Ribasphere (Ribavirin) - Description and Clinical Pharmacology

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DESCRIPTION

RIBASPHERE (ribavirin, USP), the Three Rivers Pharmaceuticals brand name for ribavirin, is a nucleoside analogue with antiviral activity. The chemical name of ribavirin is 1-β-D-ribofuranosyl-1 H -1,2,4-triazole-3-carboxamide and has the following structural formula:

The molecular formula of ribavirin is C₈H₁₂N₄O₅ and the molecular weight is 244.2. Ribavirin is a white to off-white powder. It is freely soluble in water and slightly soluble in anhydrous alcohol.

RIBASPHERE (ribavirin, USP) is available as a blue-colored (shade depending on strength), capsule-shaped, film-coated tablet for oral administration. Each tablet contains 200 mg, 400 mg, or 600 mg of ribavirin and the following inactive ingredients: microcrystalline cellulose, lactose monohydrate, croscarmellose sodium, povidone K27-33, magnesium stearate, and purified water. The coating of the 200-mg tablet contains partially hydrolyzed polyvinyl alcohol, titanium dioxide, polyethylene glycol 3350, talc, FD&C blue #2 [indigo carmine aluminum lake], and carnauba wax. The coating of the 400- and 600-mg tablet contains partially hydrolyzed polyvinyl alcohol, titanium dioxide, polyethylene glycol 3350, talc, FD&C blue #1 [brilliant blue FCF aluminum lake], and carnauba wax.

Mechanism of Action

Ribavirin is a synthetic nucleoside analogue. The mechanism by which the combination of ribavirin and an interferon product exerts its effects against the hepatitis C virus has not been fully established.

CLINICAL PHARMACOLOGY

Pharmacokinetics

Multiple dose ribavirin pharmacokinetic data are available for HCV patients who received ribavirin in combination with peginterferon alfa-2a. Following administration of 1200 mg/day with food for 12 weeks mean±SD (n=39; body weight >75 kg) AUC_0-12hr was 25,361±7110 ng·hr/mL and C_max was 2748±818 ng/mL. The average time to reach C_max was 2 hours. Trough ribavirin plasma concentrations following 12 weeks of dosing with food were 1662±545 ng/mL in HCV infected patients who received 800 mg/day (n=89), and 2112±810 ng/mL in patients who received 1200 mg/day (n=75; body weight >75 kg).

The terminal half-life of ribavirin following administration of a single oral dose of ribavirin is about 120 to 170 hours. The total apparent clearance following administration of a single oral dose of ribavirin is about 26 L/h. There is extensive accumulation of ribavirin after multiple dosing (twice daily) such that the C_max at steady state was four-fold higher than that of a single dose.

Effect of Food on Absorption of Ribavirin

Bioavailability of a single oral dose of ribavirin was increased by co-administration with a high-fat meal. The absorption was slowed (T_max was doubled) and the AUC_0-192h and C_max increased by 42% and 66%, respectively, when ribavirin was taken with a high-fat meal compared with fasting conditions (see PRECAUTIONS and DOSAGE AND ADMINISTRATION).

Elimination and Metabolism

The contribution of renal and hepatic pathways to ribavirin elimination after administration of ribavirin is not known. In vitro studies indicate that ribavirin is not a substrate of CYP450 enzymes.

Special Populations

Race

A pharmacokinetic study in 42 subjects demonstrated there is no clinically significant difference in ribavirin pharmacokinetics among Black (n=14), Hispanic (n=13) and Caucasian (n=15) subjects.

Renal Dysfunction

The pharmacokinetics of ribavirin following administration of ribavirin have not been studied in patients with renal impairment and there are limited data from clinical trials on administration of ribavirin in patients with creatinine clearance <50 mL/min. Therefore, patients with creatinine clearance <50 mL/min should not be treated with ribavirin (see WARNINGS and DOSAGE AND ADMINISTRATION).

Hepatic Impairment

The effect of hepatic impairment on the pharmacokinetics of ribavirin following administration of ribavirin has not been evaluated. The clinical trials of ribavirin were restricted to patients with Child-Pugh class A disease.

Pediatric Patients

Pharmacokinetic evaluations in pediatric patients have not been performed.

Elderly Patients

Pharmacokinetic evaluations in elderly patients have not been performed.

Gender

Ribavirin pharmacokinetics, when corrected for weight, are similar in male and female patients.

Drug Interactions

In vitro studies indicate that ribavirin does not inhibit CYP450 enzymes.

Nucleoside Analogues

In vitro data indicate ribavirin reduces phosphorylation of lamivudine, stavudine, and zidovudine.

In vitro, didanosine or its active metabolite (dideoxyadenosine 5’– triphosphate) is increased when didanosine is co-administered with ribavirin, which could cause or worsen clinical toxicities (see PRECAUTIONS: Drug Interactions).

Drugs Metabolized by Cytochrome P450

There was no effect on the pharmacokinetics of representative drugs metabolized by CYP 2C9, CYP 2C19, CYP 2D6 or CYP 3A4.

Treatment with peginterferon alfa-2a once weekly for 4 weeks in healthy subjects was associated with an inhibition of P450 1A2 and a 25% increase in theophylline AUC (see PRECAUTIONS: Drug Interactions).

