Introduction

The discovery of rapamycin represents one of the most remarkable translational success stories in modern pharmacology. Originally isolated in the 1970s from Streptomyces hygroscopicus recovered from soil samples collected on Rapa Nui (Easter Island), rapamycin was first recognised for its antifungal properties before its profound immunosuppressive and antiproliferative activities became apparent [1].

Subsequent research identified the mechanistic target of rapamycin (mTOR) pathway as its principal molecular target. The mTOR signalling network is now recognised as a master regulator of cellular metabolism, growth, proliferation, autophagy, senescence, and survival (Figure 1). Dysregulation of this pathway contributes to numerous pathological conditions, including cancer, tuberous sclerosis complex (TSC), lymphatic disorders, metabolic diseases, and age-related degeneration [2].

The increasing recognition of mTOR as a central biological hub has substantially expanded the therapeutic scope of sirolimus. Today, sirolimus occupies a unique position at the intersection of immunology, oncology, pulmonology, vascular medicine, and translational geroscience.

Molecular mechanisms and pharmacology

Sirolimus exerts its biological effects through binding to the intracellular immunophilin FK-binding protein 12 (FKBP12). The resulting FKBP12-sirolimus complex inhibits mTOR complex 1 (mTORC1), thereby suppressing downstream signalling pathways involved in protein synthesis, cell-cycle progression, and cellular proliferation [3].

Unlike calcineurin inhibitors such as cyclosporine and tacrolimus, which suppress cytokine production, sirolimus inhibits cytokine-driven lymphocyte proliferation. This distinction provides a mechanistic rationale for its use in transplantation and diseases characterised by pathological cellular growth.

The effects of mTOR inhibition extend beyond immune modulation. Experimental studies demonstrate regulation of the following: autophagy, mitochondrial function, angiogenesis, stem-cell maintenance, cellular senescence, and metabolic homeostasis.

These pleiotropic actions explain the increasingly diverse clinical applications of sirolimus.

Pharmacokinetically, sirolimus exhibits variable oral bioavailability and undergoes extensive metabolism through cytochrome P450 3A4 and P-glycoprotein pathways [4]. Its prolonged elimination half-life of approximately 60 hours permits once-daily administration but necessitates careful therapeutic drug monitoring due to substantial interindividual variability.

Sirolimus in transplantation medicine

Organ transplantation remains the foundation of sirolimus therapy. Since its regulatory approval in 1999, sirolimus has been incorporated into immunosuppressive regimens for kidney transplantation and selected liver, heart, and lung transplant recipients.

The principal advantage of sirolimus over calcineurin inhibitors is its lack of direct nephrotoxicity. Numerous studies have demonstrated improved renal preservation when patients are converted from calcineurin inhibitor-based regimens to sirolimus-containing protocols [5].

Furthermore, sirolimus possesses antineoplastic properties that may reduce post-transplant malignancy risk, a particularly important consideration among long-term transplant survivors.

Nevertheless, delayed wound healing, dyslipidaemia, proteinuria, and haematological toxicity have limited universal adoption. Contemporary practice increasingly favours individualised immunosuppressive strategies that balance efficacy with toxicity profiles.

Rare diseases and precision medicine

Lymphangioleiomyomatosis

The emergence of sirolimus as a treatment for lymphangio­leiomyomatosis (LAM) represents a landmark achievement in precision medicine.

LAM is a rare progressive cystic lung disease predominantly affecting women and characterised by inactivation of TSC genes leading to constitutive activation of mTOR signalling [6]. The pivotal MILES trial demonstrated that sirolimus stabilised pulmonary function, improved quality of life, and reduced disease progression in patients with moderate-to-severe disease.

Subsequent long-term studies have confirmed sustained benefits and established sirolimus as standard-of-care therapy for patients with declining lung function or symptomatic disease [7].

Tuberous sclerosis complex

Tuberous sclerosis complex is a multisystem genetic disorder caused by pathogenic variants in TSC1 or TSC2. Loss of function of these genes results in persistent activation of mTOR signalling and uncontrolled cellular proliferation.

By directly targeting the underlying molecular defect, sirolimus and related mTOR inhibitors reduce the size of renal angiomyolipomas, subependymal giant-cell astrocytomas, and other disease manifestations [8].

The success of sirolimus in TSC represents one of the clearest examples of mechanism-based therapy in contemporary medicine.

Vascular and lymphatic anomalies

Complex vascular anomalies constitute another rapidly expanding indication for sirolimus therapy.

These disorders frequently involve dysregulated PI3K–AKT–mTOR signalling, resulting in abnormal vascular and lymphatic growth.

Lymphatic malformation is a noncancerous, congenital birth defect where lymphatic vessels form abnormally, creating fluid-filled cysts that most commonly appear as spongy soft lumps on the head and neck, but they may involve any part of the body. They are usually detected by ultrasound and followed by MRI to map the size and depth of the tissue involved. In some cases, just routine monitoring is applied, and in selected cases sclerotherapy is used in an attempt to shrink macrocystic spaces. Postnatal surgery is used for partial removal or debulking but without very good cosmetic results. Old treatment options limited to surgery, embolisation, or supportive care are no longer used, and treatment with sirolimus is opening a new era for patients, both prenatally and postnatally.

