Expert Opinion on Investigational Drugs
`
`ISSN: 1354-3784 (Print) 1744-7658 (Online) Journal homepage: http://www.tandfonline.com/loi/ieid20
`
`Rapalogs in viral cancers
`
`Dirk P Dittmer, Aadra P Bhatt & Blossom Damania
`
`To cite this article: Dirk P Dittmer, Aadra P Bhatt & Blossom Damania (2012)
`Rapalogs in viral cancers, Expert Opinion on Investigational Drugs, 21:2, 135-138, DOI:
`10.1517/13543784.2012.642369
`
`To link to this article: http://dx.doi.org/10.1517/13543784.2012.642369
`
`Published online: 04 Jan 2012.
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`Date: 30 October 2016, At: 08:43
`
`Ex. 1068-0001
`
`

`
`Editorial
`
`Rapalogs in viral cancers
`
`†
`, Aadra P Bhatt & Blossom Damania
`Dirk P Dittmer
`The University of North Carolina at Chapel Hill, Lineberger Comprehensive Cancer Center,
`Department of Microbiology and Immunology, NC, USA
`
`At present, 150 clinical trials are registered with the National Cancer Institute,
`which investigate the efficacy of inhibitors of the PI3K/Akt/mTOR pathway
`against multiple cancers. Efficacy varies not so much with drug action, but
`with tumor type, as different cancer types (and different pre-clinical models)
`exhibit widely differing susceptibilities to mTOR inhibitors, such as rapamycin.
`Viral cancers appear to be among the most mTOR-addicted and most
`rapamycin-sensitive cancers. We discuss the different mTOR inhibitors that
`are currently available and in clinical trials. We also speculate how the mole-
`cular makeup of viral cancers could guide the selection and use of known
`and novel mTOR inhibitors to treat virus-associated malignancies.
`
`Keywords: BEZ235, everolimus, Kaposi sarcoma, mTOR, rapalog, rapamycin, sirolimus, virus
`
`Expert Opin. Investig. Drugs (2012) 21(2):135-138
`
`The term ‘Rapalogs’ describes a class of allosteric inhibitors that target the mamma-
`lian target of rapamycin (mTOR) pathway. More recently, ATP-competitive inhibi-
`tors of mTOR as well as so-called ‘dual-kinase’ inhibitors have been developed,
`which target other kinases in the mTOR signaling cascade in addition to mTOR
`itself. Rapamycin (sirolimus) was discovered in the 1970s, and is in widespread
`use as a second-generation oral immune suppressant in solid organ transplantation.
`Rapamycin inhibits IL-2 translation and secretion in T cells and thus T cell prolif-
`eration (Figure 1). In addition, it also inhibits IL-2-dependent (and other ligand)-
`dependent signaling in the same cells. In this context, the cell-autonomous G1 arrest
`phenotype induced by protein translation arrest is augmented by inhibition of IL-2,
`which is a paracrine and autocrine growth factor for T cells. The first-generation
`immune suppressants, cyclosporine and FK506, also inhibit IL-2 expression in
`T cells and thereby T cell proliferation. However, their inhibition is T cell specific,
`because the inhibitory mechanism ultimately depends on NFAT (nuclear factor of
`activated T cells), a T cell lineage-restricted transcriptional transactivator of the
`IL-2 promoter. By contrast, rapalogs inhibit the ubiquitously required mTOR
`kinase and thereby inhibit protein translation in all cell types, including cancer cells.
`Rapamycin is ‘tumorstatic’ rather than ‘tumortoxic’ because mTOR controls pro-
`tein synthesis and volume growth rather than DNA replication-driven cell prolifer-
`ation. This mechanism of action limits rapamycin’s potency as an anti-cancer agent,
`except in those cancers where mTOR does not just regulate translation in general,
`but regulates translation of specific autocrine-acting cytokines required for cancer
`cell survival. Virus-associated cancers (predominantly herpesvirus-associated B and
`T cell lymphomas) are examples of this tumor class. Here, rapalogs display nanomo-
`lar IC50s in cell culture and in pre-clinical models [1-5]. The efficacy of rapalogs
`against other subtypes of cancer have been observed in clinical trials, notably in
`sarcomas, mantle cell lymphoma and renal cell carcinoma, and most dramatically
`in Kaposi sarcoma (KS), which is associated with human herpesvirus 8 or Kaposi
`sarcoma-associated herpesvirus (KSHV).
`In transplant-associated KS, switching from the immunosuppressant drug cyclo-
`sporine A to the immunosuppressant drug rapamycin (sirolimus) resulted in resolution
`of cutaneous KS [6]. All tumor lesions disappeared but graft function did not decline.
