Safety, pharmacokinetics, and potential neurological interactions of ivermectin, tafenoquine, and chloroquine in Rhesus macaques

Ivermectin (IVM) could be used for malaria control as treated individuals are lethal to blood-feeding Anopheles, resulting in reduced transmission. Tafenoquine (TQ) is used to clear the liver reservoir of Plasmodium vivax and as a prophylactic treatment in high-risk populations. It has been suggested to use ivermectin and tafenoquine in combination, but the safety of these drugs in combination has not been evaluated. Early derivatives of 8-aminoquinolones (8-AQ) were neurotoxic, and ivermectin is an inhibitor of the P-glycoprotein (P-gp) blood brain barrier (BBB) transporter. Thus, there is concern that co-administration of these drugs could be neurotoxic. This study aimed to evaluate the safety and pharmacokinetic interaction of tafenoquine, ivermectin, and chloroquine (CQ) in Rhesus macaques. No clinical, biochemistry, or hematological outcomes of concern were observed. The Cambridge Neuropsychological Test Automated Battery (CANTAB) was employed to assess potential neurological deficits following drug administration. Some impairment was observed with tafenoquine alone and in the same monkeys with subsequent co-administrations. Co-administration of chloroquine and tafenoquine resulted in increased plasma exposure to tafenoquine. Urine concentrations of the 5,6 orthoquinone TQ metabolite were increased with co-administration of tafenoquine and ivermectin. There was an increase in ivermectin plasma exposure when co-administered with chloroquine. No interaction of tafenoquine on ivermectin was observed in vitro. Chloroquine and trace levels of ivermectin, but not tafenoquine, were observed in the cerebrospinal fluid. The 3''-O-demethyl ivermectin metabolite was observed in macaque plasma but not in urine or cerebrospinal fluid. Overall, the combination of ivermectin, tafenoquine, and chloroquine did not have clinical, neurological, or pharmacological interactions of concern in macaques; therefore, this combination could be considered for evaluation in human trials.

Strong positive selection biases identity-by-descent-based inferences of recent demography and population structure in Plasmodium falciparum

Malaria genomic surveillance often estimates parasite genetic relatedness using metrics such as Identity-By-Decent (IBD), yet strong positive selection stemming from antimalarial drug resistance or other interventions may bias IBD-based estimates. In this study, we use simulations, a true IBD inference algorithm, and empirical data sets from different malaria transmission settings to investigate the extent of this bias and explore potential correction strategies. We analyze whole genome sequence data generated from 640 new and 3089 publicly available Plasmodium falciparum clinical isolates. We demonstrate that positive selection distorts IBD distributions, leading to underestimated effective population size and blurred population structure. Additionally, we discover that the removal of IBD peak regions partially restores the accuracy of IBD-based inferences, with this effect contingent on the population's background genetic relatedness and extent of inbreeding. Consequently, we advocate for selection correction for parasite populations undergoing strong, recent positive selection, particularly in high malaria transmission settings.

Strong Positive Selection Biases Identity-By-Descent-Based Inferences of Recent Demography and Population Structure in Plasmodium falciparum

Malaria genomic surveillance often estimates parasite genetic relatedness using metrics such as Identity-By-Decent (IBD). Yet, strong positive selection stemming from antimalarial drug resistance or other interventions may bias IBD-based estimates. In this study, we utilized simulations, a true IBD inference algorithm, and empirical datasets from different malaria transmission settings to investigate the extent of such bias and explore potential correction strategies. We analyzed whole genome sequence data generated from 640 new and 4,026 publicly available Plasmodium falciparum clinical isolates. Our findings demonstrated that positive selection distorts IBD distributions, leading to underestimated effective population size and blurred population structure. Additionally, we discovered that the removal of IBD peak regions partially restored the accuracy of IBD-based inferences, with this effect contingent on the population's background genetic relatedness. Consequently, we advocate for selection correction for parasite populations undergoing strong, recent positive selection, particularly in high malaria transmission settings.

The parasite intraerythrocytic cycle and human circadian cycle are coupled during malaria infection

During infections with the malaria parasites Plasmodium vivax, patients exhibit rhythmic fevers every 48 h. These fever cycles correspond with the time the parasites take to traverse the intraerythrocytic cycle (IEC). In other Plasmodium species that infect either humans or mice, the IEC is likely guided by a parasite-intrinsic clock [Rijo-Ferreiraet al., Science 368, 746-753 (2020); Smith et al., Science 368, 754-759 (2020)], suggesting that intrinsic clock mechanisms may be a fundamental feature of malaria parasites. Moreover, because Plasmodium cycle times are multiples of 24 h, the IECs may be coordinated with the host circadian clock(s). Such coordination could explain the synchronization of the parasite population in the host and enable alignment of IEC and circadian cycle phases. We utilized an ex vivo culture of whole blood from patients infected with P. vivax to examine the dynamics of the host circadian transcriptome and the parasite IEC transcriptome. Transcriptome dynamics revealed that the phases of the host circadian cycle and the parasite IEC are correlated across multiple patients, showing that the cycles are phase coupled. In mouse model systems, host-parasite cycle coupling appears to provide a selective advantage for the parasite. Thus, understanding how host and parasite cycles are coupled in humans could enable antimalarial therapies that disrupt this coupling.

