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`STATE
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`v
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`This material may be protected by Copyright law (Title 17 U.S. Code)
`
`Immunoconjugates Against Solid Tumors:
`Mind the Gap
`
`AD Ricart1
`
`The objective of immunoconjugate development is to combine the specificity of immunoglobulins with the efficacy
`0f cytotoxic molecules. This therapeutic approach has been validated in hematologic malignancies; however, several
`obstacles to achieving efficacy in treating solid tumors have been identified.These include insufficient specificity of
`targets and poor antibody delivery, most specifically to the tumor core. Heterogeneous antigen expression, imperfect
`vascular supply, and elevated interstitial fluid pressure have been suggested as the factors responsible for the poor
`delivery of antibodies. Promising immunoconjugates are in development: immunoconjugates targeting the prostate—
`specific membrane antigen, trastuzumab-DM‘l, lorvotuzumab mertansine, and 551 P. Advances in cancer biology and
`antibody engineering may overcome some ofthe challenges. New small antibody formats, such as single—chain Fv,
`Fab, and diabodies, may improve penetration within tumor masses. Nevertheless, the cost of treatment might require
`justification in terms of demonstrable improvement in quality of life in addition to efficacy; further economic evaluation
`might be necessary before this approach can replace the current standards of care in clinical practice.
`
`“Mind the gap” is a warning to train passengers to remind them
`of the sometimes significant gap between the train door and the
`station platform.
`
`REVIEW CRITERIA
`The data for this review were obtained by searching PubMed
`and MEDLINE databases without any date limitation. The
`search terms included “immunoconjugate," “tumor-targeting
`agent,” “radioimmunotherapy,” “antibody-drug conjugate,”
`and “immunotoxin.” The abstracts of retrieved citations were
`reviewed and prioritized by relative content. Full-length arti—
`cles that were deemed relevant were analyzed before being
`included in this review, and references were checked for
`additional material where appropriate. In addition, relevant
`abstracts (that were not yet reported in the form of full—length
`articles) were identified using an electronic search of the pro—
`ceedings of the American Society ofClinical Oncology, the
`American Association for Cancer Research, the European
`Society for Medical Oncology, and the San Antonio Breast
`Cancer Symposia meetings.
`
`development of monoclonal antibody (mAb) technology.l
`Several anticancer mAbs have been introduced in clinical
`practice since approval was received for the use of the first
`such mAb, rituximab, and these are now established as a new
`component of cancer treatment (Table l).2 The success of anti—
`body cancer therapy has depended mainly on the ability to
`generate a desired mAb and the characterization of suitable
`tumor targets (or suitable targets in the tumor environment),
`opening an unprecedented opportunity for designer anticancer
`and their
`drugs (Table 2). Antibodies are complex molecules
`itment of
`effects can be ascribed to multiple mechanisms: recru
`immune cells, activation of complements, sequestration, and
`cross—linking of targets.3 There are three principal mechanisms
`ofaction of anticancer mAbs: blocking the function ofspecific
`molecules, targeting specific cells (generating cytotoxicity in
`the cells that express the antigen), and functioning as signal-
`ing molecules.“*5 In early clinical trials, mouse mAbs had lim-
`ited serum stability because of a human antimouse antibody
`response, rendering repeat dosing ineffective and more toxic"6
`But the ability to create more human variants finally made
`mAbs suitable for use as repeated treatment. Moreover, the task
`INTRODUCTION
`of selecting fully human variable domains was simplified dur—
`ing the 19805 through the isolation of genes encoding human
`Until recently, drug development in oncology was mostly
`variable regions, their successful expression in Escherichia
`empirical. The mouse hybridoma technology described
`coli, and the introduction of phage—display technologyfij“)
`by Milstein and Kohler was the instrumental step for the
`
`
`'BioTherapeutics Research and Development, La Jolla l aboratories, Pfizer, San Diego, California, USA. Correspondence: AD fiicart (alejandro.d.ricar?@0pfiaer.com)
`lirr‘rjeivred 1| fir lober .20lf);al'cepleil L: JanuaryLZOl harlvante Dollie publication 2 Menu l 2m l,«ioi:lO iOBEl/ClpiQm i 8
`
`CLINICAL PHARMACOLOGY &THERAPEUTlCS l VOLUME 89 NUMBER 4 l APRIL 20H
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`Table 1 Food and Drug Administration—approved anticancer monoclonal antibodies (mAbs)
`
`Antibody
`Antigen
`Indication
`Type of mAb Mechanism ofaction
`
`Brand name and manufacturer
`
`Naked antibodies
`_,. ._.,_.._..w..v.. ,
`
`Wiiituxim; m c5 ”VHF” ' w “’
`”T'Eiainie'rrc’m “ABE—cm“ T’TAWgcsnd Genentechl
`
`Trastu‘rurnabfl
`WHHERZ
`HEhZVt breastcancer‘ IIIIII— Humanized
`ADCC,receptorblocl:a’cleu Herceptin-(Eenenwtech)
`“a
`
`
`Alemtuzumabwm
`C652
`B~ce|lCLL
`--
`Humanigedw ADCC
`7
`h GfflflrflL___,w _,
`Bevacizumab
`\7EGF
`Metastatic coloricancer,
`‘ Hvu‘manized
`Ligand blockade
`“Avastin (Genentecm
`
`H
`W_
`,2 ,
`breast cancer,and NSCLC
`MW ,___,..
