Safety and Tolerability
Summary demographic data for all study subjects are shown in Table 1.
Study Protocol 1
In Study 1A, a total of 72 subjects were dosed, and all completed the study without relevant protocol deviations. Fifty-four subjects received the active drug. A total of eight mild AEs were reported, only three of which were considered to be possibly related to fexinidazole administration, the most frequent being headache. Two headaches were considered as probably related, and one case of pruritus was considered as unlikely to be related. No subjects were discontinued from the study due to AEs; no subject had SAEs. Neither trends nor relevant changes from baseline were observed in vital signs, ECG parameters, physical examination, or in any laboratory parameters assessed. The maximum tolerated dose was not reached and the highest administered dose, defined as the maximum administered dose, was 3,600 mg.
Further to replacement of one subject who withdrew consent for personal reasons, a total of 13 subjects were dosed in Study 1B with fexinidazole 1,200 mg, either as a suspension under fasted conditions (x) or as tablets under fasted (y) or fed conditions (z).
Twelve subjects completed the study without any relevant protocol deviation. Eleven AEs were reported, none of which were serious according to the definitions given by International Conference on Harmonisation (ICH) guidelines . Six were considered possibly related to fexinidazole (five mild and one moderate), the most frequent being headache. Neither trends nor relevant changes from baseline were observed in vital signs, ECG parameters, physical examination, or in any laboratory parameters assessed.
In Study 1C, 27 subjects were given ascending doses of fexinidazole/placebo tablets, 17 of whom received active drug for 14 days, and nine received placebo for 14 days. One received treatment for 7 days only (the last subject in the study). It was decided to interrupt the treatment early after an SAE (before blind break) was reported for another subject—it was subsequently discovered that the subject had actually received placebo. Of the 27 subjects, 15 experienced at least one AE. All AEs recovered spontaneously or following single episodic symptomatic treatment. The most frequent AE was headache, with a total of 17 episodes reported by 11 subjects.
Two SAEs were reported by subjects who received fexinidazole: one subject (2,400 mg) asked to stop the study after 9 days of treatment due to intermittent headache, anxiety, vomiting, liquid stool episodes and myalgia of inferior limbs. Moderate anxiety had started the day preceding the first study drug administration and appeared to be the only cause of hospitalisation. The other SAE, on day 15, was observed in a second subject (3,600 mg) who exhibited a marked elevation in AST (10 times the normal upper limit) and ALT (7.4 times the normal upper limit). The volunteer was kept in the unit for surveillance for 48 h as the decrease was as strong as the increase. The subject was followed up for an additional 15 days until transaminase values were back to normal. Bilirubinaemia remained normal throughout the follow-up period.
Other laboratory abnormalities were observed in subjects under active treatment. Most of these were considered to be incidental and not clinically relevant, but it was also noted that, whilst remaining within normal ranges, creatinine levels increased progressively from the fourth day of treatment to its end, and returned to baseline values thereafter. This increase was not dose related and no other renal parameters were found to be similarly increased. Variations in creatinine were considered likely to be related to the drug class . In addition, transaminase levels increased above normal upper limits, generally after treatment completion, regardless of the dose, even though the extent of the increase appeared to be dose related. Neither trends nor relevant changes from baseline were observed in vital signs and physical examination. Changes in ECG parameters consisting of an increase of heart rate and prolongation of the QT interval (within normal range) were observed across several dose levels, with no clear dose effect seen. These changes were of limited magnitude and generally remained strictly within normal ranges. None of the changes were considered to be of clinical relevance.
Study Protocol 2
Among the 12 subjects in Study 2 receiving fexinidazole 1,200 mg as oral tablets under fasted conditions or with one of the two meals, one withdrew consent and was not replaced. A total of nine AEs were reported, none of which was serious. Seven of the reported AEs were considered to be possibly related to fexinidazole administration (one under fasted conditions, one with concomitant Plumpy’Nut® intake, and five with concomitant rice and beans consumption). The intensity of AEs were mild (n = 5) or moderate (n = 2). The most frequent AE reported was headache (four cases reported by three subjects). No subjects were discontinued from the study due to AEs. Neither trends nor relevant changes from baseline were observed in vital signs, ECG parameters, physical examination, or in any laboratory parameters assessed.
