key: cord-1032306-yh96792l
authors: Reichert, Janice M.
title: Trends in the Development and Approval of Monoclonal Antibodies for Viral Infections
date: 2012-08-16
journal: BioDrugs
DOI: 10.2165/00063030-200721010-00001
sha: 19aa2504d4d33b58d04e7895217e8ef62675f437
doc_id: 1032306
cord_uid: yh96792l

Monoclonal antibodies (mAbs) developed for either the prevention or treatment of viral diseases represent a small, but valuable, class of products. Since 1985, commercial firms have initiated clinical studies involving a total of 28 mAbs. To date, one product (palivizumab) has been approved and eight candidates are currently in clinical study. Most commercial mAbs studied as antiviral agents in the clinic have either directly or indirectly targeted human immunodeficiency virus, respiratory syncytial virus, or hepatitis C virus infections. However, the ability of mAbs to bind to specific targets and utilize various anti-infective modes of action would seem to make them well suited for the prevention and/or treatment of a wider variety of viral diseases. A number of factors, including the continuing need for innovative medicines for viral infections, the global spread of viral infections, and increased government funding for the study of pathogen countermeasures, have prompted companies to reconsider mAbs as antiviral agents. Public sector research into the use of mAbs against emerging pathogens, such as severe acute respiratory syndrome coronavirus, may have already provided candidates for further development.

Antibodies are produced by the immune system to combat

To inform future efforts in the research and development of these innovative agents, an overview of trends in the commercial invading organisms such as viruses. Prior to the development of development of mAbs for viral infections, with a focus on mAbs monoclonal antibodies (mAbs), polyclonal antibody preparations for HIV, RSV, and hepatitis C virus (HCV) infections, is provided, derived from human serum were used for both prophylaxis and the and the possibility of increased efforts to develop mAbs for treatment of a number of viral infections. [1] mAbs, which can be emerging pathogens is discussed. designed to function using various modes of action, seem to be well suited to use as antiviral interventions. However, mAbs are 1. Analysis Criteria inconvenient to administer compared with oral antibiotics and provide protection from infection for much shorter time periods

Since it was founded in 1976, the Tufts Center for the Study of compared with vaccines. mAbs also tend to be more expensive Drug Development has collected data on the development and than either antibiotics or vaccines. As a consequence, in the past approval of therapeutics and vaccines. Data for mAbs sponsored in mAbs have not been the interventions of choice for infectious clinical and preclinical study by commercial firms were collected diseases. In fact, mAbs for infectious diseases have comprised by the survey of pharmaceutical and biotechnology firms and from only 13% of the total number of mAbs in clinical development, [2] information in the public domain (e.g. press releases and the and only one product (palivizumab for the prevention of respiratomedical literature). Of all commercial mAbs studied in the clinic, ry syncytial virus [RSV] infection in high-risk infants) is currently 28 candidates studied primarily for their ability to prevent or treat marketed. Due to the recent focus on emerging viral diseases such viral infections were identified (nonspecific immunomodulatory as severe acute respiratory syndrome (SARS), for which limited mAbs were excluded). The 28 mAbs entered clinical study betreatment options and no vaccines exist, this situation might be tween 1985 and 2005, and were composed of either a single mAb changing.

or a combination of mAbs (mAb cocktails).

As of July 2006, the development status of mAbs was that eight also be designed to pursue their quarry into the interior of cells, were in clinical studies and not yet approved in any country (five though this is a more complicated mode of action compared with in phase I, one in phase I/II, one in phase II, and one in phase III the other mechanisms. development), one was approved in the US and other countries, Between 1985 and 2005, companies sponsored clinical studies and the development of 19 had been discontinued (mAbs were of 28 mAbs as either monotherapies or mAb cocktails for the considered discontinued in clinical study if no studies were identiprevention or treatment of infection with 6 viruses; these viruses fied as ongoing or recently completed and the sponsoring compawere HIV (15 mAbs), RSV (4 mAbs), HCV and cytomegalovirus ny indicated that no additional studies were planned in the near (3 mAbs each), hepatitis B virus (2 mAbs), and human rhinovirus future). The 'preclinical' category comprised candidates that had

