The invention relates to the use of nucleotide substitutes for increasing the in vivo efficacy of nucleic acid molecules and also for inhibiting inflammation in mammals. More particularly, the present invention relates to the use of 2'6'diaminopurine (DAP) and analogs thereof per se in anti-inflammatory compositions, and also for preparing nucleic acid molecules having an increased in vivo physiological efficiency and a reduced toxicity as compared to conventional oligos. The invention is particularly useful for the preparation of antisense oligonucleotides for treating pulmonary/respiratory diseases such as cystic fibrosis, asthma, chronic bronchitis, chronic obstructive lung disease, eosinophilic bronchitis, allergies, allergic rhinitis, pulmonary fibrosis, adult respiratory distress syndrome, sinusitis, respiratory syncytial virus or other viral respiratory tract infection and cancer.