Molecular characterization of multidrug-resistant ESKAPEE pathogens from clinical samples in Chonburi, Thailand (2017-2018)

BACKGROUND

ESKAPEE pathogens Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. and Escherichia coli are multi-drug resistant (MDR) bacteria that present increasing treatment challenges for healthcare institutions and public health worldwide.

METHODS

431 MDR ESKAPEE pathogens were collected from Queen Sirikit Naval Hospital, Chonburi, Thailand between 2017 and 2018. Species identification and antimicrobial resistance (AMR) phenotype were determined following CLSI and EUCAST guidelines on the BD Phoenix System. Molecular identification of antibiotic resistant genes was performed by polymerase chain reaction (PCR), real-time PCR assays, and whole genome sequencing (WGS).

RESULTS

Of the 431 MDR isolates collected, 1.2% were E. faecium, 5.8% were S. aureus, 23.7% were K. pneumoniae, 22.5% were A. baumannii, 4.6% were P. aeruginosa, 0.9% were Enterobacter spp., and 41.3% were E. coli. Of the 401 Gram-negative MDR isolates, 51% were carbapenem resistant, 45% were ESBL producers only, 2% were colistin resistance and ESBLs producers (2%), and 2% were non-ESBLs producers. The most prevalent carbapenemase genes were bla_OXA-23 (23%), which was only identified in A. baumannii, followed by bla_NDM (17%), and bla_OXA-48-like (13%). Beta-lactamase genes detected included bla_TEM, bla_SHV, bla_OXA, bla_CTX-M, bla_DHA, bla_CMY, bla_PER and bla_VEB. Seven E. coli and K. pneumoniae isolates showed resistance to colistin and carried mcr-1 or mcr-3, with 2 E. coli strains carrying both genes. Among 30 Gram-positive MDR ESKAPEE, all VRE isolates carried the vanA gene (100%) and 84% S. aureus isolates carried the mecA gene.

CONCLUSIONS

This report highlights the prevalence of AMR among clinical ESKAPEE pathogens in eastern Thailand. E. coli was the most common MDR pathogen collected, followed by K. pneumoniae, and A. baumannii. Carbapenem-resistant Enterobacteriaceae (CRE) and extended spectrum beta-lactamases (ESBLs) producers were the most common resistance profiles. The co-occurrence of mcr-1 and mcr-3 in 2 E. coli strains, which did not affect the level of colistin resistance, is also reported. The participation of global stakeholders and surveillance of MDR remain essential for the control and management of MDR ESKAPEE pathogens.

Co-Infection with Plasmodium vivax and COVID-19 in Thailand

With the emergence of SARS-CoV-2, healthcare systems not only had to address the pressing clinical needs of the COVID-19 pandemic but anticipate the effect on and of other conditions and diseases. This was of particular concern in areas of the world endemic with malaria, a disease which takes hundreds of thousands of lives each year. This case report from Thailand describes a 25-year-old man diagnosed with Plasmodium vivax, who was then found to be co-infected with COVID-19. Both conditions can have overlapping acute febrile illness symptoms which may delay or complicate diagnoses. He had no prior history of malaria and had received two vaccinations against COVID-19. His clinical course was mild with no pulmonary complications or oxygen requirement, and he responded well to treatments for both conditions. Three months after cure, he again contracted COVID-19 but did not experience any P. vivax relapse. Review of the available literature produced less than 10 publications describing co-infections with P. vivax and COVID-19; nonetheless, in endemic areas, vigilance for both diseases should continue, as co-infections could significantly alter the course of clinical management and prognosis as well as affect the healthcare staff caring for these patients.

A single point mutation in the Plasmodium falciparum 3'-5' exonuclease does not alter piperaquine susceptibility

Background

The rise in Plasmodium falciparum resistance to dihydroartemisinin–piperaquine (DHA–PPQ) treatment has been documented in the Greater Mekong Subregion with associations with mutations in the P. falciparum chloroquine resistance transporter (pfcrt) and plasmepsin 2 (pfpm2) genes. However, it is unclear whether other genes also play a role with PPQ resistance, such as the E415G mutation in the exonuclease (pfexo) gene. The aim of this study was to investigate the role of this mutation in PPQ resistance by generating transgenic parasites expressing the pfexo-E415G mutant allele.

Methods

Transgenic parasite clones carrying the E415G mutation in PfEXO of the B5 isolate were derived by CRISPR-Cas9 gene editing and verified using PCR and gene sequencing. Polymorphisms of pfkelch-13, pfcrt, and pfexo were examined by PCR while the copy number variations of pfpm2 were examined by both relative quantitative real-time PCR and the duplication breakpoint assay. Drug sensitivity against a panel of antimalarials, the ring-stage survival assay (RSA), the PPQ survival assay (PSA), and bimodal dose-response curves were used to evaluate antimalarial susceptibility.