`W
`_...
`Cetuximab
`EGFR
`Metastatic colon and
`Chimeric VVVVVVVVVVVVReceptor blockade
`Erbitux(lmc|0ne Systems)
`
`head and neck cancer
`,
`
`_ Panitumumab
`EGFR
`Metastatic colon cancer
`Human
`ReceptorblOCkade
`Vectibix(Amgen)
`_, W,
`,
`2
`
`Ofatumumab
`VVVVV
`CDZOW ELL
`V
`H i
`Humanm ifriaocccoc m
`Arzerra(G|a;o;n1ithKline)
`f
`
`immW-Ocanjugafes
`7777777
`_
`__ _
`___
`”,7
`,,
`,
`. ,7 ,, "_::<
`
`_, 90Y—Ibritur—noma'btliux‘vetan
`CD20
`helapsed/refractoryNHL- ”Murine
`Radiation (semis—5mm : Zevalin(CellTherapeutics—l—
`13ll~Tositurri<>mab
`C020
`Relapsed/refractory NHL Murine
`W Radiation (B-and
`BexxarlGlaxoSmithKline)
`v—emissions)MW
`
`.,._._,\
`
`Gemtuzumab Ozogamicin (Mylotarg, Pfizer), directed against the C033 antigen present on leukemic myeloblasts in most patients with acute myelogenous leukemia (AML), has
`been recently withdrawn in the United States because a required postapproval study failed to confirm the drug’s clinical benefit.
`ADCC. antibody-dependent cell-mediated cytotoxicity; CDC, complement—dependent cytotoxicity; CLL, chronic lymphoid leukemia: EGFR, epidermal growth factor receptor:
`NHL, nonrHodgkin’s lymphoma; NSCLC, nonesmall-cell lung cancer;VEGF, vascular endothelial growth factor.
`
`..._
`
`
`Table 2 Tumor-associated antigens targeted by immunoconjugates
`
`Target
`Type of molecule
`Expression
`
`PSMA
`Cells-surface glycoprotein
`Prostate cancer, vasculature of solid tumors
`7 a
`
`TenascineC
`Extracellular matrix protein
`Glioblastoma multiforme, breastand lungrcaricergCC, and NHL stroma
`
`G250
`Membrane-associated carbonic anhydrase (CA IX)
`Clear—cell RCC
`r
`r
`
`MUCl
`Glycoprotein (mucifln)
`Ovarian, colorectal, and gastric cancer
`
`EanAg
`Glycoprotein (mucin)
`I
`r
`7
`7
`Pancreatic, colorectal,‘biliary andwgastric cancer-,"and NSCL—Ew
`IIIIII
`am
`Member ofth-e EGEiltamily:
`_
`’
`"
`"ares; cancer
`" ' " ’
`’7
`' T """"""
`
`
`CD56
`Neural cell adhesion molecule
`7
`SCLC, Merkelcell carcinoma, neiiroblastoma, ovarian cancer, and—MM”
`T
`
`GPNMBW“
`_
`.
`.
