Christine Stephan

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

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.

Identifying Optimal Biologic Doses of Everolimus (RAD001) in Patients With Cancer Based on the Modeling of Preclinical and Clinical Pharmacokinetic and Pharmacodynamic Data

PURPOSE To use preclinical and clinical pharmacokinetic (PK)/pharmacodynamic (PD) modeling to predict optimal clinical regimens of everolimus, a novel oral mammalian target of rapamycin (mTOR) inhibitor, to carry forward to expanded phase I with tumor biopsy studies in cancer patients. PATIENTS AND METHODS Inhibition of S6 kinase 1 (S6K1), a molecular marker of mTOR signaling, was selected for PD analysis in peripheral blood mononuclear cells (PBMCs) in a phase I clinical trial. PK and PD were measured up to 11 days after the fourth weekly dose. A PK/PD model was used to describe the relationship between everolimus concentrations and S6K1 inhibition in PBMCs of cancer patients and in PBMCs and tumors of everolimus-treated CA20948 pancreatic tumor-bearing rats. RESULTS Time- and dose-dependent S6K1 inhibition was demonstrated in human PBMCs. In the rat model, a relationship was shown between S6K1 inhibition in tumors or PBMCs and antitumor effect. This allowed development of a direct-link PK/PD model that predicted PBMC S6K1 inhibition-time profiles in patients. Comparison of rat and human profiles simulated by the model suggested that a weekly 20- to 30-mg dose of everolimus would be associated with an antitumor effect in an everolimus-sensitive tumor and that daily administration would exert a greater effect than weekly administration at higher doses. CONCLUSION A direct-link PK/PD model predicting the time course of S6K1 inhibition during weekly and daily everolimus administration allowed extrapolation from preclinical studies and first clinical results to select optimal doses and regimens of everolimus to explore in future clinical trials.

Role of mitochondria and C-terminal membrane anchor of Bcl-2 in Bax induced growth arrest and mortality in Saccharomyces cerevisiae.

In mammalian cells, the Bcl‐2 and Bcl‐x(L) proteins suppress programmed cell death whereas the topographically similar Bax protein accelerates the apoptotic process. Recently published data suggest that expression of the human Bax‐α gene is lethal for the yeast can be overcome by co‐expressing Bcl‐2 or Bcl‐x(L). Our findings corroborate these results. However, we find that although Bax induction invariably stops cell growth under all circumstances, it does not lead to death in ‘petite’ cell. Petites cannot respire because they lack functional mitochondria. It seems that in ‘grande’ cells, which do possess normal mitochondrial DNA, nutritional limitation is critical for increased mortality. Surprisingly, murine Bcl‐2 lacking the membrane anchor of human Bcl‐2 has no effect on grande cells, but can efficiently rescue petites in rich medium. It has been suggested that the C‐terminal membrane anchor of human Bcl‐2 may have a crucial role in rescuing apoptosis in mammalian cells. When murine Bcl‐2 is fused to the membrane anchor of yeast mitochondrial Mas70 protein, the Bcl‐2 variant mBcl‐2‐mma rescues not only petites but also grandes, just like human Bcl‐x(L). The rescuing ability of Bcl‐x(L), which contains its own membrane anchor, surpasses that of mBcl‐2‐mma. Our results indicate that the process involving Bax‐induced growth inhibition followed by possible lethality, and the rescuing effect of Bcl‐2 or Bcl‐x(L) is linked to yeast mitochondrial function. We propose a model which is consistent with these observations.

Increased AKT S473 phosphorylation after mTORC1 inhibition is rictor dependent and does not predict tumor cell response to PI3K/mTOR inhibition

Mammalian target of rapamycin (mTOR) regulates cellular processes important for progression of human cancer. RAD001 (everolimus), an mTORC1 (mTOR/raptor) inhibitor, has broad antitumor activity in preclinical models and cancer patients. Although most tumor lines are RAD001 sensitive, some are not. Selective mTORC1 inhibition can elicit increased AKT S473 phosphorylation, involving insulin receptor substrate 1, which is suggested to potentially attenuate effects on tumor cell proliferation and viability. Rictor may also play a role because rictor kinase complexes (including mTOR/rictor) regulate AKT S473 phosphorylation. The role of raptor and rictor in the in vitro response of human cancer cells to RAD001 was investigated. Using a large panel of cell lines representing different tumor histotypes, the basal phosphorylation of AKT S473 and some AKT substrates was found to correlate with the antiproliferative response to RAD001. In contrast, increased AKT S473 phosphorylation induced by RAD001 did not correlate. Similar increases in AKT phosphorylation occurred following raptor depletion using siRNA. Strikingly, rictor down-regulation attenuated AKT S473 phosphorylation induced by mTORC1 inhibition. Further analyses showed no relationship between modulation of AKT phosphorylation on S473 and T308 and AKT substrate phosphorylation patterns. Using a dual pan-class I phosphatidylinositol 3-kinase/mTOR catalytic inhibitor (NVP-BEZ235), currently in phase I trials, concomitant targeting of these kinases inhibited AKT S473 phosphorylation, eliciting more profound cellular responses than mTORC1 inhibition alone. However, reduced cell viability could not be predicted from biochemical or cellular responses to mTORC1 inhibitors. These data could have implications for the clinical application of phosphatidylinositol 3-kinase/mTOR inhibitors. [Mol Cancer Ther 2009;8(4):742–53]

