How mTOR Inhibitors Work: Rapalogs and the Feedback Problem
mTOR is a central controller of cell growth that acts through two complexes, mTORC1 and mTORC2. The approved mTOR inhibitors in oncology — everolimus and temsirolimus — are 'rapalogs', analogues of rapamycin that inhibit mTORC1 partially and indirectly. That partial action explains much of their behaviour.
Quick Answer
mTOR is a central controller of cell growth that acts through two complexes, mTORC1 and mTORC2. The approved mTOR inhibitors in oncology — everolimus and temsirolimus — are 'rapalogs', analogues of rapamycin that inhibit mTORC1 partially and indirectly. That partial action explains much of their behaviour.
Side-by-side comparison
How allosteric rapalogs compare with the ATP-competitive mTOR kinase inhibitors being developed to overcome their limitations.
| Property | Rapalogs (everolimus, temsirolimus) | ATP-competitive mTOR kinase inhibitors |
|---|---|---|
| Mechanism | Allosteric, via FKBP12 | Bind the mTOR kinase active site |
| mTORC1 inhibition | Partial: strong on S6K, weak on 4E-BP1 | Complete |
| mTORC2 inhibition | Minimal | Yes |
| AKT feedback rebound | Prominent | Largely avoided |
| Status in oncology | Approved in breast, kidney, neuroendocrine and TSC settings | Investigational; more on-target toxicity |
What mTORC1 Controls
mTORC1 integrates growth-factor signalling (through PI3K-AKT), nutrient and energy status, and stress. When active it promotes protein and lipid synthesis and blocks autophagy, largely by phosphorylating S6 kinase and 4E-BP1. The TSC1-TSC2 complex is the main brake on mTORC1; loss of either releases it.
mTORC2, a separate complex, phosphorylates and helps fully activate AKT.
How Rapalogs Inhibit It
Rapalogs bind the protein FKBP12, and the complex then binds mTOR at a site outside the kinase active site, allosterically dampening mTORC1. This inhibits phosphorylation of S6 kinase strongly but 4E-BP1 only weakly and transiently, so cap-dependent translation is not fully shut down.
mTORC2 is largely unaffected by short-term rapalog exposure. So a rapalog blocks part of mTORC1 and leaves mTORC2 intact.
The Feedback Loop
Active mTORC1 normally suppresses signalling back up to PI3K through S6 kinase. When a rapalog relieves that suppression, PI3K-AKT activity rebounds, and with mTORC2 still working, AKT can be more active than before.
This feedback reactivation blunts single-agent rapalog efficacy and is a reason for combining them with endocrine therapy or PI3K/AKT inhibitors.
Where They Are Used
Everolimus is used with exemestane in hormone-receptor-positive advanced breast cancer after progression on a non-steroidal aromatase inhibitor (BOLERO-2), in advanced renal cell carcinoma, in pancreatic and lung neuroendocrine tumours, and in tuberous sclerosis complex-associated tumours where TSC loss makes cells mTOR-dependent. Temsirolimus is used in renal cell carcinoma.
Class effects include mouth ulcers (stomatitis), non-infectious pneumonitis, hyperglycaemia and hyperlipidaemia, and immunosuppression.
Dosing and Class Effects in Detail
Everolimus is an oral tablet taken once daily at a dose that varies by indication; temsirolimus is a weekly intravenous infusion given with antihistamine premedication. Both are CYP3A substrates with substantial drug-interaction potential, and everolimus blood levels are sometimes measured to guide dosing in its non-oncological uses.
The class effects follow from the biology: stomatitis is often the first and most limiting problem and is reduced by a corticosteroid mouthwash; non-infectious pneumonitis can appear on imaging before symptoms; hyperglycaemia and hyperlipidaemia reflect metabolic effects; and immunosuppression raises infection risk. Wound healing is impaired, so dosing is paused around surgery.
Key Takeaways
- ·Rapalogs allosterically and partially inhibit mTORC1, strongly reducing S6K but not 4E-BP1 signalling.
- ·Relieving mTORC1's feedback on PI3K reactivates AKT, limiting single-agent efficacy.
- ·They work best where cells are mTOR-dependent (TSC loss) or combined with endocrine or PI3K-axis therapy.
Put these genes in pathway context
Frequently asked questions
Why do rapalogs only partially block mTOR?
They act allosterically through FKBP12 at a site outside the kinase pocket, strongly reducing S6-kinase signalling but only weakly and transiently affecting 4E-BP1, and they leave mTORC2 largely intact.
What is the feedback loop that limits rapalogs?
Active mTORC1 normally restrains signalling back up to PI3K. When a rapalog lifts that restraint, PI3K-AKT activity rebounds — and with mTORC2 still working AKT can end up more active than before, blunting single-agent effect.
Where do mTOR inhibitors work best?
Where cells are genuinely mTOR-dependent, such as tumours with TSC1/TSC2 loss, or when combined with endocrine therapy or PI3K-axis inhibitors rather than used alone.
References
Continue Reading
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