Major Global Vaccine Challenges: Recent Progress in Malaria Vaccine Development

2.1

Targeting the Circumsporozoite Protein

The P. falciparum circumsporozoite protein (CS) is a 412 amino acid (7G8 clone) protein with a characteristic central NANP repeat region and nonrepeat flanking regions. CS is present on the sporozoite surface and early liver forms, and plays an important role in motility, attachment, and entry into hepatocytes, modulating intracellular biochemical pathways. CS was an early target of malaria vaccine research, as passive transfer of CS-specific antibodies or T-cell effectors was shown to protect rodents from experimental infection, but initial constructs targeting the central repeat region of the CS failed to provide conclusive protection.

RTS,S is a chimeric protein including NANP repeats and the C-terminal flanking region of CS fused to the hepatitis B virus surface antigen (HBsAg), coexpressed in yeast with free HBsAg, yielding a spontaneously assembling viral-like particle. CHMI studies showed the critical role of adjuvantation and RTS,S/AS01 emerged as the most immunogenic formulation. AS01 is a liposomal suspension and includes the immune-enhancers such as monophosphoryl lipid A, a detoxified lipopolysaccharide derivative, and QS-21 Stimulon® (Quillaja saponaria Molina, fraction 21) (Licensed by GSK from Antigenics Inc.), a saponin molecule purified from the bark of a tree, Quillaja saponaria. While both anti-CS antibodies and cell-mediated responses have been associated with protection, there is no established correlate of protection. Antibodies likely prevent sporozoite entry into hepatocytes, while cell-mediated immunity is assumed to play a helper role supporting the humoral response and possibly an effector role against infected hepatocytes, although there is no robust evidence for the latter.

Since 2001, the RTS,S pediatric development program has been under the leadership of a public-private product development partnership between GSK and the PATH Malaria Vaccine Initiative, in collaboration with multiple academic collaborators. The overall objective of the program is to reduce the burden of P. falciparum malaria in young children in sub-Saharan Africa. The vaccine would ideally be implemented through the WHO Expanded Program on Immunization (EPI), in conjunction with other malaria control interventions. Phase 2 RTS,S studies have confirmed partial protection against malaria in children, and demonstrated favorable safety down to infants in the EPI age-range.

In 2009, a multicentre Phase 3 RTS,S/AS01 trial was undertaken in 11 African research centers with different malaria intensity and seasonality. The study included children aged 5–17 months at first vaccination, and infants 6–12 weeks of age immunized together with routine EPI vaccines. Vaccine efficacy against clinical malaria over 1 year was about 50% in the older age category, and about 30% in the younger age category. The final results from the study, including evaluation of a booster dose at Month 20, have now been published. Without a booster dose, when considering children in the older age category over the whole follow-up period (median 48 months), primary RTS,S/AS01 vaccination provided 28% (95% CI 23–33) protection against clinical malaria, and the total number of cases averted ranged in different study sites between 215 and 4443 (1774 overall) per 1000 children vaccinated. No protection against severe malaria was seen when considering the total follow-up period. There was evidence of waning of immunity over time. The reduction of early exposure to blood stage infection associated with vaccination may have delayed acquisition of blood-stage immunity, with a displacement of incidence of severe malaria toward older age. The overall effect was nevertheless favorable, with evidence of a reduction in malaria hospitalizations and all-cause hospitalizations. When a booster dose at Month 20 was given, protection against clinical malaria and severe malaria over the whole follow-up was 36% (95% CI 32–41) and 32% (95% CI 14–47), respectively, and depending on study site, 205 to 6565 cases of malaria were averted per 1000 children vaccinated. When considering other endpoints of public health interest, vaccination with a booster dose provided overall partial protection against incident severe malaria anemia and blood transfusion, malaria hospitalization and all-cause hospitalization.

