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Division of Human Gene Therapy, Departments of Medicine, Pathology and Surgery, and Gene Therapy Center, The University of Alabama at Birmingham, Birmingham, Alabama 35294-3300
| ABSTRACT |
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| Ad3 Vectors for Cancer Gene Therapy Applications |
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The recent delineation of key aspects of the cell entry pathway of
adenovirus has provided insight into these observations. Adenovirus
achieves initial recognition of target cells via the primary receptor
CAR (4)
. In this regard, alternative cellular receptors
for adenovirus have been proposed; however, recent work has clearly
established the primacy of CAR in dictating viral tropism
(5)
. After anchoring at this site by virtue of the knob
domain of the fiber capsid protein, the virus achieves internalization
via interaction of the capsid penton protein with integrins
vß3 and
vß5 present on target
cells (6)
. On this basis, a relative deficiency of either
of these target cell factors could potentially limit the capacity of
the Ad vector to accomplish efficient gene delivery. Indeed, recent
studies in the context of the airway epithelium and various tumor cell
targets have noted a virtual absence of CAR on these target cells
(7)
. This observation clearly explains the unfavorable
outcomes noted in these clinical trials. Thus, it may likewise be
understood that CAR deficiency may be a significant factor limiting
vector functions in other disease contexts whereby adenoviral vector
inefficiency is the key factor limiting practical clinical usefulness.
| Retargeting Ad Vectors to Achieve CAR-independent Gene DeliveryRetargeting Complexes |
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Use of this retargeting approach has established several key concepts with respect to the goal of achieving improvements in Ad vector efficiency: (a) it could be shown unequivocally that adenovirus could achieve effective gene delivery via CAR-independent cellular entry pathways. Thus, the interaction of the targeted virion with its native receptor CAR did not appear to be crucial to its effective cellular entry capacity; (b) the achievement of CAR-independent cell infection could allow augmented levels of gene transfer. Indeed, retargeting the vector appeared an efficient means to generally improve the susceptibility of target cells in vitro and in vivo; and (c) the internalization ability of the primary receptor was not a relevant factor predicating its utility for Ad retargeting. In this regard, cross-linking of Ad to internalizing, as well as noninternalizing, receptors allowed CAR-independent gene transfer with comparable enhancement of efficiency (20) .
The technical achievement of Ad retargeting via protein complexes has
been approached by a variety of methods. In this regard, the bispecific
antibody approach has been used with viral linkage accomplished at
sites other than the fiber knob, including the penton base
(24)
. In addition, further refinements of the strategy of
antifiber retargeting complexes have been proposed. For example, a
recombinant fusion protein consisting of an antiknob single chain
antibody (scFv) and epidermal growth factor has been derived
(25)
. Recombinant molecules such as this may indeed offer
advantages for adenovirus retargeting in terms of vector production and
validation. To this end, we have recently developed such an approach
based on achieving a physiological linkage to the vector particle.
Specifically, we have derived retargeting complexes consisting of the
ectodomain of the Ad receptor CAR in fusion with retargeting ligands
(Fig. 1)
. These recombinant fusion
proteins possess the ability to effectively retarget the vector via
non-CAR pathways with enhancement of gene transfer efficiency (Fig. 1)
.
In addition, this class of fusions may allow the derivation of
recombinant retargeting complexes not achievable with incorporated
antibodies as structural components (26)
.
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| Retargeting Adenoviral Vectors to Achieve CAR-independent Gene DeliveryGenetic Capsid Modifications |
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Therefore, we sought to alter adenoviral tropism by means of exploiting
an alternate locale on the fiber knob. In this regard, we have used the
HI loop of the fiber knob. This choice was based upon the crystal
structure of the knob domain proposed by Xia et al.
(29)
, whereby the HI loop appeared to present a locale
accessible for targeting purposes (Fig. 2)
. Furthermore, other aspects of the
knob structure predicted its utility for incorporation of targeting
ligands pursuant to our goal of rerouting the Ad to non-CAR pathways.
Specifically, the fact that the HI loop is not involved in
intramolecular interactions between fiber monomers suggested that it
might be altered without deleterious effects on quaternary structure.
In addition, the length variability among adenovirus serotypes
suggested that the HI loop did not subserve a critical function. On
this basis, initial studies to establish the feasibility of
incorporating heterologous peptides within the HI loop were deemed
rational. We demonstrated that an incorporated FLAG peptide was not
deleterious to viral rescue and propagation. Furthermore, the
heterologous peptide within the HI loop of the knob was accessible at
the surface of the virion (30)
. Subsequent studies have
established that peptides of up to 63 amino acids in size could be
incorporated at the HI loop without deleterious effects with respect to
the quaternary structure of the fiber or viral infection dynamics.
