Studies on anti-human immunodeficiency virus oligonucleotides that have alternating methylphosphonate/phosphodiester linkages

Preliminary investigations of the physical properties of oligonucleotide analogs that contain alternating methylphosphonate/phosphodiester linkages are described. An alternating oligo-2'-O-methylribonucleoside methylphosphonate, oligomer 1676, whose sequence is complementary to the upper hairpin region of human immunodeficiency virus TAR RNA, has been synthesized. This 15-mer forms a very stable duplex with its complementary RNA target, whose melting temperature is 71°C. Introduction of two mismatched bases reduces the melting temperature by 16°C. Similar results were obtained with the all-phosphodiester version of oligomer 1676, which demonstrates that introduction of the methylphosphonate linkages does not significantly perturb the ability of the oligo-2'-O-methylribonucleoside methylphosphonate to bind to RNA. Unlike the phosphodiester oligomer, however, oligomer 1676 is completely resistant to hydrolysis by the 3'-exonuclease activity found in mammalian serum. The interactions between nuclease-resistant, 5'-psoralen-derivatized, alternating oligo-2'-deoxypyrimidine methylphosphonates and double-stranded DNA were also studied. A 15-mer that contains thymine, 5-methylcytosine, and 5-propynyl-uracil forms a triplex with a polypurine tract found in the env gene of human immunodeficiency virus proviral DNA with an apparent dissociation constant of 400 nM at 22°C. Maximal triplex formation by these oligomers is observed at approximately 2.5 mM magnesium, whereas maximal triplex formation by the corresponding all-phosphodiester oligomers occurs between 10 and 20 mM magnesium. This reduced magnesium dependence most likely results from reduced charge repulsion between the backbones of the methylphosphonate oligomer and purine strand of the target. The nuclease stability and ability of the methylphosphonate oligomers to form stable complexes with their target nucleic acids suggest that these oligomers are potential candidates for use as antisense or antigene agents in cell culture Copyright (C) 2000 Elsevier Science Inc.