Antiviral (RNA) activity of selected amaryllidaceae isoquinoline constituents and synthesis of related substances

A series of 23 Amaryllidaceae isoquinoline alkaloids and related synthetic analogues were isolated or synthesized and subsequently evaluated in cell culture against the RNA-containing flaviviruses (Japanese encephalitis, yellow fever, and dengue viruses), bunyaviruses (Punta Toro, sandfly fever, and Rift Valley fever viruses), alphavirus (Venezuelan equine encephalomyelitis virus), lentivirus (human immunodeficiency virus-type 1) and the DNA-containing vaccinia virus. Narciclasine {1}, lycoricidine {2}, pancratistatin {4}, 7-deoxypancratistatin {5}, and acetates 6–8, isonarciclasine {13a}, cis-dihydronarciclasine {14a}, trans-dihydronarciclasine {15a}, their 7-deoxy analogues 13b–15b, lycorines 16 and 17, and pretazettine {18} exhibited consistent in vitro activity against all three flaviviruses and against the bunyaviruses, Punta Toro and Rift Valley fever virus. Activity against sandfly fever virus was only observed with 7-deoxy analogues. In most cases, however, selectivity of the active compounds was low, with toxicity in uninfected cells (TC₅₀) occurring at concentrations within 10-fold that of the viral inhibitory concentrations (IC₅₀). No activity was observed against human immunodeficiency virus-type 1, Venezuelan equine encephalomyelitis virus, or vaccinia viruses. Pancratistatin {4} and its 7-deoxy analogue 5 were evaluated in two murine Japanese encephalitis mouse models (dififering in viral dose challenge, among other factors). In two experiments (low LD₅₀ viral challenge, variant I), prophylactic administration of 4 at 4 and 6 mg/kg/day (2% EtOH/saline, sc, once daily for 7 days, day -1 to +5) increased survival of Japanese-encephalitis-virus-infected mice to 100% and 90%, respectively. In the same model, prophylactic administration of 5 at 40 mg/kg/day in hydroxypropylcellulose (sc, once daily for 7 days, day -1 to +5) increased survival of Japanese-encephalitis-virus-infected mice to 80%. In a second variant (high LD₅₀ viral challenge), administration of 4 at 6 mg/kg/day (ip, twice daily for 9 days, day -1 to +7) resulted in a 50% survival rate. In all cases, there was no survival in the diluent-treated control mice. Thus, 4 and 5 demonstrated activity in mice infected with Japanese encephalitis virus but only at near toxic concentrations. To our knowledge, however, this represents a rare demonstration of chemotherapeutic efficacy (by a substance other than an interferon inducer) in a Japanese-encephalitis-virus-infected mouse model.