Lower estimates of efficacy were seen in the younger age category. Vaccine efficacy against clinical malaria over the whole study (median 38 months) was 18% (95% CI 12–24) without a booster dose, and 26% (95% CI 20–32) with a booster dose. There was no evidence of protection against severe malaria. Vaccination with a booster dose reduced malaria hospitalizations by 25% (95% CI 6–40), but there was no evidence of a reduction in other endpoints of public health interest. Overall, approximately 1000 cases of clinical malaria were averted per 1000 infants vaccinated. The reasons for lower protective immunity in young infants relative to older children are unknown, but immaturity of the immune system, passively transferred maternal antibodies, past hepatitis B vaccination, and EPI vaccine coadministration may have played a role.

Safety results were favorable, although vaccination was associated with a risk of febrile seizures when children were vaccinated at a susceptible age. An unexplained increased reporting of cases of meningitis due to a heterogeneous group of pathogens, with no cluster in time-to-event, was reported in children in the older age category, but not in the younger age category.

Results from safety and immunogenicity studies including vaccination of neonates and HIV-infected children will be available in the near future. An RTS,S/AS01 vaccine regulatory application package is currently under European Medicine Agency review for scientific evaluation of the quality, efficacy and safety through the Article 58 procedure, before WHO consideration for recommendation for use and prequalification. If the outcome of these reviews is favorable, submission to national regulatory authorities in African countries will follow.

Other approaches targeting the CS protein are in early evaluation, with the hope that they may represent improvements over RTS,S. The role of the N-terminal region of the protein is being assessed using a full length CS protein antigen. Alternative immunization regimens may generate qualitatively better humoral responses or protective T-cell effectors. CS is not expressed for very long in the intracellular liver forms and strategies using recombinant viral vectors to generate cell-mediated immunity against CS only conferred limited protection in CHMI studies. Replacement of the first dose of RTS,S/AS01 by a CS-expressing recombinant adenovirus promoting CS-specific CD4+ response failed to increase vaccine efficacy. An alternative approach for qualitatively improved immunogenicity is to use a fractional third dose of RTS,S/AS01 (NCT01857869).

2.2

Whole Plasmodium falciparum Sporozoite (PfSPZ) Vaccine Candidates

In contrast to natural infection, experimental exposure to large numbers of irradiated sporozoites can protect from subsequent experimental malaria challenge. While irradiated sporozoites can initiate liver stage infection, full differentiation is aborted, leading to protective preerythrocytic immunity which appeared predominantly mediated by CD8+ T cells. Irradiated sporozoites were historically administered through the bites of a minimum of 1000 mosquitoes. Recently, what was a clinical laboratory experiment has inspired a biotechnology company (Sanaria) to develop a candidate vaccine approach based on attenuated whole P. falciparum sporozoites (PfSPZ) immunization stored in liquid nitrogen, after dissection of a large number of mosquito salivary glands for sporozoite isolation. In terms of administration route, needle injection had to replace experimental mosquito bites. Only the intravenous route led to protective immunity, while intradermal or intramuscular needle injections failed. A total of 6.75 × 10 ⁵ irradiated PfSPZ injected in five doses intravenously protected 6/6 subjects against CHMI. Protection was dose-dependent. Research is ongoing to determine duration of protection, whether cross-strain protection can be achieved, evaluate the role of preexisting malaria immunity and whether more practical storage and injection techniques are possible.

The use of genetically attenuated parasites which are able to initiate liver stage infection but arrested before blood release, or unable to multiply in the blood, is an alternative to irradiation. Exposure to a limited number of PfSPZ under chloroquine coverage, allowing for complete preerythrocytic development but preventing blood stage multiplication, leads to long-standing high protection, suggesting that the longer the preerythrocytic portion of the cycle is allowed to progress, the higher the protection generated. Several candidate gene targets for attenuation have been identified, with the objective to generate the right parasite mutant displaying enough attenuation to ensure no break-through clinical infection occurs.

Whatever the PfSPZ approach considered, manufacturing according to regulatory standards for Phase 3 evaluation and commercialization will need to be developed. If successful, field implementation of an immunization program relying on a liquid nitrogen-based cold chain and intravenous injection will be a new challenge.