Stimulation of the insulin and insulin-like growth factor I (IGF-I) receptor activates the phosphoinositide-3-kinase/Akt/mTOR pathway causing pleiotropic cellular effects including an mTOR-dependent loss in insulin receptor substrate-1 expression leading to feedback down-regulation of signaling through the pathway. In model systems, tumors exhibiting mutational activation of phosphoinositide-3-kinase/Akt kinase, a common event in cancers, are hypersensitive to mTOR inhibitors, including rapamycin. Despite the activity in model systems, in patients, mTOR inhibitors exhibit more modest antitumor activity. We now show that mTOR inhibition induces insulin receptor substrate-1 expression and abrogates feedback inhibition of the pathway, resulting in Akt activation both in cancer cell lines and in patient tumors treated with the rapamycin derivative, RAD001. IGF-I receptor inhibition prevents rapamycin-induced Akt activation and sensitizes tumor cells to inhibition of mTOR. In contrast, IGF-I reverses the antiproliferative effects of rapamycin in serum-free medium. The data suggest that feedback down-regulation of receptor tyrosine kinase signaling is a frequent event in tumor cells with constitutive mTOR activation. Reversal of this feedback loop by rapamycin may attenuate its therapeutic effects, whereas combination therapy that ablates mTOR function and prevents Akt activation may have improved antitumor activity.

https://cancerres.aacrjournals.org/content/canres/66/3/1500.full.pdf

mTOR Inhibition Induces Upstream Receptor Tyrosine Kinase Signaling and Activates Akt

Rapamycin, a macrocyclic lactone, is a highly specific inhibitor of the serine/threonine protein kinase target of rapamycin (TOR). Although it is clear that TOR controls initiation of protein translation, recent results indicate that TOR is a central controller, integrating a plethora of signalling pathways that respond to growth factors and nutritional status. In addition to the role of rapamycin as an immune suppressant, emerging data indicate that genetic and metabolic changes accompanying malignant transformation might cause hypersensitivity to TOR inhibition.

The tor pathway: a target for cancer therapy

AKT/PKB (protein kinase B) kinases mediate signaling pathways downstream of activated tyrosine kinases and phosphatidylinositol 3-kinase AKT kinases regulate diverse cellular processes including cell proliferation and survival, cell size and response to nutrient availability, tissue invasion and angiogenesis Many oncoproteins and tumor suppressors implicated in cell signaling/metabolic regulation converge within the AKT signal transduction pathway in an equilibrium that is altered in many human cancers by activating and inactivating mechanisms, respectively, targeting these inter-related proteins We review a burgeoning literature implicating aberrant AKT signaling in many sporadic human cancers as well as in several dominantly inherited cancer syndromes known as phakomatoses The latter include disorders caused by germline mutations of certain tumor suppressor genes, that is, PTEN, TSC2/TSC1, LKB1, NF1, and VHL, encoding proteins that intersect with the AKT pathway We also review various pathogenic mechanisms contributing to activation of the AKT pathway in human malignancy as well as current pharmacologic strategies to target therapeutically components of this pathway

Perturbations of the AKT signaling pathway in human cancer

Natural products to drugs: natural product derived compounds in clinical trials

Following the development of the first mTOR inhibitor that successfully improves therapeutic survival in cancer, Raymond and colleagues look at how novel biomarker identification and the use of multitargeted and multimodality therapies could advance the next generation of these drugs. Mammalian target of rapamycin (mTOR) is a kinase that functions as a master switch between catabolic and anabolic metabolism and as such is a target for the design of anticancer agents. The most established mTOR inhibitors — rapamycin and its derivatives — showed long-lasting objective tumour responses in clinical trials, with CCI-779 being a first-in-class mTOR inhibitor that improved the survival of patients with advanced renal cell carcinoma. This heralded the beginning of extensive clinical programmes to further evaluate mTOR inhibitors in several tumour types. Here we review the clinical development of this drug class and look at future prospects for incorporating these agents into multitarget or multimodality strategies against cancer.

http://dipbsf.uninsubria.it/monti/BFPN%202009/mTOR2.pdf

Current development of mTOR inhibitors as anticancer agents.

The orally bioavailable rapamycin derivative RAD001 (everolimus) targets the mammalian target of rapamycin pathway and possesses potent immunosuppressive and anticancer activities. Here, the antitumor activity of RAD001 was evaluated in the CA20948 syngeneic rat pancreatic tumor model. RAD001 demonstrated dose-dependent antitumor activity with daily and weekly administration schedules; statistically significant antitumor effects were observed with 2.5 and 0.5 mg/kg RAD001 administered daily [treated tumor versus control tumor size (T/C), 23% and 23–30%, respectively], with 3–5 mg/kg RAD001 administered once weekly (T/C, 14–36%), or with 5 mg/kg RAD001 administered twice weekly (T/C, 36%). These schedules were well tolerated and exhibited antitumor potency similar to that of the cytotoxic agent 5-fluorouracil (T/C, 23%). Moreover, the efficacy of intermittent treatment schedules suggests a therapeutic window allowing differentiation of antitumor activity from the immunosuppressive properties of this agent. Detailed biochemical profiling of mammalian target of rapamycin signaling in tumors, skin, and peripheral blood mononuclear cells (PBMCs), after a single administration of 5 mg/kg RAD001, indicated that RAD001 treatment blocked phosphorylation of the translational repressor eukaryotic initiation factor 4E-binding protein 1 and inactivated the translational activator ribosomal protein S6 kinase 1 (S6K1). The efficacy of intermittent treatment schedules was associated with prolonged inactivation of S6K1 in tumors and surrogate tissues (≥72 h). Furthermore, detailed analysis of the dose dependency of weekly treatment schedules demonstrated a correlation between antitumor efficacy and prolonged effects (≥7 days) on PBMC-derived S6K1 activity. Analysis of human PBMCs revealed that S6K1 also underwent a concentration-dependent inactivation after RAD001 treatment ex vivo (>95% inactivation with 20 nm RAD001). In contrast, human PBMC-derived eukaryotic initiation factor 4E-binding protein 1 was present predominantly in the hypophosphorylated form and was unaffected by RAD001 treatment. Taken together, these results demonstrate a correlation between the antitumor efficacy of intermittent RAD001 treatment schedules and prolonged S6K1 inactivation in PBMCs and suggest that long-term monitoring of PBMC-derived S6K1 activity levels could be used for assessing RAD001 treatment schedules in cancer patients.

https://cancerres.aacrjournals.org/content/canres/64/1/252.full.pdf

Antitumor Efficacy of Intermittent Treatment Schedules with the Rapamycin Derivative RAD001 Correlates with Prolonged Inactivation of Ribosomal Protein S6 Kinase 1 in Peripheral Blood Mononuclear Cells

Christine Stephan

Papers

Antitumor Efficacy of Intermittent Treatment Schedules with the Rapamycin Derivative RAD001 Correlates with Prolonged Inactivation of Ribosomal Protein S6 Kinase 1 in Peripheral Blood Mononuclear Cells