Rapamycin as a potential intervention to promote longevity and extend healthspan in companion dogs

Abstract

Importance

Aging is a complex process mediated by highly interconnected cellular, molecular and systemic pathways with genetic, environmental, dietary and lifestyle inputs, among others. Pharmacologic manipulation of aging’s molecular and cellular pathways—including those mediated by the mechanistic target of rapamycin (mTOR)—is an important ongoing area of study.

Observations

Inhibition of mTOR has been associated with lifespan extension and delayed or slowed age-related pathologies in several laboratory species, with cited mechanisms including improved autophagy, improved proteostasis, and reduced mRNA translation, among others. The effects of the mTOR inhibitor rapamycin on longevity in non-laboratory species is an area of emerging study, and our understanding of mTOR signaling pathways and their effects on aging continues to evolve. In people, owing to its dose-dependent immunomodulatory effects, rapamycin is currently approved as an anti-cancer agent and for prevention of organ transplant rejection or restenosis of vascular stents. Its potential use as an intervention to increase life- and healthspan in companion dogs is an area of active research.

Conclusions and Relevance

Rapamycin is a promising manipulator of multiple cellular and molecular hallmarks of aging. The positive results of longevity studies in laboratory species have paved the way for similar studies of rapamycin in non-human primates and companion veterinary species. The results of these and ongoing studies that evaluate the effects of mTOR inhibition for disease treatment or longevity promotion will facilitate greater understanding of the aging process and clinically meaningful ways to leverage relevant pathways.

INTRODUCTION

Aging is universal, inevitable, and a prominent risk factor for chronic illness. Therefore, that aging is a highly prevalent focus of current medical research is unsurprising. Aging has been characterized as “the lifelong accumulation of damage” and “a gradual decline of physiological functions” that involves processes at many levels, from molecular to that of entire organ systems [1,2]. Within geroscience—the field focused on aging biology and its effects on disease and health—the so-called “Hallmarks of Aging,” which comprise interconnected factors that play a role in this decline, have been proposed [3,4] (Table 1). As age-related declines in one or more of these hallmarks occur, overall loss of function can confer increased vulnerability to disease and death. Greater understanding of the factors that influence aging might allow for the discovery of ways to modify, slow, or even reverse its processes and prevent or delay the development of disease.

Table 1. Overview of the “Hallmarks of Aging” as proposed by López-Otín et al.^a.

Category	Hallmark	Examples of functional consequences
Primary	Genomic instability	Accumulation of genomic damage, ectopic accumulation of DNA in the cytosol
	Telomere attrition	Induction of genomic instability and cellular apoptosis or senescence
	Epigenetic alterations	Impacts on gene expression resulting in development and progression of age-related pathologies (e.g., cancer, neurodegeneration)
	Loss of proteostasis	Accumulation of misfolded, oxidized, glycated or ubiquitinylated proteins that form aggregates or plaques
	Disabled autophagy	Inflammasome activation, accumulation of dysfunctional organelles, cellular damage, oncogenesis
Antagonistic	Cellular senescence	Altered tissue repair
	Mitochondrial dysfunction	Reduced cellular fitness, activation of inflammasomes, excessive cell death
	Dysregulated nutrient-sensing	Excessive anabolic signaling, unrestrained inflammation, inhibition of autophagy
Integrative	Stem cell exhaustion	Loss of cellular plasticity required for tissue repair
	Altered intracellular communication	Compromised homeostatic and hormone regulation due to deficiencies in neural, neuroendocrine and hormonal signaling pathways
	Chronic inflammation	Increased circulating concentrations of inflammatory cytokines and biomarkers, immune function decline
	Dysbiosis	Disruption of gut microbiota-host bidirectional communication contributing to several age-related pathologies (e.g., cancer, cardiovascular disease, neurological disorders)

^aFrom reference [3].

Although the success of an aging intervention can be measured by its ability to extend lifespan (i.e., the total duration of an organism’s life), recent geroscience research has emphasized the value of maximizing healthspan (i.e., the period during which an organism is unaffected by chronic diseases or disabilities) [5]. In people, balancing these two measures is critical, as lifespan extension without a corresponding delay in the onset of chronic diseases can negatively impact the physical and mental wellbeing of the individual and their support systems [6].

Although aging has been most extensively studied in laboratory species and people, companion dogs represent an important translational model for aging research [7]. Dogs experience similar environmental exposures as their human counterparts. Like people, dogs are inherently variable in size, conformation, and disease susceptibility. The healthcare system for pets is more advanced than that of laboratory species and more closely aligned with the human healthcare system. Finally, dogs age at a rate that is on average seven to ten times faster than that of human beings, allowing studies carried out in the former to be completed in less time and with fewer resources. For these reasons, companion dogs have been proposed as a valuable model that might facilitate greater understanding of the hallmarks of aging and the effects of their modulation in people [7]. Simultaneously and of interest to veterinarians and pet owners, information gleaned from these studies can directly benefit dogs and their families.

