechanisms, demonstrating
that many cellular pathways might be exploited to influence lifespan and aging. Rapamycin
modulates the nutrient-sensing pathways via its interaction with mTOR (Harrison et
al., 2009). Acarbose was anticipated to work as a caloric restriction mimetic due
to its ability to reduce the rate of absorption of carbohydrates, but its mechanism
of action appears more complex, since caloric restriction results in significant lifespan
extension in both male and female UM-HET3 mice (Flurkey et al., 2010), while the effects
of acarbose were much larger in males (Harrison et al., 2014). Aspirin is known for
its anti-inflammatory and antioxidant activities, NDGA also has anti-inflammatory
and antioxidant activities, 17αEstradiol has neuro-protective properties independent
of binding to the estrogen receptor, and Protandim® activates Nrf2 transcriptional
regulator (Strong et al., 2008, Strong et al., 2016). This diverse group of interventions
demonstrates the complex nature of the biology of aging.
Another major surprise is the extent of sex differences in response to the interventions.
Four of the six positive interventions only worked in one sex, and the two that had
an effect in both sexes showed sex-specific differences in the extent of the effect.
Blood levels of a compound sometimes differed between males and females, but that
did not always explain the sex difference in lifespan extension. For rapamycin, achieving
approximately equivalent blood levels in males and females by treating with different
doses did result in similar increases in lifespan (Miller et al., 2014). But for NDGA,
even at doses giving similar blood levels in males and females, females still did
not respond (Harrison et al., 2014). The ITP's findings illustrate how important it
is to examine the effects of interventions in both sexes and suggest that further
studies on the mechanism of these sex effects may yield important insights into the
underlying biology, and guidance for eventually clinical studies.
Lifespan, while a valuable measurement in rodent model studies, does not entirely
capture the impact of aging interventions. Aging is a complex process, with many physiological
systems affected, and not all at the same time or rate. Measurements of a wide range
of endpoints relevant to health and maintenance of function will help to clarify the
relationship of aging to function and disease, and may also document drug effects
pertinent to human health maintenance. The ITP is developing a panel of measurements
to assess health and function that will be used across cohorts to add to the information
the lifespan studies provide.
Studies in the mouse have contributed significantly to our understanding of aging
and interventions to promote healthy aging. Testing both sexes and avoiding the use
of a single inbred strain are key features that will enhance translation of the findings.
A power analysis is essential to get the most value from experiments, and standardized
protocols allow for cross-experimental comparisons. Multi-site testing protocols also
add value to the design because some site-to-site variation is unavoidable even with
every effort made to minimize differences between sites. For example, the ITP has
consistently found that control male mice at one site weigh less and live longer than
the control males at the other two sites, even though each site uses the same food
preparations and standardized husbandry (Strong et al., 2012). Positive findings replicated
in different labs are inherently stronger than a finding from one lab, while disparate
findings convey a valuable caution and emphasize the need for replications in other
laboratories, other mouse stocks, and other drug doses.
In summary, the ITP program, now in its 13th year, has become a major contributor
to the biogerontology research community, providing new insights into the biology
of aging, new tools for probing the relationships among cells, aging, and disease,
and new ideas about how best to translate studies in basic biogerontology into human
investigation, and, eventually, into the clinic.
The ITP is funded by the National Institutes of Health grants U01-AG022303 to RAM,
U01-AG022308 to DEH, and U01-AG022307 to RS.