Immunization of humans with whole sporozoites confers complete, sterilizing immunity against malaria infection. However, achieving consistent safety while maintaining immunogenicity of whole parasite vaccines remains a formidable challenge. We generated a genetically attenuated Plasmodium falciparum (Pf) malaria parasite by deleting three genes expressed in the pre-erythrocytic stage (Pf p52/p36/sap1). We then tested the safety and immunogenicity of the genetically engineered (Pf GAP3KO) sporozoites in human volunteers. Pf GAP3KO sporozoites were delivered to 10 volunteers using infected mosquito bites with a single exposure consisting of 150 to 200 bites per subject. All subjects remained blood stage-negative and developed inhibitory antibodies to sporozoites. GAP3KO rodent malaria parasites engendered complete, protracted immunity against infectious sporozoite challenge in mice. The results warrant further clinical testing of Pf GAP3KO and its potential development into a vaccine strain.

Radiolabeled fluoromisonidazole has been characterized as a probe for hypoxic cells in vitro and in vivo. The uptake and retention of [3H]fluoromisonidazole and [3H]misonidazole were compared in V-79 cell monolayers and spheroids by varying incubation time and O2 levels in contact with the medium. The two labeled drugs were retained similarly in cell populations isolated from different depths in spheroids, and the amount of each drug bound in cells at the spheroid periphery increased with decreasing O2 level. The labeling patterns in autoradiographs were similar for spheroids incubated with the two labeled drugs, with most silver grains located over a zone of viable and presumed hypoxic cells intermediate between the necrotic center and the periphery of the spheroid. Biodistribution of the two tritiated drugs was compared in C3H mice bearing KHT tumors with 15% radiobiologically hypoxic cells. Tumor:blood and tumor:muscle ratios greater than 5.0 were achieved in mice sacrificed 4 h after the last of three injections of 5 or 20 mumol/kg of [3H]fluoromisonidazole. These ratios are compatible with imaging and are higher than those obtained with 50 mumol/kg misonidazole in a similar administration protocol. TLC analysis of plasma from mice injected with [3H]fluoromisonidazole indicated that the drug was stable in vivo for up to 2 h and that the metabolites formed were too polar to be dehalogenation products. Fluoromisonidazole labeled with 18F at the end of the alkyl side chain would retain the label on metabolites that bind in hypoxic cells in vivo. Fluoromisonidazole binds stably in the same populations of hypoxic cells as does misonidazole, and we conclude that [18F]fluromisonidazole has potential use as a hypoxia imaging agent in vivo.

Omeprazole, a widely used and potent gastric proton pump inhibitor, induces cytochrome P450 (CYP) 1A2 in humans. Induction is most pronounced in slow metabolizers of S-mephenytoin because CYP2C19 (S-mephenytoin hydroxylase) is responsible for the elimination of omeprazole. Acetaminophen (INN, paracetamol), a widely used and effective analgesic and antipyretic agent, causes serious hepatic and renal toxicity at high doses by conversion of acetaminophen to the toxic intermediate N-acetyl-p-benzoquinone imine (NAPQI) through CYP1A2, CYP2E1, and CYP3A4. This study evaluated whether omeprazole pretreatment in five rapid and five slow metabolizers of S-mephenytoin could increase thioether (an estimate of NAPQI production) metabolite formation from acetaminophen. The results of this study show that, despite induction of CYP1A2 activity in slow metabolizers (a 75% increase in plasma clearance of caffeine), the formation of NAPQI from acetaminophen was not increased after 7 days of omeprazole administration (40 mg/day). This suggests that induction of CYP1A2 activity by omeprazole is unlikely to increase the risk of acetaminophen hepatotoxicity.

Genetically engineered live Plasmodium falciparum sporozoites constitute a potential platform for creating consistently attenuated, genetically defined, whole-parasite vaccines against malaria through targeted gene deletions. Such genetically attenuated parasites (GAPs) do not require attenuation by irradiation or concomitant drug treatment. We previously developed a P. falciparum (Pf) GAP with deletions in P52 , P36 , and SAP1 genes (PfGAP3KO) and demonstrated its safety and immunogenicity in humans. Here, we further assessed safety, tolerability, and immunogenicity of the PfGAP3KO vaccine and tested its efficacy against controlled human malaria infection (CHMI) in malaria-naïve subjects. The vaccine was delivered by three ( n = 6) or five ( n = 8) immunizations with ~200 PfGAP3KO-infected mosquito bites per immunization. PfGAP3KO was safe and well tolerated with no breakthrough P. falciparum blood stage infections. Vaccine-related adverse events were predominately localized urticaria related to the numerous mosquito bites administered per vaccination. CHMI via bites with mosquitoes carrying fully infectious Pf NF54 parasites was carried out 1 month after the last immunization. Half of the study participants who received either three or five PfGAP3KO immunizations remained P. falciparum blood stage negative, as shown by a lack of detection of Plasmodium 18 S rRNA in the blood for 28 days after CHMI. Six protected study participants received a second CHMI 6 months later, and one remained completely protected. Thus, the PfGAP3KO vaccine was safe and immunogenic and was capable of inducing protection against sporozoite infection. These results warrant further evaluation of PfGAP3KO vaccine efficacy in dose-range finding trials with an injectable formulation.

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