Abstract
The purity of a drug substance can influence its toxicity and potency, so impurities must be specifically determined. In the case of synthetic oligodeoxyribonucleotide drugs, however, product complexity makes complete impurity speciation difficult. The goal of the present work was to develop a new analytical method for speciation of individual internal (n-1)mer impurities arising from formal nucleotide deletion in synthetic oligodeoxyribonucleotides. A complete series of oligodeoxyribonucleotide probes were designed, each complementary to an (n-1)mer deletion sequence of the drug in question. Glass plates were used as a solid support for individually immobilizing the entire probe array. The total mixture of internal (n-1) length impurities was isolated from a synthetic oligodeoxyribonucleotide by PAGE and labeled with 35S. Under stringently optimized conditions, only the perfectly sequence-matched oligodeoxyribonucleotide hybridized to each probe, while all other deletion sequences were removed by washing with buffer. The 35S signal intensity of the bound oligodeoxyribonucleotide was proportional to the concentration of each (n-1)mer deletion sequence in the analyte solution. This method has been applied to a number of synthetic phosphorothioate oligodeoxy-ribonucleotide lots and shown to be reliable for speciation and relative quantitation of the internal (n -1)mer deletion sequences present.