Pilot Phase IB Clinical Trial of an Alhydrogel-Adsorbed Recombinant Ricin Vaccine

Vitetta, Ellen S.; Smallshaw, Joan E.; Schindler, John

doi:10.1128/cvi.00381-12

Cited by 49 publications

(45 citation statements)

References 18 publications

(31 reference statements)

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“…Although RiVax and RVEc are currently being tested in phase I clinical trials as aluminum-adsorbed vaccines (18)(19)(20)(21)(22), our study is the first to compare the two vaccines sideby-side in an animal model. We compared RiVax and RVEc adsorbed to Alhydrogel and administered to mice s.c. in a 2-or 3-dose immunization regimen across a range of doses (0.3 to 20 g).…”

Section: Resultsmentioning

confidence: 99%

“…route elicits toxin-specific serum IgG antibodies (Abs) that are sufficient to confer protection against a lethal dose of ricin (8)(9)(10)(15)(16)(17). Phase I clinical trials have demonstrated that RiVax is safe and immunogenic in healthy human volunteers (18,19). Similarly, RVEc is effective at eliciting toxin-neutralizing antibodies in mice and rabbits (20)(21)(22)(23) and is now in phase I clinical trials.…”

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confidence: 99%

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Comparative Efficacy of Two Leading Candidate Ricin Toxin A Subunit Vaccines in Mice

O’Hara

Brey

Mantis

2013

Clin Vaccine Immunol

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The two leading ricin toxin vaccine candidates, RVEc and RiVax, are recombinant derivatives of the toxin's 267-amino-acid enzymatic A chain (RTA). RVEc is truncated at the C terminus (residues 199 to 267) to improve protein thermostability, while RiVax has two point mutations (V76M and Y80A) that eliminate the RNA N-glycosidase activity of RTA, as well as its ability to induce vascular leak syndrome. The two vaccines have never been directly compared in terms of their ability to stimulate RTAspecific antibodies (Abs), toxin-neutralizing activity (TNA), or protective immunity. To address this issue, groups of female BALB/c mice were immunized two or three times with Alhydrogel-adsorbed RiVax or RVEc at a range of doses (0.3 to 20 g) and then challenged with 10 50% lethal doses (LD 50 s) of ricin. We found that the vaccines were equally effective at eliciting protective immunity at the doses tested. There were, however, quantitative differences in the antibody responses. RVEc tended to elicit higher levels of ricin-specific RTA IgG and TNA than did RiVax. Pepscan analysis revealed that serum Abs elicited by RVEc were skewed toward a solvent-exposed immunodominant ␣-helix known to be the target of potent toxin-neutralizing Abs. Finally, immunodepletion experiments suggest that the majority of toxin-neutralizing Abs elicited by RiVax were confined to residues 1 to 198, possibly explaining the equal effectiveness of RVEc as a vaccine. R icin, one of the most potent biological toxins known, consists of two subunits, RTA and RTB. RTA is a 267-amino-acid RNA N-glycosidase that selectively and irreversibly inactivates eukaryotic ribosomes (1). RTB is a galactose/N-acetylgalactosamine-specific lectin that facilitates the delivery of RTA into the cytoplasm of eukaryotic cells in general (2). Once in the cytoplasm, it is estimated that a single molecule of RTA can inactivate Ͼ1,000 ribosomes per minute.Despite the history of ricin as an agent of biological warfare and bioterrorism, there is currently no available ricin toxin vaccine (3, 4). As early as the 1940s, the United States military focused on the development of a simple formalin-treated holotoxin toxoid vaccine. Although ricin toxoid is highly efficacious in rodents and nonhuman primates, its use in humans was abandoned because of manufacturing problems and safety concerns (5). For those reasons, current efforts are aimed at the development of a recombinant subunit vaccine. While RTB is an obvious candidate, RTB immunization confers only partial protection against ricin challenge (6, 7). In contrast, immunization of mice with RTA or nontoxic derivatives of RTA is sufficient to protect mice against a 10 50% lethal dose (LD 50 ) ricin challenge (8).There are currently two RTA-based vaccines under development: RiVax and RVEc. RiVax is a recombinant derivative of RTA with two point mutations at residues Y80 and V76. The Y80A mutation abolishes the RNA N-glycosidase activity of the toxin, while the V76M mutation eliminates the ability of RTA to elicit vascular leak syndr...

