Both formulations of AMA1-C1/Alhydrogel with CPG 7909 were well tolerated, with only one severe local adverse event of short duration reported and no volunteers withdrawn from vaccinations due to adverse events. This is in contrast to a previous Phase 1 study of AMA1-C1/Alhydrogel with CPG 7909 in a phosphate buffer, where local and solicited adverse events were more severe when CPG 7909 was added to the formulation and several volunteers were withdrawn due to adverse events [8
]. The excellent safety profile in this study increases confidence in the tolerability of the vaccine. Nearly all volunteers had transient decreases in neutrophil counts after first and second vaccinations, with one decrease reaching Grade 1. Transient fluctuations in white blood cell counts and platelet count are expected adverse events with CPG 7909 and other TLR 9 agonists [19
], and are likely due to redistribution of leukocytes as part of the immune response. No clinical events associated with these fluctuations have been reported to date and, given the rapid return to baseline, their clinical significance is likely to be minimal. There were no laboratory or clinical signs of autoimmunity, which is a theoretical concern with the use of CPG oligonucleotides.
While the safety profile for CPG 7909 as an adjuvant is limited, novel formulations and/or adjuvants are likely to be needed to induce high antibody levels for malaria proteins, which are generally poor immunogens in humans. The only malaria vaccine shown to have a consistent benefit in young children in Africa, RTS,S, is a novel formulation (virus-like particle) combined with a novel adjuvant (AS02 or AS01) [21
]. Adjuvants such as CPG 7909, which induces a Th1 biased response, may be of particular use with malarial antigens [23
]. Additional clinical studies in malaria exposed populations and in children should proceed in a cautious step-wise fashion, to expand the safety base for CPG adjuvants in the target population and to demonstrate benefit against clinical outcomes.
AMA1-C1/Alhydrogel® + CPG 7909 in saline was shown to have similar immunogenicity as the AMA1-C1/Alhydrogel® + CPG 7909 buffered with phosphate, and will be used in future clinical trials. Peak responses were superior when the vaccines were given on a 0,2 month versus 0,1 month dosing schedule, although given the small numbers in each individual dose group (n=6) it is possible that there may be a difference in kinetics between the phosphate and saline groups which the study was not powered to detect. However, studies in animals and humans also support a longer vaccination interval as more likely to induce higher antibody levels and cellular responses [24
]. AMA1 specific antibody levels dropped rapidly after vaccination. It is possible that, when used in malaria endemic areas, infection will boost antibody responses. However, a longer lasting high level antibody response is more likely to be protective. Future studies will examine the use of a two month initial dosing interval, with a boost at 6 or 12 months.
The in vitro growth-inhibitory activity in this study was the highest in our human vaccine trials with AMA1 or other antigens to date. However, the range of antibody responses and corresponding growth inhibition was wide, with some volunteers only achieving 30–40% inhibition. While growth inhibition activity greater than 60% was associated with protection in Aotus
monkeys vaccinated with MSP142
], the extent to which in vitro growth-inhibitory activity correlates with in vivo parasite multiplication rates, or with protection in the field, remains to be demonstrated. Furthermore, inhibition against heterologous parasites is markedly less than that against homologous parasites [8
], and the extent to which this combination vaccine would protect against the highly diverse AMA1 variants in the field is also unknown. The addition of other proteins, either to more broadly cover AMA1 polymorphism or to broaden the individual response to various antigens, may be an effective strategy for future investigation.