Study Protocol 3
Thirty subjects were randomised in Study 3. In the first cohort of nine subjects receiving fexinidazole 1,800 + 1,200 mg as oral tablets, five subjects withdrew consent during the course of the study due to digestive disorders (nausea and vomiting). As a cohort effect was suspected, all were replaced and dosed in subgroups of three subjects per group. Using this dosing schedule, all subjects completed the dosing regimen. In the second cohort receiving fexinidazole 2,400 + 1,200 mg as oral tablets, two subjects withdrew due to AEs and were not replaced. This second dosing regimen was stopped after the inclusion of six subjects into the cohort. Overall, 22 of the 30 included subjects completed the study (18/24 in the first cohort and 4/6 in the second cohort). A total of 98 AEs were reported, of which only three were not considered to be related to the study drug (nasopharyngitis, and one episode of dizziness and headache; see details in Table 3). The incidence of AEs was higher within the first days following treatment initiation (loading dose). Of the AEs with a possible relationship to the study drug (83 %), the most frequent were gastrointestinal and nervous system disorders. Gastrointestinal disorders consisted mainly in vomiting (23/50), while central nervous system disorders were essentially headache (27/32). Vomiting episodes generally occurred up to 6–9 h post-dose. Given that the t
max of fexinidazole is 2–4 h, the drug has long been absorbed by the time vomiting starts, suggesting that it is due to central effects. Two subjects in the second cohort experienced sustained anxiety with isolated episodes of panic attacks on the fifth day of dosing, which disappeared over the following 2–3 days after cessation of study drug treatment.
Mean haematology parameters remained within normal ranges throughout the study duration and, whilst sporadic individual abnormalities were observed, none was considered of clinical relevance. Mean biochemistry parameters also remained within normal limits, except for creatinine and creatine phosphokinase (CPK). Creatinine elevations had also been observed in Study 1C and, as no other abnormalities in renal parameters were observed, they were considered likely to be related to the drug class . It is also possible that the observed CPK increases were related to physical exertion; the abnormal values were not considered clinically relevant. A total of ten post-dose elevations of liver function parameters were observed in both cohorts. Mean bilirubin (conjugated, free and total) values were comparable between the placebo and the two active dose groups, with no major trend in changes from baseline. Most of these were limited, and none of these abnormalities in laboratory data were considered to be of clinical relevance.
No abnormalities or out of range values were detected in the safety ECG analyses. Only one Holter-extracted ECG out of more than 4,000 revealed an increase from baseline of 60 ms, but remained within the normal ranges. The 24 h Holter recordings were analysed using an analytical method similar to that of Malik et al.  using five replicates per time point; double delta from baseline for corrected QT (QTc) interval and individual corrections based on QT/RR pattern at D-1.
As the regimen to be implemented in the planned pivotal efficacy study had been completed (cohort 1), and due to the poor tolerability observed in subjects in the second cohort, it was decided not to expose more subjects to this second dosage regimen and to stop the study.
A summary of AEs is provided in Tables 2 (single dose) and 3 (multiple-dose studies).
Pharmacokinetics of Fexinidazole and its Metabolites Fexinidazole Sulfoxide (M1) and Fexinidazole Sulfone (M2)
The pharmacokinetics of fexinidazole and its principle metabolites are summarised in Tables 4, 5, 6, 7 and 8. Examples of mean fexinidazole, M1 and M2 plasma concentrations versus time profiles under different conditions of administration are illustrated in Figs. 1 and 2. Overall, the profile of plasma concentrations of fexinidazole and its two principle metabolites is similar to that seen in laboratory animals [9, 12, 14].
Single-Dose Pharmacokinetics of Fexinidazole (Study 1A, Study 1B and Study 2)
After administration of single doses of fexinidazole in fasted conditions, the drug was rapidly absorbed (median t
max between 3 and 4 h post-dose). Thereafter, plasma concentrations exhibited a multiphasic decline, with a comparable elimination (geometric mean t
½β) for all dose levels of between 9 and 15 h. The primary metabolite, M1, exhibited a pharmacokinetic profile more consistent with the parent drug, since the C
max as well as the terminal plasma t
½β occurred rapidly and in a similar range (median t
max between 2 and 5 h post-dose and a geometric mean t
½ at around 8–15 h). Fexinidazole sulfone (M2) was found to be the major metabolite following oral administration of fexinidazole. Its plasma concentrations increased more slowly than the parent compound (or M1), reaching a maximum with a median time between 18 and 24 h post-dose, following which plasma concentrations decreased progressively with a geometric mean t
½β of about 18–25 h (Fig. 1). Inter-individual variability of C
max and AUCs were slightly higher for the parent compound than for the metabolites and ranged between 19 and 77 % for fexinidazole, 8 and 59 % for M1, and 14 and 55 % for M2. Summary data are shown in Tables 4, 5 and 6.
Between 0.75 and 3.15 % of the fexinidazole dose was recovered in urine over the 168 h measured (sum of unchanged and transformed drug), after oral administration of ascending single doses, indicating that the elimination route of fexinidazole is almost entirely extra-renal. The corresponding renal clearance ranged from 1.23 to 6.01 mL/h.
The rate and extent of absorption of fexinidazole was less than dose proportional over the studied dose range (100–3,600 mg). When the administered dose was increased twofold, C
max and AUC∞ increased 1.46- and 1.65-fold, respectively. The same held true for the metabolites, as a doubling of the dose resulted in an increase in C
max and AUC∞ by 1.33- and 1.65-fold for M1 and 1.47 and 1.55-fold for M2, respectively. The observed dose proportionality of C
max and AUC∞ is illustrated in Fig. 2 for fexinidazole and the M2 metabolite.
