Phosphorothioate oligonucleotides (S-ODNs) have the ability to modulate gene expression selectively and thus have potential therapeutic capabilities. This potential led us to investigate the protein binding characteristics of selected S-ODNs. We evaluated S-ODN interactions with bovine serum albumin (BSA) and human serum albumin (HSA) in vitro. The equilibrium dissociation constants Km for the binding of a 20 mer S-ODN with BSA and HSA range between 1.1-5.2 x 10(-5) and 2.4-3.1 x 10(-4) M, respectively. The Km for an unrelated 15 mer S-ODN binding with HSA ranges between 3.7 and 4.8 x 10(-5) M. Studies with a fluorescently labeled 27 mer S-ODN suggest cooperative binding (Hill slope = 1.67) and/or the presence of secondary binding sites on the S-ODN. HSA or BSA linked to Sepharose was incubated with a 15, 20, or 24 mer S-ODN followed by the addition of selected drugs known to be highly protein bound (nifedipine, warfarin, midazolam, probenecid, indomethacin, and mitoxantrone). Up to 30% of S-ODN was displaced by warfarin in competition binding assays. Conversely, HSA-bound warfarin was incubated with a variety of oligonucleotides, including RNA and genomic dsDNA. Maximum displacement of warfarin-bound HSA was observed following incubation with 5'-cholesterol-conjugated 20 mer S-ODN. In summary, S-ODNs are likely to interact and displace other therapeutic agents that bind to albumin, particularly those binding at site I.

A synthetic phosphorothioate oligonucleotide was administered systemically to five patients with either relapsed or refractory acute myelogenous leukemia (AML), or myelodysplastic syndrome (MDS). Patients received a 10-day continuous intravenous infusion of this compound, which is complementary to p53 mRNA. No major toxicity attributable to a dose of 0.05 mg/kg/hr was observed. A range of approximately 9 to 18% of the administered dose was recovered in the urine as intact oligonucleotide. Evaluation of malignant cells recovered from bone marrow and peripheral blood at intervals before, during, and after treatment reveals no enhanced growth potential following oligonucleotide administration. Hence, a phosphorothioate oligonucleotide complementary to p53 mRNA can be administered at this dose level to humans without major toxicity. Higher doses need to be evaluated for toxicity and potential clinical efficacy.

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