Results

The transgenic line, B5-rexo-E415G-B8, was successfully generated. The PPQ-IC₉₀, %PPQ survival, and the bimodal dose-response clearly showed that E415G mutation in PfEXO of B5 isolate remained fully susceptible to PPQ. Furthermore, growth assays demonstrated that the engineered parasites grew slightly faster than the unmodified parental isolates whereas P. falciparum isolates harbouring pfkelch-13, pfcrt, and pfexo mutations with multiple copies of pfpm2 grew much more slowly.

Conclusions

Insertion of the E415G mutation in PfEXO did not lead to increased PPQ-IC₉₀ and %PPQ survival, suggesting that this mutation alone may not be associated with PPQ resistance, but could still be an important marker if used in conjunction with other markers for monitoring PPQ-resistant parasites. The results also highlight the importance of monitoring and evaluating suspected genetic mutations with regard to parasite fitness and resistance.

Supplementary information

The online version contains supplementary material available at 10.1186/s12936-022-04148-z.

Chronic Wound Infection Model of Acinetobacter baumannii in Outbred Mice

INTRODUCTION

We established a murine wound infection model with doxycycline treatment against multidrug-resistant Acinetobacter baumannii (AB5075) in Institute of Cancer Research (ICR) outbred mice.

METHODS

Using three groups of neutropenic ICR mice, two full-thickness dorsal dermal wounds (6 mm diameter) were made on each mouse. In two groups, wounds were inoculated with 7.0 × 104 colony-forming units of AB5075. Of these two groups, one received a 6-day regimen of doxycycline while the other was sham treated with phosphate-buffered saline as placebo control. Another uninfected/untreated group served as a control. Wound closure, clinical symptoms, bacterial burden in wound beds and organs, and wound histology were investigated.

RESULTS

Doxycycline-treated wounds completely healed by day 21, but untreated, infected wounds failed to heal. Compared to controls, wound infections without treatment resulted in significant reductions in body weight and higher bacterial loads in wound beds, lung, liver, and spleen by day 7. Histological evaluation of wounds on day 21 revealed ulcerated epidermis, muscle necrosis, and bacterial presence in untreated wounds, while wounds treated with doxycycline presented intact epidermis.

CONCLUSIONS

Compared to the previously developed BALB/c dermal wound model, this study demonstrates that the mouse strain selected impacts wound severity and resolution. Furthermore, this mouse model accommodates two dorsal wounds rather than only one. These variations offer investigators increased versatility when designing future studies of wound infection. In conclusion, ICR mice are a viable option as a model of dermal wound infection. They accommodate two simultaneous dorsal wounds, and upon infection, these wounds follow a different pattern of resolution compared to BALB/c mice.

Repeatability and reproducibility of a handheld quantitative G6PD diagnostic

BACKGROUND

The introduction of novel short course treatment regimens for the radical cure of Plasmodium vivax requires reliable point-of-care diagnosis that can identify glucose-6-phosphate dehydrogenase (G6PD) deficient individuals. While deficient males can be identified using a qualitative diagnostic test, the genetic make-up of females requires a quantitative measurement. SD Biosensor (Republic of Korea) has developed a handheld quantitative G6PD diagnostic (STANDARD G6PD test), that has approximately 90% accuracy in field studies for identifying individuals with intermediate or severe deficiency. The device can only be considered for routine care if precision of the assay is high.

METHODS AND FINDINGS

Commercial lyophilised controls (ACS Analytics, USA) with high, intermediate, and low G6PD activities were assessed 20 times on 10 Biosensor devices and compared to spectrophotometry (Pointe Scientific, USA). Each device was then dispatched to one of 10 different laboratories with a standard set of the controls. Each control was tested 40 times at each laboratory by a single user and compared to spectrophotometry results. When tested at one site, the mean coefficient of variation (CV) was 0.111, 0.172 and 0.260 for high, intermediate, and low controls across all devices respectively; combined G6PD Biosensor readings correlated well with spectrophotometry (rs = 0.859, p<0.001). When tested in different laboratories, correlation was lower (rs = 0.604, p<0.001) and G6PD activity determined by Biosensor for the low and intermediate controls overlapped. The use of lyophilised human blood samples rather than fresh blood may have affected these findings. Biosensor G6PD readings between sites did not differ significantly (p = 0.436), whereas spectrophotometry readings differed markedly between sites (p<0.001).

CONCLUSIONS

Repeatability and inter-laboratory reproducibility of the Biosensor were good; though the device did not reliably discriminate between intermediate and low G6PD activities of the lyophilized specimens. Clinical studies are now required to assess the devices performance in practice.

Antimicrobial Susceptibility of Neisseria gonorrhoeae in Adult Patients Seeking Care at Military Hospitals in Thailand From 2014 to 2020

INTRODUCTION

The effective dual antibiotics ceftriaxone (CRO) and azithromycin (AZM) have successfully treated Neisseria gonorrhoeae (GC) infection, however, the CRO- and AZM-resistant strains have been sporadically detected globally and in Thailand. Furthermore, there are no currently antimicrobial susceptibility profiles of the GC isolates obtained from soldiers reported in Thailand. Hence, this is the first study to describe the antimicrobial susceptibility profiles of GC isolates obtained from predominately soldiers who seeking care at Military Camp Hospitals, in Thailand from 2014 to 2020.