`Melanomabre—a—stcancer
`7’
`‘
`’
`EphA2
`Member ofthe erythropoietin-producing hepatoma (Eph)
`Breast,prostate, lungfiand ovarian cancer, and glioblastoma multifoiriie
`
`family ofTK receptors
`,_
`,.
`
`lntegrins
`--~ Transmembrane receptors for-proteins of the ECM
`W Solid tumors and-"blood vessels-m
`MIN
`
`Eripto
`GPHinked cell-surface glycoprotein
`Breast, colonigastric, pancreatic, lungtovaria ncerani'QrfiéuwiicanEEr
`
`SLC44A4
`Choline transporter—like protein
`Pancreatic, prostate, and gastric cancer —
`I”
`CD70 Lymphomas, RCC, and glioblastoma Member oftheTNF superfamily
`
`
`Mesothelin
`Glycosylphosphatidylinositoleanchoredantigen
`Mesothelioma, pancreatic and ovarian cancer, and NSCLC (adenocarcinoma)
`A33
`
`Colorectal cancer
`Glycoprotein with homology to lg superfarnily ,
`Member ofthe lg superfamily
`
`CEA
`
`SCLC, colorectal cancer, medullary thyroid carcinoma
`ECM, extracellular matrix; EGFR,epidermal growth factor receptor; lg, immunoglobulin; MM, multiple myeloma; NHL, noneHodgkin’s lymphomas, NSCLC, non—small-cell lung
`cancer; PSMA, prostateespecific membrane antigen; RCC, renal cell carcinoma; SCC, squamous cell carcinoma; SCLC, small-cell lung concenTK, tyrosine kinase7TNF, tumor necrosis
`factor.
`
`Humanized mAbs are more efiective in inducing antibody—
`dependent cellular cytotoxicity and complement-dependent
`cytotoxicity, and they are less immunogenic.
`
`FACTORS REGULATING ANTIBODY-TARGETED THERAPY
`
`The efficacy ofa particular mAb depends on different variables.
`These include not only the characteristics of the mAb itself (fine
`specificity, avidity, and isotype) but also those of the targeted
`
`antigen: its function, cell-surface density, presence ofsecreted
`isoforms, shedding and/0r internalization, normal tissue distri—
`bution, and phenotypic expression in the cancer cell population.“
`Antibody—targeted therapy attempts to induce an unprece—
`dented degree ofanticancer specificity; however, there are sig—
`nificant obstacles that prevent an ideal targeting of solid tumors
`(Table 3). Overall, the use of intact mAbs is associated with prac—
`tical limitations because of their pharmacokinetics: their slow
`
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`rate of clearance causes significant exposure to normal organs,
`limits the quantities delivered to tumors, and results in relatively
`poor diffusion from the vasculature into and through the tumor
`(Figure 1).10 They are large proteins and are therefore charac-
`terized by slower kinetics of distribution as compared to small
`molecules.4 Particularly in solid tumors, heterogeneous anti—
`gen expression and imperfect vascular supply can limit uniform
`delivery of antibodies. Impaired clearance of fluid from tumors
`(due to lack of lymphatic vessels) also leads to increased inter-
`stitial pressure within the extracellular matrix.11 This elevated
`interstitial pressure in the centers of tumors opposes inward dif-
`fusion and induces a net outward gradient from the center of
`the tumor, thereby slowing the diffusion of immunoglobulin G
`(IgG) molecules from their extravasation site. Consequently, this
`gradient within solid tumors differentially inhibits the diffusion
`of larger molecules in comparison to smaller molecules. 12 It has
`been observed in experimental studies that tumor penetration
`seems to be directly related to the size of the antibody molecules,
`with faster, deeper, more extensive, and more uniform tumor
`penetration being achieved by single—chain Fvs than by intact
`
`Table 3 Obstacles to achieving efficacy with monoclonal
`antibody (mAb) therapy
`Impaired mAb distributiond
`Limited delivery to tumor sites3
`insufficient trafficking ofeffector cells to tumor6
`Antigenic heterogeneity (intratumoral and intertumoral)a
`Shedding and internalization oftarget antigensa
`insufficient tumor specificity oftarget antigensd
`Immunogenicity: human antimouse and antichimeric antibody
`resPonses, immune response to peptide cytotoxins
`aThese obstacles either are not seen or are less critical in hematologic malignancies.
