First-generation senotherapeutics and their activities are summarized in Table 1 and described in further detail here.

Dasatinib is an FDA approved tyrosine kinase inhibitor used to treat chronic myeloid leukemia and acute lymphoblastic leukemia, as well as refractory non-Hodgkin lymphoma, metastatic gastrointestinal stromal tumors, and metastatic prostate cancer in humans (64, 65). Dasatinib functions as a senotherapeutic by interfering with ephrin ligand dependent suppression of apoptosis (30–32). When administered, dasatinib decreases viability of senescent human preadipocytes by 30–40% (30) at concentrations of 50 nM and murine embryonic fibroblasts (MEFs) at concentrations of 250 nM (30). However, with an apparent cell line specific effect, dasatinib has not demonstrated efficacy against senescent human umbilical vein cells (HUVECs) nor senescent bone marrow-derived mesenchymal cells (30–32), which prompted evaluation of co-administration with other senotherapeutics, such as quercetin, to enhance efficacy.

Quercetin is a naturally occurring flavonoid found in certain fruits, vegetables, onions, and kale. Quercetin inhibits kinases and serpines integral to modulating cell growth and arrest such as phosphoinositide 3-kinases (PI3K) (46, 47). Clinically, quercetin alone has cardioprotective effects (66, 67) by reducing obesity, restoring plasma thyroid hormone levels, mitigating cardiac oxidative stress (66, 68), and promoting angiogenesis (69). ApoE-/- hypercholesterolemic mice that were treated with quercetin had a significant reduction in left ventricular (LV) hypertrophy (70). Quercetin enhanced antioxidant defenses and improved cardiac bioenergetics in rats that were fed a high fat diet (67). Likewise, quercetin attenuated oxidant-induced endothelial dysfunction in high cholesterol mice fed a high fat diet by enhancing nitric oxide bioavailability (71). Mouse atherosclerosis studies have shown variable pro-inflammatory and non-specific systemic effects of both genetic and pharmacological senolysis (72). While quercetin has demonstrated efficacy to induce senolysis on senescent HUVECs, MEFs, and BM-MSC in vitro, but with limited significant therapeutic effect on preadipocytes (30, 46, 47), demonstrating an efficacy profile opposite that of dasatinib. Therefore, quercetin has been investigated further in combination with dasatinib (3, 14, 30, 73–80).

The combination of dasatinib and quercetin has proven effective in mice at a dosage of 5 mg/kg of dasatinib and 50 mg/kg for quercetin (30, 81), and clears from systemic circulation within 48 h after the last dose (82, 83).

Relevant to cardiovascular disease, dasatinib and quercetin has demonstrated significant improvements in vascular smooth muscle sensitivity to nitroprusside and an enhanced ventricular left ejection fraction in mouse models of cardiac disease (84). This senotherapeutic combination also activates cardiac progenitor cells following treatment in a mouse model of cardiovascular disease (85). Both aged mice and hypercholesterolemic mice treated with dasatinib and quercetin had significant reductions in telomere associated foci, staining of aortic calcification, and osteogenic gene and protein expression (86). Another study demonstrated that dasatinib and quercetin treatment not only reduced SC numbers, but also promoted cardiomyocyte regeneration in aged mice (85). In an arteriovenous fistula chronic kidney disease mouse model, dasatinib and quercetin treatment significantly reduced p16^Ink4a venous expression, indicating reduction in SC (81). Pre-treatment with this senotherapeutic combination has also been shown to reduce the quantity of SC in arterial walls and lessened the severity of abdominal aortic aneurysms in mice given angiotensin II (87).

Relevant to obesity, evaluation in mouse models have indicated that the combination of dasatinib and quercetin reduces inflammation, alleviates metabolic dysfunction pre-adiposity, and facilitates the differentiation of adipose cells into mature, insulin-responsive ones (88). Aged mice treated with dasatinib and quercetin had decreased SA-ß-Gal, p16 and p21 expression in white adipose tissue. Dasatinib and quercetin also suppressed age-related increases in pro-inflammatory SASP genes (MCP-1, TNA-α, IL-1α, IL-1β, IL-6, CXCL-2, and CXCL-10) (76). That study also demonstrated improved fasting blood glucose, glucose tolerance, reduced plasma triglycerides, and improved systemic lipid tolerance in old mice treated with dasatinib and quercetin (76). Human adipocytes treated with dasatinib and quercetin demonstrated reduced insulin resistance following xenotransplanation. Because insulin resistance is a risk factor of type 2 diabetes, these findings indicate that senotherapeutics may offer a new treatment avenue to target adipocytes expressing elevated p21^Cip (89).