Comparison of the tablet formulation and the oral suspension (fexinidazole 1,200 mg dose: Study 1B) showed that the dissolution and distribution profiles of fexinidazole were very comparable, but that there was a decrease of about 25 % of the relative bioavailability of the tablet form.
Unexpectedly, when fexinidazole 1,200 mg tablets were administered with a high-fat breakfast (Study 1B), the relative bioavailability of the drug compared with fasted conditions showed a threefold increase for both rate and extent of absorption, as expressed by C
max and AUC∞ (Table 4; Fig. 3). In parallel, the levels of M1 and M2 increased proportionally.
Comparable results were obtained when fexinidazole was taken with either a high-fat (Plumpy’Nut®) meal or a non-fat meal of rice and beans likely to be available to HAT patients in the field (Study 2), as summarised in Table 4. These results were unexpected and could not be predicted on the basis of known data with other drugs of the same class [21, 22]. Overall, concomitant food intake, such as Plumpy’Nut® or rice and beans, induced a marked increase of the relative bioavailability of fexinidazole: 2.8- and 1.6-fold increase for C
max and AUC∞, respectively, with Plumpy’Nut® and 2.9- and 2-fold increase for C
max and AUC∞ with rice and beans. The concentrations of M1 and M2 increased proportionally. In addition, systemic concentrations of fexinidazole and both metabolites were increased to a slightly higher extent after dosing with the rice and beans meal than with Plumpy’Nut®; on average, by +20, +19, +16 % (based on AUCs) for fexinidazole, M1 and M2, respectively. The metabolic ratios (metabolite vs. fexinidazole) of C
max and AUCs were comparable between the three treatments, although there was a slight reduction under both fed conditions. This decrease seemed to be limited, suggesting that the increase of relative bioavailability induced by the concomitant food intake did not lead to significant changes in the rate of biotransformation of fexinidazole to M1, or of M1 into M2.
Protein binding was estimated from the ratio of the free fraction concentration over the corresponding total concentration measured, and appeared to be constant across time with the free fractions accounting for about 2 % for fexinidazole, 59 % for M1 and 43 % for M2, reflecting the known in vitro data .
Multiple-Dose Pharmacokinetics of Fexinidazole (Study 1C and Study 3)
After multiple oral administrations of fexinidazole tablets with doses ranging from 1,200 to 3,600 mg for 14 days, steady-state plasma drug concentrations were reached between 5 and 7 days for fexinidazole and M1, and between days 9 and 10 for M2. Overall, pharmacokinetic parameters remained unchanged when compared to single-dose studies. The rate and extent of absorption of fexinidazole and both metabolites were markedly less than dose proportional over the studied dose range since, after a 14-day administration, mean C
max and AUClast increased by 1.17 and 1.34, respectively, when the dose doubled and by 1.50 and 1.61, respectively, for a threefold increase in dose. The same held true for the metabolites, as the mean C
max and AUClast of M1 increased by 1.72 and 1.86 when the dose increased from 1,200 to 3,600 mg, while the mean C
max and AUClast of M2 increased by 2.08 and 2.005. Summary data are shown in Table 7.
Based on the area over the dosing interval AUClast data there was an accumulation on day 7 compared with day 1 of 1.5, 2.1 and 7.3 for fexinidazole, M1 and M2, respectively. On day 14 the accumulation was comparable with day 7.
As reported with single-dose administration, urinary elimination of fexinidazole, M1 and M2 was negligible.
The effect of a loading dose and concomitant food intake following multiple dosing were assessed in Study 3. In terms of the simulated concentrations that could be expected, the median M2 C
max of the highest regimen was predicted to be around 35,000 ng/mL and the median maximum AUC to be around 793,000 ng·h/mL compared with the plasma concentration observed in healthy volunteers after administration of 3,600 mg for 14 days, which was close to the maximum AUC observed (603,000 ng·h/mL) at maximum dose of Study 1C. As the dose was well-tolerated, the SRC authorised continuation to the second dose regimen of 2,400 mg, followed by 1,200 mg. The study shows that the dosage regimen used for cohort 1 yields high M2 concentrations, with plasma concentrations of at least 10,000 ng/mL, which is consistent with potential efficacy in the second stage of the disease. The study shows a peak in M2 concentrations between day 4 (C
max 18,800 ng/mL) and day 7, with elevated levels maintained until day 10 (C
max 12,000 ng/mL) (Table 8). Taking into account the high free fraction of the two metabolites, these results indicate that this drug regimen would yield M2 concentrations above required levels for efficacy, deduced from the mice study (>10,000 ng/mL for at least 2 days ), for at least 6 days (day 4 to day 10) in at least 83 % of subjects. These data are illustrated in Fig. 4, and are compared to the predicted concentration of the drugs from animal models required to kill 100 % of parasites in the central nervous system.