Recent clinical studies have demonstrated significant improvements in pain, inflammation, lesion size, bleeding complications, and quality of life among patients treated with sirolimus [9]. Increasing evidence supports its incorporation into multidisciplinary treatment algorithms for refractory vascular malformations.

Oncology applications

Aberrant mTOR activation is a hallmark of numerous cancers. Consequently, mTOR inhibition has emerged as an important therapeutic strategy in oncology.

Although sirolimus itself is not widely used as a conventional anticancer agent, its derivatives – including everolimus and temsirolimus – have become established therapies for renal-cell carcinoma, neuroendocrine tumours, breast cancer, and TSC-associated neoplasms [10].

More recently, nanoparticle albumin-bound sirolimus (nab-sirolimus) has demonstrated efficacy in malignant perivascular epithelioid cell tumours (PEComas), highlighting the continued evolution of rapamycin-based therapeutics [11].

Future precision-oncology approaches may further exploit mTOR pathway alterations as predictive biomarkers for therapeutic response.

Cardiovascular applications

Sirolimus has critical cardiovascular applications primarily as an antiproliferative agent in drug-eluting stents to prevent restenosis, and as immunosuppressant in heart transplants to reduce graft vasculopathy and left ventricular mass. The first sirolimus-eluting stents (SES) were introduced in 2002, followed by sirolimus-coated balloons (2024).

The introduction of sirolimus-eluting coronary stents transformed interventional cardiology by dramatically reducing restenosis rates following angioplasty.

Through inhibition of smooth-muscle proliferation and neointimal hyperplasia, localised delivery of sirolimus prevents pathological vascular remodeling while preserving long-term vessel patency. Drug-eluting stents remain one of the most successful translational applications of mTOR inhibition.

Safety and adverse effects

Sirolimus is available in two main oral dosage forms: tablets (0.5 mg, 1 mg, and 2 mg) and oral solution (1 mg/ml). The dosage, usually once a day (1-6 mg), is tailored based on the condition being treated and is adjusted according to blood test to maintain safe therapeutic levels (5-15 ng/ml). Sirolimus has a narrow therapeutic index, and small changes in blood levels can lead to treatment failure or toxicity

The most common toxicities include the following: hypercholesterolaemia, hypertriglyceridaemia, peripheral oedema, hypertension, acneiform eruptions, oral ulceration, and delayed wound healing.

Among these, mTOR inhibitor-associated stomatitis is particularly common and frequently affects treatment adherence [12].

Haematological complications such as anaemia, thrombocytopaenia, and leukopaenia are also observed. As an immunosuppressive agent, sirolimus increases susceptibility to opportunistic infections.

Less common but potentially serious complications include the following: interstitial pneumonitis, noninfectious lung injury, proteinuria, and thrombotic microangiopathy.

Long-term observational studies indicate that adverse events are generally manageable through dose adjustment and therapeutic drug monitoring [13].

Sirolimus and human aging

Perhaps the most intriguing contemporary application of sirolimus concerns aging biology.

Clinical trial proof for longevity in healthy humans is still lacking, but it is known that sirolimus is a regulator of cell growth and metabolism. It plays a role in autophagy stimulation: it promotes cellular cleanup of damaged proteins and dysfunctional organelles. It also influences inflammatory signalling linked to tissue decay, promoting inflammation reduction.

Among all pharmacological interventions studied in animal models, mTOR inhibition consistently produces some of the most robust lifespan-extending effects. Rapamycin increases lifespan in yeast, nematodes, fruit flies, and mammals while simultaneously delaying multiple age-associated pathologies [14].

Early human studies suggest potential improvements in immune function and resilience against infection; however, evidence remains insufficient to support routine clinical use for longevity enhancement outside research settings.

The challenge for future investigations lies in determining whether the healthspan benefits observed in experimental systems can be translated safely into human populations.

Future perspectives

The future of sirolimus therapy will probably be shaped by four major developments:

1) Precision medicine approaches will increasingly identify patients most likely to benefit from mTOR-targeted interventions.

2) Novel formulations—including nanoparticle and tissue-specific delivery systems—may improve efficacy while reducing systemic toxicity.

3) Combination therapies incorporating mTOR inhibitors with targeted molecular agents may expand treatment opportunities in oncology and rare diseases.

4) Ongoing clinical trials will determine whether modulation of aging biology through mTOR inhibition can become a realistic therapeutic strategy for age-related disease prevention.

Conclusions

Few drugs have undergone a therapeutic evolution as remarkable as sirolimus. Initially developed as an immunosuppressive agent for organ transplantation, sirolimus has become a cornerstone of precision medicine across multiple specialties. Its success stems from targeting mTOR, a signalling pathway that occupies a central position in cellular physiology and disease pathogenesis.

Evidence accumulated over the past two decades supports the efficacy of sirolimus in transplantation, lymphangioleiomyomatosis, tuberous sclerosis complex, vascular anomalies, and selected neoplasms. Meanwhile, emerging research continues to explore its role in aging, neurodegeneration, and immune modulation.

As our understanding of mTOR biology deepens, sirolimus is poised to remain one of the most influential translational therapeutics of the twenty-first century; however, research should continue.

Disclosures

Ethical considerations: none.

This research received no external funding.

The authors declare no conflict of interest.

References