`
`lgGs.13 Complete human antibodies have prolonged half—lives
`(fl/2) owing to their ability to bind to the neonatal Fc recep-
`tor. This receptor is expressed on placenta and blood vessel lin—
`ings and protects serum IgG from degradation. Because Fab
`and single~chain Fv fragments lack the Fc region, they are not
`protected by this receptor.3 There is also a difference in biodis-
`tribution kinetics between fragments and whole antibodies, as
`observed in experimental studies and mathematical modelslf’15
`This variation in biodistribution has an impact on the eifective-
`ness of drug delivery to tumors. “Retention” (the percentage Of
`the injected dose found in the tumor throughout a range of time
`points) is influenced by several factors, including affinity, but is
`greater for intact IgGs. Higher retention in the tumor would be
`important for immunoconjugates with “bystander” effect (such
`as radioimmunoconjugates), whereas rapid clearance from the
`bloodstream is preferable for peptide cytotoxins. Immunotoxins
`with longer circulating t1 ,2 lead to increased vascular endothelial
`injury and more severe “vascular leak syndrome.” Rapid elimi-
`nation from blood would allow repeated and frequent admin-
`istration, or even short continuous infusions. Hence, the size
`of the delivery vehicle should be selected on the basis ofthe
`mechanism of action of the payload. However, several ques—
`tions remain unanswered in comparing the use of intact IgGs
`versus fragments in therapeutic application to solid tumors,
`because clinical experience is still limited. Antigen shedding can
`also limit the delivery within the tumor and reduce the clinical
`activity of mAbs. Given that shedding of membrane proteins
`is a physiologic process used by cells to modulate the function
`of surface proteins, soluble antigen in the extracellular fluid of
`tumors has recently been identified as a significant additional
`16
`barrier to the activity of mAbs.
`
`
`
`—r
`
`saiun
`
`Figure 1 Direct visualization of heterogeneous extravascular distribution oftrastuzumab in human HER2-overexpressing xenografts. Staining for bound
`trastuzum
`ab and HER2.Tumor cryosections are shown from MDAv435-LCC6HER2-overexpressing xenografts treated with 20 mg/kg trastuzumab for 3 h
`or left untreated.
`(a) Overlaid images ofa treated tumorshow bound trastuzumab (black) relative to blood vessels (CD3i;dark blue) and the perfusion
`marker DioC7(3) (cyan). (b) Additional staining ofthe same section for HERZ (red) shows that areas with no bound trastuzumab are overexpressing HERZ. An
`untreated MDA»435—LCC6HER2 tumor shows (c) no bound trastuzumab but id) relatively homogeneous HER2 expression Similarly stained MDA7435—LCC6
`vectortumors display no bound trastuzumab or unbound HER2 in treated or untreated tumors (data not shown). Reprinted with permission from Baker, J.H,
`eta/., Direct visualization of heterogeneous extravascular distribution oftrastuzumab in human epidermal growth factor receptor type 2 overexpressing
`xenografts, Clin. Cancer Res. 14, 21 71 —21 79 (2008). Copyright ©2008 American Association for Cancer Research.
`
`CLINICAL PHARMACOLOGY &THERAPEUTICS l VOLUME 89 NUMBER 4 i APRIL 201i
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`ENHANCING ACTIVITYTHROUGH THE USE
`OF IMMUNOCONJUGATES
`
`To date, all mAbs approved for use in the treatment of solid
`tumors are naked antibodies (Table 1). Targeting implies direct-
`ing antibodies toward cells expressing tumor-associated antigens
`and is an adaptable approach; antibodies can be engineered to
`carry moieties (payloads) such as radionuclides, chemothera-
`peutic agents, toxins, or cytokines. In particular, immunoconju-
`gation can be perceived as a strategy for improving the specificity
`of cytotoxic drugs or radiation and for enhancing the efficacy
`of passive immunotherapy, the aim being to integrate the best
`characteristics of both therapeutic approaches.3’17 Moreover,
`this approach does not depend on the host immune system
`status and is not adversely affected by internalization of mAb—
`antigen complexes in the case of radioimmunoconjugates. The
`delivery of the payload can also be increased by several orders
`of magnitude to target tumor antigens whose expression on
`the cell membrane is measured in the millions (e.g., HER 2).”