With respect to neurologic disease, mouse models of Alzheimer's disease have found that Tau-containing neurofibrillary tangles display senescent phenotypes with increased CDKN2A expression and brain atrophy (90). When 23-month-old tau transgenic mice were treated with dasatinib and quercetin, neurofibrillary tangle burden, ventricular enlargement, and neurodegeneration was significantly reduced (90). Furthermore, treatment with dasatinib and quercetin in a mouse model of Alzheimer's showed reduction of SC in amyloid-β plaques, reduced neuroinflammation (IL1-β, IFN-γ and TNFα), and significantly improved cognitive function (91). Another study investigating dasatinib and quercetin to treat age-related cognitive decline in male Wistar rats found that this therapeutic combination alleviated learning deficits, memory impairment, and markers of peripheral inflammation (IL-1α, IL-β, IL-4, IL-2, IL-10, MCP-1 and TNF-α) (92). Furthermore, these differences were associated with cellular changes to dendritic spine morphology, specifically hippocampal CA1 neurons and molecular alterations of histone H3 trimethylation at lysine 9 and 27 (92). While quercetin alone was found to ameliorate the progression of intervertebral disc disease in mice via the Nrf2/NF-κB axis (93), the combination of dasatinib and quercetin have shown decreases in p16^INK4a, p19^ARF, IL-6, and MMP13 expression while preserving cell viability (94). The neuroprotective effects of dasatinib and quercetin have also been demonstrated in a controlled cortical impact mouse model of traumatic brain injury over the time of 4 months (95). Treatment with dasatinib and quercetin significantly reduced SASP pro-inflammatory factors, IL-1β, IL-6, and attenuated neurodegeneration in mice with cortical impact traumatic brain injury (95). Furthermore, clinical behavior testing of treated mice revealed significantly improved spatial reference memory and improvement in depression-like behavior (95). Following treatment with dasatinib and quercetin, obese mice showed improved neurogenesis and a significant decrease in anxiety-like behavior (96). Specifically, dasatinib and quercetin treatment increased Nestin-positive neuronal precursor cells, double-courtin positive immature neurons, and CD133 positive ependymal cells (96).

Relevant to pulmonary disease, in a mouse model of idiopathic pulmonary fibrosis, dasatinib and quercetin treatment significantly improved lung compliance (97). This was further supported by a study evaluating dasatinib, quercetin, or dasatinib and quercetin in a mouse model of hyperoxia induced airway smooth muscle senescence (98). Mice treated with quercetin (25 mg/kg) had improved pulmonary compliance but not resistance, while dasatinib (1 mg/kg dasatinib) improved both compliance and resistance (98). To model premature neonatal pulmonary disease, fetal smooth muscle explants exposed to moderate hyperoxia (40% O₂) for 7 days display significant increases in senescent markers such as SA-ß-Gal, p16, p21, p53, and the DNA damage marker gamma-H2AX (99). Following treatment with dasatinib and quercetin, there was a reduction in cells expressing SA-ß-Gal, p21, p16, and phosphorylated γ-histone family member X (99).

With respect to renal disease, treatment with dasatinib and/or quercetin reduced renal tubular senescence and ameliorated renal fibrosis in multiple mouse models of acute kidney injury (100). In models involving a high-fat diet and renal fibrosis, treatment quercetin alone decreased SC burden, reduced protein secretome markers (p16, p19, and p53), and improved renal function shown by a decreased plasma creatinine (101).

Relevant to orthopedic disease, one study found that treatment with dasatinib and quercetin decreased osteoclastic activity while promoting osteoblastic differentiation in mouse models of osteoporosis (102). Conversely, in another mouse model (accelerated aging Z24–/– model) dasatinib and quercetin treatment did not mitigate trabecular bone loss (103) while another study indicated dasatinib and quercetin treated mice may actually improve bone production of aged bone marrow derived mesenchymal stem cells (104). Dasatinib and quercetin treatment have facilitated growth of aged muscles through SC clearance and altered Igf1, Ddit4, Mmp14 gene expression in a mouse model of blunted muscle hypertrophy (105).