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Ad vectors containing the RGD-4C peptide within the HI loop (AdRGD) were thus rescued and analyzed for their efficiency and for their mechanism of target cell binding. In this regard, binding studies demonstrated that these genetically modified Ad vectors were capable of achieving specific interaction with target cells via the recognition by the incorporated peptide of its cognate receptors (32) . This achievement of CAR-independent gene transfer allowed dramatic enhancements in gene delivery to CAR-negative cell lines that were otherwise Ad vector refractory. These levels of augmentation were noted across a range of primary tumor types including carcinoma of the ovary, carcinoma of the pancreas, cholangiocarcinoma, colon cancer, and squamous cell carcinoma of the head and neck (32, 33, 34) . Retrospective analysis of these primary tumor materials (primary lines, primary explants, and intact primary tumor samples) confirmed that profound CAR deficiency was present as a nearly universal feature of epithelial neoplasms. Importantly, this key aspect of tumor biology has not been noted in the various studies of Ad-mediated gene delivery to counterpart immortalized cell lines. Thus, for the achievement of efficient gene delivery to tumor cells in clinically relevant contexts, the exploitation of CAR-independent cell entry pathways may provide a generalized means to circumvent CAR deficiency that may be broadly relevant to tumor targets as well as to normal parenchymal cellular targets. We have also evaluated the infection capacity of AdRGD vectors after systemic vascular administration (35) . In this most stringent delivery context, the modified vector demonstrated a distribution profile distinct from the unmodified control. In addition, significant augmentations of infection of selected organs were noted. This in vivo utility of AdRGD vectors thus distinguishes the modification of the HI loop of the fiber from other proposed fiber alteration schemes in the demonstrated capacity to achieve retargeted delivery in the setting of systemic administration. Nevertheless, issues of biodistribution based on tumor access via the vascular circuit will clearly be relevant in considering adenovirus-based gene therapy approaches for disseminated disease (5) . Other approaches to alter tropism via genetic capsid modification have also been proposed. In this regard, chimeras have been generated by substitution of the fiber, or fiber knob, of the serotype 5 with corresponding components from alternate Ad serotypes (36 , 37) . Such modifications have allowed CAR-independent delivery with an enhanced ability to infect in selected instances. Furthermore, genetic modifications of the major capsid hexon has allowed incorporation of targeting ligands at defined sites within the ecodomain of that capsid component (38) . Clearly, additional possibilities for incorporation of targeting ligands within the capsid will be realized as further advancements of precise capsid structure are defined (39 , 40) .
In the aggregate, the data obtained with tropism-modified Ad vectors have established key concepts with respect to the adenovirus-based gene therapy approaches:
(a) It is clear that the evaluation of vector efficiency must be accomplished in model systems with the highest level of relevance to the intended delivery scheme and target cell. The substantial differences in vector efficiency noted with respect to immortalized cell lines compared with primary tissue render questionable the value of any data achieved with the former substrate. On this basis, vector modifications designed to improve vector efficiency must establish their utility in the context of stringent, and relevant, substrate systems; and (b) the recognition that CAR levels may play an overriding role in limiting vector efficiency predicates the development of vectors possessing the capacity to circumvent this barrier. Indeed, the recognition of a new class of Ad vectors with an enhanced ability to infect calls into question the basis of promoting further human trials with the significantly less potent Ad vectors that have been used heretofore.
It must be further noted that the enhancement of infection achieved herein clearly has relevance with respect to other vector limits noted in the use of recombinant adenovirus. In this regard, vector-associated toxicity observed in human clinical trials is closely linked to the magnitude of particle dose. The means to use significantly lower vector doses, as will logically accrue to the advanced generation vectors described here, will clearly impact this aspect of Ad vector science. On this basis, it may likewise be anticipated that vectors with an enhanced ability to infect may evoke diminished immune response consequent to the lower vector burdens used. Furthermore, the means to "untarget" antigen-presenting cells, as may logically derive from these tropism-modification maneuvers, may additionally accrue beneficial outcomes with respect to a reduction in antivector immunology.
This gain in potency will clearly also impact the basic means by which host immune responses may affect any gene therapy approach. Although the capacity of these vectors to achieve gene delivery in novel ways raises important biological and safety questions, the utility gains embodied by these agents cannot be ignored. On this basis, the rapid evaluation of these vectors in human systems is clearly warranted. Such studies will establish the importance of engineered vector design in achieving meaningful clinical outcomes in human clinical gene therapy approaches.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 This research was supported by Grants R01
HL50255, R01 CA74242, and R01 CA68245-01 from the NIH and grants from
the American Lung Association, American Heart Association, Muscular
Dystrophy Association, and the Fanconi Anemia Association. ![]()
2 To whom requests for reprints should be
addressed, at Division of Human Gene Therapy, The University of Alabama
at Birmingham, 1824 6th Avenue South, 620 Lurleen Wallace Tumor
Institute, Birmingham, AL 35294-3300. Phone: (205) 934-8627; Fax:
(205) 975-7476; E-mail: david.curiel{at}ccc.uab.edu ![]()
3 The abbreviations used are: Ad, adenoviral; CAR,
coxsackie adenovirus receptor. ![]()
Received 4/11/00. Accepted 10/16/00.
| REFERENCES |
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vß3 and
vß5 promote adenovirus internalization but not virus attachment. Cell, 73: 309-319, 1993.[Medline]
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