`
`10.1517/13543784.2012.642369 © 2012 Informa UK, Ltd. ISSN 1354-3784
`All rights reserved: reproduction in whole or in part not permitted
`
`135
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`Ex. 1068-0002
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`

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`Rapalogs in viral cancers
`
`Transplant
`
`Cancer
`
`IL-2
`
`HVS
`
`IL-2
`
`T cell
`
`TL
`
`TL
`
`FK506
`
`Rapamycin
`
`FK506
`
`Rapamycin
`
`IL-6
`
`KSHV
`
`IL-6
`
`B cell
`
`PEL
`
`BC-1
`
`Rapamycin
`- resistant
`
`PEL
`
`BCBL-1
`
`Figure 1. Model of rapamycin modes of action in transplantation (left) and cancer (right). If used as immune suppressants in
`solid organ transplantation, both rapamycin and FK506 inhibit translation of essential cytokines for activated T cells (IL-2).
`Rapamycin also inhibits the translation of essential cytokines for activated B cells (IL-6). If used as anti-cancer drugs for viral
`cancers, both rapamycin and FK506 inhibit IL-2 in herpesvirus saimiri (HVS)-induced T cell lymphoma (TL). Rapamycin also
`inhibits IL-6 in KSHV-induced primary effusion lymphoma (PEL). Eventually, clones of TL and PEL evolve, which no longer
`depend on IL-6 or in which IL-6 expression is rapamycin insensitive [3].
`
`This study thus separated rapamycin’s immunosuppressive
`function (on T cells) from its anti-cancer effects on the endothe-
`lial lineage tumor KS. Since then, similar results have been
`reported by others [7,8], although exceptions have been noted as
`well [9]. Discordant case studies are part of the norm, particularly
`in a highly pre-treated patient population. This should not
`detract from the general mechanism. A randomized clinical trial
`to formally establish the efficacy of any rapalog against KS is
`still missing.
`KS tumor cells are firmly addicted to mTOR signaling. KS
`lesions are characterized molecularly by high-level phosphoryla-
`tion of Akt, mTOR and the mTORC1 targets, p70 S6 kinase
`and ribosomal protein S6 [6,10,11]. In other systems, rapamycin
`blocked focus formation induced by oncogenic alleles of PI3K
`or of Akt [12]. These observations place mTOR downstream of,
`and epistatic to, PI3K and Akt.
`Modern mTOR inhibitors promise to improve on the clin-
`ical efficacy of rapamycin in several ways. The first class of
`modern mTOR inhibitors or rapalogs are allosteric inhibitors
`of mTORC1. They display better bioavailability and pharma-
`cokinetics than sirolimus, but they follow the same molecular
`mechanism. Everolimus,
`temsirolimus
`and ridaforolimus
`form a mTORC1:FKBP:rapalog complex analogous to rapa-
`mycin (sirolimus). Prior binding to FKBP is required and
`mTORC1 is the direct target of inhibition; a second complex,
`mTORC2, is not affected. The interactions are a bit more
`complicated, since the same catalytic subunit of mTOR kinase
`
`participates in both mTORC1 and mTORC2 complexes, with
`specificity being contributed by different co-factors (raptor in
`case of mTORC1 and rictor in case of mTORC2). Depending
`on tumor type and stoichiometry, this can lead to opposing
`indirect effects: sequestering mTOR kinases into an inactive
`mTORC1:FKBP:rapalog complex may deplete mTOR and
`thus diminish mTORC2 kinase activity as well; alternatively
`mTORC2 activity may increase and in effect compensate for
`loss of mTORC1 through feedback activation of Akt [13].
`Which scenario ultimately prevails is cell type-specific and
`may help explain the wide range of rapamycin responsiveness
`among different tumors.
`Direct clinical comparisons among the rapalogs have not
`been reported, but in our animal studies, all allosteric-
`acting rapalogs exhibit equivalent efficacy across a wide
`range of
`tumor xenograft models
`(unpublished obser-
`vation); cell
`lines/tumor types that exhibit a high IC50
`against rapamycin tend to also be partially resistant to
`allosteric-acting rapalogs.
`The second class of modern mTOR inhibitors target the active
`site of mTOR kinase (Torin/PP242/Ku-0063794/WYE-354)
`and in some cases have been shown to inhibit tumor cells
`more effectively than rapamycin [14]. It appears that active site
`mTOR inhibitors have weaker effects on the proliferation and
`function of normal lymphocytes than rapalogs. Additionally,
`one would expect active site inhibitors to be associated with the
`rapid development of resistance as well as correspondingly greater
`
`136
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`Expert Opin. Investig. Drugs (2012) 21(2)
`
`Ex. 1068-0003
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`

`
`Dittmer, Bhatt & Damania
`
`Action
`Allosteric
`Dual
`mTOR
`PI3K
`AKT
`
`A.
`
`40
`
`30
`
`20
`
`10
`
`0
`
`No. of trials
`
`Other
`
`HD
`
`Endome.
`
`Hem
`
`HNC
`
`Sarcoma
`
`Carcinoma
`
`HCC
`
`NHL
`
`Solid
`
`B.