Finally, dasatinib and quercetin have been investigated in several other unrelated disease processes in rodent models. In a murine model of sclerodermatous graft vs. host disease, the administration of a combination of dasatinib and quercetin led to a reduction in peripheral senescence-associated secretory phenotype (SASP) cytokines, specifically IL-4, IL-6, and IL-8Rα (106). In a mouse model of doxorubicin-induced ovarian injury, both dasatinib and quercetin and fisetin treatments reduced SC presence, however treatment was unable to restore normal ovarian function (107). A study conducted in aged mice revealed that dasatinib and quercetin treatment significantly decreased SC markers p16 and p21 expression, as well as the expression of inflammatory markers Cxcl1, Il1β, Il6, Mcp1, and Tnfα in the small and large intestine (108). Notably, dasatinib and quercetin treatment also induced alterations in specific microbial signatures across ileal, cecal, and colonic regions, and within feces.

Fisetin is a natural flavonoid with several reported modes of action that target the anti-apoptotic pathways of senescent cells, including BCL-2/BCL-xL inhibition (48, 109) and induction of hypoxia–inducible factor−1α (HIF-1α) (3, 110). Promoting apoptosis through increased caspase activity (48, 49), fisetin successfully targeted senescent HUVECs (48). Studies have shown that treatment with 500 nM fisetin induces apoptosis in senescent cells derived from subcutaneous adipose tissue (48) and that the effect is larger in adipose cells expressing p16^Ink4a and lower in p21^CIP1-expressing cells (49).

In vivo, fisetin has been studied at dosages ranging from 60 to 100 mg/kg, administered orally for up to five consecutive days, exhibiting efficacy in reducing the number of senescent cells in white adipose tissue. One study evaluated the impact of fisetin on neuronal cellular senescence in a randomized controlled trial using aged sheep (111). There was a significant decrease in senescent neurons, astrocytes, and microglia in the neural tissue of sheep treated with fisetin (111). The treatment group also exhibited decreased senescent mRNA expression of SA-ß-Gal in the lung, heart, and spleen. Furthermore, the senescent marker p21 was also decreased in the liver and lung, and inflammatory markers in the lung, liver, heart and spleen following treatment (111).

A mouse model of systemic lupus erythematosus found that senescent neural cells accumulate in a hippocampal region of the brains of mice with depressive behavior (112). Oral administration of fisetin successfully reduced the number of senescent neural cells, reduced depressive behavior and limited SASP factors in the hippocampal region (112). Mouse models of chronic wounds also support a potential benefit to the senotherapeutic fisetin in chronic wound management and elimination of dermal SC (113). Targeting SC, treatment with fisetin improved muscle function and myogenic phenotypes in a mouse model of muscular dystrophy (114). Conversely, in a mouse model of chronic inflammatory myopathy, a benefit to senescent fibro-adipogenic-progenitor cells was demonstrated in mitigating exercise-induced muscle degeneration through pro-inflammatory regenerative mediators while mice without senescent adipogenic-progenitor cells exhibited muscle degeneration, which warrants further investigation (115).

Navitoclax (ABT263) is a BCL-2 inhibitor with demonstrated efficacy to reduce the number of senescent bone marrow hematopoietic stem cells and muscle stem cells (50). Navitoclax reduced the senescent cell burden in HUVECs, IMR90 human lung fibroblasts, and MEFs by targeting BCL-2 proteins (116). However, navitoclax has reported side effects of neutrophil toxicity and thrombocytopenia (50, 117, 118). To mitigate these adverse effects, galacto-conjugation of navitoclax has shown a significant reduction in platelet toxicity and increased senotherapeutic specificity in both human blood samples and a murine lung cancer model (119). Explant studies have also shown that human pulmonary endothelial cells from patients with pulmonary arterial hypertension have a significant increase in SC burden and these cells undergo apoptosis when exposed to navitoclax in vitro (120).