(1 mAb). The candidates were in-licensed as well as self-originatbeen tested in animals; a total of nine such antiviral mAbs sponed. As a result, they most likely represent the most promising sored at least in part by commercial firms were identified. All mAbs with commercial potential for these indications. Nearly half mAbs either originated at companies or were licensed from com-(46%) of the mAbs were human, a quarter were humanized mAbs, mercial, government, or academic sources. Data were updated and the remainder were either murine (14%), chimeric (11%), or with all changes noted through July 2006. bispecific (4%) antibodies. The human, humanized, and chimeric mAbs were all either IgG1 (87%) or IgG4 (13%). Four product candidates were mAb cocktails. A trend towards the study of more 2. Antibodies as Antiviral Agents mAbs was observed -of 24 candidates that entered clinical study in the four 5-year periods shown in figure 1, the largest number (8 Antibodies are complex glycoproteins produced by B-lympho-mAbs) began studies between 2000 and 2004. A further four cytes. The molecules bind to, and help eliminate, foreign and mAbs for viral infections entered clinical study in 2005. infectious pathogens in the body. Antibodies are Y-shaped, having two branches attached to a single stem. The tips of the branches 3. Current Commercial Clinical Development bind to the target, while the stem, also called the crystallizable fragment (Fc), can perform several functions, including the activa-Of the 28 mAbs in the data set, the majority (79%) were for tion of components of the human immune system. Specifically, HIV, RSV, or HCV infections. In fact, all the mAb candidates now after binding to a target, antibodies can recruit effector cells (e.g.

under clinical study are for these three infections (see table I) . neutrophils, macrophages, natural killer cells) or activate comple-Most mAbs currently in studies are potential treatments for HIV ment to destroy the target. [3] These two modes of action are infection, though none has progressed further than phase II studreferred to as antibody-dependent cell cytotoxicity (ADCC) and

ies. The only antiviral mAb in phase III is an anti-RSV mAb, complement-dependent cytotoxicity (CDC), respectively, and are motavizumab, which is an improved version of the approved antimediated through the Fc portion of the molecule. There are five RSV agent palivizumab. Two mAbs intended as treatments for classes of antibodies, the most abundant of which is IgG, and there patients with HCV infection are currently at phase I. It is important are four isotypes (IgG1-4) within the IgG class. Human IgG to note that results from phase I and II studies, which are generally isotypes vary in their ability to induce ADCC and CDC; this is most likely because of their differential affinities for immune system components. [4] Of the four isotypes, IgG1 most effectively induces ADCC and CDC.

Monoclonal versions of antibodies were originally produced from single B-lymphocyte cells. Genetic manipulation now allows genes from multiple sources of B-lymphocytes (e.g. mouse and human) to be combined, resulting in chimeric or humanized mAbs. In addition, a number of methods are currently available for the production of fully human mAbs, including the use of transgenic animals and phage display. mAbs are naturally well suited to intercepting hostile invaders and prompting responses from the immune system. However, mAbs can also simply block entry of an infectious agent into host cells by targeting human cellular components. This mode of action might be desirable against viruses that mutate rapidly and, thus, do not provide stable targets. mAbs can designed to assess safety and target-dosage regimens, do not When fusion is complete, the core of viral RNA and proteins provide sufficient data to be able to draw conclusions about are released into the cell. Once inside, the core disassembles and efficacy or comparisons with study results (e.g. reduction in viral the contents then reform as a complex. Viral RNA is transcribed load) for other agents.