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Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

Comparative Efficacy of Two Leading Candidate Ricin Toxin A Subunit Vaccines in Mice

O’Hara

Brey

Mantis

2013

Clin Vaccine Immunol

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“…Ricin's enzymatic subunit (RTA) is of particular interest to the biodefense research community as two recombinant RTAbased vaccines, RiVax and RVEc, are being evaluated in preclinical and clinical studies (38)(39)(40). Predicting the efficacy of these two vaccine candidates in humans is challenging, because neither the anti-ricin serum IgG titer nor the TNA is necessarily a reliable predictor of protective immunity (41).…”

Section: Discussionmentioning

confidence: 99%

High-Resolution Epitope Positioning of a Large Collection of Neutralizing and Nonneutralizing Single-Domain Antibodies on the Enzymatic and Binding Subunits of Ricin Toxin

Vance

Tremblay

Rong

et al. 2017

Clin Vaccine Immunol

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We previously produced a heavy-chain-only antibody (Ab) VH domain (V H H)-displayed phage library from two alpacas that had been immunized with ricin toxoid and nontoxic mixtures of the enzymatic ricin toxin A subunit (RTA) and binding ricin toxin B subunit (RTB) (D. J. Vance, J. M. Tremblay, N. J. Mantis, and C. B. Shoemaker, J Biol Chem 288:36538 -36547, 2013, https://doi.org/ 10.1074/jbc.M113.519207). Initial and subsequent screens of that library by direct enzyme-linked immunosorbent assay (ELISA) yielded more than two dozen unique RTAand RTB-specific V H Hs, including 10 whose structures were subsequently solved in complex with RTA. To generate a more complete antigenic map of ricin toxin and to define the epitopes associated with toxin-neutralizing activity, we subjected the V H Hdisplayed phage library to additional "pannings" on both receptor-bound ricin and antibody-captured ricin. We now report the full-length DNA sequences, binding affinities, and neutralizing activities of 68 unique V H Hs: 31 against RTA, 33 against RTB, and 4 against ricin holotoxin. Epitope positioning was achieved through cross-competition ELISAs performed with a panel of monoclonal antibodies (MAbs) and verified, in some instances, with hydrogen-deuterium exchange mass spectrometry. The 68 V H Hs grouped into more than 20 different competition bins. The RTA-specific V H Hs with strong toxin-neutralizing activities were confined to bins that overlapped two previously identified neutralizing hot spots, termed clusters I and II. The four RTB-specific V H Hs with potent toxin-neutralizing activity grouped within three adjacent bins situated at the RTA-RTB interface near cluster II. These results provide important insights into epitope interrelationships on the surface of ricin and delineate regions of vulnerability that can be exploited for the purpose of vaccine and therapeutic development.KEYWORDS antibody, biodefense, epitope, neutralizing, toxins, vaccines R icin is the prototype of the type II ribosome-inactivating protein (RIP) family of toxins (1). Ricin toxin A subunit (RTA) is an RNA N-glycosidase that catalyzes the hydrolysis of a conserved adenine residue within the sarcin/ricin loop (SRL) of 28S rRNA, resulting in ribosome arrest and apoptosis (2-4). Ricin toxin B subunit (RTB) is a galactose-and N-acetylgalactosamine (Gal/GalNAc)-specific lectin that promotes toxin entry into mammalian cells, including the epithelial cells that line the respiratory tract

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“…RiVax, for example, is a recombinant, attenuated derivative of the enzymatic subunit of ricin toxin (RTA), which has been deemed to be safe for human use (4,5). Unfortunately, phase I clinical trials indicated that even repeated high-dose immunizations were relatively poor at stimulating ricin-specific IgG antibodies and toxin-neutralizing activity (TNA) in sera of volunteers.…”

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confidence: 99%

“…immunization model. We chose to use RiVax and RVEc for these studies, because they are each well-characterized investigational vaccine antigens that have been deemed safe in phase I clinical trials (4,5). Both are recombinant, nontoxic derivatives of ricin's enzymatic subunit, RTA.…”

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confidence: 99%

Comparative Adjuvant Effects of Type II Heat-Labile Enterotoxins in Combination with Two Different Candidate Ricin Toxin Vaccine Antigens

Vance

Greene

Rong

et al. 2015

Clin. Vaccine Immunol.