Relevant to cardiovascular disease, mice with angiotensin II or doxorubicin induced heart failure had decreased cardiac fibrosis, hypertrophy, improved cardiac function and decreased inflammation following treatment with navitoclax (121, 122). Aged mice treated with navitoclax had a significantly reduced number of telomere associated foci, fibrosis and reduced SC, but there was no difference in cardiac ejection fraction or left ventricle mass compared with controls (123). These findings were also supported in a mouse model of ischemia-reperfusion cardiac injury, where mice treated with navitoclax had significant reduction in pro-inflammatory, profibrotic, and anti-angiogenic cytokines (interferon gamma-induced protein-10, TGF-β3, interleukin-11, interleukin-16, and fractalkine) (124). Senotherapeutic clearance of β-cells in obese metabolically dysregulated transgenic mice treated with navitoclax had decreased SASP factors, increased glucose tolerance and increased β-cell metabolism. This study suggests that pancreatic β-cell senescence may also play a role in peripheral insulin intolerance and predisposition to type 2 diabetes (125).

With relation to age related neurodegenerative disorders, navitoclax treated mice had increases in neurogenesis of hippocampal neuronal precursors and increases spatial memory (126). Cellular senescence also plays a role in chronic skin diseases like scleroderma (127–130) and psoriasis (131, 132) by promoting inflammation, fibrosis, and tissue dysfunction. One study used young and old hairless mice treated with navitoclax showed selective clearance of senescent dermal fibroblasts (133). Furthermore, aged mouse skin treated with navitoclax had increased collagen density, epidermal thickness, proliferation of keratinocytes, and decreased SASP factors such as MMP-1 and IL-6 (133).

Relevant to pulmonary disease, a mouse model of idiopathic pulmonary fibrosis found that radiation induced pulmonary fibrosis could be reversed by the clearance of senescent type II pneumocytes with Navitoclax treatment (134). However, secondary effects following clearance of heavy SC burden in chronic disease have been described, such as vessel remodeling, increased right ventricular systolic pressure, and increased cardiac hypertrophy index in rodents following navitoclax treatment (24). These findings indicate that consideration should be given to the disease severity and potential for systemic reaction following senotherapeutic treatment, particularly in more chronic disease processes. With respect to renal disease, navitoclax has been shown to specifically target senescent proximal tubular epithelial cells and resulted in improved renal function with decreased renal fibrosis demonstrated in mice (135).

The effects of navitoclax on clinical bone health are mixed when it comes to osteoporosis and bone loss with age. One study indicated that navitoclax decreased the senescent cell burden and decreased the trabecular bone in aged mice by up 60.1% in females (136). With increased cytotoxicity, use of navitoclax in vivo should be carefully considered.

Luteolin is a flavonoid found in celery, broccoli, dandelion, carrots, and olive oil (137) that exerts senotherapeutic properties through modulation of sirtuin 1 (SIRT1) and p53. Luteolin has largely been studied in mouse auditory cells (House Ear Institute-Organ of Corti 1) with 50 % efficacy in reversing senescence induced with hydrogen peroxide, although it appears less effective in reducing senescence in MEF (49, 51, 52). Through the upregulation of SIRT1, luteolin effectively protects against senescence induced by hydrogen peroxide (52). This mode of action was confirmed when the knockout of SIRT1 resulted in induced senescence. Luteolin also protected cells from peroxide induced senescence by decreasing p53 phosphorylation and p21 expression (52).

Curcumin is a senotherapeutic that selectively targets apoptotic pathways such as nuclear factor NF-kB, mitogen-activated protein kinases (MAP-kinase), p53, nuclear factor erythroid 2-related factor 2 (NRF2), AKT, COX-2 and EGFR (49, 53, 54), although its effect is relatively weak relative to other senotherapeutics due to limited bioavailability (54, 138). When attempting to concentrate curcumin above 10 μM, it exhibits genotoxic and cytotoxic effects. To mitigate these toxicity risks, a curcumin analog EF24 has shown promise against SC through the proteasomal degradation of Bcl-2 family proteins and production of reactive oxygen species (ROS), although further investigation is indicated (55).

