Advanced morpholino oligomers: a novel approach to antiviral therapy

Abstract

Phosphorodiamidate morpholino oligomers (PMOs) are synthetic antisense oligonucleotide analogs that are designed to interfere with translational processes by forming base-pair duplexes with specific RNA sequences. Positively charged PMOs (PMOplus™) are effective for the postexposure protection of two fulminant viral diseases, Ebola and Marburg hemorrhagic fever in nonhuman primates, and this class of antisense agent may also have possibilities for treatment of other viral diseases. PMOs are highly stable, are effective by a variety of routes of administration, can be readily formulated in common isotonic delivery vehicles, and can be rapidly designed and synthesized. These are properties which may make PMOs good candidates for use during responses to emerging or reemerging viruses that may be insensitive to available therapies or for use during outbreaks, especially in regions that lack a modern medical infrastructure. While the efficacy of sequence-specific therapies can be limited by target-site sequence variations that occur between variants or by the emergence of resistant mutants during infections, various PMO design strategies can minimize these impacts. These strategies include the use of promiscuous bases such as inosine to compensate for predicted base-pair mismatches, the use of sequences that target conserved sites between viral strains, and the use of sequences that target host products that viruses utilize for infection.

Fig. 1

Comparison of translational arrest mechanisms mediated by antisense molecules. During typical translational processes, assembly of the ribosomal complex at the AUG start codon facilitates de novo protein synthesis from template RNA. Complementary antisense molecules can arrest translation either by sterically blocking ribosomal assembly or by supporting RNAse H cleavage activity. ASO mediated translational arrest may interfere with critical processes required for virion production and assembly and could promote the formation of defective interfering viral particles. The presence of these altered proteins or defective particles may serve to stimulate productive immunological clearance mechanisms.

Fig. 2

Nucleic acids and ASO analogs discussed in this review.

Fig. 3

Accommodation of sequence mismatches by PMOplus^TM therapeutics. Sequences represent an alignment of a generalized MARV consensus genome with the PMOplus^TM molecules comprising AVI-6003. Positively charged piperazine groups (+) and inosine bases (I) are positioned in the molecules at sites where mismatches occur. Genomic sequences from representative MARV variants/isolates are shown as cDNA complementary to genomic RNA. The MARV isolates shown include those against which AVI-6003 has exhibited antiviral activity in in vivo models (mouse-adapted Ravn virus, guinea-pig adapted MARV-Musoke, and non-adapted MARV-Musoke), or which were isolated during outbreaks (Ang0998, 07DRC, 09DRC), or which may have been involved in weaponization programs (Poppinga).