Assessing the tolerability of a potentially safer radical curative regimen of primaquine in healthy volunteers with glucose 6 phosphate dehydrogenase
Lead Research Organisation:
University of Oxford
Department Name: Tropical Medicine
Abstract
Malaria, the most important parasite disease of Man, is transmitted by mosquitoes and has two types: Plasmodium falciparum (Pf) and P. vivax (Pv). Outside Africa, where Pf predominates, Pv is the most common malaria species causing an estimated 6-12 million cases and 1800-4900 deaths.
Malaria occurs when the parasites enter the red blood cells where they multiply and produce male and female forms that need to be taken up by mosquitoes to complete their life cycle. Pv has found a way to avoid mosquito transmission by developing a sleeping form in the liver, called a hypnozoite which wake up periodically to cause new illnesses, called relapses. In some areas, notably SE Asia, hypnozoites frequently relapse and account for about 80% of all Pv illness.
Repeated Pv leads to childhood anaemia, growth retardation, developmental delay, poor school performance, time off work and economic loss. So the hypnozoite reservoir is an important focus of our research.
The only available drug to kill hypnozoites is primaquine, a drug developed seventy years ago. Unfortunately, it causes red cell destruction (haemolysis) in people whose red cells are deficient in the enzyme, glucose -6-phosphate dehydrogenase (G6PD). G6PD deficiency (G6PDd) is common and inherited through defective X chromosomes. Thus, in some regions, up to 30% of men have severe G6PDd and some 75% of women have milder G6PDd.
G6PDd red cells cannot produce enough antioxidants; the more the red cells are deficient, the fewer antioxidants they produce. When primaquine is metabolised, it produces oxidants that damage red cells resulting in haemolysis. The higher the primaquine dose, the more haemolysis. If severe, patients can become anaemic very quickly which can be life threatening.
We can test for G6PDd in the laboratory, which is cumbersome. There are easy to use rapid tests but they are expensive and governments cannot afford to deploy them throughout the health service. So, primaquine is considered dangerous and is not used and this hinders the elimination of Pv.
About 60 years ago, doctors found that giving primaquine every week for 8 weeks to African Americans with the mild African form of G6PDd was safe and effective at stopping relapses. However, when this dose was tested in Cambodia, a country with moderately severe G6PDd, some Pv patients had significant falls in haemoglobin (the red pigment in red cells) and one needed to be transfused. Therefore, treated patients should be monitored, another costly precaution. This would apply also in the Middle East, Pakistan and Afghanistan where the most severe G6PDd is present.
We asked ourselves this question: "Could primaquine be given more safely to G6PDd patients without requiring G6PD testing ? Primaquine destroys older rather than younger red blood cells. Combining this with knowledge of how red cells are produced, and haemolysis data in healthy individuals and Pv patients given primaquine, we used mathematical modelling to find a primaquine dose that led to a 'slow burn' haemolysis. We found that increasing the dose of primaquine every 5 days for 20 days gave that slow burn haemolysis and the fall in haemoglobin would be less compared to the weekly dose given to the Cambodian patients.
The next step is to test this dose in G6PDd males in Thailand where G6PD Mahidol is common and similar to Cambodian G6PD Viangchan. The study is designed so that we can alter the primaquine dose according to the fall in the haemoglobin, called an adaptive design. There will be a tight safety net and all study participants will be monitored closely in hospital and have their haemoglobin checked every day. If this study is successful, we will test the dose in Pv patients and will also adapt the primaquine dose for studies in G6PD Mediterranean countries.
Ultimately, if we show that dosing primaquine in steps is safe and effective, it will be a giant leap forward and governments could adopt this new dose quickly.
Malaria occurs when the parasites enter the red blood cells where they multiply and produce male and female forms that need to be taken up by mosquitoes to complete their life cycle. Pv has found a way to avoid mosquito transmission by developing a sleeping form in the liver, called a hypnozoite which wake up periodically to cause new illnesses, called relapses. In some areas, notably SE Asia, hypnozoites frequently relapse and account for about 80% of all Pv illness.
Repeated Pv leads to childhood anaemia, growth retardation, developmental delay, poor school performance, time off work and economic loss. So the hypnozoite reservoir is an important focus of our research.
The only available drug to kill hypnozoites is primaquine, a drug developed seventy years ago. Unfortunately, it causes red cell destruction (haemolysis) in people whose red cells are deficient in the enzyme, glucose -6-phosphate dehydrogenase (G6PD). G6PD deficiency (G6PDd) is common and inherited through defective X chromosomes. Thus, in some regions, up to 30% of men have severe G6PDd and some 75% of women have milder G6PDd.
G6PDd red cells cannot produce enough antioxidants; the more the red cells are deficient, the fewer antioxidants they produce. When primaquine is metabolised, it produces oxidants that damage red cells resulting in haemolysis. The higher the primaquine dose, the more haemolysis. If severe, patients can become anaemic very quickly which can be life threatening.
