Available online at www.sciencedirect.com

Biologicals 36 (2008) 277e286www.elsevier.com/locate/biologicals

Replicating viral vectors as HIV vaccines:Summary Report from IAVI Sponsored Satellite Symposium,

International AIDS Society Conference, July 22, 2007

W.C. Koff a, C.L. Parks a, B. Berkhout b, J. Ackland c, S. Noble a,*, I.D. Gust d

a International AIDS Vaccine Initiative, New York, USAb Academic Medical Center of the University of Amsterdam, Amsterdam, The Netherlands

c Global BioSolutions, Melbourne, Australiad University of Melbourne, Melbourne, Australia

Received 3 April 2008; accepted 19 April 2008

Abstract

At the International AIDS Society Conference on Pathogenesis, Treatment and Prevention held in Sydney, Australia, in July 2007, theInternational AIDS Vaccine Initiative (IAVI) convened a satellite symposium entitled ‘Accelerating the Development of Replicating ViralVectors for AIDS Vaccines.’ Its purpose was to highlight the rationale for accelerating the development of replicating viral vectors for useas vaccines against HIV-1, and to bring together vaccine scientists, regulatory officials, and public health specialists from industrialized anddeveloping nations to discuss the major issues facing the development and testing of replicating viral vector-based vaccines.� 2008 The International Association for Biologicals. Published by Elsevier Ltd. All rights reserved.

Keywords: HIV; AIDS; Vaccine; Replicating vector

1. Introduction

Vaccine candidates developed to date have been unable toelicit broadly neutralizing antibodies or immune responsescapable of controlling HIV following infection. This contrastswith multiple studies demonstrating that vaccination ofmacaques with live-attenuated SIV vaccine protects againstdisease after challenge with pathogenic SIV [1e4].Thesenon-human primate (NHP) studies have demonstrated thatlive-attenuated SIV vaccines outperform current vaccine can-didates based on non-replicating vaccine platforms, includingprotein subunit, peptide, whole inactivated virus or virus-likeparticle, DNA, or replication-defective vector (e.g. adenovirus,poxvirus) [5e25]. The efficacy of vaccination with a live-attenuated SIV in non-human primates is consistent with thehistorical performance of licensed live-attenuated vaccines,

* Corresponding author. Tel.: þ1 212 847 1056; fax: þ1 212 847 1112.

E-mail address: snoble@iavi.org (S. Noble).

1045-1056/08/$34.00 � 2008 The International Association for Biologicals. Publi

doi:10.1016/j.biologicals.2008.04.004

which are among the most effective for human infectiousdiseases (e.g. measles, mumps, rubella).

Many in the AIDS vaccine field were hopeful that the mostpromising of the current vaccine candidatesdMerck’s replica-tion-defective adenovirus serotype 5 (MRK-Ad5) expressingthe HIV gag, pol, and nef genesdwould provide some benefitto vaccinated individuals, and that multiple vaccinations withMRK-Ad5 would elicit immunity capable of suppressing viralload and slowing disease progression in those individuals thatwere later infected with HIV. However, these hopes weredashed when recent results from a Phase IIb proof-of-conceptefficacy trial demonstrated no benefit to vaccinees [26]. Thisfailure of the leading candidate has increased the need fordevelopment of novel, more effective candidate HIV vac-cines. Moreover, the clinical trial results proved to be consis-tent with findings from preclinical studies conducted inmacaques infected with SIV, indicating that this model systemshould be utilized as a primary research tool for vaccinedevelopment [27].

shed by Elsevier Ltd. All rights reserved.

Table 1

AIDS vaccine development pipeline: candidates in clinical trials

DNA vectors

Clade B’C electroporation IAVI ADARC

Clade B minigenes þ protein Epimmune Pharmexa

DNA polyepitopic, MVA boost Epimmune Bavarian Nordic

278 W.C. Koff et al. / Biologicals 36 (2008) 277e286

Development of a live-attenuated HIV vaccine is notcurrently a viable option because of the difficulty in demon-strating adequate attenuation, the risk associated with genomeintegration, and the risk of reversion to a virulent phenotype[28e30]. Attenuated SIV can revert to virulence and causedisease over time in vaccinated animals [31e34] and,similarly, an HIV-1 deletion variant eventually producedAIDS in some infected persons among the Sydney BloodBank Cohort [35]. HIV strains with multiple attenuating muta-tions (the HIV-1D3 variant with deletions in the vpr, nef, andLTR sequences) can also regain replicative capacity throughcompensatory mutations accrued during long-term cell culturepassage [36]. The capacity of attenuated HIV and SIV topersist in the host and evolve compensatory mutations posesa serious safety risk that currently prevents consideration oflive-attenuated HIV vaccines in humans.