Several highly conserved and interconnected molecular signaling pathways have been implicated in the aging process and chronic disease development [8,9]. In many instances, these are normal homeostatic pathways whose deregulation during excessive stress or in response to other factors can accelerate cellular aging [10]. Such pathways include, but are not limited to, the sirtuin, insulin/insulin growth factor-1, AMP-activated kinase, and mechanistic/mammalian target of rapamycin (mTOR) pathways [8,10,11,12]. It follows that interventions (e.g., dietary, pharmaceutical, physical) targeting one or more of the signaling pathways related to aging might have the ability to extend healthspan. Rapamycin, an mTOR-inhibiting macrolide antibiotic whose effects on longevity extension and age-related pathologies have been most extensively evaluated in laboratory species, is one such intervention that has received considerable attention in recent years [13]. In the present review, we summarize current knowledge regarding mTOR signaling and the effects of mTOR inhibition; review the history and current use of rapamycin as a method of mTOR inhibition; and discuss evidence for, and future directions of, rapamycin use as an intervention to promote longevity in veterinary medicine.

METHODS

A literature search using electronic databases was conducted to identify studies on the discovery and current understanding of the mTOR pathway and rapamycin as an mTOR inhibitor. The search strategy utilized single keywords or combinations of keywords including “mTOR,” “mTOR inhibition,” “rapamycin,” “sirolimus,” “dog,” “domestic animal,” and additional words related to aging. All studies included in this review were published in peer-reviewed journals and written in English.

OBSERVATIONS

The mechanistic/mammalian target of rapamycin (mTOR)

The target of rapamycin (TOR) complex was first discovered in yeast three years prior to identification of its mammalian (also: “mechanistic”) analog (mTOR) in 1994 [14,15]. Highly conserved across mammalian species, mTOR is an intracellular serine-threonine kinase of the phosphoinositide 3-kinase family that functions as an essential and central regulator of cellular metabolism, growth, autophagy, and senescence in tissues throughout the body [8,13,16,17,18]. In brief, mTOR behaves as a complex signaling “hub” that senses and integrates environmental and hormonal cues (e.g., cellular nutrients; indicators of energy and stress status), each of which indicate whether the prevailing conditions—including available resources and organismal needs—support cellular growth and proliferation or senescence. In response, it enacts the downstream cellular processes necessary to meet these goals [13]. mTOR is essential both for embryonic development and later in life.

The mTOR protein kinase makes up the active site of two distinct multi-protein complexes, mTOR-complex-1 (mTORC1) and mTOR-complex-2 (mTORC2) [8,13,16,17,18], which differ not only structurally, but also in their upstream stimuli and inhibitors, substrates, and downstream signaling pathways (Table 2). These complexes also differ in their acute sensitivity (mTORC1 > mTORC2) to rapamycin.

Table 2. Characteristics of the mTORC1 and mTORC2.

Characteristics		mTORC1	mTORC2
Upstream signals
	Signals that increase mTORC activity	• Growth factors (insulin/IGF-1) via PI3K/Akt	• Growth factors (insulin/IGF-1) via PI3K
		• Environmental nutrient (e.g., amino acids, glucose) abundance	• Translocation to assembled ribosomes
		• Wnt, mitogens	• Foxo1
		• Pro-inflammatory cytokines
	Signals that decrease mTORC activity	• Energy depletion (decreased ATP:ADP, via AMPK)	• Chronic treatment with rapamycin or rapalogs
		• Prolonged hypoxia
		• Decreased amino acid availability
		• Calorie/dietary restriction (without malnutrition)
		• Genotoxic/proteotoxic stress
		• Treatment with rapamycin or rapalogs (acute)
Downstream effects of increased activity		• Stimulation of anabolic cellular processes (e.g., mRNA translation) to encourage growth and replication	• Increased glucose uptake
		• Increased mitochondrial activity and cellular metabolism (via HIF-1)	• Increased glycolysis
		• Inhibition of catabolic processes (e.g., autophagy, lysosomal degradation)	• Increased ribosomal capacity/activity
		• Vascularization (via HIF-1 and VEGF)	• Actin/cytoskeleton assembly
		• Inflammation (with hyperactivation)	• Inflammation
			• Stress resistance
Sensitivity to rapamycin		Sensitive	Relatively insensitive; disrupted by chronic exposure in some tissues

Boldface text reflects major regulators of mTOR activity.

mTORC, mechanistic/mammalian target of rapamycin-complex; IGF-1, insulin-like growth factor 1; PI3K/Akt, phosphoinositide 3-kinase/protein kinase B pathway; Foxo1, forkhead box protein O1; ATP, adenosine triphosphate; ADP, adenosine diphosphate; AMPK, AMP-activated protein kinase; HIF-1, hypoxia-inducible factor 1; VEGF, vascular endothelial growth factor.

mTORC1

The mTORC1 complex comprises 5 subunits (Fig. 1). Three—the catalytic subunit mTOR, the regulatory-associated protein of mTOR, and the mammalian lethal with Sec13 protein 8 (mLST8)—are essential to the assembly and function of the complex. The remaining subunits, proline-rich AKT substrate 40kDA (PRAS40) and DEP-domain-containing mTOR-interacting protein (Deptor) [16], negatively modulate mTORC1’s activity [9]. Because mTORC1 is sensitive to acute inhibition by rapamycin, research using rapamycin has facilitated a greater understanding of mTORC1 pathways compared to those of its less sensitive counterpart, mTORC2.

