Assuring immune globulin potency in a world of changing pathogen challenges

Unlike any other biologic product, plasma-derived immune globulins (IGs) contain a diversity of potencies. IGs are made from thousands of plasma donors, most of whom express at least a million antibody specificities.1, 2 Patients with primary (PI) and secondary humoral immunodeficiencies receiving IG rely on this collective antibody repertoire for protection against infectious diseases. Given the impracticality of testing for each specificity, the US Food and Drug Administration (FDA) originally selected three potency tests to represent the more general repertoire. Since the 1960s, potency testing for antibodies against measles, polio, and diphtheria has been required by regulation for all IGs that are licensed to prevent infections in patients with primary or secondary humoral immunodeficiencies.3 At that time, all three pathogens were of concern for people with PI, as measles, polio, and diphtheria were still endemic in the United States. Neutralization assays were feasible and well established. Titers against these pathogens did not greatly fluctuate from lot to lot. Now, more than 50 years after IG potency specifications were established, near-term polio eradication is likely, and the World Health Organization has announced plans for measles eradication.4 Is it time to reinvent IG potency testing for the future? The 2017 workshop, “Immune Globulin Potency in the 21st Century” was initiated by the FDA to discuss emerging challenges affecting measles and poliovirus antibody specifications and to seek possible solutions. The broader issue of whether IG testing might be changed or improved in the future was examined by considering what specificities are currently important for patients with PI, and the availability and feasibility of testing. Patients with PI as well as those with acquired immunosuppression can be at risk for lethal measles infection.5-7 Hypogammaglobulinemic patients with intact T-cell responses typically clear measles infection.7 Severity of measles disease is dictated by intactness of both cellular immune and antibody functions. Measles antibodies provide several levels of defense that are important for protection, although cellular immune responses are critical for viral clearance.8 Protective antibody levels against measles disease in immunocompetent individuals are around 120 mIU/mL, whereas sterilizing immunity occurs around 1000 mIU/mL.9 FDA regulations dictate that IG products must meet FDA specifications for measles antibodies. By 2007, IG products were noted to have declining measles antibody levels due to increasing proportions of vaccinated donors born after 1963, who had not been naturally infected. To avert IG shortages, manufacturers were allowed to lower the measles antibody specification from 0.6 to 0.48× Center for Biologics Evaluation and Research Standard Lot 176 in 2008. This was contingent on provision of trough measles antibody titer data from patients with PI, along with modeling, to show that if these patients received a lot at the lower specification, their trough serum measles antibody titer would be at least 240 mIU/mL.10-12 The serum level of 240 mIU/mL (double the protective level in immunocompetent individuals) was proposed by the FDA, as the level of protection for patients with PI is unknown and also may vary across different categories of these genetic immunodeficiencies. Individual patients with deficits in humoral and/or cellular immunity may need different levels of antibody for protection, and higher levels may be required for the latter category. The doubling of the minimum titer is also justified based on assay variation.13 As predicted in 2007, measles titers have continued to decline, and a similar but not identical predicament must now be addressed.11 If the measles specification is lowered again, it is unlikely that all patients will maintain a trough level of 240 mIU/mL, based both on extrapolation from trough titer data, and on separate pharmacokinetic modeling assuming IG doses of 300 to 400 mg/kg.11, 12 A range of possible solutions were discussed. Reimmunization of plasma donors was shown not to provide strong or sustained increases in measles antibodies.14 Labeling and segregating IG lots with higher measles antibodies for patients with PI was felt to be logistically infeasible, and might also lead to short supply for the non-PI hematologic, neurologic, and Kawasaki disease indications. Labeling specific lots as “not for use” in PI or secondary immunodeficiencies was predicted to be, at the least, confusing for physicians and pharmacists because the same brands would carry either full or limited indications. At most, it could result in shortages of IG for patients with PI if anti-measles titers continue their downward drift, because relatively few lots would be available for the PI indication. Monoclonal measles antibodies for postexposure prophylaxis are currently not licensed but may provide a future option for measles prevention. Another strategy would be to provide postexposure prophylaxis to patients with PI consisting of an extra dose of IG intravenous or subcutaneous as soon as possible. The amount of the dose, and relationship to timing of prior infusions would need to be considered in detail. An infusion of IG intravenous at 400 mg/kg following measles exposure would likely provide immediate peak serum levels around 1200 mIU/mL.13 This strategy would remove pressure from the need to maintain a trough level of 240 mIU/mL and could be communicated through labeling recommendations. Measles outbreaks continue to occur worldwide.15 In theory, preexposure prophylaxis for travelers with PI or secondary antibody deficiencies could be provided by a dose of IG intravenous that would provide at least 4 weeks of serum measles antibody titers greater than 240 mIU/mL. Labeling would be a means to provide instructions related to measles prevention for travelers. It is clear that action will be needed in the near term, to protect patients with primary and secondary immunodeficiency from measles disease. If the worldwide effort to eradicate measles meets with success in the coming decade, the current concern will lose relevancy.4 Potency testing for poliovirus antibodies using neutralization assays requires use of live virus to demonstrate antibody functionality. As well described by Oberste,16 World Health