As a result some vaccine developers are now beginning tofocus on replicating viral vectors, aiming to mimic the efficacyof live-attenuated vaccines while ensuring a safety profileappropriate for mass immunization campaigns. But replicatingviral vectors raise new challenges for vaccine developers,regulatory authorities, and the general public regarding riskand benefit. With close to 7000 new HIV infections everyday [37] there is an urgent need for an effective AIDS vaccine,and risk-benefit assessments may differ in regions of the worldwhere HIV incidence is greatest, such as in sub-SaharanAfrica, compared with other regions with minimal HIVincidence.

Clade B, MVA boost GeoVax

Multiclade A, B, C, Ad-5 boost NIH VRC

Multiclade A, B, C, MVA boost Karolinska

Clade B’C, MVA boost Johns Hopkins Guangxi

Clade B/C Changchun Baike

Clade B, C, NYVAC boost EuroVacc

Clade B þ IL12, IL-15, peptide boost Wyeth

DNA clade C, MVA boost SAAVI

Clade B electroporation University of Pennsylvania/

Penvax

Viral vectors

Adenovirus

Ad-5 clades A, B, C, DNA prime NIH VRC

Ad-5 clade B Merck

Ad-6 clade B Merck

Ad-35 clade A, Ad-5 prime NIH VRC GenVec

Ad-26 clade A NAIDS Harvard

Poxvirus

Canarypox clades B, E, gp120 prime Aventis US Army

MVA clade B, DNA prime GeoVax

MVA clade B/C Changchun Baike

MVA clade C IAVI

MVA clades A, E, DNA prime WRAIR

Vaccinia multiclade

(DNA and protein cocktail)

St. Jude’s Hospital

MVA clade B Bavarian Nordic

Other viral vectors

VEE clade C AlphaVax

AAV clade C Targeted Genetics/IAVI

Proteins

gp140 clade C mucosal St. George’s University

gp140 þ LTK63/MF59 Novartis/St George’s University

p24 þ fragment gp41 Ivanovsky Institute

2. Rationale for consideration of replicating viral vectorsas HIV vaccines

To begin constructive dialog, the IAVI satellite symposiumsought to address the issues surrounding development andevaluation of replicating viral vectors. Ian Gust (Universityof Melbourne) chaired the symposium and began by settingthe stage for the scientific presentations. He noted that safetyhas become the overriding concern of regulatory authoritiesdealing with vaccine development, and illustrated this byrecounting two examples; the recently-licensed human papil-lomavirus vaccine that had been tested in clinical trials involv-ing more than 70,000 volunteers at a cost of over US$200million, and the promising live-attenuated hepatitis A vaccinein the early 1980s that was effective in NHPs but occasionallyled to increased transaminase levels, suggesting moderate liverinflammation. The company (Smith Kline) developing thelive-attenuated hepatitis A vaccine judged the potential safetyrisks too great and, fearing that some recipients might sufferjaundice, decided not to develop this candidate. Gust alsonoted that although today there is a great emphasis on thedevelopment of inactivated and replication-defective vectors,many of the most effective vaccines currently in usedmeasles,mumps, rubella, chickenpox, and oral polio vaccinesdarebased upon live-attenuated viruses. Based on the paucity ofprotection observed in SIV/macaque studies and the lack ofrobust immunogenicity of candidates tested so far in clinical

trials, Gust declared that it is time to reconsider the use ofreplication-competent viral vectors for use in HIV vaccines.