Fig. 1. Overview of relevant mTOR signaling pathways. The major regulators of mTORC1 pathways include: 1) intracellular amino acid levels sensed at the lysosome surface by Rag; 2) intracellular energy balance (i.e., ATP:AMP) sensed by AMPK; and 3) extracellular growth factors and insulin via the PI3/Akt pathway. Increased mTORC1 activity is associated with downstream effects that favor cell growth and proliferation and inhibit autophagy. Increased mTORC2 activity is associated with downstream effects that regulate cytoskeletal organization, inflammation, and resistance to stress.

mTOR, mechanistic/mammalian target of rapamycin; mTORC, mechanistic/mammalian target of rapamycin-complex; Rag, ragulator; ATP, adenosine triphosphate; AMP, adenosine monophosphate; AMPK, AMP-activated protein kinase; PI3/Akt, phosphoinositide 3-kinase/protein kinase B; GDP, guanosine diphosphate; GTP, guanosine triphosphate; TSC1, tuberous sclerosis complex 1 (i.e., hamartin); TSC2, tuberous sclerosis complex 2 (i.e., tuberin); Rheb, Ras homolog enriched in brain GTP-binding protein; Rictor, rapamycin-insensitive companion of mTOR; Deptor, DEP-domain-containing mTOR-interacting protein; mLST8, mammalian lethal with Sec13 protein 8; mSIN1, mammalian stress-activated protein kinase interacting protein; Protor-1, protein observed with Rictor-1; FKBP, FK506-binding protein; Raptor, regulatory-associated protein of mTOR; PRAS40, proline-rich AKT substrate 40 kDA; HIF-1, hypoxia-inducible factor 1. Created in BioRender. Coleman M. (2025) https://BioRender.com/zpolra3.

Upstream regulators and the effects of mTORC1 activity are summarized in Table 2. In general, mTORC1 activity is greatest during periods of energy and nutrient abundance, and on balance, increased mTORC1 activity promotes higher rates of cell growth and replication. This is achieved by simultaneous 1) increases in anabolic cellular processes (e.g., protein, lipid, and organelle biosynthesis), mitochondrial activity, and glucose metabolism [19], and 2) reduction in catabolic processes such as autophagy (i.e., the organized lysosomal process by which cells degrade and recycle aged cellular components) (Fig. 1). Negative modulators or inhibitors of mTORC1 logically decrease these downstream effects. In addition, improvements to stem-cell function and cellular resistance to insults have been linked to inhibition of mTORC1 activity, highlighting its role in these functions [17].

The mTORC1 pathway has a prominent role in the regulation of autophagy, which involves the intracellular sequestration of damaged organelles, proteins or portions of the cell membrane and their delivery to lysosomes, where they are degraded [2]. In health, basal levels of autophagy are important for recycling cytoplasmic contents. During periods of nutrient deprivation or stress (e.g., high concentrations of reactive oxygen species, genotoxic or proteotoxic insults, intracellular pathogens), increases in autophagy help to provide substrates critical for cell survival [2]. One signal for increased autophagy in these situations is mTOR inhibition [20]. As discussed below (“Effects of mTOR inhibition on autophagy”), effects on autophagy represent a major link between mTORC1 signaling pathways and aging and age-related diseases.

Pathways involving mTORC1 are also important in immune signaling. For example, following antigenic stimulation, mTORC1 mediates T-cell activation through interleukin, leptin and Toll-like receptor signaling pathways, among others [21], and participates in the differentiation and activation of dendritic cells [22]. The prominent role of mTORC1 in the immune response has helped position it as a clinically useful target of inhibition; as such, mTORC1 inhibitors (e.g., rapamycin) have been widely studied as immunosuppressants since the early 1990s [23]. While this has significant therapeutic relevance – for example, rapamycin and its derivatives (i.e., “rapalogs”) are commonly used to prevent rejection of transplanted organs in human beings—it also represents an area of potential risk during pharmacologic mTORC1 inhibition for other reasons (e.g., to promote longevity).

Increased mTORC1 activity has been implicated in several age-related diseases of human beings. These include cancer [24], several neurodegenerative conditions (e.g., Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis), and certain inherited cardiovascular diseases (e.g., hypertrophic cardiomyopathy [HCM]) [25]. In dogs, mTOR appears to play a major role in canine pulmonary carcinoma [26]. Among the proposed mechanisms linking increased mTORC1 activity and these conditions are autophagic dysfunction and the subsequent accumulation of damaged intracellular proteins [8,13,16,17,18]. Interestingly, whether normal “healthy” aging is associated with general hyperactivity of mTORC1 pathways appears unresolved. Results from mouse models suggest heterogeneity across tissues, with some displaying increased mTORC1 signaling and others experiencing no change or even a decrease in signaling with age [13,27,28]. This, along with the measurable benefits of mTOR inhibition on lifespan, has led some authors to conclude that “even normal” mTORC1 activity levels might be too high to maintain health in aging tissues [13,29].