MATERIALS AND METHODS

A total of 624 symptomatic gonococcal samples were received from 10 military hospitals during 2014-2020. They were collected from urethral swabs and inoculated into selective media. The suspected GC isolates were subcultured and presumptively identified using conventional microbiology techniques. Antimicrobial susceptibility test was performed by Etest to determine minimal inhibitory concentration (μg/mL) against AZM, benzylpenicillin, cefepime, cefixime, ceftriaxone (CRO), ciprofloxacin, spectinomycin, and tetracycline using the criteria outlined in the Clinical and Laboratory Standards Institute guidelines. This study was approved by Institutional Review Board, Royal Thai Army Medical Department under protocol number S036b/56 and Walter Reed Army Institute of Research, and Silver Spring, MD under protocol number WR #2039.

RESULTS

A total of 624 samples were collected from symptomatic gonococcal infectious patients with 91.5% (571/624) of samples obtained from soldiers. Of those, 78% (488/624) were identified as GC and 92% (449/488) of them were isolated from soldiers. All GC samples collected were susceptible to CRO (first-line treatment) with only one GC isolate identified as non-susceptible to cefepime and three isolates identified as non-susceptible to AZM.

CONCLUSION

The recommended dual treatment of GC infections with CRO and AZM is currently an effective empirical treatment for patients who are seeking care at military hospitals in Thailand. Nevertheless, cefepime is a fourth-generation cephalosporin with documented high activity against GC strains equal to other "third-generation" cephalosporins such as CRO. Due to the active duty of military personnel, they concerned about the confidentiality and frequently seek treatment at civilian clinics. Additionally, due to the availability of antibiotics over the counter in Thailand, many choose the option to self-medicate without a physician's prescription. These could be subsequently driven the gradual increase of multidrug-resistant gonococcal strains throughout the country. Thus, the GC surveillance would be needed for further Force Health Protection and public health authorities in response to the drug-resistant GC threats.

Civilian-military malaria outbreak response in Thailand: an example of multi-stakeholder engagement for malaria elimination

BACKGROUND

In April 2017, the Thai Ministry of Public Health (MoPH) was alerted to a potential malaria outbreak among civilians and military personnel in Sisaket Province, a highly forested area bordering Cambodia. The objective of this study was to present findings from the joint civilian-military outbreak response.

METHODS

A mixed-methods approach was used to assess risk factors among cases reported during the 2017 Sisaket malaria outbreak. Routine malaria surveillance data from January 2013 to March 2018 obtained from public and military medical reporting systems and key informant interviews (KIIs) (n = 72) were used to develop hypotheses about potential factors contributing to the outbreak. Joint civilian-military response activities included entomological surveys, mass screen and treat (MSAT) and vector control campaigns, and scale-up of the "1-3-7" reactive case detection approach among civilians alongside a pilot "1-3-7" study conducted by the Royal Thai Army (RTA).

RESULTS

Between May-July 2017, the monthly number of MoPH-reported cases surpassed the epidemic threshold. Outbreak cases detected through the MoPH mainly consisted of Thai males (87%), working as rubber tappers (62%) or military/border police (15%), and Plasmodium vivax infections (73%). Compared to cases from the previous year (May-July 2016), outbreak cases were more likely to be rubber tappers (OR = 14.89 [95% CI: 5.79-38.29]; p < 0.001) and infected with P. vivax (OR=2.32 [1.27-4.22]; p = 0.006). Themes from KIIs were congruent with findings from routine surveillance data. Though limited risk factor information was available from military cases, findings from RTA's "1-3-7" study indicated transmission was likely occurring outside military bases. Data from entomological surveys and MSAT campaigns support this hypothesis, as vectors were mostly exophagic and parasite prevalence from MSAT campaigns was very low (range: 0-0.7% by PCR/microscopy).

CONCLUSIONS

In 2017, an outbreak of mainly P. vivax occurred in Sisaket Province, affecting mainly military and rubber tappers. Vector control use was limited to the home/military barracks, indicating that additional interventions were needed during high-risk forest travel periods. Importantly, this outbreak catalyzed joint civilian-military collaborations and integration of the RTA into the national malaria elimination strategy (NMES). The Sisaket outbreak response serves as an example of how civilian and military public health systems can collaborate to advance national malaria elimination goals in Southeast Asia and beyond.