Wayne Koff (IAVI) provided a comprehensive overview ofthe state of the field, emphasizing that almost all of the currentcandidates in the pipeline are focused primarily on inductionof cell-mediated immunity and that none of them elicitsbroadly neutralizing antibodies (Table 1). Also, none arelikely to induce effective mucosal immune responses at thegut-associated lymphoid tissue (GALT), a critical site forHIV amplification following initial infection.

Koff noted that studies with replication-defective SIVvaccine that undergoes only a single round of replication failedto provide protection against challenge with pathogenic SIV, incontrast to the robust protection conferred with live-attenuatedSIV [6,7,38,39]. These studies prompted IAVI, through itsLive Attenuated Consortium, to try to elucidate the mecha-nism for protection with live-attenuated SIV and use this infor-mation to develop the next generation of replicating viralvectors [3].

He then presented a summary of published NHP studiesdemonstrating the effectiveness of live-attenuated SIV (Dnef,

279W.C. Koff et al. / Biologicals 36 (2008) 277e286

D3, or D5G) protection against challenge with homologouspathogenic SIV, showing that 94% of the vaccinated animalsmaintained a >3 log suppression of viral load at setpoint; incontrast, all other types of vaccine tested in NHPs protectedonly 7% of the animals as measured by the same criterion[5,8,9,11,12]. Encouragingly, recent data indicates thata live-attenuated SIVmac239Dnef vaccine can lower viralsetpoint by 2 logs even after heterologous pathogenic SIVE660 challenge, providing further support for investigatingvaccines based on replication-competent viral vectors (MRReynolds et al., J. Exp. Med., in press). Koff noted that accel-erating the development and testing of replicating viral vectorsfor HIV vaccines will require a partnership among vaccinedevelopers, regulatory agency scientists, developing countryscientists, public health officials, institutional review boards,and communities where the clinical trials will take place.

3. Case study: vesicular stomatitis virus(VSV)-vectored HIV vaccine development

Chris Parks (IAVI) provided an overview of replicatingviral vectors in the pipeline, and a case study that illustratessome of the hurdles faced during development of replica-tion-competent VSV vectors in HIV vaccines. He noted thatthere are a variety of technological platforms that can beused for candidate HIV vaccines (see Fig. 1) and that this isone of the most influential factors in determining the safetyand efficacy of the final vaccine. Currently, the vast majority

Fig. 1. The technology platforms used to make vaccines are arranged in a circular

efficacy. The figure is subdivided by the dashed line into vaccines that replicate in

of HIV vaccine resources are devoted to development andtesting of non-replicating vector vaccines (Table 2).

The emphasis placed on investigating non-replicatingvaccine platforms (Fig. 1) has led to the development and test-ing of an impressive number of sophisticated and imaginativevaccines and vaccination regimens. Yet results from clinicaltrials have failed to validate any non-replicating viral vector,DNA vaccine, or subunit antigen as the platform of choicewith which to aggressively develop an HIV vaccine.

Parks then gave a brief historical review of viral vaccines toprovide insight into why current HIV vaccine strategies haveso far failed to produce promising candidates. He emphasizedthat a number of extremely effective viral vaccines have beendeveloped and used safely for decades to control highly infec-tious pathogens that cause devastating disease (Table 3 andreference [40]). It could be argued that HIV is a far moreformidable opponent because it has evolved sophisticatedmechanisms to evade host antiviral defenses (Table 4). Onthe other hand, many of these evasion strategies have beenovercome by vaccination before: for example, variola virus[41] and others [42,43] encode polypeptides that modulatespecific host pathways required to engage innate antiviraldefenses or mount an effective immune response; measlesvirus, like HIV, is lymphotropic and infection causes consider-able lymphopenia and an acute state of immune suppression[44e46]; both varicella zoster virus [27,47] and hepatitis Bvirus [48,49] have evolved mechanisms by which theirgenomes persist for the life of the host; and a number of viral

diagram to illustrate that their unique characteristics influence both safety and

the vaccinated host and those that do not.