by reverse transcriptase into DNA as this complex travels to the nucleus of the cell. After entering the nucleus, the DNA encoding the viral protein is then incorporated into the host genome by an 4. Monoclonal Antibodies (mAbs) for HIV Infection integrase enzyme. [9] At this point, the viral DNA can either lie dormant or the protein-production machinery generates the viral HIV infection is currently a global pandemic affecting millions proteins from the integrated DNA, though studies suggest that of people. The genetic composition of the virus is highly variable, HIV proteins can also be produced from non-integrated viral which has greatly increased the difficulty of designing a drug that DNA. [10] The coordinated activity of both viral components and effectively treats HIV infection. There are two main types of virus, cellular components is needed to produce the new viral ele-HIV-1 and HIV-2, but many genetically distinct viral subtypes.

ments. [11] The RNA and proteins comprising the new viral particle HIV-1, the most prevalent variety, can be subdivided into three are packaged and, through a procedure involving both viral and groups (M, N, and O); the most prevalent of these, group M, can be cellular components, the virus buds from the cell while maturing further subdivided into at least nine subgroups or clades, the most into a new infectious agent. [12] wide-spread of which are clades B and C. [5] The virus itself is

The current standard of care for patients with HIV is highly composed of a viral envelope of glycoproteins (gp) and a core active antiretroviral therapy (HAART), which comprises a drug containing the viral RNA and enzymes necessary for replication.

cocktail of two nucleoside reverse transcriptase inhibitors and A clear understanding of how HIV enters cells, replicates, and either a non-nucleoside reverse transcriptase inhibitor or protease then emerges has been critical to designing interventions. HIV is inhibitor. [13] Although the therapy can suppress viral replication to an RNA-based virus and as such cannot replicate without host low levels, the virus is not eliminated. As a consequence, life-long cellular machinery. As a consequence, the virus must bind to and adherence to a drug regimen is necessary. Furthermore, viral enter host cells. Infection is initiated by envelope protein spikes [6] resistance to drugs in the cocktail can occur and most of the drugs and occurs via a two step procedure: available for HAART have associated toxicities (e.g. mitochondri-1. surface envelope gp120 binds to the CD4 receptor and a coal dysfunction, hepatotoxicity, lipodystrophy). receptor such as chemokine (C-C motif) receptor 5 (CCR5); [7] 2. transmembrane envelope gp41 mediates fusion of the viral Because of the limitations of the existing treatments, alternaenvelope with the cell membrane. [8] tives to the components of the currently available drug cocktails are needed. To date, a total of 15 commercially sponsored mAbs gests that other factors are involved. [17] The host's own activated have been studied as treatments for HIV infection. The majority immune system has been implicated in CD4+ cell loss, although (67%) of candidates have targeted HIV envelope proteins -six the exact mechanism for this phenomenon is not well under-mAbs have targeted gp120, two have targeted gp41, and two mAb stood. [18, 19] One theory suggests that the persistent decline in cocktails have targeted both gp120 and gp41. However, only one CD4+ lymphocytes is due partially to the action of cytotoxic T of these mAbs (anti-gp120 mAb KD-247; Kaketsuken, Japan) lymphocytes. [20] Therefore, blocking the activity of lymphocyte remains in clinical study and as yet no efficacy data are available.

