Methaemoglobinaemia
Methaemoglobin results when the haem iron in haemoglobin is converted from the ferrous state (Fe++), the reduced form, to the (Fe+++) ferric ion, the oxidized form (20). This process is coupled to redox cycles in the red cell. In the main cycle, driven by the NAD-cytochrome b5 reductase (the main methaemoglobin reductase), haemoglobin and methaemoglobin are cycled (Figure 1). In the second, a cell redox cycle system is driven by the oxidation of haemoglobin, with methaemoglobin as the product. Although the main enzyme responsible for intraerythrocytic methaemoglobin reduction is the NADH-cytochrome b5-reductase, there are alternative pathways. These include an NADPH-dependent methaemoglobin reductase (which has substantially reduced activity in G6PD deficiency), and direct reduction by intracellular ascorbate and glutathione (Figure 1). Methaemoglobin reduction is a first order process. Under normal steady state conditions approximately 3% of the body’s haemoglobin is oxidized each day to methaemoglobin but, because of back-conversion, the average proportion of methaemoglobin is less than 2% of the corresponding haemoglobin concentration. Numerous factors influence this balance (including foods, drugs, exercise, smoking, hypoxia).
Methaemoglobin is a dark blue-brown colour compared with the bright red of oxygenated haemoglobin, so the skin colouration in methaemoglobinaemia resembles that in cyanosis caused by increased concentrations of deoxygenated haemoglobin. Methaemoglobin has a distinct spectrum from haemoglobin. Methaemoglobinaemia can be measured by spectrophotometry of fresh blood samples in the laboratory or, utilizing the same principle, by continuous transcutaneous oximetry devices. During exposure to the oxidizing agent methaemoglobinaemia increases, although there is substantial interindividual variation in iatrogenic methaemoglobinaemia with a skew distribution of steady state values. Interquartile ranges typically extend from approximately 60 to 140% of the median values and, for a given drug exposure, individual values can range tenfold in large series. With daily primaquine dosing methaemoglobinaemia has an estimated elimination half-life of approximately 1.5 days (Figure 2) i.e. 90% of the new steady state is reached in approximately 6 days.
Several different oxidant chemicals and drugs can cause methaemoglobinaemia. Although methaemoglobin production (i.e. haem oxidation) by 8-aminoquinolines is generally considered not to lie in the causal pathway to haemolysis or antimalarial activity (26), it does provide an approximate correlate of these activities within the 8-aminoquinoline class. Under the rigorous test of the experimental challenge studies conducted in the USA in the late 1940s and early 1950s drugs, or drug concentrations, which produced less than 6% steady state methaemoglobinaemia were associated with sub-optimal radical cure rates (23-25) (Figure 3). As for iatrogenic haemolysis, there is a slight delay before intraerythrocytic methaemoglobin levels begin to rise following 8-aminoquinoline administration. This presumably reflects depletion of the oxidant defences (notably reduced glutathione) (27). In a large recent study of vivax malaria conducted on the Thailand-Myanmar border the relationships between primaquine and carboxyprimaquine plasma concentrations, relapses, CYP2D6 polymorphisms and methaemoglobinaemia were investigated. After adjusting for age and partner drug, the day 7 concentrations of primaquine and carboxyprimaquine were not associated with the risk of recurrence, but a 1% absolute increase in day 7 methemoglobin was associated with a hazard ratio for recurrence of 0.9 (95% CI: 0.85-0.99, p=0.02) (28).
Tafenoquine is a slowly eliminated 8-aminoquinoline which has been introduced recently. It is notable that the currently recommended dose of tafenoquine (adult dose 300mg), which provides radical cure rates which are inferior to the lower dose of primaquine (total dose 3.5mg/kg) in South-East Asia (29), is associated with methaemoglobin concentrations which are less than half those associated with the lower dose primaquine regimen (30,31) (Figure 4).