`Nonetheless, conjugation to radionuclides or cytotoxic drugs
`considerably increases the toxicity profile of mAbs, as exempli-
`fied by the comparison of the safety profiles oftrastuzumab and
`trastuzumab-DMI.18‘20
`
`The requirement for tumor—specific antigen expression is
`critical for the success of these agents, because sequestration
`of the cytotoxic payload to nontumor cells might occur even
`with low antigen expression if the antigen is widely distributed
`among normal tissues. This process, known as “antigen sink,”
`could manifest as toxicity in an anatomically distant organ and
`result in lack of antitumor activity. This outcome was graphically
`illustrated during the clinical development of BR96—doxorubicin
`conjugate in the mid 19905. The LewisY antigen, the target antie
`gen for a family of BR96—constructs (BR96 mAb linked to cyto~
`toxic drugs), is expressed with great intensity on a number of
`carcinomas. Nonetheless, during the phase I clinical studies of
`BR96—doxorubicin, an unanticipated constellation of symptoms
`and signs was observed, including intractable nausea, vomitr
`ing, and hematemesis, which were dose-limiting.21 In a subse—
`quent phase II study in patients with metastatic breast cancer,
`no significant antitumor activity was observed in those receiving
`BR96—doxorubicin as compared with the control group. Further
`investigation demonstrated that BR96—d0xorubicin binds to
`Lewisy antigen expressed on gastric mucosa cells and that this
`binding was responsible for both the toxicity and the absence
`ofantitumor activity. Attempts to develop this agent have so
`far been unsuccessful. The target requirements ofa solid tumor
`antigen are likely to be much more stringent.”
`
`Radioimmunoconjugates
`Radioimmunotherapy is an attractive approach as a treatment
`for lymphomas because the cells in lymphomas are inherently
`sensitive to radiation. It is also well known that lymphomas
`metastasize to areas such as the lymph nodes and bone mar—
`row, sites that are readily accessible to circulating mAbs, The
`two currently available radiolabeled mAbs, yttrium-90—labeled
`(WY) ibritumomab tiuxetan (Zevalin) and iodinerl31—labeled
`(1311) tositumomab (Bexxar), target CD20, the same antigen
`516
`
`recognized by rituximab, and show more clinical activity than
`the naked antibody?2 They are fully mouse molecules, but this
`is not a major concern with respect to tositumomab or ibritu-
`momab, which are intended for one-time dosing.3
`While radioimmunotherapy has shown success in lymphomas,
`responses in refractory adenocarcinomas have been infrequent.
`This is attributable, in part, to their relative lack of sensitivity
`to radiation and the consequent failure to deliver an adequate
`radiation dose to tumor masses. The physical properties of iso-
`topes (path length, energy of emission, and physical t1 /2) should
`be selected on the basis of lesion size and the mAb’s properties.
`For solid tumors, [E—emitters would be the optimal choice for
`lesions that are larger than 273 mm, whereas (it—emitters might
`be best suited for treating micrometastasis. 90Y has a higher
`B—particle energy and longer range than lutetium-177 (177Lu);
`however, this renders it more toxic.23 like 1311, 177Lu has a longer
`t”2 than 90Y and is therefore more suitable to the pharmacoki—
`netics of mAbs, but its chemistry is similar to that of WY and,
`when internalized, it is retained by the tumor whereas 13’I can
`be quickly released. 177Lu seems to be more effective in treating
`small lesions.23
`
`Radioimmunoconjugates against PSMA. Antigen expression on
`prostate cancer cells has been studied at length in recent years.
`Prostate-specific membrane antigen (PSMA) has emerged as
`one of the most promising targets for mAb—based therapy.
`PSMA has many of the paramount characteristics of a tumor
`target antigen, although its functional
`role is still unclear
`{Table 4).23 Interestingly, endothelial cells of tumor-associated
`neovasculature can express PSMA (including carcinoma of the
`colon, breast, bladder, pancreas, and kidney, and melanoma).