Table 2

HIV vaccine vectors advanced into clinical trials

Vector Group Site of replication Notes

Vaccinia virus Poxviruses, enveloped Cytoplasmic replication cycle Host-restricted (cowpox or cowpox-variola recombinant)

NYVAC (vaccinia) DNA viruses Host-restricted, further attenuated vaccinia virus

ALVAC (canarypox) Host-restricted; abortive replication in mammalian cells

Fowlpox Host-restricted; abortive replication in mammalian cells

MVA Host-restricted; abortive replication in mammalian cells

Adenovirus type 5 (E1�E3�) Non-enveloped DNA viruses Nuclear replication cycle Non-replicating

Adeno-associated virus (AAV)

Venezuelan equine encephalitis virus Enveloped (þ) sense RNA virus Cytoplasmic replication cycle Viral replicon

Plasmid DNA Naked nucleic acid Transcription in the nucleus Does not propagate

Sources included on line databases maintained by the International AIDS Vaccine Initiative (http://www.iavireport.org/trialsdb/), the HIV Vaccine Trials Network

(http://www.hvtn.org/science/trials.html), and the Los Alamos National Laboratory (http://www.hiv.lanl.gov/content/index). Literature sources included.

280 W.C. Koff et al. / Biologicals 36 (2008) 277e286

pathogens, notably influenza A virus, have highly mutableRNA genomes that can promote rapid antigenic evolutionand diversity [50e52].

Parks also emphasized that many successful viral vaccinesare live-attenuated viruses (Table 5). Arguably, this indicatesthat vaccination with an attenuated virus is a well-establishedmeans by which to elicit a protective immune response againsta complex viral pathogen. As noted above, there is a growingbody of preclinical data that also supports this argument forSIV and, by extension, HIV.

But it would be very difficult to demonstrate that an HIVvaccine is completely attenuated and unable to revert to viru-lence, making further development of this concept in humansproblematic. The vaccine developer would have to assemblean exceptionally convincing dossier of safety data to convincethe scientific community, regulatory agencies, vaccinemanufacturer executives, activists, and the public that a live-attenuated HIV vaccine could be administered safely tohumans. Given that there is no relevant preclinical animalmodel to study the HIVehost interaction, quantifying theresidual virulence of an attenuated HIV strain and determiningits relationship to vaccination risk in humans will be tremen-dously complicated and could take decades. Similarly, therisk of genetic reversion that restores pathogenicity or therisk associated with retroviral DNA integration into the hostchromosome will be difficult to determine. Consequently,the possibility that a live-attenuated HIV strain, or any live

Table 3

Vaccine-preventable morbidity in the United Statesa

Baseline cases Cases

Time period Average number of cases 1970

Smallpox 1900e2004 48164 0

Paralytic Polio 1951e1954 16316 33

Measles 1958e1962 503282 47351

Mumps 1968 152902 104953

Rubella 1966e1968 47745 56552

Congenital Rubella 1966e1968 823 67

Varicella 1972 164114 nrb

a Contents derived from online sources including: Fenner F (1988) Smallpox an

Morbidity and Mortality Weekly Report (April 2, 1999; http://www.cdc.gov/mmw

Deaths (http://www.cdc.gov/vaccines/pubs/pinkbook/downloads/appendices/G/caseb nr, not recorded.

retrovirus vaccine, can be advanced into clinical trials seemsto be remote. This reasoning would rule out the two live-attenuated and ‘‘Jennerian’’ HIV vaccines (Fig. 1), makingreplicating viral vector vaccines the logical next choice.

Such replication-competent vectors are promising becausethey retain many of the traits that make viruses immunogenic:efficient delivery through natural virus receptor-mediatedpathways; immunogen expression within infected cellsprovides natural context for polypeptide synthesis, post-translational processing and folding, and subsequent antigenpresentation to the immune system; and, importantly, replica-tion resulting in abundant and sustained expression of antigen.There are additional characteristics that will determinewhether a virus is a promising vector platform for deliveryof an HIV vaccine (Table 6). Two of the more influentialdthatthe virus is not a human pathogen and neutralizing antibodiesagainst it are rare in humansdfavor animal viruses as candi-date vectors and there are a growing number in development,including chimp adenoviruses [53], monkey parvoviruses [54]vesicular stomatitis virus (VSV) [55], Sendai virus [56], bo-vine parainfluenza virus [57], and some animal pox and herpesviruses [58,59].

