Antisense oligonucleotides comprising 5-aminoalkyl pyrimidine nucleotides

A modified oligonucleotide having the formula:

• ##STR18## wherein n is an integer of from 4 to 30;
• and
• each B is a radical independently selected from the group consisting of, ##STR19## adenine, guanine, thymine and cytosine, wherein
• each X is a linking group independently selected from the group consisting of C.sub.1 -C.sub.10 alkyl, C.sub.1 -C.sub.10 unsaturated alkyl, dialkyl ether and dialkylthioether;
• each Y is a cationic moiety independently selected from the group consisting of --(NH.sub.3).sup.+, --(NH.sub.2 R.sup.1).sup.+, --(NHR.sup.1 R.sup.2).sup.+, and --(NR.sup.1 R.sup.2 R.sup.3).sup.+ ;
• and
• R.sup.1, R.sup.2, and R.sup.3 are each independently lower alkyl having from one to ten carbon atoms,
• with the proviso that when n is from 4 to 8, no more than 30% Bs are adenine, guanine, cytosine or thymine, and when n is from 9 to 30, no more than 50% Bs are adenine, guanine, cytosine or thymine, wherein said modified oligonucleotide forms chimetic duplexes when subjected to a complementary natural oliogonucleotide in H.sub.2 O at pH 7 in the presence of 10 mM sodium phosphate, 0.1 mM EDTA and 50 mM sodium chloride.