Probenecid is packaged as 500 mg tablets. It is a weak acid (pKa 3.7) [44], and its oral bioavailability is > 90% [45]. In healthy adults, probenecid is 85-95% bound to plasma albumin and has a small apparent volume of distribution of 0.003-0.014 L/kg [44,46]. The maximum adult dose of probenecid is 3 g, and a single oral dose of 2 g (approximate 25 mg/Kg) yields peak plasma concentrations of 150-200 mcg/mL within four hours; concentrations above 50 mcg/mL are sustained for eight hours [31,47]. Following a 2 g dose, the half-life is 4-17 h; the half-life is dose-dependent, decreasing as the dose decreases to 500 mg [31]. Probenecid is metabolized in the liver via oxidation and glucuronidation and is primarily excreted in the urine (75-85%) [27]. No dosage adjustment is necessary for patients with mild renal failure (glomerular filtration rate greater than 50 mL/min); probenecid is not effective in severe renal failure and should be avoided [48,49]. No data are available on the pharmacokinetics of probenecid in pregnant women.

SP is packaged as a fixed dose combination containing 500 mg sulphadoxine and 25 mg pyrimethamine and costs approximately US$1 per treatment (adult dosage is three tablets). In the non-pregnant population, both sulphadoxine and pyrimethamine have an oral bioavailability of > 90%, and are 85-90% protein bound [25]. The volume of distribution for sulphadoxine and pyrimethamine is 0.14 L/kg and 2.3 L/kg, respectively [50]. Oral administration of one tablet yields peak plasma levels of approximately 0.2 mg/L for pyrimethamine and approximately 60 mg/L for sulphadoxine at approximately four hours [50]. Both compounds have a very long half-life; the half-life of sulphadoxine is approximately 169 h (range 100-230 h), while that of pyrimethamine is approximately 111 h (range 54-148 h) [51]. Sulphadoxine is metabolized via glucuronidation and excreted primarily in urine, while pyrimethamine is metabolized to several unidentified metabolites but also excreted primarily in the urine [25,52].

Several studies have examined the pharmacokinetics of SP in pregnant women. All examined the standard dose for IPTp of three tablets (1500 mg of sulphadoxine and 75 mg of pyrimethamine). Nyunt et al. studied women in four African countries both during pregnancy and six to eight weeks post-partum, and found that, at Day 7 following administration of SP, there was an increased blood concentration of pyrimethamine, with an area under the curve (AUC) of 1,906 ng·day/ml during pregnancy vs 849 ng·day/ml postpartum, with geometric mean ratio of pharmacokinetic values between pregnancy and postpartum period 1.38 (1.22-1.56), p-value < 0.0001. In addition, they found decreased concentration of sulphadoxine with an AUC of 877 μg·day/ml during pregnancy versus 884 μg·day/ml postpartum, with geometric mean ratio of pharmacokinetic values between pregnancy and postpartum period 0.88 (0.80-0.96), p-value = 0.004, also on Day 7. However, there was significant variation in the exact pharmacokinetic parameters among the different study sites [25]. Findings from a study done in Kisumu, Kenya by Green et al., looking at levels of SP during pregnancy compared to eight-12 weeks post-partum, found similar results for sulphadoxine [53]. However, pyrimethamine pharmacokinetics were not significantly different in pregnant vs non-pregnant women [53]. In a third study conducted in Papua New Guinea where pregnant women were matched to non-pregnant controls, plasma concentrations of both sulphadoxine and pyrimethamine were found to be lower in the pregnant women compared to the non-pregnant controls (AUC of 22,315 vs 33,284 mg·h/l for sulphadoxine and 72,115 vs 106,065 μg·h/l for pyrimethamine; p-value < 0.001 for both) [54]. These varying results may be due to genetic differences in the women that contribute to differential drug metabolism, as well as to differences in body weight.

