Ribavirin

Overview
Ribavirin (Copegus®; Rebetol®; Ribasphere®; Vilona®,Virazole®, also generics from Sandoz, Teva, Warrick) is an anti-viral drug which is active against a number of DNA and RNA viruses. It is a member of the nucleoside antimetabolite drugs that interfere with duplication of viral genetic material. Though not effective against all viruses, ribavirin is remarkable as a small molecule for its wide range of activity, including important activities against influenzas, flaviviruses and agents of many viral hemorrhagic fevers.

Ribavirin is a pro-drug, meaning that it is a chemical precursor for the actual pharmacologically active molecule. When the pro-drug is administered, the body converts it into the desired chemical. Ribavirin is activated by cellular kinases which change it into the 5' triphosphate nucleotide. In this form it interferes with aspects of RNA metabolism related to viral reproduction. A number of mechanisms have been proposed for this (see Mechanisms of Action, below) but none of these is proven. More than one mechanism may be active.

In the U.K. & the U.S. the oral (capsule or tablet) form of ribavirin is used in the treatment of hepatitis C, in combination with interferon drugs. The aerosol form is used to treat respiratory syncytial virus-related diseases in children. In Mexico, ribavirin ("ribavirina") has been sold for use against influenza.

The primary observed serious adverse side-effect of ribavirin is hemolytic anemia, which may worsen preexisting cardiac disease. The mechanism for this effect is unknown. It is dose-dependent and may sometimes be compensated by decreasing dose. Ribavirin is also a teratogen in some animals species and thus poses a theoretical reproductive risk in humans, remaining a hazard as long as the drug is present, which can be as long as 6 months after a course of the drug has ended.

Marketing
Oral ribavirin, as Rebetol®, was marketed in the U.S. until 2005 by Schering Plough with royalty payments for licensing made to Valeant Pharmaceuticals International (see History below). It was also marketed as Copegas ® tablets by Roche Pharmaceuticals under a separate license to Valeant Pharmaceuticals International. After concluding patent disputes over generic ribavirin availability in 2003, Three Rivers Pharmaceuticals, LLC in conjunction with Par Pharmaceutical, was approved in 2005 to market ribavirin as Ribosphere® capsules. Generic ribavirin (200 mg, no brand name) became available in 2005 from Sandoz, Teva Pharmaceutical Industries, and Warrick Pharmaceuticals, which is the generic arm of Schering Plough. More generics are expected, including one from Geneva. These products are expected to displace the brand name products paying license fees to Valeant Pharmaceuticals International. The only present FDA-approved indication for these products is in conjunction with interferon against chronic hepatitis C with hepatic damage.

In Mexico, oral ribavirin has been available since the 1980s as an over-the-counter drug ("ribavirina," ICN pharmaceuticals Spanish tradename Vilona®), for treating influenza. In this form it was occasionally brought into the U.S. for HIV/AIDS patients. However, ribavirin has proven to have little if any clinical usefulness against HIV, and it can greatly increase blood levels and also toxicity of the HIV antiviral didanosine (ddI, Videx®). Other interactions with nucleoside antivirals for HIV should be considered when HIV/AIDS patients use ribavirin to treat hepatitis C (see "aidsinfo" external link).

History
Ribavirin (originally also known as Virazole) is a synthetic chemical not found in nature. It was first synthesized in 1970 (Lau, 2002-- see hepcassoc.org external link below) at ICN Pharmaceuticals, Inc. (later Valeant Pharmaceuticals International) by chemist Joseph T. Witkowski, under the direction of laboratory director Roland K. Robins. (Robins [1926-92], a purine chemist, had earlier been the inventor of the highly successful purine-analogue pharmaceutical allopurinol). Ribavirin was discovered as part of a systematic ICN search of antiviral and antitumor activity in synthetic nucleosides. This was inspired in part by discovery (in the 1960s) of antiviral activity from naturally-occurring purine-like nucleoside antibiotics like showdomycin, coformycin, and pyrazomycin. These agents had too much toxicity to be clinically useful (and the antiviral activity of them may be incidental), but they served as the starting point for pharmaceutical chemists interested in antivirals and antimetabolic chemotherapeutic agents.

In 1972 it was reported that ribavirin was active against a variety of RNA and DNA viruses in culture and in animals, without undue toxicity. Ribavirin protected mice against mortality from both A and B strains of influenza, and ICN originally planned to market it as an anti-influenza drug. Results in human trials against experimental influenza infection were mixed, however, and the FDA ultimately did not approve this indication for ribavirin use in humans, thereby causing a severe financial shock to ICN.

