Activation and Deactivation of a Broad-Spectrum Antiviral Drug by a Single Enzyme: Adenosine Deaminase Catalyzes Two Consecutive Deamination Reactions

The pharmacokinetics of [14 C]viramidine, a prodrug of ribavirin, were studied in rats (30 mg/kg of body weight) and monkeys (10 mg/kg) following intravenous (i.v.) and oral administration. The levels of oral absorption and bioavailabilities were 61.7 and 9.91%, respectively, in rats and 43.9 and 13.6%, respectively, in monkeys. Following i.v. administration, the elimination half-lives were 2.7 h in rats and 28.9 h in monkeys. Total body clearances were 14.0 liters/h/kg in rats and 1.23 liters/h/kg in monkeys; the apparent volumes of distribution were 15.6 liters/kg in rats and 18.6 liters/kg in monkeys. Following oral administration, viramidine was extensively converted to ribavirin, followed by further metabolism of ribavirin in both species, with a faster rate of metabolism in rats than in monkeys. In rats, excretion of total radioactivity in urine accounted for 77.0% of the i.v. dose and 60.8% of the oral dose, while in monkeys it accounted for 44.4% of the i.v. dose and 39.0% of the oral dose. The amount of unchanged viramidine and ribavirin in urine was small in both species after i.v. and oral administration of viramidine.Ribavirin (1-␤-D-ribofuranosyl-1,2,4-triazole-3-carboxamide) is a purine nucleoside analog with activity against a variety of DNA and RNA viral infections (10, 11). The clinical efficacies of interferon alfa-2b and pegylated interferon alfa-2b in combination with ribavirin are about 40% (6, 9) and 54% (5), respectively, in terms of a sustained virologic response when they are used to treat chronic hepatitis C virus infection. At present, combination therapy with ribavirin and pegylated interferon alfa-2b is the "gold standard" for the treatment of chronic hepatitis C. However, ribavirin has a dose-limiting side effect, hemolytic anemia. After absorption into the circulation, a significant portion of ribavirin is transported into red blood cells (RBCs) (2) and phosphorylated into the triphosphate form (8). Due to the lack of phosphatase activity in erythrocytes, phosphorylated derivatives of ribavirin are retained intracellularly and accumulate over time, leading to hemolytic anemia (1, 3). This side effect is dose limiting and necessitates dose reduction and withdrawal in some patients. A new ribavirin which retains those properties deemed critical in the treatment of chronic hepatitis C, but with less potential for hemolytic anemia, would be highly desirable.Viramidine is a prodrug of ribavirin. Preliminary studies in our laboratories indicated that viramidine can be retained and converted to ribavirin in the liver (J. AASLD, abstr. 1123AASLD, abstr. , 2001. Since the liver is the target for hepatitis C virus infection and since RBCs are the target for ribavirin toxicity, the use of viramidine may provide an opportunity to improve the efficacy and reduce the toxicity associated with ribavirin.The aim of this study was to determine the absorption, pharmacokinetics, metabolism, and excretion of viramidine in rats and monkeys. MATERIALS AND METHODS Compound. The compound [5-14 C]virami...

Section: Discussionmentioning

confidence: 99%

Pharmacokinetics and Metabolism of [ ¹⁴ C]Viramidine in Rats and Cynomolgus Monkeys

Lin¹,

Luu²,

Lourenco³

et al. 2003

“…1) may be converted to ribavirin by adenosine deaminase (21). In rats and monkeys following oral administration, viramidine was extensively converted to ribavirin, followed by further metabolism to ribofuranosyl triazole carboxylic acid and triazole carboxamide (TCONH 2 ) (8).…”

mentioning

confidence: 99%

Disposition and Metabolic Profiles of [ ¹⁴ C]Viramidine and [ ¹⁴ C]Ribavirin in Rat and Monkey Red Blood Cells and Liver

