Beyond Blood Smears: Qualification of Plasmodium 18S rRNA as a Biomarker for Controlled Human Malaria Infections.
18S rRNA is a biomarker that provides an alternative to thick blood smears in controlled human malaria infection (CHMI) trials. We reviewed data from CHMI trials at non-endemic sites that used blood smears and Plasmodium 18S rRNA/rDNA biomarker nucleic acid tests (NATs) for time to positivity. We validated a multiplex quantitative reverse transcription-polymerase chain reaction (qRT-PCR) for Plasmodium 18S rRNA, prospectively compared blood smears and qRT-PCR for three trials, and modeled treatment effects at different biomarker-defined parasite densities to assess the impact on infection detection, symptom reduction, and measured intervention efficacy. Literature review demonstrated accelerated NAT-based infection detection compared with blood smears (mean acceleration: 3.2-3.6 days). For prospectively tested trials, the validated Plasmodium 18S rRNA qRT-PCR positivity was earlier (7.6 days; 95% CI: 7.1-8.1 days) than blood smears (11.0 days; 95% CI: 10.3-11.8 days) and significantly preceded the onset of grade 2 malaria-related symptoms (12.2 days; 95% CI: 10.6-13.3 days). Discrepant analysis showed that the risk of a blood smear-positive, biomarker-negative result was negligible. Data modeling predicted that treatment triggered by specific biomarker-defined thresholds can differentiate complete, partial, and non-protective outcomes and eliminate many grade 2 and most grade 3 malaria-related symptoms post-CHMI. Plasmodium 18S rRNA is a sensitive and specific biomarker that can justifiably replace blood smears for infection detection in CHMI trials in non-endemic settings. This study led to biomarker qualification through the U.S. Food and Drug Administration for use in CHMI studies at non-endemic sites, which will facilitate biomarker use for the qualified context of use in drug and vaccine trials.
Seilie, AM; Chang, M; Hanron, AE; Billman, ZP; Stone, BC; Zhou, K; Olsen, TM; Daza, G; Ortega, J; Cruz, KR; Smith, N; Healy, SA; Neal, J; Wallis, CK; Shelton, L; Mankowski, TV; Wong-Madden, S; Mikolajczak, SA; Vaughan, AM; Kappe, SHI; Fishbaugher, M; Betz, W; Kennedy, M; Hume, JCC; Talley, AK; Hoffman, SL; Chakravarty, S; Sim, BKL; Richie, TL; Wald, A; Plowe, CV; Lyke, KE; Adams, M; Fahle, GA; Cowan, EP; Duffy, PE; Kublin, JG; Murphy, SC
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