Organization Global Action Plan requirements for continued use of polioviruses will require costly retrofitting, physical locations within a radius of a population with proven high levels of immunity, and generally unwieldy requirements for existing quality control and research laboratories. Use of a hyperattenuated poliovirus strain that is incapable of causing paralytic disease can provide a pathway that would permit testing without Global Action Plan requirements. Promising advances in development and testing of hyperattenuated S19 poliovirus strains were described at the workshop.17 Such strains would need to be 1) noninfectious in people; 2) incapable of causing paralytic disease; 3) genetically stable and unable to regain paralytic potential; 4) easily cultured on conventional cells to high titers usable in neutralization assays; and 5) antigenically comparable to polioviruses currently used in neutralization assays. The S19 strain described at the workshop is modified to eliminate gut infectivity and does not cause paralysis in susceptible mice when introduced into the spinal cord or in primates given high oral doses of virus.17 Based on this work, the S19 poliovirus strain described by Farcet et al17 was approved in June 2018, for use under Biosafety Level 2 conditions by the World Health Organization Containment Advisory Group, thus providing an attractive option for continued testing of IGs for poliovirus antibodies in IG products.18 Other, less well developed options include pseudotype virus neutralization assays or competitive enzyme-linked immunosorbent assay (ELISA). In the latter case, concerns would be whether any ELISA, under all likely conditions, faithfully reflects neutralizing activity and is not susceptible to well-known polio ELISA cross-reactivity with other enteroviruses. How relevant are poliovirus antibodies to patients with PI? In the early 1950's, IG was studied for postexposure prevention of paralytic disease in the epidemic setting.19 Use of IG to prevent poliovirus-induced paralysis fell out of favor due to contradictory results of clinical trials and the advent of efficacious vaccines.20-22 Then and now, patients with PI who contract vaccine-derived or wild-type poliovirus are prone to persistent excretion that may last for years, even when regular IG treatment is given.23-25 Affected patients most often have common variable immunodeficiency or severe combined immunodeficiency.23 As case reports attest, IG treatment does not reliably eliminate enteric poliovirus excretion and it is not known whether or how much poliovirus antibody in IG might prevent spread to the central nervous system. At the workshop, McKinlay reported that trials with a single antiviral treatment did not eliminate prolonged poliovirus excretion in patients with PI and that new studies are planned using dual antiviral therapy.26 Poliovirus antibodies in IG continue to serve as a useful specification for measuring antibody functionality and manufacturing process consistency. In the near future, replacement of the live virus with a Biosafety Level 2 attenuated virus for neutralization assays will allow for continuation of this potency specification. Eventually, poliovirus eradication may culminate in a decision to cease vaccination, which will surely prompt reevaluation of this potency test for IG. Workshop participants identified potential candidates based on ability to cause disease in patients with PI. These were mostly respiratory pathogens, and included Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, respiratory syncytial virus, parainfluenza, influenza, adenovirus, mycoplasma, and rhinovirus.14, 28, 29 Nonrespiratory pathogens discussed included cytomegalovirus, herpes simplex virus, enteroviruses, and meningococcus. Relatively limited data are available on the viral respiratory infection epidemiology of patients with PI in part because diagnostic cultures are not routinely performed. Widespread vaccination against a pathogen was seen by some attendees as a positive quality, as prevalence of antibodies (such as tetanus, pertussis, and hepatitis B) in the donor population should be consistent over time. Others pointed out that this might be a disadvantage based on the current example of measles antibody levels. Influenza antibodies were also of interest but may present challenges for strain selection and variability over time. Considerable attention was given to S. pneumoniae because it has been a major cause of pneumonia and invasive disease in untreated and undertreated patients with PI. Test methods and standards have evolved since 2007, although some commercial assays yield conflicting results, and ELISA titers do not always correlate with functional opsonophagocytic assays.30-32 It was generally agreed that an opsonophagocytosis assay or a multiplex version of the opsonophagocytosis assay were most attractive, as they provide antibody functionality, are reproducible, and may be relatively stable over time. H. influenzae titers (by ELISA) were also stable across IG products in one study.28 Preliminary studies of neutralizing antibodies in IG to viral pathogens were also discussed. Titers against human adenovirus 5, respiratory syncytial virus, and parainfluenza 3 across IG lots in the United States showed relatively low lot-to-lot variation.14 Because many antiviral antibodies have a subclass-specific predilection, participants suggested that a specification with this characteristic might be advantagous and could represent general stability and function of IgG1 or IgG3 antibodies. It became clear that more information and discussion, followed by preliminary laboratory studies of the most promising candidates, would be fundamental to wise selection of any new specification. As stated by Arceniega,33 “We should not be afraid of selecting a potency test, if the choice is based on the best science available and common sense.” In the coming years, it is important to make the effort to explore specifications that will be meaningful to patients for whom IG is the essential health- and life-sustaining treatment. FDA posted a letter to manufacturers concerning lowering specification for measles antibodies in IG products. [posted and cited 2018 Nov 26]. Available at: https://www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/ucm594350.htm. The author has disclosed no conflict of interest and is employed by the Food and Drug Administration.