Although ICN was allowed in 1980 to market ribavirin, in inhalant form, for RSV infection in children, the U.S. market for this indication was small. By the time oral ribavirin was finally approved by the FDA as part of a combination treatment (with interferon) for hepatitis C in 1998, the original ICN patents on ribavirin itself had expired, and (notwithstanding subsequent patent disputes) ribavirin had become essentially a generic drug.

Future: Other Viral Activities
Experimental data indicate that ribavirin may have useful activity against many viruses of interest, including avian influenza, hepatitis B, polio, measles and smallpox. Ribavirin is the only known treatment for a variety of viral hemorrhagic fevers, including Lassa fever, Crimean-Congo hemorrhagic fever, and Hantavirus infection. Ribavirin is active in a hamster model of yellow fever, a finding which is not surprising, given the familial relationship of yellow fever and hepatitis C viruses as flaviviridae. Ribavirin is active against other important flaviviridae such as West Nile virus and dengue fever.

Ribavirin's present generic status is expected to slow research into new uses, however.

Chemistry
Physically ribavirin is similar to the sugar D-ribose from which it is derived. It is freely soluble in water, and is re-crystallized as fine silvery needles from boiling methanol. It is only sparingly soluble in anhydrous ethanol.

Classically ribavirin is prepared from natural D-ribose by blocking the 2', 3' and 5' OH groups with benzyl groups, then derivatizing the 1' OH with an acetyl group which acts as a suitable leaving group upon nucleophilic attack. The ribose 1' carbon attack is accomplished with 1,2,4 triazole-3-carboxymethyl ester, which directly attaches the 1' nitrogen of the triazole to the 1' carbon of the ribose, in the proper 1-β- D isomeric position. The bulky benzyl groups hinder attack at the other sugar carbons. Following purification of this intermediate, treatment with ammonia in methanolic conditions then simultaneously deblocks the ribose hydroxyls, and converts the triazole carboxymethyl ester to the carboxamide. Following this step, ribavirin may be recovered in good quantity by cooling and crystallization.

Derivatives
Ribavirin is possibly best viewed as a ribosyl purine analogue with an incomplete purine 6-membered ring. This structural resemblance historically prompted replacement of the 2' nitrogen of the triazole with a carbon (which becomes the 5' carbon in an imidazole), in an attempt to partly "fill out" the second ring--- but to no great effect. Such 5' imidazole riboside derivatives show antiviral activity with 5' hydrogen or halide, but the larger the substituent, the smaller the activity, and all proved less active than ribavirin. Note that two natural products were already known with this imidazole riboside structure: substitution at the 5' carbon with OH results in pyrazomycin/pyrazofurin, an antibiotic with antiviral properties but unacceptable toxicity, and replacement with an amino group results in the natural purine synthetic precursor 5-aminoimidazole-4-carboxamide-1-ß-D-ribofuranoside (AICAR), which has only modest antiviral properties.

Derivatization of the triazole 5' carbon, or replacement of it with a nitrogen (i.e., the 1,2,4,5 tetrazole 3-carboxamide) also results in substantial loss of activity, as does alkyl derivatization of the 3' carboxamide nitrogen.

The 2' deoxyribose version of ribavirin (the DNA nucleoside analogue) is not active as an antiviral, suggesting strongly that ribavirin requires RNA-dependent enzymes for its antiviral activity.

Antiviral activity is retained for acetate and phosphate derivation of the ribose hydroxyls, including the triphosphate and 3', 5' cyclic phosphates, but these compounds are no more active than the parent molecule, reflecting the high efficiency of esterase and kinase activity in the body.

Viramidine
The most successful ribavirin derivative to date is the 3-carboxamidine derivative of the parent 3-carboxamide, first reported in 1973 by Witkowski, and now generally called viramidine (also "Ribamidine"). This drug shows a similar spectrum of antiviral activity to ribavirin, which is not surprising as it is now known to be a pro-drug for ribavirin. Viramidine, however, has useful properties of less erythrocyte-trapping and better liver-targeting than ribavirin. The first property is due to viramidine's basic amidine group which inhibits drug entry into RBCs, and the second property is probably due to increased concentration of the enzymes which convert amidine to amide, in liver tissue. Viramidine is in phase III human trials and may one day be used in place of ribavirin, at least against certain kinds of viral hepatitis (see links in viramidine article). Viramidine's slightly superior toxicological properties may eventually cause it to replace ribavirin in all uses of ribavirin. For example, see PMID 16087250.