The compounds A1331852 and A1155463 are both BCL-XL inhibitors, but with a lower relative risk of BCL-2 mediated neutrophil toxicity compared to navitoclax (48, 117). Treatment with A1331852 and A1155463 resulted in apoptosis, shown through enhanced caspase3/7 activity, of senescent HUVECs and IMR90 cells, but not preadipocytes (48). Further mechanistic insights of A1331852 have shown caspase-dependent apoptosis of senescent chondrocytes and increased expression of a pro-apoptotic Bcl-2 family member called BAK (139). Utilizing live cell fluorescence resonance imaging, A1331852 was shown to interfere with binding of BCL-XL (139). In vivo, treatment of genetically modified mice with A1331852 resulted in clearance of 80% of senescent cholangiocytes, reduced expression of fibrosis-inducing growth factors, and subsequent reduction in liver fibrosis (140).

Heat shock protein 90 (HSP90) inhibitors influence protein stability and function, impacting p53′s ability to regulate apoptosis and DNA repair (141). Geldanamycin and tanespimycin (17-AAG) reduce SC viability, although geldanamycin in not particularly water soluble, while alvespimycin (17-DMAG) is (53). The targeted effects of geldanamycin and Tanespimycin are specific to HSP90. All HSP90 inhibitors have a dose-dependent senotherapeutic effect that is not cell type-specific (53). The in vivo effect of alvespimycin treatment in age related symptoms in mice, resulted in a significant reduction in kyphosis, dystonia, tremor, loss of forelimb grip strength, coat condition, ataxia, gait disorder, and overall body condition when compared to sex matched untreated mice (53). Similarly, azythromycin has been briefly studied for its ability to reduce senescent human fibroblasts by 97% (142). However, many of the senotheraputics in the HSP90 inhibitor class were initially developed and FDA approved for their antimicrobial action. Therefore, selection of these senotherapeutics must be made with antimicrobial stewardship in mind.

Piperlongumine is a therapeutic agent often paired with chemotherapeutics because of its established pro-apoptotic properties (143). The precise mechanism of piperlongumine in unknown (56). It was previously thought that piperlongumine promoted the production of reactive oxygen species; however, it has since been proven to be an ROS-independent mechanism (56). Piperlongumine has been shown to promote caspase activity and kill senescent human WI-38 fibroblasts (56). Piperlongumine, has been assessed in an ex vivo goat osteoarthritis model, demonstrating decreased p53 and p16 gene and protein expression in senescent chondrocytes in a concentration-dependent manner following treatment (57). Furthermore, piperlongumine treatment rescued the oxidative stress cause by IL-1β in cartilage explants, indicating that it has potential benefit to rescue senescent chondrocytes in OA (57).

The FOXO-related peptide was engineered to be a permeable peptide in p53-interaction domain in FOXO4. Treatment with this synthetic peptide induces apoptosis in senescent fibroblasts through the nuclear exclusion of p53 (58). In a concentration-dependent manner, FOXO-related peptide reduced the viability of senescent by 11.73-fold compared with control IMR90 cells (58). Importantly, the targeted design of FOXO-related peptide allows it to be safe to normal cells. In vivo, the FOXO-related peptide restored fitness, hair density, and renal function in aged mice (58).

Nutlin-3a acts as an inhibitor of MDM2, a ubiquitin ligase responsible for downregulating p53 (59). Interestingly, Nutlin-3a has been described as both a senotherapeutic (60, 144) and a senescence inducing agent (59, 145, 146). Originally studied for its anti-cancer properties, Nutlin-3a has proven effective in inducing apoptosis in carcinomas (147, 148), melanomas, T-cell lymphoma (149) and adult T-cell leukemia (145). The senescence inducing properties of Nutlin-3a have been described in normal human and mouse fibroblasts (146, 150), non-small cell lung cancers (151), adult T-cell leukemia cells (145), cutaneous T-cell lymphoma (149), glioblastomas (152), renal carcinoma (153). It is important to note that a dose dependent cytotoxic effect has been described in both senescent and non-senescent melanoma cells at concentrations ranging from 2.5 to 10 μmol/L in vitro (60). Further studies are needed to evaluate the pharmacodynamics and pharmacokinetics of Nutlin-3a in different cellular phenotypes. In vivo, The senotherapeutic nutlin-3a has been investigated as a treatment for age-related macular degeneration (144). Treatment of a mouse model with nutlin-3a showed a significant recovery of visual function (144) and ameliorated retinal degeneration (154).