mTORC2

The mTORC2 complex comprises six subunits, three of which (mTOR, mLST8, and Deptor) overlap with mTORC1 and three of which (rapamycin-insensitive companion of mTOR [Rictor], protein observed with Rictor-1, and mammalian stress-activated protein kinase interacting protein) are unique to mTORC2 [16,30] (Fig. 1). As mentioned above and implied by “Rictor,” mTORC2 is less sensitive than mTORC1 to the acute inhibitory effects of rapamycin; consequently, mTORC2’s downstream effects are more poorly characterized. However, chronic rapamycin administration does relevantly impact mTORC2, as discussed below (“Rapamycin”) [30]. Upstream regulators and the effects of mTORC2 activity are listed in Table 2. Like mTORC1, mTORC2 appears to play a role in cell survival, metabolism, and proliferation in response to stimulation by growth factors [16,30,31], with glucose metabolism downstream of insulin signaling likely constituting one of its most important metabolic roles [16,32]. Unlike mTORC1, mTORC2 also controls “spatial aspects” of cell growth by its involvement in cytoskeletal organization (i.e., actin polymerization) and regulation of cell polarity [16]. The mTORC2 complex plays an important role in macrophage activation and dendritic cell function through regulation of T-cell function, B-cell signaling, and natural killer cell development and function [33,34,35,36,37,38,39,40]. mTORC2’s effects on immunity and glucose and lipid metabolism likely explain the adverse effects (i.e., immunosuppression, dysregulated glucose and lipid metabolism) that can occur with chronic mTOR inhibition.

mTOR inhibition, longevity and healthspan

Investigations in several organisms—from budding yeast to nematodes to mice—have linked mTOR pathways to the aging process. Studies using deletion or knockdown models in which the expression of mTOR substrates or mTOR complex components are reduced compared to wild type, as well as studies involving treatment with an mTOR inhibitor (e.g., rapamycin), have demonstrated lifespan extension across several evolutionarily divergent species [17,41,42]. In addition, mTOR pathways have been connected to chronic age-related disease processes ranging from osteoporosis to neurodegeneration to heart disease to cancer [9,24,25,43].

The beneficial effects of mTOR inhibition on lifespan and healthspan seem to be largely attributable to decreased mTORC1 activity, whereas mTORC2 inhibition can be generally (but not always) associated with negative effects on lifespan. Several mechanisms linking mTORC1 inhibition to increased longevity and health-span have been proposed. The best characterized include its effects on mRNA translation and autophagy, briefly discussed below. Complex, yet beneficial, effects on mitochondrial function and mitophagy, improvements in stem cell function, and reduction in age-related inflammation and improved function of the aging immune system likely underpin the benefits of mTOR inhibition, as well, although these proposed mechanisms are currently less well characterized [2].

Effects of mTOR inhibition on mRNA translation

A primary process by which mTOR signaling favors cell growth is increased mRNA translation and protein synthesis. In contrast, decreases in mRNA translation have been directly associated with increased lifespan in yeast, fruit flies, and nematodes [44]. It has been proposed that the reduced rates of protein synthesis associated with mTOR inhibition might support healthy protein homeostasis (i.e., “proteostasis”), which becomes disordered with increasing age and in several age-related diseases [45]. Impaired proteostasis has also been associated with the accumulation of misfolded or aggregated proteins [45]. Specifically, lessening of “baseline” cellular resource allocation to protein repair or degradation might mean that the cell has a greater capacity to respond to transient, proteotoxic stressors (e.g., oxidative damage) by redirecting the “saved” energy and machinery [2,44]. Other work suggests that during mTOR inhibition, differential translation of mRNAs from genes that favor enhanced stress responses might have more important effects on lifespan than global reductions in translation [17,46].

Effects of mTOR inhibition on autophagy

As cells age, they accumulate damaged proteins, lipids and organelles. This could reflect inefficient, inadequate or reduced autophagy, the major mechanism by which these products are degraded and recycled, as discussed above [47,48]. Increased rates of autophagy and lysosomal degradation in response to mTOR inhibition are a necessary element for the prolongation of lifespan in yeast and nematodes [49,50]. In addition, stimulation of autophagy prevents neurodegeneration in models of age-related neurodegenerative conditions (i.e., Alzheimer’s diseases and Parkinson’s disease) that are characterized by the accumulation of damaged intracellular macromolecules and protein aggregates [51].

Whether mTOR inhibition extends lifespan in these species by delaying the onset or rate of age-related changes or by preventing age-related pathology is yet to be determined.

Rapamycin

Rapamycin (also known as sirolimus) is an mTOR inhibitor that was first discovered in the 1970s in soil samples collected from Easter Island (Rapa Nui) [52]. These samples contained the gram-positive bacterium Streptomyces hygroscopicus (now, Streptomyces rapamycinicus) which produces an antifungal macrocyclic lactone compound, rapamycin, that was named in honor of its site of origin. Because the discovery of rapamycin pre-dated that of TOR and the mTOR complexes by nearly two decades, these structures were relatively unconventionally named after the inhibitor that facilitated their discovery. Given rapamycin’s relatively poor water solubility, several derivative compounds, referred to as “rapalogs” (e.g., everolimus, temsirolimus), were subsequently developed to improve the drug’s pharmacologic properties [53,54,55].