function-associated antigen-1 (LFA-1), a molecule on cytotoxic T lymphocytes, might help reduce CD4+ depletion that is not direct-Due to issues associated with directly targeting HIV (e.g. high ly due to HIV. An anti-LFA-1 mAb has been administered at low viral mutation rate), alternative methods to block the progression doses (≤2 mg/kg) in phase I and I/II studies, although no definitive of HIV infection using mAbs have been explored. Three mAbs efficacy data is yet available. Since the anti-LFA-1 mAb does not currently in clinical development target cellular receptors used by affect HIV directly, it must be used in conjunction with antiviral the virus to gain entry into lymphocytes, and so are in the entry drugs. inhibitor (EI) class of HIV drugs. One mAb (TNX-355; Tanox, Houston, TX, USA) targets CD4 while two other mAbs (PRO-140, Progenics, Tarrytown, NY, USA; CCR5mAb004, 5. mAbs for Respiratory Syncytial Virus Infection Human Genome Sciences, Rockville, MD, USA) target CCR5. The anti-CD4 mAb has undergone preliminary safety and efficacy RSV is a common human pathogen that primarily infects (phase I and II) studies. In a phase II study, [14] TNX-355 was respiratory epithelial cells. Like HIV and HCV, RSV is an enadministered to HIV-1 infected patients intravenously at doses of veloped RNA virus. The viral envelope gpF mediates fusion of the 10 mg/kg and 15 mg/kg. The mAb was well tolerated and reduced envelope with host cell membranes. In a process that also requires viral load at both doses, although the reduction was greater at the F proteins, infected cells subsequently fuse with each other, result-10 mg/kg dose. The US FDA requested an additional dose-finding ing in the formation of multi-nucleated masses called syncytia. [21] study. TNX-355 has been given fast track designation, which

The F protein of RSV is an attractive target for therapeutic entitles the sponsoring company to additional input from the FDA intervention, because the protein sequence is similar across viral during the clinical study period. To date, the two anti-CCR5 mAbs strains. RSV causes lower respiratory tract infections in infants have been subjected to preliminary safety studies only. One of and children, although severe RSV disease usually develops in these mAbs (PRO-140) has also been given fast track designation infants and immunosuppressed individuals. High-risk groups inby the FDA.

clude infants under six months of age, premature infants with or The EI class of mAbs already faces competition from drugs without chronic lung disease, and infants with congenital heart with similar modes of action. One EI peptide drug (enfuvirtide; disease or immunodeficiency. [22] Trimeris/Hoffmann LaRoche, USA) is already approved and a To date, a total of four commercially sponsored mAbs have small molecule EI drug (maraviroc; Pfizer, USA) is in late-stage been studied for prophylaxis or the treatment of RSV infection. All clinical development. Enfuvirtide, a 36 amino acid synthetic pepfour mAbs have targeted the RSV F protein; two anti-RSV mAbs tide approved by the FDA in 2003, binds viral gp41 and disrupts (felvizumab, HNK-20) were discontinued, one (motavizumab) is the fusion of the virus to the cell membrane. [15, 16] In two phase III currently in phase III, and another (palivizumab) is already marstudies in which enfuvirtide 90mg was administered twice daily, keted worldwide. Interestingly, despite having the same target as the mean viral load was reduced by 1.67 log10 and 1.43 log10 at 24 palivizumab, felvizumab did not demonstrate efficacy in a phase weeks. [16] Maraviroc, an orally-delivered small molecule that III study. Subsequent direct comparisons between in vitro and in binds CCR5, is currently in a phase III study. The drug reduced vivo activities of these agents revealed that palivizumab was viral load by at least 1.6 log10 at doses over 100mg administered notably more potent than felvizumab in antigen binding, virus twice daily in earlier studies. The FDA has given maraviroc fast neutralization, and fusion inhibition assays, and also displayed track designation. enhanced activity in a cotton rat model of RSV infection. [23] An alternative approach to treating HIV does not involve Palivizumab (MedImmune, USA) was approved by the FDA in targeting the virus. HIV infection is characterized by the indis-1998, and is indicated for the prevention of serious lower respiracriminate destruction of CD4+ lymphocytes, which are critical tory tract disease caused by RSV in pediatric patients at high risk components of the human immune system. Further adding to the of RSV disease. Palivizumab targets an epitope in the A antigenic complexity of the disease, the decline in CD4+ lymphocytes site of gpF of RSV and prevents viral binding to cells. The mAb is cannot be predicted from the presenting RNA levels, which sug-administered intramuscularly at a dose of 15 mg/kg on a monthly basis for up to 5 months. A total of 14 clinical studies involving Irsael), was in phase II studies, but was discontinued in favor of a 1281 patients were performed with palivizumab between 1994 and mAb cocktail (XTL-6865) comprising AB68 and another anti-1997 -5 studies in adult volunteers, 4 prophylaxis studies con-HCV E2 mAb, AB65. The mAb cocktail is considered a separate ducted in pediatric patients, and 5 treatment studies in both adult and distinct product from AB68. AB68 was tested as a single agent and pediatric patients. In the phase III IMpact-RSV study, [24] at doses of up to 480mg in a phase II study. [34] However, preclinipalivizumab reduced the risk of RSV-associated hospitalizations cal data suggested that the mAb cocktail might be more efficacious by 55% (10.6% placebo vs 4.8% palivizumab) in high risk infants and so a phase I study of that product candidate was initiated. and RSV-associated hospitalizations were reduced by 78% in XTL-6865 has been given fast track designation by the FDA. children with prematurity but without bronchopulmonary dyspla-