Mechanism(s) of Action
Ribavirin's carboxamide group can make the native nucleoside drug resemble adenosine or guanosine, depending on its rotation. For this reason, when ribavirin is incorporated into RNA, as a base analog of either adenine or guanine, it pairs equally well with either uracil or cytosine, inducing mutations in RNA-dependent replication in RNA viruses. Such hypermutation can be lethal to RNA viruses.

Ribavirin 5' mono- di- and tri-phosphates, in addition, are all inhibitors of certain viral RNA-dependent RNA polymerases which are a feature of RNA viruses (save for retroviruses).

Neither of these mechanisms explains ribavirin's effect on many DNA viruses, which is more of a mystery. Ribavirin 5'-monophosphate inhibits cellular inosine monophosphate dehydrogenase, thereby depleting intracellular pools of GTP. This mechanism may be useful in explaining the drug's general cytotoxic and anti-DNA replication effect (i.e. its toxicity) as well as some effect on DNA viral replication.

Ribavirin is an inhibitor of some viral RNA guanylyl transferase and (guanine-7N-)-methyl transferase enzymes, and this may contribute to a defective 5'-cap structure of viral mRNA transcripts and therefore inefficient viral translation for certain DNA viruses, such as vaccinia virus (a complex DNA virus). It has been suggested that incorporation of ribavirin into the 5' end of mRNA transcripts would mimic the 7-methyl guanosine endcap of cellular mRNAs, causing poor cellular translation of these. This would be a cell-toxic effect, but it does not seem to be important at therapeutic ribavirin concentrations. Any difference between cellular and viral enzyme handling of ribavirin-containin mRNA transcripts, is a potential mechanism of differential inhibition of ribavirin to translation of mRNAs from viruses (including DNA viruses).

Finally, ribavirin is known to enhance host T-cell-mediated immunity against viral infection through helping to switch the host T-cell phenotype from type 2 to type 1. This may explain rivavirin's antiviral activity against some viruses such as hepatitis C, at doses which do not clearly interfere with replication of the virus when used without interferon (see hepcassoc.org external link below).

Pharmacology
Ribavirin is absorbed from the GI tract probably by nucleoside transporters. Absorption is about 45%, and this is modestly increased (to about 75%) by a fatty meal. Once in the plasma, ribavirin is transported through the cell membrane also by nucleoside transporters.

Ribavirin is widely distributed in all tissues, including the CSF and brain. The pharmacokinetics of ribavirin is dominated by trapping of the phosphated form inside cells, particularly red blood cells (RBCs) which lack the enzyme to remove the phosphate once it has been added by kinases, and therefore attain high concentrations of the drug. Most of the kinase activity which converts the drug to active nucleotide form, is provided by adenine kinase. This enzyme is more active in virally infected cells.

The volume of distribution of ribavirin is large (2000 L/kg) and the length of time the drug is trapped varies greatly from tissue to tissue. The mean half-life for multiple doses in the body is about 12 days, but very long-term kinetics are dominated by the kinetics of RBCs (half-life 40 days). RBCs store ribavirin for the lifetime of the cells, releasing it into the body's systems when old cells are degraded in the spleen.

About a third of absorbed ribavirin is excreted into the urine unchanged, and the rest is excreted into urine as the de-ribosylated base 1,2,4-triazole 3-carboxamide, and the oxidation product of this, 1,2,4- triazole 3-carboxylic acid.

Adverse effects
Ribavirin is not substantially incorporated into DNA, but does have a dose-dependent inhibiting effect on DNA synthesis, as well as having other effects on gene-expression. Possibly for these reasons, significant teratogenic effects have been noted in all non-primate animal species on which ribavirin has been tested. Ribavirin did not produce birth defects in baboons, but this should not be an indication that it is safe in humans. Therefore, two simultaneous forms of birth control are recommended during treatment of either partner and continued for six months after treatment. Women who are pregnant or planning to become pregnant are advised not to take ribavirin. Of special concern as regards teratogenicity is the ribavirin's long half-life in the body. Red blood cells (erythrocytes) concentrate the drug and are unable to excrete it, so this pool is not completely eliminated until all red cells have turned over, a process estimated to take as long as 6 months. Thus in theory, ribavirin might remain a reproductive hazard for as long as 6 months after a course of the drug has ended. Drug packaging information materials in the U.S. now reflect this warning.

Ribavirin should not be given with zidovudine because of the increased risk of anaemia ; concurrent use with didanosine should likewise be avoided because of an increased risk of mitochondrial toxicity.