We can test for G6PDd in the laboratory, which is cumbersome. There are easy to use rapid tests but they are expensive and governments cannot afford to deploy them throughout the health service. So, primaquine is considered dangerous and is not used and this hinders the elimination of Pv.
About 60 years ago, doctors found that giving primaquine every week for 8 weeks to African Americans with the mild African form of G6PDd was safe and effective at stopping relapses. However, when this dose was tested in Cambodia, a country with moderately severe G6PDd, some Pv patients had significant falls in haemoglobin (the red pigment in red cells) and one needed to be transfused. Therefore, treated patients should be monitored, another costly precaution. This would apply also in the Middle East, Pakistan and Afghanistan where the most severe G6PDd is present.
We asked ourselves this question: "Could primaquine be given more safely to G6PDd patients without requiring G6PD testing ? Primaquine destroys older rather than younger red blood cells. Combining this with knowledge of how red cells are produced, and haemolysis data in healthy individuals and Pv patients given primaquine, we used mathematical modelling to find a primaquine dose that led to a 'slow burn' haemolysis. We found that increasing the dose of primaquine every 5 days for 20 days gave that slow burn haemolysis and the fall in haemoglobin would be less compared to the weekly dose given to the Cambodian patients.
The next step is to test this dose in G6PDd males in Thailand where G6PD Mahidol is common and similar to Cambodian G6PD Viangchan. The study is designed so that we can alter the primaquine dose according to the fall in the haemoglobin, called an adaptive design. There will be a tight safety net and all study participants will be monitored closely in hospital and have their haemoglobin checked every day. If this study is successful, we will test the dose in Pv patients and will also adapt the primaquine dose for studies in G6PD Mediterranean countries.
Ultimately, if we show that dosing primaquine in steps is safe and effective, it will be a giant leap forward and governments could adopt this new dose quickly.
Technical Summary
Relapse is the main cause of Plasmodium vivax malaria illness. Prevention of relapse and ultimately elimination of P. vivax can be achieved only by killing the liver stage parasites (hypnozoites) that cause relapse. Primaquine (PQ) is currently the only registered hypnozoitocidal drug but it causes acute haemolysis in glucose-6-phosphate dehydrogenase deficiency (G6PDd), the most common enzymopathy of man (prevalence up to 30%). As G6PD testing is generally unavailable radical cure with primaquine is seldom used.
PQ haemolyses older more G6PD deficient red cells so controlled haemolysis allows gradual replacement of vulnerable erythrocytes without dangerous anaemia. Based on studies in the mild G6PDd (African) A- variant, 0.75 mg PQ/kg/week for 8 weeks is recommended by WHO for radical cure in G6PDd, however this regimen may be unsafe in the more severe variants prevalent outside Africa. A safer regimen, not requiring G6PD testing, would be a major advance.
Using all available data, we developed a mathematical model of red cell dynamics. It suggests an ascending PQ regimen of four 5 day cycles (7.5, 15, 22.5, then 30 mg/day) would provide a gradual and safe haemoglobin (Hb) decline because haemolysis of older cells induces reticulocytosis and these young cells are resistant to oxidant haemolysis. The total PQ dose, 6.25 mg/kg, approximates the 7 mg/kg recommended for tropical P. vivax.
We propose an adaptive Phase 1-type, dose finding study designed to maximise safety in healthy G6PDd Mahidol or Viangchan variant Thai adults. In 5 cohorts of five, the 1st cohort will receive the modelled regimen which may be modified in the next cohort following a pre-specified Hb response algorithm.
If this trial is successful, we will: (i) proceed to a safety and efficacy study in G6PDd vivax patients, and (ii) design an adaptive trial for the severe G6PDdMed variant. We aim to provide safer immediately deployable antirelapse regimens that do not require G6PDd testing.
PQ haemolyses older more G6PD deficient red cells so controlled haemolysis allows gradual replacement of vulnerable erythrocytes without dangerous anaemia. Based on studies in the mild G6PDd (African) A- variant, 0.75 mg PQ/kg/week for 8 weeks is recommended by WHO for radical cure in G6PDd, however this regimen may be unsafe in the more severe variants prevalent outside Africa. A safer regimen, not requiring G6PD testing, would be a major advance.
Using all available data, we developed a mathematical model of red cell dynamics. It suggests an ascending PQ regimen of four 5 day cycles (7.5, 15, 22.5, then 30 mg/day) would provide a gradual and safe haemoglobin (Hb) decline because haemolysis of older cells induces reticulocytosis and these young cells are resistant to oxidant haemolysis. The total PQ dose, 6.25 mg/kg, approximates the 7 mg/kg recommended for tropical P. vivax.
We propose an adaptive Phase 1-type, dose finding study designed to maximise safety in healthy G6PDd Mahidol or Viangchan variant Thai adults. In 5 cohorts of five, the 1st cohort will receive the modelled regimen which may be modified in the next cohort following a pre-specified Hb response algorithm.