Developing a replicating viral vector for use in humans istime-consuming, complex, and costly. This is exemplifiedwell by the NIAID/DAIDS-sponsored HIV vaccine programrun by Wyeth Vaccines in which a candidate vaccine basedon VSV is under development [55,60]. VSV-HIV vaccine

Vaccine usage

1980 1990 2000 2005 2006

0 0 0 0 0 1798

9 6 0 1 0 1955

13506 27786 86 66 45 1963

8576 5292 338 314 6339 1967

3904 1125 176 11 8 1969

14 32 8 1 1 1969

190894 173099 27382 32242 42173 1995

d its eradication (WHO; http://whqlibdoc.who.int/smallpox/9241561106.pdf),

r/preview/mmwrhtml/00056803.htm) and the Pink Book Reported Cases and

s&deaths.pdf).

Table 4

Scientific challenges in HIV vaccine development: a summary of mechanisms by which HIV evades immune responses [83]

Genetic persistence Integration of the retroviral genome into host chromosome is an underlying mechanism of persistence that is difficult

for the host to overcome

Inactivation of antiviral defenses HIV encodes a number of proteins that modulate the host’s antiviral responses

Deception HIV Env rarely elicits broadly neutralizing antibodies because critical epitopes are poorly accessible to the immune system

and the antibody repertoire is directed towards non-neutralizing epitopes

Evolution RNA genome is highly mutable, providing a mechanism by which to generate immense genetic and antigenic diversity

Immune system destruction HIV propagation eventually causes T-cell depletion, immune system dysregulation, and eventual immune response failure

281W.C. Koff et al. / Biologicals 36 (2008) 277e286

prototypes, one encoding HIV Env and a second encoding SIVGag, were first developed in John Rose’s laboratory at YaleUniversity [61e64]. Preclinical studies demonstrated thatvectors were highly immunogenic and that vaccination ofmacaques protected them from disease after intravenouschallenge with SHIV89.6P. The vectors also exhibited promis-ing safety characteristics; no adverse reactions were attributedto intramuscular, intranasal, or oral administration.

Clinical development of the VSV-HIV vaccine prototypeappeared straightforward given the favorable preclinicalresults and the fact that VSV is not typically associated withhuman disease, but Wyeth researchers proceeded cautiouslysince VSV had been shown to be neuroinvasive and neurovir-ulent when administered to neonatal mice at peripheral sitesand that both neonate and older mice were susceptible to directintracranial instillation [55,65e67]. A pilot neurovirulencestudy in macaques with vector prototypes [60,65,68] revealedthat intranasal administration produced no detectable histolog-ical changes in the central nervous system. However, viralreplication was evident in the central nervous system whenadministered directly into the brain by intrathalamic injection.Consequently the Wyeth team spent years engineering andtesting modified vectors, producing attenuated variants that

Table 5

Viral vaccines licensed for use in the United Statesa

Disease Virus

Smallpoxb Variola virus

Measles Measles virus

Mumps Mumps virus

Rubella and congenital rubella syndrome Rubella virus

Poliomyelitis and paralytic polio Polio virus

Hepatitis A Hepatitis A virus

Hepatitis B Hepatitis B virus

Influenza Influenza virus

Yellow fever Yellow fever virus

Japanese encephalitis Japanese encephalitis virus

Varicella Varicella zoster virus

Acute viral gastroenteritis Rotavirus

Rabies Rabies virus

Cervical dysplasia Human papillomavirus

a Contents derived from http://www.fda.gov/cber/vaccine/licvacc.htm.b mass vaccination discontinued after smallpox eradication; see Fenner F (198

9241561106.pdf).c Now that the risk of poliovirus infection has been minimized in the United Stat

risk of vaccine-associated paralytic poliomyelitis. Vaccination with oral poliovirus

live vaccine was instrumental in controlling poliovirus infection in the United State

easy to administer it is still the primary vaccine used in the world-wide poliovirus

were minimally neurovirulent when directly instilled into thebrains of neonatal mice or macaques, one of which (VSV-N4CT1-gag1) is now being advanced towards Phase I studies[69e72]. This highly attenuated VSV-HIV candidate shouldbe safe for human vaccination, although it remains to be deter-mined whether it will be significantly more immunogenic orefficacious than non-replicating vectors given that it now hasa considerably restricted replicative capacity.