The Ret receptor tyrosine kinase pathway functionally interacts with the ERalpha pathway in breast cancer.

A limited number of receptor tyrosine kinases (e.g., ErbB and fibroblast growth factor receptor families) have been genetically linked to breast cancer development. Here, we investigated the contribution of the Ret receptor tyrosine kinase to breast tumor biology. Ret was expressed in primary breast tumors and cell lines. In estrogen receptor (ER)alpha-positive MCF7 and T47D lines, the ligand (glial-derived neurotrophic factor) activated signaling pathways and increased anchorage-independent proliferation in a Ret-dependent manner, showing that Ret signaling is functional in breast tumor cells. Ret expression was induced by estrogens and Ret signaling enhanced estrogen-driven proliferation, highlighting the functional interaction of Ret and ER pathways. Furthermore, Ret was detected in primary cancers, and there were higher Ret levels in ERalpha-positive tumors. In summary, we showed that Ret is a novel proliferative pathway interacting with ER signaling in vitro. Expression of Ret in primary breast tumors suggests that Ret might be a novel therapeutic target in breast cancer.

A modified Kex2 enzyme retained in the endoplasmic reticulum prevents disulfide-linked dimerisation of recombinant human insulin-like growth factor-1 secreted from yeast

The majority of the recombinant human insulin‐like growth factor‐1 (IGF1) molecules, secreted from yeast using the prepro sequence of the prepro‐α‐factor, are not active monomers but inactive, disulfide‐linked dimers. The prepro sequence of the prepro‐α‐factor, usually referred to as the α‐factor leader (αFL), consists of a pre or signal sequence and a proregion. After signal sequence removal during translocation into the endoplasmic reticulum (ER) the proregion is still attached to IGF1 when it folds to acquire a tertiary structure. Mature IGF1 is released only in a late Golgi compartment by the membrane‐bound endoprotease Kex2p. We find that co‐expression of a novel ER‐retained Kex‐2p variant, soluble Kex2pHDEL, can prevent intermolecular disulfide bond formation between two IGF1 molecules, implying that the presence of the proregion during the folding of IGF1 in the ER could be a reason for disulfide‐linked dimerisation. This result indicates that the proregion of the αFL may have a role in the folding of some heterologous proteins in yeast, and that the ER‐retained Kex2p mutant could be used as a convenient tool to study the cellular function of the proregions present naturally in various eucaryotic precursor proteins.

A C‐terminal domain, which prevents secretion of the neuroendocrine protein 7B2 in Saccharomyces cerevisiae, inhibits Kex2 yet is processed by the Yap3 protease

Recent reports reveal that the C‐terminal half of the neuroendocrine polypeptide 7B2 selectively inhibits and binds PC2, a mammalian prohormone converting enzyme that is homologous to the yeast pro‐α‐factor processing protease Kex2. During attempted secretion of the 185 amino‐acid human 7B2 in Saccharomyces cerevisiae, we observe that the protein is mostly retained inside the cell. However a mutant polypeptide (7B2Δ1), where the C‐terminal 48 amino acids of 7B2 are deleted, is efficiently secreted. Two shorter C‐terminal truncations either permit poor secretion or no secretion at all. Surprisingly, full‐length 7B2 but not 7B2Δ1 abolishes the catalytic activity of Kex2, indicating that C‐terminal residues of 7B2 might also be important for inhibition of the yeast protease. When the KEX2 gene is disrupted, yeast cells unexpectedly secrete a 7B2 variant similar in size to 7B2Δ1, suggesting involvement of the alternate yeast prohormone convertase Yap3 in processing. Secretion is enhanced by overexpression of Yap3 and by the presence of a Lys‐Arg residue at the processing site of precursor 7B2. These results purport that, in neuroendocrine cells too, secretion of 7B2 could be mediated by a homologue of Yap3.