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12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Nitazoxanide is an antiprotozoal [ see Microbiology (12.4)].

12.3 Pharmacokinetics

Absorption

Single Dosing:

Following oral administration of Nitazoxanide Tablet, the parent drug, nitazoxanide, is not detected in plasma. The pharmacokinetic parameters of the metabolites, tizoxanide and tizoxanide glucuronide are shown in Table 1 below.

Table 1. Mean (+/- SD) plasma pharmacokinetic parameters of tizoxanide and tizoxanide glucuronide following administration of a single dose of one 500 mg Nitazoxanide Tablet with food to subjects ≥12 years of age

	Tizoxanide			Tizoxanide Glucuronide
Age	C max (µg/mL)	T max (hr)*	AUC t (µg•hr/mL)	C max (µg/mL)	T max (hr)*	AUC t (µg•hr/mL)
12 — 17 years	9.1 (6.1)	4.0 (1-4)	39.5 (24.2)	7.3 (1.9)	4.0 (2-8)	46.5 (18.2)
>18 years	10.6 (2.0)	3.0 (2-4)	41.9 (6.0)	10.5 (1.4)	4.5 (4-6)	63.0 (12.3)

*T _max is given as a Mean (Range)

Multiple dosing:

Following oral administration of a single Nitazoxanide Tablet every 12 hours for 7 consecutive days, there was no significant accumulation of nitazoxanide metabolites tizoxanide or tizoxanide glucuronide detected in plasma.

Bioavailability:

Nitazoxanide for oral suspension is not bioequivalent to Nitazoxanide Tablets. The relative bioavailability of the suspension compared to the tablet was 70%.

When Nitazoxanide Tablets are administered with food, the AUC _t of tizoxanide and tizoxanide glucuronide in plasma is increased almost two-fold and the C _max is increased by almost 50%.

Nitazoxanide Tablets were administered with food in clinical trials and hence they are recomended to be administered with food [see Dosage and Administration (2.1)]

Distribution

In plasma, more than 99% of tizoxanide is bound to proteins.

Elimination

Metabolism

Following oral administration in humans, nitazoxanide is rapidly hydrolyzed to an active metabolite, tizoxanide (desacetyl-nitazoxanide). Tizoxanide then undergoes conjugation, primarily by glucuronidation.

Excretion

Tizoxanide is excreted in the urine, bile and feces, and tizoxanide glucuronide is excreted in urine and bile. Approximately two-thirds of the oral dose of nitazoxanide is excreted in the feces and one-third in the urine.

Specific Populations

Pediatric Patients

The pharmacokinetics of tizoxanide and tizoxanide glucuronide following administration of Nitazoxanide Tablets in pediatric patients 12-17 years of age are provided above in Table 1.

Drug Interaction Studies

In vitro studies have demonstrated that tizoxanide has no significant inhibitory effect on cytochrome P450 enzymes.

12.4 MICROBIOLOGY

Mechanism of Action

The antiprotozoal activity of nitazoxanide is believed to be due to interference with the pyruvate:ferredoxin oxidoreductase (PFOR) enzyme-dependent electron transfer reaction which is essential to anaerobic energy metabolism. Studies have shown that the PFOR enzyme from G. lamblia directly reduces nitazoxanide by transfer of electrons in the absence of ferredoxin. The DNA-derived PFOR protein sequence of C. parvum appears to be similar to that of G. lamblia. Interference with the PFOR enzyme-dependent electron transfer reaction may not be the only pathway by which nitazoxanide exhibits antiprotozoal activity.

Resistance

A potential for development of resistance by C. parvum or G. lamblia to nitazoxanide has not been examined.

Antimicrobial Activity

Nitazoxanide and its metabolite, tizoxanide, are active in vitro in inhibiting the growth of (i) sporozoites and oocysts of C. parvum and (ii) trophozoites of G. lamblia.

Susceptibility Test Methods

For protozoa such as C. parvum and G. lamblia , standardized tests for use in clinical microbiology laboratories are not available.

13 NONCLINICAL TOXICOLOGY

13.1 Carcinogenesis, Mutagenesis, Impairment of Fertility

Carcinogenesis

Long-term carcinogenicity studies have not been conducted.