Rapamycin potently inhibits mTORC1 by binding several FK506-binding proteins (e.g., FKBP12, FKBP51) to form a functional complex that blocks mTORC1 activity through allosteric binding [56,57]. While the rapamycin-FKBP51 complex can directly inhibit mTORC1 only, the rapamycin-FKB12 complex (and as recently noted, other rapamycin-FKBP complexes) can also interfere with mTORC2 during chronic administration by sequestering mTOR and inhibiting the assembly of new mTORC2 complexes in some cell types [57,58,59] (Fig. 1). Relative expression of these binding proteins is one determinant of whether and the extent to which a given tissue is affected by chronic rapamycin administration [57].

While rapamycin’s longevity-promoting effects are likely mediated by mTORC1 inhibition, its adverse effects on glucose and lipid metabolism—which can lead to hyperglycemia and hyperlipidemia—are likely mediated by the mTORC2 inhibition that occurs with chronic drug exposure [46]. Therefore, intermittent and low-dose strategies, designed to improve safety, have gained substantial attention from researchers. The development of agents that more specifically block mTORC1, possibly by non-allosteric binding, has been suggested as an alternative to circumvent these effects [46].

Approved uses of rapamycin and rapalogs in human medicine

In the United States, rapamycin and its derivatives are currently approved for several indications (Table 3). These relate to:

Table 3. Approved indications for rapamycin and rapalogs in humans (US).

Condition type	Disease/condition	Indication	Drug	First approved (US)
Cancer	Renal cell carcinoma	Disease failing VEGF-inhibitor therapy	EVR (Afinitor) oral	2009
			TSL (Torisel) IV	2007
	Pancreatic neuroendocrine tumor	Progressive or metastatic unresectable disease	EVR (Afinitor) oral	2011
	Perivascular epithelioid cell tumor	Advanced, unresectable or metastatic disease	SRL (Fyarro) IV protein-bound particles	2021
	Nonfunctional GI or lung neuroendocrine tumors	Advanced, unresectable or metastatic disease	EVR (Afinitor) oral	2016
	Hormone receptor-positive, HER2-negative advanced breast cancer (with exemestane)	Advanced disease failing aromatase inhibition	EVR (Afinitor) oral	2012
Organ transplantation	Kidney transplantation	Rejection prophylaxis	SRL (Rapamune)	1999
			EVR (Zortress)	2010
	Liver transplantation	Rejection prophylaxis	EVR (Zortress)	2013
Genetic disorder characterized by mTOR hyperactivity	Tuberous sclerosis complex	Facial angiofibromas	SRL (Hyftor) topical	2022
		Unresectable subependymal giant cell astrocytoma	EVR (Afinitor) oral tablets	2010
		Epilepsy	EVR (Afinitor DISPERZ)	2018
		Lymphangioleiomyomatosis	SRL (Rapamune)	2015
		Renal angiomyolipoma	EVR (Votubia)	2012
Cardiovascular	Vascular stenoses	Coronary artery disease; inhibition of smooth muscle cell proliferation to reduce risk of in-stent restenosis	SRL-eluting coronary stent (Cypher)	2003
			EVR-eluting coronary stents (several)	2008
			RFL-eluting coronary stent (EluNIR)	2017

US, United States; VEGF, vascular endothelial growth factor; EVR, everolimus; TSL, temsirolimus; SRL, siroliumus/rapamycin; GI, gastrointestinal; HER2, human epidermal growth factor receptor 2; mTOR, mechanistic/mammalian target of rapamycin; RFL, ridaforolimus.

1) Prevention of organ transplant rejection

Rapamycin’s immunosuppressive effects were among its earliest to be demonstrated, and approval of rapamycin for prevention of organ rejection following renal transplantation was first obtained in 1999 [60].

2) Cancer

Excessive mTORC1 activity is a hallmark of some cancers caused by gene mutations (i.e., gain-of-function mutations in oncogenes; loss-of-function mutations in tumor suppressor genes) for various upstream regulators of mTORC1. These oncogenes use the mTOR pathway to alter nutrient uptake, energy metabolism, and general metabolic reprogramming, which bestows a competitive advantage over normal cells [61]. Therefore, drugs that decrease mTORC1 activity can have anti-neoplastic properties in certain cancers.