Another previously established drug class, cardiac glycosides has recently been described for their senotherapeutic properties (61, 62, 155). This class of drug is FDA approved for the treatment of heart failure and arrythmias such as atrial fibrillation (156, 157). Acting with Na+/K+ ATPase pump inhibition, cardiac glycosides target SCs with higher H+ concentrations and slightly depolarized membranes (62, 155). On the in vitro scale, the cardiac glycoside digoxin had significant senotherapeutic activity against A549 tumor cells, primary human BJ fibroblasts, and osteoarthritic chondrocytes (62). However, efficacy in mouse embryo fibroblasts was not seen (62). Treatment with Digoxin had significant senotherapeutic activity in a mouse model of IPF (62). However, research on the senotherapeutic uses of cardiac glycosides is still in its infancy and further investigation is needed to determine efficacy against different senescent phenotypes in vivo.

The non-steroidal anti-inflammatory, aspirin, has an established repertoire in reducing endothelial senescence (158–160). Recently, aspirin has been investigated for its senotherapeutic abilities to ameliorate the long term effects on patients that have received chemotherapy and radiation (63). This study found that aspirin suppresses p53 and p21 accumulation in doxorubicin induced senescent human fibroblasts and murine embryonic fibroblasts (63). Cyclooxygenase 2 (COX2) knockout mouse embryonic fibroblasts that underwent the same treatment had a significant reduction in p53 accumulation (63). This data suggests that aspirin's senotherapeutic activity is through the inhibition of COX2. In vivo, aspirin treatment significantly reduce amyloid-β₄₂ induced senescent neuronal cells by upregulating sirtuin-1 (SIRT1), a key regulator in cell aging (161). Aspirin has also demonstrated senotherapeutic potential in doxorubicin-treated mouse models through the reduction of SA-ß-Gal staining in liver, spleen, pancreas, and lung tissues when compared to controls (63).

Following the identification of readily available “first generation” senotherapeutics, the so-called “second generation” of these drugs have been more recently identified and engineered compounds (Table 2). The information available pertaining to preclinical evidence for their application in veterinary medicine is summarized below.

The development of oligosaccharide coated nanoparticles containing drug encapsulated beads such as doxorubicin or navitoclax are capable of inducing apoptosis in senescent cells (162). Direct targeting through endocytosis allows these drugs to be delivered intracellularly with minimal reported systemic side effects to date (162).

Senotherapeutic vaccinations have evolved at the intersection of oncology and immunotherapy. Chemotherapy-induced tumor senescence has been demonstrated to limit further tumor growth and can allow for immunomodulation through vaccination against the static tumor cell type, leading to the development of senotherapeutic vaccination (163–166). A murine model of senescence-related aging implemented a CD153 vaccine to effectively clear senescent T-cells (164). In another mouse model, transcriptomic analyses were used to evaluate vascular endothelial cells for senescent cell markers which identified transmembrane glycoprotein nonmetastatic melanoma protein B (GPNMB) as a sero-antigen candidate (165). Following vaccination with GPNMB, improvement in aging phenotypes and male mouse lifespans were noted (165). While these targeted approaches are promising, additional preclinical modeling and Phase 1 clinical testing are needed to ensure their safety and efficacy (164–168).

Recent attention has focused on chimeric antigen receptor T cell (CAR T cell) modulated treatments targeting SC accumulation. The urokinase-type plasminogen activator receptor (uPAR) was identified as an expressed SC surface antigen, making it a potential target for immunomodulatory senotherapeutics (169). Treatment with uPAR-specific CAR T cell therapy demonstrated efficacy in reducing liver fibrosis and improving treatment outcomes in mouse models of lung adenocarcinoma, which warrants further investigation (169).

To highlight a few disease processes for example, osteoarthritis (OA) is found to be a common cause of pain and lameness in horses (186, 187). Approximately 33% of the equine population overall in the US is estimated to be affected by OA (188), and that prevalence climbs to 50% at 15 years of age and 80 to 90% in horses over 30 years of age (189–191). Therefore, OA risk is highly associated with aging, which has been similarly reported in humans (175, 176). In conditions such as osteoarthritis, rheumatoid arthritis, and age-related frailty, senescent cells can accumulate in musculoskeletal tissues. Transplantation of SC has precipitated an OA-like phenotype in mice when compared to transplantation of non-SC (192). A recent study evaluating synovial fluid in healthy horses and horses with OA found an increased number of senescent mesenchymal stem cells (193). The senescent cells displayed impaired chondrogenic differentiation (193) when compared with a non-senescent cell population, making them potential targets for senotherapeutics. Murine models of rheumatoid arthritis (RA) found that dasatinib was protective against RA by inhibiting osteoclastogenesis through immunomodulatory effects (194). Additional murine studies have shown that SC interact with synovial cells and that dasatinib and/or quercetin have been effective in ameliorating cartilage damage and pain due to OA, as well as alleviating post-menopausal osteoporosis (73, 75, 195, 196). These findings collectively support further investigation of senotherapeutics as a potential disease modifying treatment in OA.