The VSV-HIV vectors example gets to the crux of thedebate: what is the appropriate balance between vaccine safetyand the need to advance potentially more efficacious HIVvaccine candidates into the clinic? This is particularly difficultsince it is likely that vaccines with increased replicative capac-ity will have significantly enhanced efficacy and the pool ofpromising vector candidates includes an increasing numberof animal viruses for which little is known about their patho-genic potential in humans. There is also an urgent need toidentify preclinical safety models that are relevant to humanvaccination. VSV-HIV vectors are a case in point; even thoughcandidate vaccines had been administered safely to more than150 macaques intranasally, orally, or intramuscularly [73],development of the prototype vaccines was discontinuedbecause, perhaps predictably, neurological pathology was

Vaccine type

Live-attenuated (immunologically related, host-restricted animal virus)

Live-attenuated

Live-attenuated

Live-attenuated

Live-attenuatedc; inactivated virus

Inactivated virus

Protein subunit

Viral protein extract vaccine; live-attenuated

Live- attenuated

Inactivated virus

Live-attenuated

Live-attenuated (immunologically related, host-restricted human/animal

virus recombinant); live-attenuated human virus

Inactivated virus

Virus-like particles

8) Smallpox and its Eradication (WHO; http://whqlibdoc.who.int/smallpox/

es, the live oral poliovirus vaccine is no longer recommended due to the small

vaccine has been replaced by a regimen of 4 doses of inactivated vaccine. The

s and elsewhere, and because it is very efficacious, relatively inexpensive, and

eradication program.

Table 6

Properties sought in replicating viral vectors considered for development of HIV vaccines

Recombinant vaccine vector development Vector can be engineered readily

Adequate capacity to accept 1 or more sizable foreign coding sequences (i.e. HIV env or pol )

Genetically stable; recombination is rare

Capable of abundant gene expression, but it can be modulated through vector design

Propagates efficiently in cell lines acceptable for vaccine development

Immunogenicity and efficacy Propagates adequately in the vaccinee to produce abundant and sustained antigen expression from infected cells

Induces durable T and B cell responses and immunologic memory

Exhibits cell and tissue tropism capable of inducing mucosal immunity

Minimal effect of pre-existing immunity

Pre-existing immunity is uncommon in humans

Or the effect of pre-existing neutralizing antibody is minimal

Or the vector can be engineered to minimize the effect of pre-existing immunity

Sufficiently immunogenic to be efficacious without use of complex heterologous prime-boost vaccination regimens

Animal models are available or can be developed to evaluate efficacy

Safety The vector propagates following administration but is restricted sufficiently to prevent disease

The vector genome does not integrate into the host chromosome

Ideally, the vector is not a human pathogen and is not neurotropic/neuroinvasive or cardiotropic

Animal models exist or can be developed to thoroughly investigate safety relevant

to human vaccination

282 W.C. Koff et al. / Biologicals 36 (2008) 277e286

evident after inoculation of 1 � 107 pfu VSV-HIV directly intothe brain of macaques. Clearly this would prevent advance-ment of the first-generation VSV-HIV vaccines into the clinic,but it also seems reasonable to ask whether the outcome ofdirect instillation of vector into the macaque brain is a relevantmeasure of safety, particularly for a vector derived from a virusthat typically does not cause human disease and that wasintended for intramuscular delivery. These questions will becommonplace if replicating viral vectors emerge as an impor-tant HIV vaccine strategy. Researchers and regulators willhave to discuss these issues early in vector development sincethe most useful safety data will likely come from preclinicalstudies tailored to each specific vaccine platform.

4. Preclinical development of next generationreplicating vectors

Ben Berkhout (University of Amsterdam) and LinquiZhang (Chinese Academy of Medical Sciences) then presentedtwo additional case studies describing replicating vectorsystems that could be either a promising tool to elucidatemechanisms of protection by live-attenuated SIV vaccinationor a potential HIV vaccine vector, respectively.

