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The demonstrated utility of the nucleoside analog ribavirin in the treatment of certain viral diseases can be ascribed to its multiple distinct properties. These properties may vary in relative importance in differing viral disease conditions and include the direct inhibition of viral replication, the promotion of T-cell-mediated immune responses via an enhanced type 1 cytokine response, and a reduction of circulating alanine aminotransferase (ALT) levels associated with hepatic injury. Ribavirin also has certain known toxicities, including the induction of anemia upon chronic administration. To determine if all these properties are linked, we compared the D-nucleoside ribavirin to its L-enantiomer (ICN 17261) with regard to these properties. Strong similarities were seen for these two compounds with respect to induction of type 1 cytokine bias in vitro, enhancement of type 1 cytokine responses in vivo, and the reduction of serum ALT levels in a murine hepatitis model. In contrast, ICN 17261 had no in vitro antiviral activity against a panel of RNA and DNA viruses, while ribavirin exhibited its characteristic activity profile. Importantly, the preliminary in vivo toxicology profile of ICN 17261 is significantly more favorable than that of ribavirin. Administration of 180 mg of ICN 17261 per kg of body weight to rats by oral gavage for 4 weeks generated substantial serum levels of drug but no observable clinical pathology, whereas equivalent doses of ribavirin induced a significant anemia and leukopenia. Thus, structural modification of ribavirin can dissociate its immunomodulatory properties from its antiviral and toxicologic properties, resulting in a compound (ICN 17261) with interesting therapeutic potential.Ribavirin (1-␤-D-ribofuranosyl-1,2,4-triazole-3-carboxamide) is a nucleoside analog that has demonstrated efficacy in treating viral disease as monotherapy (respiratory syncytial virus [RSV] [15]) and in combination with alpha interferon (IFN-␣) (hepatitis C virus [HCV] [27,36]). Ribavirin has multiple biologic properties that are favorable for treating viral diseases. It can directly inhibit the replication of many DNA and RNA viruses (38). More recently, studies have shown that it can also act as an immunomodulator and thus promote T-cell-mediated immunity against viral infection (18,25,30,39,40). The central focus of this effect of ribavirin is the augmentation of antiviral type 1 cytokine expression (interleukin-2 [IL-2], gamma interferon [IFN-␥], and tumor necrosis factor alpha [TNF-␣]) and concomitant suppression of type 2 cytokine levels by activated T cells in both human and murine systems. Finally, ribavirin, alone or in combination with IFN-␣, can lower serum alanine aminotransferase (ALT) levels during the course of treatment of HCV infection (11). Elevated serum ALT levels are a marker for liver damage and progressive hepatitis, and hence the ribavirin-mediated lowering of ALT levels is a distinct liver-specific effect of this nucleoside analog.The therapeutic use of ribavirin is restric...

A series of 1,2,4-triazole L-nucleosides were synthesized and evaluated for their ability to stimulate type 1 cytokine production by activated human T cells in direct comparison to the known active agent ribavirin. Among the compounds prepared, 1-beta-L-ribofuranosyl-1,2,4-triazole-3-carboxamide (5, ICN 17261) was found to be the most uniformly potent compound. Conversion of the 3-carboxamide group of 5 to a carboxamidine functionality resulted in 1-beta-L-ribofuranosyl-1,2,4-triazole-3-carboxamidine hydrochloride (10), which induced cytokine levels comparable to 5 for two of the three type 1 cytokines examined. Modification of the carbohydrate moiety of 5 provided compounds of reduced activity. Significantly, ICN 17261 offers interesting immunomodulatory potential for the treatment of diseases where type 1 cytokines play an important role.

A series of pyrrolo[2,3-d]pyrimidone nucleosides were synthesized and evaluated for their ability to enhance Type 2 cytokines and to suppress Type 1 cytokines in human T cells activated in vitro. Compounds 16b, 16c, 16d, 18c, and 19b induced substantial enhancement of IL-4 (a Type 2 cytokine) levels while three compounds (16b, 16c, and 16f) showed significant suppression of IFNgamma (a Type 1 cytokine) levels. The results revealed a strict structural requirement for the nucleoside-mediated enhancement of IL-4. Modifications of the ribofuranose moiety of the nucleosides either abolished or dramatically reduced the activity. Both the 5'-hydroxy and 5-carboxamidine are crucial for the activity. Of the few nucleoside analogues that demonstrated enhancement on Type 2 cytokine production, 7-(beta-D-ribofuranosyl)pyrrolo[2, 3-d]-4-pyrimidone-5-carboxamidine (16c) showed a dramatic enhancement (>200%) of IL-4 levels and a significant enhancement (36%) of IL-5 levels. Moreover, this compound showed substantial suppression of the Type 1 cytokines, IFNgamma (42%), IL-2 (54%), and TNFalpha (55%). Similarly, compound 16b showed a substantial enhancement of IL-4 (46%) and suppression of IL-2 (35%), IFNgamma (30%), and TNFalpha (26%). To our knowledge, these are the first nucleoside analogues that induce a Type 2 cytokine bias in T cells. The cytokine modulation property of 16c and 16b merits the therapeutic evaluation of these compounds in treating diseases in which immunopathology is associated with polarized Type 1 cytokine responses.

Monocyclic L-Nucleosides with Type 1 Cytokine-Inducing Activity.-The synthesis of a series of L-nucleosides structurally related to ribavirin, a D-nucleoside in which the heterocycle is a triazole rather than a purine or pyrimidine, is reported. The introduction of the triazole moiety is in general achieved by acid catalyzed fusion of triazole (II) and the appropriate protected sugars, which are derived by different routes. The conversion of the resulting esters into the amides combined with deprotection is accomplished by methanolic ammonia. A study of the ability of the L-nucleosides to stimulate type 1 cytokine production by activated human T cells shows that selected L-nucleosides possess immunomodulatory activity. The replacement of the D-ribose nucleus of ribavirin by L-ribose in derivative (V) retains the activity whereas additional structural changes can have significant effect on this biologic activity.