Aim: Imetelstat, a 13-mer oligonucleotide with a lipid tail is being evaluated for treating hematologic myeloid malignancies. This report describes the development of extraction and quantification methods for imetelstat. Methodology & results: Imetelstat was extracted using SPE (rat plasma) or by hybridization using a biotinylated capture probe (human plasma) and was quantified by LC-MS/MS. Calibration curves were established (0.1-50 μg/ml). Stability of imetelstat in plasma was demonstrated. Concentrations of imetelstat extracted using either of the methods and quantified with LC-MS/MS were comparable with a validated ELISA. Conclusion: Two extraction methods (solid phase and hybridization) were developed for quantifying imetelstat in plasma using LC-MS/MS. The hybridization extraction in combination with LC-MS/MS is a novel extraction approach. In recent years, oligonucleotide drugs (ODNs) have gained popularity owing to their ability to target specific genes and provide specificity. ODNs comprise an oligonucleotide sequence, containing 10-50 nucleotides [1,2]. Numerous ODNs have been investigated so far [1][2][3][4]. But only six ODNs (fomivirsen, pegaptanib, mipomersen, eteplirsen, nusinersen and defibrotide sodium) have been approved by the US FDA since 1998 [5].Pharmaceutical development of ODNs has several technical challenges such as maintaining in vivo stability, ensuring delivery and bioavailability, and minimizing off-target effects. These challenges can be minimized by chemical modifications of ODNs to enhance their pharmacokinetic and pharmacodynamic properties [4,6]. These chemical modifications include: sulfurization of the phosphodiester bond to avert degradation by enlindogenous exonucleases, addition of methoxy or methoxyethyl groups to sugar moieties [7,8], locked and unlocked nucleic acids, and alterations in the internucleotide linkage (e.g., amide linkage) resulting in peptide nucleic acids [1]. Sulfurization results in phosphorothioate analogs that are more hydrophobic, exhibit more complex secondary structures with higher protein binding and greater accumulation in organs than phosphodiester analogs [9][10][11].Imetelstat is a novel, first-in-class telomerase inhibitor ODN [12], being investigated for the treatment of hematologic myeloid malignancies [13,14] and certain solid tumors [12,15,16]. Imetelstat is a 13-mer oligonucleotide N3 -P5 thio-phosphoramidate (NPS) with a covalently linked C16 (palmitoyl) lipid moiety at the 5 -end (Figure 1; Supplementary Table 1). Addition of a lipid chain and the modified oligonucleotide backbone enables imetelstat to penetrate cells and tissues, with high biodistribution into normal and malignant cells [12].Imetelstat differs from antisense oligonucleotides in its mechanism of action. Imetelstat is complementary to the template region of the RNA component of telomerase, to which it binds with high affinity and specificity, and directly competes with telomere binding, thereby inhibiting telomerase activity [17]. Thus, imetelstat acts as a classical ac...