Na-GST-1 adsorbed on Alhydrogel co-administered with different Toll-like receptor agonists in hookworm-naive adults...

Necator americanus glutathione S-transferase-1 (Na-GST-1) performs a crucial enzymatic step in hookworm blood feeding and is a leading target for vaccine development. Phase 1 trials in Brazil, Gabon, and the USA of recombinant Na-GST-1 adsorbed on Alhydrogel (Na-GST-1/Al), co-administered with or without AP 10-701 (Toll-like receptor [TLR] 4 agonist) or CpG 10104 (TLR9 agonist), showed the vaccine was well tolerated, safe, and immunogenic. A controlled human hookworm infection (CHHI) model was previously developed to evaluate the effect of candidate hookworm vaccines on infection. We aimed to assess the effect of vaccination with three different Na-GST-1 vaccine formulations on subsequent CHHI challenge with infectious N americanus larvae (NaL3), compared with placebo, and to evaluate vaccine safety.

In this double-blind, randomised, placebo-controlled phase 2 trial, we enrolled non-pregnant, hookworm-naive adults aged 18-49 years who were deemed healthy by means of screening procedures at The George Washington University Medical Faculty Associates (Washington, DC, USA) and The George Washington University School of Medicine and Health Sciences (Washington, DC, USA). Participants were randomly assigned (1:1:1:1) in blocks of four to receive three intramuscular injections at 2-month intervals of Na-GST-1/Al, Na-GST-1/Al in combination with CpG 10104 (Na-GST-1/Al-CpG), Na-GST-1/Al in combination with AP 10-701 (Na-GST-1/Al-AP), or placebo. Participants, vaccinators, investigators conducting safety assessments, and laboratory personnel were all blinded to assignment. Only pharmacy personnel were unblinded. 4 weeks after the final vaccination, participants underwent CHHI challenge with 50 infectious NaL3. Faecal samples were collected every second week until treatment with three consecutive daily oral doses of 400 mg albendazole starting on day 280. After albendazole administration, clearance of infection was documented by three negative post-treatment faecal examinations, collected once per week. Primary outcomes were efficacy and safety. The primary efficacy outcome was infection incidence at any timepoint up to day 380 detected by faecal microscopy in the per-protocol efficacy population. Safety was assessed in the vaccine safety analysis population (ie, all participants who received at least one dose of a vaccine or placebo) and CHHI safety analysis population (ie, all participants who underwent CHHI challenge) from day 0 to day 380. Secondary endpoints were infection intensity and vaccine immunogenicity. The trial is registered at ClinicalTrials.gov (NCT03172975) and is complete.

Between May 24, 2018, and Dec 9, 2020, 57 volunteers were assessed for eligibility, 39 of whom were enrolled (mean age 26·7 years [SD 5·4], 22 [56%] self-reported as female, 17 [44%] self-reported as male). Ten (26%) participants were randomly assigned to Na-GST-1/Al, nine (23%) to Na-GST-1/Al-CpG, ten (26%) to Na-GST-1/Al-AP, and ten (26%) to placebo. All 39 participants received at least one dose and were included in the vaccine safety analysis; 33 received CHHI, of whom 31 were included in the per-protocol efficacy analysis. Participants in the Na-GST-1/Al-CpG group had lower incidence of infection (two [40%] of five) compared to participants in the placebo (four [57%] of seven), Na-GST-1/Al (five [56%] of nine), and Na-GST-1/Al-AP (six [60%] of ten) groups, although differences were not significant. Maximum faecal hookworm egg counts were significantly lower in the Na-GST-1/Al-CpG (median 0·0 eggs per g) group compared with the placebo group (median 66·7 eggs per g; rate ratio 0·00, 95% credible interval 0·00-0·00). Maximum eosinophil counts were significantly lower in the Na-GST-1/Al-CpG group (median 0·6 × 10³ cells per μL) compared with the placebo group (median 3·1 × 10³ cells per μL; geometric mean ratio 0·33, 95% CI 0·13-0·88; p=0·027). Vaccine-induced anti-Na-GST-1 IgG responses were highest in the Na-GST-1/Al-CpG group. No related serious adverse events were observed and most adverse events were mild.

Based on the protection observed against infection, Na-GST-1/Al-CpG has been selected for further clinical testing. The higher levels of anti-Na-GST-1 IgG seen in association with protection against challenge infection suggests that humoral immune responses might correlate with protection.

US National Institute of Allergy and Infectious Diseases.