The third mAb, bavituximab (Peregrine Pharmaceutical, Tussia.

tin, CA, USA), is in phase I development as a treatment for HCV Motavizumab, a next generation anti-RSV mAb currently in infection, although this mAb has the potential to target other phase III studies, is derived from palivizumab and also targets the viruses. During the process of exiting the host cell, enveloped RSV gpF. The mAb has more potent anti-RSV neutralizing activi-viruses such as HCV, HIV, and RSV incorporate components of ty than palivizumab; it has a 70-fold greater binding affinity, the host cell membrane into the viral envelope. [35] Bavituximab 18-fold increased neutralizing activity, and is 50-100 times more targets phosphatidylserine, which resides on the inside of cellular potent in a cotton rat model. [25] However, details on the efficacy of membranes. The mAb should, thus, bind to any extracellular virus motavizumab in humans are limited. A phase III study assessing with phosphatidylserine-modified envelope proteins without afthe reduction of hospitalizations due to RSV in high-risk infants fecting host cells. Bavituximab has been administered at a dose of has been completed. [26] In the study, motavizumab was compared 3 mg/kg in a phase Ia study; the mAb showed evidence of antiwith palivizumab in over 6600 children, but full results of the HCV activity and was well tolerated. Interestingly, the mAb may study have not yet been released. Preliminary results indicate that also be useful for other indications. Since phosphatidylserine is motavizumab showed non-inferiority in the primary endpoint by also exposed on the outside of tumor vasculature, [36, 37] bavituxreducing the incidence of hospitalizations caused by RSV (1.4% imab is also in clinical studies as a treatment for solid tumors. motavizumab vs 1.9% palivizumab) and showed superiority in a 7. Commercial Preclinical Development secondary endpoint by reducing the incidence of RSV-specific medically attended outpatient lower respiratory infection (1.9%

The type and number of mAbs that might enter clinical study motavizumab vs 3.9% palivizumab). A number of other studies sponsored by companies can be assessed, but not necessarily are on-going. accurately predicted, based on commercial preclinical pipelines. A degree of unpredictability is present because preclinical mAbs 6. mAbs for Hepatitis C Virus Infection developed by academic and academic institutions were excluded from the data set, but discoveries by these institutions might be Like HIV, HCV is an enveloped RNA virus that mutates licensed for commercialization in the future. As a consequence, rapidly and comprises numerous genetically distinct viral subthe results discussed here have probably undercounted the total types. Infection with HCV causes chronic inflammation of the possible number of candidates to an unknown degree. Nevertheliver (hepatitis), which can lead to reduced liver function and the less, examination of commercial preclinical mAbs indicates that need for liver transplantation. The viral genome was described in only a few new targets are of interest (table II) . Furthermore, the 1989, [27] but study of the virus life-cycle was hampered until size of the group (nine mAbs) suggests that, as a whole, the systems suitable for culturing the virus in the laboratory were industry has dedicated few resources to candidate anti-viral mAb developed more than a decade later. [28] [29] [30] The HCV genome enprograms. The majority (56%) of the preclinical mAbs targeted codes two envelope proteins, E1 and E2, and nonstructural proeither SARS coronavirus (SCV) or West Nile virus. The remaining teins required for replication (e.g. protease), but little is known four products targeted rabies (two mAbs), hepatitis B, or influenza about the virion structure. [31] HCV gains entry into hepatocytes (one mAb each) virus. through interaction of the viral envelope proteins with cell surface receptors. [32, 33] 8. Discussion To date, a total of three commercially sponsored mAbs have been studied for the treatment of HCV infection. Two mAbs target