3) Neurodegenerative disorders

Tuberous sclerosis complex (TSC) is a genetic disorder of human beings that leads to reduced absolute or functional levels of the mTOR-inhibiting proteins TSC1 and TSC2 (Fig. 1). In healthy individuals, these proteins inactivate an important mTOR cofactor (Rheb). Therefore, their absence is associated with excessive mTORC1 activity, and affected individuals can experience several neurologic (e.g., epilepsy) and non-neurologic disorders [62]. The accumulation of misfolded and aggregated proteins is a common feature of aging and several age-related neurodegenerative diseases (i.e., Alzheimer’s disease, Parkinson’s disease, Huntington’s disease), as discussed above. Although rapamycin has shown benefit (e.g., prevention or reversal of cognitive deficits, attenuated intracellular changes, improved neuronal function) in laboratory models of these conditions—likely due to its autophagy-enhancing effects—it is not currently approved for these indications [63,64,65]. At the time of writing, a small phase II clinical trial designed to evaluate the efficacy of rapamycin in people with Alzheimer’s disease is ongoing [63].

4) Drug-eluting medical devices

Rapamycin-eluting vascular stents significantly decrease in-stent (luminal) neointimal tissue proliferation and are associated with reduced rates of stent restenosis [66,67]. The first rapalog-eluting stent was approved by the United States Food and Drug Administration in 2003 [68], with others receiving approval subsequently.

Rapamycin for longevity and health span: evidence in laboratory species and people

Rapamycin’s ability to extend lifespan has been studied extensively in laboratory species. Foundational studies in yeast, nematodes, fruit flies and mice first documented significantly extended lifespan in individuals with genetic mutations leading to decreased function of the TOR complex or components downstream of TOR, compared to those without [69,70,71], and later in those treated with rapamycin [72,73]. Since the 2009 publication of findings from the first study demonstrating longevity extension in wild-type mice treated with rapamycin [72], others have consistently reproduced these results in healthy, genetically diverse laboratory mice and genetically modified mice used as models for common human diseases [46,74,75,76]. Initiation of rapamycin therapy in middle age [72] appears to be equally or nearly as effective as lifelong administration in mice, and remarkably, studies have demonstrated substantial increases (9%–14%) in lifespan after transient treatment during middle age [28,77]. Interestingly, rapamycin-treated female mice seem to experience greater increases in lifespan than rapamycin-treated male mice for reasons that are presently unclear [75,78].

As evidenced by reduction of spontaneous tumorigenesis and rate of development of detrimental age-related organ changes, rapamycin treatment has also been associated with health-span increases in laboratory species [77,79]. Improvements in cardiac, cognitive, and kidney function have also been demonstrated in aged mice and aged rats administered rapamycin [80,81,82].

Combined with the discovery that mTOR and its signaling pathways are highly conserved across mammalian species, demonstration of rapamycin’s positive healthspan and lifespan effects in rodents has opened the door to similar ongoing studies in non-human primates [83,84]. To date, there are no published studies evaluating rapamycin or its derivatives as an anti-aging intervention (i.e., with lifespan as a primary outcome) in healthy human beings. However, the results of recent human studies have suggested that low dose rapalog therapy might improve the function of the aging immune system as evidenced by improved vaccination response, or lower influenza or viral respiratory infection rates [85,86,87]. Currently, there are multiple ongoing clinical trials evaluating the effects of mTOR inhibition on age-related pathologies, including cardiac, metabolic, cognitive, and physical function in elderly humans [88,89,90].

In people receiving rapamycin or rapalogs at relatively high dosages for prevention of organ transplant rejection or for the treatment of cancer, adverse effects are noted. This has logically raised concern around the use of these drugs to promote longevity in otherwise healthy individuals. The most common adverse effects reported with clinical use of rapamycin include stomatitis, gastrointestinal upset, hyperlipidemia, impaired wound healing, and infection [91]. In mice (but not human beings), rapamycin is also known to increase cataract formation [92]. Intermittent, transient, low-dose strategies, which are associated with enhanced immune function, might avoid these side effects while conserving the lifespan-promoting effects of the drug, as discussed above.

Rapamycin in companion animals

Cats

In March 2025, a delayed-release oral tablet formulation of rapamycin (Felycin-CA1, PRN Pharmacal, USA) was conditionally approved by the United States Food and Drug Administration for the management of ventricular hypertrophy in cats with subclinical HCM. Conditional approval was granted after the results of a randomized placebo-controlled pilot field study of 43 cats with HCM suggested that once-weekly rapamycin (0.3 mg/kg PO) for 6 months was well tolerated and might prevent or delay progressive left ventricular hypertrophy [93]. A larger pivotal field trial is ongoing [94]. At the time of writing, this is the only rapamycin or rapalog formulation approved (conditionally or otherwise) for any indication in a veterinary species.

Dogs

In the 1990s, dogs were used as a human model to study the efficacy and safety of rapamycin for prevention of renal transplant rejection [95]. In more recent years, additional studies have evaluated the safety (and for some, efficacy) of rapamycin administration in dogs (Table 4).

Table 4. Summary of published studies evaluating the safety of systemic rapamycin in dogs.