Molecular Detection of Mutations in the Propeller Domain of Kelch 13 and pfmdr1 Copy Number Variation in Plasmodium falciparum Isolates from Thailand Collected from 2002 to 2007

We determined the prevalence of Kelch 13 mutations and pfmdr1 copy number in samples collected from the Thailand-Myanmar border, the Thailand-Cambodia border, and southern Thailand from 2002 to 2007. C580Y was the most prevalent in Trat (Thailand-Cambodia border) and Ranong (Thailand-Myanmar border) at 42% (24/57) and 13% (6/48), respectively. Less predominant mutations were also identified including R539T (7%, 4/57) and Y493H (2%, 1/57) in Trat, P574L (6%, 3/48) and P553L (2%, 1/48) in Ranong, and N537I and D452E (7%, 1/15) in Sangkhlaburi (Thailand-Myanmar border). Samples from Mae sot (33%, 11/33) harbored the highest percentage of multiple pfmdr1 copies, followed by Trat (18%, 10/57), Chiang Dao in 2003 (13%, 4/30), Phang Nga (5%, 2/44), and Chiang Dao in 2002 (4%, 1/26). This retrospective study provides geographic diversity of K13 and pfmdr1 copies and the emergence of these molecular markers in Thailand, an important background information for future surveillance in the region.

Plasmodium falciparum phenotypic and genotypic resistance profile during the emergence of Piperaquine resistance in Northeastern Thailand

Malaria remains a public health problem in Thailand, especially along its borders where highly mobile populations can contribute to persistent transmission. This study aimed to determine resistant genotypes and phenotypes of 112 Plasmodium falciparum isolates from patients along the Thai-Cambodia border during 2013-2015. The majority of parasites harbored a pfmdr1-Y184F mutation. A single pfmdr1 copy number had CVIET haplotype of amino acids 72-76 of pfcrt and no pfcytb mutations. All isolates had a single pfk13 point mutation (R539T, R539I, or C580Y), and increased % survival in the ring-stage survival assay (except for R539I). Multiple copies of pfpm2 and pfcrt-F145I were detected in 2014 (12.8%) and increased to 30.4% in 2015. Parasites containing either multiple pfpm2 copies with and without pfcrt-F145I or a single pfpm2 copy with pfcrt-F145I exhibited elevated IC₉₀ values of piperaquine. Collectively, the emergence of these resistance patterns in Thailand near Cambodia border mirrored the reports of dihydroartemisinin-piperaquine treatment failures in the adjacent province of Cambodia, Oddar Meanchey, suggesting a migration of parasites across the border. As malaria elimination efforts ramp up in Southeast Asia, host nations militaries and other groups in border regions need to coordinate the proposed interventions.

Distribution and temporal dynamics of P. falciparum chloroquine resistance transporter mutations associated with piperaquine resistance in Northern Cambodia

BACKGROUND

Newly emerged mutations within the Plasmodium falciparum chloroquine resistance transporter (PfCRT) can confer piperaquine resistance in the absence of amplified plasmepsin II (pfpm2). In this study, we estimated the prevalence of co-circulating piperaquine resistance mutations in P. falciparum isolates collected in northern Cambodia from 2009-2017.

METHODS

The sequence of pfcrt was determined for 410 P. falciparum isolates using PacBio amplicon sequencing or whole genome sequencing. Quantitative PCR was used to estimate pfpm2 and pfmdr1 copy number.

RESULTS

Newly emerged PfCRT mutations increased in prevalence after the change to dihydroartemisinin-piperaquine in 2010, with >98% of parasites harboring these mutations by 2017. After 2014, the prevalence of PfCRT F145I declined, being out-competed by parasites with less resistant, but more fit PfCRT alleles. After the change to artesunate-mefloquine, the prevalence of parasites with amplified pfpm2 decreased, with nearly half of piperaquine-resistant PfCRT mutants having single copy pfpm2.

CONCLUSIONS

The large proportion of PfCRT mutants that lack pfpm2 amplification emphasizes the importance of including PfCRT mutations as part of molecular surveillance for piperaquine resistance in this region. Likewise, it is critical to monitor for amplified pfmdr1 in these PfCRT mutants, as increased mefloquine pressure could lead to mutants resistant to both drugs.

Piperaquine resistant Cambodian Plasmodium falciparum clinical isolates: in vitro genotypic and phenotypic characterization

BACKGROUND

High rates of dihydroartemisinin-piperaquine (DHA-PPQ) treatment failures have been documented for uncomplicated Plasmodium falciparum in Cambodia. The genetic markers plasmepsin 2 (pfpm2), exonuclease (pfexo) and chloroquine resistance transporter (pfcrt) genes are associated with PPQ resistance and are used for monitoring the prevalence of drug resistance and guiding malaria drug treatment policy.

METHODS

To examine the relative contribution of each marker to PPQ resistance, in vitro culture and the PPQ survival assay were performed on seventeen P. falciparum isolates from northern Cambodia, and the presence of E415G-Exo and pfcrt mutations (T93S, H97Y, F145I, I218F, M343L, C350R, and G353V) as well as pfpm2 copy number polymorphisms were determined. Parasites were then cloned by limiting dilution and the cloned parasites were tested for drug susceptibility. Isobolographic analysis of several drug combinations for standard clones and newly cloned P. falciparum Cambodian isolates was also determined.