`
`Allosteric
`
`Dual
`
`mTOR
`
`PI3K
`
`AKT
`
`Phase
`
`II
`
`I
`I/II
`III
`
`40
`
`30
`
`20
`
`10
`
`0
`
`No. of trials
`
`MK2206
`
`SF1126
`PX-866
`GDC-4254
`GDC-0349
`BKM120
`AMG319
`
`OSI-027
`INK128
`CC-223
`AZD8055
`
`XL765
`PF-04691502
`PF-05212384
`GSK2126458
`GDC-0980
`DS-7423
`NVP-BEZ235
`
`Ridaforolimus
`Sirolimus
`Temsirolimus
`Everolimus
`
`Figure 2. Current NCI registered clinical trials using inhibitors of the PI3K/Akt/mTOR pathway. (A). Stacked bar chart showing
`the number of trials (on the vertical axis) for different cancer categories (on the horizontal axis). The class of inhibitor is
`indicated by color. NHL, non-Hodgkin lymphoma; HCC, any liver cancer including hepatocellular carcinoma; HNC, head and
`neck cancer; Hem, hematopoietic malignancies including multicentric Castleman disease and leukemia; endome., endometrial
`cancer; HD, Hodgkin disease. (B) Stacked bar chart showing the number of trials (on the vertical axis) for different mTOR
`inhibitors (on the horizontal axis) grouped by mechanism of action. The phase of the trial is indicated by color.
`
`potential for dose-limiting side effects than rapalogs, which are
`allosteric inhibitors.
`Some active site mTOR inhibitors (e.g., NVP-BEZ335,
`PI-103) also inhibit PI3K. Dual inhibitors like NVP-BEZ235
`are being tested in clinical trials (Figure 2) (reviewed in [15]).
`These tend to be associated with greater efficacy as well as a
`wider range of susceptible tumor types based on pre-clinical
`experiments [1,2,14,16-18]. The exact molecular mechanism for
`this increased potency probably lies in the expanded range of
`kinase targets
`that are inhibited. Inhibiting PI3K affects
`many more targets (e.g., mTOR-independent targets of Akt).
`Additionally, PI3K inhibition results in Akt inhibition as
`well. Consequently, mTORC2-driven Akt activation (due to
`mTORC1 inhibition-induced feedback)
`should also be
`
`suppressed. At present, the biological significance of only a few
`targets of mTORC1 and mTORC2 has been established.
`A very recent study combined an allosteric rapalog with a dual
`PI3K/mTOR active site inhibitor [14,19], which resulted in addi-
`tive efficacy. It remains to be seen if the increased tumor efficacy
`of dual inhibitors is associated with increased suppression of nor-
`mal lymphocyte proliferation.
`Recently, a new molecular phenotype of mTOR inhibitors has
`been uncovered. Inhibition of mTOR either by allosteric or active
`site inhibitors, as well as the dual PI3K/mTOR inhibitors has
`been shown to induce autophagy [20,21]. In cancer cells, autophagy
`is one mechanism used by tumor cells to survive anti-neoplastic
`chemotherapies. Interestingly, although rapamycin by itself could
`induce autophagy in glioma cells, competitive mTOR inhibitors
`
`Expert Opin. Investig. Drugs (2012) 21(2)
`
`137
`
`Ex. 1068-0004
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`

`
`Rapalogs in viral cancers
`
`like Ku-0063794 induced autophagy to a greater degree than
`rapamycin. Chloroquine is an inhibitor of autophagy and the
`combination of chloroquine with dual inhibitors of PI3K and
`mTOR was found to effectively induce apoptosis in glioma [21].
`Thus, the addition of autophagy inhibitors to PI3K/mTOR
`inhibitors may be warranted in the future.
`Currently, extensive safety studies on active site mTOR
`inhibitors or PI3K/mTOR dual
`inhibitors have not been
`reported. Hence, our predictions of clinical utility are highly
`speculative. Several questions remain to be answered. Should
`we prioritize individualized therapy and find out when, where
`and why the established,
`safe allosteric rapalogs work?
`Or should we bet on the power of combinatorial therapy,
`assuming that dual PI3K/mTOR inhibitors will be inherently
`
`more efficacious? What is the safety profile when combination
`inhibitors are added to the dual inhibitors? In this era of tar-
`geted therapies, targeting multiple pathways critical for tumor
`cell survival is warranted. However, as we target more path-
`ways, the potential for side effects and cytotoxicity is greatly
`increased and the safety profiles of such combinations need
`to be considered in equal proportion to their efficacy.
`
`Declaration of interest
`
`This work was supported by NIH (CA163217 and CA096500).
`The authors have no other competing interests to declare. The
`authors state no conflict of interest and have received no
`payment in preparation of this manuscript.
`
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`Affiliation
`†
`Dirk P Dittmer
`, Aadra P Bhatt &
`Blossom Damania
`†
`Author for correspondence
`The University of North Carolina at Chapel Hill,
`Lineberger Comprehensive Cancer Center,
`Department of Microbiology and Immunology,
`CB# 7290, 715 Mary Ellen Jones Bldg,
`Chapel Hill, NC 27599-7290, USA
`Tel: +919 966 7960; Fax: +919 962 8103;
`E-mail: ddittmer@med.unc.edu
`
`Ex. 1068-0005