Sample	Number of dogs	Age (yr)	Rapamycin dose/route	Duration of administration	Safety monitoring performed	Findings relative to potential side effects	Reference
Purpose-bred dogs with glycogen storage disease	3	0.17 (2 mon)	1 mg/kg/d PO	14 mon	Monthly serum biochemistry	No effect on biochemical parameters; normal serum triglyceride and cholesterol concentrations	Yi et al. (2014) [97]
	2	0.67 (8 mon)	0.5–1.0 mg/kg/d PO	8 mon
Purpose-bred healthy dogs	5	11.0–15.0	0.1 mg/kg/d PO	5 d	Clinical status	Vomiting (once; n = 1)	Larson et al. (2016) [96]
Client-owned dogs with osteosarcoma	22	3.7–11.9	0.01–0.08 mg/kg IM q24h	7 d	Weekly CBC, serum biochemistry, urinalysis	Thrombocytopenia (n = 1)	Paoloni et al. (2010) [99]
Healthy client-owned dogs	8	7.2–12.7	0.05 mg/kg PO, three times weekly (MWF)	10 wk	Weekly clinical status	• Similar frequencies of owner-reported clinical changes in rapamycin vs. placebo groups	Urfer et al. (2017) [103]
	13	6.8–11.5	0.1 mg/kg PO, three times weekly (MWF)	10 wk	Baseline and study end: CBC, serum biochemistry, urinalysis, TT4/fT4	• Decreased MCV vs. baseline in rapamycin-treated dogs
	8	6.6–10.4	Placebo (MWF)	10 wk		• No abnormalities or change vs. baseline in CBC/chem parameters
Healthy client-owned dogs	9	7.8 ± 1.0	0.025 mg/kg PO, three times weekly (MWF)	6 mon	Bi-weekly clinical status	• Similar frequencies of mild, transient thrombocytopenia in rapamycin and placebo groups	Barnett et al. (2023) [104]
	8	8.5 ± 1.7	Placebo (MWF)	6 mon	Baseline and at months 3, 6 and 12: CBC, serum biochemistry, urinalysis	• Severe asymptomatic hypertriglyceridemia in one rapamycin-treated dog
Client-owned dogs with osteosarcoma treated with amputation + carboplatin	92	1.5–13.0	0.1 mg/kg PO, four times weekly (4 days on + 3 days off)	Up to 4 mon	Clinical status q8w: physical exam, thoracic radiographs	Non-serious AE in 18%; most common: lethargy/fatigue (n = 7), diarrhea (n = 6), anorexia (n = 5), nausea (n = 4)	LeBlanc et al. (2021) [102]

PO, per os; IM, intramuscular; CBC, complete blood count; MWF, Monday, Wednesday, Friday; MCV, mean corpuscular volume; AE, adverse events.

In two laboratory studies, orally administered rapamycin at dosages of up to 1 mg/kg/day for up to 14 months were reportedly well-tolerated by a small number of dogs, with no significant changes in surveyed biochemical parameters [96,97].

Three studies have evaluated the pharmacokinetics of rapamycin in dogs. In the first, the single- and multiple-dose pharmacokinetics of short-term, orally administered rapamycin (0.1 mg/kg/day once or for 5 consecutive days) in healthy dogs were reported [96]. Measured ranges of maximum plasma concentration (C_max) values (5.86–10.6 ng/mL and 3.70–8.18 ng/mL after 1 or 5 doses, respectively) were within the range (< 10 ng/mL) of those associated with anti-tumor, but not immunosuppressive, properties in mice [98]. The second study evaluated safety and pharmacokinetic and pharmacodynamic parameters in a cohort of client-owned dogs with appendicular osteosarcoma [99]. Rapamycin was well-tolerated when administered intramuscularly at doses of up to 0.08 mg/kg/day for 1 week, and pharmacokinetic exposures were similar to those known to be therapeutic in human cancer patients. These findings supported the safety and antineoplastic therapeutic potential of rapamycin at the doses evaluated. A subsequent small study [100] treated four healthy companion dogs with a low, less frequent dosage (0.025 mg/kg PO three times weekly) for a longer period (1–5 months). Blood rapamycin concentrations (median [min–max] C_max, 1.47 ng/mL [0.9–2.1 ng/mL]) in those dogs were within the range previously shown to inhibit mTOR [98] but were substantially lower than the concentrations (8–15 ng/mL) targeted for therapeutic immunosuppression in human organ transplant recipients [101]. These results provided important information to guide rational dosing in subsequent studies.

To date, the results of three randomized controlled trials (RCT) of orally administered rapamycin in client-owned dogs have been published: one evaluating dogs with cancer [102], and two with the purpose of establishing safety of low-dose rapamycin in healthy, client-owned dogs [103,104]. In the largest, dogs with appendicular osteosarcoma underwent limb amputation and carboplatin chemotherapy, with (n = 92) or without (n = 149) rapamycin (0.1 mg/kg PO, four times per week). Dogs treated with rapamycin tolerated it well, with non-serious adverse events recorded in 18% of this sample.