To date, the majority of commercial clinical development of the E2 envelope protein and interfere with viral entry into host mAbs for viral infections has focused on products that might meet cells. One of these, AB68 (XTL Biopharmaceuticals, Rehovot, existing medical needs for new therapies and provide sufficient on emerging pathogens such as Crimean-Congo hemorrhagic fever virus, rabies, and SCV. Importantly, the emphasis has been on development of human mAbs, which, along with humanized products, comprise most of the commercial mAbs now in clinical study. Ties between the public and private sectors are notoriously intricate. [40] In fact, the preclinical mAbs for SCV and West Nile virus infections attributed here to the commercial sector were all developed with at least some input from the public sector. However, it remains to be seen whether mAbs for infections that occur at high levels only sporadically, or those that would be likely to provide poor return on investment, will be commercialized. MAbs could potentially be critical as a first response measure in the case of a public health crisis, but substantial public sector input might be required to get such products to the market. The NIH's current emphasis on translational medicine might ease the progress of less commercially attractive products through the process of preclinical and clinical development. Despite obstacles to the development of innovative mAbs for the prophylaxis or treatment of viral diseases, there is reason for return on investment. Opportunities of this type exist for numerous cautious optimism. Scientific advances have uncovered potential viral diseases for which (i) current treatment options are not new viral targets and mAb modes of action. New possibilities exist optimal; (ii) no effective vaccines are available; and (iii) markets, for designing safer and more efficacious mAbs. An example of the at least those in the US and Europe, are defined. Examples of success potentially achievable with improved design can be found agents that meet such criteria are the mAbs directed against HIV, in the case of the mAbs for RSV -felvizumab did not prove RSV, and HCV infections. Examination of the commercial efficacious, palivizumab was sufficiently efficacious to be appreclinical pipeline suggests that similar selection criteria have proved, motavizumab shows some improved efficacy compared been applied to the majority of candidates that might enter clinical with palivizumab, and even more potent anti-RSV mAbs can be study in the near future. However, in the case of mAbs targeting designed. [41] In addition, increased emphasis from both the public SCV, the current low incidence of new infections begs the quesand private sectors on the study of mAbs as viral countermeasures tion of whether the preclinical candidates will progress, although might serve the immediate purpose of providing mAbs useful as as a defensive public health measure, products for these infections either preventative measures or as treatments, but might also should be available.

provide information potentially useful in the development of vac-The current dearth of commercial research and development of cines. In either case, the results could greatly benefit public health. mAbs for emerging pathogens might be ameliorated somewhat by: human studies would not be ethical or feasible. [2] The author thanks Matthew Dewitz and Julia Wenger for their assistance with data collection and colleagues at Tufts CSDD for critical review of the While these factors might indeed bolster the somewhat anemic manuscript. efforts of industry, the public sector appears to have already The author has no conflicts of interest that are directly relevant to the responded to the challenge in a more vigorous way. Numerous content of this article. mAbs for emerging viral diseases are in early research and preclinical stages at academic and government institutions. [38, 39] Work References has focused on priority pathogens such as Hanta and Ebola viruses 1. Sawyer LA. Antibodies for the prevention and treatment of viral diseases. Antiviral that are easily disseminated or transmitted person-to-person, and Res 2000; 47: 57-77

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