Discussion
Although current research foreshadows a promising perspective for using
metformin and rapamycin as anti-aging drugs, there are still some
concerns that need to be highlighted, and they apply not only to the
researches of metformin and rapamycin, but to other anti-aging mechanism
and anti-aging drug researches as well.
First, despite the positive outcomes from many studies, it is not
uncommon to find a change in dosage turning the result from
life-extending to life-ending. When a low dose of metformin (0.1%) was
given to middle-aged male mice with their diet, their lifespans were
extended by 5.83% on average, but a higher concentration (1%) became
toxic.21 In another study, although metformin
activated AMPK and suppressed lipid storage in fruit flies, their
lifespan did not increase. At higher doses (25mM and 50mM), metformin
reduced the survival rates. The authors reasoned the causes to be
excessive starvation, disrupted intestinal fluid homeostasis, or
metformin toxicity.97 In the PD study with mice models
created with MPTP, when MPTP and metformin were given in the same day,
75% lethality ensued in the mice. Furthermore, although metformin
increased the levels of BDNF and GDNF, two neurotrophic factors, the
high dosage (400 mg/kg) killed all the mice.95 The
issues with dosage along with physical and genetic differences between
humans and animals make scaling the positive lab results for human use a
tricky matter. In the study that showed metformin’s beneficial effects
for treating CVDs in mice, the dosage was 200mg/kg, a number that can no
way be applicable to humans.41 Hence, conducting human
clinical trials may be a more efficient approach to find a safe and
effective dosage for human use. Encouragingly, when anti-diabetic doses
of metformin were given to 12 pre-operative endothelial cancer patients
and comparison of their tissue samples before and after the operation
were made, the same effects observed in vitro were found – increased
AMPK phosphorylation, decreased tumor cell proliferation, and decreased
H19 levels.19
Another issue that stands in the way is the side effects associated with
chronic use of drugs. About 25% of patients treated with metformin have
gastrointestinal side effects associated with the phenotype of organic
cation transporter 1 (OCT1).98 Besides, chronic use of
metformin can cause dose-dependent vitamin B12 deficiency, increasing
the risk for anemia and neuropathy.99,100 Lactic
acidosis has been reported as a side effect of metformin, but there has
been controversies, and in the study using diabetes model mice to study
AD-like brain changes, metformin did not further increase the serum
lactate concentrations.81 Whether this holds true in
healthy mice or humans is yet to be seen. As for rapamycin, over a third
of users have reported diarrhea and nausea, accounting for around 5% of
treatment discontinuation.101 The issues with side
effects can be addressed in four ways: the first is to selectively take
supplements, such as vitamin B12, to make up for the loss. The second is
to reduce the dose and increase the interval between every dose. A small
RCT suggests short-term use of rapamycin to be relatively safe
approach.102 Intermittently administering 2mg/kg of
rapamycin every 5 days has also reduced incidence of side effects in
mice and extended their lifespan.103 More clinical
trials are needed to calibrate the balance between safety and anti-aging
effectiveness. The third way is taking a variety of anti-aging drugs
(also known as drug cocktail therapy), each with a very low dose,
instead of taking only metformin or rapamycin since the side effects are
dose-dependent. Although cultured cells and a mice study both showed
metformin combined with insulin reduced A-β peptide
levels,80,81 the GPRD study showed long-time combined
exposure to metformin and other anti-diabetic drugs increased the risk
of AD while using only metformin did not show any
difference.104 Therefore, this method requires further
validation as well. The fourth way is to find analogs with fewer side
effects. Recent study found DL001, an effective mTORC1 specific rapalog,
does not induce metabolic disruption and
immunesuppression.105 Furthermore, a low dose of
rapalog (RAD001) combined with the catalyst BEZ235 reduced infection by
40% and improved response to influenza vaccination in healthy elderly
(aged 65 and older) subjects. More importantly, this combination therapy
was well-tolerated in majority of the subjects.106Rapalogs have been approved for treating multiple cancers, including
renal cell carcinoma, hepatocellular carcinoma and mantle cell lymphoma,
making it by far the most promising way to circumvent the side effects
of rapamycin.107,108 Nevertheless, as reported
clinical benefits have been modest,58,59 the quest for
more effective rapalogs is still ongoing. On the other hand, metformin
had fewer available analogs with well-studied side effects. Phenformin
and buformin, which are biguanide drugs like metformin, were withdrawn
from the market due to fatal lactic acidosis.109Mito-metformin, synthesized by adding a positively charged
triphenylphosphonium group to metformin, showed 100-fold to 1000-fold
more anti-proliferative effects depending on alkyl chain lengths, but
how the drastically improved potency will impact healthy cells is poorly
understood at the moment.110
Future research should also work to elucidate how gender influences drug
effectiveness. Metformin increased mean lifespan of female mice by 4.4%
while decreased that of male mice by 13.4%.111 Male
pre-diabetic patients who received metformin had a significantly lower
coronary calcium score compared with control while the female group did
not.46 Such sexual dimorphism also affects the
lifespan of mice that took rapamycin – female mice had greater lifespan
increase than male mice did.24,112 These studies have
showed varied amount by which gender influences drug effectiveness, and
studying drug-hormone interactions could help finding the reason.
Besides these issues, much more can be found about the genetic
mechanisms that regulate lifespan. Although many positive outcomes have
come out of attempts to control DNA methylation with metformin and
rapamycin, the full picture of epigenetic modifications have not been
understood. Metformin treatment led to a combination of hyper, hypo, and
non-differentially methylated CpG sites, and this was due to a
combination of direct and indirect effects. For example,
hypermethylation of one site can lead to reduced expression of a
protein, and this can have downstream effects that alter methylation
status of other sites.19 Understanding this complex
network of interactions will not only promote further understanding of
metformin and rapamycin, but also help developing more anti-aging
measures. APOE, a locus on chromosome 5q33.3, and FOXO3A are all known
to correlate with longevity. It has also been mentioned above that a
GWAS identified 5 SNPs related to cerebellum aging.83In addition, SNPs in the human genome also affects the efficacy of
drugs. For example, rs2740574, located in CYP3A4 changes breast cancer
cells’ response to rapamycin by altering drug metabolisms in liver, and
rs2282143, located in SLC22A1, changes breast cancer cells’ response to
metformin by affecting the rate of drug entering
cells.113 A locus on chromosome 11 (rs11212617) is
associated with the glycemic response to metformin.114Three SNPs (rs8111699, rs11212617, rs9803799), which are located in the
LKB1, ATM and PRKAA2, have been identified as significant influencers on
metformin therapy by affecting the AMPK
pathway.115–117 As DNA sequencing becomes more
convenient and accessible, it is reasonable to assume that increasing
amount of genetic data and research efforts will reveal many more such
connections, and they can point to novel genes and drug targets or be
used for precision medicine to improve current treatments.
The issues of dosage, side effects, sexual dimorphism, and genetic
regulatory mechanisms all point to the need for a large-scale clinical
trial. The Targeting Aging with Metformin (TAME) trial is a large
placebo-controlled trial that has been designed to enroll 3000 subjects
to test whether metformin delays age-related
diseases.118 The TAME trial received FDA approval in
2015, and after receiving all the required budget in 2019, it was set to
start at the end of the same year. The TAME trial may make metformin the
first approved drug for anti-aging, but more importantly, since it is
not testing metformin against a single disease but a collection of
age-related ones, it establishes aging as a medical condition that can
be intervened or treated instead of an irreversible process outside
human control. The shift in the notion of aging will make future
anti-aging clinical trials proceed with much more
ease.119