In the first of two RCT of rapamycin in healthy middle-aged dogs, participants were treated with oral rapamycin at a dosage of 0.05 mg/kg or 0.1 mg/kg three times per week (i.e., on Monday, Wednesday, and Friday) or a placebo, for 10 weeks [103]. In a second, longer study, dogs were given oral rapamycin at an even lower dosage (0.025 mg/kg PO three times weekly) or a placebo, for a total of 6 months [104]. In rapamycin-treated dogs of both studies, the incidence of adverse events was low and comparable to that of the placebo groups. No clinically meaningful changes to biochemical or hematologic parameters were noted in the first study. In the second, at least one instance of mild, transient thrombocytopenia was noted in 5 rapamycin-treated and 4 placebo-treated dogs. The same numbers in each group also had at least one instance of visible lipemia during the study, and one rapamycin-treated dog developed a severe asymptomatic hypertriglyceridemia that resolved within 15 days of drug discontinuation [105]. The results of these studies strengthened the notion that low-dose rapamycin can be administered to apparently healthy companion dogs for several months with a low risk of adverse effects.

Along with evaluating safety, the two RCT of healthy dogs mentioned above also measured several echocardiographic parameters before and after rapamycin treatment [103,104]. In the first, dogs treated with rapamycin (0.05 or 0.1 mg/kg; n = 21) had a significantly greater increase in left ventricular fractional shortening (a measure of systolic performance) compared to placebo-treated dogs (n = 8), suggesting that mTOR inhibition might improve or prevent decline of cardiovascular function in this species [1,103]. This finding was not repeated in the subsequent longer-term study, in which there were no significant differences in any echocardiographic parameter between rapamycin- (n = 9) or placebo- (n = 8) treated dogs [104].

Ongoing efforts: the test of rapamycin in aging dogs (TRIAD) study

Several of the studies of dogs presented above were performed by or in conjunction with the Dog Aging Project (DAP), a long-term, longitudinal, community science, open-data project of United States companion dogs and their owners [7]. The scientific goals of the DAP are to define normal aging, to identify genetic and environmental determinants of morbidity and mortality, and to increase duration of healthy lifespan in companion dogs, among others [7]. In 2021, the DAP launched the TRIAD RCT [106], which will be the first large-scale study to test the ability of rapamycin to prolong lifespan in a non-laboratory species.

The TRIAD RCT is a prospective, randomized, double-masked, placebo-controlled parallel group study evaluating client-owned dogs aged 7 years or older that are of typical health status for their age. Participants are randomized in a 1:1 ratio to receive rapamycin (0.15 mg/kg PO) or a placebo once weekly for 12 months, followed by a 24-month post-treatment monitoring period. The study’s primary objective is to determine whether low-dose, intermittent administration of rapamycin increases lifespan in companion dogs. Secondary objectives include evaluation of rapamycin’s effects on healthspan as assessed by functional measures (i.e., physical function, neurological and cognitive status, cardiovascular health), and evaluation of treatment safety. Every 6 months for the duration of the 3-year study, participating dogs are evaluated at a specialty cardiology or specialty neurology clinic, where they undergo comprehensive assessment of cardiovascular function (approximately one-half of all dogs) or neurocognitive function (approximately one-half of all dogs), respectively. Adverse events are closely monitored at in-hospital visits and through regular at-home electronic observational surveys. A total of 580 dogs (n = 290 rapamycin-treated, n = 290 placebo-treated) will be enrolled and randomized, and for the primary outcome measure of survival time from randomization, the trial will have > 80% power to detect a 6% or greater difference in mean lifespan between treatment groups. At the time of writing, 210 dogs have been enrolled and randomized into the TRIAD RCT.

The results of the TRIAD RCT will provide direct information about the safety and potential use of low-dose rapamycin to extend healthspan and lifespan, and to prevent or delay the onset of age-related pathologies in dogs. In addition, given the companion dog’s role as a promising translational geroscience model, positive outcomes from the TRIAD RCT would provide justification for similar trials in people.

Future directions

Studies evaluating the safety, efficacy and optimal dosing strategy of rapamycin or rapalogs in dogs, cats, and human beings are necessary, especially as off-label mTOR inhibition becomes increasingly popular. Even if proven useful by the TRIAD RCT, the optimal rapamycin dosing protocol—one that delays age-related conditions and maximizes healthspan prolongation while minimizing side effects—will still need determination [107].

While over 100 clinical trials of rapamycin use in people are ongoing, to the authors’ knowledge, none plans to evaluate lifespan as a primary outcome. Should the TRIAD RCT prove rapamycin’s efficacy in prolonging lifespan in dogs, subsequent trials in people can be considered to determine translational relevance.

Finally, the development of mTORC1 specific inhibitors that work (for example) by decreasing or increasing FKB12 or FKB51 affinity, respectively, might allow for safer chronic use of rapamycin as a therapeutic drug or as a longevity modulator with decreased risk of mTORC2 related adverse effects [13]. Recently discovered novel mTOR-inhibitors, including piperazine-based compounds, hold potential promise as specific, dose-dependent inhibitors of mTORC1 [108,109].

DISCUSSION

Rapamycin represents a promising intervention by which to manipulate the so-called “Hallmarks of Aging” [2] and, in doing so, to improve health span and extend longevity. The results of ongoing and previous studies evaluating the effects of mTOR inhibition in non-laboratory species, both for disease treatment and longevity promotion, will allow us to better understand aging processes and clinically meaningful ways to leverage the relevant pathways.