Mutagenesis

Nitazoxanide was not genotoxic in the Chinese hamster ovary (CHO) cell chromosomal aberration assay or the mouse micronucleus assay. Nitazoxanide was genotoxic in one tester strain (TA 100) in the Ames bacterial mutation assay.

Impairment of Fertility

Nitazoxanide did not adversely affect male or female fertility in the rat at 2400 mg/kg/day (approximately 20 times the clinical adult dose adjusted for body surface area).

14 CLINICAL STUDIES

14.1 Diarrhea Caused by G. lamblia

Diarrhea caused by G. lamblia in adults and adolescents 12 years of age or older:

In a double-blind, controlled trial (Study 1) conducted in Peru and Egypt in adults and adolescents with diarrhea and with one or more enteric symptoms (e.g., abdominal pain, nausea, vomiting, fever, abdominal distention, loss of appetite, flatulence) caused by G. lamblia , a three-day course of treatment with Nitazoxanide Tablets administered 500 mg BID was compared with a placebo tablet for 3 days. A second double-blind, controlled trial (Study 2) conducted in Egypt in adults and adolescents with diarrhea and with or without enteric symptoms (e.g., abdominal colic, abdominal tenderness, abdominal cramps, abdominal distention, fever, bloody stool) caused by G. lamblia compared Nitazoxanide Tablets administered 500 mg BID for 3 days to a placebo tablet. For both of these studies, clinical response was evaluated 4 to 7 days following the end of treatment. A clinical response of ‘well’ was defined as ‘no symptoms, no watery stools and no more than 2 soft stools with no hematochezia within the past 24 hours’ or ‘no symptoms and no unformed stools within the past 48 hours.’ The following clinical response rates were obtained:

Table 2. Adult and Adolescent Patients with Diarrhea Caused by G. lamblia

Clinical Response Rates* 4 to 7 Days Post-therapy

% (Number of Successes/Total)

	Nitazoxanide Tablets		Placebo Tablets
Study 1	85% (46/54) ¶		44% (12/27)
Study 2	100% (8/8)		30% (3/10)

* Includes all patients randomized with G. lamblia as the sole pathogen. Patients failing to complete the studies were treated as failures.

^¶ Clinical response rates statistically significantly higher when compared to placebo.

Some patients with ‘well’ clinical responses had G. lamblia cysts in their stool samples 4 to 7 days following the end of treatment. The relevance of stool examination results in these patients is unknown. Patients should be managed based upon clinical response to treatment.

14.2 Diarrhea Caused by C. parvum

Diarrhea caused by C. parvum in adults and adolescents 12 years of age or older:

In a double-blind, controlled trial conducted in Egypt in adults and adolescents with diarrhea and with or without enteric symptoms (e.g., abdominal pain/cramps, nausea, vomiting) caused by C. parvum , a three-day course of treatment with Nitazoxanide Tablets administered 500 mg BID was compared with a placebo tablet for 3 days. Clinical response was evaluated 4 to 7 days following the end of treatment. A clinical response of ‘well’ was defined as ‘no symptoms, no watery stools and no more than 2 soft stools within the past 24 hours’ or ‘no symptoms and no unformed stools within the past 48 hours.’ The following clinical response rates were obtained:

Table 3. Clinical Response Rates in Adult and Adolescent Patients 4 to 7 Days Post-therapy

% (Number of Successes/Total)

	Nitazoxanide Tablets		Placebo Tablets
Intent-to-treat analysis*	96% (27/28) ¶		41% (11/27)

* Includes all patients randomized with C. parvum as the sole pathogen. Patients failing to complete the study were treated as failures.

^¶ Clinical response rates statistically significantly higher when compared to placebo.

In a second double-blind, placebo-controlled trial of Nitazoxanide Tablets conductd in Egypt in adults and adolescents with diarrhea and with or without enteric symptoms (e.g., addominal colic, abdominal cramps, epigastric pain) caused by C. parvum as the sole pathogen, clinical and parasitological reponse rates showed a similar trend to the first study. Clinical response rates, evaluted 2 to 6 days following the end of treatment, were 71% (15/21) in the nitazoxanide group and 42.9% (9/21) in the placebo group.

Some patients with ‘well’ clinical responses had C. parvum oocysts in their stool samples 4 to 7 days following the end of treatment. The relevance of stool examination results in these patients is unknown. Patients should be managed based upon clinical response to treatment.