Rapamycin therapy is considered a promising approach for lifespan extension and the delay of age-related disease, with numerous preclinical studies documenting benefit [1–19]. These benefits have inspired some patients to seek rapamycin therapy from specialty practitioners. Yet, the clinical evidence of benefit associated with low-dose rapamycin use in healthy human adults has not been established, and there may exist signals indicating caution with off-label use at non-immunosuppressive doses.
While the benefit of rapamycin therapy has been demonstrated in non-human models, nonetheless, the clinical evidence for low-dose mTOR inhibitors as a therapy for extending lifespan or delaying the onset of age-related disease in healthy adults remains unestablished. Here, we provide a critical appraisal of studies evaluating low-dose rapamycin therapy in healthy adults and offer considerations for its potential use as an off-label longevity drug in humans.
Clinical evidence in healthy participants
As the concept of aging as a modifiable risk factor for illness and death continues to gain traction, repurposing previously approved drugs, such as rapamycin, metformin, and acarbose to delay age-related disease, is becoming increasingly prevalent.
Longevity data in humans is difficult to acquire. Any well-designed trial that attempts to assess the longevity impact for any drug in people will be time consuming, expensive, and complicated by uncertainties in clinically valid endpoints. Since rapamycin is a generic medication, there is little incentive for any private group to fund such a study, which further complicates acquisition of high quality evidence with regard to low-dose rapamycin therapy. Accordingly, the clinical evidence evaluating low-dose rapamycin, or its analogues, in healthy participants is scant, with less than a dozen known trials exploring a variety of biomarkers, including immune function, protein synthesis, and hematologic parameters.
Evidence favoring low-dose mTOR inhibition was established by Mannick et al. (2014) using everolimus (RAD001), an mTOR complex 1 (mTORC1) rapalog, to evaluate markers of immune function in older adults. Mannick et al. evaluated a cohort of 218 healthy older adults receiving everolimus therapy, which was discontinued two weeks prior to influenza vaccination [23]. The results of this analysis suggested that low-dose everolimus therapy (0.5 mg/day and 5 mg weekly) induced a 20% increase in immune titers, while circulating T-cell inhibiting PD-1 positive CD4 and CD8 counts declined relative to placebo, a finding which is associated with enhanced T-cell function, or a more youthful immune phenotype [23, 43]. Rather than being immune suppressive, low-dose mTOR inhibition was associated with signals of enhanced immune function in this population. While safety parameters were acceptable, benign apthous ulcers were significantly more common in the treatment arm. The ability of mTOR inhibition (everolimus) in combination with an ATP-competitive kinase inhibitor with secondary mTORi effects (RTB101) in reducing respiratory tract infection was confirmed by Mannick’s group in 2018 in a phase 2 trial of 264 healthy adults [24]. Caution is appropriate in interpreting these results, as the study did not detect a significant difference in annualized rates of respiratory tract infection, though the study may have been underpowered to detect this [25].
In a follow-up phase 2b and phase 3 analysis, 10 mg/day of RTB101 demonstrated a similar effect and was associated with a marked reduction in respiratory tract infection incidence [25]. Laboratory analysis supported a phenotypic improvement in immune function, mediated by an upregulation of interferon-gamma (IFN-γ), an inflammatory cytokine whose induction stems from NF-kB activation. Several caveats apply in this trial. In the 2b phase, participants (n = 652) were initially randomized to RTB101 at 5mg/day, 10 mg/day, or placebo. Importantly, the incidence of respiratory tract infections did not differ significantly between the 5 mg/day dose versus placebo. In part 2 of this trial, investigators instead randomized the remaining participants (473) to 10 mg/day, 20 mg/day, 10mg/day + everolimus 10 mg/day, or placebo, noting that only the 10 mg/day dose of RTB101 was sufficient to reduce the respiratory tract infection rate relative to control. No significant safety events occurred and, similarly to the 2a trial, isolated mTORC1 inhibition was confirmed, and there were no significant differences in hyperglycemia or hyperlipidemia. The relative success of lower doses versus higher doses is of potential interest and may point towards the existence of an immunologic “threshold” at which the immune enhancing effects of low dose mTOR inhibition are replaced by immunosuppressive effects observed at higher doses.
Other limitations should be noted, including the increase in respiratory tract infection incidence in smokers or those with COPD. Although “intermittent” mTOR inhibition achieved a clinically significant effect, continuous therapy did not. In the phase 3 arm of this study, the results of the phase 2b study were not replicated, though this finding was complicated by an endpoint alteration from laboratory-confirmed infections in phase 2 to patient-reported infections in phase 3, as requested by the FDA. This decision muddied their findings, limiting the ability to detect a significant difference between groups. The authors suggested that this unexpected observation may be the result of improper symptom logging and/or a healthier cohort composition. The latter consideration may be significant because the effect of mTOR inhibition was greatest in subjects >85y, and in subjects >65y with evidence of asthma, when compared with cohort controls. Thus, rapamycin therapy may have an outsized, positive effect on hematopoietic parameters in immunologically-challenged participants. In sum, the evidence favoring the beneficial effect of rapamycin therapy on the incidence of respiratory tract infection is compelling, but not convincing at present.