RESULTS

The characterization of culture-adapted isolates revealed that the presence of novel pfcrt mutations (T93S, H97Y, F145I, and I218F) with E415G-Exo mutation can confer PPQ-resistance, in the absence of pfpm2 amplification. In vitro testing of PPQ resistant parasites demonstrated a bimodal dose-response, the existence of a swollen digestive vacuole phenotype, and an increased susceptibility to quinine, chloroquine, mefloquine and lumefantrine. To further characterize drug sensitivity, parental parasites were cloned in which a clonal line, 14-B5, was identified as sensitive to artemisinin and piperaquine, but resistant to chloroquine. Assessment of the clone against a panel of drug combinations revealed antagonistic activity for six different drug combinations. However, mefloquine-proguanil and atovaquone-proguanil combinations revealed synergistic antimalarial activity.

CONCLUSIONS

Surveillance for PPQ resistance in regions relying on DHA-PPQ as the first-line treatment is dependent on the monitoring of molecular markers of drug resistance. P. falciparum harbouring novel pfcrt mutations with E415G-exo mutations displayed PPQ resistant phenotype. The presence of pfpm2 amplification was not required to render parasites PPQ resistant suggesting that the increase in pfpm2 copy number alone is not the sole modulator of PPQ resistance. Genetic background of circulating field isolates appear to play a role in drug susceptibility and biological responses induced by drug combinations. The use of latest field isolates may be necessary for assessment of relevant drug combinations against P. falciparum strains and when down-selecting novel drug candidates.

IMRAS-A clinical trial of mosquito-bite immunization with live, radiation-attenuated P. falciparum sporozoites: Impact of immunization parameters on protective efficacy and generation of a repository of immunologic reagents

Background

Immunization with radiation-attenuated sporozoites (RAS) by mosquito bite provides >90% sterile protection against Plasmodium falciparum (Pf) malaria in humans. RAS invade hepatocytes but do not replicate. CD8+ T cells recognizing parasite-derived peptides on the surface of infected hepatocytes are likely the primary protective mechanism. We conducted a randomized clinical trial of RAS immunization to assess safety, to achieve 50% vaccine efficacy (VE) against controlled human malaria infection (CHMI), and to generate reagents from protected and non-protected subjects for future identification of protective immune mechanisms and antigens.

Methods

Two cohorts (Cohort 1 and Cohort 2) of healthy, malaria-naïve, non-pregnant adults age 18–50 received five monthly immunizations with infected (true-immunized, n = 21) or non-infected (mock-immunized, n = 5) mosquito bites and underwent homologous CHMI at 3 weeks. Immunization parameters were selected for 50% protection based on prior clinical data. Leukapheresis was done to collect plasma and peripheral blood mononuclear cells.

Results

Adverse event rates were similar in true- and mock-immunized subjects. Two true- and two mock-immunized subjects developed large local reactions likely caused by mosquito salivary gland antigens. In Cohort 1, 11 subjects received 810–1235 infected bites; 6/11 (55%) were protected against CHMI vs. 0/3 mock-immunized and 0/6 infectivity controls (VE 55%). In Cohort 2, 10 subjects received 839–1131 infected bites with a higher first dose and a reduced fifth dose; 9/10 (90%) were protected vs. 0/2 mock-immunized and 0/6 controls (VE 90%). Three/3 (100%) protected subjects administered three booster immunizations were protected against repeat CHMI vs. 0/6 controls (VE 100%). Cohort 2 uniquely showed a significant rise in IFN-γ responses after the third and fifth immunizations and higher antibody responses to CSP.

Conclusions

PfRAS were generally safe and well tolerated. Cohort 2 had a higher first dose, reduced final dose, higher antibody responses to CSP and significant rise of IFN-γ responses after the third and fifth immunizations. Whether any of these factors contributed to increased protection in Cohort 2 requires further investigation. A cryobank of sera and cells from protected and non-protected individuals was generated for future immunological studies and antigen discovery.

Trial registration

ClinicalTrials.gov NCT01994525.

Prevalence of CYP2D6 Genotypes and Predicted Phenotypes in a Cohort of Cambodians at High Risk for Infections with Plasmodium vivax

Clinical failure of primaquine (PQ) has been demonstrated in people with CYP450 2D6 genetic polymorphisms that result in reduced or no enzyme activity. The distribution of CYP2D6 genotypes and predicted phenotypes in the Cambodian population is not well described. Surveys in other Asian countries have shown an approximate 50% prevalence of the reduced activity CYP2D6 allele *10, which could translate into increased risk of PQ radical cure failure and repeated relapses, making interruption of transmission and malaria elimination difficult to achieve. We determined CYP2D6 genotypes from 96 volunteers from Oddor Meanchey Province, Cambodia, an area endemic for Plasmodium vivax. We found a 54.2% frequency of the *10 allele, but in approximately half of our subjects, it was paired with a normal activity allele, either *1 or *2. The prevalence of *5, a null allele, was 9.4%. Overall predicted phenotype percentages were normal metabolizers, 46%; intermediate metabolizers, 52%; and poor metabolizers, 1%.

Evaluation of the CareStart™ glucose-6-phosphate dehydrogenase (G6PD) rapid diagnostic test in the field settings and assessment of perceived risk from primaquine at the community level in Cambodia

Background

Primaquine is an approved radical cure treatment for Plasmodium vivax malaria but treatment can result in life-threatening hemolysis if given to a glucose-6-phosphate dehydrogenase deficient (G6PDd) patient. There is a need for reliable point-of-care G6PD diagnostic tests.