Berkhout described the construction and evaluation ofconditional replication-competent HIV and SIV vectors[69e72,74,75] that he and his colleagues developed, which arecontrolled by doxycycline (dox). HIV and SIV gene expressionis normally activated by the Tat protein through its interactionwith the trans-acting responsive (TAR) region in the 50 end of na-scent RNA [76]. Conditional-live HIV variants were constructedby mutating Tat and the TAR element and functionally replacingthe transcriptional activation pathway with the dox-induciblegene expression system (Tet-On system) [77]. Upon infectionwith this virus, replication can be temporarily activated and con-trolled to the extent needed for induction of immune responsesby transient dox administration. Berkhout and colleaguesrecently constructed a similar dox-dependent SIV variant that can

be used to study the efficacy and safety of a conditional-live virusvaccine in macaques [78]. This SIV variant might also be anattractive tool to study mechanisms by which attenuated SIVinduces immunity; in particular, conditional-live viruses mightbe used to determine whether persistence is required to elicitprotective immunity. Based on information from such studies,vaccine designers may focus greater efforts on persisting repli-cating vectors (e.g. cytomegalovirus) rather than non-persistingreplicating vectors (e.g. reovirus).

Linqui Zhang presented studies on a replication-competentattenuated strain of vaccinia virus (Tian Tan) that is currentlybeing developed as a candidate HIV vaccine vector. The TianTan strain was used as a smallpox vaccine in China, effectivelyimmunizing millions of people [79]. Although less virulentthan the WR strain of vaccinia virus, the Tian Tan strain causessignificant weight loss in mice after intranasal administrationand consequently a further attenuated variant was considereddesirable for HIV vaccine vector development. The completeTian Tan genome has been sequenced making it possible tointroduce targeted mutations, and Zhang described the devel-opment of a genetically modified further attenuated derivativeof Tian Tan in which HIV genes have been inserted. Thisvector induced systemic and mucosal immune responses inpreclinical studies against the encoded HIV proteins, andZhang and his colleagues plan to advance this modified TianTan-based vector to clinical development.

5. Replicating vectors: addressing the regulatory issues

A panel of regulatory experts addressed the regulatoryissues associated with accelerating the development of repli-cating viral vectors. Jim Ackland (Global BioSolutions) notedthat, as with any novel product, there are both predictable andunpredictable potential risks. Unpredictable risks can only beassessed during clinical development, whereas many of thepredictable risks can be assessed during preclinical develop-ment by in vitro and in vivo assays.

283W.C. Koff et al. / Biologicals 36 (2008) 277e286

Predictable risks associated with the virus vector involvethe potential for pathogenicity and transmission of the virus.A thorough understanding of virulence in both animals and hu-mans is important in trying to assess the risk to both recipientsand contacts of recipients. Vaccine developers also need toremain vigilant about contamination with adventitious agents,genetic stability of the vector and its antigenic insert, toxicity,the distribution and persistence of viral vector and HIVgenome, and recombination with wild type or other virusesthat might facilitate reversion to virulence. Some of these riskscan be minimized by utilizing a well designed and controlledmanufacturing process, and appropriate selection of the virusand design of the product. Ackland noted that there are onlylimited regulatory authority guidelines available for viralvaccines and he counseled vaccine developers to engage regu-latory authorities early and often throughout the developmentprocess.

Keith Peden (US Food and Drug Administration) indicatedthat replicating virus vectors presented the regulatory authori-ties with significant challenges due to their potential to evolverapidly in the human host. He concurred with Ackland’scounsel on the importance of continually engaging regulatoryauthorities, and emphasized the importance of virus tropismwhen choosing a viral vector, cautioning that neurotropic orcardiotropic viruses would present the greatest challenges toregulators. He also noted that it was critically important toselect and characterize a suitable cell line for viral vectorproduction. He elaborated on the advantages of producingvectors from cloned DNA rather than using isolates thatwere derived by other biological methods, because this elimi-nates some of the concerns related to adventitious agents andprovides a means by which to rapidly re-derive the vector ifnecessary.