If this trial is successful, we will: (i) proceed to a safety and efficacy study in G6PDd vivax patients, and (ii) design an adaptive trial for the severe G6PDdMed variant. We aim to provide safer immediately deployable antirelapse regimens that do not require G6PDd testing.
Planned Impact
We are proposing a novel regimen of primaquine which mathematical modelling suggests is very promising. The study in volunteers is the first of several studies which, if successful, will lead to field and healthy volunteer studies in SE Asia and in countries where the severe G6PD Mediterranean variant is present. In this way, we will have covered the moderately severe and severe G6PD variants, allowing for the global use of our regimen and a substantial impact on vivax elimination.
Vivax malaria is a global disease. The estimated population at risk is 2.5 billion [Howes, AJTMH 95 (supp 6) 2016]. Although there are data from the WHO on the estimated number of cases per year (6-12 million) and deaths (1800-4900), the number of individuals with hypnozoites is unknown because no hypnozoite biomarker exists. Nevertheless, the hypnozoite burden is likely to be far greater than the number of reported cases because many individuals harbour asymptomatic blood stage infections most of which are derived from hynozoites. Recent research suggests the asymptomatic vivax reservoir is substantial in diverse geographical settings (Rovira-Vallbona PLoS Negl Trop Dis. 2017 Jul 3;11(7):e0005674, Tadesse Malar J. 2017 Mar 3;16(1):99, Ghinai Malar J. 2017 Jan 5;16(1):16).
Eliminating the global burden of hypnozoites will be a massive undertaking. Tafenoquine, a drug in the final steps of development, will contribute because it has the distinct advantage of being a one dose, effective treatment thanks to its long elimination half-life (mean 14 days). However, this is also a major weakness.
If given to a G6PD deficient individual or a heterozygous female with low G6PD activity, the resulting haemolysis could be prolonged and severe. Thus, tafenoquine will only be registered for use in G6PD normal individuals or heterozygous females with a G6PD activity 70% or more of the median of a G6PD normal population.
For tafenoquine to be effective, G6PD testing will need to extend to the most peripheral parts of the health service where malaria treatment is given. It is naive to think that countries will be able to achieve this in a timely fashion, let alone afford it.
Whenever G6PD testing becomes widely available for tafenoquine use, many individuals will not be able to receive it - all hemizygous G6PD males, all homozygous G6PDd females, 70% of heterozygous females, and 40% of normal females. This is a substantial gap and the absolute numbers of untreated individuals in a given population will increase as the G6PDd prevalence increases, especially for heterozygous females.
If our research leads to a deployable regimen, namely, a regimen proven to be safe and effective that does not require G6PD testing, this massive gap will be closed, assuming tafneoquine is deployed. Moreover, our regimen could be used exclusively in countries that: (i) cannot afford to deploy tafenoquine coupled with G6PD testing, or (ii) prefer the simplicity and convenience of one regimen.
Vivax malaria is a global disease. The estimated population at risk is 2.5 billion [Howes, AJTMH 95 (supp 6) 2016]. Although there are data from the WHO on the estimated number of cases per year (6-12 million) and deaths (1800-4900), the number of individuals with hypnozoites is unknown because no hypnozoite biomarker exists. Nevertheless, the hypnozoite burden is likely to be far greater than the number of reported cases because many individuals harbour asymptomatic blood stage infections most of which are derived from hynozoites. Recent research suggests the asymptomatic vivax reservoir is substantial in diverse geographical settings (Rovira-Vallbona PLoS Negl Trop Dis. 2017 Jul 3;11(7):e0005674, Tadesse Malar J. 2017 Mar 3;16(1):99, Ghinai Malar J. 2017 Jan 5;16(1):16).
Eliminating the global burden of hypnozoites will be a massive undertaking. Tafenoquine, a drug in the final steps of development, will contribute because it has the distinct advantage of being a one dose, effective treatment thanks to its long elimination half-life (mean 14 days). However, this is also a major weakness.
If given to a G6PD deficient individual or a heterozygous female with low G6PD activity, the resulting haemolysis could be prolonged and severe. Thus, tafenoquine will only be registered for use in G6PD normal individuals or heterozygous females with a G6PD activity 70% or more of the median of a G6PD normal population.
For tafenoquine to be effective, G6PD testing will need to extend to the most peripheral parts of the health service where malaria treatment is given. It is naive to think that countries will be able to achieve this in a timely fashion, let alone afford it.
Whenever G6PD testing becomes widely available for tafenoquine use, many individuals will not be able to receive it - all hemizygous G6PD males, all homozygous G6PDd females, 70% of heterozygous females, and 40% of normal females. This is a substantial gap and the absolute numbers of untreated individuals in a given population will increase as the G6PDd prevalence increases, especially for heterozygous females.
If our research leads to a deployable regimen, namely, a regimen proven to be safe and effective that does not require G6PD testing, this massive gap will be closed, assuming tafneoquine is deployed. Moreover, our regimen could be used exclusively in countries that: (i) cannot afford to deploy tafenoquine coupled with G6PD testing, or (ii) prefer the simplicity and convenience of one regimen.