There are no data on how the combination of probenecid, sulphadoxine, and pyrimethamine affects pharmacokinetics or pharmacodynamics of the individual drugs. Further studies are needed to define these effects and to determine optimal dosing regimens. Given the significant differences in the half lives of SP and probenecid, there is also a need to optimize the probenecid regimen to maximize efficacy of SP while minimizing the frequency of administration to facilitate adequate compliance and adherence.

Adverse reactions with probenecid are quite rare, occurring in approximately 0.1-0.3% of the general population, although the rate in pregnancy is unknown [55]. While rare, adverse reactions have been reported in almost all organ systems, including gastrointestinal, dermatologic, hematologic, renal, and immunologic (anaphylaxis) [26]. Isolated cases of aplastic anaemia, leukopenia, neutropenia, and thrombocytopenia have been reported [56]. Early reports suggested that probenecid may be associated with haemolysis in individuals with glucose-6-phosphatase dehydrogenase (G6PD) deficiency [57]. However, this has not been shown to be the case in subsequent studies [58,59]. Furthermore, recent reviews report that probenecid is not likely to cause haemolysis in patients with G6PD deficiency [60,61]. Probenecid has, however, been associated with immune mediated haemolytic anaemia in several case reports [62,63]. Nephrotic syndrome, which typically resolves on removal of drug, has been reported in a very small number of cases, with one case progressing to death [64-68]. There has also been a single case report of fatal haepatic necrosis [69]. Hypersensitivity reactions to probenecid in the general population may occur in as many as 2% to 4%. The incidence of probenecid hypersensitivity is much higher in HIV-infected patients ranging from 11% to 25% [70]. These reactions ranged from a mild rash that resolved with continued drug administration to more severe reactions consisting of a diffuse rash accompanied by constitutional symptoms, including fever and hypotension, leading to discontinuation of the drug [71-73]. There has been a single case report of anaphylactoid reaction after a single oral dose of probenecid [74].

Despite a long list of potentially severe side effects associated with sulphonamides, the dose of SP used for IPTp is generally well tolerated [50,51]. Bone marrow suppression including agranulocytosis, aplastic anaemia, megaloblastic anaemia, thrombocytopenia, leukopenia, haemolytic anaemia, and eosinophilia may be seen. Haematologic adverse events are related to folic acid antagonism, and can generally be reversed with folinic acid [51]. Mild adverse reactions, such as nausea, vomiting, rash, pruritus, and fatigue have been reported in a small proportion (1-2%) of patients following a single dose of SP [1,75-77]. Parise et al. reported an increased incidence of adverse events in women with HIV, but this was not confirmed in a study by Filler et al. [75,76]. Severe adverse events are rare [1]. Cutaneous hypersensitivity reactions (i.e. Stevens-Johnson syndrome and toxic epidermal necrolysis) are the most common severe adverse events. In studies of travellers taking weekly SP for malaria prophylaxis, the rate of cutaneous hypersensitivity reactions has been reported to be approximately one per 5,000-8,000, with one fatality per 11,000-25,000 [78-80]. The crude rate of cutaneous hypersensitivity reaction was reported to be approximately 1.2 per 100,000 exposures to SP in passive surveillance data from 22 health facilities in Blantyre, Malawi [81]. In that study, the rate of cutaneous hypersensitivity in HIV-infected individuals was approximately four times the crude rate, at 4.9 cases per 100,000 [81]. In a study comparing IPTp with standard SP to SP plus azithromycin, Luntamo et al. reported 14 maternal severe adverse events, including one maternal death, among 1,320 women (1%). However, the majority of these were considered to be unrelated to the study drugs [82]. It is unclear whether any of the reported adverse events were severe cutaneous reactions. In a systematic review of IPTp-SP, ter Kuile et al. report a total of 21 maternal deaths among 11,379 doses of SP given to 4,911 women; nine of these occurred in women receiving placebo [1]. Only one of these deaths was reported to be due to a severe cutaneous hypersensitivity reaction, and it occurred in an HIV-positive woman three weeks after the dose of SP [1,77]. A more comprehensive review of cutaneous hypersensitivity reactions was conducted by Peters et al. [51]. Other, less common, severe reactions that have been seen with SP use include liver toxicity (e.g. cholestatic hepatotoxicity and fulminant hepatic necrosis), fever, and respiratory problems such as hypersensitivity pneumonitis [51]. Patients with G6PD deficiency can safely receive SP-IPTp or malaria treatment with SP [51,83].