CLINICAL STUDIES

HCV Patients

The safety and effectiveness of peginterferon alfa-2a in combination with ribavirin for the treatment of hepatitis C virus infection were assessed in two randomized controlled clinical trials. All patients were adults, had compensated liver disease, detectable hepatitis C virus, liver biopsy diagnosis of chronic hepatitis, and were previously untreated with interferon. Approximately 20% of patients in both studies had compensated cirrhosis (Child-Pugh class A). Patients coinfected with HIV were excluded from these studies.

In Study NV15801 (described as Study 4 in the PEGASYS^®¹ Package Insert), patients were randomized to receive either peginterferon alfa-2a 180 µg sc once weekly (qw) with an oral placebo, peginterferon alfa-2a 180 µg qw with ribavirin 1000 mg po (body weight <75 kg) or 1200 mg po (body weight ≥75 kg) or REBETRON^™² (interferon alfa-2b 3 MIU sc tiw plus ribavirin 1000 mg or 1200 mg po). All patients received 48 weeks of therapy followed by 24 weeks of treatment-free follow-up. Ribavirin or placebo treatment assignment was blinded. Sustained virological response was defined as undetectable (<50 IU/mL) HCV RNA on or after study week 68. Peginterferon alfa-2a in combination with ribavirin resulted in a higher SVR compared to peginterferon alfa-2a alone or interferon alfa-2b and ribavirin (Table 1). In all treatment arms, patients with viral genotype 1, regardless of viral load, had a lower response rate to peginterferon alpha-2a in combination with ribavirin compared to patients with other viral genotypes.

Table 1  Sustained Virologic Response (SVR) to Combination Therapy (Study NV15801*)

	Interferon alfa-2b + Ribavirin 1000 mg or 1200 mg	Peginterferon alfa-2a + placebo	Peginterferon alfa-2a + Ribavirin 1000 mg or 1200 mg
Difference in overall treatment response (Peginterferon alfa-2a /ribavirin – Interferon alfa-2b/ribavirin) was 9% (95% CI 2.3, 15.3). *Described as Study 4 in the PEGASYS Package Insert.
All Patients	197/444 (44%)	65/224 (29%)	241/453 (53%)
Genotype 1	103/285 (36%)	29/145 (20%)	132/298 (44%)
Genotypes 2–6	94/159 (59%)	36/79 (46%)	109/155 (70%)

In Study NV15942 (described as Study 5 in the PEGASYS Package Insert), all patients received peginterferon alfa-2a 180 µg sc qw and were randomized to treatment for either 24 or 48 weeks and to a ribavirin dose of either 800 mg or 1000 mg/1200 mg (for body weight <75 kg/≥75 kg). Assignment to the four treatment arms was stratified by viral genotype and baseline HCV viral titer. Patients with genotype 1 and high viral titer (defined as >2 x 10⁶ HCV RNA copies/mL serum) were preferentially assigned to treatment for 48 weeks.

HCV Genotypes

HCV 1 and 4 – Irrespective of baseline viral titer, treatment for 48 weeks with peginterferon alfa-2a and 1000 mg or 1200 mg of ribavirin resulted in higher SVR (defined as undetectable HCV RNA at the end of the 24-week treatment-free follow-up period) compared to shorter treatment (24 weeks) and/or 800 mg ribavirin.

HCV 2 and 3 – Irrespective of baseline viral titer, treatment for 24 weeks with peginterferon alfa-2a and 800 mg of ribavirin resulted in a similar SVR compared to longer treatment (48 weeks) and/or 1000 mg or 1200 mg of ribavirin (see Table 2).

The numbers of patients with genotype 5 and 6 were too few to allow for meaningful assessment.

Table 2  Sustained Virologic Response as a Function of Genotype (Study NV15942*)

	24 Weeks Treatment		48 Weeks Treatment
	Peginterferon alfa-2a + Ribavirin 800 mg (N=207)	Peginterferon alfa-2a + Ribavirin 1000 mg or 1200 mg** (N=280)	Peginterferon alfa-2a + Ribavirin 800 mg (N=361)	Peginterferon alfa-2a + Ribavirin 1000 mg or 1200 mg** (N=436)
* Described as Study 5 in the PEGASYS Package Insert. **1000 mg for body weight <75 kg; 1200 mg for body weight ≥75 kg.
Genotype 1	29/101 (29%)	48/118 (41%)	99/250 (40%)	138/271 (51%)
Genotypes 2,3	79/96 (82%)	116/144 (81%)	75/99 (76%)	117/153 (76%)
Genotype 4	0/5 (0%)	7/12 (58%)	5/8 (63%)	9/11 (82%)

Other Treatment Response Predictors

Treatment response rates are lower in patients with poor prognostic factors receiving pegylated interferon alpha therapy. In studies NV15801 and NV15942, treatment response rates were lower in patients older than 40 years (50% vs. 66%), in patients with cirrhosis (47% vs. 59%), in patients weighing over 85 kg (49% vs. 60%), and in patients with genotype 1 with high vs. low viral load (43% vs. 56%). African-American patients had lower response rates compared to Caucasians.

Paired liver biopsies were performed on approximately 20% of patients in studies NV15801 and NV15942. Modest reductions in inflammation compared to baseline were seen in all treatment groups.

In studies NV15801 and NV15942, lack of early virologic response by 12 weeks (defined as HCV RNA undetectable or >2log₁₀ lower than baseline) was grounds for discontinuation of treatment. Of patients who lacked an early viral response by 12 weeks and completed a recommended course of therapy despite a protocol-defined option to discontinue therapy, 5/39 (13%) achieved an SVR. Of patients who lacked an early viral response by 24 weeks, 19 completed a full course of therapy and none achieved an SVR.