Senescent cells have been found to accumulate in adipose tissue which is associated with chronic low-grade inflammation and insulin resistance (17, 76, 197–200). Senotherapeutics have been shown to reduce senescent cell burden in diet-induced obesity, to alleviate metabolic dysfunction, and to restore the capacity of preadipocytes to differentiate into functional insulin-responsive fat cells (88). Obesity in companion animals is prevalent in the US (e.g., 20–45% of riding horses) and has increasingly been recognized as a factor associated with insulin resistance and OA progression. Implementation of senotherapeutics in the population of horses commonly treated for OA has potential benefit to simultaneously alleviate multiple co-morbidities (2, 201–203). Displaying an age-related phenotype, metabolic dysregulations may precipitate insulin resistance (204, 205). Hyperinsulinemia induces human hepatocyte senescence (206), which has been shown to be attenuated by dasatinib and quercetin. Interestingly, increased cellular senescence has also been observed in equine adipose-derived stem cells in horses with equine metabolic syndrome and is associated with impaired antigen stability and clonogenic potential (207). SC have altered metabolism compared to non-SC (208) and likely contribute to impaired fat metabolism and insulin resistance. Elimination of these cells may aid in healthy weight loss and return to metabolic homeostasis, potentially improving treatment of insulin resistance and associated disease processes (76).

Senescent cells have been found to contribute to the progression of atherosclerosis, arterial stiffness, and heart failure in human cardiovascular disease (120, 122, 209–211). The risk of heart disease increases with age and precipitates hemodynamic instability. Senotherapeutics have been investigated in canine myxomatous mitral valve disease (212). Valve interstitial cells that were treated with quercetin or quercetin plus dasatinib showed a decrease in SC and p53 expression (212). The decrease in SC is achieved through PI3K/AKT/mTOR antagonism which facilitates the reversal of myofibroblast senescence and promoting autophagy (213). Therefore, current therapeutic approaches for mitral valve disease could be expanded to include senotherapeutics which may slow disease progression (212).

Senescent cells can accumulate in renal tissue, contributing to inflammation, fibrosis, and impaired function (100, 135, 214). Eliminating dysfunctional cells may reduce inflammation, decrease fibrosis, slow down the progression of kidney damage, and preserve renal function (100, 214). Spontaneous chronic kidney disease (CKD) is well-described in cats (182–185). Feline patients suffering from CKD have renal senescence, telomere shortening, and nitrosative stress in renal cells (215). With similar pathologic findings to humans (216, 217), they represent a naturally occurring model for study of senotherapeutics (215). In vitro studies utilizing feline renal cells have shown a dose dependent correlation with radiation and SC (218). Further studies can utilize this data to understand the etiopathology of radiotoxicity induced renal senescence and investigation of senotherapeutics in vivo in this context (Figure 4).

Despite the potential benefits, there are prominent limitations that call for collaborative research to fully understand the breadth and depth of senotherapy in veterinary species. Studies in laboratory species and human clinical trials are still examining the full extent of side effects and long-term effects of senotherapeutics. It remains crucial to also examine the impact this drug class has on healthy cells and to not lose focus on the unknown effects on cell and tissue homeostasis. This raises concerns for off-target effects and toxicity in veterinary species since there are metabolic nuances depending on which species is studied. SC have different markers and characteristics depending on the tissue and the cause of senescence. The heterogeneity in SC makes it difficult to develop universally effective protocol, especially when considering effective dosing across species. Lastly, it is important to recognize that senescent cells could become resistant to these therapies. Similar with anthelmintic use and antibiotic stewardship, judicious use of medication relies on accurate diagnostics, clinical monitoring, and continued research.