Probenecid is known to cross the placental barrier and appears in cord blood [48]. There are limited data on the use of probenecid in pregnancy, with approximately 460 cases described in the literature (Table 1). In most of these cases, no data are provided on the outcome of pregnancy. Of the few studies that describe foetal outcomes, none shows a statistically significant increase in foetal adverse events in infants exposed to probenecid compared to controls [84], although the small sample size of most of these studies is a major limitation to effectively evaluating infant outcomes. Due to the paucity of data, the US Food and Drug Administration classifies probenecid as Pregnancy Category B. However, the data available do not suggest any evidence of teratogenic effects of probenecid when used in pregnancy [48,55].

Probenecid is a ucosuric agent that inhibits the tubular secretion of organic anion derivatives. These organic anions (such penicillin and analogs), which are primarily derivatives of carboxylic acid (DCA), are secreted in the kidney by organic anion transporters (OATs). Probeneicd, which is also a DCA, interacts with these OATs, leading to a decrease in DCA secretion [92,93]. Thus, the mechanism of action of probenecid depends on the inhibition of OATs, reducing the secretion of DCA. Most of the drugs reported in the list below are organic anion compounds that are secreted through OATs, hence their interaction with probenecid. In the next section, we detail specific probenecid-drug interaction.

Neither sulphadoxine nor pyrimethine, nor their primary metabolites, are DCA [95,96], thus the mechanisms of secretion of these two drugs are not likely to involve OATs. Therefore, their pharmacokinetics would not be affected by probenecid. In support of this hypothesis, the clinical evaluation of probenecid with SP did not indicate any increase in sulfadoxine or pyrimethamine toxicity [19]. However, further investigations need to be carried out to establish the pharmacokinetics, safety, and efficacy of probenecid + SP, before this combination could be used routinely for IPTp.

SP should not be used in combination with other antifolates (i.e. trimethoprim), sulphonamides, or other agents associated with myelosuppression, as this can increase the risk of bone marrow suppression (Table 2). The concurrent use of gold salts and anti-malarial agents is also contraindicated due to an increased risk of blood dyscrasias. Sulphadoxine decreases the serum concentration of cyclosporine while enhancing its nephrotoxic effect. Potassium P-aminobenzoate and its derivatives (Procaine, benzocaine, and tetracaine) can decrease the therapeutic effect of sulphadoxine, while folate derivatives can decrease the therapeutic effect of pyrimethamine. The concomitant administration of antacids or absorbent anti-diarrhoeals may significantly reduce the bioavailability of pyrimethamine by reducing its absorption. Pyrimethamine inhibits CYP2C9 and CYP 2D6, and may therefore interact with other compounds, which are metabolized via these pathways (Table 3).

SP-IPTp works both through clearance of existing asymptomatic infections as well as prophylactically by preventing new infections [1]. The prophylactic effect of SP is believed to be of primary importance [1]. Due to the long half-life of SP, its parasitocidal drug levels are sustained for weeks in the case of sensitive parasites, providing a prolonged period of prophylaxis. As parasite resistance to SP increases, the period of prophylactic efficacy becomes shorter [21]. Even if probenecid can increase the concentration of SP, given the very short half-life of probenecid (less than 20 h) compared to SP (approximately 100 h), this effect is unlikely to be sustained. Therefore, while the addition of a single dose of probenecid to SP may be useful for treatment, it is unclear to what extent it will provide a prolonged period of prophylaxis in the context of IPTp. Multiple doses of probenecid may be needed to provide a sustained benefit of SP-IPTp. However, a multiple-dose regimen would be less ideal as compliance with such a regimen is likely to be low.