Development of human monoclonal antibodies to diphtheria toxin: A solution for the increasing lack of equine DAT for therapeutic use?

KEYWORDS:

• diphtheria
• equine diphtheria antitoxin

This article refers to:

Potency of a human monoclonal antibody to diphtheria toxin relative to equine diphtheria anti-toxin in a guinea pig intoxication model

Clinical symptoms and diphtheria antitoxin (DAT) treatment

Most diphtheria infections in highly vaccinated populations are asymptomatic or result in a mild clinical course and therefore may remain undiagnosed and hence underreported. However the overall case-fatality rate for systemic diphtheria is 5%-10%, with higher death rates (up to 20%) among persons younger than 5 and older than 40 years of age.

As of today, successful treatment of diphtheria depends on rapid administration of equine-derived diphtheria antitoxin (DAT) in combination with antibiotics. DAT should be administered upon clinical suspicion of diphtheria, whether or not there are systemic toxic symptoms present, as it binds to circulating toxin but does not neutralise toxin that has already bound to, or entered into, cells. DAT treatment initiated later than 48 hours after onset of systemic toxic symptoms has limited impact on the clinical outcome although DAT is, when necessary, offered at any stage of the disease. Administration of equine DAT can cause delayed hypersensitivity reactions, the so-called serum sickness, and more rarely acute anaphylactic shock

Several European countries stopped manufacturing DAT following the significant decline in incidence of the disease after the introduction of mass vaccination.² The situation is similar in the US, where no supplier of a licensed DAT product exists and there is a quality assurance issue for non-licensed pharmaceutical products. The current lack of DAT is of great concern. There is an urgent need to find solutions that will allow countries to have immediate access to DAT, in the 48 hours following the initial symptoms, in case a diphtheria patient is suspected.

The use of a human antitoxin preparation (as has been the case for tetanus) with the aim to minimize the risk of anaphylaxis, has been envisaged as an alternative for the equine product, but the antibody enrichment from blood donor sera that would be required is unrealistic. Indeed the amount of antibodies needed for curing symptomatic diphtheria, is ranging between 5,000 to 50,000 IU, amount much larger than that required to prevent infection. Thus, even selection of high-titered donors (with anti-diphtheria toxin levels in general in the order of 3 I.U./ml) from human plasma pools and enrichment could result in a product containing at most 50 IU/ml, meaning that effective treatment would require excessive doses up to thousand ml !²

The development of recombinant monoclonal antibodies against diphtheria exotoxin is another important option has been considered.

In 2006, Kakita et al reported on a human monoclonal antibody with strong neutralizing activity against diphtheria toxin.³ The neutralizing activity was only assessed in a rabbit skin test and to the best of my knowledge, further development of the product was halted.

In 2013, researchers at MassBiologics of the University of Massachusetts Medical School, Boston, reported on the identification of a human monoclonal IgG1 anti-diphtheria toxin antibody (S315), binding to the receptor-binding domain of the toxin and thus blocking the interaction with its putative receptor.⁴ In this issue of Virulence, they now report on a further characterization of this S315 antibody.⁵ A study of the diphtheria toxin neutralizing capacity of increasing doses of equine DAT as compared to S315 antibody in 44, respectively 54 guinea pigs, enabled Smith et al to determine a relative potency of S315 at 48 µg/IU. In this study, animals received a single injection of antibody premixed with toxin. Before an Investigational New Drug Application (IND) may eventually be submitted, it is now essential to determine whether the S315 antibody will also have an effect when administered after toxin exposure, modeling the anti-toxin therapy in humans. As compared to polyclonal antibody products, the advantage of monoclonal antibodies is the fact that they consist of one antibody molecule with singular specificity, the neutralizing activity of which contained in one microgram of purified mAb can be determined by evaluating the mAb relative to the FDA diphtheria anti-toxin standard expressed in I.U.s

Vaccination

Vaccination against diphtheria effectively protects against the effects of the exotoxin produced by C. diphtheria and C. ulcerans , but vaccinated individuals can still be infected by the bacteria, become asymptomatic carriers of toxin-producing strains and may transmit these to others.

Diphtheria vaccines are effective and have essentially eliminated clinical diphtheria disease in countries with high vaccination coverage such as Western Europe. Despite this success of routine vaccination, diphtheria remains a serious health problem with 5000 diphtheria cases reported in 2012 by WHO, particularly in South-East Asia.

Antibody levels against diphtheria toxin below 0.01 IU/ml are considered non-protective, levels of 0.01-0.1 IU/ml are considered to provide basic protection and levels superior to 0.1 IU/ml are considered to provide full protection against diphtheria.⁶ Although basic primary immunisation will result in protective antibody levels, there are unresolved issues about waning immunity and the need for booster doses. Limited data on population level immunity in Western Europe published in 2000 reported on significant proportions of susceptible individuals particularly among adults and the elderly.⁷ As another example, Theeten et al reported that protective antibody levels of > 0.1 I.U./ml in serum samples collected in Belgium in 2006 were detected in >70% in all age groups < 30 years. However, the seroprotection rate decreased steeply with age in subjects aged >30 years, to a minimum of 20% in the 55–59 years.⁸

Furthermore, there remain groups of people refraining from vaccination for philosophical or religious reasons, such as the orthodox protestant community in the Netherlands.⁹ In addition, there are families from under-served population groups, including migrants who stay ‘under the radar’ and who, for one reason or another, are not covered by the national vaccination programs. A fatal case of diphtheria reported in Belgium March 2016 in a 3-year old unvaccinated child of Chechnyan origin, born in Belgium, was a tragic example (http://ecdc.europa.eu/en/publications/Publications/communicable-disease-threats-report-13-19-mar-2016.pdf).

Conclusion

Incentives to promote further research on the S315 antibody may help to accelerate the development of a human product that could be used for diphtheria therapy. However, this will take time and as previously mentioned by Karen Wagner et al, the only reasonable alternative for the moment would be a large scale production of freeze-dried equine antitoxin (with a long shelf life) on a contract basis negotiated on a global scale, that would facilitate the maintenance of individual country stocks²

Finally, the scarcity of equine DAT stock and the absence for the moment of validated human anti-diphtheria antibodies (being polyclonal or monoclonal) emphasizes the importance of the maintenance of high vaccine coverage in all countries. Booster doses every 10 years with combined diphtheria- tetanus-pertussis vaccines should be recommended. Furthermore, boosting with Tdap vaccines during pregnancy, aiming to close the susceptibility gap of young infants for pertussis, may offer at the same time the needed protection for women and neonates against diphtheria and tetanus.^10,11

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.