`The humanized mAb 1591, which targets the extracellular
`domain of PSMA, has emerged as one of the most promising
`carriers. Selective targeting of I ‘ 'In—labcied 1591 to tumors has
`been seen in clinical studies, and the naked mAb is well toler—
`ated in repetitive administration.24
`Prostate cancer is the most common noncutaneous cancer
`
`in men in the United States and is the second leading cause of
`cancer-related death in men. For more than 60 years, hormo—
`nal therapy has been the cornerstone of treatment for advanced
`prostate cancer. Unfortunately, hormonal therapy is mostly pal—
`liative, with little impact on survival; consequently, improved
`systemic therapies are necessary. Phase I trials of 177Lu—JS91 and
`
`Table 4 Prostate—specific membrane antigen (PSMA) has many
`ofthe ideal characteristics ofan antigen for antibody~based
`therapy
`Stable cell surface glycoprotein with minimal shedding or isoform secretion
`Abundantly expressed in prostate cancer (magnitude of expression)
`Expression increases with highvgrade tumors and hormone-refractory
`disease (upregulated by androgen deprivation)
`Highly specific: low expression in normal tissues (most notably small
`intestine, proximal renal tubule cells, and salivary glands) as compared
`with tumor
`Little phenotypic variation in expression in prostate cancer metastases
`Rapid internalization ofthe PSMA antibody complex along with any
`payload carried by the antibody
`
`VOLUME 89 NUMBER 4 | APRIL 2011 |www.nature.com/cpt
`
`IMMUNOGEN 2124, pg. 6
`Phigenix v. Immunogen
`|PR2014-00676
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`IMMUNOGEN 2124, pg. 6
`Phigenix v. Immunogen
`IPR2014-00676
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`

`

`STATE
`
`ART
`'—
`
`90Y—IS9] have been performed in castration—resistant prostate
`cancer. In a phase I trial with 177Lu—IS91, an 11% decline rate
`(250%) in prostate—specific antigen was reported, which is a
`satisfactory result in previously treated patients.25 Despite this
`biochemical indication of biological activity, significant tumor
`regressions were not seen. Another early»phase study, with 90Y-
`1591, reported that 2 0129 patients had objective responses after
`treatment.26 Although the two trials had similar patient eligibil-
`ity criteria, the study populations were not equivalent (e.g., there
`was a smaller number of patients with measurable lesions in the
`l”Lu-1591 trial), and any formal comparison must be avoided.
`However, we can hypothesize that 177Lil—1591 may he a better
`candidate for smallevolume lesions (5mm), whereas (”Y-1591
`may be more effective in larger (21 cm) tumors.23 Bone mar—
`row is the dose-limiting organ in radioimmunotherapy targeting
`PSMA. Further clinical examination, including research into
`fractionated dose regimens and combination therapy, is needed
`in proofeofeconcept studies.
`
`Antitenascin mAb. Neuradiab (previously referred to in the liter-
`ature as antitenascin radiolabeled mAb 131I-81C6) is a murine
`mAb conjugated to 13'I and is delivered directly into the surgi-
`cal resection cavity in a separate procedure after the initial sur-
`gery for glioblastoma multiforme. The intention is to deliver a
`concentrated level of radiation specifically to cancer cells that
`remain after surgery. The target, tenascin, is a protein that is
`overexpressed by 99% of all glioblastoma multiforme but is
`virtually absent from normal brain tissues. Neuradiab was
`proven to be safe in early phases of development, and phase
`II data showed a significant increase in overall survival as
`compared to currently approved therapies.27 The therapeutic
`regimen consists of a small dose of the mAb instilled into the
`surgical cavity, and the external measurements of the resultant
`absorbed radioactivity are used to calculate the patient-specific
`amount of neuradiab required to achieve the optimal targeted
`absorbed dose. This specific dose is then administered into the
`surgical cavity. Acute reversible neurotoxicity and hematologic
`toxicity are the most common adverse events. Patient—specific
`dosing decreases the chances of irreversible neurotoxicity or
`radionecrosis. A phase III study (the GLASS»ART trial, http://
`www.glassarttrial.com) comparing neuradiab plus chemora—
`diotherapy vs. the standard of care alone commenced in 2008,
`but the sponsor of the study has closed enrollment because of
`unforeseen delays in patient recruitment.
`
`Targeting carbonic anhydrase IX. G250 is a membrane—associated
`carbonic anhydrase (CA IX) that is thought to play a role in
`the regulation of cell proliferation in response to hypoxic
`conditions and may be involved in carcinogenesis and tumor
`progression. It is ubiquitously expressed in ~95% of clear-cell
`renal