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Type II heat-labile enterotoxins (HLTs) constitute a promising set of adjuvants that have been shown to enhance humoral and cellular immune responses when coadministered with an array of different proteins, including several pathogen-associated antigens. However, the adjuvant activities of the four best-studied HLTs, LT-IIa, LT-IIb, LT-IIb T13I , and LT-IIc, have never been compared side by side. We therefore conducted immunization studies in which LT-IIa, LT-IIb, LT-IIb T13I , and LT-IIc were coadministered by the intradermal route to mice with two clinically relevant protein subunit vaccine antigens derived from the enzymatic A subunit (RTA) of ricin toxin, RiVax and RVEc. The HLTs were tested with low and high doses of antigen and were assessed for their abilities to stimulate antigen-specific serum IgG titers, ricin toxin-neutralizing activity (TNA), and protective immunity. We found that all four HLTs tested were effective adjuvants when coadministered with RiVax or RVEc. LT-IIa was of particular interest because as little as 0.03 g when coadministered with RiVax or RVEc proved effective at augmenting ricin toxin-specific serum antibody titers with nominal evidence of local inflammation. Collectively, these results justify the need for further studies into the mechanism(s) underlying LT-IIa adjuvant activity, with the long-term goal of evaluating LT-IIa's activity in humans.T he absence of effective adjuvants constitutes a critical bottleneck to the development of protein subunit vaccines for biodefense and emerging infectious diseases (1-3). RiVax, for example, is a recombinant, attenuated derivative of the enzymatic subunit of ricin toxin (RTA), which has been deemed to be safe for human use (4, 5). Unfortunately, phase I clinical trials indicated that even repeated high-dose immunizations were relatively poor at stimulating ricin-specific IgG antibodies and toxin-neutralizing activity (TNA) in sera of volunteers. Adsorption of RiVax to aluminum salts adjuvant only marginally improved the antibody response to the vaccine antigen, indicating that the future success of RiVax and other ricin toxin subunit vaccine antigens under investigation, like RVEc, may depend on the identification of morepotent adjuvants (6).The type I and type II bacterial heat-labile enterotoxins (HLTs) are among the most potent adjuvants described to date. The HLTs are AB 5 subunit toxins, consisting of a single ADP-ribosylating ("A") subunit joined noncovalently to pentamers of gangliosidebinding ("B 5 ") subunits. The well-characterized type I HLTs include cholera toxin (CT), expressed by Vibrio cholerae, and LT enterotoxin, expressed by enterotoxigenic strains of Escherichia coli. The type II enterotoxins, on the other hand, are less well characterized and more diverse than CT or LT (7,8). The beststudied type II HLTs are LT-IIa, LT-IIb, and LT-IIc, each of which has been shown to enhance antigen-specific immune responses when coadministered with a model antigen like ovalbumin (OVA) (9). Adjuvant activities of the type I and II HL...

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Pilot Phase IB Clinical Trial of an Alhydrogel-Adsorbed Recombinant Ricin Vaccine

Cited by 49 publications

References 18 publications

Comparative Efficacy of Two Leading Candidate Ricin Toxin A Subunit Vaccines in Mice

Comparative Efficacy of Two Leading Candidate Ricin Toxin A Subunit Vaccines in Mice

High-Resolution Epitope Positioning of a Large Collection of Neutralizing and Nonneutralizing Single-Domain Antibodies on the Enzymatic and Binding Subunits of Ricin Toxin

Comparative Adjuvant Effects of Type II Heat-Labile Enterotoxins in Combination with Two Different Candidate Ricin Toxin Vaccine Antigens

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