Objectives

To evaluate the performance of the CareStart™ rapid diagnostic test (RDT) in the hands of healthcare workers (HCWs) and village malaria workers (VMWs) in field settings, and to better understand user perceptions about the risks and benefits of PQ treatment guided by RDT results.

Methods

This study enrolled 105 HCWs and VMWs, herein referred to as trainees, who tested 1,543 healthy adult male volunteers from 84 villages in Cambodia. The trainees were instructed on G6PD screening, primaquine case management, and completed pre and post-training questionnaires. Each trainee tested up to 16 volunteers in the field under observation by the study staff.

Results

Out of 1,542 evaluable G6PD volunteers, 251 (16.28%) had quantitative enzymatic activity less than 30% of an adjusted male median (8.30 U/g Hb). There was no significant difference in test sensitivity in detecting G6PDd between trainees (97.21%), expert study staff in the field (98.01%), and in a laboratory setting (95.62%) (p = 0.229); however, test specificity was different for trainees (96.62%), expert study staff in the field (98.14%), and experts in the laboratory (98.99%) (p < 0.001). Negative predictive values were not statistically different for trainees, expert staff, and laboratory testing: 99.44%, 99.61%, and 99.15%, respectively. Knowledge scores increased significantly post-training, with 98.7% willing to prescribe primaquine for P.vivax malaria, an improvement from 40.6% pre-training (p < 0.001).

Conclusion

This study demonstrated ability of medical staff with different background to accurately use CareStart™ RDT to identify G6PDd in male patients, which may enable safer prescribing of primaquine; however, pharmacovigilance is required to address possible G6PDd misclassifications.

Gametocyte Carriage, Antimalarial Use, and Drug Resistance in Cambodia, 2008-2014

Gametocytes are the malaria parasite stages responsible for transmission from humans to mosquitoes. Gametocytemia often follows drug treatment, especially as therapies start to fail. We examined Plasmodium falciparum gametocyte carriage and drug resistance profiles among 824 persons with uncomplicated malaria in Cambodia to determine whether prevalent drug resistance and antimalarial use has led to a concentration of drug-resistant parasites among gametocyte carriers. Although report of prior antimalarial use increased from 2008 to 2014, the prevalence of study participants presenting with microscopic gametocyte carriage declined. Gametocytemia was more common in those reporting antimalarial use within the past year, and prior antimalarial use was correlated with higher IC₅₀s to piperaquine and mefloquine, as well as to increased pfmdr1 copy number. However, there was no association between microscopic gametocyte carriage and parasite drug resistance. Thus, we found no evidence that the infectious reservoir, marked by those carrying gametocytes, is enriched with drug-resistant parasites.

Atovaquone-Proguanil in Combination With Artesunate to Treat Multidrug-Resistant P. falciparum Malaria in Cambodia: An Open-Label Randomized Trial

Background

Recent artemisinin-combination therapy failures in Cambodia prompted a search for alternatives. Atovaquone-proguanil (AP), a safe, effective treatment for multidrug-resistant Plasmodium falciparum (P.f.), previously demonstrated additive effects in combination with artesunate (AS).

Methods

Patients with P.f. or mixed-species infection (n = 205) in Anlong Veng (AV; n = 157) and Kratie (KT; n = 48), Cambodia, were randomized open-label 1:1 to a fixed-dose 3-day AP regimen +/-3 days of co-administered artesunate (ASAP). Single low-dose primaquine (PQ, 15 mg) was given on day 1 to prevent gametocyte-mediated transmission.

Results

Polymerase chain reaction-adjusted adequate clinical and parasitological response at 42 days was 90% for AP (95% confidence interval [CI], 82%-95%) and 92% for ASAP (95% CI, 83%-96%; P = .73). The median parasite clearance time was 72 hours for ASAP in AV vs 56 hours in KT (P < .001) and was no different than AP alone. At 1 week postprimaquine, 7% of the ASAP group carried microscopic gametocytes vs 29% for AP alone (P = .0001). Nearly all P.f. isolates had C580Y K13 propeller artemisinin resistance mutations (AV 99%; KT 88%). Only 1 of 14 treatment failures carried the cytochrome bc1 (Pfcytb) atovaquone resistance mutation, which was not present at baseline. P.f. isolates remained atovaquone sensitive in vitro but cycloguanil resistant, with a triple P.f. dihydrofolate reductase mutation.

Conclusions

Atovaquone-proguanil remained marginally effective in Cambodia (≥90%) with minimal Pfcytb mutations observed. Treatment failures in the presence of ex vivo atovaquone sensitivity and adequate plasma levels may be attributable to cycloguanil and/or artemisinin resistance. Artesunate co-administration provided little additional blood-stage efficacy but reduced post-treatment gametocyte carriage in combination with AP beyond single low-dose primaquine.