Kaeberlein et al. (2023) reported on community use of rapamycin for longevity purposes, finding that users had a significantly lower likelihood of COVID infection and long-COVID incidence along with subjective increases in various measures of well-being and physical stamina [26]. This cohort of rapamycin users also documented self-reported benefit in abdominal cramps, depression, abdominal pain, muscle tightness, anxiety, and eye pain relative to non-users. It is important to note this cohort was not blinded to their intervention and may have been impacted by some degree of placebo effect. Importantly, this cohort reported no safety signals. Nonetheless, the Kaeberlein study supports observations made by previous human and animal studies with respect to immune enhancement. Thus, while promising, this study does not constitute firm evidence that rapamycin can extend healthspan or lifespan in humans.
Trials of rapamycin have yielded ambiguous evidence with respect to muscle protein synthesis. For example, Gunderman et al. (2014) demonstrated that 16 mg of rapamune blunted post-exercise increases in protein synthesis [27]. In contrast, Dickinson et al. (2013) found that the same dose did not alter rates of synthesis in skeletal muscle as assessed via muscle biopsy, nor did it affect circulating markers of autophagy up to 4 hours post-ingestion relative to baseline [28]. However, this study only considered basal post-absorptive and non-post-exercise rates. Thus, it is possible that sirolimus may demonstrate anti-sarcopenic effects that should be considered, particularly because sarcopenia has been repeatedly associated with adverse outcomes in the context of age-related decline [29, 30]. However, it is also plausible that muscle protein synthesis exerts age-dependent effects and that mTOR inhibition by sirolimus may in fact benefit other aspects of muscle preservation including reduced catabolic activity. The relative absence of surveillance and testing for this finding in other cohorts limits the conclusions derived from these two clinical studies.
In a trial by Horbelt et al. (2020), 22 healthy young men were subjected to RAD001 (everolimus) at various dosing schedules to evaluate its effect in 4 doses over 12-hour periods for 15 days [31]. Several days post-administration, significant reductions for two interleukins (IL), IL-2 and IL-10 were identified. Reductions in IL-10 are noteworthy and warrant attention given that IL-10 is recognized as a key anti-inflammatory cytokine, as it promotes homeostasis (resolution of inflammation) and immune surveillance (preparation for robust response given need, e.g. cancer cells or pathogens) [32]. IL-10 expression has also been associated with increased longevity [32]. With respect to psychological parameters, lower and medium doses (5-10 mg) of everolimus significantly increased self-reported anxiety and increased noradrenaline. While these changes cannot be interpreted in isolation, the net effect of rapamycin therapy in this cohort was significant and merits further consideration.
One of the most detailed trials of low-dose rapamycin use in healthy participants comes from Kraig et al.’s (2018) examination of 25 healthy older adults (aged 70-95 years), which evaluated the safety of 1 mg/day sirolimus for 8 weeks. A mean 7.2 ng/dL circulating level of sirolimus was achieved, with authors documenting changes in several hematologic, hormonal and physical parameters [33]. The limitations of this study are (1) a relatively short duration and (2) the use of continuous (1 mg/day) rather than intermittent rapamycin dosing schedules that may lead to off-target effects. Kraig et al.’s results reported no significant improvement in various metabolic parameters, while noting several potentially undesirable findings between groups, including a significant decrease in plasma albumin, increased triglycerides and Hemoglobin A1C (HbA1C) and a near-significant (p=0.06) increase for very-low-density lipoprotein (VLDL) within rapamycin-treated subjects. Albumin declines during aging [34], which could suggest an unfavorable age-related change with rapamycin use, though liver function biomarkers appeared unaffected. Increases in triglycerides, HbA1C, and VLDL are also concerning, however, insulin sensitivity was not altered, nor were results from oral glucose tolerance tests or fasting glucose. Thus, Kraig et al. concluded that rapamycin therapy did not lead to significant adverse outcomes in the short term. Alterations in various hematologic parameters on rapamycin therapy included a significantly decreased red cell distribution width (RDW), a marker that has been associated with a “youthful” biological age [35]. Other hematologic parameters were consistent with previous trials of rapamycin therapy that identified decreases for mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and hemoglobin (Hgb) [36–38]. A higher MCV and lower hemoglobin are associated with an increased all-cause mortality risk [36–38], which may suggest an opposing impact on health-related biomarkers. With regard to physical parameters, weight was reduced in the rapamycin cohort, however, handgrip strength was unchanged, which may support the complex role of mTOR inhibition in muscle preservation. There is also evidence that handgrip strength may be related to cognitive capacity, thus suggesting a possible protective role against neurological decline [39]. Further, though walking speed declined markedly in the control group, the rapamycin cohort maintained gait speed without a significant functional decline.
Rapamycin users demonstrated no significant reduction on any individual inflammatory marker in this small cohort. Instead, inflammatory cytokines broadly increased, including a significant TNF-α elevation. This cohort was not appropriately powered and thus non-significant changes in inflammatory parameters such as IL-2, IL-6, and monocyte chemoattractant protein 1 (MCP-1), while interesting, may not necessarily denote harm and instead could be artifacts of the study. As a caveat, TNF-α is a marker of autophagy; thus, we cannot conclude whether this finding is a confound associated with an underlying, healthy alteration [40]. Some circulating immune cells (CD) increased, with a signal indicating regulatory T cell (Treg) clusters may have improved if more appropriately powered. Treg expression is considered a marker of “healthy” tolerance, as it promotes homeostasis and immune surveillance [41, 42]. Further work to better understand the complex relationship between various immune components and aging is needed. Kraig et al. concluded that rapamycin therapy did not demonstrate any significant adverse outcome in the short term, nor was a signal of clear benefit identified.