Gary Grohmann (Therapeutic Goods Administration, Aus-tralia) said that replicating viral vectors present new groundbeyond conventional vaccines and thus the importance for di-alog, not only with the regulatory agencies but also with thegeneral public. The panel noted collectively that regulatorypositions do change when presented with compelling data,and provided the recent example of products being manufac-tured in continuous cell lines such as PER.C6 and MDCKadvancing through various stages of clinical development.

Since some AIDS vaccine efficacy trials will be conductedin regions of the developing world with greatest HIV inci-dence, understanding the perspective of their regulatoryagencies is critical for accelerating the development of repli-cating vectors. Helen Rees (University of Witwatersrand,Johannesburg, South Africa) addressed the issue of regionaldifferences in vaccination risk-benefit analyses, pointing outthat some of the inequities in global health research greatlyaffect attitudes in developing countries, including biomedicalregulatory attitudes. She further noted that the ‘guinea pig’perceptiondthe testing of vaccines in developing countries,which are not evaluated in developed countriesdcontinuesto be a major concern for regulators and local institutionalreview committees, and affects the political attitudes towardclinical research conducted in developing countries.

Rees noted that although there has been a significantincrease in the number of clinical trials conducted in develop-ing countries, clinical trial review is still an emerging area ofexpertise and regulatory agencies in developing countries stillrely heavily on guidelines and decisions made by the US Foodand Drug Administration (FDA) and European Agency for theEvaluation of Medicinal Products (EMEA) as the default stan-dard. Similarly, the capacity to evaluate trial ethics is stillemerging. So the onus remains on sponsors to ensure that trialsare being conducted with sufficient ethical and regulatoryoversight. Finally, she noted that many developing countrieslook toward the World Health Organization (WHO) as anindependent and trustworthy body that can facilitate accep-tance of new concepts, and strongly recommended that theWHO be engaged in an expert consultation to continue thediscussion about strategies for accelerating the developmentof replicating viral vectors.

6. Summary

The relationship between vaccine development, efficacy,safety and acceptable vaccination risk is extraordinarilycomplex and goes well beyond scientists and regulatorsdiscussing the pros and cons of relevant preclinical modelsand vaccination riskebenefit projections; public opinion andpolicymakers will be essential to this debate as well. In thecurrent public opinion environment, there is very little toler-ance for vaccination risk. Ironically, the huge success of publichealth vaccination programs in many countries has fosteredintolerance of vaccination risk by controlling serious patho-gens and removing them from the public’s awareness fordecades [80,81]. When faced with a lethal pathogen likesmallpox, the public was remarkably tolerant of adversevaccination events, some of which were severe [75]. Bycontrast, a safe and efficacious vaccine against measles virusused for decades has been blamed for a host of side effects[76] despite the absence of any credible scientific evidenceto support these claims and in spite of the fact that loweredvaccination coverage quickly leads to re-emergence ofmeasles virus and severe consequences [82]. This intensefocus on side-effects already has been and will continue tobe an overwhelming consideration in vaccine development.For the vaccine manufacturer, there are daunting financialburdens associated with the very large clinical trials neededto estimate vaccination risk and detect rare but potentiallyserious adverse events. For example, Merck conducted studiesthat included over 40,000 children while testing their recently-licensed live rotavirus vaccine. Expectation that the market-ability of future vaccines will incur the expense associatedwith clinical studies of this magnitude will be an unavoidableelement of future considerations in the vaccine industry. Thisclinical trial burden, combined with the knowledge that HIVis a tremendously difficult vaccine target and the fact thatmore reactogenic live vaccines will commonly fail currentpreclinical safety testing standards, makes the prospect offuture HIV vaccine development very challenging for leadersin industry. Thus, if more aggressive HIV vaccine strategies

284 W.C. Koff et al. / Biologicals 36 (2008) 277e286

are to be tested in a meaningful way, vaccine developers andregulators will need to engage in thoughtful dialog focusedon modification and refinement of the methodology to assesssafety, efficacy and risk-benefit. Twenty-five million individ-uals have already died of AIDS, 33 million are living withHIV, and almost 7000 are infected every day, providing theurgency for accelerating AIDS vaccine development. In thiscontext, IAVI will continue its efforts towards discovering,comparing, and prioritizing the most effective viral-basedreplication-competent vectors for development and testing asHIV vaccines.

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