Lin

Lourenco

et al. 2004

The disposition and metabolic profiles of [14 C]viramidine and [ 14 C]ribavirin were compared in rat and monkey red blood cells and liver. Our data reveal that the total ribavirin-related components (ribavirin plus its mono-, di-, and triphosphate metabolites) may account for most of the drug in monkey liver following prolonged oral administration of viramidine.Ribavirin is a purine nucleoside analog ( Fig. 1) with broadspectrum activity against a variety of DNA and RNA viral infections (17,19). In combination with either alpha interferon or pegylated alpha interferon, the clinical efficacy of ribavirin in the treatment of chronic hepatitis C virus infection, in terms of a sustained virologic response, has been shown to be about 41 to 47% (13, 16) and 54 to 56% (4, 11, 12), respectively. However, ribavirin had a dose-limiting side effect. After entering the circulation, a significant portion of ribavirin is transported into erythrocytes (RBC) (6) and metabolized into various phosphorylated derivatives (15). Owing to the lack of phosphatase activity in RBC, these phosphorylated metabolites of ribavirin are trapped intracellularly and accumulate over time, leading to hemolytic anemia (6,7,15). This adverse effect often necessitates dose reduction and discontinuation of ribavirin therapy in a significant proportion of patients. Therefore, a new form of ribavirin that retains ribavirin's clinical efficacy but has less potential for hemolytic anemia would be highly desirable.Viramidine ( Fig. 1) may be converted to ribavirin by adenosine deaminase (21). In rats and monkeys following oral administration, viramidine was extensively converted to ribavirin, followed by further metabolism to ribofuranosyl triazole carboxylic acid and triazole carboxamide (TCONH 2 ) (8). Ribavirin has also been reported to undergo metabolism to ribofuranosyl triazole carboxylic acid and TCONH 2 (9). Despite the lower absorption of viramidine compared to that of ribavirin, the plasma ribavirin AUC after viramidine administration in rats was similar to or slightly higher than the plasma ribavirin AUC after ribavirin administration (8,9). This is in good agreement with the rapid conversion of viramidine to ribavirin in rats. In monkeys, however, the plasma ribavirin AUC following viramidine administration was lower than the plasma ribavirin AUC following ribavirin administration. This is probably related to either lower absorption of viramidine compared to that of ribavirin in monkeys and/or the slower rate of conversion of viramidine to ribavirin (8, 9). [ 14 C]viramidine administration in rats produced a 32% higher liver radioactivity AUC than did [14 C]ribavirin administration (10). Cynomolgus monkeys with portal vein cannulas that were given [ 3 H]viramidine retained three times the liver radioactivity of those given [ 3 H]ribavirin (10). However, no metabolic profile in rat and monkey liver has been evaluated. The aim of this study was to compare the disposition and metabolic profiles of viramidine and ribavirin in the RBC and livers of ...

“…Recent studies revealed that viramidine mainly acts as a prodrug and is converted to ribavirin by adenosine deaminase ( Fig. 1) (14). Animal studies indicate that viramidine is not efficiently taken up by RBCs like ribavirin (5).…”

mentioning

confidence: 99%

Dual-Action Mechanism of Viramidine Functioning as a Prodrug and as a Catabolic Inhibitor for Ribavirin

Larson

Hong

2004

Self Cite

An investigational nucleoside analogue drug, viramidine, has recently emerged as a potentially safer alternative to ribavirin for the treatment of hepatitis C viral infection. We have reported that viramidine mainly functions as a prodrug of ribavirin that is enriched in the liver. This in vitro study further explores viramidine's activity against nucleoside phosphorylase, a host enzyme that is responsible for phosphorolysis of ribavirin in vivo. Our experiments show that viramidine inhibits ribavirin phosphorolysis with a K i of 2.5 M. This result suggests that viramidine may act through a dual-action mechanism by serving as a prodrug of ribavirin and concomitantly as an inhibitor for nucleoside phosphorylase catabolism of ribavirin.