Plasmodium falciparum Clearance Is Pitting-Dependent With Artemisinin-Based Drugs but Pitting-Independent With Atovaquone-Proguanil or Mefloquine

Pitting, the removal of dead parasites from their host erythrocyte, has been studied in patients with severe malaria treated parenterally with quinine or artesunate, and was recently shown to contribute to delayed hemolysis, a frequent adverse event of artesunate. We quantified pitting in 81 travelers treated with oral antimalarial therapy. Pitting rate was high (55.8%) with artemisinin-based combinations, but <10% with the nonartemisinin drugs quinine, mefloquine, and atovaquone-proguanil. This may, in part, explain the slower parasite clearance in patients treated with antimalarial drugs lacking an artemisinin component, as well as the absence of posttreatment hemolysis with these drugs.

Measuring ex vivo drug susceptibility in Plasmodium vivax isolates from Cambodia

Background

While intensive Plasmodium falciparum multidrug resistance surveillance continues in Cambodia, relatively little is known about Plasmodium vivax drug resistance in Cambodia or elsewhere. To investigate P. vivax anti-malarial susceptibility in Cambodia, 76 fresh P. vivax isolates collected from Oddar Meanchey (northern Cambodia) in 2013–2015 were assessed for ex vivo drug susceptibility using the microscopy-based schizont maturation test (SMT) and a Plasmodium pan-species lactate dehydrogenase (pLDH) ELISA. P. vivax multidrug resistance gene 1 (pvmdr1) mutations, and copy number were analysed in a subset of isolates.

Results

Ex vivo testing was interpretable in 80% of isolates using the pLDH-ELISA, but only 25% with the SMT. Plasmodium vivax drug susceptibility by pLDH-ELISA was directly compared with 58 P. falciparum isolates collected from the same locations in 2013–4, tested by histidine-rich protein-2 ELISA. Median pLDH-ELISA IC₅₀ of P. vivax isolates was significantly lower for dihydroartemisinin (3.4 vs 6.3 nM), artesunate (3.2 vs 5.7 nM), and chloroquine (22.1 vs 103.8 nM) than P. falciparum but higher for mefloquine (92 vs 66 nM). There were not significant differences for lumefantrine or doxycycline. Both P. vivax and P. falciparum had comparable median piperaquine IC₅₀ (106.5 vs 123.8 nM), but some P. falciparum isolates were able to grow in much higher concentrations above the normal standard range used, attaining up to 100-fold greater IC₅₀s than P. vivax. A high percentage of P. vivax isolates had pvmdr1 Y976F (78%) and F1076L (83%) mutations but none had pvmdr1 amplification.

Conclusion

The findings of high P. vivax IC₅₀ to mefloquine and piperaquine, but not chloroquine, suggest significant drug pressure from drugs used to treat multidrug resistant P. falciparum in Cambodia. Plasmodium vivax isolates are frequently exposed to mefloquine and piperaquine due to mixed infections and the long elimination half-life of these drugs. Difficulty distinguishing infection due to relapsing hypnozoites versus blood-stage recrudescence complicates clinical detection of P. vivax resistance, while well-validated molecular markers of chloroquine resistance remain elusive. The pLDH assay may be a useful adjunctive tool for monitoring for emerging drug resistance, though more thorough validation is needed. Given high grade clinical chloroquine resistance observed recently in neighbouring countries, low chloroquine IC₅₀ values seen here should not be interpreted as susceptibility in the absence of clinical data. Incorporating pLDH monitoring with therapeutic efficacy studies for individuals with P. vivax will help to further validate this field-expedient method.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-017-2034-2) contains supplementary material, which is available to authorized users.

Fractional Third and Fourth Dose of RTS,S/AS01 Malaria Candidate Vaccine: A Phase 2a Controlled Human Malaria Parasite Infection and Immunogenicity Study

BACKGROUND

Three full doses of RTS,S/AS01 malaria vaccine provides partial protection against controlled human malaria parasite infection (CHMI) and natural exposure. Immunization regimens, including a delayed fractional third dose, were assessed for potential increased protection against malaria and immunologic responses.

METHODS

In a phase 2a, controlled, open-label, study of healthy malaria-naive adults, 16 subjects vaccinated with a 0-, 1-, and 2-month full-dose regimen (012M) and 30 subjects who received a 0-, 1-, and 7-month regimen, including a fractional third dose (Fx017M), underwent CHMI 3 weeks after the last dose. Plasmablast heavy and light chain immunoglobulin messenger RNA sequencing and antibody avidity were evaluated. Protection against repeat CHMI was evaluated after 8 months.

RESULTS

A total of 26 of 30 subjects in the Fx017M group (vaccine efficacy [VE], 86.7% [95% confidence interval [CI], 66.8%-94.6%]; P < .0001) and 10 of 16 in the 012M group (VE, 62.5% [95% CI, 29.4%-80.1%]; P = .0009) were protected against infection, and protection differed between schedules (P = .040, by the log rank test). The fractional dose boosting increased antibody somatic hypermutation and avidity and sustained high protection upon rechallenge.

DISCUSSIONS

A delayed third fractional vaccine dose improved immunogenicity and protection against infection. Optimization of the RTS,S/AS01 immunization regimen may lead to improved approaches against malaria.

CLINICAL TRIALS REGISTRATION

NCT01857869.